Hybrid system and method for characterizing stress in chemically strengthened transparent substrates

Abstract

A scattered light polarization measurement (LSP) subsystem for a hybrid system for characterizing stress in a chemically strengthened (CS) substrate having a top surface and a near-surface waveguide includes an LSP light source system, an LSP light source actuator coupled to the LSP light source system, and an optical compensator within an optical path of an LSP laser beam emitted by the LSP light source system. The optical compensator includes a half wave plate, a half wave plate actuator, a diffuser, and a diffuser actuator. The LSP subsystem further includes an LSP detector system in optical communication with the optical compensator through an LSP coupling prism having an LSP coupling surface, a focusing lens and a focusing lens actuator, and a support plenum having a surface and a measurement aperture, the support plenum is configured to support the CS substrate at a measurement plane at the measurement aperture, and to support the LSP coupling prism in an operable manner.

Description

[0001] This application claims priority benefit under 35 U.S.C. §119 to U.S. Provisional Application No. 63 / 385,475, filed on November 30, 2022, the contents of which are relied upon and incorporated herein by reference in their entirety. Technical Field

[0002] The present disclosure relates to characterizing stress in chemically strengthened transparent substrates, and in particular, to a hybrid system and method for characterizing stress in chemically strengthened transparent substrates. Background Art

[0003] Chemically strengthened transparent substrates exhibit enhanced scratch and breakage resistance. These substrates are useful for a variety of display applications, from television screens and computer screens to mobile handheld device screens and watches. An example chemical strengthening process is ion exchange (IOX), whereby ions in the near-surface region of the glass substrate are exchanged with external ions (e.g., from a salt bath).

[0004] Fabricating chemically strengthened (CS) transparent substrates requires characterizing their stress properties to ensure that the CS substrate possesses the desired level of chemical strengthening suitable for a given application. Characterization typically involves measuring the stress profile of the CS substrate from the surface to the center, as well as related stress parameters such as surface compressive stress, knee stress, peak layer depth, total layer depth, compressive depth, and central tension. Other stress-related parameters include the variation of birefringence with depth within the CS substrate.

[0005] There are two main methods for characterizing stress in transparent CS substrates. The first method utilizes evanescent prism coupling spectroscopy (EPCS). The EPCS method uses a coupling prism to couple light into a guided mode supported by a near-surface waveguide (NSWG) formed in the substrate, for example, through an IOX process. The coupling prism is also used to couple light from the NSWG to form a guided mode spectrum. The guided mode spectrum contains a transverse electric (TE) mode spectrum with TE mode lines and a transverse magnetic (TM) spectrum with TM mode lines. The TE and TM mode lines are analyzed to extract stress-related properties, including stress profiles. The EPCS method is particularly suitable for characterizing stress in the near-surface region of the CS substrate (such as surface compressive stress and peak layer depth), but is not suitable for characterizing central tension (CT) and depth of compression (DOC) deeper within the substrate.

[0006] The second main method utilizes light scattering polarimetry (LSP). In LSP, an input laser illuminates a CS substrate at a relatively shallow angle through a coupling prism. Using an optical compensator, the laser polarization is continuously changed between different polarization states. The scattered light is detected by an image sensor. Stress in the CS substrate causes optical delay along the optical path, where the amount of stress is proportional to the derivative of the optical delay. The amount of optical delay can be determined based on the detected scattered light intensity distribution, which varies due to constructive and destructive interference of different effective path lengths of the detected light. The LSP method is suitable for measuring certain stress-related properties, such as central tension (CT) and depth of compression (DOC), but is not suitable for measuring near-surface stress-related properties.

[0007] Currently, to fully characterize the stress profile of a CS substrate from the surface to the center, the CS substrate is first measured using an EPCS measurement system, then moved to an LSP measurement system and the two measurements stitched together. This is time-consuming and requires handling the CS substrate when moving it between the two measurement systems, which carries the risk of breakage.

[0008] Therefore, it would be more advantageous to have a single measurement system capable of performing both EPCS and LSP measurements. Summary of the Invention

[0009] The hybrid measurement system and method disclosed in this paper can perform comprehensive stress characterization of transparent CS substrates, including surface stress S(0), near-surface compressive stress profile S(x) (including knee stress S k =S(x k )), depth of layer DOL, central tension CT, and depth of compression DOC. A complete stress characterization is obtained by combining stress calculations using EPCS and LSP measurements.

[0010] Embodiments of the present disclosure relate to a scattered light polarimetry (LSP) subsystem of a hybrid system for characterizing stress in a chemically strengthened (CS) substrate having a top surface and a near-surface waveguide, the subsystem comprising an LSP light source system; an LSP light source actuator coupled to the LSP light source system, wherein the LSP light source actuator is operable to rotate the orientation of the LSP light source system; and an optical compensator within an optical path of an LSP laser beam emitted by the LSP light source system. The optical compensator comprises a half-wave plate and a half-wave plate actuator operable to rotate the half-wave plate, and a diffuser and a diffuser actuator operable to translate the diffuser along the optical path. The LSP subsystem further comprises an LSP detector system in optical communication with the optical compensator via an LSP coupling prism having an LSP coupling surface; a focusing lens and a focusing lens actuator operable to translate the focusing lens along the optical path; and a support gas cell having a surface and a measurement aperture, the support gas cell being configured to support the CS substrate at a measurement plane at the measurement aperture and operable to support the LSP coupling prism.

[0011] Another embodiment of the present disclosure relates to a method for measuring first and second stress properties of a chemically strengthened (CS) substrate having a surface, a near-surface waveguide proximate the surface, and a backing surface opposite the surface, the method comprising applying at least one ink line to the backing surface of the CS substrate; operably positioning the surface of the CS substrate at a measurement position relative to a coupling prism assembly, the coupling prism assembly comprising an evanescent wave prism coupling spectroscopy (EPCS) coupling prism and a light scattering polarimetry (LSP) coupling prism to define adjacent EPCS and LSP coupling interfaces, respectively; performing an EPCS measurement of the CS substrate using the EPCS coupling interface to obtain a first stress property and performing an LSP measurement of the CS substrate using the LSP coupling interface to obtain a second stress property, without removing the coupling prism assembly or the CS substrate from the measurement position; and combining the first and second stress properties to define a complete stress characterization of the CS substrate, wherein the first stress property is selected from a first stress property group comprising: surface compressive stress S(0), total layer depth DOLT, peak layer depth DOLsp, knee stress CSk, and birefringence B, and the second stress property comprises at least one of compressive depth DOC and central tension CT.

[0012] Another embodiment of the present disclosure relates to a method for processing a light scattering polarimetry (LSP) image, comprising forming an LSP image on a digital detector; processing the LSP image to form an optical retardation (OR) versus depth (D) curve of a retardation profile, the curve including OR data points defining first and second inflection points and first and second end regions; and determining a top window and a bottom window of the OR versus D curve by: determining a first derivative curve of the OR versus D curve; selecting a starting edge and an ending edge of the retardation profile corresponding to a first minimum and a second minimum of the first derivative curve, respectively, wherein the first region includes a depth less than or equal to the starting edge and the second region includes a depth greater than or equal to the ending edge; determining a curvature of the OR versus D curve; determining a centroid to the left of the curvature defining the first inflection point and a centroid to the right of the curvature defining the second inflection point. The top window is defined by a top line defined by a maximum retardation of the OR versus D curve, a bottom line defined by a maximum retardation percentage of the OR versus D curve, a starting edge, and a ending edge. The bottom window is defined by the following: the lowest line defined by the minimum delay of the OR and D curve, the upper line defined by the maximum delay percentage of the OR and D curve, the starting edge, and the ending edge.

[0013] In a first embodiment, a scattered light polarimetry (LSP) subsystem of a hybrid system for characterizing stress in a chemically strengthened (CS) substrate having top surface and near-surface waveguides comprises:

[0014] LSP light source system;

[0015] an LSP light source actuator coupled to the LSP light source system, wherein the LSP light source actuator is operable to rotate the orientation of the LSP light source system;

[0016] An optical compensator, which is within the optical path of an LSP laser beam emitted by the LSP light source system, the optical compensator comprising:

[0017] a half-wave plate and a half-wave plate actuator operable to rotate the half-wave plate, and

[0018] a diffuser and a diffuser actuator operable to translate the diffuser along the optical path;

[0019] an LSP detector system in optical communication with the optical compensator via an LSP coupling prism having an LSP coupling surface;

[0020] a focus lens and a focus lens actuator operable to translate the focus lens along the optical path; and

[0021] A support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture and operably support the LSP coupling prism.

[0022] In a second embodiment, the first embodiment further includes an evanescent wave prism coupled spectroscopy (EPCS) subsystem comprising an EPCS light source system and an EPCS detector system in optical communication via an EPCS coupling prism having an EPCS coupling surface.

[0023] In a third embodiment, for the second embodiment, the EPCS subsystem further comprises a polarizer positioned within the optical path at a distance from the detector device of the EPCS detector system in the range of XX to XX, inclusive.

[0024] In a fourth embodiment, for any one of the first to third embodiments, the half-wave plate actuator automatically rotates the half-wave plate so that the LSP laser beam reaches a predetermined beam intensity.

[0025] In a fifth embodiment, for any of the first to fourth embodiments, wherein the translatable diffuser actuator comprises a stepper motor.

[0026] In a sixth embodiment, for any of the first to fifth embodiments, wherein the translatable diffuser actuator automatically adjusts the position of the diffuser.

[0027] In a seventh embodiment, for any one of the first to sixth embodiments, the focus lens actuator comprises a stepping motor.

[0028] In an eighth embodiment, for any one of the first to seventh embodiments, the focus lens actuator automatically adjusts the position of the focus lens.

[0029] In a ninth embodiment, a method of measuring first and second stress characteristics of a chemically strengthened (CS) substrate having a surface, a near-surface waveguide proximate the surface, and a backing surface opposite the surface comprises:

[0030] applying at least one line of ink to the backing surface of the CS substrate;

[0031] operably positioning a surface of the CS substrate at a measurement position relative to a coupling prism assembly comprising an evanescent wave prism coupling spectroscopy (EPCS) coupling prism and a light scattering polarimetry (LSP) coupling prism to define adjacent EPCS and LSP coupling interfaces, respectively;

[0032] performing an EPCS measurement of the CS substrate using the EPCS coupling interface to obtain a first stress characteristic and performing an LSP measurement of the CS substrate using the LSP coupling interface to obtain a second stress characteristic without removing the coupling prism assembly or the CS substrate from a measurement position; and

[0033] The first and second stress characteristics are combined to define a complete stress characterization of the CS substrate, wherein the first stress characteristic is selected from a first stress characteristic group including: surface compressive stress S(0), total layer depth DOLT, peak layer depth DOLsp, knee stress CSk and birefringence B, and the second stress characteristic includes at least one of compression depth DOC and central tension CT.

[0034] In a tenth embodiment, with respect to the ninth embodiment, performing LSP measurement includes:

[0035] forming the LSP image on a digital detector to define a raw digital LSP image;

[0036] Performing Gaussian blur processing on the original digital LSP image to form a Gaussian blurred LSP image;

[0037] performing Otsu thresholding on the Gaussian blurred image to define a threshold image; and

[0038] The optical delay versus depth into the CS substrate was calculated using the thresholded image to obtain the second stress characteristic.

[0039] In an eleventh embodiment, with respect to the ninth embodiment or the tenth embodiment, performing LSP measurement includes:

[0040] forming the LSP image on a digital detector to define a raw digital LSP image;

[0041] Performing Gaussian blur processing on the original digital LSP image to form a Gaussian blurred LSP image;

[0042] performing a binarization method on the Gaussian blurred LSP image to define an image contour; and

[0043] The image profile is used to calculate the optical delay and the depth into the CS substrate to obtain the second stress characteristic.

[0044] In a twelfth embodiment, for any one of the ninth to eleventh embodiments, performing LSP measurement includes:

[0045] forming an LSP image on a digital detector;

[0046] processing the LSP image to form an optical retardation (OR) versus depth (D) curve comprising OR data points; and

[0047] A combination of linear and quadratic functions was used to obtain the fitting curve of the OR data points.

[0048] In a 13th embodiment, for any one of the 9th to 12th embodiments, performing LSP measurement includes:

[0049] forming an LSP image on a digital detector;

[0050] processing the LSP image to form an optical retardation (OR) versus depth (D) curve of the retardation profile, the curve including OR data points including first and second inflection points; and

[0051] Do at least one of the following:

[0052] i) obtaining a fitting curve of the OR data points using a power-spike function; and

[0053] ii) shifting the OR data point so that the first and second inflection points are symmetric about the mid-plane of the CS substrate, thereby defining a shifted OR data point; and

[0054] The shifted OR data points were used to obtain the depth of compression DOC measurements for the CS substrate.

[0055] In a 14th embodiment, for any one of the 9th to 13th embodiments, further comprising:

[0056] forming an LSP image on a digital detector;

[0057] processing the LSP image to form an optical retardation (OR) versus depth (D) curve of the retardation profile, the curve including OR data points defining first and second inflection points and first and second end regions; and

[0058] Do at least one of the following:

[0059] i) performing first and second curve fits on the OR data points defining the first and second inflection points, respectively, to determine the central tension CT of the CS substrate;

[0060] ii) performing curve fitting on the OR data points between the first inflection point and the second inflection point to determine the depth of compression DOC of the CS substrate; and

[0061] iii) curve fitting the OR data points within a span excluding the first and second end regions to define an OR fit curve, and determining at least one of a central tension CT and a depth of compression DOC of the CS substrate using the OR fit curve.

[0062] In a 15th embodiment, according to the 14th embodiment, the curve fitting comprises performing a polynomial optimization process that determines an optimal polynomial order by applying a cost function.

[0063] In a sixteenth embodiment, for the fourteenth embodiment, curve fitting is performed within the top window and the bottom window of the OR vs. D curve.

[0064] In the seventeenth embodiment, with respect to the sixteenth embodiment, the top window and the bottom window are determined by the following operations:

[0065] Determine the first derivative curve of the OR and D curves;

[0066] selecting a starting edge and an ending edge of the delay profile corresponding to a first minimum and a second minimum of the first-order derivative curve, respectively, wherein the first region includes a depth less than or equal to the starting edge and the second region includes a depth greater than or equal to the ending edge;

[0067] Determine the curvature of the OR and D curves;

[0068] determining a centroid of a left side defining the curvature of a first inflection point and a centroid of a right side defining the curvature of a second inflection point; and

[0069] The top window is defined by:

[0070] The highest line defined by the maximum delay of the OR and D curves;

[0071] The lower line defined by the maximum delay percentage of the OR vs. D curve;

[0072] Starting edge; and

[0073] Termination edge;

[0074] The bottom window is defined by:

[0075] The lowest line defined by the minimum delay of the OR and D curves

[0076] Upper line defined by the maximum delay percentage of the OR vs. D curve

[0077] Starting edge; and

[0078] Terminate edge.

[0079] In an eighteenth embodiment, the seventeenth embodiment further includes applying a smoothing filter to the OR and D curves.

[0080] In a 19th embodiment, based on the 18th embodiment, the smoothing filter is a LOESS filter.

[0081] In a 20th embodiment, for any one of the 14th to 19th embodiments, further comprising identifying an entry point and an exit point of the LSP laser beam on the CS substrate.

[0082] In a 21st embodiment, for the 20th embodiment, wherein the LSP image includes a plurality of bright spots, and identifying the entry point and the exit point includes:

[0083] filtering a subset of the plurality of bright spots that are outside the upper and lower 50% of the CS substrate thickness and outside ±10 degrees around the laser propagation angle;

[0084] forming a plurality of bright spot combinations of the remaining bright spot sets among the plurality of bright spots;

[0085] Scoring each of the plurality of bright spot combinations to determine its proximity to the CS substrate thickness, its proximity to the laser propagation angle, and the median light intensity between each bright spot in the bright spot combination;

[0086] Normalize the scores of highlight combinations;

[0087] For each bright spot combination, sum the normalized scores of proximity to the CS substrate thickness, proximity to the laser propagation angle, and the median intensity between each bright spot in the bright spot combination; and

[0088] The highlight combinations with the highest total scores among the highlight combinations are selected as the entry point and the exit point.

[0089] In a 22nd embodiment, for any one of the 14th to 21st embodiments, the ink line is drawn with a black permanent marker.

[0090] In a 23rd embodiment, a method of processing a light scattering polarimetry (LSP) image includes:

[0091] forming an LSP image on a digital detector;

[0092] processing the LSP image to form an optical retardation (OR) versus depth (D) curve of the retardation profile, the curve including OR data points defining first and second inflection points and first and second end regions;

[0093] Determine the top and bottom windows of the OR and D curves by doing the following:

[0094] Determine the first derivative curve of the OR and D curves;

[0095] selecting a starting edge and an ending edge of the delay profile corresponding to a first minimum and a second minimum of the first-order derivative curve, respectively, wherein the first region includes a depth less than or equal to the starting edge and the second region includes a depth greater than or equal to the ending edge;

[0096] Determine the curvature of the OR and D curves;

[0097] determining a centroid of a left side defining the curvature of a first inflection point and a centroid of a right side defining the curvature of a second inflection point; and

[0098] The top window is defined by:

[0099] The highest line defined by the maximum delay of the OR and D curves;

[0100] The lower line defined by the maximum delay percentage of the OR vs. D curve;

[0101] Starting edge; and

[0102] Termination edge;

[0103] The bottom window is defined by:

[0104] The lowest line defined by the minimum delay of the OR and D curves;

[0105] The upper line defined by the maximum delay percentage of the OR vs. D curve;

[0106] Starting edge; and

[0107] Terminate edge.

[0108] In a 24th embodiment, the 23rd embodiment further includes:

[0109] Do at least one of the following:

[0110] i) performing first and second curve fits on the OR data points defining the first and second inflection points, respectively, to determine the central tension CT of the CS substrate;

[0111] ii) performing curve fitting on the OR data points between the first inflection point and the second inflection point to determine the depth of compression DOC of the CS substrate; and

[0112] iii) curve fitting the OR data points within a span excluding the first and second end regions to define an OR fit curve, and determining at least one of a central tension CT and a depth of compression DOC of the CS substrate using the OR fit curve.

[0113] In a 25th embodiment, according to the 24th embodiment, the curve fitting comprises performing a polynomial optimization process to determine an optimal polynomial order by applying a cost function.

[0114] In a 26th embodiment, according to the 23rd or 24th embodiment, further comprising applying a smoothing filter to the OR and D curves.

[0115] In a 27th embodiment, based on the 26th embodiment, the smoothing filter is a LOESS filter.

[0116] In a 28th embodiment, for any one of the 23rd to 27th embodiments, further comprising identifying an entry point and an exit point of the LSP laser beam on the CS substrate.

[0117] In a 29th embodiment, with respect to the 28th embodiment, wherein the LSP image includes a plurality of bright spots, and identifying the entry point and the exit point includes:

[0118] filtering a subset of the plurality of bright spots that are outside the upper and lower 50% of the CS substrate thickness and outside ±10 degrees around the laser propagation angle;

[0119] forming a plurality of bright spot combinations of the remaining bright spot sets among the plurality of bright spots;

[0120] Scoring each of the plurality of bright spot combinations to determine its proximity to the CS substrate thickness, its proximity to the laser propagation angle, and the median light intensity between each bright spot in the bright spot combination;

[0121] Normalize the scores of highlight combinations;

[0122] For each bright spot combination, sum the normalized scores of proximity to the CS substrate thickness, proximity to the laser propagation angle, and the median intensity between each bright spot in the bright spot combination; and

[0123] The highlight combinations with the highest total scores among the highlight combinations are selected as the entry point and the exit point.

[0124] The described embodiments may be combined in any suitable combination. Additional features and advantages will be set forth in the detailed description that follows, and in part will be apparent to those skilled in the art from this specification or learned by practicing the embodiments described in the written description and claims hereof as well as in the accompanying drawings. It should be understood that both the foregoing general description and the following detailed description are merely exemplary and are intended to provide an overview or framework for understanding the nature and character of the claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0125] The accompanying drawings are included to provide a further understanding and are incorporated into and constitute a part of this specification. The drawings illustrate one or more embodiments and, together with the detailed description, serve to explain the principles and operation of the various embodiments. Thus, the present disclosure will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0126] Figure 1A is a front view of an example transparent CS substrate in the form of a planar sheet.

[0127] Figure 1B Example refractive index profile n(z) vs. z for an example transparent CS substrate, showing a near-surface peak region (R1), a deeper region (R2), and a bulk region (R3), with a knee (KN) at the transition between regions R1 and R2.

[0128] Figure 2A Schematic diagram of the hybrid EPCS-LSP measurement system disclosed in this article for comprehensive stress characterization of transparent CS substrates.

[0129] Figure 2B for Figure 2A A more detailed schematic diagram of a hybrid EPCS-LSP system showing example configurations of the EPCS measurement subsystem and the LSP measurement subsystem.

[0130] Figure 3A for Figure 2A Schematic diagram of an example EPCS subsystem of a hybrid EPCS-LSP system.

[0131] Figure 3B FIG. 4 is a schematic diagram of an exemplary EPCS mode spectrum obtained by the EPCS subsystem, wherein the EPCS mode spectrum includes a TM mode spectrum having TM mode lines (fringe) and a TE mode spectrum having TE mode lines (fringe).

[0132] Figure 3C This is an EPCS image when the polarizer is relatively far away from the digital detector.

[0133] Figure 3D is the position ratio of the polarizer Figure 3C The polarizer is placed closer to the digital detector for EPCS images.

[0134] Figures 4A to 4C for Figure 2A Schematic diagram of an example LSP subsystem of a hybrid EPCS-LSP system.

[0135] Figure 4D is a close-up view of an LSP image formed on a digital detector of an LSP subsystem, wherein the LSP image includes two line images forming a cross or "X" pattern, and wherein the LSP image and the digital detector form a digital LSP image.

[0136] Figure 4E Schematic diagram of an example LSP light source test fixture.

[0137] Figure 4F Schematic diagram of an example optical model for generating optimal optical power at an optimal wavelength from an LSP light source.

[0138] Figure 4G A plot showing how filters block light due to a mismatch in laser wavelength.

[0139] Figures 4H to 4J Schematic diagram of an example LSP subsystem with adjustable components.

[0140] Figure 4KAn example LSP image showing entry and exit points.

[0141] Figure 4L for Figure 4K An example LSP image showing the algorithmically determined indications of entry and exit points.

[0142] Figure 4M for Figure 4B Simplified diagram of the LSP subsystem.

[0143] Figure 4N A diagram showing an LSP beam that appears thicker to the LSP detector system.

[0144] Figure 4O A diagram showing an LSP beam that appears thinner to an LSP detector system.

[0145] Figures 4P to 4S Diagram showing various LSP light source and LSP detector system orientations.

[0146] Figure 5A Front view of an example prism support structure for supporting EPCS and LSP coupling prisms.

[0147] Figure 5B For installation in Figure 5A Front view of the cover plate on the prism support structure.

[0148] Figure 6A and 6B is a side view of an EPCS and LSP coupling prism supported on a stable platform and illustrates an example method of forming an integrally molded prism support structure for the coupling prism assembly.

[0149] Figure 6C is a side view of an example coupling prism assembly in which the prism support structure is configured such that at least one of the EPCS and LSP coupling prisms is movable relative to the other in one direction (eg, the z-direction, as shown).

[0150] Figure 6D Schematic diagram of an example hybrid EPCS-LSP measurement system, in which a single coupling prism is used for both the EPCS subsystem and the LSP subsystem instead of two separate coupling prisms.

[0151] Figure 7 is a cross-sectional view of an example prism support structure attached to an example support plenum of a hybrid system and showing an example movable substrate holder for adjusting the measurement position on a CS substrate.

[0152] Figure 8AA front view of the supporting gas cell is shown showing the measurement aperture and a pressure vacuum (PV) strip of the vacuum system operably disposed within the measurement aperture to pneumatically engage the CS substrate, thereby pulling the CS substrate onto the coupling surface of the EPCS and LSP coupling prism.

[0153] Figure 8B A close-up cross-sectional view of the supporting gas cell and measurement aperture is shown, showing an example configuration of the coupling prism assembly and vacuum system.

[0154] Figure 8C LSP images without backing material.

[0155] Figure 8D An image of LSP using a black permanent marker as the backing material.

[0156] Figure 8E is another LSP image without backing material.

[0157] Figure 8F Figure 2 is an image of LSP using standard oil and an immersion lens as backing material.

[0158] Figure 8G is the average LSP image using standard oil and immersion lens as backing material.

[0159] Figure 8H Another LSP image using black permanent marker line as backing material.

[0160] Figure 8I The average LSP image was obtained using a black permanent marker line as the backing material.

[0161] Figure 8J Image of LSP using polarization filter and immersion oil as backing material.

[0162] Figure 8K Image of LSP using black plastic capsules and immersion oil as backing material.

[0163] Figure 8L An LSP image using a dry erase marker as the backing material.

[0164] Figure 8M Image of LSP using red permanent marker as backing material.

[0165] Figure 8N An image of LSP using a black permanent marker as the backing material.

[0166] Figure 9Schematic diagram of an example user interface presented to a system controller, wherein the user interface includes an EPCS section displaying an EPCS mode spectrum and an LSP section displaying an LSP line image of a digital LSP image.

[0167] Figure 10A As an example of the LSP section of the user interface, an example digital LSP image and an intensity histogram of the digital LSP image are shown.

[0168] Figure 10B Example original or raw digital LSP images and Gaussian blurred ("blurred") LSP images are shown.

[0169] Figure 10C Show that by applying Ostu thresholding to Figure 10B Example thresholded image obtained from a Gaussian blurred image.

[0170] Figure 10D and 10E An example showing contour detection on a sample Gaussian blurred LSP image.

[0171] Figure 11A A close-up view of the CS substrate and the focused LSP beam direction.

[0172] Figure 11B It is a close-up view of the edge portion of the CS substrate and shows the viewing angle of the LSP detector system relative to the focused LSP beam.

[0173] Figure 11C Similar to Figure 11B , and shows the scattered light beam reaching the LSP detector system and forming a line image.

[0174] Figure 11D Another view showing the LSP detector system and CS substrate with the focused LSP beam.

[0175] Figure 11E Schematic diagram showing the dimensions and angles used to determine the thickness of the CS substrate.

[0176] Figure 12A To extract the phase of the noisy LSP signal Plot of the required average computation time T (in milliseconds (ms)) versus the noise factor N for the lock method (lock or L) and the sine method (sine or S).

[0177] Figure 12B is the absolute phase difference between the lock method (lock or L) and the sine method (sine or S) used to process the noisy LSP signal Plot of φ vs. noise factor.

[0178] Figure 13A and 13B is a plot of optical retardation OR (radians) versus depth D (mm) into the CS substrate (“OR vs. D Plot”), where Figure 13A FIG. 13 shows OR data collected by the LSP subsystem without speckle noise suppression and FIG. 14 shows OR data collected by the LSP subsystem with speckle noise suppression.

[0179] Figure 14A and 14B FIG2 is an OR versus D plot illustrating an example method of shifting OR data to make bending points BP1 and BP2 symmetric about the mid-plane of the CS substrate.

[0180] Figure 15A is an example OR vs. D plot containing discrete data points (circles) and a fitted line for the OR vs. D data points, where the fitted line is formed using the "LinQuad" method disclosed herein.

[0181] Figure 15B Based on Figure 15A Plot of stress S (MPa) versus depth D (mm) from the LinQuad fit of the OR and D data points.

[0182] Figure 16A is an example OR versus D plot containing discrete data points (circles) and a fitted line for the OR versus D data points, where the fitted line is formed using the power-spike method disclosed herein.

[0183] Figure 16B Based on Figure 16A Plot of stress S (MPa) versus depth D (mm) for power-spike fits of OR versus D data points (“S versus D Plot”).

[0184] Figure 17A and 17B To show the original (raw) OR and D data points ( Figure 17A ) and OR with the symmetric component removed with D data ( Figure 17B ) plotted against OR and D using the LinQuad curve fit.

[0185] Figure 18A and 18B To illustrate the OR versus D plot using reduced area fitting regions when calculating selected stress parameters, Figure 18A shows the reduced area fitting region for calculating the depth of compression DOC at the bending points BP1 and BP2 and Figure 18B The reduced area fitting region for calculating the central tension CT between the bending points BP1 and BP2 is shown.

[0186] Figure 19APlot OR vs. D and Figure 19B ] is the corresponding S vs. D plot, where a curve fit is performed for the entire OR data set.

[0187] Figure 19C Plot OR vs. D and Figure 19D ] are corresponding S vs. D plots where the curve fit is performed for a simplified OR data set that does not include the data portion near the opposite endpoints.

[0188] Figure 19E is a plot of R versus D after 20% smoothing filtering of the OR data.

[0189] Figure 19F for Figure 19E Plot of the derivative of the smoothed R data and Figure 19E Plot of the R vs. D plot.

[0190] Figure 19G for Figure 19E Plot of the curvature of the smoothed R data and Figure 19E Plot of the R vs. D plot.

[0191] Figure 19H Plot R vs. D with top and bottom windows.

[0192] Figure 19I For use Figure 19H The top and bottom windows are plotted with the cost of the polynomial fit versus the order.

[0193] Figure 19J R and D plots of the polynomial fit for the two inflection point regions.

[0194] Figure 20 Similar to Figure 3A , and shows an embodiment of an EPCS subsystem wherein the detector system includes an adjustable focus lens, wherein the adjustability includes at least one of axial movement and changing focal length.

[0195] Figure 21A and 21B Schematic diagram of example support components used to form a focusing lens assembly of an EPCS subsystem to provide a method for adjusting the contrast of a captured modal spectrum. DETAILED DESCRIPTION

[0196] Reference will now be made in detail to various embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same or similar reference numerals and symbols are used throughout the drawings to refer to the same or similar parts. The drawings are not necessarily drawn to scale, and those skilled in the art will recognize that the drawings have been simplified to illustrate key aspects of the present disclosure.

[0197] The claims set forth below are incorporated into and constitute a part of this detailed description.

[0198] Cartesian coordinates are shown in some of the drawings for reference purposes and are not intended to limit direction or orientation.

[0199] In some parts of the discussion, the z coordinate is used for the depth direction into the substrate, while in other parts of the discussion, different coordinates are used.

[0200] The acronym "IOX" stands for "ion exchange" or "ion exchanged," depending on the context of discussion.

[0201] The acronym “CS” when used to describe a substrate type (eg, “CS substrate”) means “chemically strengthened.” The acronym CS may also mean “compressively stressed,” and the meaning of this acronym will be apparent from the context of the discussion.

[0202] The term "strengthening" of a CS substrate as considered herein means that the original CS substrate has undergone a process to create some stress profile, which can have various shapes, generally intended to make the CS substrate stronger and therefore harder to break. Example strengthening processes include ion exchange, tempering, annealing, and similar thermal processes.

[0203] The term "transparent" used with respect to a CS substrate means that the CS substrate is transparent at a given measurement wavelength (i.e., the EPCS wavelength λ A or LSP wavelength λ B ) to make satisfactory measurements on CS substrates (i.e., EPCS measurements or LSP measurements) and thus to make sufficiently accurate measurements of the stress characteristics associated with a given measurement.

[0204] The abbreviation "ms" stands for "milliseconds."

[0205] The abbreviation "nm" stands for "nanometer".

[0206] The term “near-surface”, for example when referring to a near-surface waveguide or a near-surface peak region of a CS substrate, refers to the portion of the substrate body that is immediately adjacent to a given surface (eg, top or measurement surface) of the CS substrate.

[0207] In the example, a glass substrate is used to form a CS substrate. As used herein, the term "glass substrate" encompasses any object made entirely or partially of glass, such as a laminate of glass and a non-glass material, a laminate of glass and a crystalline material, and glass ceramics (including both amorphous and crystalline phases). Thus, in the example, a "glass CS substrate" may be composed entirely of a glass material, while in another example, it may be composed entirely of a glass ceramic material.

[0208] The terms "image" and "line image" are used in this article to describe the light distribution (i.e., intensity distribution) of a portion of an X-shaped LSP image formed by light scattered by the LSP subsystem at a digital detector (CCD camera or CMOS sensor, etc.), and an imaging system is not required to form the LSP image considered in this article.

[0209] In the following discussion, the LSP subsystem is configured to cycle between two or more polarization states (or simply "polarizations"). In an example, up to eight different polarization states may exist per cycle, combining linear, elliptical, and circular polarizations known in the art. A greater number of polarizations may be used, for example, up to one hundred or more polarizations.

[0210] As used herein, the term "stress" may generally mean compressive stress or tensile stress. Figure 15B 、 16B In the plots of 19B, 19D, compressive stress is negative and tensile stress is positive. Whether the stress is compressive or tensile depends on the location or depth region of the CS substrate under consideration. Positive values of compressive stress should be understood to mean the magnitude of the compressive stress. Stress is represented by S or σ, and unless otherwise indicated or the context of the discussion indicates otherwise, it refers to compressive stress. In some cases, compressive stress is represented as CS, such as knee stress CS k The stress profile is the stress S as a function of depth into the CS substrate, and the depth coordinate can be any local coordinate, and in the discussion below, both z and x are used as local coordinates.

[0211] In an example, "characterization" of the CS substrate includes determining one or more stress-based properties of the CS substrate, such as stress profile S(z), depth of layer DOL, surface stress S(0), depth of compression DOC, central tension CT, and birefringence profile B(z). In an example, the characterization utilizes EPCS and LSP measurements, which provide first and second stress properties, respectively, that, when combined, provide a "comprehensive characterization" of the stress properties of the CS substrate, wherein the term "comprehensive characterization" means a more complete characterization of stress and stress-related properties than using only the first stress property measured by EPCS or only the second stress property measured by LSP.

[0212] Unless otherwise stated, the acronym "OR" stands for "optical retardation" and is measured in radians ("rad"). A plot of optical retardation versus depth into a CS substrate is hereinafter referred to as an "OR versus D" curve or plot, where D is understood to be the depth from the top (measurement) surface into the bulk of the CS substrate.

[0213] The term "index matching fluid" means a fluid having a refractive index substantially the same as another material to facilitate optical coupling. In an example, the index matching fluid comprises an oil or a mixture of oils. The refractive index of the index matching fluid is represented by n f or n oil These two expressions are used interchangeably in the following text.

[0214] CS substrate

[0215] Figure 1A FIG1 is a front view of an exemplary type of CS substrate 10 in the form of a planar sheet. CS substrate 10 has a body 11, a top surface 12, a bottom surface 14, and side surfaces 16. CS substrate 10 has a thickness TH and a midplane MP intermediate and parallel to top and bottom surfaces 12 and 14.

[0216] In some cases, the thickness TH can be in the range of 0.020 mm≤TH≤2 mm, such as 0.050 mm≤TH≤2 mm, 0.20 mm≤TH≤2 mm, 0.25 mm≤TH≤2 mm, 0.3 mm≤TH≤2 mm, or 0.3 mm≤TH≤1 mm, and any and all subranges formed therebetween.

[0217] An example type of CS substrate 10 is a glass substrate, and is used as a protective cover for a display and / or housing of a mobile device such as a smartphone, tablet computer, laptop computer, GPS device, etc. Figure 1A As shown in , these CS substrates 10 tend to be thin and flat.

[0218] The CS substrate 10 includes a near surface waveguide (NSWG) 18 in the body 11 near the top surface 12. In an example, the NSWG 18 is formed using an IOX process and is defined by at least one IOX region of different refractive index.

[0219] Figure 1B The refractive index n of an example NSWG 18 is plotted against the depth z into the CS substrate. The surface refractive index is denoted as n S , while the bulk refractive index, i.e., the refractive index of the substrate material not affected by the chemical strengthening process, is expressed as n B .

[0220] Figure 1BThe plot of FIG shows an example refractive index profile n(z) defining two (10X) regions, a first near-surface peak region R1 and a second depth region R2. There is also a third region R3 that is deeper than the second depth region and is referred to herein as having a refractive index n(z). B The near-surface peak region R1 has the maximum refractive index n at the surface. s , and the refractive index decreases rapidly with depth (z) to a relatively shallow depth z = D1 (which defines the first "peak" layer depth DOL sp ) when the value n k The refractive index of the depth region R2 changes from n k The depth D2 is reduced more slowly downwards, which defines the total layer depth DOL from which the third body region R3 begins. T The first region R1 and the second region R2 are at z=z k The refractive index n=n k And its relationship with knee (compression) stress CS k associated.

[0221] Because of the two regions of different refractive index, R1 and R2, in NSWG 18, some guided modes propagate only in the uppermost peak region, R1, while other guided modes travel in both regions R1 and R2, and still other guided modes travel only in the deep region, R2. The remaining refractive index profile, n(z), contains a more uniform refractive index variation. Some of the deep guided modes may extend into the bulk region, R3.

[0222] Figure 1B The refractive index profile n(z) may be formed by a double IOX (DIOX) process, wherein one IOX process forms the depth region R2 and another IOX process different from the first IOX process forms the peak region R1. Figure 1B The plot of φ represents a DIOX process performed in a lithium-containing glass substrate 10, wherein lithium ions are exchanged with potassium and sodium ions in two different IOX processes, wherein the potassium IOX process produces a peak region R1.

[0223] Hybrid EPCS-LSP system

[0224] Figure 2A Schematic diagram of a hybrid EPCS-LSP measurement system (“hybrid system”) 20 disclosed herein, shown along with an example CS substrate 10. The hybrid system 20 includes a coupling prism assembly 40, an EPCS measurement subsystem (“EPCS subsystem”) 100, an LSP measurement subsystem (“LSP subsystem”) 200, and a system controller 400. The coupling prism assembly 40 defines a measurement location ML on the CS substrate 10.

[0225] The EPCS subsystem 100 generates an EPCS measurement signal SA representing a first stress characteristic of the CS substrate at the measurement location ML, as reflected by the mode spectrum of the guided mode of the NSWG 18. The first stress characteristic may include one or more of the following: surface compressive stress S(0), total layer depth DOL T , peak layer depth DOL sp , knee stress CS k and birefringence B.

[0226] The LSP subsystem 100 generates an LSP measurement signal SB representing a second stress characteristic of the CS substrate at a measurement location ML, as embodied by optical delay (OR) information as a function of depth into the CS substrate (including a depth region R2). The second stress characteristic may include one or more of the following: a stress profile, a compression depth DOC, and a central tension CT.

[0227] In an example, EPCS and LSP measurements of the first and second stress characteristics are performed without moving the measurement location ML. In another example, EPCS and LSP measurements of the first and second stress characteristics are performed by translating the coupling prism assembly 40 so that the EPCS and LSP measurements are performed at the same location on the substrate rather than at a portion of the measurement location slightly spaced apart as defined by the configuration of the coupling prism assembly.

[0228] In the example, EPCS and LSP measurements of the first and second stress characteristics are performed without removing the coupling prism assembly 40 or the CS substrate 10 from the measurement location ML. This is advantageous over the prior art in that EPCS and LSP measurements can be performed in a single system without having to remove or otherwise handle the CS substrate to bring it into different measurement systems.

[0229] The EPCS and LSP measurement signals SA and SB are sent to a system controller 400 for processing. The system controller 400 may include, for example, a microcontroller, a computer, a programmable logic controller (PLC), etc. In an example, the system controller 400 is configured with instructions embodied in a non-transitory computer-readable medium (e.g., software) for controlling the operation of the hybrid system 20 and performing calculations to determine the first and second stress characteristics of the CS substrate 10 based on the EPCS and LSP measurement signals SA and SB.

[0230] In an example, system controller 400 processes EPCS and LSP measurement signals SA and SB to define a stress profile and associated stress characteristics from top surface 12 of CS substrate 10 down to at least the bottom of depth region R2. In other words, system controller combines the first and second stress characteristics obtained from EPCS subsystem 100 and LSP subsystem 200 to produce a more complete or "global" stress profile of the CS substrate than would be possible using only one of the measurement subsystems.

[0231] The coupling prism assembly 40 includes an EPCS coupling prism 42A and an LSP coupling prism 42B operably supported by a prism support structure 46. The coupling prism assembly 40 is operably disposed on or near the top surface 12 of the CS substrate 10. In the examples discussed below, the EPCS coupling prism 42A and the LSP coupling prism 42B can be separate coupling prisms or different sections of a single (common) coupling prism.

[0232] Continue to refer Figure 2A , the hybrid system 20 includes an example housing 21 having dimensions L1 and L2. For a relatively compact embodiment of the hybrid system 20, the example dimensions L1 and L2 are in the range of 8 inches to 12 inches.

[0233] The EPCS subsystem 100 includes an EPCS light source system 110 and an EPCS detector system 140 optically coupled via an EPCS coupling prism 42A. The LSP subsystem 200 includes an LSP light source system 210, an optical compensator 230, and an LSP detector system 240 optically coupled to the optical compensator via an LSP coupling prism 42B. Detector systems 140 and 240 are operably connected to a system controller 400. Examples of the EPCS subsystem 100 are described in U.S. Patent Nos. 9,534,981 and 9,696,207, which are incorporated herein by reference. Examples of the LSP subsystem 100 are described in U.S. Patent No. 4,655,589 and U.S. Provisional Patent Application No. 62 / 753,388, which are incorporated herein by reference.

[0234] Figure 2B for Figure 2A FIG. 1 is a more detailed schematic diagram of a hybrid EPCS-LSP system, showing example configurations of the EPCS measurement subsystem 100 and the LSP measurement subsystem 200. Figure 3A FIG. 1 is a schematic diagram of an example EPCS subsystem 100 . Figures 4A to 4C is a schematic diagram of an example LSP subsystem 200.

[0235] EPCS subsystem

[0236] refer to Figure 2B and Figure 3AThe EPCS light source system 110 of the EPCS subsystem 100 includes an EPCS light source 112 that generates a first wavelength λ along a first axis A1. A EPCS beam 116 with a first wavelength λ A Also known as EPCS wavelength.

[0237] The EPCS light source system 110 further includes, along the first optical axis A1, an optional polarizer 118, a light diffuser 122 located downstream of the EPCS light source 112, and a focusing lens 120 located downstream of the light diffuser. In an example, the light source comprises a light emitting diode (LED), and further, in an example, the LED emits light at an EPCS measurement wavelength λ of 365 nm. A The EPCS detector system 140 exists along the second axis A2 and includes, in sequence along the second axis: a focusing lens 142, a lens 143 with a wavelength λ A A bandpass filter 144 is centered, an attenuator 146, a TM-TE polarizer 148 (which has TM and TE segments, not shown) and a digital detector (such as a digital camera, image sensor, CCD array, etc.) 150, which has TM and TE segments defined by the TM-TE polarizer 148 (not shown).

[0238] EPCS light beam 116 from EPCS light source 112 is diffused by light diffuser 120 and focused by focusing lens 120 to form focused EPCS light beam 116F. Focused EPCS light beam 116F is incident on EPCS coupling prism 42A at input surface 43A. This couples the focused EPCS light beam into NSWG 18 at a first (EPCS) coupling interface INT1, which is defined by the top surface 12 of the CS substrate and the bottom or "coupling" surface 45A of the EPCS coupling prism 42A. The first coupling interface INT1 may include a refractive index matching fluid 5A, as discussed in more detail below.

[0239] The reflected EPCS beam 116R is formed by the focused EPCS beam 116F at the first EPCS coupling interface INT1 and emerges from the output surface 44A of the EPCS coupling prism 42A to travel along the second axis A2. The first axis A1 and the second axis A2 are located in a common plane (e.g., Figure 3A The reflected EPCS beam 116R contains information about the modal spectrum of the guided mode of the NSWG 18. The reflected EPCS beam 116R is focused by a focusing lens 142 in the EPCS detector system 140 to form an image of the modal spectrum of the guided light at the EPCS digital detector 150.

[0240] The bandpass filter 144 ensures that only the reflected EPCS beam 116R reaches the digital detector 150. The attenuator 146 ensures that the detected reflected EPSC beam 116R has an appropriate intensity distribution for efficient digital detection. The TM-TE polarizer 148 defines the TM and TE segments of the digital detector so that the TM and TE mode spectra can be captured by the digital detector 150. The TM and TE mode spectra are reflected in the first detector signal SA that is sent to the system controller 400 for processing. It should be noted that the order of the bandpass filter 144, attenuator 146, and focusing lens 142 is not important and is Figure 2B and 3A The two are intentionally shown as different to illustrate this point.

[0241] Figure 3B FIG1 is a schematic diagram of an idealized mode spectrum 160 captured by digital detector 150. Local (x, y) Cartesian coordinates are shown for reference. Mode spectrum 160 has TM and TE total internal reflection (TIR) segments 161TM and 161TE associated with TM and TE guided modes, respectively, and non-TIR segments 162TM and 162TE associated with TM and TE radiative and leakage modes, respectively. TIR segment 161TM includes one or more TM mode lines or TM "stripes" 163TM, while TIR segment 161TE includes one or more TE mode lines or TE "stripes" 163TE. TM mode lines 163TM and TE mode lines 163TE are generally aligned in the x-direction and spaced apart in the y-direction.

[0242] The transition regions (“transition regions”) 166TM and 166TE between the TIR segments 161TM, 161TE and the non-TIR segments 162TM, 162TE define the critical angle for optical coupling of TM and TE polarized light into and out of the NSWG 18 of the CS substrate 10 and are referred to as critical angle transition regions. The difference in the starting positions of the critical angle transition regions 166TM and 166TE is related to the knee stress CS. k is proportional to Figure 3B In the "about CS k "instruct.

[0243] Depending on the configuration of the EPCS subsystem 100, the TM mode line 163TM and the TE mode line 163TE may be a light line or a dark line. Figure 3B , for ease of illustration, the TM mode line 163TM and the TE mode line 163TE are shown as dark lines.

[0244] The stress characteristics measured by EPCS are calculated based on the difference in the y position of the TM mode line 163TM and the TE mode line 163TE in the mode spectrum 160. The birefringence B is the difference between the effective refractive index for TM polarization and TE polarization, where the effective refractive index is represented by the y position of the mode line. The surface compressive stress S(0)=CS is calculated from the y distance between the mode lines (effective refractive index) and the ratio B / SOC, where SOC is the stress optical coefficient. At least two TM mode lines 163TM and TE mode lines 163TE are required to calculate the surface stress S(0). Additional mode lines are required to calculate the compressive stress profile S(z). Depth of layer DOL T TE mode lines 163TM and 163TE are a measure of the stress penetration or ion penetration length into the bulk 11 of the CS substrate 10 and, in the case of an IOX process, can also be calculated from the y-position and number of mode lines 163TM and 163TE. Therefore, the TM and TE mode line positions along the y-axis are the most basic measurements used to infer stress-related properties of the CS substrate 10. Calculations used to determine stress properties of the CS substrate 10 based on EPCS measurements using the EPCS subsystem 100 are performed in the system controller 400.

[0245] In some cases, defects in the image are present in the form of horizontal bands as images of the split polarizer lines of the TE-TM polarizer 148, which is located in the optical path of the reflected EPCS beam 116R. By moving the TE-TM polarizer 148 closer to the digital detector 150, the size of the horizontal bands can be reduced, which in turn helps reduce noise in the fringe detection. Figure 3C A horizontal band having a width indicated by the arrow is shown resulting from the TE-TM polarizer 148 being positioned at a distance from the digital detector 150 . Figure 3D Horizontal bands having widths indicated by arrows are shown resulting from positioning the TE-TM polarizer 148 at a closer distance to the digital detector 150. Thus, closer placement of the TE-TM polarizer 148 to the digital detector 150 results in smaller horizontal bands.

[0246] LSP subsystem

[0247] Reference Figure 2B and Figures 4A to 4C The LSP light source system 210 of the LSP subsystem 200 includes an LSP light source 212 that generates a wavelength λ along the third axis A3. B In the example, the LSP light source 212 is configured to emit light at a second wavelength λ B = Laser diode operating at 415nm. Second wavelength λ B Also called LSP wavelength.

[0248] The LSP light source system 210 includes, in order along the third axis A3: an optional neutral density filter 218 (shown in FIG. Figure 2B and 4A ), a first focusing lens 220, a movable light diffuser 222, and a second focusing lens 224. The movable light diffuser 222 may include a lens configured to focus at a wavelength λ B In some embodiments, the LSP subsystem 200 may be folded to form a holographic element that diffuses light under the PBS. In some examples, the movable light diffuser may include a rotating light diffuser or an oscillating light diffuser. One or more folding mirrors FM may be used to fold the LSP subsystem 200 to make it more compact. In some embodiments, the neutral density filter 218 may be replaced with a half-wave plate, which will control the principal axis of polarization so that there will be more or less steering at the PBS.

[0249] The optical compensator 230 lies along a (folded) third axis A3 and includes a polarizer 232, which may be in the form of a polarizing beam splitter PBS. The optical compensator 230 also includes a half-wave plate 234H and a quarter-wave plate 234Q, one of which may be rotated relative to the other to change the polarization state of the LSP beam 216. In an example, the optical compensator 230 may include an electronically controlled polarization modulator, such as a liquid crystal-based modulator, a ferroelectric liquid crystal-based modulator, or the like.

[0250] In an example, optical compensator 230 is operably connected to a controller (not shown) or otherwise includes a controller that controls the polarization switching operation performed by the optical compensator. In an example, optical compensator 230 may include a single liquid crystal device. In another example, optical compensator 230 may include multiple components, such as polarizers, wave plates, filters, prisms (e.g., wedge prisms), etc. In an example, optical compensator 230 causes LSP beam 216 to undergo a complete polarization cycle (i.e., change between two or more selected polarizations) in less than 1 second to 10 seconds. In an example, optical compensator 240 may be operably connected to and controlled by system controller 400.

[0251] A third focusing lens 236 is located downstream of the optical compensator 230 and is used to form a focused LSP beam 216F, which is directed to an LSP coupling prism 42B. The LSP coupling prism has respective input and output surfaces 43B, 44B, and a bottom or "coupling" surface 45B. The coupling surface 45B and the top surface 12 of the CS substrate 10 define a second (LSP) coupling interface IF2. In the example, the second coupling interface INT2 comprises an index matching fluid 5B, as discussed below.

[0252] The LSP detector system 240 exists along a fourth axis A4 that is orthogonal to the third axis A3 , ie, the fourth axis A4 exists in the YZ plane.

[0253] In an example, the LSP detector system 240 includes a collection optical system 243 and a digital detector (eg, a CCD camera) 246. In an example, the collection optical system 243 is telecentric and has unity magnification. The LSP detector system 240 may also include a second wavelength λ B The center of the bandpass filter 344. Figure 4C In the example shown in , digital detector 246 includes an array of imaging pixels 247 , which in examples may have dimensions between 1.1 microns and 10 microns or between 1.8 microns and 10 microns.

[0254] In operation of the LSP subsystem 200, a focused LSP beam 216F is incident on the input surface 43B of the LSP coupling prism 42B and travels to the coupling surface 45B and then travels through the index matching fluid 5B to the top surface 12 of the CS substrate 10 to enter the body 11 of the CS substrate. The focused LSP beam 216F has a selected polarization defined by the optical compensator 230 at any given time. The (polarized) input LSP beam 216F is scattered by features in the body 11 of the CS substrate 10 to form a scattered LSP beam 216S. The scattered LSP beam 216S exits the CS substrate 10 at the top surface 12, passes back through the second coupling interface INT2 and then exits the LSP coupling prism 42B at the output surface 44B. The scattered LSP beam 216S travels to the LSP detector system 240 and is directed by the collection optical system 243 to the digital detector 246. The scattered LSP beam 216S forms an LSP image 248 on the digital detector 246, as shown in FIG. Figure 4D 248 is shown in a close-up view of FIG. This defines a digital LSP image. Unless otherwise noted, the LSP image 248 discussed below is considered a digital LSP image. The characteristic "X" shape of the LSP image 248 is known in the LSP art and is due to the reflection of the scattered light beam 216S from different interfaces associated with the LSP interface INT2 defined by the CS substrate 10, the LSP coupling prism 242B, and the refractive index matching fluid 5B.

[0255] like Figure 4D As shown in FIG, the X shape of the LSP image 248 is composed of the X-shaped LSP images 248 having local length coordinates x along their length. L Each line image LI has an intensity distribution I(x L ), which is measured by pixel 247 that coincides with the line image. The digital detector converts the intensity distribution I(x L) is converted into a second detector signal SB, which is sent to the system controller 400. Only one of the line images LI is required for measurement. In an example, image processing is used to identify a portion of the LSP image 248 for subsequent processing to extract optical delay information, as explained below.

[0256] In an example, taking a given measurement of the CS substrate 10 using the LSP subsystem 200 includes taking a measurement for a time t M Measurement between 1 second and 10 seconds. At the measurement time t M During this time, the polarization state of the LSP beam 216 changes between different polarization states, preferably performing one or more cycles between polarization states. At the same time, for each polarization state, the digital detector 246 is set to the exposure time t E During this time, the LSP image 248 is captured. In this example, the exposure time t E is approximately the same as the frame rate FR of the digital detector 246. Example exposure time t E = 50ms, which corresponds to a frame rate FR = 20 frames / second. Exposure time t E It can also be smaller than the frame rate.

[0257] Depending on the polarization state of the input LSP beam 216R and the stress-induced optical delays induced along the beam path, the electronically captured LSP image 248 has a range of values in its intensity distribution I(x L ) are different. The differences are due to differences in destructive and constructive interference along the length of the scattered LSP beam 216S as a function of the depth D into the CS substrate 10 between the different polarization states. The system controller 400 uses multiple intensity distributions I(x L ) is used to calculate the optical retardation OR as a function of the depth D into the CS substrate body 11 using relationships well known in the art. Similarly, the intensity distribution I(x L ) to calculate multiple optical delay OR and depth D curves (“OR and D plots”). For example, for a 3 second measurement time t with an image sensor frame rate FR of 20 frames / second M , which can generate a total of 60 I(x L ) and D plot to calculate OR and D, and to calculate one or more stress-related characteristics of the CS substrate 10.

[0258] When stress exists in the CS substrate 10, although the intensity distribution I(x L) will necessarily differ between the polarization states of the input light beam 112, but for a given CS substrate whose stress profile is (ideally) constant, at a given measurement position, the different OR vs. D curves (plots) calculated from the measured intensity distribution should ideally be the same.

[0259] While the LSP measurement technique can produce a stress profile S(z), it is generally unable to produce an accurate representation of the stress profile in the near-surface region of the CS substrate 10. At least two problematic effects pose challenges to extracting an accurate representation of the near-surface stress profile of the CS substrate 10 using the LSP measurements from the LSP subsystem 200. One problematic effect is known as the "fireball" effect, which is caused by excessive light scattering at the LSP interface INT2. Excessive light scattering creates noise that corrupts the LSP measurement data in the near-surface region, which in this example is the first 60 to 100 microns below the top surface 12 of the CS substrate 10.

[0260] Another problematic effect is caused by the convolution of photons scattered from different depths into a signal corresponding to a specific depth. This convolution significantly alters the signal in the region of rapid stress change, which is typically located in the near-surface compression zone, most commonly the first 80, 100, or 150 microns, but sometimes as high as 200 microns. The thicker the lithium-based glass, the thicker the rapid stress change region.

[0261] Some prior art LSP systems attempt to reduce these convolution effects by using a highly focused beam (beam diameter as small as 10 microns) near the surface of the CS substrate. Unfortunately, this leads to other problems, such as increased laser noise (e.g., speckle) in the region of interest at the same depth, making the stress profiles extracted in the near-surface region less reliable.

[0262] LSP light source test

[0263] The reliability of the LSP light source 212 affects the overall performance of the hybrid system 20 described herein. Therefore, the LSP light source 212 should be thoroughly evaluated before being included in the hybrid system 20. The disclosed embodiments also include a test fixture 300 and rigorous laser specifications to quantify and qualify each laser used in the hybrid system 20.

[0264] refer to Figure 4E, shows an example test fixture 300. Test fixture 300 may include a base 302 along which a plurality of optical components may be mounted. Test fixture 300 further includes a laser mount 303 configured to hold LSP light source 212 on the same reference surface as when installed in LSP subsystem 200. Laser mount 303 may be mounted on a rotating bearing such that the bearing rotation axis is collinear with the laser axis. Test fixture 300 further includes a collimating lens 308 for collimating the laser beam generated by LSP light source 2121, a focusing lens 309 mounted such that its optical axis is also collinear with the laser axis, and a sensor 310 mounted perpendicular to the laser axis. This sensor 310 is precisely positioned at the focal point of focusing lens 309 when illuminated by a collimated beam at the precise operating wavelength of LSP light source 212, centered about focusing lens 309 and aimed along its optical axis. The various optical components may be mounted on rails 306 so that their positions along base 302 can be adjusted.

[0265] Before installing the LSP light source 212 in the LSP subsystem of the hybrid system 20, it can be evaluated using the test fixture 300. If the LSP light source 212 is properly collimated, the laser beam will be focused to a near-diffraction-limited spot on the sensor 310. By specifying the maximum allowable spot size, the collimation quality of the LSP light source 212 can be controlled. As the rotary bearing rotates, the focused spot describes a circle on the sensor 310. The radius of this circle measures the aiming error of the LSP light source 212 relative to its reference surface. By specifying the maximum circumferential rotation, the aiming quality of the laser can be controlled.

[0266] High-efficiency laser power

[0267] It will be appreciated that when making precise measurements on materials that produce less scattering (e.g., glass), a laser with greater effective laser power, lower measurement noise, or both may be required to obtain a sufficient signal-to-noise ratio. Figure 4F , higher laser power is achieved by using a semiconductor laser die that provides higher power as the LSP light source 212, and using a collimator lens 231 to collect the power from the LSP light source 212 into a clean Gaussian beam (which can be focused to a small beam spot within the sample 10 by a focusing lens 236).

[0268] like Figure 4FAs shown in , for example, this can be achieved by using a laser die that produces a single-mode astigmatic Gaussian beam with half-angles of approximately 15 degrees and 5 degrees, respectively, and a waist at the diode of approximately 0.4um×1.4um, respectively. The light from this laser can be collected, for example, by a 7mm focal length, 0.5NA molded aspheric collimator lens 231. If the collimator lens 231 and subsequent optics maintain beam quality and given a 100mm focusing lens 236, this will provide a beam waist BW of approximately 6um×21um at the object position in the sample 10. This small beam size provides high spatial measurement resolution.

[0269] Active laser temperature control for wavelength tuning

[0270] To reduce measurement noise, all extraneous light that is not part of the measurement laser beam should be eliminated. One example method for achieving this is to use a spectral filter in front of the camera to block light with wavelengths other than the illuminating laser. This filter will block both ambient light and any fluorescence stimulated by the laser. For best results, the spectral width of this filter should be comparable to that of the laser.

[0271] However, the wavelength of a diode laser can vary from its nominal wavelength from die to die and can also vary with changes in die temperature. This can lead to mismatches between the laser and the filter, such as Figure 4G As shown in , some of the light from the laser is blocked by the filter. This reduces the effective power of the laser. In some embodiments, this problem is alleviated by selecting the laser and actively tuning the wavelength by setting the temperature of the die. As a non-limiting example, this can be achieved by using a Peltier cooler and an active control system so that the die is tuned to a precise temperature to radiate at the desired wavelength and is maintained at that temperature regardless of the ambient temperature and the power dissipated by the laser die. Figure 4F An example cooler and active control system 215 is shown for optimally setting the temperature of the die of the LSP light source 212 to ensure the power of the laser passes through the filter.

[0272] LSP subsystem calibration

[0273] Calibration of the LSP subsystem 200 should be performed to ensure proper operation of the hybrid system 20. However, manual calibration of the LSP subsystem 200 can result in user setup errors and potential damage to components within the LSP subsystem 200. In manual calibration, system adjustments are made by an operator manually adjusting components, analyzing images or instrumentation provided via software, and then readjusting as needed. This user-based feedback loop can cause problems due to the sensitivity of the system instrumentation and user errors caused by perceived image quality and visualization.

[0274] Additionally, current scattered light polarization measurement setups use basic manually adjustable sliders, knobs, and set screws to control the placement of components in the LSP subsystem 200 for system calibration. These manual adjustments and locking mechanisms can cause damage to components, such as flat spots created by set screws, which can lead to errors when calibrating the system in the future.

[0275] Therefore, in some embodiments, the LSP subsystem 200 includes actuators that enable automatic calibration with minimal human intervention. The actuators enable adjustment of at least five SLP subsystem 200 components: a rotatable laser orientation, a rotatable half-wave plate, a translatable diffuser, a translatable focus lens, and a translatable / rotatable camera. System calibration involves adjusting each of these components to set various parameters, such as, but not limited to, spot percentage, beam width, intensity, beam focus, camera focus, and camera tilt.

[0276] Reference again Figure 4A , the LSP light source 212 can be rotated within its mount by an actuator (e.g., a motor), as shown by arrow A. This changes the orientation of its elliptical laser beam. Figure 4I An example LSP subsystem 200 is shown with a rotatable SLP light source 212. The LSP light source 212 is rotated so that its minimum waist is oriented toward the digital detector 246 to minimize the beam width. The LSP light source 212 is rotated to a repeatable stable condition during initial system setup and is typically not adjusted during system calibration.

[0277] In addition to the LSP light source 212, the diffuser 222 is also mounted on an actuator (e.g., a stepper motor) that enables the position of the diffuser 222 to be controlled relative to the focus of the fixed lens within a range of + / - 5 mm, as shown by arrow B. The positioning device may have sensors to reduce the risk of over-adjustment and damage to components of the LSP subsystem 200. Adjusting this position affects the beam width and speckle percentage of the beam within the sample. Figure 4H Shown with a translatable diffuser 222 Figure 4I Example LSP subsystem 200.

[0278] The half-wave plate 234H can also be coupled to an actuator (e.g., a motor) that enables the half-wave plate to be rotated, as shown by arrow C. Rotation of the half-wave plate 234H changes the intensity of the laser beam across the thickness of the sample. A user can manually input a "forward" or "backward" command to adjust the beam intensity, or the software can automatically position the half-wave plate to the correct position to achieve optimal beam intensity during an automated calibration process. Figure 4H Shows how a half-wave plate can be rotated.

[0279] Furthermore, the focusing lens 236 is mounted on an actuator (e.g., a stepper motor) that enables translation of the position of the focusing lens 236, as indicated by arrow D. The beam focus of the LSP laser source 212 can be controlled by translating the fixed-focal-length focusing lens 236 relative to the position of the sample on the prism, which affects the beam focus parameters. Figure 4I and 4J Shows how the focus lens can be rotated.

[0280] The detector assembly 150 may also be secured to one or more actuators that enable translation and / or rotation of the detector assembly, as indicated by arrows E and F. The detector assembly 150 may be mounted to a translation stage that is used to move the detector assembly 150 relative to the position of the sample on the prism, thereby changing the camera focus parameters. Movement of the detector assembly 150 takes into account the fixed focal length of the telecentric lens used on the detector assembly 150. The motors may be controlled via software input to adjust the position of the detector assembly 150 to achieve optimal image focus. The detector assembly 150 may also be rotated in its mount, which may affect camera tilt. The detector assembly 150 may be rotated during initial system setup, may be accurately adjusted using software instruments, and is typically not adjusted during normal system calibration. Figure 4H and 4I It shows how the detector device 150 (ie the camera) can be rotated and translated.

[0281] The motorized components described above enable automation of calibration. Embodiments implement a single-step calibration method where a single software action automatically sets all component positions for optimal performance.

[0282] Selection of beam entry / exit positions

[0283] For measurements of glass samples, LSP analysis selects the entry and exit points of the laser light passing through the sample in the image. These points are typically seen as bright "fireballs" and can be selected using an image contour detection algorithm. However, if the sample contains spots of dirt, bubbles, or fingerprints, the image contour algorithm may have difficulty selecting which bright spots are true fireballs and, therefore, the true entry and exit locations. Embodiments of the present disclosure may employ a scoring system to select which point combinations are most likely candidates for entry and exit fireballs.

[0284] First, if a point combination is not within a certain threshold of the known limits of the system, it is excluded from the selection process. Because the general thickness of the sample and the angle of laser propagation can be estimated, all point combinations outside the upper and lower 50% of the thickness and outside ±10 degrees around the laser propagation angle are excluded from the selection process. For the remaining point combinations, each combination is scored based on: (1) the degree of closeness to the sample thickness; (2) the degree of closeness to the laser propagation angle; and (3) the median intensity between the two points. Because the laser propagates between the entry and exit points, it is assumed that the median intensity will be greater than the median intensity between any two random points on the image.

[0285] Scoring is calculated by taking all values and normalizing them to a range of 0 to 1, with 1 being the best score. For example, the brightest combination of points receives a score of 1, while the dimmest combination receives a score of 0. Once scoring is complete, the highest combined score across all three requirements is reported as the entry and exit points for the laser.

[0286] Figure 4K A digital image of a sample acquired by the system is shown, from which the entry and exit points were selected. Figure 4L A digital image is shown with the final point combination selected as a cross, and the yellow outlines represent all points that were initially detected and subsequently rejected by the image processing algorithm.

[0287] LSP laser orientation optimization

[0288] The LSP beam 216 orientation affects the accuracy of the central tension measurement of the part. There is an optimal orientation where the beam width BW is minimized in the middle of the sample 10. Once the optimal orientation is identified, all hybrid system 20 instruments can be assembled in a similar manner to reduce inter-device measurement variation.

[0289] Figure 4M for the reason Figure 4B A simplified diagram of the subsystem 200 is shown. The focused LSP beam 216F is oriented so that the beam width BW is minimized from the focused laser minimum waist at the center of the sample thickness (BS) and the optimal focus of the LSP detector system 240.

[0290] Because the focused LSP beam 216F and the LSP detector system 240 view of the focused LSP beam 216F are at an angle relative to the stress profile (shown by the horizontal line in the sample 10), a three-dimensional analysis is required to determine the proper orientation of the LSP beam 216. The analysis includes the intersection of the following four three-dimensional features:

[0291] 1. The stress profile of the sample (ie, the "layers" or gradients through the thickness of the sample).

[0292] 2. Cross section of LSP beam 216F relative to the sample layer.

[0293] 3. View of the LSP detector system 240 perpendicular to the LSP beam 216F relative to the sample layer.

[0294] 4. In the view of the LSP detector system 240, pixels are selected to be grouped (by averaging or various fitting methods) to reduce the data noise associated with some points in the thickness.

[0295] Ideally, the grouped values all represent light integrated from the same depth in the thickness. Minimizing the size of the LSP beam 216F and the illumination collection depth of the LSP detector system 240 is one way to optimize the measurement. In situations where the illumination is sufficient to observe scattered light using the LSP detector system 240, a thin sheet of illumination can be used instead of a light beam. Light sheets have been used in microscopy, primarily for fluorescence, and this will be useful for scattered light polarization analysis.

[0296] The cross-sectional profile of the LSP beam 216F has an elliptical power distribution. The elliptical orientation (and beam shape) may be selected to improve the quality of the delay data used for measurements in the disclosed embodiments. The elliptical view of the LSP detector system 240 changes the integrated light to the sensor. Figure 4N In , LSP beam 216F appears “coarse” to LSP detector system 240, but the light on the sensor pixel comes from a specific area. On the other hand, in Figure 4O , the LSP beam 216F appears thinner to the LSP detector system 240, but the light on the sensor pixel comes from a larger area along the axis of the LSP detector system 240. Figures 4P to 4S Various arrangements for directing the LSP beam 216F relative to the LSP detector system 240 are shown.

[0297] Focusing helps improve the quality of the delay data used for measurement, and as described above, minimizing the beam width also helps, especially when control of the beam axis and focusing astigmatism (and other) artifacts also affect the depth of the light source integrated into the LSP detector system 240 in the sample.

[0298] Directional optimization can be assessed using several analytical tools. A simple analysis can be performed to optimize the beam width at the center of the sample by focusing the laser beam and the detector.

[0299] First, a more detailed analysis of the per-pixel delay (the phase of the sine-fit modulated delay) and peak-to-peak intensity (the amplitude of the sine-fit modulated delay) in the camera view was performed. For a given sample, the delay values grouped for noise reduction would ideally be identical. The incremental delay values from adjacent groups (relative to the stress magnitude in the "layer") are expected to be identical to similarly adjacent individual values.

[0300] Coupling prism assembly

[0301] The hybrid system 20 utilizes the aforementioned coupling prism assembly 40 , which operably supports an EPCS coupling prism 42A and an LSP coupling prism 42B to provide prism coupling for the EPCS subsystem 100 and the LSP subsystem 200 when performing EPCS measurements and LSP measurements on the CS substrate 10 .

[0302] Figure 5A 4 is a front view of a top portion of an example coupling prism assembly 40 showing an example support frame 48. Figure 5B For similar Figure 5A and further includes a front view of the cover plate 60. The example support frame 48 includes an EPCS frame section 48A that supports the EPCS coupling prism 42A and an LSP frame section 48B that supports the LSP coupling prism 42B. The support frame 48 also includes an isolation member 50 disposed between the EPCS frame section 48A and the LSP frame section 48B, which is configured to optically isolate the EPCS coupling prism 42A from the LSP coupling prism 42B. In an example, the isolation member 50 also prevents the refractive index matching fluids 5A and 5B used with the EPCS coupling prism 42A and the LSP coupling prism 42B, respectively, from mixing. In another example, the isolation member 50 allows a single refractive index matching fluid to be used with the EPCS coupling prism 42A and the LSP coupling prism 42B, that is, the single refractive index matching fluid can flow between the first interface INT1 and the second interface INT2 defined by the two different prisms. In one example, the isolation member 50 is a separate portion of the support frame 48 and is attached thereto. In another example, the isolation member 50 is part of the support frame 48 , ie, is formed integrally therewith during formation of the support frame.

[0303] In an example, the EPCS and LSP frame sections 48B and the isolation member 50 including the fastening tabs 52 (including the mounting holes 53) allow a cover plate 60 to be fastened to the frame sections using fastening members (not shown). The cover plate 60 includes a first aperture 62A sized to accommodate the coupling surface 45A of the EPCS coupling prism 42A and a second aperture 62B sized to accommodate the coupling surface 45B of the LSP coupling prism 42B.

[0304] Figure 6A and 6B An example method is shown in which a resin molding process is used to form the EPCS frame section 48A and the LSP frame section 48B. The process achieves precise alignment of the EPCS coupling prism 42A and the LSP coupling prism 42B. In the example, the molding process is performed with the example EPCS prism 42A and the LSP prism 42B in place on a stable platform 75. This process is discussed in more detail below.

[0305] Figure 7 FIG2 is an xz cross-sectional view of an example prism support structure 46 attached to an example support plenum 70 of a hybrid system 20 using fastening tabs 52 and fastening members 54 (e.g., screws) extending through mounting holes 53. The support plenum 70 has a top surface 71 and a measurement aperture 72. The top surface 71 defines an example measurement plane MP at the measurement aperture 72. The prism support structure 46 is supported by the support plenum 70 such that the EPCS coupling prism 42A and the LSP coupling prism 42B are located at the measurement aperture 72. In the example, the EPCS coupling surfaces 45A and the LSP coupling surfaces 45B of the EPCS coupling prism 42A and the LSP coupling prism 42B are located at or substantially at the measurement plane MP.

[0306] In the example, the CS substrate 10 is operably supported by a movable substrate holder 80, which holds the CS substrate on the measurement aperture 72 so that the EPCS coupling prism 42A and the LSP coupling prism 42B can interface with the top surface 12 of the CS substrate 10 to establish a first coupling interface INT1 and a second coupling interface INT2 at or substantially at the measurement plane MP. In the example, the movable substrate holder 80 is transferred above the upper surface 71 of the support gas chamber 70 using a transfer element 73 (e.g., a roller, wheel, slider, bearing, etc.). In the example, the CS substrate 10 is supported by the movable substrate holder 80 at an inner lip 82, which supports an outer (peripheral) portion of the top surface 12 of the CS substrate. In the example, the plane of the inner lip 82 defines the example measurement plane MP. Therefore, Figure 7 Two different example positions of the measurement plane MP are shown.

[0307] Figure 8A To illustrate a front view of an example of a support air chamber 70 in the form of a plate, the plate includes a pressure vacuum (PV) conduit or PV element 90 (e.g., a PV strip) for pneumatically engaging the CS substrate 10 to pull the CS substrate onto the coupling surfaces 45A and 45B of the EPCS coupling prism 42A and the LSP coupling prism 42B via vacuum (negative pressure) and then release the CS substrate from the prisms via pressure (positive pressure). Figure 8B for Figure 8A A cross-sectional view of a support chamber and measurement aperture of a configuration of FIG. 1 shows an example vacuum system 91 including a PV element (PV strip) 90 and a vacuum source 92.

[0308] It should be noted that the inner lip 82 of the movable substrate holder 80 defines a stop for limiting vertical movement of the CS substrate 10 when vacuum is applied to the CS substrate via the vacuum system 91 .

[0309] Hybrid system using a single refractive index matching fluid.

[0310] like Figure 6D The exemplary embodiment of the hybrid system 20 shown in FIG. 1 employs a refractive index n for the EPCS subsystem 100 and the LSP subsystem 200. f This is a counter-intuitive approach because a single index matching fluid 5 would generally be considered incapable of producing good measurements from both subsystems simultaneously, for at least the following reasons.

[0311] If an index matching fluid is selected based on EPCS measurement considerations, then the refractive index n of the index matching fluid f Significantly higher than the surface refractive index n of the CS substrate S (e.g., 0.1 or more higher) to facilitate coupling of light into the guided modes and obtain good fringe contrast in the captured TM and TE mode spectra.

[0312] On the other hand, this refractive index level contrast (difference) Δn between the refractive index of the index matching fluid 5 and the refractive index of the surface 12 of the CS substrate 10 can lead to significant surface scattering at the coupling interface INT2 due to the beam deflection caused by the refractive index mismatch associated with the micro-roughness of the surface. This is problematic for accurately extracting delay and stress measurements at medium depths based on receiving and processing scattered light from the CS substrate. High surface scattering produces "fireballs", such as large bright spots on the image of the scattered beam, where the pixels 247 of the digital detector (CCD camera) 246 are saturated with photons. This leads to a large loss of stress-related information. Well-polished surfaces or pristine surfaces (e.g. formed by melt stretching) tend to scatter less.

[0313] If the refractive index matches the fluid n f The surface refractive index n of the CS substrate 10 S If the pattern spectrum 160 is approximately matched (e.g., similar, slightly higher, or slightly lower) to ensure low surface scattering, the fringe contrast in the pattern spectrum 160 is generally poor when there is a sharp change in refractive index near the surface, such as the peak region R1 caused by a shallow, concentrated peak in K2O concentration produced by the IOX process (see Figure 1B ). In addition, the position and contrast of the TM fringes 163TM and the TE fringes 163TE become dependent on the thickness of the index matching fluid. These two effects make it difficult to accurately measure surface CS and peak DOL using the EPCS subsystem 100.

[0314] If the refractive index matching fluid 5 is selected to have a lower refractive index n than the substrate (body) of the CS substrate 10 B (This usually also means a refractive index lower than the surface refractive index) f, then the thickness of the index matching fluid must be extremely small (e.g., less than 0.4 microns) to enable light to be coupled into the waveguide mode in the near-surface portion (peak region R1) of the NSWG 18 in order to perform surface CS measurements. A small thickness is also required in order to measure the critical angle of the coupled light traveling in the depth region R2 between the surface peak region R1 and the bulk region R3. This is difficult to achieve consistently in a production environment due to small particle contamination issues. These issues result in an inability to accurately measure the surface (compressive) stress S(0) and the "knee stress" S at the bottom of the surface refractive index peak region R1 for dual IOX lithium-containing glasses and glass-ceramics. k .

[0315] It has been shown that EPCS and LSP measurements can be performed using a single refractive index matching oil 5 under the selected conditions, where the peak region R1 of the CS substrate 10 has a normalized slope S n =|(λ / n)dn(z) / dz|<0.0005, or more preferably S n <0.0004, where λ is the measurement wavelength, and n(z) is the refractive index of the CS substrate 10 at the measurement wavelength.

[0316] In one embodiment, the refractive index n of the index matching fluid 5 is found to be f Higher (greater) than the surface refractive index n of the CS substrate 10 glass S The amount Δn=n f -n S , ranging from 0.02 to 0.06, to produce adequate measurement results for EPCS and LSP measurements. n <0.0004, preferably, Δn is at the high end of the above range, for example from 0.05 to 0.06.

[0317] In one aspect of the invention, the measurement wavelength λ of the EPCS measurement is reduced A , to reduce the normalized slope S n , so that it is easier to meet the above conditions. In one example, the measurement wavelength λ of the EPCS measurement A The measurement wavelength λ of the LSP measurement B 5% or more shorter to help achieve a smaller normalized slope S n In an example, one or more light blocks (not shown) can be selectively positioned in the beam path of the EPCS subsystem 100 to preferentially block light rays propagating at larger angles of incidence corresponding to higher effective refractive indices. This enhances the contrast of the TM and TE fringes of the guided modes captured for the near-surface peak region R1 of the NSWG 18.

[0318] In another embodiment, the surface peak region R1 may have a normalized slope S n>0.0005. In this example, the refractive index matching fluid can be selected so that it is at the EPCS measurement wavelength λ B The refractive index n f Very close to the knee point KN at position z k (i.e., the effective refractive index of the bottom of the peak region R1). In this case, n f ≈n crit , where n crit is the refractive index associated with the critical angle of the peak region, ie the angle below which light does not travel in a guided wave form within the peak region R1.

[0319] In many practical cases, the effective refractive index difference between the TM and TE guided waves at a position corresponding to the bottom of the surface peak region R1 is relatively small. For example, in most practical cases, the difference is less than 0.0006 refractive index units (RIU), and most commonly between 0.00015 and 0.0005 RIU. In the example,

[0320]

[0321] In some examples, only n f ≈n crit ,mean and / or More specifically, n f is not significantly less than the smaller of the TM and TE critical indices, nor is it significantly greater than the largest of the TM and TE critical indices. Therefore, in the example (and expressing the above mathematically):

[0322]

[0323] The upper limit of the above equation is defined when there is no index matching fluid, by making n f The probability of missing fringes associated with the peak region R1 is reduced by being greater than the effective refractive index of the fringes. Therefore, in order to be able to properly consider all modes in order to accurately calculate the depth of the surface peak region R1 (defined by the potassium 10X process in the example), it is preferred that n f Not significantly greater than the larger of the TM and TE critical refractive indices Ideally, however, it should not be significantly greater than the smaller of the two critical refractive indices.

[0324] In one embodiment, the mode fringes in the TM and TE mode spectra associated with the peak region R1 are spaced far apart in the effective refractive index space, for example, when the effective refractive index of the last fringe in a particular polarization state (TM or TE) is close to the corresponding critical refractive index ( or ) between the refractive index of the fluid n, the separation is greater than 0.0015 RIU or preferably greater than 0.002 RIU or even more preferably greater than 0.0025 RIU. In this embodiment, the refractive index of the refractive index matching fluid n is greater than 0.0015 RIU or preferably greater than 0.002 RIU or even more preferably greater than 0.0025 RIU. oil It can be chosen to be closer to the higher of the two critical refractive indices, and possibly higher than the larger of these:

[0325]

[0326] or

[0327]

[0328] Using the EPCS subsystem 100, these differences in effective refractive index can be readily determined by measuring the differences in critical angle positions (corresponding to the intensity transition regions 166TM and 166TE on the sensor from bright total internal reflection to dark (partial reflection)) and / or fringe positions, and taking into account the calibration of the instrument (per RIU angle, or per RIU pixel, or spacing of points on the sensor plane per RIU).

[0329] In another embodiment having more general application, the refractive index n of the index matching fluid is oil The effective refractive index is selected to be closer to the lower of the TM and TE. This enables the capture of TM and TE fringes whose effective refractive index can be close to the critical refractive index, but may require relatively close proximity (e.g., a few wavelengths) between the coupling surface 45A of the EPCS coupling prism 42A and the top surface 12 of the CS substrate 10.

[0330] More specifically, in this embodiment, preferably,

[0331]

[0332] or

[0333]

[0334] Furthermore, in order to reduce the significant change in the shape of the critical angle transition region, it may be preferred that

[0335]

[0336] or even

[0337]

[0338] In some practical cases, the effective index of the lowest effective index guided mode is very close to the critical index, within about 0.0002RIU. In such cases, it may be desirable to also impose a more stringent requirement on the refractive index of the index matching fluid, constraining it from above:

[0339]

[0340] In n oil When the refractive index is less than at least one of the two critical refractive indices, the critical refractive index n crit Obtaining a high contrast transition region may require the aforementioned close proximity (e.g., several wavelengths) between the coupling surface 45A of the EPCS coupling prism 42A and the top surface 12 of the CS substrate 10. In an example, such close proximity is achieved by drawing the sample toward the prism via a PV conduit 90 pneumatically connected to a PV source 92 using a vacuum system.

[0341] In another embodiment, when the refractive index matches the refractive index of the fluid n oil When the effective refractive index of the light-guiding mode used to calculate the surface compressive stress CS is not much different, the systematic error in the calculation of the surface compressive stress S(0)=CS is corrected. Specifically, this correction can be preferably used in the following cases:

[0342]

[0343] In one exemplary embodiment, the correction is specified by calibrating out the systematic error, for example by comparing the surface compressive stress CS measured using the preferred dual-purpose index matching fluid of the present invention with that measured using the refractive index n oil The CS measured with a relatively large more conventional index matching fluid, such as that used to measure the bulk refractive index n B The λ of the CS substrate 10 is in the range of 1.45 to 1.55. A =n at 590nm oil =1.72 oil.

[0344] In a related embodiment, the systematic error can also be calibrated based on the width of the TM fringes 163TM and the TE fringes 163TE, because the width can be related to the thickness of the refractive index matching fluid and, at the same time, to the amount of systematic error in the surface compressive stress CS measurement. It should be noted that the systematic error also depends on the refractive index slope S of the refractive index profile of the surface peak region R1 of the CS substrate 10. n This means that the systematic error can be calculated for the surface refractive index slope S n The specific types of CS substrates are defined within a relatively narrow range. This narrow range is typical for CS substrates employing lithium-based glasses that have been strengthened using the IOX process.

[0345] The high-power nature of the laser diodes used in the system causes the refractive index oil to degrade over time. This is another source of measurement noise and variation. To mitigate this issue, laser-mixed refractive index oils are available that are strong enough to sustain the laser energy without degradation.

[0346] A hybrid system using two fluids with different refractive indices.

[0347] The exemplary embodiment of the hybrid system 20 employs two different refractive index matching fluids 5 (denoted as 5A and 5B) for the EPCS subsystem 100 and the LSP subsystem 200, respectively, wherein the two different refractive index matching fluids 5A and 5B have corresponding refractive indices n fA and n fB (or n oil-A and n oil-B ).

[0348] The use of two different refractive index matching fluids 5A and 5B requires that the two refractive index matching fluids be kept separate so that they do not mix with each other. Figure 5A and 5B In one example discussed, the prism support structure 46 includes an isolation member 50 disposed between the EPCS coupling prism 42A and the LSP coupling prism 42B to keep the two index matching fluids 5A and 5B fluidly separated, ie, fluidly isolated from each other.

[0349] In another embodiment, pressurized gas (e.g., air) is introduced into the small gap between the EPCS coupling prism 42A and the LSP coupling prism 42B to define an "air curtain" 30 (see Figure 2B ), ensuring that the index matching fluids 5A and 5B do not interact when measuring the CS substrate 10 in the hybrid system 20. This separation then enables the respective index matching fluids 5A and 5B to be automatically dripped onto their respective EPCS coupling prisms 42A and LSP coupling prisms 42B simultaneously, thereby allowing simultaneous measurements. In an example, the air curtain 30 can be formed using the PV system 91 (see, e.g., Figure 8B ).

[0350] Hybrid systems with reduced crosstalk.

[0351] Due to the proximity of EPCS coupling prism 42A and LSP coupling prism 42B, crosstalk may occur between EPCS subsystem 100 and LSP subsystem 200. This crosstalk may reduce the accuracy of stress measurements made by each subsystem. The various embodiments described below for reducing (including eliminating) crosstalk may be used individually or in combination.

[0352] In one example, the EPCS detector system 140 of the EPCS subsystem 100 includes an EPCS detector system 140 configured to measure wavelength λ at EPCS. A Meanwhile, the LSP detector system 240 of the LSP subsystem 200 includes a LSP measuring wavelength λ B A bandpass filter 244 is provided, centered at . In an example, the respective bandwidths of bandpass filters 144 and 244 are sufficiently narrow to substantially filter out the wavelengths measured by the other subsystems. Since the bandpass filters can be very narrow (e.g., a few nanometers), a slight difference in the measured wavelengths (e.g., 10 nm) is sufficient to reduce or eliminate crosstalk using the bandpass filters. In an example, a given bandpass filter can be inserted at any location between the corresponding coupling prism and the detector system.

[0353] In another embodiment, for measuring the EPCS wavelength λ A and LSP wavelength λ B An optically opaque barrier is disposed between the EPCS coupling prism 42A and the LSP coupling prism 42B. In the example, the barrier is in the form of an isolation member 50, as described above in conjunction with Figure 5A As discussed above, the isolation member 50 can be formed of a rigid material such as aluminum or a non-rigid material such as rubber, as long as it can prevent the EPCS and LSP measurement light from propagating between the EPCS coupling prism and the LSP coupling prism. As described above, the isolation member 50 can also be configured to perform both optical isolation and fluid isolation.

[0354] Reduction of laser entrance noise and exit noise

[0355] Noise in the delay data can be caused by many factors, including laser speckle, surface roughness, and fireballs. As described above, fireballs are bright light scattering that occurs at the glass prism and glass-air interface. Laser speckle noise can be mitigated using a rotating holographic diffuser 222, but fireballs remain a challenge even with index-matching fluids.

[0356] The embodiment of the present disclosure is achieved by forming a Figure 1A A layer of ink was applied to the top surface 12 of the sample 10 shown in FIG, not the incident surface of the sample 10 relative to the light beam 216, to mitigate the fireball effect. Thus, drawing a black ink line on the top of the tested sample mitigated the fireball at the glass-air interface. Specifically, it was found that a black permanent marker line on the sample 10 provided the best results. Figure 8C 20 images of the hybrid system with no permanent marker lines on the back surface of the sample, and Figure 8D Image the hybrid system 20 with a permanent marker line on the back surface of the sample. Figure 8D Showing a significant reduction in the fireball effect.

[0357] Before using the permanent marker backing method, the inventors applied a lens and immersion oil to the back of the glass sample to reduce image reflection and laser speckle. The use of the permanent marker backing method further reduced reflections on the back surface and laser speckle, thereby reducing noise during measurement. Figure 8E The image is shown without backing material. Figure 8F Images with standard oil and L3 lens are shown. Figure 8G The average image with the L3 lens is shown. Figure 8H The image is shown with a permanent marker line on the back surface as described above. Figure 8I Shown is the average image with a permanent marker line on the back surface as described above.

[0358] Experiments were conducted to find alternatives to the original method of using an L3 lens and immersion oil atop a glass sample. Throughout the experiments, various backing materials were applied and measurements attempted. Most materials were less effective at reducing reflections and backscatter than the original L3 lens method, and while some did allow measurements, black permanent marker proved most effective at reducing reflections, backscatter, and noise during SLSS measurements. Figure 8J An image of a sample is shown where the backing material includes a polarizing filter and immersion oil. Figure 8K Images showing samples where the backing material comprises a black plastic capsule and immersion oil. Figure 8L An image showing a sample where the backing material includes a black dry erase marker. Figure 8M Digital image showing a sample where the backing material includes a black permanent marker. Figures 8J to 8N As shown, black permanent marker is the most effective backing material to minimize the fireball effect.

[0359] Coupling prism alignment

[0360] Hybrid system 20 achieves the most accurate measurements when EPCS coupling prism 42A and LSP coupling prism 42B are aligned with each other and their coupling surfaces 45A and 45B lie in a common plane.

[0361] To achieve this alignment, the coupling prism assembly 40 utilizes the aforementioned prism support structure 46. In the example of forming the prism support structure 40, the coupling surfaces 45A and 45B of the EPCS coupling prism 42A and the LSP coupling prism 42B are first ground and polished to have high flatness and verticality. Figure 6A , the EPCS coupling prism 42A and the LSP coupling prism 42B are then placed on a stable platform 75, such as a precision flat granite bar, with the coupling surfaces 45A and 45B resting on a surface 76 of the stable platform.

[0362] Reference Figure 6B , the mold 49 is mounted on a stable platform 75 at the surface 76, and then the resin 49R is poured into the mold. After the resin hardens, the wall of the mold 49 is removed to define the prism support structure 46 of the coupling prism assembly 40, as shown in FIG. Figure 5B In the example, the molded prism support structure 46 includes an isolation member 50 in the form of a thin wall 47 between the EPCS coupling prism 42A and the LSP coupling prism 42B, as shown in FIG. Figure 6C In an example, the molded prism support structure 46 is formed so that at least one of the prisms is partially encased to avoid crosstalk. In an example, the molded prism support structure 46 comprises or consists of a unitary molded structure, i.e., a single piece (i.e., a monolithic piece) made of a single material and thus not formed by joining two or more components.

[0363] In an example, the molded prism support structure 46 includes a fastening tab 52 that includes a mounting hole 53 for fastening the prism support structure 46 to the support plenum 70 (see also FIG. Figure 5A ).like Figure 7 As shown in FIG and described above, the use of the movable substrate holder 82 enables EPCS and LSP measurements to be performed at the same location on the CS substrate 10. The movable substrate holder 82 can be moved under the operation of the system controller 40 by using a precision linear motor (e.g., a piezoelectric actuator) to set the measurement positions of the EPCS subsystem 100 and the LSP subsystem 200.

[0364] In an example, the prism support structure 46 includes sections that are movable relative to each other, such that the EPCS prism 42A and the LSP prism 42B are movable relative to each other, e.g. Figure 6C In the example, the support frame 48 of the prism support structure includes adjacent walls 47 that are configured so that one wall can slide relative to the other wall in a controlled manner. Figure 6C In the example of FIG. 4 , the EPCS coupling prism 42A is shown as being moved in the z direction relative to the LSP prism 42B.

[0365] Figure 6D Similar to Figure 4CAn embodiment of a hybrid system 20 is also shown in which the EPCS subsystem 100 and the LSP subsystem 200 share a common coupling prism 42, i.e., the common coupling prism 42 acts as both the ECSP coupling prism 42A and the LSP coupling prism 42B. A single refractive index matching fluid 5 is also used. Various surfaces of the coupling prism 42 serve dual purposes, for example, coupling surfaces are designated as 45A and 45B because they serve dual purposes of performing both EPCS coupling and LSP coupling. In this example, bandpass filters 144 and 244 of the EPCS subsystem 100 and the LSP subsystem 200 are used, as well as different wavelengths λ. A and λ B Crosstalk between the subsystems can be significantly reduced or eliminated by using a common coupling prism 42 (e.g., separated in wavelength by at least the bandwidth of one of the bandpass filters 144 and 244). In the example of a common coupling prism 42, the coupling prism can have an ECSP segment PS1 and an LSP segment PS2, and in further examples, the segments can be separated, that is, the ECSP measurement light 116 and the LSP measurement light 216 generally remain in their respective segments except for a small amount of scattered light.

[0366] Reduce substrate warpage

[0367] The CS substrate 10 may be large enough that it may warp to the extent that accurate EPCS and LSP stress measurements are difficult to make. Specifically, warping the CS substrate 10 may make it difficult to establish the EPCS coupling interface INT1 and the LSP coupling interface INT2 required for EPCS and LSP measurements.

[0368] Reference again Figure 8A and 8B , the PV system 91 is used to reduce or eliminate substrate warpage. The PV conduit (PV strip) 90 is pneumatically connected to the top surface 12 of the CS substrate 10 through the aperture 72 in the support air chamber 70, which supports the CS substrate so that the top surface 12 is substantially located at the measurement plane MP. Activation of the PV source 92 produces a reduced pressure near the coupling prism assembly 40 via the PV strip 90, resulting in the surrounding high pressure generating a downward force FD on the CS substrate, as shown by the two large arrows. The PV strip 90 enables the height of the CS substrate relative to the top surface 72 of the support air chamber 70 (and therefore relative to the measurement plane MP) to be controlled within an accuracy range of ±5 microns. The use of the PV system 91 also reduces vibrations and enables non-contact control of the CS substrate for dynamic handling and inspection without the need to stabilize the CS substrate on a vacuum chuck.

[0369] PV strips 90 are commercially available and can be configured to reduce warping, such as Figure 8A and 8B. It may be desirable to omit some of the PV strips 90 proximate to the coupling prism assembly 40 so as not to interfere with the EPCS measurement beam 116 and the LSP measurement beam 216 and the various components of the EPCS subsystem 100 and the LSP subsystem 200 immediately below the support plenum 70. In an example, one or more stop members 94 may be used to hold the CS substrate 10 in place on the support plenum 70.

[0370] In some cases, it may be desirable for at least one of the EPCS coupling prism 42A and the LSP coupling prism 42B to be adjustable independently of the other. In such cases, the coupling prism assembly 40 may include two separate prism support structures 46, one or both of which may be adjustable. In one example, the EPCS coupling prism 42A may be adjusted in the z-direction to optimize the contrast between the TM and TE mode fringes in the modal spectrum. This may be achieved using a single-axis micropositioner operably attached to the prism support structure 46, which holds the EPCS coupling prism in a movable configuration.

[0371] Processing of EPCS and LSP measurements

[0372] Figure 9 4 is a schematic diagram of an example user interface 410 displayed by the system controller 400 of the hybrid system 20. The user interface 410 includes an EPCS section 412A showing the modal spectrum 160 generated by the EPCS subsystem 100 and an LSP section 412B showing the digital LSP image 248 generated by the LSP subsystem 200. The software in the system controller 400 is configured to calculate a first stress characteristic of the CS substrate using EPCS measurements from the EPCS subsystem 100 (i.e., the modal spectrum 160) and to calculate a second stress characteristic of the CS substrate using LSP measurements from the LSP subsystem 200 (i.e., the digital LSP image 248D), and then combine the measurements to generate a complete or comprehensive stress characterization of the CS substrate.

[0373] Processing LSP measurements

[0374] In an example, the system controller 400 is configured to process (e.g., using software) the LSP image 248 to extract a “second” or LSP stress characteristic obtained from the LSP subsystem 200. This includes using Gaussian blur Otsu thresholding to digitally characterize the contours of the LSP image 248 as part of a contour detection method to facilitate calculation of optical delay and depth (OR and D).

[0375] Figure 10A4 is an example representation of an LSP image 248 as shown in the LSP section 412B of the user interface 410. The LSP image 248 is detected by the digital detector 246 to form a digital LSP image 248D, which may be referred to as a raw LSP image or a raw digital LSP image. The LSP section 412B of the user interface also shows a histogram of the scattered light intensity that constitutes the digital LSP image 248D and some related statistical measurements. In this example view, the main light beam entrance into the CS substrate is from the lower right to the center of the cross. From the center of the cross to the upper right, the digital camera sees reflections from the air surface of the CS substrate on one side of the light beam due to total internal reflection (see below). Figure 11C From the center to the lower left of the crosshairs, the direct beam reflects off the air surface of the CS substrate and traverses the thickness of the CS substrate back toward the LSP coupling prism. From the center to the upper left, reflections of the reflected beam are seen by the digital camera.

[0376] The digital LSP image 248D is composed primarily of very bright pixels and pixels that are barely exposed. Figure 10B As part of the contour detection method, a Gaussian blur is applied to the original (raw) digital LSP image to reduce any residual noise. The result is a blurred LSP image. Image 248D is an example of an original (raw) digital LSP image. Image 248B is an example of a blurred LSP image. The Gaussian blur is applied in a manner that does not obscure the optical delay information encoded in the intensity variations of digital LSP image 248D.

[0377] Reference Figure 10C , Otsu thresholding is applied to Figure 10B The (Gaussian) blurred LSP image 248B is obtained to obtain the threshold LSP image 248T. The Otsu thresholding mechanism uses the image histogram (see Figure 10A ) to select an intensity value below which all pixels are set to zero. Figure 10C The bright segments in represent all pixels with intensities above the threshold.

[0378] Figure 10D The next process step is shown, which involves using the thresholded LSP image 248T to define an LSP image outline 248C using a binarization method, such as by applying a known open-source binarization algorithm, such as available from open-source image processing algorithms (e.g., via OpenCV). The example uses an image coordinate system with the upper left being 0.0 and increasing values in the right (x) and downward (y) directions. The LSP image outline 248C is composed of a series of points that can be split into quadrants to find the following five critical points of the cross-shaped image: upper left, upper right, lower left, lower right, and center. Figure 10DThe close-up view of shows an example of the bottom left point detection, which is obtained by finding the lowest X value and the highest Y value in the region. The same process is repeated for all four corners, and the center is determined by averaging the corner X and Y values.

[0379] Figure 10E The final LSP image profile 248C is shown with the contours and processed regions fully defined.In the example, the lower right leg (see the trapezoidal region) of the processed "X" LSP image profile 248C is then used to calculate LSP stress properties. Figure 10E The horizontal lines in the LSP image profile 248C are at a constant depth. The Gaussian blurred intensity (e.g., sum, peak, or average) on the horizontal lines of each of the images acquired when the polarization of the light source is modulated is used as input for subsequent analysis to obtain OR and D data.

[0380] Thus, the threshold LSP image 248T and LSP image profile 248C are used to define a “mask” that identifies one or more portions of the captured or Gaussian-smoothed LSP image 248B for use in calculating the optical delay OR as a function of depth (D) into the CS substrate 10, as explained above.

[0381] CS substrate thickness extraction and beam angle calculation

[0382] Figure 11A is a view of the CS substrate 10. Figure 11A Also shown is the beam path of a portion of the focused LSP beam 216F inside the body 11 of the CS substrate 10 after passing through the LSP coupling prism 42B (not shown). Figure 11B A close-up view showing an edge portion of the CS substrate 10 as a region of interest for calculating the CS substrate thickness TH. Figures 11C to 11E 1 is an additional view of the path of the focused LSP beam 216F within the CS substrate. For ease of illustration, the LSP coupling prism 42B is not shown.

[0383] By viewing the edge of the CS substrate 10 along the propagation direction of the focused LSP beam 216F, the thickness of the CS substrate 10 as seen by the digital detector 246 of the LSP detector system 240 can be highlighted, as shown in FIG. Figure 11B As shown in FIG. 4 , since the digital detector 246 is coupled to the prism 42B through the tilted LSP (e.g., tilted 45 ° ) to view the focused LSP beam 216F, the actual thickness TH of the CS substrate 10 can be calculated as

[0384] TH=x / {Cos(45 ° )

[0385] where x represents the path length in the plane of the digital detector 246 .

[0386] Once the thickness TH is calculated, the propagation angle A of the focused LSP beam 216F within the CS substrate (see Figure 11E ) can be determined by looking at the edge of the CS substrate 10 in the direction of the digital detector 246 and using Figure 11E The propagation angle A is determined using the following formula:

[0387] A=ArcTan(W / TH)

[0388] Where W is the horizontal distance between the center cross C of the image profile 248C obtained by the profile detection method described above and the lower right (LR) critical point of the image profile. Once the processing area is selected, the digital detector 246 records several images 248 according to the input polarization. The optical delay information as a function of depth into the CS substrate is then extracted using techniques known in the art.

[0389] Lock detection method

[0390] The lock-in detection method is a signal analysis technique that has proven to be very adept and fast at retrieving signals that are obscured by noise. For this method to be effective, the period of the signal must be known.

[0391] The period of the measurement (detector) signal SB from the LSP subsystem 200 depends on the polarization rotation rate of the optical compensator 230. When a rotating half-wave plate 234H is used in the optical compensator 150, one complete rotation corresponds to four oscillations of the polarization state of the scattered light 216S.

[0392] The locking method applied to the derivation of the LSP measurement signal SB = s(t), where t is time, is as follows. Assume that the LSP measurement signal s(t) is centered around zero and has an amount of noise ("noise factor") of N. The measurement data D(t) received by the system controller 400 can be expressed as:

[0393] D(t)=s(t)+N

[0394] The measured signal s(t) can be summarized as follows

[0395]

[0396] in is the phase value to be extracted, and f is the known frequency of the signal. This signal can be "locked" by multiplying it with a universal test wave W(t) = cos(-f-θ) of equal and negative period (and arbitrary phase) to obtain the following equation:

[0397]

[0398] The first two terms of the D(t)*W(t) equation above oscillate according to the time variable t. However, the last term is a constant and can be extracted by strongly low-pass filtering the product wave. This is achieved by averaging the product wave, as the average value of a wave approximates its deviation over multiple oscillations.

[0399] If the measured signal s(t) has a small number of oscillations (e.g., less than one complete oscillation) or if the signal has a non-integer number of half-cycles, this approximation can produce a slight error. This error can be reduced by averaging only the signal with the largest number of half-cycles. For example, if the signal has approximately 3.7 oscillations, then the average value can be taken for a maximum of 3.5 cycles.

[0400] Once low-pass filtered using known methods, the product D(t)*W(t) reduces to a constant term Think back, is the desired phase value, and θ is an arbitrary phase of the test wave. Therefore, if θ is incremented by a series of numbers, the constant obtained by low-pass filtering the product wave at each increment will be calculated according to the time-invariant cosine function Oscillation. The wave number of this cosine wave is -1 and the amplitude is A / 2, and the phase is Knowing this, one can then perform a cosine fit (e.g., using a least squares fit) to these constants and extract the phase The amplitude of signal A can also be extracted.

[0401] It turns out that the lock-in method for signal extraction is much faster than regular sine fitting. Figure 12A To extract the phase of the noise signal Plot of the required average computation time T (in milliseconds (ms)) versus the noise factor N for the lock method (L or black curve) and the sinusoidal method (S or gray curve). Figure 12A Data was collected through a series of tests. In these tests, random noise was added to the setup signal, and the phase was extracted based on the signal using a sine fit and lock-in detection method. 100 tests were performed using the randomized noise at each noise level. The lock-in method took roughly half the time to calculate compared to the sine fit method.

[0402] Figure 12B is the absolute phase difference Plotted against the noise factor, N, for the lock method (L or black curve) and the sine method (S or gray curve). Figure 12B demonstrated that both methods maintained approximately the same level of accuracy and precision across all tests.

[0403] The locking method eliminates the need to predict the sinusoidal parameters for fitting. The only fitting performed is that of the cosine wave with the low-pass filter constant, which is strictly limited and rarely results in a bad fit. However, if a sinusoidal fit is used, it has been found that the fit is much more accurate when the sinusoid fitted to the data has a constant period. If the period can be fitted with other parameters, the processing time will be longer and the results will generally be less accurate.

[0404] Noise Reduction in LSP Measurement

[0405] Extracting the secondary stress characteristic using LSP measurements from the LSP subsystem 200 consists of two main parts: a data acquisition part and a data analysis part. In the data acquisition part of the measurement, the scattered light 216S is imaged as a function of the input polarization state of the initial LSP beam 216 from the LSP light source 212. Imaging is achieved by recording the intensity of scattered light from features (e.g., particles, refractive index changes) within the host CS substrate 10 due to one or more IOX processes at a digital detector 246.

[0406] The recorded image 248 is processed by the system controller 400 to extract the intensity along the laser beam, which is analyzed according to the input polarization to extract the amount of optical delay between the two orthogonal states of the beam. By modeling the observed delay, the stress profile is reconstructed. Therefore, the quality of the stress profile measured by LSP is fundamentally limited by the noise in the imaging process, which is generally dominated by laser-based noise. An example of such laser-based noise is speckle, which arises from the high coherence of the LSP light source 212, as well as defects in the optical surface (roughness, flatness, etc.) and the bulk properties of the optical element (impurities, density inhomogeneities, heterogeneity, etc.).

[0407] As the LSP beam 216 propagates through the LSP subsystem 200, the interaction of the beam (laser) 216 with system imperfections results in random amplitude and phase variations within the beam's wavefront. When the LSP beam 116 is coherently imaged via Rayleigh scattering, this wavefront distortion results in a static interference pattern in the image plane, known as a speckle pattern. This pattern is characterized by large intensity variations at high spatial frequencies superimposed on the desired signal. Deviations in the intensity of the desired signal are considered noise in LSP measurements. To reduce the effects of laser speckle, individual speckle patterns can be imaged and averaged by modulating the polarization, amplitude, or phase of the beam's wavefront.

[0408] In one embodiment, laser-based noise is reduced in the LSP subsystem 200 by passing the initial beam (laser) 216 through a movable light diffuser 222, which in one example may comprise a holographic diffuser. This "stirs" the beam rays within the diffusion angle, depending on the local structure of the diffuser. To minimize the beam divergence caused by this "ray stirring," the light diffuser 222 is placed in the image plane of a Keplerian telescope configuration, such as Figure 4A The LSP beam 216 is first focused by a first focusing lens 220 onto a light diffuser 222 , and the transmitted beam is re-collimated by a second focusing lens 224 .

[0409] After undergoing light diffusion, the divergence of the LSP beam 216 is mitigated to provide a more efficient (i.e., less distorted) focused LSP beam 216F at the CS substrate. Using the moving light diffuser 222, the laser-based noise (e.g., speckle pattern) at the digital detector 246 is reduced at the rotational speed v of the rotating diffuser. D Under ν D τ C >1, the maximum effect of noise averaging is achieved, where τ C is the exposure time of the digital detector 246. This condition also eliminates potential imaging flicker caused by optical transmission changes on the light diffuser 222. The implementation of the diffuser-based noise reduction improves the measurement of optical delay. Figure 13A and 13B , which is a plot of optical retardation OR (radians) versus depth D (mm) into the CS substrate. The plot of 13A was obtained without using the noise reduction apparatus and method described above. The plot of 13A was obtained by using the noise reduction apparatus and method described above. Figure 13B 's plotted diagram. Figure 13B The smoothness of the plot is a direct result of applying the noise reduction apparatus and methods disclosed herein.

[0410] Another way to reduce noise is by moving the substrate.

[0411] OR plot using inflection points and CS substrate mid-plane shift

[0412] Since the location of the surface 12 of the CS substrate 10 may be difficult to determine from the LSP image 248, the stress profile may be shifted to an appropriate location based on the general shape of the delay curves (OR and D). The OR delay curve has two inflection points where the derivative is zero. Figure 14A An example actual OR vs. D curve is shown in Figure 1 along with two inflection points, BP1 and BP2. The data points are shown as open circles. The two inflection points correspond to where the stress profile changes from compression to tension or vice versa.

[0413] If the stress profile is symmetrical, then the two bending points BP1 and BP2 should also be symmetrical about the mid-plane MP of the CS substrate (see Figure 1A ). Therefore, if the thickness TH of the CS substrate 10 is known and the two bending points BP1 and BP2 of the optical retardation OR curve can be found, the OR profile can be horizontally shifted to the correct position. This allows for a more accurate determination of the compression depth DOC because the position of the surface 12 of the CS substrate 10 is selected based on the known symmetry and thickness of the CS substrate. Figure 14B Similar to Figure 14A , but the display is compared to Figure 14A The OR curve is shifted to the left using the plot shifting (data shifting) technique described above.

[0414] Shift OR plot using curve fitting

[0415] An alternative method to extract the DOC for a symmetric stress profile involves analyzing the shape of the delay profile, i.e., the OR vs. D curve. If the thickness TH of the CS substrate is known and the relative positions of the inflection points BP1 and BP2 can be determined by polynomial fitting, the compression depth DOC of the CS substrate can be determined by the following expression:

[0416] DOC=[TH-(BP2-BP1)] / 2

[0417] BP1 and BP2 are the relative depth positions of the bending points.

[0418] Curve fitting of OR and D curves

[0419] Embodiments of the present disclosure relate to methods for obtaining an excellent fit of OR vs. D curve data. The method involves using a combination of linear and quadratic functions to obtain the curve fit. This method is hereinafter referred to as the LinQuad method.

[0420] Figure 15A is a plot of OR versus D data (circles) and shows an example fit curve FC (solid line) for the OR data using the LinQuad method. The LinQuad method assumes the following model stress function, where σ is stress, x is the depth coordinate into the CS substrate 10, and R is defined as follows:

[0421]

[0422] The corresponding delay can be extracted and fitted to the raw data of interest to regenerate the stress profile. Here, C represents the normalized model concentration of ions in the CS substrate. Its expression is as follows.

[0423]

[0424] where d lis the depth of the linear region, d c is the depth of the curved region, C0 is a constant multiplier, and t is the CS substrate thickness.

[0425] An alternative expression is given by:

[0426]

[0427] Here, CT is the central tension of the stress profile, and is a (partially arbitrary) constant approximately The true LinQuad function is defined as above, where only d c d l However, this latest expression for σ(x) allows for a fourth parameter, the central tension CT, to vary, which may help the function fit the data more closely.

[0428] Figure 15B Based on Figure 14A Plot of stress S(x)=σ(x) versus depth D(mm) (or x-coordinate) obtained by LinQuad fitting of the OR and D curves.

[0429] Power-Spike Function

[0430] The power-peak function is defined as:

[0431]

[0432]

[0433] CT sp is the central tension of the peak in the peak region R1, mid is half the thickness TH, CS sp is the peak compressive stress, and DOL sp is the layer depth of the peak. Parameter L eff is the effective length (depth) of the peak region R1. This function is the concatenation of the power profiles with two error function peaks at both ends. CS sp and DOL sp The values are specific to each glass type and are entered as constants. The only parameters that need to be fitted are the function power p and the peak central tension CT p .

[0434] Figure 16A OR versus D plot showing an example fit curve FC using a power-spike function. Figure 16B Based on Figure 16A A plot of the stress profile S(x) (MPa) fitted by the power-spike function of the OR and D curve versus the depth D into the CS substrate 10 is shown.

[0435] Remove systematic errors to fit a symmetrical stress profile

[0436] The stress profile of a CS substrate using LSP measurement data is obtained by differentiating the OR vs. D curve. Thus, a symmetric stress profile will always correspond to an asymmetric OR vs. D curve. However, systematic errors from various components in the LSP subsystem 200 can introduce symmetric components into the OR vs. D delay data, hindering accurate stress profile extraction. This effect can be mitigated by decomposing the delay data into symmetric and antisymmetric components and fitting only the antisymmetric portion (i.e., the asymmetric data points).

[0437] Given an optical delay OR in the form of a function f(x), the decomposition can be achieved as follows.

[0438] f(x)=f s (x)+f a (x)

[0439] where f s and f a are the symmetric and antisymmetric components of the delay f and are represented by the following equations:

[0440]

[0441] Figure 17A is a plot of OR and D based on the original OR data, and Figure 17B is a plot of OR versus D fitted using the technique described above to remove the symmetric component of the data. Figure 17B The fitting error of the fitting curve FC of the measured data is 0.006, while Figure 17A The fitting error in is about 0.46.

[0442] Adjustable fitting area for accurate CT and DOC

[0443] A single fit to the OR vs. D curve may not always accurately determine the central tension CT and the depth of compression DOC because scattering from the LSP coupling prism 42B or the coupling interface INT2 may hinder data collection close to the top surface 12 of the CS substrate 10 .

[0444] In an example, fitting of the OR versus D curve is performed using fitting of separate regions of the curve associated with central tension CT and depth of compression DOC, respectively, and adjusting the fitting range of the OR data to accurately extract CT and DOC.

[0445] Figure 18A and 18B Example OR vs. D curves are shown where the regions around the inflection points BP1 and BP2 defined by the data (circles) are fitted to extract the depth of compression DOC. Figure 18B The central linear region between the inflection points BP1 and BP2 fitted to extract the central tension CT is shown.In both cases, the range of the OR vs. D data is significantly reduced to the portion of the OR vs. D curve associated with a given stress parameter.

[0446] Figures 19A to 19D The effect of data range selection (indicated by the vertical dashed line) on the quality of the fit is further shown. Figure 19A is a plot of OR versus D, where the full data range is considered and where the fitted curve does not fit the inflection points BP1 and BP2 very closely. Figure 19B for Figure 19A The corresponding plot of stress S(x) and D(depth) shows the compressive stress CT and the compression depth DOC.

[0447] Figure 19C For similar Figure 19A 2. The OR and D plots are shown in FIG. 2, except that the data range is reduced to the region between the vertical dashed lines and therefore the first and second “end regions” ER1 and ER2 of the measured data are omitted. Figure 19C The fitting curve FC closely follows the bending points BP1 and BP2. The corresponding S(x) and D plots are shown in Figure 19C and the values of compressive stress CT and compression depth DOC are significantly different from those using the full data range. Figure 19B The value in .

[0448] Optimizing bending point detection

[0449] Embodiments also enable accurate detection of the two bending points, BP1 and BP2, of the delay profile. The bending points occur where the delay profile plateaus, i.e., where the glass stress changes from compression to tension. These points are reported as the depth of compression (DOC). Previously, the locations of these points were roughly estimated, and the region around these estimates was fitted using a polynomial of a predefined order. Third or fourth order polynomials have been used in the past, but for some delay profiles, these orders have proven to be less accurate than other orders. To achieve an accurate fit for both bending points, the optimal polynomial order and the best fit range should be determined.

[0450] In a first step, the profile is smoothed to roughly estimate the locations of the two bending points. Figure 19E In the non-limiting example shown in , a LOESS filter with a smoothness of 20% is used to smooth out most of the noise in the delay profile.

[0451] Next, two steps are performed on this smoothed profile. First, the derivative of the smoothed profile is calculated, such as Figure 19FThe starting and ending edges of the delay profile are considered as the minimum points of the first derivative of the profile. This edge selection can be seen in Figure 19F These edge points are used to constrain the polynomial fit so that it does not overextend into the surface of the delay profile that would be damaged by the fireball. Next, the curvature of the smoothed profile is calculated. The curvature is calculated using the following equation:

[0452]

[0453] where y' and y" are the first and second derivatives of the input profile, functions of the variable x.

[0454] Because the curvature of the delay profile is highest at the two bending points, this calculated profile can be used to roughly estimate BP1 and BP2. To calculate this rough estimate, the curvature profile is split in half and a weighted average of the left and right sides is taken. This effectively determines the centroid of the left and right curvatures, as Figure 19G As depicted, it roughly matches the locations of the two inflection points.

[0455] Next, the profile is windowed vertically to isolate the region to be fitted. To window the profile, the difference between the vertical positions of the rough BP1 and BP2 points obtained by the two methods described above is used. A certain percentage (e.g., 15%) of this total vertical range from BP1 to BP2 is taken from the top and bottom, and data is selected within these regions for polynomial fitting, as shown in Figure 2. Figure 19H As shown in .

[0456] After determining the fitting regions for BP1 and BP2, the optimal polynomial order can be selected for fitting. Choosing a fixed order that not only fits the BP1 and BP2 regions (which may have slightly different shapes) well but also fits every type of delay profile that can be measured can be challenging. If the polynomial order is too low, the fit may not match the data well, potentially biasing the measurement results. If the polynomial order is too high, the fit may be too close to the noise, also biasing the measurement results. Therefore, the polynomial order should be optimized based on the fitted data.

[0457] The polynomial optimization process works by fitting the input data with polynomials of successive orders (starting with zero, then first, and so on). For each order, the fitting cost is calculated using the following equation:

[0458]

[0459] where n is the total number of data points, m is the degrees of freedom of the fitted function (for a polynomial, m=o+1, where o is the order), and Error is the sum of the squared differences between the original data and the fit.

[0460]

[0461] It should be noted that as the order of the polynomial increases, the numerator and denominator of the cost equation decrease. Plotting the cost equation for fitting the first bending point region will show how the cost decreases rapidly in the first three orders and then stabilizes around the fourth to fifth order, as shown in Figure 19I As shown in . At these orders, increasing the polynomial order only slightly improves accuracy. Therefore, the algorithm will choose the fourth or fifth order as the optimal order (fifth order can be chosen because it has the lower cost of the two orders). The same analysis is performed on the second bending point region, and the optimal order is ten. These fits can be seen in Figure 19J Once these fits are completed, the minimum point of the BP1 fit and the maximum point of the BP2 fit are considered the two inflection points of the delay profile.

[0462] Therefore, using this method, the inflection point region can be windowed and fitted to optimize for any profile being measured. This advancement not only improves the detection of the two inflection points (which are reported as DOC parameters for the profile), but also improves the rest of the analysis, as the delay profile is shifted, filtered, and fitted based on the locations of these inflection points.

[0463] Considerations for Simultaneous EPCS and LSP Measurements

[0464] One method of achieving good accuracy in measuring the depth of compression (DOC) using the LSP subsystem 200 is to press the CS substrate 10 against a stop surface (e.g., the support plenum 70) to ensure that the top surface 12 of the CS substrate 10 is coplanar with a predefined surface that can be assigned a depth z = 0. This pressing can be achieved by pushing the CS substrate 10 against a stop, or by applying a vacuum so that the ambient atmospheric pressure provides a force to push the top surface 12 of the CS substrate 10 to the z = 0 position (see, e.g., FIG. Figure 8A 、 8B ).

[0465] On the other hand, achieving a clear (ie, high contrast) mode spectrum 160 using the EPCS subsystem 10 to obtain accurate stress measurements of the near-surface region R1 of the NSWG 18 typically also requires good CS substrate flatness in the EPCS measurement region, which can also be achieved using the vacuum system 91 .

[0466] Because the EPCS and LSP measurement regions are located at different locations on the CS substrate, applying a vacuum at the LSP measurement region can, in some cases, deform the CS substrate at the EPCS measurement region, leading to suboptimal or even poor flatness in the EPCS measurement region, or significant surface deformation. This causes the EPCS mode spectrum 160 to have poor contrast and be "out of focus." These conditions can lead to reduced accuracy and precision, as well as measurement failures, because the poor contrast can prevent the system controller from identifying some of the target features in the mode spectrum 160 for stress calculations.

[0467] In an exemplary embodiment, the EPCS detector system 140 of the EPCS subsystem 100 utilizes adaptive focusing to achieve proper alignment of the CS substrate 10 on the supporting gas chamber 70 so that the best (most accurate) DOC measurement and near-surface stress measurement can be performed using the EPCS subsystem when the CS substrate is aligned to achieve the best LSP measurement of the LSP subsystem 200.

[0468] exist Figure 20 In one embodiment shown in FIG, this is achieved by mounting the focusing lens 142 of the EPCS detector system 140 on a translation stage 143 so that the focusing lens is adjustable (e.g., axially movable), which, in an example, is operably connected to and controlled by the system controller 400. In an example, the translation stage 143 includes a precision linear actuator, such as a piezoelectric actuator. In another example, the translation stage 143 includes a ball screw actuator. This allows the focusing lens 142 to be translated along the second optical axis A2 to improve or maximize the contrast of the mode spectrum 160 captured by the digital detector 150. In an example, the contrast of the mode spectrum 160 is improved to enhance target spectral features, such as the TM fringes 163TM and the TE fringes 163TE and the critical angle transition regions 166TM and 166TE.

[0469] The system controller 400 can electronically monitor the position of the axially movable focusing lens 142 to correct the EPCS subsystem calibration by calculating the "optical path length" or OPL (e.g., the distance from the focusing lens 142 to the digital detector 150). In one embodiment, the calculation can be simplified so that the original calibration remains accurate as long as the OPL does not exceed a predefined acceptable range. In another embodiment, the calibration is corrected based on the OPL, and the surface stress S(0)=CS and / or the depth of layer DOL are calculated based on the corrected calibration.

[0470] In another embodiment, the focusing lens f1 has a variable effective focal length that is actively controlled by the system controller 400 to obtain a high contrast pattern spectrum 160 when the sample is aligned, thereby ensuring the most precise or accurate measurement of the depth of compression DOC by the LSP subsystem 200. The variable focal length focusing lens 142 may include a composite lens (similar to a photographic multi-component lens having more than one optical element), or may additionally include an adaptive lens, such as a fluid-filled lens, where changing the pressure of the fluid changes the shape of the lens and, therefore, the focal length. When using a variable focal length lens 142, it may not be necessary to shift the position of the focusing lens 142, as changing the focal length may in many cases be sufficient to compensate for deformation of the sample shape in the EPCS measurement region caused by aligning the sample for optimal measurement in the LSP measurement region.

[0471] In another embodiment, variation in the effective focal length of the focusing lens 142 can be achieved by an adaptive lens surface in the form of a mirror, which can be combined with a fixed single lens to produce a net effective focal length that can be varied over a range sufficient to produce a high contrast mode spectrum 160 even if the CS substrate alignment is optimized for the LSP subsystem 200.

[0472] Since deformation of the CS substrate 10 is usually not large, the change in the refractive power of the variable focus lens 142 does not need to be particularly large to compensate. In an example, the focal length of the focusing lens 142 can change by at most 15%, or by at most 10% in another example.

[0473] On the other hand, when the thickness of the CS substrate 10 is less than 0.6 mm, it may be necessary to change the refractive power by more than 15%, as much as 20%, or even 25%. Therefore, in the example, the adaptive system for changing the focal length of the focusing lens 142 is configured to change the focal length within a focal length range representing 25% of the average focal length, but in many cases, a range of 20%, 15%, or even 10% of the average focal length may be sufficient.

[0474] Similarly, since for measurements on flat CS substrates, the focusing lens 142 system is focused at infinity, when the focusing lens 142 has a fixed focal length and the position of the focusing lens is axially adjusted to produce a high contrast mode spectrum 160, the range of axial positions that the focusing lens can achieve ideally represents approximately 25% of the focal length of the lens, but in some cases, 20%, 15% or even 10% of the focal length may represent a sufficient position range.

[0475] Figure 21A and 21B1 is a schematic diagram of an example embodiment in which two or more focusing lenses 142 having slightly different focal lengths are mounted on a support member 152 to define a focusing lens assembly 153. The support member 152 is movable so as to position a selected one of the focusing lenses 142 in the optical path of the reflected light beam 116R (i.e., along the second axis A2). This allows a user to select the focal length of the focusing lens 142 from a discrete set of focal lengths. Figure 21A An example is shown in which the support member 152 is in the form of a rotatable wheel rotatable about an axis of rotation AW. Figure 21B An example of a support member 152 in the form of a linearly translatable support frame is shown. Four example focus lenses 142 are shown. In general, the focus lens assembly 153 can support two or more focus lenses 142.

[0476] If the contrast of the features of interest in the modal spectrum 160 (e.g., TM mode line 163TM and TE mode line 163TE, TM critical angle transition region 166TM and TE critical angle transition region 166TE, etc.) is deemed sufficient, the measurement proceeds as normal. If the contrast of the features of interest is deemed insufficient, a focusing lens 142 of a different focal length is moved into the optical path of the reflected light beam 116R, and a new modal spectrum 160 is captured by the EPCS digital detector 150 and analyzed for contrast. This process is repeated until a modal spectrum 160 with sufficient contrast is obtained.

[0477] In an example, the difference in focal length of the focusing lenses 142 can be set by the desired total focal length coverage and the total number of lenses on the support assembly. In one example, the support assembly supports six focusing lenses, with the focusing lenses covering a range of 20% to 30% of the average focal length of the entire focusing lens set, and the focal lengths being spaced apart by 3% to 7% of the average focal length.

[0478] In another example, the focal lengths are not evenly spaced such that each pair of adjacent focal lengths is spaced approximately a fixed percentage of the average of the adjacent focal lengths, wherein the percentage is between 2% and 20%, and more preferably between 3% and 10%.

[0479] In another related embodiment, some or all of the focusing lenses 142 comprise Fresnel lenses. In another embodiment, the focusing lenses 142 need not have different focal lengths, but can be positioned on a movable support member such that, when a selected focusing lens is placed in the optical path, it is at a different distance from the digital detector 150 than the other focusing lenses. In this embodiment, rather than having a complete set of discrete, closely spaced, custom-selected focal lengths, the set of distances from the digital detector and / or available focal lengths is sufficient to ensure a spectrum with sufficient contrast for the features of interest. This can reduce the cost of the EPCS subsystem 100 by utilizing standard, off-the-shelf focusing lenses and positioning each focusing lens to produce a clear image for a specific warp / curvature range of the CS substrate 10.

[0480] In an example, the system controller 400 may be configured to select one of the focus lenses 142 based on a measure of contrast of a feature of interest of the captured pattern spectrum 160 .

[0481] In another embodiment, a measurement may be performed using two or three preferred pattern spectra 160 with the best contrast among all captured pattern spectra, and a preferred result may be calculated as the average of the two or three preferred pattern spectra. In an example, the preferred result may be calculated as a weighted average of the two or three preferred pattern spectra. In a related example, the weight of each preferred spectrum may be proportional to the contrast of the feature of interest in the preferred pattern spectrum.

[0482] Strain measurement calibration using independent strain measurements

[0483] The EPCS subsystem 100 is well suited to obtain high contrast mode spectra 160 of CS substrates formed using an IOX process of lithium-based glass, for example, where K ions replace Li and / or Na ions in the glass near the surface. This in turn allows for good measurement of knee stress CS by measuring birefringence based on the relative positions of the TM critical angle transition region 166TM and the TE critical angle transition region 166TE. k (See Figure 3B ).

[0484] On the other hand, knee stress CS k The relative accuracy of EPCS measurement is usually lower than that of surface stress S(0). Specifically, the knee stress CS k The standard deviation of the measurements is typically a few percentage points of its mean, while the standard deviation of the surface stress S(0) is typically about 1% to 2% of its mean. Alternatively, the knee stress CS can be obtained as simply as the ratio of the birefringence B at the critical angle detected to the stress optical coefficient (SOC). k The values of CS and CS obtained from the destructive RNF measurements of the stress profile are similar.k The values are slightly different.

[0485] When knee stress CS k When the accuracy of the EPCS measurement is lower than it could or should be, it may be due to a systematic error in the birefringence measurement at the critical angle. This systematic error may be caused by the TM mode line 163TM and the TE mode line 163TE being too close to the TM critical angle transition region 166TM and the TE critical angle transition region 166TE, and may also be caused by the specific shapes of the TM and TE refractive index profiles.

[0486] When performing quality control measurements, the knee stress CS is calibrated by using the corresponding independent reference stress measurement. k These systematic errors are reduced by measuring the reference stress based on EPCS measurements, which may be destructive measurements taken on a set of CS substrates formed using the same process or from the same batch during the same process. In an example, this is achieved by applying a calibration multiplier K based on the independent measurements via the following relationship: cal To achieve:

[0487] CS k (EPCS, calibration) = K cal CS k (independent).

[0488] In the example, the calibration multiplier K cal It can be used as a general calibration factor for the stress profile calculated by the EPCS subsystem 100 via the following equation:

[0489] S(EPCS, calibration) = K cal S (original)

[0490] where S(orig) is the originally measured (uncalibrated) stress profile S(z).

[0491] Extraction of stress profile in tension zone

[0492] The 10X process used to form the CS substrate 10 forms a compression region that defines the NSWG 18. This compression region extends into the substrate and reaches a zero value, defining a depth of compression, DOC. At the DOC, the compression region ends and the tension region begins.

[0493] If the stress profile in the tension region can be accurately extracted, it can be used as a powerful tool to help extract a substantially accurate representation of the stress profile in the compression region. This can be done by utilizing a force balance of the stresses in the entire CS substrate 10 or in half of the CS substrate (i.e., a so-called "half-force balance").

[0494] In one embodiment, in addition to the area of the stress profile in the tension region (which is represented by the depth integral of the tensile stress from one compression depth to the opposite compression depth), a reliable value of the slope of the stress profile at the reliable slope extraction depth can also be obtained from the LSP-based measurement.

[0495] In an example, the reliable slope extraction depth may be the compression depth DOC. In the compressive stress region, the surface compressive stress is determined by the EPCS method. In some cases, when there are not enough guide patterns for reliable IWKB extraction, a portion of the compressive stress profile may also be extracted from the EPCS method using existing techniques (e.g., IWKB, or linear profile, erfc-shaped profile, exponential profile, or LinQuad profile approximation). The EPCS-based method then provides a target connection point that is located at the surface with a surface stress value S(0) or a deeper connection point (e.g., knee depth z k ; See Figure 1B ), until the connection point, only the surface part of the stress profile S(z) can be extracted from the EPCS measurement. In the latter case, due to the limitations of the EPCS measurement, it may not be possible to specify the knee stress CS with high accuracy. k .

[0496] Nevertheless, knee stress CS k This value can be extracted by iterative improvement of the compression area (basically, for example Figure 1B The stress profiles in regions R1 and R2 in the first iteration can provide sufficient starting points. In the first iteration, the near-surface connection point with surface stress value S(0) can be connected to the depth connection point (for example, knee stress CS k or depth of compression DOC) and reliably extract the stress slope using a second-order polynomial. This determines a first approximation of the stress profile in the compression zone, the first part of which is obtained from the EPCS up to the first connection point (e.g. at the knee depth z k The second part is obtained via polynomial interpolation between the two connection points, where at the second connection point not only the surface stress S(0) matches but also the stress profile slope matches.

[0497] In certain examples, the second connection point can be the depth of compression DOC, but this need not be the case. A first approximation of the stress profile S(z) is integrated. If the stress profile is asymmetric, EPCS measurements can be made on both sides of the specimen, and a first approximation of the stress profile for each side can be obtained as described above. If the stress profile S(z) is symmetric in design and implementation, it can be assumed that the stress profile in the backside compression region of the specimen is the same as the stress profile in the frontside compression region.

[0498] A first approximation of the stress profiles from the front and back compression zones is integrated relative to the depth of the respective compression zones and compared to the depth integral of the tension zone. If the absolute value of the difference is greater than a predefined acceptable limit, a correction step is performed to reduce the difference. In this example, the predefined acceptable limit is 5% of the tension zone stress area, but progressively higher acceptable limits include 3%, 2%, 1%, and 0.5%.

[0499] The acceptable limit can be determined based on an estimate of the accuracy of the extracted stress profile in the tension zone. In one embodiment, several first approximations of the stress profile are obtained by different methods, all of which are compared with the knee stress CS at the first connection point. k Different types of first approximations can include second-, third-, and fourth-order polynomials, exponential profiles, erfc-shaped profiles, Gaussian profiles, and Lorentzian profiles, as well as matching the stress value and stress slope at a second connection point (e.g., depth of compression DOC). Subsequently, for each of these first approximations, the difference between the stress area in the first approximation compression region and the stress area in the tension region extracted using LSP-based measurements is found. A linear combination of these first approximation stress profiles is then found such that the stress area of the linear combination stress profile is equal to the stress area in the tension region.

[0500] In another embodiment, by allowing the knee stress CS k The range is set at knee stress CS k Consider the knee stress CS based on the initial estimate of EPCS k In the first approximation of the compressive stress part of the stress profile, a preferred target shape function of the interpolated area of the compression zone is used, which is related to the knee stress CS. k The connection is made based on the initial value of EPCS. In the example, the preferred target shape is a second-order polynomial.

[0501] After each iteration, the stress area from the two combined compression zones (one on each side of the specimen) is subtracted from the stress area in the tension zone. If the absolute value of the difference is greater than a predefined acceptable target limit, then the knee stress CS is k The target value can be found at the knee point CS k The knee stress measurement may vary within a predefined acceptable range determined based on the estimated accuracy of knee stress measurements obtainable from the EPCS-based method.

[0502] In the example, the estimated knee stress accuracy is about 10 MPa, but in some cases is better at 7 MPa or 5 MPa or 3 MPa. When there are no surface spikes and the guided mode is not available, then the same technique can be used to connect to the target surface stress S(0), allowing it to vary within the range determined by the accuracy of the surface stress measurement.

[0503] In the example, the target surface stress S(0) or knee stress CS k The width of the acceptable range of values for can be up to 6 standard deviations, for example 3 standard deviations on either side of the measured value of the surface stress or knee stress. In one embodiment, the target surface value S(0) need not be varied iteratively but can be determined algebraically using the measured area difference between the first approximate stress profile and the tension region stress profile, and the preferred functional form selected for the interpolated portion of the compressive stress region.

[0504] It will be apparent to those skilled in the art that various modifications may be made to the preferred embodiments of the present disclosure described herein without departing from the spirit or scope of the present disclosure as defined in the appended claims. Therefore, the present disclosure covers modifications and variations that fall within the scope of the appended claims and their equivalents.

Claims

1. A scattered light polarimetry (LSP) subsystem for a hybrid system for characterizing stress in a chemically strengthened (CS) substrate having top surface and near-surface waveguides, comprising: LSP light source system; an LSP light source actuator coupled to the LSP light source system, wherein the LSP light source actuator is operable to rotate the orientation of the LSP light source system; an optical compensator within an optical path of an LSP laser beam emitted by the LSP light source system, the optical compensator comprising: a half-wave plate and a half-wave plate actuator operable to rotate the half-wave plate, and a diffuser and a diffuser actuator operable to translate the diffuser along the optical path; an LSP detector system in optical communication with the optical compensator via an LSP coupling prism having an LSP coupling surface; a focus lens and a focus lens actuator operable to translate the focus lens along the optical path; and A support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at a measurement plane at the measurement aperture and to operably support the LSP coupling prism.

2. The LSP subsystem of claim 1 , further comprising an evanescent wave prism coupled spectroscopy (EPCS) subsystem, the EPCS subsystem comprising an EPCS light source system and an EPCS detector system in optical communication via an EPCS coupling prism having an EPCS coupling surface.

3. The LSP subsystem according to claim 1 or claim 2, wherein the half-wave plate actuator automatically rotates the half-wave plate so that the LSP laser beam reaches a predetermined beam intensity.

4. The LSP subsystem of any preceding claim, wherein the translatable diffuser actuator automatically adjusts the position of the diffuser.

5. A method of measuring first and second stress characteristics of a chemically strengthened (CS) substrate having a surface, a near-surface waveguide proximate the surface, and a backing surface opposite the surface, comprising: applying at least one ink line to the backing surface of the CS substrate; operatively positioning the surface of the CS substrate at a measurement position relative to a coupling prism assembly, the coupling prism assembly comprising an evanescent wave prism coupling spectroscopy (EPCS) coupling prism and a light scattering polarimetry (LSP) coupling prism to define adjacent EPCS and LSP coupling interfaces, respectively; performing an EPCS measurement of the CS substrate using the EPCS coupling interface to obtain the first stress characteristic and performing an LSP measurement of the CS substrate using the LSP coupling interface to obtain the second stress characteristic without removing the coupling prism assembly or the CS substrate from the measurement position; as well as The first stress characteristic and the second stress characteristic are combined to define a complete stress characterization of the CS substrate, wherein the first stress characteristic is selected from a first stress characteristic group comprising: surface compressive stress S(0), total layer depth DOLT, peak layer depth DOLsp, knee stress CS k and birefringence B, and the second stress characteristic comprises at least one of a depth of compression DOC and a central tension CT.

6. The method according to claim 5, wherein performing the LSP measurement comprises: forming the LSP image on a digital detector to define a raw digital LSP image; Performing Gaussian blur processing on the original digital LSP image to form a Gaussian blurred LSP image; performing Otsu threshold processing on the Gaussian blurred image to define a threshold image; as well as The optical delay and the depth into the CS substrate are calculated using the threshold image to obtain the second stress characteristic.

7. The method according to claim 5, wherein performing the LSP measurement comprises: forming the LSP image on a digital detector to define a raw digital LSP image; Performing Gaussian blur processing on the original digital LSP image to form a Gaussian blurred LSP image; performing a binarization method on the Gaussian blurred LSP image to define an image contour; as well as The optical delay and the depth into the CS substrate are calculated using the image profile to obtain the second stress characteristic.

8. The method according to claim 5, wherein performing the LSP measurement comprises: forming an LSP image on a digital detector; processing the LSP image to form an optical delay (OR) versus depth (D) curve comprising OR data points; as well as A combination of linear and quadratic functions was used to obtain a fitting curve for the OR data points.

9. The method according to claim 5, wherein performing the LSP measurement comprises: forming an LSP image on a digital detector; processing the LSP image to form an optical delay (OR) versus depth (D) curve of the delay profile, the curve including OR data points including first and second inflection points; and Do at least one of the following: i) obtaining a fitting curve of the OR data points using a power-peak function; and ii) shifting the OR data point so that the first bending point and the second bending point are symmetrical about a mid-plane of the CS substrate, thereby defining a shifted OR data point; as well as The shifted OR data points are used to obtain depth of compression DOC measurements of the CS substrate.

10. The method according to claim 5, further comprising: forming an LSP image on a digital detector; processing the LSP image to form an optical retardation (OR) versus depth (D) curve of the retardation profile, the curve including OR data points defining first and second inflection points and first and second end regions; as well as Do at least one of the following: i) performing first and second curve fits on the OR data points defining the first and second inflection points, respectively, to determine the central tension CT of the CS substrate; ii) performing curve fitting on the OR data points between the first inflection point and the second inflection point to determine a depth of compression DOC of the CS substrate; as well as iii) performing curve fitting on the OR data points within a span excluding the first end region and the second end region to define an OR fitting curve, and determining at least one of a central tension CT and a depth of compression DOC of the CS substrate using the OR fitting curve. 11 . The method of claim 10 , wherein the curve fitting comprises performing a polynomial optimization process that determines an optimal polynomial order by applying a cost function.

12. The method of claim 10, wherein the curve fitting is performed within a top window and a bottom window of the OR vs. D curve.

13. The method according to claim 12, wherein the top window and the bottom window are determined by: determining a first derivative curve of the OR and D curve; selecting a starting edge and an ending edge of the delay profile corresponding to a first minimum and a second minimum of the first-order derivative curve, respectively, wherein a first region includes a depth less than or equal to the starting edge and a second region includes a depth greater than or equal to the ending edge; determining the curvature of the OR vs. D curve; determining a centroid of a left side of the curvature defining the first inflection point and a centroid of a right side of the curvature defining the second inflection point; as well as The top window is defined by: the highest line defined by the maximum delay of the OR vs. D curve; a lower line defined by percentages of the maximum delay of the OR versus D curve; the starting edge; and said terminating edge; The bottom window is defined by: the lowest line defined by the minimum delay of the OR vs. D curve; an upper line defined by a percentage of the maximum delay of the OR versus D curve; the starting edge; and The terminating edge.

14. The method according to any one of claims 5 to 13, further comprising identifying an entry point and an exit point of an LSP laser beam on the CS substrate.

15. The method of claim 14, wherein the LSP image comprises a plurality of bright spots, and the identifying of the entry point and the exit point comprises: filtering a subset of the bright spots among the plurality of bright spots that are outside upper and lower 50% of the thickness of the CS substrate and outside ±10 degrees around the laser propagation angle; forming a plurality of bright spot combinations of the remaining bright spot sets among the plurality of bright spots; Scoring each of the plurality of bright spot combinations to determine its proximity to the thickness of the CS substrate, its proximity to the laser propagation angle, and a median light intensity between each bright spot in the bright spot combination; Normalizing the scores of the highlight combinations; For each bright spot combination, summing the normalized scores of the proximity to the thickness of the CS substrate, the proximity to the laser propagation angle, and the median light intensity between each bright spot in the bright spot combination; as well as The highlight combination with the highest total score among the highlight combinations is selected as the entry point and the exit point.

16. A method for processing a light scattering polarimetry (LSP) image, comprising: Forming LSP images of chemically strengthened (CS) substrates on a digital detector; processing the LSP image to form an optical retardation (OR) versus depth (D) curve of the retardation profile, the curve including OR data points defining first and second inflection points and first and second end regions; The top window and the bottom window of the OR vs. D curve are determined by the following operations: determining a first derivative curve of the OR and D curve; selecting a starting edge and an ending edge of the delay profile corresponding to a first minimum and a second minimum of the first-order derivative curve, respectively, wherein a first region includes a depth less than or equal to the starting edge and a second region includes a depth greater than or equal to the ending edge; determining the curvature of the OR vs. D curve; determining a centroid of a left side of the curvature defining the first inflection point and a centroid of a right side of the curvature defining the second inflection point; as well as The top window is defined by: the highest line defined by the maximum delay of the OR vs. D curve; a lower line defined by percentages of the maximum delay of the OR versus D curve; the starting edge; and said terminating edge; The bottom window is defined by: the lowest line defined by the minimum delay of the OR vs. D curve; an upper line defined by a percentage of the maximum delay of the OR versus D curve; the starting edge; and The terminating edge.

17. The method according to claim 16, further comprising: Do at least one of the following: i) performing first and second curve fits on the OR data points defining the first and second inflection points, respectively, to determine the central tension CT of the CS substrate; ii) performing curve fitting on the OR data points between the first inflection point and the second inflection point to determine a depth of compression DOC of the CS substrate; as well as iii) performing curve fitting on the OR data points within a span excluding the first end region and the second end region to define an OR fitting curve, and determining at least one of a central tension CT and a depth of compression DOC of the CS substrate using the OR fitting curve.

18. The method of claim 17, wherein the curve fitting comprises performing a polynomial optimization process that determines an optimal polynomial order by applying a cost function.

19. The method of any one of claims 16 to 18, further comprising identifying an entry point and an exit point of an LSP laser beam on the CS substrate.

20. The method of claim 19, wherein the LSP image comprises a plurality of bright spots, and the identifying of the entry point and the exit point comprises: filtering a subset of the bright spots among the plurality of bright spots that are outside upper and lower 50% of the thickness of the CS substrate and outside ±10 degrees around the laser propagation angle; forming a plurality of bright spot combinations of the remaining bright spot sets among the plurality of bright spots; Scoring each of the plurality of bright spot combinations to determine its proximity to the thickness of the CS substrate, its proximity to the laser propagation angle, and a median light intensity between each bright spot in the bright spot combination; Normalizing the scores of the highlight combinations; For each bright spot combination, summing the normalized scores of the proximity to the thickness of the CS substrate, the proximity to the laser propagation angle, and the median light intensity between each bright spot in the bright spot combination; as well as The highlight combination with the highest total score among the highlight combinations is selected as the entry point and the exit point.

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