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

Abstract

The 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 includes an LSP light source system, an LSP light source actuator coupled to the LSP light source system, and an optical compensator in 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 via 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 configured to support the CS substrate at a measurement plane at the measurement aperture and to operably support the LSP coupling prism.

Description

[Technical Field]

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

[0002] The present disclosure relates to characterizing stress in transparent, chemically strengthened substrates, and in particular to hybrid systems and methods for characterizing stress-chemically strengthened transparent substrates. [Background technology]

[0003] Transparent substrates that undergo chemical strengthening processes exhibit improved resistance to scratches and cracks. Such substrates are highly useful for a variety of display applications, from television screens to computer screens, and from mobile handheld device screens to wristwatches. An exemplary chemical strengthening process is the ion-exchange (IOX) process, in which ions in the near-surface region of a glass-based substrate are exchanged with external ions, for example, from a salt bath.

[0004] Producing transparent chemically strengthened (CS) substrates requires characterizing the stress properties of the CS substrate to ensure that the CS substrate has 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, along with related stress parameters such as surface compressive stress, knee stress, spike depth of layer, total depth of layer, compressive depth, and center tension. Other stress-related parameters include the variation of birefringence with depth in the CS substrate.

[0005] There are two main methods used to characterize stress in transparent CS substrates. The first utilizes evanescent prism coupling spectroscopy (EPCS). EPCS uses a coupling prism to couple light into guided modes supported by near-surface waveguides (NSWGs) formed in the substrate, for example, by the IOX process. The coupling prism is also used to couple light from the NSWGs to form a guided-mode spectrum. The guided-mode spectrum includes a transverse electric (TE) mode spectrum with transverse magnetic (TM) mode lines and a transverse magnetic (TM) mode spectrum with transverse TE mode lines. The TE and TM mode lines are analyzed to extract stress-related features, including the stress profile. While the EPCS method is particularly useful for characterizing stress in the near-surface region of CS substrates (e.g., surface compressive stress and the layer depth of spikes), it is not useful for characterizing the central tension (CT) and depth of compression (DOC) that reside deeper within the substrate.

[0006] The second main method utilizes light scattering polarimetry (LSP). In LSP, a CS substrate is illuminated with input laser light at a relatively shallow angle through a coupling prism. An optical compensator is used to continuously change the polarization of the laser light between different polarization states. The scattered light is detected by an image sensor. Stress in the CS substrate causes optical retardation along the optical path, and the amount of stress is proportional to the derivative of the optical retardation. The amount of optical retardation can be determined from the detected scattered light intensity distribution, which varies due to constructive and destructive interference for different effective path lengths of the detected light. The LSP method is useful for measuring certain stress-related properties, such as central tension (CT) and depth of compression (DOC), but not 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 transferred to an LSP measurement system, and the two measurements are spliced ​​together. This is time-consuming and introduces the risk of damage due to the handling of the CS substrate when transferring it between the two measurement systems.

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

[0009] The hybrid measurement system and method disclosed herein calculates the surface stress S(0) and the knee stress S k =S(x k This allows for a complete stress characterization of the transparent CS substrate, including the near-surface compressive stress profile S(x), including the depth of layer DOL, the central tension CT, and the compression depth DOC. The complete stress characterization is obtained by combining stress calculations using both EPCS and LSP measurements.

[0010] Embodiments of the present disclosure are directed to an 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 LSP subsystem including a scattered light polarimetry (LSP) light source system, an LSP light source actuator coupled to the LSP light source system, the LSP light source actuator operable to rotate the orientation of the LSP light source system, and an optical compensator in an optical path of an LSP laser beam emitted by the LSP light source system. The optical compensator includes 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 includes 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 back into the optical path, and a support plenum having a surface and a measurement aperture, the support plenum configured to support the CS substrate at the measurement plane at the measurement aperture and to operably support the LSP coupling prism.

[0011] Another embodiment of the present disclosure is directed to a method for measuring first and second stress characteristics of a chemically strengthened (CS) substrate having a surface, a near-surface waveguide proximate the surface, and a back surface opposite the surface, the method comprising: inscribing at least one inclined line on the back surface of the CS substrate; operatively disposing the surface of the CS substrate relative to a coupling prism assembly at a measurement location, the coupling prism assembly including an evanescent prism coupling prism and an LSP coupling prism for defining adjacent evanescent prism coupling spectroscopy (EPCS) and light scattering polarimetry (LSP) coupling interfaces, respectively; performing an EPCS measurement of the CS substrate using the EPCS bonding interface to obtain a first stress characteristic and performing an LSP measurement of the CS substrate using the LSP bonding interface to obtain a second stress characteristic, without removing either the rhythm assembly or the CS substrate from the measurement location; and combining the first stress characteristic and the second stress characteristic to define a complete stress characteristic of the CS substrate, wherein the first stress characteristic is selected from a group of first stress characteristics including surface compressive stress S(0), total depth of layer DOLT, spike depth of layer DOLsp, knee stress CSk, and birefringence B, and the second stress characteristic includes at least one of compression depth DOC and central tension CT.

[0012] Another embodiment of the present disclosure is directed to a method for processing a light scattering spectroscopy (LSP) image, the method including: forming an LSP image on a digital detector; processing the LSP image to form an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points defining first and second inflection points and first and second end regions; determining a first derivative curve of the OR vs. D curve; selecting a start edge and an end edge of the retardation profile corresponding to a first minimum and a second minimum of the first derivative curve, respectively, wherein the first region has a depth less than or equal to the start edge and the second region has a depth greater than or equal to the end edge; determining a curvature of the OR vs. D curve; and determining centers of mass of a left side of the curvature defining the first inflection point and a right side of the curvature defining the second inflection point, thereby determining upper and lower windows. The upper window is defined by a maximum line defined by the maximum retardation of the OR vs. D curve, a lower line defined by a percentage of the maximum retardation of the OR vs. D curve, a start edge, and an end edge. The lower window is defined by a minimum line defined by the minimum retardation of the OR vs. D curve, an upper line defined by a percentage of the maximum retardation of the OR vs. D curve, a start edge, and an end 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 a top surface and a near-surface waveguide includes: LSP light source system, an LSP light source actuator coupled to the LSP light source system, the LSP light source actuator operable to rotate an orientation of the LSP light source system; an optical compensator in an optical path of the 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; an optical compensator comprising 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 through 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; 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.

[0014] In a second embodiment, the first embodiment further includes an evanescent prism coupled spectroscopy (EPCS) subsystem, the EPCS subsystem including an EPCS light source system and an EPCS detector system in optical communication via an EPCS coupling prism having an EPCS coupling surface.

[0015] In a third embodiment, for the second embodiment, the EPCS subsystem further comprises a polarizer positioned in the optical path at a distance from the detector device of the EPCS detector system within the range of XX to XX, inclusive of the end points.

[0016] In the fourth embodiment, in any of the first to third embodiments, a half-wave plate actuator automatically rotates the half-wave plate so that the LSP laser beam has a predetermined beam intensity.

[0017] In a fifth embodiment, in any one of the first to fourth embodiments, the translatable diffuser actuator includes a stepping motor.

[0018] In a sixth embodiment, for any of the first to fifth embodiments, a translatable diffuser actuator automatically adjusts the position of the diffuser.

[0019] In the seventh embodiment, in any one of the first to sixth embodiments, the focusing lens actuator includes a stepping motor.

[0020] In the eighth embodiment, in any of the first to seventh embodiments, a condenser lens actuator automatically adjusts the position of the condenser lens.

[0021] In a ninth embodiment, a method for measuring first and second stress characteristics of a chemically strengthened (CS) substrate having a front surface, a near-surface waveguide proximate the front surface, and a back surface opposite the front surface, the method comprising: providing at least one ink line on the backside of the CS substrate; operatively disposing a surface of the CS substrate relative to a coupling prism assembly at the measurement location, the coupling prism assembly comprising an adjacent evanescent prism coupling spectroscopy (EPCS) coupling prism and a light scattering polarimetry (LSP) coupling prism for defining an EPCS coupling interface and an LSP coupling interface, respectively; performing an EPCS measurement of the CS substrate using the EPCS bonding interface to obtain a first stress characteristic and an LSP measurement of the CS substrate using the LSP bonding interface to obtain a second stress characteristic without removing either the bonding prism assembly or the CS substrate from the measurement location; combining the first stress characteristic and the second stress characteristic to define a complete stress characteristic of the CS substrate, wherein the first stress characteristic is selected from a group of first stress characteristics including surface compressive stress S(0), total depth of layer DOLT, spike depth of layer DOLsp, knee stress CSk, and birefringence B, and the second stress characteristic includes at least one of compression depth DOC and central tension CT.

[0022] In the tenth embodiment, for the ninth embodiment, performing LSP measurement is forming an LSP image on a digital detector to define a raw digital LSP image; performing a Gaussian blur of the raw digital LSP image to form a Gaussian blurred LSP image; performing Otsu thresholding on the Gaussian blurred image to define a threshold image; and using the threshold image to calculate the optical retardation versus depth into the CS substrate to obtain a second stress characteristic.

[0023] In an eleventh embodiment, for the ninth or tenth embodiment, performing LSP measurement is forming an LSP image on a digital detector to define a raw digital LSP image; performing a Gaussian blur of the raw digital LSP image to form a Gaussian blurred LSP image; performing a binarization method on the Gaussian blurred LSP image to define image contours; and using the image contour to calculate the optical retardation versus depth into the CS substrate to obtain a second stress characteristic.

[0024] In the twelfth embodiment, an LSP measurement is performed for any of the ninth to eleventh embodiments, forming an LSP image on a digital detector; processing the LSP image to form an optical retardation (OR) vs. depth (D) curve containing OR data points; and employing a combination of linear and quadratic functions to obtain a curve fit to the OR data points.

[0025] In the thirteenth embodiment, for any of the ninth to twelfth embodiments, LSP measurement is performed, forming an LSP image on a digital detector; processing the LSP image to form an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points including a first inflection point and a second inflection point; i) employing a power-spike function to obtain a fitted curve for the OR data points; and ii) shifting the OR data points such that the first inflection point and the second inflection point are symmetric about the mid-plane of the CS substrate, thereby defining shifted OR data points; and employing the shifted OR data points to obtain a measurement of the compression depth DOC of the CS substrate.

[0026] In the fourteenth embodiment, for any of the ninth to thirteenth embodiments, forming an LSP image on a digital detector; processing the LSP image to generate an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points defining first and second inflection points and first and second end regions; i) first and second curve fits of the OR data points defining a first inflection point and a second inflection point, respectively, to determine a central tension, CT, of the CS substrate; ii) curve fitting the OR data points between the first and second inflection points to determine the compression depth DOC of the CS substrate; and iii) performing a curve fit to the OR data points over the span excluding the first end region and the second end region to define an OR fit curve, and using the OR fit curve to determine at least one of a central tension CT and a compression depth DOC of the CS substrate.

[0027] In a fifteenth embodiment, for the fourteenth embodiment, the curve fitting includes performing a polynomial optimization process that determines the optimal polynomial order by applying a cost function.

[0028] In a sixteenth embodiment, as for the fourteenth embodiment, curve fitting is performed within the upper and lower windows of the OR vs. D curve.

[0029] In the seventeenth embodiment, the upper and lower windows of the sixteenth embodiment are: determining the first derivative curve of the OR vs. D curve; selecting a start edge and an end edge of the retardation profile corresponding to a first minimum and a second minimum of the first derivative curve, respectively, wherein the first region has a depth equal to or less than the start edge and the second region has a depth equal to or greater than the end edge; Determining the curvature of the OR vs. D curve; determining a center of mass of a left side of the curvature defining a first inflection point and a right side of the curvature defining a second inflection point; The upper window is a maximum line defined by the maximum retardation of the OR vs. D curve; a lower line defined by a certain percentage of maximum retardation of the OR vs. D curve; A starting edge and and the end edge, defined by The bottom window is a minimum line defined by the minimum retardation of the OR vs. D curve; an upper line defined by a certain percentage of maximum retardation of the OR vs. D curve; A starting edge and The end edge is defined by

[0030] In an eighteenth embodiment, the seventeenth embodiment further includes applying a smoothing filter to the OR vs. D curve.

[0031] In the 19th embodiment, the smoothing filter of the 18th embodiment is a LOESS filter.

[0032] In a twentieth embodiment, for any of the fourteenth to nineteenth embodiments, the method further includes identifying an entry point and an exit point of the LSP laser beam on the CS substrate.

[0033] In a 21st embodiment, for the 20th embodiment, the LSP image comprises a plurality of bright points, and the identification of the entry points and exit points is performed by: filtering a subset of the plurality of bright spots that are outside of 50% above and below the thickness of the CS substrate and ±10 degrees around the angle of laser propagation; creating a plurality of light point combinations of the remaining set of light points of the plurality of light points; Scoring each bright point combination of the plurality of bright point combinations for proximity to the thickness of the CS substrate, proximity to the angle of laser propagation, and median light intensity between each bright point in the bright point combination; normalizing the scores of the bright spot combinations; For each bright spot combination, summing the normalized scores for proximity to the CS substrate thickness, proximity to the angle of laser propagation, and median light intensity between each bright spot of the bright spot combination; selecting the selected light point combinations having the highest total scores among the light point combinations as the entry and exit points.

[0034] In a twenty-second embodiment, for any of the fourteenth to twenty-first embodiments, the ink lines are marked with a black permanent marker.

[0035] In a twenty-third embodiment, a method for processing a light scattering polarimetry (LSP) image comprises: forming an LSP image on a digital detector; processing the LSP image to generate an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points defining first and second inflection points and first and second end regions; The upper and lower windows of the OR vs. D curve are determining the first derivative curve of the OR vs. D curve; selecting a start edge and an end edge of the retardation profile corresponding to a first minimum and a second minimum of the first derivative curve, respectively, wherein the first region has a depth equal to or less than the start edge and the second region has a depth equal to or greater than the end edge; Determining the curvature of the OR vs. D curve; determining a center of mass of a left side of the curvature defining a first inflection point and a right side of the curvature defining a second inflection point; The upper window is a maximum line defined by the maximum retardation of the OR vs. D curve; a lower line defined by a certain percentage of maximum retardation of the OR vs. D curve; A starting edge and and the end edge, defined by The bottom window is a minimum line defined by the minimum retardation of the OR vs. D curve; an upper line defined by a certain percentage of maximum retardation of the OR vs. D curve; A starting edge and The end edge is defined by

[0036] In the 24th embodiment, the 23rd embodiment is i) first and second curve fits of the OR data points defining a first inflection point and a second inflection point, respectively, to determine a central tension, CT, of the CS substrate; ii) curve fitting the OR data points between the first and second inflection points to determine the compression depth DOC of the CS substrate; and iii) performing a curve fit to the OR data points over the span excluding the first end region and the second end region to define an OR fit curve, and using the OR fit curve to determine at least one of a central tension CT and a compression depth DOC of the CS substrate.

[0037] In a twenty-fifth embodiment, for the twenty-fourth embodiment, the curve fitting includes performing a polynomial optimization process that determines the optimal polynomial order by applying a cost function.

[0038] In a twenty-sixth embodiment, for the twenty-third or twenty-fourth embodiment, further comprising applying a smoothing filter to the OR vs. D curve.

[0039] In the 27th embodiment, the smoothing filter of the 26th embodiment is a LOESS filter.

[0040] In a twenty-eighth embodiment, for any of the twenty-third to twenty-seventh embodiments, further comprising identifying an entry point and an exit point of the LSP laser beam on the CS substrate.

[0041] In a 29th embodiment, for the 28th embodiment, the LSP image comprises a plurality of bright points, and the identification of the entry point and the exit point is performed by: filtering a subset of the plurality of bright spots that are outside of 50% above and below the thickness of the CS substrate and ±10 degrees around the angle of laser propagation; creating a plurality of light point combinations of the remaining set of light points of the plurality of light points; Scoring each bright point combination of the plurality of bright point combinations for proximity to the thickness of the CS substrate, proximity to the angle of laser propagation, and median light intensity between each bright point in the bright point combination; normalizing the scores of the bright spot combinations; For each bright spot combination, summing the normalized scores for proximity to the CS substrate thickness, proximity to the angle of laser propagation, and median light intensity between each bright spot of the bright spot combination; selecting the selected light point combinations having the highest total scores among the light point combinations as the entry and exit points.

[0042] The described embodiments may be combined in any suitable combination. Additional features and advantages will be set forth in the following detailed description, and in part will be apparent to those skilled in the art from this description, or may be learned by practicing the embodiments as set forth in the written description and claims herein, as well as the accompanying drawings. It is to be understood that both the foregoing general description and the following detailed description are exemplary only and are intended to provide an overview or framework for understanding the nature and characteristics of the claims.

[0043] The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s) and, together with the detailed description, explain the principles and operation of various embodiments. Thus, the present disclosure will be more fully understood when the following detailed description is taken in conjunction with the accompanying figures. [Brief explanation of the drawings]

[0044] [Figure 1A] FIG. 1 is an overhead view of an exemplary transparent CS substrate in the form of a planar sheet. [Figure 1B] FIG. 10 is an exemplary refractive index profile, n(z) versus z, of an exemplary transparent CS substrate showing the near-surface spike region (R1), the deeper region (R2), and the bulk region (R3), with a knee (KN) at the transition between regions R1 and R2. [Figure 2A] FIG. 1 is a schematic diagram of the hybrid EPCS-LSP measurement system disclosed herein for fully characterizing stress in transparent CS substrates. [Figure 2B] 2B is a more detailed schematic diagram of the hybrid EPCS-LSP system of FIG. 2A showing an exemplary configuration for the EPCS measurement subsystem and the LSP measurement subsystem. [Figure 3A] FIG. 2B is a schematic diagram of an example EPCS subsystem of the hybrid EPCS-LSP system of FIG. 2A. [Figure 3B] FIG. 1 is a schematic diagram of an exemplary EPCS mode spectrum acquired by the EPCS subsystem, which includes a TM mode spectrum with TM mode lines (fringes) and a TE mode spectrum with TE mode lines (fringes). [Figure 3C] EPCS image with the polarizer positioned relatively far from the digital detector. [Figure 3D] EPCS image with the polarizer positioned closer to the digital detector than the polarizer arrangement in FIG. 3C. [Figure 4A] FIG. 2B is a schematic diagram of an example LSP subsystem of the hybrid EPCS-LSP system of FIG. 2A. [Figure 4B] FIG. 2B is a schematic diagram of an example LSP subsystem of the hybrid EPCS-LSP system of FIG. 2A. [Figure 4C] FIG. 2B is a schematic diagram of an example LSP subsystem of the hybrid EPCS-LSP system of FIG. 2A. [Figure 4D] FIG. 1 is a close-up view of an LSP image formed on a digital detector of the LSP subsystem, the LSP image including two line images forming a cross or "X" pattern, the LSP image and the digital detector forming a digital LSP image. [Figure 4E] FIG. 1 is a schematic diagram of an exemplary LSP light source test fixture. [Figure 4F] FIG. 1 is a schematic diagram of an exemplary optical model for generating optimal light output at an optimal wavelength from an LSP light source. [Figure 4G] 10 is a plot showing blocked light from a filter due to laser wavelength mismatch. [Figure 4H] FIG. 1 is a schematic diagram of an example LSP subsystem with tunable components. [Figure 4I] FIG. 1 is a schematic diagram of an example LSP subsystem with tunable components. [Figure 4J] FIG. 1 is a schematic diagram of an example LSP subsystem with tunable components. [Figure 4K]1 is an exemplary LSP image showing entry and exit points. [Figure 4L] FIG. 4K is the example LSP image of FIG. 4K showing indications of algorithmically determined entry and exit points. [Figure 4M] FIG. 4C is a simplified diagram of the LSP subsystem of FIG. 4B. [Figure 4N] FIG. 1 illustrates an LSP light beam appearing thick to an LSP detector system. [Figure 4O] FIG. 1 illustrates an LSP light beam that appears faint in an LSP detector system. [Figure 4P] 1A-1C illustrate various LSP source and detector system orientations. [Figure 4Q] 1A-1C illustrate various LSP source and detector system orientations. [Figure 4R] 1A-1C illustrate various LSP source and detector system orientations. [Figure 4S] 1A-1C illustrate various LSP source and detector system orientations. [Figure 5A] FIG. 1 is an overhead view of an exemplary prism support structure for supporting an EPCS coupling prism and an LSP coupling prism. [Figure 5B] FIG. 5B is an overhead view of a cover plate attached to the prism support structure of FIG. 5A. [Figure 6A] FIG. 1 is a side view of an EPCS coupling prism and an LSP coupling prism supported on a stable platform, illustrating an exemplary method of forming a unitary molded prism support structure for the coupling prism assembly. [Figure 6B] FIG. 1 is a side view of an EPCS coupling prism and an LSP coupling prism supported on a stable platform, illustrating an exemplary method of forming a unitary molded prism support structure for the coupling prism assembly. [Figure 6C]FIG. 1 is a side view of an exemplary coupling prism assembly, in which the prism support structure is configured such that at least one of the EPCS coupling prism and the LSP coupling prism is movable in one direction (e.g., the z-direction as shown) relative to the other. [Figure 6D] 1 is a schematic diagram of an exemplary hybrid EPCS-LSP measurement system in which a single coupling prism is used for the EPCS and LSP subsystems instead of two separate coupling prisms. [Figure 7] FIG. 10 is a cross-sectional view of an exemplary prism support structure mounted in an exemplary support plenum of a hybrid system, showing an exemplary movable substrate holder used to adjust the measurement position on the CS substrate. [Figure 8A] FIG. 1C is an overhead view of the support plenum showing the measurement aperture and pressure- and vacuum-resistant (PV) bars of a vacuum system operatively disposed within the measurement aperture for pneumatically engaging the CS substrate to pull it onto the coupling surfaces of the EPCS and LSP coupling prisms. [Figure 8B] FIG. 10 is an enlarged cross-sectional view of the support plenum and measurement aperture showing an exemplary configuration of the coupling prism assembly and vacuum system. [Figure 8C] This is an LSP image without a backing material. [Figure 8D] This is an LSP image using a black permanent marker line as the backing material. [Figure 8E] Another LSP image without the backing material. [Figure 8F] LSP images using standard oil as the backing material and an immersion lens. [Figure 8G] Average LSP images using standard oil as the backing material and an immersion lens. [Figure 8H] Another LSP image using black permanent marker lines as the backing material. [Figure 8I] Average LSP image using black permanent marker lines as backing material. [Figure 8J]LSP images using polarized filer and immersion oil as backing materials. [Figure 8K] LSP images using black plastic capsules and immersion oil as backing materials. [Figure 8L] This is an LSP image using dry erase marker lines as the backing material. [Figure 8M] This is an LSP image using a red permanent marker line as the backing material. [Figure 8N] This is an LSP image using a black permanent marker line as the backing material. [Figure 9] FIG. 10 is a schematic diagram of an exemplary user interface presented by the system controller, the user interface including an EPCS section showing EPCS mode spectra and an LSP section showing LSP line images of the digital LSP image. [Figure 10A] 1 is an example of an LSP section of a user interface showing an example digital LSP image and an intensity histogram of the digital LSP image. [Figure 10B] An exemplary initial or raw digital LSP image is shown along with a Gaussian-blurred ("blurred") LSP image. [Figure 10C] 10C shows an exemplary threshold image obtained by applying Otsu thresholding to the Gaussian blurred image of FIG. 10B. [Figure 10D] 10 shows an example of performing contour detection on an exemplary Gaussian blurred LSP image. [Figure 10E] 10 shows an example of performing contour detection on an exemplary Gaussian blurred LSP image. [Figure 11A] FIG. 10 is a close-up view of the CS substrate and the direction of the focused LSP light beam. [Figure 11B] 1 is a close-up view of an edge portion of a CS substrate showing the field of view of the LSP detector system relative to the focused LSP light beam. [Figure 11C] 11B, showing the scattered light beam reaching the LSP detector system and forming a line image. [Figure 11D] 1 shows another view of the LSP detector system and CS substrate with a focused LSP light beam. [Figure 11E] FIG. 1 is a schematic diagram showing the dimensions and angles used to determine the thickness of a CS substrate. [Figure 12A] 1 is a plot of the average computation time T in milliseconds (ms) required to extract the phase φ of a noisy LSP signal versus the noise figure N for both the lock-in method (lock or L) and the sine method (sine or S). [Figure 12B] 1 is a plot of absolute phase difference |Δφ| versus noise figure for the lock-in method (lock or L) and the sine method (sine or S) for processing a noisy LSP signal. [Figure 13A] 1 is a plot of optical retardation OR (radians) versus depth D (mm) into the CS substrate ("OR vs. D plot") showing OR data collected by the LSP subsystem without speckle reduction. [Figure 13B] 1 is a plot of optical retardation OR (radians) versus depth D (mm) into the CS substrate ("OR vs. D plot") showing OR data collected by the LSP subsystem using speckle reduction. [Figure 14A] 10 is an OR vs. D plot illustrating an exemplary method for shifting OR data to make inflection points BP1 and BP2 symmetric about the mid-plane of a CS substrate. [Figure 14B] 10 is an OR vs. D plot illustrating an exemplary method for shifting OR data to make inflection points BP1 and BP2 symmetric about the mid-plane of a CS substrate. [Figure 15A] 1 is an exemplary OR vs. D plot including discrete data points (circles) and a fitted line to the OR vs. D data points, the fitted line being generated using the "LinQuad" method disclosed herein. [Figure 15B] FIG. 15B is a plot of stress S (MPa) versus depth D (mm) based on a LinQuad fit to the OR versus D data points of FIG. 15A. [Figure 16A]1 is an exemplary OR vs. D plot including discrete data points (circles) and a fitted line to the OR vs. D data points, where the fitted line was generated using the power-spike method disclosed herein. [Figure 16B] 16B is a plot of stress S (MPa) versus depth D (mm) ("S vs. D plot") based on a power-spike fit to the OR vs. D data points of FIG. 16A. [Figure 17A] 1 is an OR vs. D plot showing the LinQuad curve fit to the original (raw) OR vs. D data points. [Figure 17B] 10 is an OR vs. D plot showing a LinQuad curve fit to OR vs. D data with the symmetric component removed. [Figure 18A] 10 is an OR vs. D plot illustrating the use of reduced area fit regions when calculating selected stress parameters, showing reduced area fit regions at inflection points BP1 and BP2 for calculating compression depth DOC. [Figure 18B] 10 is an OR vs. D plot illustrating the use of a reduced-area fit region when calculating selected stress parameters, showing the reduced-area fit region between inflection point BP1 and inflection point BP2 for calculating central tension force CT. [Figure 19A] OR vs. D plot, curve fitting is performed on the entire set of OR data. [Figure 19B] Corresponding S vs. D plot, curve fitting is performed on the entire set of OR data. [Figure 19C] OR vs. D plot, where the curve fit is performed on a reduced OR data set that excludes a portion of the data near either endpoint. [Figure 19D] Corresponding S vs. D plot, where the curve fit is performed on a reduced OR data set that excludes a portion of the data near either end point. [Figure 19E] R vs. D plot after a 20% smoothing filter on the OR data. [Figure 19F] 19E, a plot of the derivative of the smoothed R data, and FIG. 19F, a plot of the R vs. D plot. [Figure 19G] 19E, a plot of the curvature of the smoothed R data, and FIG. 19E, a plot of the R vs. D plot. [Figure 19H] R vs. D plot with upper and lower windows. [Figure 19I] 19H is a cost versus degree plot of the polynomial fit using the upper and lower windows of FIG. 19H. [Figure 19J] 10 is an R vs. D plot with a polynomial fit for the two inflection point regions. [Figure 20] 3B is similar to FIG. 3A and illustrates an embodiment of an EPCS subsystem in which the detector system includes an adjustable focusing lens, and the adjustability includes at least one of axial movement and changing the focal length. [Figure 21A] FIG. 1 is a schematic illustration of an exemplary support member used to form a focusing lens assembly for an EPCS subsystem to provide a means for adjusting the contrast of the captured mode spectrum. [Figure 21B] FIG. 1 is a schematic illustration of an exemplary support member used to form a focusing lens assembly for an EPCS subsystem to provide a means for adjusting the contrast of the captured mode spectrum. DETAILED DESCRIPTION OF THE INVENTION

[0045] 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 reference numbers and symbols will be used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and those skilled in the art will recognize where the drawings have been simplified to illustrate key aspects of the disclosure.

[0046] The following claims are incorporated into and constitute a part of this Detailed Description.

[0047] Cartesian coordinates are shown in some of the figures for reference purposes and are not intended to be limiting with respect to direction or orientation.

[0048] In some parts of the discussion, the z coordinate is used for depth into the substrate, while in other parts of the discussion, a different coordinate is used.

[0049] The acronym "IOX" stands for "ion exchange" or "ion-exchanged," depending on the context of the discussion.

[0050] The acronym "CS," when used to describe a type of substrate (as in "CS substrate"), means "chemically strengthened." The acronym CS can also mean "compressive stress," and the context of the discussion will make clear which meaning is used for this acronym.

[0051] The term "strengthened" with respect to the CS substrate contemplated herein means that the original CS substrate has undergone processing that creates several stress profiles, which can have a variety of shapes, typically intended to make the CS substrate stronger and therefore less susceptible to fracture. Exemplary strengthening processes include ion exchange, tempering, annealing, and similar thermal processes.

[0052] The term “transparent” as used with respect to a CS substrate means that the CS substrate is transparent to light at a given measurement wavelength (i.e., EPCS wavelength λ ) in order to make a sufficient measurement (i.e., EPCS measurement or LSP measurement) of the CS substrate that will result in a sufficiently accurate measurement of the stress properties associated with the given measurement. A or LSP wavelength λ B ) means a CS substrate having sufficient light transmittance.

[0053] The abbreviation "ms" stands for "milliseconds."

[0054] The abbreviation "nm" stands for "nanometer."

[0055] The term "near-surface," such as when referring to a near-surface waveguide or near-surface spike region of a CS substrate, refers to the portion of the substrate body that lies directly adjacent to a given surface (e.g., top surface or measurement surface) of the CS substrate.

[0056] In one example, a glass-based substrate is used to form the CS substrate. As used herein, the term "glass-based substrate" includes any object made entirely or partially of glass, such as a stack of glass and non-glass materials, a stack of glass and crystalline materials, and glass-ceramics (including amorphous and crystalline phases). Thus, in one example, a "glass-based CS substrate" may be entirely made of glass material, while in another example, it may be entirely made of glass-ceramic material.

[0057] The terms "image" and "line image" are used herein to describe the light distribution (i.e., intensity distribution) of a portion of an X-shaped LSP image formed by scattered light at a digital detector (such as a CCD camera or CMOS sensor) by the LSP subsystem; an imaging system is not required to form the LSP images considered herein.

[0058] In the following discussion, the LSP subsystem is configured to cycle between two or more polarization states (or "polarizations" for short). In one example, there may be up to eight different polarization states per cycle, combining linear, elliptical, and circular polarizations, as known in the art. Many more polarizations may also be used, e.g., up to 100 or more.

[0059] The term "stress" as used herein can generally refer to compressive or tensile stress. In the plots of Figures 15B, 16B, 19B, and 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. A positive value of compressive stress is understood to mean the magnitude of compressive stress. Stress is denoted by S or σ and is interpreted to refer to compressive stress unless otherwise specified or understood by the context of the discussion. In some instances, compressive stress is also referred to as knee stress CS. k The stress profile is denoted as CS, as for (stress S). The depth coordinate can be any local coordinate; in the following discussion, both z and x are used as local coordinates.

[0060] In one example, "characterization" of the CS substrate includes determining one or more stress-based properties of the CS substrate, such as the stress profile S(z), depth of layer DOL, surface stress S(O), compression depth DOC, central tension CT, and birefringence profile B(z). In one example, the characterization utilizes both EPCS and LSP measurements, which provide first and second stress characteristics, respectively, that when combined provide a "complete characterization" of the stress properties of the CS substrate, the term "complete characterization" referring to a more complete characterization of stress and stress-related properties than either the first stress characteristic from the EPCS measurement alone or the second stress characteristic from the LSP measurement alone.

[0061] The acronym "OR" stands for "optical retardation" and is measured in radians ("rad") unless otherwise specified. A plot of optical retardation versus depth into the CS substrate is hereinafter referred to as an "OR vs. D" curve or plot, where D is understood to be the depth from the top (measurement) surface into the CS substrate body.

[0062] The term "index-matching fluid" refers to a fluid that has substantially the same refractive index as another material to facilitate optical coupling. In one example, the index-matching fluid includes an oil or a mixture of oils. The refractive index of the index-matching fluid is n f or n oil These two formulas are used interchangeably below.

[0063] CS base material 1A is an overhead view of an exemplary type of CS substrate 10 in the form of a planar sheet. CS substrate 10 has a body 11, an upper surface 12, a lower surface 14, and side surfaces 16. CS substrate 10 has a thickness TH and a mid-plane MP that is midway between and parallel to upper surface 12 and lower surface 14.

[0064] In some cases, the thickness TH may be 0.020 mm≦TH≦2 mm, and any and all subranges formed between these endpoints, 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.

[0065] An exemplary type of CS substrate 10 is glass-based and is used as a protective cover for the display and / or housing for mobile devices such as smartphones, tablets, laptop computers, GPS devices, etc. Such CS substrates 10 tend to be thin and planar, as shown in FIG. 1A.

[0066] The CS substrate 10 includes a near-surface waveguide (NSWG) 18 present in the body 11 proximate the top surface 12. In one example, the NSWG 18 is formed using an IOX process and is defined by at least one IOX region of varying refractive index.

[0067] FIG. 1B is a plot of refractive index, n, versus depth, z, in a CS substrate for an exemplary NSWG18. The surface refractive index is n SWhile the bulk refractive index, i.e., the refractive index of the substrate material unaffected by the chemical strengthening process, is expressed as n B It is written as follows.

[0068] The plot of FIG. 1B shows an exemplary refractive index profile n(z) that defines two (IOX) regions: a first near-surface spike region R1 and a second deep region R2. There is also a third region R3 that is deeper than the second deep region and is referred to herein as having a refractive index n B The near-surface spike region R1 is referred to as the "bulk" region, which has the maximum refractive index n s and the first "spike" layer depth DOL sp Define the refractive index value n as a function of depth (z) over a relatively shallow depth z=D1. k The deep region R2 has a rapid decrease to n k The total layer depth DOL from which the third bulk region R3 begins T The first region R1 and the second region R2 have a refractive index n=n k and the knee (compression) stress CS k z=z k intersects (and therefore defines) KN at

[0069] Because of the two distinct refractive index regions R1 and R2 of NSWG 18, certain guided modes propagate only in the top spike 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. Other refractive index profiles n(z) include more uniform variations in refractive index. Some of the deep guided modes can extend into the bulk region R3.

[0070] The refractive index profile n(z) of Figure 1B can be formed by a dual IOX (DIOX) process in which one IOX process forms deep region R2 and another IOX process different from the first IOX process forms spike region R1. The plot of Figure 1B represents a DIOX process performed on a Li-containing glass substrate 10 in which Li ions are exchanged with potassium and sodium ions in two distinct IOX processes, with the potassium IOX process producing spike region R1.

[0071] Hybrid EPCS-LSP system 2A is a schematic diagram of a hybrid EPCS-LSP measurement system ("hybrid system") 20 disclosed herein shown with an exemplary CS substrate 10. 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. Coupling prism assembly 40 defines a measurement location ML on CS substrate 10.

[0072] The EPCS subsystem 100 generates an EPCS measurement signal SA representing a first stress characteristic of the CS substrate at the measurement location ML, embodied in the mode spectrum of the guided modes of the NSWG 18. The first stress characteristic includes: a surface compressive stress S(0), a total layer depth DOL, and a surface compressive stress S(0). T , spike layer depth DOL sp , knee stress CS k , and birefringence B.

[0073] The LSP subsystem 100 generates an LSP measurement signal SB representing a second stress characteristic of the CS substrate at the measurement location ML, embodied in optical retardation (OR) information as a function of depth into the CS substrate including the deep region R2. The second stress characteristic can include one or more of the following: a stress profile, a compression depth DOC, and a central tension CT.

[0074] In one example, the EPCS and LSP measurements of the first and second stress characteristics are made without moving the measurement location ML. In another example, the EPCS and LSP measurements of the first and second stress characteristics are made by translating the coupling prism assembly 40 so that the EPCS and LSP measurements are made at the same location on the substrate rather than at slightly spaced apart sub-locations at the measurement location defined by the configuration of the coupling prism assembly.

[0075] In one example, the EPCS and LSP measurements of the first and second stress properties are made without removing either the coupling prism assembly 40 or the CS substrate 10 from the measurement location ML. This represents an advantage over the prior art in that both the EPCS and LSP measurements may be made in a single system without having to remove or otherwise handle the CS substrate to transport it to a different measurement system.

[0076] The EPCS measurement signals SA and the LSP measurement signals SB are transmitted 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 one example, the system controller 400 is configured with instructions (e.g., software) embodied in a non-transitory computer-readable medium to control the operation of the hybrid system 20 and to perform calculations to determine first and second stress characteristics of the CS substrate 10 based on the EPCS measurement signals SA and the LSP measurement signals SB.

[0077] In one example, system controller 400 processes EPCS measurement signals SA and LSP measurement signals SB to define a stress profile and associated stress characteristics from top surface 12 to at least the bottom of deep region R2 of CS substrate 10. In other words, the system controller combines the first and second stress characteristics obtained from EPCS subsystem 100 and LSP subsystem 200 to generate a more complete or “complete” stress profile of the CS substrate than would be possible from either one of the measurement subsystems alone.

[0078] 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 adjacent to 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 may be separate coupling prisms or different sections of a single (common) coupling prism.

[0079] 2A, hybrid system 20 includes an exemplary housing 21 having dimensions L1 and L2. Exemplary dimensions of L1 and L2 are in the range of 8 inches to 12 inches for a relatively compact embodiment of hybrid system 20.

[0080] EPCS subsystem 100 includes EPCS light source system 110 and EPCS detector system 140 optically coupled via EPCS coupling prism 42A. LSP subsystem 200 includes LSP light source system 210, optical compensator 230, and LSP detector system 240 optically coupled to the optical compensator via LSP coupling prism 42B. Detector systems 140 and 240 are operably connected to system controller 400. Examples of EPCS subsystem 100 are described in U.S. Pat. Nos. 9,534,981 and 9,696,207, which are incorporated herein by reference. Examples of LSP subsystem 100 are described in U.S. Pat. No. 4,655,589 and U.S. Provisional Patent Application No. 62 / 753,388, which are incorporated herein by reference.

[0081] Figure 2B is a more detailed schematic diagram of the hybrid EPCS-LSP system of Figure 2A, showing exemplary configurations for EPCS measurement subsystem 100 and LSP measurement subsystem 200. Figure 3A is a schematic diagram of an exemplary EPCS subsystem 100. Figures 4A-4C are schematic diagrams of an exemplary LSP subsystem 200.

[0082] EPCS Subsystem 2B and 3A, the EPCS light source system 110 of the EPCS subsystem 100 emits light at a first wavelength λ along a first axis A1. A The EPCS light source 112 generates an EPCS light beam 116 of a first wavelength λ A can also be referred to as the EPCS wavelength.

[0083] EPCS light source system 110 also includes, along first optical axis A1, an optional polarizer 118, an optical diffuser 122 downstream from EPCS light source 112, and a focusing lens 120 downstream from the optical diffuser. In one example, the light source includes a light emitting diode (LED), and in one example, the LED emits light at an EPCS measurement wavelength λ of 365 nm. AThe EPCS detector system 140 lies along a second axis A2 and includes, in order along the second axis, a focusing lens 142, a focusing lens 144, a focusing lens 146, a focusing lens 148, a focusing lens 149 ... A , a bandpass filter 144 centered on the TM section, an attenuator 146, a TM-TE polarizer 148 (having a TM section and a TE section, not shown), and a digital detector (e.g., a digital camera, an image sensor, a CCD array, etc.) 150 having a TM section and a TE section (not shown) defined by the TM-TE polarizer 148.

[0084] An EPCS light beam 116 from the EPCS light source 112 is diffused by an optical diffuser 120 and focused by a focusing lens 120 to form a focused EPCS light beam 116F. The focused EPCS light beam 116F enters the EPCS coupling prism 42A at its input face 43A, which couples the EPCS focused light beam into the NSWG 18 at a first (EPCS) coupling interface INT1 defined by the top surface 12 of the CS substrate and the bottom or "coupling" face 45A of the EPCS coupling prism 42A. The first coupling interface INT1 may include an index-matching fluid 5A, as discussed in more detail below.

[0085] A reflected EPCS light beam 116R is formed from the focused EPCS light beam 116F at the first EPCS coupling interface INT1 and exits the output face 44A of the EPCS coupling prism 42A, traveling along a second axis A2. The first axis A1 and the second axis A2 lie in a common plane (e.g., the xz plane in FIG. 3A ). The reflected EPCS light beam 116R contains information about the modal spectrum of the guided modes of the NSWG 18. The reflected EPCS light 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 on the EPCS digital detector 150.

[0086] The bandpass filter 144 ensures that only the reflected EPSC light beam 116R passes through it to the digital detector 150. The attenuator 146 ensures that the detected reflected EPSC light beam 116R has the appropriate intensity distribution for efficient digital detection. The TM-TE polarizer 148 defines the TM and TE sections of the digital detector so that the TM and TE mode spectra may be captured by the digital detector 150. The TM and TE mode spectra are embodied in the first detector signal SA, which is sent to the system controller 400 for processing. Note that the order of the bandpass filter 144, attenuator 146, and focusing lens 142 is not important and is intentionally shown as different between Figures 2B and 3A to illustrate this point.

[0087] FIG. 3B is a schematic representation of an idealized mode spectrum 160 captured by digital detector 150. Local (x, y) Cartesian coordinates are shown for reference. Mode spectrum 160 has a TM total internal reflection (TIR) ​​section 161™ and a TE total internal reflection (TIR) ​​section 161TE associated with TM guided and TE guided modes, respectively, and a non-TIR section 162™ and a non-TIR section 162TE associated with TM radiative and leaky modes, respectively. TIR section 161™ includes one or more TM mode lines or TM “fringes” 163™, while TIR section 161TE includes one or more TE mode lines or TE “fringes” 163TE. TM mode lines 163™ and TE mode lines 163TE are generally aligned in the x-direction and spaced apart in the y-direction.

[0088] The transition regions ("transitions") 166TM and 166TE between the TIR sections 161TM, 161TE and the non-TIR sections 162TM, 162TE define the critical angles for light coupling into and out of the NSWG 18 of the CS substrate 10 for TM-polarized and TE-polarized light, and are referred to as the critical angle transitions. The difference in the location of the onset of the critical angle transitions 166TM and 166TE determines the knee stress CS k This is proportional to the "approximately Cs" in Figure 3B. k " is indicated.

[0089] The TM mode lines 163TM and the TE mode lines 163TE can be either bright or dark lines, depending on the configuration of the EPCS subsystem 100. In Figure 3B, the TM mode lines 163TM and the TE mode lines 163TE are shown as dark lines for ease of illustration.

[0090] The stress characteristics for EPCS measurements 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 of TM and TE polarizations, and the effective refractive index is represented by the y-position of the mode line. The surface compressive stress S(0) = CS is calculated by the y-distance between the mode line (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 is a measure of the stress penetration or ion penetration length into the body 11 of the CS substrate 10, and in the case of the IOX process, can also be calculated from the y location and the number of TM and TE mode lines 163. Therefore, the location of the TM and TE mode lines along the y axis is the most fundamental measurement for inferring the stress-related properties of the CS substrate 10. Calculations for determining the stress properties of the CS substrate 10 based on EPCS measurements using the EPCS subsystem 100 are performed in the system controller 400.

[0091] In some cases, defects in the form of horizontal bands across the image are present as an image of the split polarizer lines of the TE-TM polarizer 148 positioned in the path of the reflected EPCS light 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 fringe detection. FIG. 3C illustrates horizontal bands with widths indicated by the arrows that result from positioning the TE-TM polarizer 148 at a certain distance from the digital detector 150. FIG. 3D illustrates horizontal bands with widths indicated by the arrows that result from positioning the TE-TM polarizer 148 at a closer distance from the digital detector 150. Therefore, placing the TE-TM polarizer 148 closer to the digital detector 150 results in smaller horizontal bands.

[0092] LSP Subsystem 2B and 4A-4C, LSP light source system 210 of LSP subsystem 200 emits light at wavelength λ along third axis A3. B In one example, the LSP light source 212 includes an LSP light source 212 that generates an LSP light beam 216 at a second wavelength λ B The second wavelength λ is configured as a laser diode operating at λ = 415 nm. B can also be referred to as the LSP wavelength.

[0093] LSP light source system 210 includes, in sequence along a third axis A3, an optional neutral density filter 218 (shown in FIGS. 2B and 4A), a first focusing lens 220, a movable optical diffuser 222, and a second focusing lens 224. Movable optical diffuser 222 is configured to focus light having a wavelength λ BThe movable optical diffuser 218 may comprise a holographic element configured to perform optical diffusion at the PBS. In one example, the movable optical diffuser may include a rotating optical diffuser or a vibrating optical diffuser. One or more fold 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 that controls the major axis of polarization so that there is some rotation at the PBS.

[0094] 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. Optical compensator 230 also includes a half-wave plate 234H and a quarter-wave plate 234Q, one of which is rotatable relative to the other to change the state of polarization of LSP light beam 216. In one example, optical compensator 230 may include an electronically controlled polarization modulator, such as a liquid crystal or ferroelectric liquid crystal modulator or similar modulator.

[0095] In one example, optical compensator 230 is operably connected to or otherwise includes a controller (not shown) that controls the polarization switching operations performed by the optical compensator. In one example, optical compensator 230 can include a single liquid crystal device. In another example, optical compensator 230 can include multiple elements such as polarizers, wave plates, filters, prisms (e.g., wedge prisms), etc. In one example, optical compensator 230 causes LSP light beam 216 to undergo a complete polarization cycle (i.e., change between two or more selected polarizations) in anywhere from less than 1 second to 10 seconds. In one example, optical compensator 240 can be operably connected to and controlled by system controller 400.

[0096] A third focusing lens 236 is downstream of the optical compensator 230 and is used to form a focused LSP light beam 216F that is directed to the LSP coupling prism 42B. The LSP coupling prisms have respective input and output faces 43B and 44B and a bottom or "coupling" face 45B. The coupling face 45B and the top face 12 of the CS substrate 10 define a second (LSP) coupling interface IF2. In one example, the second coupling interface INT2 includes an index-matching fluid 5B, as discussed below.

[0097] The LSP detector system 240 lies along a fourth axis A4 that is orthogonal to the third axis A3, ie, the fourth axis A4 lies in the YZ plane.

[0098] In one example, LSP detector system 240 includes collection optics 243 and a digital detector (e.g., a CCD camera) 246. In one example, collection optics 243 is telecentric and has unity magnification. LSP detector system 240 also detects a second wavelength, λ B 4C, the digital detector 246 comprises an array of imaging pixels 247, which in one example may have dimensions between 1.1 microns and 10 microns, or between 1.8 microns and 10 microns.

[0099] In operation of LSP subsystem 200, focused LSP light beam 216F enters input face 43B of LSP coupling prism 42B, travels to coupling face 45B, then travels through index-matching fluid 5B to top face 12 of CS substrate 10 and into body 11 of the CS substrate. At any given time, focused LSP light beam 216F has a selected polarization defined by optical compensator 230. The (polarized) input LSP light beam 216F is scattered by features in body 11 of CS substrate 10 to form scattered LSP light beam 216S. Scattered LSP light beam 216S exits CS substrate 10 at top face 12, passes back through second coupling interface INT2, and then exits LSP coupling prism 42B at output face 44B. Scattered LSP light beam 216S travels to LSP detector system 240 and is directed by collection optics 243 to digital detector 246. Scattered LSP light beam 216S forms LSP image 248 on digital detector 246, as shown in the enlarged view of FIG. 4D . This defines a digital LSP image. LSP images 248 discussed below are considered digital LSP images unless otherwise specified. The characteristic “X” shape of LSP image 248 is known in the LSP art and results from reflections of scattered light beam 216S from different interfaces associated with LSP interface INT2, which is defined by CS substrate 10, LSP coupling prism 242B, and index-matching fluid 5B.

[0100] As shown in FIG. 4D, the X shapes of the LSP image 248 each have a local length coordinate x along their length. L Each line image LI is defined by two intersecting line images LI with the intensity distribution I(x L ) The digital detector detects the intensity distribution I(x L) into a second detector signal SB, which is sent to the system controller 400. Only one of the line images LI is needed to perform the measurement. In one example, image processing is used to identify a portion of the LSP image 248 in order to extract optical retardation information using subsequent processing, as described below.

[0101] In one example, a given measurement of the CS substrate 10 using the LSP subsystem 200 may take place over a measurement time t M This involves taking measurements over a measurement time t M During exposure time t, the polarization state of LSP light beam 216 varies between different polarization states, preferably through one or more cycles. Meanwhile, for each polarization state, digital detector 246 measures the polarization state over an exposure time t E In one example, an LSP image 248 is captured during an exposure time t E is approximately the same as the frame rate FR of the digital detector 246. An exemplary exposure time t E = 50 ms, which corresponds to a frame rate FR = 20 frames / s. E can also be smaller than the frame rate.

[0102] The electronically captured LSP images 248 have an intensity distribution I(x L ) are different. This difference is due to the difference in destructive and constructive interference along the length of the scattered LSP light beam 216S as a function of depth D into the CS substrate 10 between the different polarization states. The multiple intensity distributions I(x L ) is used by the system controller 400 to calculate the optical retardation OR as a function of depth D into the CS substrate body 11 using relationships well known in the art. Similarly, a plurality of optical retardation curves OR versus depth D ("OR vs. D plots") can be plotted against the intensity distribution I(x LFor example, for a measurement time t of 3 seconds with an image sensor frame rate FR of 20 frames / second, M Over the period, I(x L A total of 60 plots of OR vs. D are generated to calculate OR vs. D, which can be used to calculate one or more stress-related properties of the CS substrate 10.

[0103] The intensity distribution I(x L ) will necessarily differ between polarization states of the input light beam 112 when stress is present in the CS substrate 10, but the different OR vs. D curves (plots) calculated from the measured intensity distributions should ideally be the same for a given measurement location for a given CS substrate as they would be for a CS substrate where the stress profile is (ideally) constant.

[0104] While the LSP measurement technique can generate a stress profile S(z), it generally does not produce an accurate representation of the stress profile in the near-surface region of the CS substrate 10. There are at least two problematic effects that present challenges for extracting an accurate characterization of the near-surface stress profile for the CS substrate 10 using LSP measurements from the LSP subsystem 200. One problematic effect is referred to as the “fireball” effect, which is caused by excessive light scattering at the LSP interface INT2. The excessive light scattering generates noise that corrupts the LSP measurement data for the near-surface region, which in one example is the first 60 to 100 micrometers below the top surface 12 of the CS substrate 10.

[0105] Another problematic effect is caused by the superposition of photons scattered from different depths on the signal corresponding to a particular depth. This superposition significantly alters the signal in regions of fast stress change, usually the compressive region near the surface, most often the first 80, 100, or 150 microns, but sometimes as high as 200 microns. The region of fast change is thicker with increasing thickness of Li-based glasses.

[0106] Some prior art LSP systems attempt to reduce these overlapping effects by using a highly focused beam near the CS substrate surface, with a beam diameter as small as 10 microns. Unfortunately, this leads to other problems, such as increased laser noise (e.g., speckle) in the region of interest at the same depth, further reducing the reliability of the extracted stress profile in the near-surface region.

[0107] LSP light source test The quality of reliability of LSP light source 212 affects the overall performance of hybrid system 20 described herein. Therefore, LSP light source 212 should be thoroughly evaluated before being included in hybrid system 20. Embodiments of the present disclosure also include a test fixture 300 with strict laser specifications for quantifying and qualifying each laser used in hybrid system 20.

[0108] Referring to FIG. 4E, an exemplary test fixture 300 is illustrated. Test fixture 300 may include a base 302 to which multiple optical components may be mounted. Test fixture 300 further includes a laser mount 303 configured to hold LSP light source 212 relative to the same reference plane as when installed in LSP subsystem 200. Laser mount 303 may be mounted on a rotational bearing such that the bearing rotation axis is collinear with the laser axis. Test fixture 300 also 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 so that it is at the focal point of the focusing lens 309 when the focusing lens is illuminated with a collimated beam of the operating wavelength of the precise LSP light source 212 that is centered on and aimed along the optical axis of the focusing lens 309. The various optical components may be mounted on rails 306 so that their positions may be adjusted along the base 302.

[0109] The LSP light source 212 can be evaluated using the test fixture 300 before being installed in the LSP subsystem of the hybrid system 20. When the LSP light source 212 is properly collimated, the laser beam focuses to a diffraction-limited spot on the sensor 310. By specifying a maximum allowable spot size, the collimation quality of the LSP light source 212 can be controlled. As the rotary bearing rotates, the focused spot traces a circle on the sensor 310. The radius of this circle measures the aiming error of the LSP light source 212 relative to the reference plane of the sensor 310. By specifying a maximum circle rotation, the quality of the laser aiming error can be controlled.

[0110] High effective laser power It will be appreciated that for accurate measurements of materials that do not scatter much, such as glass, a laser with either higher effective laser power or lower measurement noise, or both, may be desired for a sufficient signal-to-noise ratio. In some embodiments, referring to FIG. 4F , which illustrates an exemplary LSP light source system 210, higher laser power is achieved by using a semiconductor laser die for LSP light source 212 that provides higher power and a collimator lens 231 that collects the output from LSP light source 212 into a clean Gaussian beam that can be focused by focusing lens 236 into a small beam spot within sample 10.

[0111] As shown in Figure 4F, this can be achieved, for example, 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 0.4 µm x 1.4 µm. Light from this laser can be collected by, for example, a 7 mm focal length, 0.5 NA molded aspheric collimator lens 231. If the collimator lens 231 and subsequent optics maintain beam quality, and given a 100 mm focusing lens 236, this provides a beam waist BW of approximately 6 µm x 21 µm at the object location on the sample 10. This small beam size provides high spatial measurement resolution.

[0112] Active laser temperature control for wavelength tuning To reduce measurement noise, all external light that is not part of the measurement laser beam should be filtered out. One exemplary technique for achieving this is to use a spectral filter in front of the camera to block light of wavelengths different from that of the illuminating laser. This filter blocks both ambient light and the fluorescent light excited by the laser. To be most effective, this filter should have a spectral width comparable to that of the laser.

[0113] However, the wavelength of a diode laser can vary from its nominal wavelength from die to die and as the die temperature changes. This can lead to a mismatch between the laser and the filter, such that some of the light from the laser is blocked by the filter, as shown in FIG. 4G. This reduces the effective output power of the laser. In some embodiments, this problem is mitigated by actively adjusting the wavelength by selecting the laser and setting the temperature of the die. As a non-limiting example, this can be achieved by using a Peltier cooler and active control system to ensure that the die is regulated to a precise temperature to emit at the desired wavelength and maintained at that temperature regardless of the ambient temperature and the power emitted by the laser die. FIG. 4F illustrates an exemplary cooler and active control system 215 for optimally setting the temperature of the die of the LSP light source 212 to ensure that the laser's output passes through the filter.

[0114] LSP Subsystem Calibration To ensure proper hybrid system 20 operation, calibration of the LSP subsystem 200 should be employed. However, manual calibration of the LSP subsystem 200 leads to user error in setup and potential damage to components within the LSP subsystem 200. In manual calibration, system adjustments are performed by an operator manually adjusting components, analyzing images or gauges provided via software, and then re-adjusting as necessary. A user-based feedback loop can cause issues due to user error resulting from the sensitivity of the system gauges and perceived image quality and visualization.

[0115] Additionally, current scattered light polarization analysis devices use basic manually adjusted slides, knobs, and locking screws to control the placement of components in LSP subsystem 200 for system calibration. These manual adjustment and locking devices can cause damage to the components, such as flat spots created by the locking screws, which can lead to future errors when calibrating the system.

[0116] Therefore, in some embodiments, LSP subsystem 200 includes actuators that enable automatic calibration with minimal manual intervention. The actuators provide adjustment of at least five SLP subsystem 200 components: a rotatable laser orientation, a rotatable half-wave plate, a translatable diffuser, a translatable focusing lens, and a translatable / rotatable camera. System calibration involves adjusting each of these components to set various parameters, such as, but not limited to, speckle percentage, beam width, intensity, beam focus, camera focus, and camera tilt.

[0117] Referring again to FIG. 4A , the LSP light source 212 can be rotated within its mount by an actuator (e.g., a motor), as indicated by arrow A. This changes the orientation of the elliptical laser beam of the LSP light source 212. FIG. 4I illustrates an exemplary LSP subsystem 200 with a rotatable LSP light source 212. To minimize the beam width, the LSP light source 212 is rotated so that the smallest waist of the LSP light source 212 is directed toward the digital detector 246. The LSP light source 212 is rotated to a reproducible, stable state during initial system setup and is generally not adjusted during system calibration.

[0118] In addition to the LSP light source 212, the diffuser 222 is mounted on an actuator (e.g., a stepper motor) that allows for + / - 5 mm control of the position of the diffuser 222 relative to the focal spot of the fixed lens, as illustrated 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 illustrates the example LSP subsystem 200 of Figure 4I with a translatable diffuser 222.

[0119] The half-wave plate 234H may also be coupled to an actuator (e.g., a motor) that allows it to rotate, as illustrated by arrow C. Rotating the half-wave plate 234H varies the intensity of the laser beam across the thickness of the sample. The user can manually input "forward" or "backward" commands to adjust the beam intensity, or the software can automatically position the half-wave plate in the correct position for optimal beam intensity during an auto-calibration process. Figure 4H illustrates how the half-wave plate can be rotated.

[0120] Additionally, focusing lens 236 is mounted on an actuator (e.g., a stepper motor) that allows the position of focusing lens 236 to be translated as illustrated by arrow D. The beam focus of LSP laser source 212 can be controlled by translating fixed focal length focusing lens 236 relative to the position of the sample on the prism, which affects the beam focus parameters. Figures 4I and 4J illustrate how the focusing lens can be rotated.

[0121] The detector device 150 may also be fixed to one or more actuators that allow it to translate and / or rotate, as indicated by arrows E and F. The detector device 150 may be attached to a translation stage that is used to move the detector device 150 relative to the position of the sample on the prism, thereby changing camera focus parameters. The movement of the detector device 150 takes into account the fixed focal length of the telecentric lens used on the detector device 150. The motors can be controlled by software input to adjust the position of the detector device 150 for best image focus. The detector device 150 may also be rotated by its mount, which affects the camera tilt. The detector device 150 may be rotated during initial system setup, where it can be precisely adjusted using software gauges, and is generally not adjusted during normal system calibration. Figures 4H and 4I illustrate how the detector device 150 (i.e., the camera) may be rotated and translated.

[0122] The motorized components described above allow for automation of calibration. Embodiments provide a single-step calibration method where a single software action can automatically set all component positions for best performance.

[0123] Beam entrance / exit location selection For measurements of glass samples, LSP analysis selects the entry and exit points in the image of the laser traveling through the sample. These points are typically considered bright "fireballs" and can be selected using an image contour detection algorithm. However, if there are stains, bubbles, or fingerprint speckles on the sample, it can be difficult for the image contour algorithm to select which bright points are true fireballs and therefore true entry and exit locations. Embodiments of the present disclosure can employ a scoring system to select which point combinations are the most likely candidates for entry and exit fireballs.

[0124] First, point combinations are eliminated from the selection process if they are not within a certain threshold of known system limitations. The overall thickness of the sample and the angle at which the laser is propagating can be estimated, so all point combinations that are outside 50% above and below the thickness and within ±10 degrees of the angle of laser propagation are eliminated from the selection process. For the remaining point combinations, each combination is scored for (1) the combination's proximity to the sample thickness, (2) the combination's proximity to the angle of laser propagation, and (3) the median light intensity between the two points. Because the laser is propagating between an entry point and an exit point, the median intensity is estimated to be greater than the intensity between any two random points on the image.

[0125] Scoring is calculated by taking the full spread of values ​​and normalizing them to a range of 0 to 1, with 1 being the highest score. For example, the point combination with the brightest inter-point intensity would be scored as 1, whereas the combination with the dimmest intensity would be scored as 0. Once scoring is complete, the combination with the highest overall score across all three requirements is reported as the laser entry and exit points.

[0126] Figure 4K illustrates a digital image of the sample taken by the system from which the entry and exit points are selected. Figure 4L illustrates a digital image with the final combination of points selected as the intersection, with the yellow outline representing all points initially detected and then discarded by the image processing algorithm.

[0127] Optimization of LSP laser orientation The orientation of the LSP light beam 216 affects the accuracy of some central tension measurements. There is an optimal orientation where the beam width BW is smallest at the center of the sample 10. Once the optimal orientation is identified, all of the instruments in the hybrid system 20 can be assembled in a similar manner, reducing measurement variability between devices.

[0128] Figure 4M is a simplified diagram of the subsystem 200 illustrated by Figure 4B. The focused LSP light beam 216F is oriented such that the beam width BW is minimized from both the minimum waist of the focused laser at the specimen thickness center (BS) and the best focus of the LSP detector system 240.

[0129] Because the focused LSP light beam 216F and its line of sight to the LSP detector system 240 are at an angle to the stress profile (exemplified by the horizontal line in the sample 10), determining the proper orientation of the LSP light beam 216 is a three-dimensional analysis that incorporates the intersection of four three-dimensional features. 1. The stress profile of the specimen (i.e., the "layers" or gradients through the specimen thickness). 2. Cross section of LSP light beam 216F relative to a layer of the sample. 3. A line of sight of the LSP detector system 240 that is perpendicular to the LSP light beam 216F with respect to the layer of the sample. 4. Pixels selected to be grouped (by averaging or various fitting methods) in the line of sight of the LSP detector system 240 to reduce noise in the data associated with a point in thickness.

[0130] Ideally, all grouped values ​​represent light integrated from the same depth in thickness. Minimizing the dimensions of LSP light beam 216F and the illumination collection depth of LSP detector system 240 is one way to optimize the measurement. Instead of a beam, a thin illumination sheet could be used, provided there is enough illumination to still observe the scattered light with LSP detector system 240. Light sheets have been applied primarily in microscopes for fluorescence, which would be for scattered light polarization analysis.

[0131] The cross-sectional profile of the LSP light beam 216F has an elliptical power distribution. In embodiments of the present disclosure, the elliptical orientation (and beam shape) can be chosen to improve the quality of the retardation data for measurements. The line of sight of the elliptical LSP detector system 240 changes the integrated light onto the sensor. In FIG. 4N, the LSP light beam 216F appears "thick" to the LSP detector system 240, but the light on the sensor pixels is from a specific area. In contrast, in FIG. 4O, the LSP light beam 216F appears thin to the LSP detector system 240, but the light on the sensor pixels is from a larger area along the axis of the LSP detector system 240. FIGS. 4P-4S illustrate various arrangements for orienting the LSP light beam 216F relative to the LSP detector system 240.

[0132] Focusing helps improve the quality of the retardation data for the measurement, and minimizing the beam width also helps, as discussed above, especially when beam axis control and (among other things) focus astigmatism artifacts also affect the depth into the sample source of the light integrated into the LSP detector system 240.

[0133] Orientation optimization can be assessed with several analytical tools: A simple analysis of the beam width at the center of the sample can be optimized through both the laser beam focus and the detector focus.

[0134] We begin with a more detailed analysis of the retardation (phase of the sine-fit modulated retardation) and peak-to-peak intensity (amplitude of the sine-fit modulated retardation) per pixel at the camera line of sight. For a known specimen, retardation values ​​grouped together for noise reduction would ideally be the same. Delta retardation values ​​(related to the magnitude of the stress in the "layer") from adjacent groups would likewise be expected to be the same as from adjacent individual values.

[0135] Coupling Prism Assembly The hybrid system 20 utilizes the aforementioned coupling prism assembly 40, which operably supports the EPCS coupling prism 42A and the LSP coupling prism 42B to provide prism coupling to the EPCS subsystem 100 and the LSP subsystem 200 when performing EPCS and LSP measurements of the CS substrate 10.

[0136] FIG. 5A is an overhead view of a top portion of an exemplary coupling prism assembly 40 showing an exemplary support frame 48. FIG. 5B is an overhead view similar to FIG. 5A , additionally including a cover plate 60. The exemplary 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 a separation member 50 disposed between the EPCS frame section 48A and the LSP frame section 48B that is configured to optically separate the EPCS coupling prism 42A and the LSP coupling prism 42B. In one example, the separation member 50 also prevents mixing of the index-matching fluids 5A and 5B used with the EPCS coupling prism 42A and the LSP coupling prism 42B, respectively. In another example, the isolation member 50 enables a single index-matching fluid to be used with both the EPCS coupling prism 42A and the LSP coupling prism 42B, i.e., a single 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 piece from and attached to the support frame 48. In another example, the isolation member 50 is part of the support frame 48, i.e., is integrally formed with the support frame during its formation.

[0137] In one example, EPCS and LSP frame sections 48B and isolation member 50 include fastening tabs 52 with mounting holes 53 that allow cover plate 60 to be secured to the frame sections using a fastening member (not shown). Cover plate 60 includes a first opening 62A sized to accommodate coupling surface 45A of EPCS coupling prism 42A and a second opening 62B sized to accommodate coupling surface 45B of LSP coupling prism 42B.

[0138] 6A and 6B illustrate an exemplary method by which EPCS frame section 48A and LSP frame section 48B are formed using a resin molding process. This process provides precise alignment of EPCS and LSP coupling prisms 42A and 42B. In one example, the molding process is performed with exemplary EPCS and LSP prisms 42A and 42B in place on a stable platform 75. This process is discussed in more detail below.

[0139] 7 is an xz cross-sectional view of an exemplary prism support structure 46 mounted to an exemplary support plenum 70 of hybrid system 20 using fastening members 54, such as fastening tabs 52 and screws, passing through mounting holes 53. Support plenum 70 has a top surface 71 and a measurement aperture 72. Top surface 71 defines an exemplary measurement plane MP at measurement aperture 72. Prism support structure 46 is supported by support plenum 70 such that EPCS coupling prism 42A and LSP coupling prism 42B reside in measurement aperture 72. In one example, EPCS coupling surfaces 45A and LSP coupling surfaces 45B of EPCS coupling prism 42A and LSP coupling prism 42B reside at or substantially reside in measurement plane MP.

[0140] In one example, the CS substrate 10 is operably supported by a movable substrate holder 80 that holds the CS substrate over the measurement aperture 72 such 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 one example, the movable substrate holder 80 is transported over the top surface 71 of the support plenum 70 using transport elements 73, such as rollers, wheels, sliders, bearings, or the like. In one example, the CS substrate 10 is supported by the movable substrate holder 80 at an inner lip 82 that supports an outer (peripheral) portion of the top surface 12 of the CS substrate. In one example, the plane of the inner lip 82 defines an exemplary measurement plane MP. Thus, FIG. 7 illustrates two different exemplary locations of the measurement plane MP.

[0141] 8A is an overhead view illustrating an example in which support plenum 70 is in the form of a plate containing pressure- and vacuum-resistant (PV) conduits or PV elements 90 (e.g., PV bars) that are used to pneumatically engage CS substrate 10, pull the CS substrate onto coupling surfaces 45A and 45B of EPCS coupling prism 42A and LSP coupling prism 42B via vacuum (negative pressure), and then release the CS substrate from the prisms via pressure (positive pressure). FIG. 8B is a cross-sectional view of the support plenum and measurement aperture of the configuration of FIG. 8A , showing an exemplary vacuum system 91 including PV elements (PV bars) 90 and a vacuum source 92.

[0142] Note that the inner lip 82 of the movable substrate holder 80 defines a stop member for limiting vertical movement of the CS substrate 10 when a vacuum is applied to the CS substrate via the vacuum system 91 .

[0143] Hybrid system employing a single refractive index matching fluid. An exemplary embodiment of the hybrid system 20 as shown in FIG. 6D uses refractive index n fThis is a counter-intuitive approach, as a single index-matching fluid 5 would typically be deemed incapable of producing good measurements from both subsystems simultaneously, for at least the following reasons:

[0144] When the index-matching fluid is selected based on the considerations of the EPCS measurement, the index-matching fluid should be selected to match the surface refractive index n of the CS substrate to facilitate light coupling into the guided modes and to obtain good fringe contrast in the captured TM and TE mode spectra. S A refractive index n that is substantially (e.g., 0.1 or more) higher than f It has.

[0145] On the other hand, this level of refractive index contrast (difference) Δn between the refractive index of the index-matching fluid 5 and the surface 12 of the CS substrate 10 causes significant surface scattering at the bonding interface INT2 from beam deflection at the refractive index mismatch associated with surface microroughness. This presents a problem for accurate extraction of retardation and stress measurements at moderate depths based on receiving and processing scattered light from the CS substrate. High surface scattering generates a "fireball," e.g., a large bright spot on the image of the scattered light beam when the pixels 247 of the digital detector (CCD camera) 246 are saturated with photons. This results in the loss of a significant amount of stress-related information. Highly polished or pristine surfaces (such as those formed by melt drawing) tend to have low scattering.

[0146] Refractive index matching fluid f However, to ensure low surface scattering, the surface refractive index n SWhen the refractive index of the EPCS subsystem 100 is approximately matched (e.g., similar, slightly higher, or slightly lower) to the surface, the fringe contrast of the mode spectrum 160 is typically poor when there is a sharp change in the refractive index near the surface, such as in the spike region R1 (see FIG. 1B), as caused by the shallow concentrated spike in K2O concentration produced by the IOX process. Furthermore, the position and contrast of the TM fringes 163TM and TE fringes 163TE depend on the thickness of the index-matching fluid. These two effects make it very difficult to accurately measure the surface CS and spike DOL using the EPCS subsystem 100.

[0147] The refractive index matching fluid 5 is set to the bulk refractive index n B A refractive index lower than n f (which also typically means lower than the surface refractive index), the thickness of the index-matching fluid must be very small (e.g., less than 0.4 microns) to allow light coupling into the waveguide modes of the near-surface portion (spike region R1) of the NSWG18 for surface CS measurements. A small thickness is also needed to measure the critical angle for coupling light traveling in the deep region R2 between the surface spike region R1 and the bulk region R3. This is difficult to achieve consistently in a production environment due to particulate contamination issues. These issues are compounded by the surface (compressive) stress S(0) and "knee stress" S at the base of the surface refractive index spike region R1 for dual-IOX Li-containing glasses and glass-ceramics. k This causes problems in measuring accurately.

[0148] A single refractive index matching oil 5 for both the EPCS and LSP measurements was used, with the spike region R1 of the CS substrate 10 being at a normalized gradient S n = |(λ / n)dn(z) / dz| < 0.0005, or more preferably S n It has been found that it can be used under selected conditions with a refractive index of <0.0004, where λ is the measurement wavelength and n(z) is the refractive index of the CS substrate 10 at the measurement wavelength.

[0149] In one embodiment, the amount Δn=n in the range of 0.02 to 0.06 f -n S The surface refractive index of the CS substrate glass is n S Higher (larger) refractive index n f It is found that an index-matching fluid 5 having S produces satisfactory measurement results for both EPCS and LSP measurements. n If <0.0004, Δn is preferably at the higher end of the range given above, for example 0.05 to 0.06.

[0150] In one embodiment of the present invention, the measurement wavelength λ for EPCS measurement A To reduce the gradient S to more easily satisfy the above conditions, n In one example, the measurement wavelength λ of the EPCS measurement is reduced. A is the smaller normalized gradient S n To help achieve this, the measurement wavelength λ of the LSP measurement B is more than 5% shorter. In one example, one or more optical blocks (not shown) can be selectively positioned on 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 increases the contrast of the trapped TM and TE fringes of the guided modes for the near-surface spike region R1 of the NSWG 18.

[0151] In another embodiment, the surface spike region R1 is determined by the normalized slope S n In one example, the index-matching fluid may have a refractive index of 0.0005. k , i.e., at the bottom of the spike region R1, the refractive index n f The EPCS measurement wavelength λ B In this case, n f ≒n crit , where n crit is the refractive index associated with the critical angle of the spike region, i.e., the angle below which light will not travel as a guided wave within the spike region R1.

[0152] In many cases of practical interest, the difference in effective refractive index between the TM and TE waveguides at the location corresponding to the bottom of the surface spike region R1 is relatively small. For example, in most cases of practical interest, the difference is less than 0.0006 refractive index units (RIU), and in most cases is between 0.00015 and 0.0005 RIU. In one example,

number

[0153] In some instances, n f ≒n crit It is sufficient to specify

number

number

number

[0154] The upper limit of the above formula is n f is defined to reduce the likelihood of missing a fringe associated with spike region R1 by making n larger than the effective refractive index of that fringe. Therefore, to allow for proper consideration of all modes for the purpose of accurately calculating the depth of surface spike region R1 (defined by the potassium IOX process in one example), n f is the TM critical refractive index and the TE critical refractive index

number

[0155] In one embodiment, the mode fringes of the TM and TE mode spectra associated with the spike region R1 are determined by the effective refractive index of the last fringe in a particular polarization state (TM or TE) and the corresponding critical refractive index (

number

number

number

number

[0156] These differences in effective refractive index are easy to establish using the EPCS subsystem 100 by measuring the differences in the locations of the bright total reflection to dark (partial reflection) intensity transitions 166TM and 166TE on the sensor and / or the critical angles corresponding to the fringe positions, and taking into account the instrument calibration (angles per RIU, or pixels per RIU, or spacing of points on the sensor plane per RIU).

[0157] In another embodiment having more general application, the refractive index of the index-matching fluid, n oil is chosen to be close to the lower of the TM and TE effective indices. This allows for capture of TM and TE fringes, which may result in an effective index close to the critical index but may require relatively close proximity (e.g., several wavelengths) between coupling surface 45A of EPCS coupling prism 42A and top surface 12 of CS substrate 10.

[0158] More specifically, in this embodiment, the following is preferred:

number

number

[0159] Furthermore, to reduce significant changes in the shape of the critical angle transition, the following may be preferred.

[0160]

number

number

[0161] In some cases of practical interest, the guided mode with the lowest effective index has its effective index very close to that of the critical index, within about 0.0002 RIU, in which case it may be desirable to impose a stricter requirement on the index of the index-matching fluid to be bounded from above.

number

[0162] n oil If is smaller than at least one of the two critical refractive indices, then the critical refractive index n critObtaining a high contrast transition for proper measurement of 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 one example, this close proximity is made possible by using a vacuum system that draws the specimen toward the prism through a PV conduit 90 pneumatically connected to a PV source 92.

[0163] In another embodiment, the index-matching fluid refractive index n oil does not differ significantly from the effective refractive index of the guided optical mode used to calculate the surface compressive stress CS. In particular, such a correction may be preferable to use in the following cases:

number

[0164] In one exemplary embodiment, the correction is performed by calibrating the systematic error, for example, by measuring the surface compressive stress Cs measured using the preferred dual-use index-matching fluid of the present invention and the bulk refractive index n in the range of 1.45 to 1.55. B Used to measure the CS substrate 10 having λ A = n at 590 nm oil A relatively large refractive index, such as oil, n = 1.72 oil It is defined by comparison of CS measured by using a more common index-matching fluid with

[0165] In a related embodiment, the systematic error may also be calibrated to the width of the TM fringes 163TM and the TE fringes 163TE, since the width may be related to the thickness of the index-matching fluid and, at the same time, to the amount of systematic error in the measurement of the surface compressive stress CS. The systematic error may also be related to the refractive index gradient S of the refractive index profile of the surface spike region R1 of the CS substrate 10. n Note that the systematic error will depend on the surface refractive index gradient S within a relatively narrow range.n This means that it can be defined for a specific type of CS substrate having a range of 0.01 to 0.01 mm. Such a narrow range is typical for CS substrates employing Li-based glass strengthened using the IOX process.

[0166] The high power nature of the laser diodes used in the system leads to degradation of the refractive index oil over time, which is another source of noise and measurement variation. To mitigate this issue, a laser blend refractive index oil can be used that is sufficient to maintain the laser energy without degradation.

[0167] A hybrid system employing two different refractive index matching fluids. An exemplary embodiment of the hybrid system 20 employs two different index-matching fluids 5 (labeled 5A and 5B) for the EPCS subsystem 100 and the LSP subsystem 200, respectively, with the two different index-matching fluids 5A and 5B having respective refractive indices n fA and n fB (or n oil-A and n oil-B )

[0168] Employing two different index-matching fluids 5A and 5B requires that the two index-matching fluids be kept separate so that they do not mix with one another. In the example discussed above in connection with Figures 5A and 5B, the prism support structure 46 includes a separation 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 fluidically separated, i.e., fluidically separated from one another.

[0169] 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, defining an "air curtain" 30 (see FIG. 2B) that ensures that the index-matching fluids 5A and 5B do not interact with other fluids while the CS substrate 10 is being measured with the hybrid system 20. This separation then allows for the simultaneous automated dispensing of the respective index-matching fluids 5A and 5B onto the respective EPCS coupling prisms 42A and LSP coupling prisms 42B, thus enabling simultaneous measurements. In one example, the air curtain 30 can be formed using a PV system 91 (see, e.g., FIG. 8B).

[0170] Hybrid system with reduced crosstalk. The proximity of EPCS coupling prism 42A and LSP coupling prism 42B can result in crosstalk between EPCS subsystem 100 and LSP subsystem 200. Such crosstalk can reduce the accuracy of the stress measurements of each subsystem. Various embodiments for reducing (including eliminating) crosstalk, described below, can be used separately or in combination.

[0171] In one example, the EPCS detector system 140 of the EPCS subsystem 100 detects an EPCS measurement wavelength λ A Meanwhile, the LSP detector system 240 of the LSP subsystem 200 includes the aforementioned bandpass filter 144 centered at the LSP measurement wavelength λ B The optical subsystems 140, 142, 144, and 244 include a bandpass filter 244 centered at . In one example, the bandwidth of each of the bandpass filters 144 and 244 is narrow enough to substantially filter out the measurement wavelength of the other subsystem. Because the bandpass filters can be made very narrow (e.g., a few nanometers), even a small difference in measurement wavelength (e.g., 10 nm) would be large enough to reduce or eliminate crosstalk using the bandpass filters. In one example, a given bandpass filter can be inserted anywhere between the corresponding coupling prism and the detector system.

[0172] In another embodiment, the EPCS wavelength λ of the measurement A and LSP wavelength λ B A barrier that is optically opaque to the EPCS and LSP measurement light is disposed between the EPCS coupling prism 42A and the LSP coupling prism 42B. In one example, the barrier takes the form of an isolation member 50, as discussed above in connection with FIG. 5A. The isolation member 50 can be formed of a rigid material, such as aluminum, or a non-rigid material, such as rubber, so long as it is capable of stopping the EPCS and LSP measurement light from transmitting between the EPCS coupling prism and the LSP coupling prism. As mentioned above, the isolation member 50 can also be configured to serve the dual purpose of optical isolation and fluid isolation.

[0173] Laser entrance and exit noise reduction Noise in the retardation data can be attributed to many aspects, including laser speckle, surface roughness, and fireball, which, as mentioned above, is bright light scattering that occurs at the glass-prism and glass-air interfaces. Laser speckle noise can be mitigated with the use of a rotating holographic diffuser 222, but fireball remains an issue even with the use of index-matching fluid.

[0174] Embodiments of the present disclosure mitigate the fireball effect by applying an ink layer to the back surface of the sample 10 (i.e., the top surface 12 of the sample 10 shown in FIG. 1A, not the incident surface of the sample 10 relative to the beam 216). Therefore, a black ink line drawn on the sample under test mitigates fireballs at the glass-air interface. In particular, a black permanent marker line on the sample 10 was found to provide the best results. FIG. 8C is an image of a hybrid system 20 without a permanent marker line on the back surface of the sample, and FIG. 8D is an image of a hybrid system 20 with a permanent marker line on the back surface of the sample. FIG. 8D illustrates a clear reduction in the fireball effect.

[0175] Prior to using the permanent marker backing method, we applied a lens and immersion oil to the back of the glass sample to reduce reflections and laser speckle in the image. The use of the permanent marker backing method further reduced reflections and laser speckle on the back surface, resulting in reduced noise during measurement. Figure 8E illustrates an image without a backing material. Figure 8F illustrates an image using standard oil and the L3 lens. Figure 8G illustrates an average image using the L3 lens. Figure 8H illustrates an image using a permanent marker line on the back surface, as described above. Figure 8I illustrates an average image using a permanent marker line on the back surface, as described above.

[0176] Experiments were conducted to find alternatives to the original method of using an L3 lens and immersion oil on a glass sample. Throughout the experiment, various backing materials were applied and measurements were attempted. Most materials were less effective at reducing reflections and backscatter than the original L3 lens method, and some materials enabled measurements. However, a black permanent marker proved to be the most effective at reducing reflections, backscatter, and noise during SLSS measurements. Figure 8J illustrates an image of a sample using a backing material with a polarizing filter and immersion oil. Figure 8K illustrates an image of a sample using a backing material with a black plastic capsule and immersion oil. Figure 8L illustrates an image of a sample using a backing material with a black dry-erase marker. Figure 8M illustrates a digital image of a sample using a backing material with a black permanent marker. As shown in Figures 8J–8N, a black permanent marker is the most effective backing material for minimizing the fireball effect.

[0177] Coupling prism alignment Hybrid system 20 provides the most accurate measurements when EPCS coupling prism 42A and LSP coupling prism 42B are aligned with each other and with their coupling surfaces 45A and 45B residing in a common plane.

[0178] To achieve such alignment, coupling prism assembly 40 employs and uses the aforementioned prism support structure 46. In one example of forming prism support structure 40, coupling surfaces 45A and 45B of EPCS coupling prism 42A and LSP coupling prism 42B are first ground and polished to a high degree of flatness and perpendicularity. Referring again to FIG. 6A , EPCS coupling prism 42A and LSP coupling prism 42B are then placed on a stable mounting 75, such as a precision flat granite mount, with coupling surfaces 45A and 45B resting on a surface 76 of the stable mounting.

[0179] Referring now to FIG. 6B , a mold 49 is placed on a stable rest 75 on a surface 76, and then a resin 49R is poured into the mold. Once the resin has hardened, the walls of the mold 49 are removed to define the prism support structure 46 of the coupled prism assembly 40, as shown in FIG. 5B . In one example, the molded prism support structure 46 includes a separation member 50 in the form of a thin wall 47 between the EPCS coupled prism 42A and the LSP coupled prism 42B, as shown in FIG. 6C . In one example, the molded prism support structure 46 is formed so that at least one of the prisms is partially encapsulated to avoid crosstalk. In one example, the molded prism support structure 46 includes or consists of a unitary molded structure, i.e., a single piece made of a single material (i.e., a monolithic molding), and therefore is not formed by joining two or more components.

[0180] In one example, the molded prism support structure 46 includes fastening tabs 52 with mounting holes 53 for securing the prism support structure 46 to the support plenum 70 (see also FIG. 5A ). The use of the movable substrate holder 82 shown in FIG. 7 and described above allows 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 precision linear motors (e.g., piezoelectric actuators) to set measurement locations for the EPCS subsystem 100 and the LSP subsystem 200.

[0181] In one example, prism support structure 46 includes sections that are movable relative to one another so that EPCS prism 42A and LSP prism 42B can be moved relative to one another, for example, in the axial or z-direction as shown in FIG. 6C. In one example, support frame 48 of the prism support structure includes adjacent walls 47 configured to allow one wall to slide relative to the other in a controlled manner. In the example of FIG. 6C, EPCS coupling prism 42A is shown moved in the z-direction relative to LSP prism 42B.

[0182] FIG. 6D is similar to FIG. 4C and illustrates an embodiment of the hybrid system 20 in which the EPCS subsystem 100 and the LSP subsystem 200 share a common coupling prism 42, i.e., the common coupling prism 42 functions as both the ECSP coupling prism 42A and the LSP coupling prism 42B. A single index-matching fluid 5 is also used. Various surfaces of the coupling prism 42 have dual purposes; for example, the coupling faces are designated 45A and 45B because the coupling faces serve the dual purpose of performing EPCS coupling and LSP coupling. In one example, the bandpass filters 144 and 244 of the EPCS subsystem 100 and the LSP subsystem 200 are configured to filter light at different wavelengths λ. A and λ B(e.g., wavelengths separated by the bandwidth of at least one of bandpass filters 144 and 244) to substantially reduce or eliminate crosstalk between the subsystems. In the example of common combining prism 42, the combining prism can have ECSP section PS1 and LSP section PS2, and further in this example, the sections can be separated, i.e., ECSP measurement light 116 and LSP measurement light 216 generally remain in their respective sections, except for a small amount of scattered light.

[0183] Reduced warpage of the substrate The CS substrate 10 can be large enough to warp to the point where making accurate EPCS and LSP stress measurements becomes problematic. In particular, a warped CS substrate 10 can make it difficult to establish the EPCS and LSP bond interfaces INT1 and INT2 necessary to make EPCS and LSP measurements.

[0184] Referring again to Figures 8A and 8B, a PV system 91 is used to reduce or eliminate substrate warpage. PV conduits (PV bars) 90 are in pneumatic communication with the top surface 12 of the CS substrate 10 through openings 72 in the support plenum 70, which supports the CS substrate so that the top surface 12 substantially resides at the measurement plane MP. Actuation of the PV source 92 generates a vacuum near the coupling prism assembly 40 via the PV bar 90, resulting in a downward force FD on the CS substrate due to the ambient high pressure, as indicated by the two large arrows. The PV bar 90 allows for accurate control of the height of the CS substrate relative to the top surface 72 of the support plenum 70 (and therefore relative to the measurement plane MP) to within ±5 microns. The use of the PV system 91 also reduces vibration and enables non-contact control of the CS substrate for dynamic processing and inspection without the need to stabilize the CS substrate on a vacuum chuck.

[0185] PV bars 90 are commercially available and can be configured to reduce warping, as shown in Figures 8A and 8B. Some of the PV bars 90 proximate to the coupling prism assembly 40 may need to be omitted to avoid interference between the EPCS measurement light beam 116 and the LSP measurement light beam 216 and various components of the EPCS subsystem 100 and the LSP subsystem 200 that reside directly below the support plenum 70. In one example, one or more stop members 94 can be used to hold the CS substrate 10 in place on the support plenum 70.

[0186] In some cases, it may be desirable to be able to adjust at least one of the EPCS coupling prism 42A and the LSP coupling prism 42B independently of the other. In this case, the coupling prism assembly 40 may include two separate prism support structures 46, one or both of which are adjustable. In one example, the EPCS coupling prism 42A is adjustable in the z-direction to optimize the contrast between the TM and TE mode fringes in the mode spectrum. This can be achieved by using a single axis operably attached to the prism support structure 46 that holds the EPCS coupling prism in a movable configuration.

[0187] Processing of EPCS and LSP measurements 9 is a schematic diagram of an exemplary 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. Software within the system controller 400 is configured to use the EPCS measurements from the EPCS subsystem 100 (i.e., the modal spectrum 160) to calculate a first stress characteristic of the CS substrate, use the LSP measurements from the LSP subsystem 200 (i.e., the digital LSP image 248D) to calculate a second stress characteristic of the CS substrate, and then combine the measurements to generate a completed or complete stress characterization of the CS substrate.

[0188] Processing LSP measurements In one example, system controller 400 is configured (e.g., using software) to process LSP image 248 to extract "second" or LSP stress characteristics obtained from LSP subsystem 200. This involves digitally characterizing the contours of LSP image 248 using Gaussian blurred Otsu thresholding as part of a contour detection method to facilitate calculation of optical retardation versus depth (OR vs. D).

[0189] FIG. 10A is an exemplary representation of LSP image 248 shown in LSP section 412B of user interface 410. Detection of LSP image 248 by digital detector 246, which can be referred to as a raw LSP image or a raw digital LSP image, forms digital LSP image 248D. LSP section 412B of the user interface also shows a histogram of the scattered light intensities and several related statistical measurements that make up digital LSP image 248D. In this exemplary illustration, the primary beam entrance into the CS substrate is from the bottom right to the center of the intersection. From the center of the intersection to the top right, the digital camera sees reflections from the air surface of the CS substrate beside the beam due to total internal reflection (see FIG. 11C below). From the center of the intersection to the bottom left, the direct beam reflects from the air surface of the CS substrate and traverses back through the thickness of the CS substrate toward the LSP coupling prism. From the center to the top left, the digital camera sees reflections of the reflected beam.

[0190] Digital LSP image 248D is composed primarily of very bright pixels and pixels with little or no exposure. Referring to FIG. 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 the blurred LSP image. Image 248D is an example of an original (raw) digital LSP image. Image 248B is an example of an LSP image after blurring. The Gaussian blur is applied in a manner that does not obscure the optical retardation information encoded in the intensity variations of digital LSP image 248D.

[0191] Referring now to Figure 10C, Otsu thresholding is applied to the (Gaussian) blurred LSP image 248B of Figure 10B to obtain a thresholded 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. The light sections in Figure 10C represent all pixels with intensities above that threshold.

[0192] FIG. 10D illustrates the next processing step, which involves using the thresholded LSP image 248T to define an LSP image contour 248C using a binarization method, such as by applying a known open-source binarization algorithm, such as one available from an open-source image processing algorithm (e.g., via OpenCV). This example uses an image coordinate system with 0.0 at the top left and increasing values ​​in the right (x) and down (y) directions. The LSP image contour 248C consists of an array of points that can be divided into quadrants to find five critical points in the intersection shape image: top left, top right, bottom left, bottom right, and center. The enlarged portion of FIG. 10D shows an example of bottom-left point detection, obtained by finding the lowest X value and highest Y value in that area. The same process is repeated for all four corners, and the center is determined by averaging the corner X and Y values.

[0193] FIG. 10E shows the final LSP image contour 248C with a fully defined contour and processed area. In one example, the lower right leg of the processed "X" LSP image contour 248C (see trapezoid area) is then used to calculate the LSP stress characteristics. The horizontal lines in the LSP image contour 248C in FIG. 10E are at a constant depth. A Gaussian blur of the intensity (e.g., total, peak, or average) across the horizontal lines from each of the images acquired while the polarization of the light source was modulated is used as input for subsequent analysis to obtain OR vs. D data.

[0194] Therefore, the threshold LSP image 248T and LSP image contour 248C are used to define a "mask" that identifies one or more portions of the captured or Gaussian-smoothed LSP image 248B that are used to calculate the optical retardation OR as a function of depth (D) into the CS substrate 10, as described above.

[0195] CS substrate thickness extraction and beam angle calculation FIG. 11A is a diagram of the CS substrate 10. FIG. 11A also shows the beam path of a portion of the focused LSP light beam 216F within the body 11 of the CS substrate 10 after passing through the LSP coupling prism 42B (not shown). FIG. 11B is an enlarged view showing the edge portion of the CS substrate 10 as an area of ​​interest for calculating the thickness TH of the CS substrate. FIGS. 11C-11E are additional views of the path of the focused LSP light beam 216F within the CS substrate. The LSP coupling prism 42B is not shown for ease of illustration.

[0196] By viewing the edge of the CS substrate 10 along the propagation direction of the focused LSP light 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. 11B. The digital detector 246 continues to view the focused LSP light beam 216F through the angled LSP coupling prism 42B (e.g., angled at 45°), and therefore determines the actual thickness TH of the CS substrate 10 as TH=x / {Cos(45°) where x represents the path length in the plane of the digital detector 246.

[0197] Once the thickness T H is calculated, the propagation angle A (see FIG. 11E ) of the focused LSP light beam 216F within the CS substrate 10 can be calculated by looking at the edge of the CS substrate 10 along the direction of the digital detector 246 and using the schematic diagram of FIG. 11E : A=ArcTan(W / TH) The polarization of the image can be determined by determining the propagation angle A using , where W is the horizontal distance between the center intersection C of the image contour 248C and the lower right (LR) critical point of the image contour obtained from the contour detection method described above. Once the processed area is selected, the digital detector 246 records several images 248 as a function of the input polarization. Optical retardation information as a function of depth into the CS substrate is then extracted using techniques known in the art.

[0198] Lock-in detection method Lock-in detection is a signal analysis technique that has proven very effective at quickly extracting signals obscured by noise. For this method to work, the period of the signal must be known.

[0199] The measurement (detector) signal SB from the LSP subsystem 200 has a period that depends on the rate of polarization rotation by the optical compensator 230. When using a rotating half-wave plate 234H in the optical compensator 150, one complete rotation corresponds to four oscillations of the polarization state of the scattered light 216S.

[0200] The derivation of the lock-in method applied to the LSP measurement signal SB=s(t) is as follows, where t is time: Consider the LSP measurement signal s(t) as being centered at zero and having a certain amount of noise ("noise figure") N. The measurement data D(t) received by the system controller 400 can be expressed as: D(t)=s(t)+N The measurement signal s(t) is s(t)=A cos(ft+φ) where φ is the phase value to be extracted and f is the known frequency of the signal. This signal can be "locked in" by multiplying it with a generic test wave of equal negative period (and arbitrary phase) W(t) = cos(-f-θ), resulting in the following equation:

number

[0201] The first two terms in the equation immediately above for D(t)*W(t) oscillate according to the time variable t, but the last term is a constant that can be extracted by strong low-pass filtering of the product wave. This is accomplished by averaging the product wave, since the average of a wave approaches its offset value over multiple oscillations.

[0202] This approximation introduces a small amount of error if the measured signal s(t) does not have many oscillations (e.g., less than one complete oscillation) or if the signal has a non-integer number of half cycles. This error can be reduced by averaging the signal over only the largest amount of half cycles in the signal. For example, if the signal has approximately 3.7 oscillations, average the signal up to 3.5 cycles.

[0203] When low-pass filtering is performed using known means, the product D(t)*W(t) reduces to a constant term [A / 2]cos(-θ+φ), where φ is the desired phase value and θ is the arbitrary phase of the test wave. Thus, when θ is incremented through a series of numbers, the constant resulting from low-pass filtering of the product wave for each increment oscillates according to a non-time-varying cosine function, [A / 2]cos(-θ+φ). This cosine wave has a wave number of -1, A φ = 1 / 2 / ...

[0204] The lock-in method for signal extraction has been found to be much faster than conventional sine fitting. Figure 12A plots the average computation time, T, in milliseconds (ms) required to extract the phase, φ, of a noisy signal versus the noise figure, N, for both the lock-in method (L, or black curve) and the sine method (S, or gray curve). The data in Figure 12A were collected over a series of tests. In these tests, random noise was added to a set signal where both the sine fitting and lock-in detection methods were used to extract the phase. At each noise level, 100 tests were performed using random noise. The lock-in method performed the calculation in approximately half the time taken by the sine fitting method.

[0205] Figure 12B is a plot of the absolute phase difference |Δφ| versus the noise figure N for the lock-in method (L or black curve) and the sine method (S or gray curve). Figure 12B shows that both methods maintained approximately the same level of accuracy and precision across all tests.

[0206] The lock-in method eliminates the need to predict the sine parameter of the fit. The only fit performed is a cosine fit to a low-pass filtering constant, which is constrained so as to rarely produce a bad fit. However, when a sine fit is used, it has been found to work much more accurately when the sine wave fitted to the data has a constant period. If the period can be fitted along with other parameters, processing time increases and the results are often not as accurate.

[0207] Noise reduction in LSP measurements Extraction of the second 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, scattered light 216S is imaged as a function of the input polarization state of the initial LSP light beam 216 from the LSP light source 212. Imaging is achieved by recording in a digital detector 246 the intensity of the scattered light from features (e.g., particles, refractive index fluctuations) within the body CS substrate 10 that result from one or more IOX processes.

[0208] The recorded image 248 is processed by the system controller 400 to extract the intensity along the laser beam and analyze this intensity relative to the input polarization to extract the amount of optical retardation between the two orthogonal states of the light beam. The stress profile is reconstructed by modeling the observed retardation. As a result, the quality of the LSP-measured stress profile is fundamentally limited by noise in the imaging process, which is typically dominated by laser-based noise. One example of such laser-based noise is speckle, which arises from the high degree of coherence of the LSP light source 212 and optical surface imperfections (e.g., roughness, flatness) and volumetric properties of the optical elements (e.g., impurities, inhomogeneities, and density inhomogeneities).

[0209] During propagation of the LSP light beam 216 through the LSP subsystem 200, interactions of the light (laser) beam 216 with system imperfections result in random amplitude and phase fluctuations in the light beam wavefront. When the LSP light beam 116 is coherently imaged by Rayleigh scattering, the wavefront distortions lead to static interference patterns in the image plane called speckle patterns, which are characterized by large intensity fluctuations with high spatial frequencies superimposed on the desired signal. Intensity deviations from the desired signal are considered noise in the LSP measurement. To reduce the effects of laser speckle, imaging can be averaged across independent speckle patterns by modulation of polarization, amplitude, or phase in the beam wavefront.

[0210] In one embodiment, laser-based noise is reduced in the LSP subsystem 200 by passing the initial light (laser) beam 216 through a movable optical diffuser 222. In one example, the movable optical diffuser 222 can include a holographic diffuser. This "stirs" the light rays within a divergent angle depending on the local structure of the diffuser. To minimize the beam divergence caused by this "beam stirring," the optical diffuser 222 is positioned within the image plane of the Keplerian telescope configuration, as shown in the exemplary configuration of FIG. 4A. The LSP light beam 216 is first focused onto the optical diffuser 222 by a first focusing lens 220, and the transmitted light beam is recollimated by a second focusing lens 224.

[0211] Reducing the divergence of the LSP light beam 216 after undergoing optical diffusion provides a more efficient (i.e., lower aberration) focused LSP light beam 216F at the CS substrate. Using the movable optical diffuser 222, the fluctuations of the laser-based noise (e.g., speckle pattern) at the digital detector 246 can be measured with a rotational speed v D The maximum effect of noise averaging is D τ C >1 condition is met, and τ C is the exposure time of the digital detector 246. This condition also eliminates potential flickering in the imaging caused by light transmission variations across the optical diffuser 222. Implementing diffuser-based noise reduction improves the measurement of optical retardation. This is illustrated in FIGS. 13A and 13B, which are plots of optical retardation OR (radians) versus depth D (mm) into the CS substrate. The plot in FIG. 13A was obtained without using the noise reduction apparatus and method described above. The plot in FIG. 13B was obtained by using the noise reduction apparatus and method described above. The smoothness of the plot in FIG. 13B is a direct result of the application of the noise reduction apparatus and method disclosed herein.

[0212] Another way to reduce noise is by moving the substrate.

[0213] Shifting the OR plot using the inflection point and mid-plane of the CS substrate Because it may be difficult to determine the location of the surface 12 of the CS substrate 10 from the LSP image 248, the stress profile can be shifted to a position based on the general shape of the retardation curve (OR vs. D). The OR retardation curve has two inflection points where the derivative is zero. An exemplary actual OR vs. D curve is shown in FIG. 14A with two inflection points BP1 and BP2. 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.

[0214] If the stress profile is symmetric, the two inflection points BP1 and BP2 should also be symmetric about the mid-plane MP of the CS substrate (see FIG. 1A). Therefore, if the thickness TH of the CS substrate 10 is known and the two inflection points BP1 and BP2 of the optical retardation OR curve can be found, the OR profile can be shifted horizontally to the correct position. This allows for a more accurate determination of the compression depth DOC, since the location of the surface 12 of the CS substrate 10 is selected based on the known symmetry and thickness of the CS substrate. FIG. 14B shows an OR curve similar to FIG. 14A, but shifted to the left compared to FIG. 14A using the plot shift (data shift) technique described above.

[0215] Shifting OR plots using curve fitting An alternative method of extracting the DOC for a symmetric stress profile involves analyzing the shape of the retardation profile, i.e., the OR vs. D curve. If the thickness of the CS substrate, T, is known and the relative positions of the inflection points, BP1 and BP2, can be determined by polynomial fitting, then the compression depth, DOC, of ​​the CS substrate can be calculated using the following equation: DOC = [TH - (BP2 - BP1)] / 2 where BP1 and BP2 are the relative depth positions of the inflection points.

[0216] Curve fitting for OR vs. D curves One embodiment of the present disclosure is directed to methods for obtaining a good fit of an OR vs. D curve to data. These methods involve employing a combination of linear and quadratic functions to obtain the curve fit. This method is hereinafter referred to as the LinQuad method.

[0217] 15A is a plot of OR versus D data (circles) and shows an exemplary fit curve, F C (solid line), to 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 as defined below:

number

[0218] The corresponding retardation can be extracted and fitted to the raw data of interest to reconstruct the stress profile. Here, C represents the normalized modeled concentration of ions in the CS substrate. The equations for these modeled concentrations are:

number

[0219] An alternative formula is given by:

number

number

number

[0220] FIG. 15B is a plot of stress S(x)=σ(x) versus depth D (mm) (or x-coordinate) based on a LinQuad fit to the OR versus D curve of FIG. 14A.

[0221] Power-spike function The power-spike function is

number

[0222] Figure 16A is an OR vs. D plot illustrating an exemplary fit curve F C using a power-spike function. Figure 16B is a plot of the stress profile S(x) (MPa) vs. depth D into the CS substrate 10 based on a power-spike function fit to the OR vs. D curve of Figure 16A.

[0223] Elimination of systematic errors for shape-fitting symmetric stress profiles The stress profile of the CS substrate using LSP measurement data is obtained by distinguishing the OR vs. D curves. Therefore, a symmetric stress profile always corresponds to an asymmetric OR vs. D curve. However, systematic errors from various components within the LSP subsystem 200 can introduce a symmetric component into the OR vs. D retardation data, thereby preventing accurate extraction of the stress profile. This effect can be mitigated by decomposing the retardation data into symmetric and anti-symmetric components and fitting only the anti-symmetric part (i.e., the asymmetric data points).

[0224] Given the optical retardation OR in the form of a function f(x), the decomposition can be achieved as follows: f(x)=f s (x)+f a (x) In the formula, f s and f a are the symmetric and antisymmetric components of the retardation f, and are expressed by the following equations:

number

[0225] Figure 17A is a fit to an OR vs. D plot based on the original OR data, while Figure 17B is a fit to an OR vs. D plot where the symmetric component of the data has been removed using the techniques described above. The fit error of the fitted curve F C to the measured data in Figure 17B is 0.006, while the fit error in Figure 17A is approximately 0.46.

[0226] Adjustable fit area for accurate CT and DOC A single fit to the OR vs. D curve may not always be sufficient to accurately determine both the central tension CT and the compression depth DOC because scattering from the LSP coupling prism 42B or the coupling interface INT2 may interfere with data collection near the top surface 12 of the CS substrate 10.

[0227] In one example, a fit to the OR vs. D curve is performed using a fit to isolate the regions of the curve associated with central tension CT and compression depth DOC, respectively, and to adjust the fit range of the OR data for accurate CT and DOC extraction.

[0228] 18A and 18B show exemplary OR vs. D curves, where the region around inflection points BP1 and BP2 defined by the data (circles) is fitted to extract the compression depth DOC. FIG. 18B shows the central linear region between inflection points BP1 and BP2, which is fitted to extract the central tension Ct. In both cases, the range of OR vs. D data is substantially reduced to that portion of the OR vs. D curve associated with a given stress parameter.

[0229] Figures 19A-19D further illustrate the effect of data range selection (indicated by the vertical dashed lines) on fit quality. Figure 19A is an OR vs. D plot where the entire data range is considered and the fit curve does not fit the inflection points BP1 and BP2 too closely. Figure 19B is a corresponding plot of stress S(x) vs. D (depth) for Figure 19A, showing the compressive stress CT and compression depth DOC.

[0230] FIG. 19C is an OR vs. D plot similar to FIG. 19A, except that the first "end region" ER1 and second "end region" ER2 of the measured data are omitted because the data range has been reduced to the region between the vertical dashed lines. The fitted curve FC in FIG. 19C closely follows the inflection points BP1 and BP2. The corresponding S(x) vs. D plot is shown in FIG. 19C, and the values ​​of compressive stress CT and compression depth DOC are substantially different from those in FIG. 19B, which used the full range of data.

[0231] Optimized inflection point detection The embodiments also provide accurate detection of two inflection points BP1 and BP2 in the retardation curve. The inflection points occur where the retardation profile levels off, i.e., where the glass stress changes from compressive to tensile. These points are reported as depth of compression (DOC). Previously, the locations of these points would be roughly estimated, and the area around these estimates would be fit with a polynomial of some predefined order. While third- or fourth-order polynomials have been used in the past, it has been found that for some retardation profiles, these orders do not fit as well as other orders. To achieve an accurate fit to the two inflection points, a determination of the optimal polynomial order should be made, along with within what range the fit is best.

[0232] In the first step, the profile is smoothed to roughly estimate the locations of the two inflection points. As a non-limiting example shown in Figure 19E, a LOESS filter with 20% smoothing is used to smooth out most of the noise in the retardation profile.

[0233] Two steps are then performed on this smoothed profile. First, the derivative of the smoothed profile is calculated, as shown in FIG. 19F. The start and end edges of the retardation profile are considered to be the minimum points of the first derivative of the profile. This edge selection can be seen in FIG. 19F. These edge points are used to limit the polynomial fit from extending too far into the surface of the retardation profile where fireball damage is present. Second, the curvature of the smoothed profile is calculated. The curvature is calculated using the following formula:

number

[0234] Because the retardation profile has the highest curvature at the two inflection points, this calculated profile can be used to provide a rough estimate of BP1 and BP2. To calculate this rough estimate, the curvature profile is divided in half and a weighted average of the left and right sides is taken. This effectively determines the center of mass of the curvature for the left and right sides, which roughly correspond to the locations of the two inflection points, as shown in Figure 19G.

[0235] The profile is then vertically windowed to isolate the region within which to fit. To window the profile, the difference between the vertical positions of the coarse BP1 and BP2 points obtained by the two methods described above is taken. As shown in Figure 19H, a percentage (e.g., 15%) of this total vertical range from BP1 to BP2 is taken from the top and bottom, and it is within these regions that data is selected for polynomial fitting.

[0236] Now that the fitting regions for BP1 and BP2 have been determined, the best polynomial order for the fitting can be selected. It can be difficult to select a constant order that not only fits both the BP1 and BP2 regions (which may have slightly different shapes) but also fits every type of retardation profile that may be measured. If the polynomial order is too low, the fit may not match the data very well and may distort the measurement results. If the polynomial order is too high, the fit may track the noise too closely and may equally distort the measurement results. Therefore, the polynomial order should be optimized based on the data being fitted.

[0237] The polynomial optimization process works by fitting the input data with successive polynomial orders (starting with order 0, then order 1, etc.). For each order, the cost of the fit is calculated by the following formula:

number

number

[0238] Note that as the polynomial order increases, both the numerator and denominator of the cost equation decrease. Graphing this cost equation for the fit of the first knee region shows that the cost decreases rapidly over the first three orders, then plateaus around orders 4-5, as shown in Figure 19I. At these orders, increasing the polynomial order provides only a slight improvement in accuracy. Therefore, the algorithm selects either order 4 or 5 as the optimal order (5 can be selected because it has the smaller cost of the two). Performing the same analysis for the second knee region yields an optimal order of 10. These fits can be seen in Figure 19J. Once these fits are completed, the minimum of the BP1 fit and the maximum of the BP2 fit are considered the two knees of this retardation profile.

[0239] Thus, using this method, the inflection point region can be windowed and fitted in a way that is optimized for any profile being measured. This advance not only improves the detection of the two inflection points (reported as the profile's DOC parameter), but also improves the rest of the analysis by shifting, filtering, and fitting the retardation profile based on the location of these inflection points.

[0240] Considerations for simultaneous EPCS and LSP measurements One way to achieve good accuracy for depth of compression (DOC) measurements using the LSP subsystem 200 is to press the CS substrate 10 against a stop surface (e.g., support plenum 70) to ensure that the top surface 12 of the CS substrate 10 is flush with a predefined surface that can be assigned a depth of z=0. This pressing can be achieved by pressing the CS substrate 10 against a stop, or by applying a vacuum so that ambient atmospheric pressure provides the force to press the top surface 12 of the CS substrate 10 into place at z=0 (see, e.g., FIGS. 8A, 8B).

[0241] On the other hand, achieving a clear (i.e., 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 requires good CS substrate flatness in the EPCS measurement area, which can also be achieved with the use of a vacuum system 91.

[0242] Because the EPCS and LSP measurement areas are located at different locations on the CS substrate, applying a vacuum to the LSP measurement area can, in some cases, deform the CS substrate at the EPCS measurement area, resulting in suboptimal or very poor flatness or a significantly deformed surface at the EPCS measurement area. This results in an EPCS mode spectrum 160 that has poor contrast and is "out of focus." These conditions can lead to reduced accuracy and precision, as well as measurement failure, because the poor contrast can cause the system controller to be unable to identify some of the target features in the mode spectrum 160 used to perform stress calculations.

[0243] In an exemplary embodiment, the EPCS detector system 140 of the EPCS subsystem 100 utilizes adaptive focusing to enable proper alignment of the CS substrate 10 on the support plenum 70 for best (most accurate) DOC and near-surface stress measurements using the EPCS subsystem once the CS substrate is aligned for best LSP measurements for the LSP subsystem 200.

[0244] 20 , this is accomplished by making the focusing lens 142 of the EPCS detector system 140 adjustable, e.g., axially movable, by mounting the focusing lens on a translation stage 143, which in one example is operably connected to and controlled by a system controller 400. In one 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 modal spectrum 160 captured by the digital detector 150. In one example, the contrast of the modal spectrum 160 is improved to enhance target spectral features, such as TM fringes 163TM and TE fringes 163TE and critical angle transitions 166TM and 166TE.

[0245] The position of the axially movable focusing lens 142 can be electronically monitored by the system controller 400 to correct the EPCS subsystem calibration by taking into account the "optical path length" or OPL, e.g., the distance from the focusing lens 142 to the digital detector 150. In one embodiment, the consideration can be simplified so that the original calibration remains accurate as long as the OPL does not fall outside a predefined tolerance. In another embodiment, the calibration is corrected based on the OPL, and the surface stress S(0)=CS and / or depth of layer DOL are calculated based on the corrected calibration.

[0246] In another embodiment, focusing lens f1 has a variable effective focal length that is actively controlled by system controller 400 to acquire high-contrast mode spectrum 160 when the specimen is aligned to ensure maximum precision or accuracy of the compression depth DOC measurement of LSP subsystem 200. Variable focal length focusing lens 142 may comprise a compound lens (similar to a photographic multi-component lens having two or more optical elements) or may otherwise comprise an adaptive lens, such as a fluid-filled lens, in which changing the pressure of a fluid changes the shape of the lens, and therefore the focal length. When using variable focal length lens 142, shifting the position of focusing lens 142 may not be necessary, as changing the focal length may often be sufficient to compensate for deformation of specimen shape in the EPCS measurement area as a result of aligning the specimen for best measurement in the LSP measurement area.

[0247] In another embodiment, variation in the effective focal length of the focusing lens 142 may be enabled by an adaptive lens surface in the form of a mirror surface that may be combined with a fixed simple lens to produce a net effective focal length that can vary over a range sufficient to produce a high contrast mode spectrum 160, even when the alignment of the CS substrate is optimized relative to the LSP subsystem 200.

[0248] Because the deformation of the CS substrate 10 tends not to be very large, the change in the optical power of the variable-focus focusing lens 142 does not need to be particularly large to compensate. In one example, the focal length of the focusing lens 142 can change by up to 15%, or in another example, by up to 10%.

[0249] On the other hand, if the CS substrate 10 has a thickness of less than 0.6 mm, it may be necessary to change the optical power by more than 15%, and even by as much as 20% or even 25%. Thus, in one example, the adaptive system for changing the focal length of the focusing lens 142 is configured to change the focal length over a focal length range representing 25% of the average focal length, although in many cases a range of 20%, 15%, or even 10% of the average focal length may be sufficient.

[0250] Similarly, for measurements of flat CS substrates, since the focusing lens 142 system focuses to infinity, when the focusing lens 142 has a fixed focal length and the position of the focusing lens is adjusted axially to produce a high contrast mode spectrum 160, the range of axial positions accessible by the focusing lens will ideally represent about 25% of the focal length of the lens, although in some cases 20%, 15%, or even 10% of the focal length may represent a sufficient range of positions.

[0251] 21A and 21B are schematic diagrams of an exemplary embodiment in which two or more focusing lenses 142 with slightly different focal lengths are mounted on a support member 152 to define a focusing lens assembly 153. The support member 152 is movable 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. FIG. 21A shows an example in which the support member 152 is in the form of a rotatable wheel that is rotatable about a rotation axis AW. FIG. 21B shows an example in which the support member 152 is in the form of a linearly translatable support frame. Four exemplary focusing lenses 142 are shown. In general, the focusing lens assembly 153 can support two or more focusing lenses 142.

[0252] If the contrast of the features of interest in the mode spectrum 160 (e.g., TM and TE mode lines 163TM and 163TE, TM and TE critical angle transitions 166TM and 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 with a different focal length is moved into the path of the reflected light beam 116R, and a new mode spectrum 160 is captured by the EPCS digital detector 150 and analyzed for contrast. This process is repeated until a mode spectrum 160 with sufficient contrast is obtained.

[0253] In one example, the focal length differences of the focusing lenses 142 can be set by the desired range of total focal length coverage and the total number of lenses on the support member. In one example, there are six focusing lenses supported by the support member, and the focusing lenses cover a range of 20% to 30% of the average focal length of the entire set of focusing lenses, with focal length intervals of 3% to 7% of the average focal length.

[0254] In another example, the focal lengths are non-uniformly spaced such that the spacing between each pair of adjacent focal lengths is an approximately constant percentage of the average of the adjacent focal lengths, the percentage being between 2% and 20%, and more preferably between 3% and 10%.

[0255] In another related embodiment, some or all of the focusing lenses 142 include Fresnel lenses. In another embodiment, the focusing lenses 142 need not have different focal lengths, but may be positioned on the movable support member in such a way that when a selected focusing lens is positioned in the optical path, its distance from the digital detector 150 is different from that of the other focusing lenses. In this embodiment, obtaining a spectrum with sufficient contrast for the features of interest is ensured by the set of distances to the digital detector and / or available focal lengths, rather than necessarily by having an entire set of discrete, closely spaced, custom-selected focal lengths. 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 sharp image for a specific range of warp / curvature of the CS substrate 10.

[0256] In one example, the system controller 400 can be configured to select one of the focusing lenses 142 based on a measurement of the contrast of a feature of interest in the captured mode spectrum 160 .

[0257] In another embodiment, measurements can be made by using the two or three preferred mode spectra 160 with the best contrast among all captured mode spectra, and then the preferred result can be calculated as the average of the two or three preferred mode spectra. In one example, the preferred result can be calculated as a weighted average of the two or three preferred mode spectra. In a related example, the weight for each preferred spectrum can be proportional to the contrast of the feature of interest to the preferred mode spectrum.

[0258] Use of independent stress measurements for stress measurement calibration The EPCS subsystem 100 can be very adept at acquiring high contrast mode spectra 160 of CS substrates formed using an IOX process using Li-based glasses, e.g., K ions replace Li and / or Na ions from the glass in the near-surface region. This, in turn, allows for the measurement of knee stress CS by measuring birefringence based on the relative positions of the TM critical angle transition 166TM and the TE critical angle transition 166TE. k This allows for a very good measurement of the ion concentration (see Figure 3B).

[0259] On the other hand, knee stress CS k EPCS measurements of S(0) are usually less accurate than measurements of surface stress S(0). k The standard deviation of the measurement of the knee stress is usually a few percent of the average value of the knee stress, while the standard deviation of the surface stress S(0) is usually about 1% to 2% of the average value of the surface stress. In addition, the knee stress CS, which is simply obtained as the ratio of the detected critical angle birefringence B to the stress optical coefficient (SOC), k The value of is the knee stress Cs obtained from destructive RNF measurements of the stress profile k slightly different from the value of

[0260] Knee Stress CS k If the EPCS measurement appears to be less accurate than it could or should be, this may be due to systematic errors in the measurement of the critical angle birefringence, which may be caused by the TM and TE mode lines being too close to the TM and TE critical angle transitions 166 and 166TE, as well as by the particular shapes of the TM and TE refractive index profiles.

[0261] When performing quality control measurements, such systematic errors can be measured from a set of CS substrates formed using the same process, or destructive measurements on CS substrates taken from the same batch during the same process, with corresponding independent reference stress measurements to determine the knee stress of the CS. kThis can be mitigated by calibrating the EPCS-based measurement of the measurement. In one example, this is achieved by using a calibration multiplier K based on an independent measurement via the following relationship: cal This is accomplished by applying CS k (EPCS, calibrated) = K cal CS k (independent) In one example, the calibration multiplier K cal can be used as a general calibration factor for the stress profile calculated by the EPCS subsystem 100 via the following equation: S(EPCS, calibrated) = K cal ·S (original) where S(original) is the originally measured (uncalibrated) stress profile S(z).

[0262] Tension zone stress profile extraction The IOX process used to form the CS substrate 10 creates a compression zone that defines the NSWG 18. This compression zone extends into the substrate and reaches a zero value that defines the compression depth DOC, where the compression zone ends and the tension zone begins.

[0263] If the stress profile in the tension zone can be accurately extracted, it can serve as a powerful tool to help extract a substantially accurate representation of the stress profile in the compression zone. This can be done by utilizing a force balance of the stresses in the entire CS substrate 10 or half of the CS substrate (i.e., a so-called "half force balance").

[0264] In one embodiment, in addition to the area of ​​the stress profile in the tension zone (represented by the depth integral of the tensile stress from one compression depth to the opposite compression depth), reliable values ​​of the gradient of the stress profile at the depth of reliable gradient extraction are also obtained from the LSP-based measurements.

[0265] In one example, the depth of reliable gradient extraction can be the compressive depth DOC. In the compressive stress region, the surface compressive stress is determined by the EPCS method. In some cases, a portion of the compressive stress profile is also extracted from the IWKB, or the EPCS method using prior techniques such as a linear profile, a complementary error function-shaped profile, an exponential profile, or a LinQuad profile approximation, if there are not enough guided modes for reliable IWKB extraction. The EPCS-based method then extracts the surface stress value S(0) at the surface or at a deeper connection point (e.g., at the knee depth z k , see Figure 1B), and the surface portion of the stress profile S(z) up to the target connection point can be extracted from the EPCS measurement. In the latter case, due to the limitations of the EPCS measurement, the knee stress Cs k may not be specified with high precision.

[0266] Nevertheless, knee stress CS k This value of S(θ) can provide a sufficient starting point for pursuing extraction of the stress profile in the compression zone (e.g., substantially zones R1 and R2 in Figure 1B) by iterative refinement. In the first iteration, near-surface connection points with surface stress values ​​S(θ) are compared to deep connection points with reliably extracted stress gradients (e.g., knee stresses C(θ)) using a second-order polynomial. k or compression depth DOC). This means that the first connection point (e.g., knee depth z k Determine a first approximation of the stress profile in the compression zone, with a first part obtained from EPCS and a second part obtained via polynomial interpolation between two connection points, such that at the second connection point not only the surface stress S(0) but also the stress profile gradient coincides.

[0267] In certain examples, the second connection point may be, but need not be, the compression depth DOC. A first approximation of the stress profile S(z) is integrated. If the stress profile is asymmetric, EPCS measurements may be performed on both sides of the specimen, and a first approximation of the stress profile is obtained as described above for each side. If the stress profile S(z) is symmetric by design and implementation, the back side of the specimen may be assumed to have the same stress profile in the back compression region as in the front compression region.

[0268] A first approximation of the stress profile from both the front and back compression zones is integrated with respect to depth across each compression zone and compared to the depth integral of the tension across the entire tension zone. If the difference is greater in absolute value than a predefined tolerance limit, a correction step is performed to reduce the difference. In one example, the predefined tolerance limit is 5% of the tension zone stress area, but progressively better tolerance limits include 3%, 2%, 1%, and 0.5%.

[0269] The tolerance limit may be determined based on an estimate of the degree of accuracy of the extraction of the tension zone stress profile. In one embodiment, several first approximations of the stress profile are obtained by different methods, all of which are based on the knee stress Cs at the first connection point. k and the stress value and stress gradient, e.g., compression depth DOC, at the second connection point. Different types of first approximations can include second-, third-, and fourth-order polynomials, exponential profiles, complementary error function-shaped profiles, Gaussian profiles, and Lorentzian profiles. Then, for each of these first approximations, the difference between the stress area in the first approximation compression zone and the stress area in the tension zone extracted using the LSP-based measurement is found. A linear combination of these first approximation stress profiles is then found, such that the stress area of ​​the linearly combined stress profile is equal to the tension zone stress area.

[0270] In another embodiment, the knee stress C.S. kThe limited accuracy of EPCS-based measurements of knee stress CS k The range of knee stress CS k This is taken into account by allowing targeting near the initial EPCS-based estimate of . In the first approximation of the compressive stress portion of the stress profile, the connection is made by using a preferred target shape function for the interpolated region of the compression zone to estimate the knee stress CS. k In one example, the preferred target shape is a quadratic polynomial.

[0271] After each iteration, the stress area of ​​the compressive stress profile from the two combined compression zones (one on each side of the specimen) is subtracted from the stress area of ​​the tension zone. If the difference is greater in absolute value than the target predefined tolerance limit, the knee stress CS k The target value of CS of the knee was determined according to the estimated accuracy of knee stress measurements available from the EPCS-based method. k can be varied within a predefined tolerance of

[0272] In one example, the estimated knee stress precision is about 10 MPa, but in some cases the estimated knee stress precision is better at 7 MPa or 5 MPa or 3 MPa. In the absence of surface spikes and no guided modes available, the same technique can be used to connect a target surface stress S(0) that is allowed to vary over a range determined by the precision of the surface stress measurement.

[0273] In one example, the target surface stress S(0) or knee stress CS k The range of acceptable values ​​for can be up to six standard deviations wide, for example, three standard deviations wide on either side of the measured surface or knee stress. In one embodiment, the target surface value S(0) does not need to be iteratively varied, but can be determined by algebraic calculations utilizing the measured difference in area between the first approximate stress profile and the tension zone stress profile and a preferred function form selected for the interpolated portion of the compressive stress region.

[0274] It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the present disclosure described herein can be made without departing from the spirit or scope of the present disclosure as defined in the appended claims. Accordingly, the present disclosure covers these modifications and variations insofar as they come within the scope of the appended claims and their equivalents.

Claims

1. 1. 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, comprising: an LSP light source system; an LSP light source actuator coupled to the LSP light source system, the LSP light source actuator operable to rotate an orientation of the LSP light source system; an optical compensator in an optical path of the 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 said half-wave plate; an optical compensator comprising 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 focusing lens and a focusing lens actuator operable to translate the focusing lens along the optical path; 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 operatively support the LSP coupling prism.

2. 10. The LSP subsystem of claim 1, further comprising an evanescent 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. 3. The LSP subsystem of claim 1, wherein the half-wave plate actuator automatically rotates the half-wave plate to adjust the LSP laser beam to a predetermined beam intensity.

4. 10. An LSP subsystem according to any one of the preceding claims, wherein the translatable diffuser actuator automatically adjusts the position of the diffuser.

5. 1. A method for measuring first and second stress characteristics of a chemically strengthened (CS) substrate having a surface, a near-surface waveguide proximate the surface, and a back surface opposite the surface, comprising: applying at least one ink line to the back surface of the CS substrate; operatively disposing the surface of the CS substrate relative to a coupling prism assembly at a measurement location, the coupling prism assembly comprising an evanescent prism coupling spectroscopy (EPCS) coupling prism and a light scattering polarimetry (LSP) coupling prism for defining adjacent EPCS and LSP coupling interfaces, respectively; performing an EPCS measurement of the CS substrate using the EPCS bonding interface to obtain the first stress characteristic and an LSP measurement of the CS substrate using the LSP bonding interface to obtain the second stress characteristic without removing either the bonding prism assembly or the CS substrate from the measurement location; and combining the first stress characteristic and the second stress characteristic to define a complete stress characteristic of the CS substrate, the first stress characteristic comprising a surface compressive stress S(0), a total depth of layer DOLT, a spike depth of layer DOL ｓｐ , knee stress CS ｋ and birefringence B, and said second stress characteristic comprises at least one of a compression depth DOC and a central tension CT.

6. performing the LSP measurement, forming an LSP image on a digital detector to define a raw digital LSP image; performing a Gaussian blur of the raw digital LSP image to form a Gaussian blurred LSP image; performing Otsu thresholding on the Gaussian blurred image to define a threshold image; and using the threshold image to calculate optical retardation versus depth into the CS substrate to obtain the second stress characteristic.

7. performing the LSP measurement, forming an LSP image on a digital detector to define a raw digital LSP image; performing a Gaussian blur of the raw digital LSP image to form a Gaussian blurred LSP image; performing a binarization method on the Gaussian blurred LSP image to define image contours; and using the image contour to calculate optical retardation versus depth into the CS substrate to obtain the second stress characteristic.

8. performing the LSP measurement, forming an LSP image on a digital detector; processing the LSP image to generate an optical retardation (OR) vs. depth (D) curve containing OR data points; and employing a combination of linear and quadratic functions to obtain a curve fit to the OR data points.

9. performing the LSP measurement, forming an LSP image on a digital detector; processing the LSP image to generate an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points including a first inflection point and a second inflection point; i) employing a power-spike function to obtain a curve fit to the OR data points; and ii) shifting the OR data points such that the first inflection point and the second inflection point are symmetric about the mid-plane of the CS substrate, thereby defining shifted OR data points; and employing the shifted OR data points to perform at least one of the following operations: obtaining a measurement of a compression depth DOC of the CS substrate.

10. forming an LSP image on a digital detector; processing the LSP image to generate an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points defining first and second inflection points and first and second end regions; i) first and second curve fits of the OR data points defining the first and second inflection points, respectively, to determine a central tension CT of the CS substrate; ii) curve fitting the OR data points between the first inflection point and the second inflection point to determine the compression depth DOC of the CS substrate; and 10. The method of claim 5, further comprising: performing at least one of: iii) performing a curve fit on the OR data points over a span excluding the first end region and the second end region to define an OR fit curve; and using the OR fit curve to determine at least one of a central tension CT and a compression depth DOC of the CS substrate.

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 an upper and lower window of the OR vs. D curve.

13. the upper window and the lower window determining a first derivative curve of said OR vs. D curve; selecting a start edge and an end edge of the retardation profile corresponding to a first minimum and a second minimum of the first derivative curve, respectively, wherein the first region has a depth equal to or less than the start edge and the second region has a depth equal to or greater than the end edge; determining the curvature of the OR vs. D curve; determining a center of mass of a left side of the curvature defining the first inflection point and a right side of the curvature defining the second inflection point; The upper window is a maximum line defined by the maximum retardation of the OR vs. D curve; a lower line defined by a percentage of the maximum retardation of the OR vs. D curve; the starting edge; the end edge, the lower window: a minimum line defined by the minimum retardation of the OR vs. D curve; an upper line defined by a percentage of the maximum retardation of the OR vs. D curve; the starting edge; The method of claim 12 , wherein the end edge is defined by:

14. The method of 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 LSP image comprises a plurality of bright points, and the identification of the entry points and the exit points is performed by: filtering a subset of bright spots of the plurality of bright spots that are outside above and below 50% of the thickness of the CS substrate and outside ±10 degrees around the angle of laser propagation; creating a plurality of light point combinations of the remaining set of light points of the plurality of light points; Scoring each bright point combination of the plurality of bright point combinations for proximity to the thickness of the CS substrate, proximity to the angle of laser propagation, and median light intensity between each bright point of the bright point combination; normalizing the scores of the bright spot combinations; For each bright spot combination, summing normalized scores for proximity to the thickness of the CS substrate, proximity to the angle of laser propagation, and the median light intensity between each bright spot of the bright spot combination; and selecting a selected light point combination having the highest total score among the light point combinations as the entry point and the exit point.

16. 1. A method for processing a light scattering polarimetry (LSP) image, comprising: forming an LSP image of the chemically strengthened (CS) substrate on a digital detector; processing the LSP image to generate an optical retardation (OR) vs. depth (D) curve of a retardation profile comprising OR data points defining first and second inflection points and first and second end regions; The upper and lower windows of the OR vs. D curve are determining a first derivative curve of said OR vs. D curve; selecting a start edge and an end edge of the retardation profile corresponding to a first minimum and a second minimum of the first derivative curve, respectively, wherein the first region has a depth equal to or less than the start edge and the second region has a depth equal to or greater than the end edge; determining the curvature of the OR vs. D curve; determining a center of mass of a left side of the curvature defining the first inflection point and a right side of the curvature defining the second inflection point; The upper window is a maximum line defined by the maximum retardation of the OR vs. D curve; a lower line defined by a percentage of the maximum retardation of the OR vs. D curve; the starting edge; the end edge, the lower window: a minimum line defined by the minimum retardation of the OR vs. D curve; an upper line defined by a percentage of the maximum retardation of the OR vs. D curve; the starting edge; the ending edge.

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

18. 20. 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 LSP image comprises a plurality of bright points, and the identification of the entry points and the exit points is performed by: filtering a subset of bright spots of the plurality of bright spots that are outside above and below 50% of the thickness of the CS substrate and outside ±10 degrees around the angle of laser propagation; creating a plurality of light point combinations of the remaining set of light points of the plurality of light points; Scoring each bright point combination of the plurality of bright point combinations for proximity to the thickness of the CS substrate, proximity to the angle of laser propagation, and median light intensity between each bright point of the bright point combination; normalizing the scores of the bright spot combinations; For each bright spot combination, summing normalized scores for proximity to the thickness of the CS substrate, proximity to the angle of laser propagation, and the median light intensity between each bright spot of the bright spot combination; and selecting a selected light point combination having the highest total score among the light point combinations as the entry point and the exit point.

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