Rapid measurement method for characterization of transparent substrate films based on angle-resolved ellipsometry

Abstract

The invention discloses a rapid measurement method for characterizing a transparent substrate film based on angle-resolved ellipsometry. Firstly, an incoherent partial superposition mathematical model (IPS) including back reflection of a transparent substrate is established to accurately describe the optical superposition effect of the substrate front reflection and the unknown back reflection; an angle-resolved ellipsometry measuring device (ARE) and a corresponding ellipsometric parameter angular spectrum (ARS) rapid acquisition method are established (or purchased); an ARE measurement reference image and its IPS model back reflection correction function are calibrated using a standard transparent plate of known thickness; finally, the ARS of a transparent substrate film sample is rapidly measured using a calibrated IPS‑ARE system, and then the attribute parameters such as the refractive index, thickness and substrate thickness of the film sample are fitted and calculated to characterize; the invention solves the problems of complex optical modeling, complex measurement operation and significant measurement error of transparent substrate films in conventional ellipsometry measurement methods, and can be applied to online inspection of mass production of advanced electronic products such as LED panels and packaging films.

Description

Technical Field

[0001] The present invention belongs to the technical field related to ellipsometry measurement, and in particular relates to a rapid measurement method for transparent substrate film characterization based on angle-resolved ellipsometry. Background Art

[0002] With the in-depth development of flexible electronics in advanced display and intelligent sensing, transparent materials represented by polymers and glass are widely used as substrates for thin film devices due to their excellent and stable surface quality, mechanical properties and physical properties. These substrates not only provide mechanical support for the attached thin film to protect it from physical and chemical damage, but also directly affect the device performance. For example, polymer substrates with high water / oxygen permeability and mechanical deformability are widely used in wearable devices to meet the requirements of high permeability and flexibility. In addition, the substrate will also affect the preparation process of thin film devices. For example, flexible transparent substrates suitable for roll-to-roll rapid manufacturing can achieve low-cost manufacturing of thin film devices. However, the thin film preparation process is inevitably affected by the processing environment, material properties, etc. For example, the different thermal expansion coefficients between the film and the substrate will produce residual stress and cause the thickness of the deposited film to change, which will further lead to changes in the performance of the final thin film device. Therefore, monitoring of film parameters, especially the changes in its parameters during processing, is crucial to thin film manufacturing.

[0003] Optical measurement methods have the advantages of being non-destructive, high-precision, and high-speed, and are the mainstream measurement methods in thin film measurement. Among them, transmission-type measurement methods, such as transmission ellipsometry and transmission spectroscopy, have good sensitivity to signals from the substrate, while the relatively low sensitivity to signals before the substrate makes these methods more suitable for bulk material measurements. Reflection-based measurement methods, such as spectral reflectance and spectral ellipsometry, are more suitable for thin film parameter measurements. Among them, ellipsometry determines thin film parameters by measuring the change in the polarization state of the sample reflection signal, which has higher accuracy and robustness and is the main method for thin film measurement.

[0004] However, transparent substrates pose challenges to these reflection-based thin film measurement methods. The reflected signal from the back of the substrate is mixed with the signal reflected from the front of the substrate and received by the detector, and cannot be separated, which makes optical modeling and data analysis during the measurement process complicated and difficult, resulting in reduced measurement accuracy and efficiency. The impact of transparent substrates is mainly reflected in two points: since the optical thickness of the substrate usually exceeds the optical coherence condition, the reflected signal from the back of the substrate and the reflected signal on the substrate experience incoherent depolarization; there is an off-axis offset between the back-reflected light of the substrate and the reflected light on the substrate caused by the incident angle. This off-axis offset causes only the part of the back-reflected signal that overlaps with the reflected signal on the substrate to be received by the detector. The incoherent overlap caused by the transparent substrate is affected by the incident angle, system aperture, and the thickness and optical refractive index of the substrate.

[0005] Existing methods usually use physical means to reduce the back reflection effect of transparent substrates, including polishing the back of the substrate to scatter the back reflection signal, processing the substrate into a wedge shape to emit the back reflection signal out of the system, or absorbing the back signal through a matching medium. Although these experimental methods can reduce the back reflection effect, they are all contact lossy measurements and the measurement efficiency is insufficient. Accurate optical modeling to describe the substrate back reflection effect is expected to achieve rapid non-destructive measurement of transparent substrate (substrate) films. However, existing optical modeling methods, such as the modeling method based on the Mueller matrix, can solve the depolarization problem caused by incoherence, but cannot describe the problem caused by partial superposition.

[0006] Therefore, there is an urgent need to establish an optical model that accurately describes the substrate back reflection effect so that rapid and non-destructive measurement of transparent base films can be achieved. Summary of the invention

[0007] The purpose of the present invention is to provide a rapid measurement method for characterizing transparent substrate films based on angle-resolved ellipsometry in order to address the deficiencies in the prior art. The method is used to solve the problem that back reflection of transparent substrate films is difficult to model. By establishing an incoherent part superposition model that accurately describes the substrate back reflection effect, an angle-resolved ellipsometry measurement device is used to achieve rapid and non-destructive measurement of transparent substrate films.

[0008] To achieve the above object, the present invention provides a rapid measurement method for transparent substrate film characterization based on angle-resolved ellipsometry, comprising the following steps:

[0009] S1. Establish an incoherent partial superposition model (IPS) including back reflection to correct the influence of back reflection of transparent substrate;

[0010] S2. Establishing an angle-resolved ellipsometric measurement device (ARE) and a corresponding angle-resolved ellipsometric parameter spectrum (ARS) rapid acquisition method;

[0011] S3, using a standard transparent plate with known thickness to calibrate the ARE measurement reference image and the IPS back reflection correction function;

[0012] S4. Use the calibrated IPS-ARE system to measure the ARS of the transparent substrate film, and then fit and characterize the optical and geometric parameters of the film and its substrate thickness.

[0013] As a further improvement of the above scheme, the overlapping area between the back-reflected light beam and the front-reflected light beam of the transparent substrate in step S1 varies with the incident angle, system aperture, substrate thickness and refractive index, and a coherent matrix is ​​used to establish an IPS model including back reflection: R t =R 012 +f h T 012 R 20 (Ifh R 210 R 20 ) -1 T 210

[0014] Where: R 012 ,T 012 ,R 210 ,T 210 and R 20 are the coherent matrices of the thin film transmittance and reflection coefficients on the substrate and the boundary reflection coefficients on the back of the substrate; I is the unit matrix; f h It is a correction function that describes the influence of the partial superposition effect of back-reflected light on the incident angle, system aperture, substrate thickness and refractive index.

[0015] As a further improvement of the above solution, step S2 specifically includes:

[0016] The S21, ARE device consists of a polarization generator, a non-polarization beam splitter, an objective lens, a polarization analyzer, an imaging lens group, and an area array camera.

[0017] Wherein: the imaging lens group is composed of a first lens and a second lens, and its front and rear focal planes coincide with the back focal plane (BFP) of the objective lens and the plane of the area array camera respectively;

[0018] The polarized light modulated by the polarization generator enters the sample at different incident angles through the objective lens and is reflected. The light beams at different incident angles converge at different spatial positions of the objective lens BFP;

[0019] The BFP image of the objective lens is modulated by the polarization generator and formed on the area array camera by the imaging lens group;

[0020] S22. Calculate the BFP reflectivity image of the film to be tested with reference to the BFP image of the standard sample, further perform Fourier transform on the reflectivity image, and quickly extract the Mueller matrix element ARS of the film to be tested.

[0021] As a further improvement of the above solution, step S3 specifically includes:

[0022] Using a transparent flat plate sample with known thickness, obtain the BFP reference image of the ARE device, calculate the angular frequency of the correction function of the IPS model of the sample with the corresponding thickness, and use polynomial fitting to establish the mathematical relationship between the angular frequency of the correction function and the substrate thickness to achieve the calibration of the back reflection correction function of the IPS model. As a further improvement of the above scheme, the step S4 specifically includes:

[0023] The ARE device is used to quickly obtain the BFP reflectivity image of the transparent substrate film and quickly obtain the ellipsometric parameter M of the sample. E , and compared it with the theoretical ellipsometric parameter M of the transparent substrate film based on the IPS modelS Matching, obtain the film optical and geometric parameters and substrate thickness.

[0024] As a further improvement of the above scheme, the nonlinear regression algorithm is used to match the M of the IPS model. S and the measured M E , directly calculate the optical and geometric parameters of transparent substrate films and substrate thickness.

[0025] As a further improvement of the above scheme, the optical and geometric parameters of the thin film on the substrate and the angular frequency of the correction function are calculated, and then the substrate thickness is obtained from the linear relationship between the angular frequency and the substrate thickness.

[0026] In general, the present invention proposes a rapid measurement method for transparent substrate film characterization based on angle-resolved ellipsometry, which solves the problem that it is difficult to eliminate the back reflection effect of transparent substrate film in the prior art. By establishing an incoherent part superposition model that accurately describes the substrate back reflection effect, the angle-resolved ellipsometry measurement device is used to achieve rapid non-destructive measurement and characterization of transparent substrate film. The method has the following beneficial effects:

[0027] 1. The present invention uses the change in the overlapping area of ​​the front and rear reflected light beams of the substrate within the system aperture to establish a mathematical relationship between the back reflection signal of the transparent substrate and the change of the incident angle, system aperture, substrate thickness and refractive index, and accurately describes the superposition effect of the back reflection part;

[0028] 2. The present invention further uses a correction function to correct the change of back reflection with the incident angle, establishes a mathematical relationship between the function angular frequency and the substrate thickness, and can realize the rapid calculation of the substrate thickness;

[0029] 3. The present invention uses the incoherent light superposition based on the coherent matrix and the correction function to correct the back reflected light, and establishes the transparent substrate back reflection incoherent part superposition (IPS) model, which can realize the accurate optical modeling of the transparent substrate film;

[0030] 4. The present invention utilizes the advantages of single-frame snapshot measurement of angle-resolved ellipsometry and can realize rapid calculation of optical and geometric parameters of thin films on transparent substrates and substrate thickness based on the IPS model. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0032] Figure 1It is a schematic diagram of the measurement process of a rapid measurement method for characterizing a transparent substrate film based on angle-resolved ellipsometry provided in accordance with a preferred embodiment of the present invention;

[0033] Figure 2 It is a schematic diagram of the structure of an incoherent part superposition model describing the back reflection effect of a transparent substrate established according to a preferred embodiment of the present invention;

[0034] Figure 3 is a schematic diagram of an angle-resolved ellipsometric measurement device constructed according to a preferred embodiment of the present invention;

[0035] Figure 4 It is a schematic diagram of measuring the reflectivity of the back focal plane of a transparent substrate film by an angle-resolved ellipsometric system constructed according to a preferred embodiment of the present invention;

[0036] Figure 5 It is a schematic diagram of the relationship between the angular frequency of the transparent substrate back reflection correction function and the substrate thickness established according to the preferred embodiment of the present invention;

[0037] Figure 6 It is a single-frame back focal plane image of a single-layer film on transparent glass constructed according to a preferred embodiment of the present invention.

[0038] Figure 7 It is a diagram showing the matching results of the IPS model constructed according to the preferred embodiment of the present invention and the angle-resolved Mueller matrix parameters of the single-layer film on the glass obtained by measurement.

[0039] Throughout the drawings, the same reference numerals are used to denote the same elements or structures, wherein:

[0040] 1-polarization generator, 2-non-polarization beam splitter, 3-first lens, 4-rear focal plane, 5-objective lens, 6-sample to be measured, 7-polarization analyzer, 8-second lens, 9-area array camera. DETAILED DESCRIPTION

[0041] In order to make the purpose, technical solution and advantages of the embodiment of the present invention clearer, the technical solution of the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings of the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all of the embodiments. Based on the described embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field belong to the scope of protection of the present invention.

[0042] It is difficult to eliminate the problem of back reflection effect of transparent substrate films with existing technologies. It is urgent to establish an optical model that accurately describes the substrate back reflection effect so as to realize rapid and non-destructive measurement of transparent substrate (substrate) films.

[0043] Therefore, an embodiment of the present invention provides a rapid measurement method for characterizing transparent substrate films based on angle-resolved ellipsometry. The method is used to solve the problem that back reflection of transparent substrate films is difficult to model. By establishing an incoherent part superposition model that accurately describes the substrate back reflection effect, an angle-resolved ellipsometry measurement device is used to achieve rapid and non-destructive measurement of transparent substrate (substrate) films.

[0044] like Figure 1 As shown, a rapid measurement method for transparent substrate film characterization based on angle-resolved ellipsometry includes the following steps:

[0045] S1. Establish an incoherent part superposition model including back reflection to correct the unknown influence of transparent substrate back reflection;

[0046] When the incident light is θ 0 When the reflected light is incident on a transparent substrate film sample with a substrate thickness of h, it will undergo multiple transflections in the substrate and finally exit the sample surface. There will be a geometric offset Z between the back-reflected light that has undergone the jth reflection in the substrate and the reflected light beam on the substrate. j =2hjtanθ s , where sinθ s =n 0 sinθ 0 / n s ,θ s is the refraction angle into the substrate, n 0 ,n s The refractive index of the ambient medium and substrate respectively; under this offset, the light intensity of the front and rear reflected light beams of the substrate entering the system aperture is different. Taking the reflected light beam on the substrate as the reference, only the overlapped part of the back reflected light of the substrate and the upper reflected light is detected and received. The overlapping area can be divided into two j The overlapping area of ​​the circles is calculated, which is equal to the system aperture area A b The ratio is F z :

[0047]

[0048] Where b is the beam radius, F z It can be further simplified to approximate a cosine function:

[0049] f h =cosωθ 0

[0050] Where ω is the corrected angular frequency, which can be obtained by using f h Describe the partial superposition effect of back reflections.

[0051] like Figure 2As shown, in this embodiment, it is assumed that the film is a single-layer structure, and the film transfer matrix or Fresnel formula can be used to establish the Jones matrix r that does not include the back reflection of the substrate. 012 ,t 012 ,r 210 ,t 210 and the Jones matrix r of the back-side boundary reflection coefficient of the substrate 20 , these Jones matrices are passed through ( The Kronecker product is converted into a coherent matrix, thereby realizing the direct calculation of incoherent superposition. The mixed signal of the substrate front and back reflections is calculated by multi-beam superposition and each back reflection signal is calculated by using f h Correction is made to establish the incoherent part superposition reflection model R of the mixed reflection signal t , and then further transformed into the Mueller matrix M S :

[0052] R t =R 012 +f h T 012 R 20 (If h R 210 R 20 ) -1 T 210

[0053]

[0054] In the formula, R 012 ,T 012 ,R 210 ,T 210 and R 20 They are the incoherent matrices of the thin film transmittance and reflection coefficients on the substrate and the boundary reflection coefficients on the back side of the substrate, respectively.

[0055] S2. Establish or purchase an angle-resolved ellipsometric snapshot measuring device and a corresponding angle-resolved ellipsometric spectrum rapid acquisition method;

[0056] like Figure 3 As shown in the schematic diagram of the angle-resolved ellipsometric measurement device constructed in an embodiment of the present invention, the angle-resolved ellipsometric measurement device includes a polarization generator 1, a non-polarizing beam splitter 2, an objective lens 5, a polarization analyzer 7, an imaging lens group and an area array camera 9. The imaging lens group consists of a first lens 3 and a second lens 8, and its front and rear focal planes coincide with the back focal plane (BFP) of the objective lens 5 and the area array camera 9, respectively.

[0057] It should be noted that, in the present embodiment, the polarization generator 1 and the polarization analyzer 7 are both composed of linear polarizers. Preferably, a Glan Taylor polarizer is selected as the linear polarizer to obtain high-purity polarization control, preferably, they are set to 0° and 45° respectively; the non-polarizing beam splitter 2 selects a beam splitter with a 50:50 splitting ratio; the objective lens 5 adopts a high NA microscope objective lens with NA=0.9 to collect signals in a wide angle range.

[0058] In this embodiment, the linearly polarized light S modulated by the polarization generator 1 P Through the objective lens 5, different incident angles θ, azimuth angles The beams with different incident angles converge at different spatial positions of the BFP of the objective lens 5, and the angle θ and the BFP radius r a The mapping relationship satisfies r a =fsinθ, f is the focal length of the objective lens 5; the BFP image of the sample is obtained by the imaging lens group and the polarization analyzer 7Γ A The modulated imaging is on the area array camera 9, and the normalized light intensity is expressed as:

[0059]

[0060] Where R is the rotation matrix, M T is the reflection Mueller matrix, M S is the Mueller matrix of the sample 6 to be tested, which is further expanded as follows:

[0061]

[0062] In the formula, m 12 ,m 21 ,m 33 is the element in the Mueller matrix of the sample 6 to be tested.

[0063] like Figure 4 As shown in the figure, a single-frame BFP image is collected, and a known sample, such as bare silicon, is used as a reference sample to calculate the BFP reflectivity map. Then, the coordinate system of the BFP reflectivity map is expanded along the azimuth angle into radius-azimuth polar coordinates with the center of the image as the origin. The fast Fourier transform method is used to quickly extract the angular resolution Mueller matrix element m of the sample to be tested 6. 12 ,m 21 ,m 33 .

[0064] S3, using a standard transparent plate with known thickness to calibrate the back reflection correction function of the angle-resolved ellipsometry reference image and its incoherent part superposition model;

[0065] In this embodiment, an angularly resolved ellipsometer is used to measure the angularly resolved Mueller matrix parameters of a transparent glass plate with a known thickness in the range of [0-250] μm, and the angular frequency of the correction function in the IPS model of the corresponding sample is calculated. The linear relationship between the substrate thickness and the angular frequency of the correction function is obtained from the several sets of thickness and angular frequency calculations.

[0066] It should be noted that the least squares fitting method is used in this embodiment to calculate the correction function f in the IPS model. h The angular frequency ω and the linear relationship between substrate thickness and angular frequency ω=0.0062×h+0.0023, such as Figure 5 shown.

[0067] S4. Use the calibrated IPS-ARE system to measure the ARS of the transparent substrate film, and then fit and characterize the optical and geometric parameters of the film and its substrate thickness.

[0068] In this embodiment, an angle-resolved ellipsometric measurement device is used to snapshot-measure the BFP light intensity image of a single-layer ITO film on a glass substrate with a thickness of 300 μm, and obtain a single-frame BFP reflectivity image of the transparent substrate film, such as Figure 6 As shown; Fast Fourier transform calculation to obtain the angle-resolved Mueller matrix parameters m 12 ,m 33 ; Theoretical Mueller matrix parameters of transparent substrate films established by matching the IPS model and measure the angle-resolved Mueller matrix parameters Extract optical and geometric parameters of thin films on substrates as well as substrate thickness.

[0069] More preferably, if Figure 7 As shown in Figure 2, the nonlinear regression algorithm is used to fit the IPS model. and measured Calculate the optical and geometrical parameters of the thin film on the substrate and f h The angular frequency of the film includes the film thickness d, the refractive index n and the angular frequency ω:

[0070]

[0071] Where N is the number of angle dimension data obtained in the experiment, M is the number of parameters to be solved, and σ is the standard deviation of the measured parameters. is the film parameter to be measured and f h is a vector of angular frequencies, Ω is the value range of the parameter vector, are the final calculated film parameters and f h The angular frequency vector value of .

[0072] By hThe angular frequency ω and the linear relationship between the angular frequency and the substrate thickness ω=0.0062×h+0.0023, and preferably, the thickness of the transparent substrate can be directly calculated.

[0073] In summary, the present invention proposes an angle-resolved ellipsometry measurement method for rapid measurement and characterization of transparent substrate (substrate) films. The method is used to solve the problem that back reflection of transparent substrate films is difficult to model. By establishing an incoherent part superposition model that accurately describes the substrate back reflection effect, angle-resolved ellipsometry is used to achieve rapid and non-destructive measurement of transparent substrate (substrate) films.

[0074] It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above and that the invention can be implemented in other specific forms without departing from the spirit or essential features of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations falling within the meaning and scope of the equivalent elements of the claims be included in the invention. Any reference numeral in a claim should not be considered as limiting the claim to which it relates.

[0075] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

[0076] The above-described embodiments are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A rapid measurement method for transparent substrate film characterization based on angle-resolved ellipsometry, characterized in that: The following steps are involved: S1. Establish an incoherent part superposition model (IPS) including back reflection to correct the influence of transparent substrate back reflection. The incoherent part superposition model (IPS) is: R t =R 012 +f h T 012 R 20 (I-f h R 210 R 20 ) -1 T 210 ; Where: R 012 ,T 012 ,R 210 ,T 210 and R 20 are the coherent matrices of the thin film transmittance and reflection coefficients on the substrate and the boundary reflection coefficients on the back of the substrate; I is the unit matrix; f h It is a correction function that describes the influence of the partial superposition effect of back-reflected light on the incident angle, system aperture, substrate thickness and refractive index; S2. Establishing an angle-resolved ellipsometric measurement device (ARE) and a corresponding angle-resolved ellipsometric parameter spectrum (ARS) rapid acquisition method, the specific steps of which are as follows: The S21, ARE device consists of a polarization generator, a non-polarization beam splitter, an objective lens, a polarization analyzer, an imaging lens group, and an area array camera. Wherein: the imaging lens group is composed of a first lens and a second lens, and its front and rear focal planes coincide with the back focal plane (BFP) of the objective lens and the plane of the area array camera respectively; The polarized light modulated by the polarization generator enters the sample at different incident angles through the objective lens and is reflected. The light beams at different incident angles converge at different spatial positions of the objective lens BFP; The BFP image of the objective lens is modulated by the polarization generator and formed on the area array camera by the imaging lens group; S22, using the BFP image of the standard sample as a reference to calculate the BFP reflectivity image of the film to be tested, further performing Fourier transform on the reflectivity image, and quickly extracting the Mueller matrix element ARS of the film to be tested; S3, using a standard transparent plate with known thickness to calibrate the ARE measurement reference image and the IPS back reflection correction function, that is, using a transparent plate sample with known thickness to obtain the BFP reference image of the ARE device, and calculate the angular frequency of the correction function of the IPS model of the sample with the corresponding thickness, and use polynomial fitting to establish the mathematical relationship between the angular frequency of the correction function and the substrate thickness, so as to realize the calibration of the back reflection correction function of the IPS model; S4. Use the calibrated IPS-ARE system to measure the ARS of the transparent substrate film, and then fit and characterize the optical and geometric parameters of the film and its substrate thickness. Use the ARE device to quickly obtain the BFP reflectivity image of the transparent substrate film and quickly obtain the ellipsometric parameter M of the sample. E , and compared it with the theoretical ellipsometric parameter M of the transparent substrate film based on the IPS model S The optical and geometric parameters of the film and the substrate thickness are obtained, and then the M of the IPS model is matched using a nonlinear regression algorithm. S and the measured M E , directly calculate the optical and geometric parameters of the transparent substrate film and the substrate thickness, and finally calculate the optical and geometric parameters of the film on the substrate and the angular frequency of the correction function, and then obtain the substrate thickness from the linear relationship between the angular frequency and the substrate thickness.

2. The rapid measurement method for transparent substrate film characterization based on angle-resolved ellipsometry according to claim 1, characterized in that: In the step S1, the overlapping area between the back-reflected light beam and the front-reflected light beam of the transparent substrate varies with the incident angle, system aperture, substrate thickness and refractive index, and a coherent matrix is ​​used to establish an IPS model including back-reflection.

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