CN-115077873-B - System and method for testing gratings

Abstract

The present disclosure relates to systems and methods for testing gratings. Gratings, such as those used in waveguide displays, may have a large aspect ratio. For example, grating characteristics (e.g., period, feature size, etc.) may be much smaller than the grating region. The variation of the grating characteristics over the grating area may look like a secondary grating with a long grating period superimposed on the primary grating for which the grating is designed. Because the variations that result in the secondary grating occur over long distances relative to the primary grating period, it may be difficult to locate and characterize these variations using test methods designed for shorter distances. The present disclosure proposes systems and methods for quickly and efficiently detecting and characterizing a secondary grating.

Inventors

• Timothy Bodia

Assignees

• 谷歌有限责任公司

Dates

Publication Date

20260508

Application Date

20220629

Priority Date

20210629

Claims (20)

1. 1. A method for testing a grating, comprising: directing light from a laser tuned to a test wavelength to a first test area on the grating; Capturing a raster image based on the light passing through the first test area; Analyzing the grating image to measure diffraction from a secondary grating and determining a characteristic of the secondary grating from the diffraction when the measured diffraction is non-zero, the secondary grating resulting from a defect of a primary grating having a primary grating period that is a sub-wavelength of the test wavelength such that the test wavelength is not diffracted, and The steps of directing, capturing and analyzing are repeated at a second test area on the grating to characterize the defect of the primary grating.
2. 2. The method of claim 1, wherein the defect of the primary grating is a spatial variation in a characteristic of the primary grating over a distance greater than 100 times a primary grating period.
3. 3. The method of claim 2, wherein the characteristic is a primary grating period, a primary grating fill factor, a primary grating feature size, a primary grating feature shape, or a primary grating refractive index.
4. 4. The method of claim 1, wherein the secondary grating has a secondary grating period that is not a sub-wavelength of the test wavelength such that the test wavelength λ t is within a range given by: Wherein: n sub is the refractive index of the grating, P Is the primary grating period, and S Is the secondary grating period.
5. 5. The method of claim 1, wherein directing the light from a laser to the first test region on the grating comprises: projecting the light from the laser such that the light is in a direction aligned with a surface normal vector of the grating, and The light is polarized such that the light is in the same plane as the surface normal vector of the grating.
6. 6. The method of claim 1, wherein analyzing the grating image to measure diffraction from the secondary grating and determining characteristics of the secondary grating from the diffraction comprises: Associating position information with the grating image when the measured diffraction is non-zero, and A secondary grating position is identified based on the position information.
7. 7. The method of claim 6, further comprising: the secondary grating positions are scanned using a microscope to measure changes in grating period, grating feature size, or grating fill factor from corresponding design values.
8. 8. The method of claim 1, wherein analyzing the grating image to measure diffraction from the secondary grating and determining characteristics of the secondary grating from the diffraction comprises: determining a diffraction distance between a diffraction order and a primary order in the grating image, and A secondary grating period of the grating image is determined based on the diffraction distance.
9. 9. The method of claim 8, further comprising: determining an orientation between the diffraction order and the primary order, and An orientation of the secondary grating is determined based on the orientation.
10. 10. A system for testing a grating, comprising: a substrate comprising a primary grating in a primary grating region; A laser configured to project light of a test wavelength to a test area within the primary grating area, the test wavelength not being diffracted by the primary grating; a camera configured to capture a raster image of the light after the light passes through the test area, and A processor configured by software instructions to: Detecting a diffraction order in the grating image, the diffraction order corresponding to diffraction from a secondary grating having a secondary grating period greater than the primary grating period, and The diffraction orders are analyzed to determine characteristics of the secondary grating.
11. 11. The system of claim 10, wherein the secondary grating corresponds to a change in primary grating characteristics over a distance at least 100 times greater than a primary grating period.
12. 12. The system of claim 11, wherein the characteristic is a primary grating period, a primary grating fill factor, a primary grating feature size, a primary grating feature shape, or a primary grating refractive index.
13. 13. The system of claim 10, wherein the processor is further configured to: Controlling a grating positioner coupled to the substrate to move the test region to a plurality of different ones of the primary grating regions, and Repeating the step of analyzing the diffraction orders detected in the grating image from each of the plurality of different test areas to determine characteristics of the secondary grating.
14. 14. The system of claim 10, further comprising: an optical turret configured to direct light in a direction aligned with a surface normal vector of the substrate, and A polarizer configured to polarize the light from the laser such that the light is in the same plane as the surface normal vector of the substrate.
15. 15. The system of claim 10, wherein: the test wavelength (lambda t ) is in a test wavelength range given by: Wherein: n sub is the refractive index of the grating, P Is the primary grating period of the primary grating, and S Is the secondary grating period of the secondary grating.
16. 16. The system of claim 10, further comprising: An imaging lens having a focal length, the imaging lens being positioned between the grating and the camera approximately one focal length away from the grating.
17. 17. The system of claim 10, wherein the substrate is a waveguide of a waveguide display system.
18. 18. A method for testing a waveguide display, comprising: Sequentially directing light from a laser to a plurality of different test areas on a primary grating of the waveguide display, the light being at a test wavelength that is greater than a primary grating period of the primary grating such that the light is not diffracted by the primary grating; sequentially capturing a raster image of the light after the light passes through each of the plurality of different test areas; Detecting a diffraction order within one or more of the plurality of grating images, the diffraction order corresponding to a secondary grating, and The diffraction orders are analyzed to determine characteristics of the secondary grating in each test region corresponding to each of the one or more grating images including the diffraction orders.
19. 19. The method of claim 18, further comprising: Determining positional information corresponding to the one or more grating images including diffraction orders, and A spatial map of the secondary grating is generated based on the location information.
20. 20. The method of claim 19, further comprising: a change in the secondary grating is determined based on the spatial map.

Description

System and method for testing gratings Technical Field The present disclosure relates to gratings, and more particularly to systems and methods for characterizing deviations from an ideal grating that occur spatially over a large area compared to the grating period. Background Gratings may be used to guide and manipulate light in an imaging system. For example, gratings may be used in a waveguide display to direct and project images from the display to the eyes of a user. When any or all of these gratings deviate from their design specifications, the user may observe artifacts in the image from the display. Disclosure of Invention In at least one aspect, the present disclosure generally describes a method for testing a grating. The method includes directing light from a laser to a first test area on a grating and capturing a grating image based on the light passing through the first test area. The method further includes analyzing the grating image to measure diffraction from the secondary grating and determining a characteristic of the secondary grating from the measured diffraction when the diffraction is non-zero. The secondary grating may be generated by a defect of the primary grating. The method also includes directing light to a second test area, capturing a grating image from the second test area, and analyzing the grating image corresponding to the second test area to characterize defects of the primary grating and/or the system (e.g., substrate, coating). In another aspect, the present disclosure generally describes a system for testing a grating. The system includes a substrate, a laser, a camera, and a processor. The substrate of the system includes a primary grating in a primary grating region. The laser of the system is configured to project light of a test wavelength that is not diffracted by the primary grating to a test area within the primary grating area. The camera of the system is configured to capture a raster image of the light after the light passes through the test area. The processor of the system is configured by software instructions to detect a diffraction order in the grating image, the diffraction order corresponding to diffraction from a secondary grating having a secondary grating period greater than the primary grating period. The processor is further configured by the software instructions to analyze the diffraction orders to determine characteristics of the secondary grating. In another aspect, the present disclosure generally describes a method for testing a waveguide display. The method includes sequentially directing light from a laser to a plurality of different test areas on a primary grating of a waveguide display. The guided light is at a test wavelength that is greater (i.e., longer) than the primary grating period of the primary grating, such that the light is not diffracted by the primary grating. The method further includes sequentially capturing a raster image of the light after the light passes through each of the plurality of different test areas. The method further includes detecting diffraction orders of one or more of the captured grating images. The detected diffraction order corresponds to the secondary grating. Thus, the method further comprises analyzing the diffraction orders to determine characteristics of the secondary grating in each test area corresponding to each of the one or more grating images comprising the diffraction orders. The foregoing illustrative overview, as well as other exemplary objects and/or advantages of the present disclosure, and the manner in which the same are accomplished, are further explained in the following detailed description and the accompanying drawings thereof. Drawings Fig. 1 graphically illustrates a waveguide display according to a possible embodiment of the present disclosure. Fig. 2 is a k-space diagram illustrating the possible effects of a grating in the waveguide display of fig. 1. Fig. 3 is a k-space diagram illustrating the effect of grating variations in the waveguide display of fig. 1. Fig. 4 is an example image corresponding to the k-space diagram of fig. 3. Fig. 5 is a k-space diagram illustrating testing of gratings according to a possible embodiment of the present disclosure. Fig. 6A-6C illustrate a system for testing gratings according to possible embodiments of the present disclosure. Fig. 7 is an example of a raster image captured by the raster test system of fig. 6C. Fig. 8 is a flow chart of a method for testing a grating according to an embodiment of the present disclosure. The components in the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding parts throughout the several views. Detailed Description The present disclosure describes methods and systems for testing gratings (i.e., gratings). A grating is a spatially periodic structure that can diffract light when the spatial wavelength of the periodic structure approximates the optica