Method and system for adjustable gradient patterning using a shadow mask

Abstract

To provide a method and a system for tunable gradient patterning using a shadow mask.SOLUTION: A method of fabricating a diffractive structure with a varying diffractive element depth includes providing a shadow mask having a first region with a first aperture dimension-to-aperture periodicity ratio and a second region with a second aperture dimension-to-aperture periodicity ratio less than the first aperture dimension-to-aperture periodicity ratio. The method also includes positioning the shadow mask adjacently to a substrate. The substrate comprises an etching mask corresponding to the diffractive structure. The method further includes exposing the substrate to an etchant, etching the substrate to form diffractive elements with a first depth adjacently to the first region, and etching the substrate to form diffractive elements with a second depth less than the first depth adjacently to the second region.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62 / 582,082, filed November 6, 2017, the disclosure of which is incorporated herein by reference in its entirety for all purposes. [Background technology]

[0002] Modern computing and display technology has facilitated the development of systems for so-called "virtual reality" or "augmented reality" experiences in which digitally reproduced images, or portions thereof, are presented to a user in a manner that appears or can be perceived as real. Virtual reality, or "VR," scenarios typically involve the presentation of digital or virtual image information without transparency to other actual, real-world visual input, while augmented reality, or "AR," scenarios typically involve the presentation of digital or virtual image information as an extension of the user's visualization of the real world around them.

[0003] Despite the advances made in these display technologies, there remains a need in the art for improved methods and systems relating to augmented reality systems, and particularly display systems. Summary of the Invention [Means for solving the problem]

[0004] The present disclosure relates to virtual reality and augmented reality imaging and visualization systems. The present disclosure generally relates to methods and systems related to the fabrication of diffractive structures in which diffractive elements have variable depths as a function of position. The embodiments described herein can be applied in the context of diffraction grating-based waveguide displays used for eyepiece displays. In certain embodiments, gratings with a gradient depth profile are utilized to improve field of view uniformity and increase light outcoupling efficiency. As described herein, some embodiments utilize a shadow mask and dry etching process to fabricate gratings with a gradient depth profile achieved by controlling plasma density to the substrate surface. The present disclosure is applicable to various applications in computer vision and image display systems.

[0005] According to an embodiment of the present invention, a method for fabricating a diffractive structure with a variable diffractive element depth is provided. The method includes providing a shadow mask having a first region with a ratio of aperture size to a first aperture periodicity and a second region with a ratio of aperture size to a second aperture periodicity that is smaller than the ratio of aperture size to the first aperture periodicity, and positioning the shadow mask adjacent to a substrate, the substrate comprising an etch mask corresponding to the diffractive structure. The method also includes exposing the substrate to an etchant, etching the substrate to form diffractive elements having a first depth adjacent to the first region, and etching the substrate to form diffractive elements having a second depth smaller than the first depth adjacent to the second region.

[0006] According to another embodiment of the present invention, a method for fabricating a master substrate is provided. The method includes providing a shadow mask having a first region characterized by a ratio of aperture size to a first gradient aperture periodicity in at least a first direction and a second region characterized by a ratio of aperture size to a second gradient aperture periodicity in at least a second direction. The method also includes providing an etching mask characterized by diffractive features and a substrate having a first exposed region and a second exposed region. The method further includes positioning the shadow mask adjacent to the substrate. The first region is initially aligned with the exposed region, and the second region is aligned with the second exposed region. Additionally, the method includes exposing the substrate to a plasma etching process and etching a first diffractive element adjacent to the first region. The first diffractive element is characterized by a first gradient depth profile in at least the first direction. The method further includes etching a second diffractive element adjacent to the second region. The second diffractive element is characterized by a second gradient depth profile in at least a second direction.

[0007] According to a specific embodiment of the present invention, a method for depositing a variable-thickness material is provided. The method includes providing a substrate and a shadow mask having a first region with a ratio of opening size to a first opening periodicity and a second region with a ratio of opening size to a second opening periodicity that is smaller than the ratio of opening size to the first opening periodicity. The method also includes positioning the shadow mask adjacent to the substrate and performing a plasma deposition process on the substrate to deposit the variable-thickness material. The layer thickness adjacent to the first region is greater than the layer thickness adjacent to the second region.

[0008] In one embodiment, the substrate comprises a growth surface including a diffractive structure, which may include a diffraction grating. The variable thickness material can include a conformal layer. In another embodiment, the shadow mask comprises a plurality of apertures and a surface parallel to the plurality of apertures, the substrate comprises a deposition surface, and positioning the shadow mask adjacent to the substrate includes placing the surface of the shadow mask parallel to the deposition surface. The shadow mask can be characterized by a variable ratio of aperture size to aperture periodicity in two directions. Furthermore, in one embodiment, the substrate comprises a uniform diffractive structure including a plurality of diffractive elements, and the variable thickness material is characterized by a diffractive element depth in a first region and a second diffractive element depth in a second region that is smaller than the diffractive element depth. Furthermore, in another embodiment, the substrate comprises a uniform diffractive structure including a plurality of diffractive elements, and the variable thickness material is characterized by a diffractive element width in a first region and a second diffractive element width that is smaller than the diffractive element width in the second region.

[0009] According to another specific embodiment of the present invention, a method for fabricating a master substrate is provided. The method includes providing a shadow mask having a first region characterized by a ratio of an aperture size to a first gradient aperture periodicity in at least a first direction and a second region characterized by a ratio of an aperture size to a second gradient aperture periodicity in at least a second direction. The method also includes providing a substrate having a mask characterized by diffractive features, a first exposed region, and a second exposed region. The method also includes positioning the shadow mask adjacent to the substrate. The first region is aligned with the first exposed region, and the second region is aligned with the second exposed region. The method further includes exposing the substrate to at least one of a plasma coating or deposition process and coating a first diffractive element adjacent to the first region. The first diffractive element is characterized by a first gradient depth and a first linewidth profile in at least the first direction. The method also includes coating a second diffractive element adjacent to the second region. The second diffractive element is characterized by a second gradient depth and a second linewidth profile in at least a second direction.

[0010] In one embodiment, the first gradient depth is greater than the second gradient depth, hi another embodiment, the first linewidth profile is characterized by a first width and the second linewidth profile is characterized by a second width that is less than the first width.

[0011] Numerous advantages are achieved by the disclosed methods over conventional techniques. For example, embodiments of the present invention provide methods and systems that adapt currently utilized plasma-based systems and tools for etching and / or deposition systems to achieve improved control of plasma density / energy, including controlled variation of plasma density / energy near the target surface. In addition, embodiments improve plasma-enhanced etching or deposition uniformity for current systems, including systems with variable levels of non-uniformity due to plasma density, gas flow, etc. Furthermore, embodiments of the present invention are characterized by reduced system complexity and offer lower costs for fabricating graded pattern templates for contact-based nanolithography and microlithography processes. By utilizing readily available materials and systems, embodiments of the present invention enable fabrication of masks / templates and materials that can be used to vary plasma density / energy in a controlled manner. Additionally, embodiments can produce nano / micro structures with graded depths. By utilizing the shadow masks described herein, variable depth structures can be achieved using a single etching step, avoiding multiple lithography-etching processes, thereby saving both time and cost. These and other embodiments of the present disclosure, along with many of its advantages and features, are described in more detail in conjunction with the following text and accompanying figures. For example, the present invention provides: (Item 1) 1. A method of fabricating a diffractive structure with variable diffractive element depth, the method comprising: providing a shadow mask having a first region with a ratio of opening size to a first opening periodicity and a second region with a ratio of opening size to a second opening periodicity that is smaller than the ratio of opening size to the first opening periodicity; positioning the shadow mask adjacent to a substrate, the substrate including an etching mask corresponding to the diffractive structures; exposing the substrate to an etching solution; Etching the substrate to form a diffractive element having a first depth adjacent the first region; etching the substrate to form a diffractive element adjacent the second region having a second depth less than the first depth; A method comprising: (Item 2) Item 10. The method of item 1, wherein the ratio of aperture size to the first aperture periodicity and the ratio of aperture size to the second aperture periodicity are defined by an aperture size of 1 in the first region being larger than an aperture size of 2 in the second region and a constant center-to-center spacing of apertures. (Item 3) Item 10. The method of claim 1, wherein the ratio of aperture size to the first aperture periodicity and the ratio of aperture size to the second aperture periodicity are defined by a first center-to-center spacing of apertures in the first region and a second center-to-center spacing of apertures in the second region, and a constant aperture size. (Item 4) Item 10. The method of item 1, wherein the shadow mask is characterized by a ratio of aperture size to aperture periodicity that varies linearly across the shadow mask. (Item 5) Item 10. The method of claim 1, wherein the diffractive structure comprises a diffraction grating and the diffractive elements comprise grating teeth. (Item 6) Item 10. The method of claim 1, wherein exposing the substrate to an etching solution comprises performing a plasma etching process. (Item 7) Item 10. The method of item 1, further comprising removing the etching mask after etching the diffractive elements in the first region and etching the diffractive elements in the second region. (Item 8) Item 10. The method of claim 1, further comprising subjecting the substrate to a plasma-enhanced coating process. (Item 9) Item 10. The method of claim 1, wherein the substrate comprises a silicon substrate. (Item 10) Item 2. The method according to item 1, wherein the first depth and the second depth range from about 10 nm to about 150 nm. (Item 11) Item 10. The method of claim 1, wherein the diffractive element varies non-linearly. (Item 12) 1. A method of fabricating a master substrate, the method comprising: 1. A shadow mask, comprising: a first region characterized by a ratio of aperture size to a first gradient aperture periodicity in at least a first direction; a second region characterized by a ratio of aperture size to a second gradient aperture periodicity in at least a second direction; and 1. Providing a substrate, the substrate comprising: an etching mask characterized by diffractive features; a first exposed area; a second exposed area; and positioning the shadow mask adjacent to the substrate, the first area aligned with the first exposed area and the second area aligned with the second exposed area; exposing the substrate to a plasma etching process; Etching a first diffractive element adjacent to the first region, the first diffractive element characterized by a first gradient depth profile in the at least first direction; etching a second diffractive element adjacent to the second region, the second diffractive element being characterized by a second gradient depth profile in the at least second direction; A method comprising: (Item 13) the first region is further characterized by a ratio of an aperture size to a third gradient aperture periodicity in at least a direction orthogonal to the first direction; the second region is further characterized by a ratio of an aperture size to a fourth gradient aperture periodicity in at least a direction orthogonal to the second direction; the first diffractive element is further characterized by a third gradient depth profile in the direction orthogonal to the first direction; the second diffractive element is further characterized by a fourth gradient depth profile in the direction orthogonal to the second direction. Item 13. The method according to item 12. (Item 14) a ratio of aperture size to said first gradient aperture periodicity is equal to a ratio of aperture size to said second gradient aperture periodicity; Item 13. The method according to item 12, wherein the first direction and the second direction are the same direction. (Item 15) a ratio of aperture size to the first gradient aperture periodicity defined by a constant center-to-center spacing of apertures and a graduated aperture size along the first direction; Item 13. The method of item 12, wherein the ratio of aperture size to the second gradient aperture periodicity is defined by a constant center-to-center spacing of apertures and a graduated aperture size along the second direction. (Item 16) a ratio of aperture size to the first gradient aperture periodicity defined by a constant aperture size along the first direction and a graduated center-to-center spacing of the apertures; Item 13. The method of item 12, wherein a ratio of aperture size to the second gradient aperture periodicity is defined by a constant aperture size along the second direction and a graduated center-to-center spacing of apertures. (Item 17) Item 13. The method of item 12, wherein the first diffractive element and the second diffractive element comprise grating teeth. (Item 18) Item 13. The method of item 12, further comprising removing the etching mask after etching the first diffractive element adjacent to the first region and etching the second diffractive element adjacent to the second region. (Item 19) Item 13. The method of item 12, wherein the substrate comprises a silicon substrate. (Item 20) the first gradient depth profile ranges in depth from about 5 nm to about 150 nm; Item 13. The method of item 12, wherein the second gradient depth profile ranges in depth from about 5 nm to about 150 nm. [Brief explanation of the drawings]

[0012] [Figure 1A] FIG. 1A diagrammatically illustrates light paths within a viewing optical assembly (VOA) that may be used to present a digital or virtual image to a viewer according to an embodiment described herein.

[0013] [Figure 1B] FIG. 1B is a simplified cross-sectional view of a diffractive structure according to one embodiment of the present invention.

[0014] [Figure 2] FIG. 2 is a simplified cross-sectional diagram illustrating variable depth etching using a shadow mask according to one embodiment of the present invention.

[0015] [Figure 3A] FIG. 3A is a simplified plan view of a substrate and exemplary shadow mask features according to one embodiment of the present invention.

[0016] [Figure 3B] FIG. 3B is an enlarged view of a portion of the area illustrated in FIG. 3A according to one embodiment of the present invention.

[0017] [Figure 3C] FIG. 3C is an enlarged view of a portion of the area illustrated in FIG. 3A in accordance with another embodiment of the present invention.

[0018] [Figure 4A] FIG. 4A is an image showing a portion of an eyepiece including a variable height grating structure according to one embodiment of the present invention.

[0019] [Figure 4B] 4B-4D are illustrations of SEM cross sections of the grating teeth at the three measurement locations shown in FIG. 4A. [Figure 4C] 4B-4D are illustrations of SEM cross sections of the grating teeth at the three measurement locations shown in FIG. 4A. [Figure 4D] 4B-4D are illustrations of SEM cross sections of the grating teeth at the three measurement locations shown in FIG. 4A.

[0020] [Figure 4E] FIG. 4E is a plot illustrating grating etch depth as a function of distance from the edge of the eyepiece illustrated in FIG. 4A.

[0021] [Figure 5A] 5A-5C are schematic plan views illustrating diffractive structures fabricated using conventional techniques. [Figure 5B] 5A-5C are schematic plan views illustrating diffractive structures fabricated using conventional techniques. [Figure 5C] 5A-5C are schematic plan views illustrating diffractive structures fabricated using conventional techniques.

[0022] [Figure 5D] 5D-5F are schematic plan views illustrating diffractive structures with one-dimensional or two-dimensional contours fabricated in accordance with embodiments of the present invention. [Figure 5E] 5D-5F are schematic plan views illustrating diffractive structures with one-dimensional or two-dimensional contours fabricated in accordance with embodiments of the present invention. [Figure 5F] 5D-5F are schematic plan views illustrating diffractive structures with one-dimensional or two-dimensional contours fabricated in accordance with embodiments of the present invention.

[0023] [Figure 6A] FIG. 6A is a simplified cross-sectional view of a diffractive structure with a grating height gradient according to one embodiment of the present invention.

[0024] [Figure 6B] FIG. 6B is a simplified plan view illustrating a digital dispense pattern with variable dispense volumes according to one embodiment of the present invention.

[0025] [Figure 6C] FIG. 6C is a simplified cross-sectional view of the diffractive structure illustrated in FIG. 6A with a blended interface region according to one embodiment of the present invention.

[0026] [Figure 7] FIG. 7 illustrates a simplified process flow diagram illustrating a method of fabricating a shadow mask according to one embodiment of the present invention.

[0027] [Figure 8] FIG. 8 is a simplified diagram illustrating the use of a shadow mask master to produce a substrate with variable depth diffractive elements according to one embodiment of the present invention.

[0028] [Figure 9] FIG. 9 is a simplified cross-sectional diagram illustrating a variable thickness deposition layer according to one embodiment of the present invention.

[0029] [Figure 10] FIG. 10 is a flowchart illustrating a method for fabricating a diffractive structure with variable diffractive element depth according to one embodiment of the present invention.

[0030] [Figure 11] FIG. 11 is a flowchart illustrating a method for fabricating a master substrate according to an embodiment of the present invention.

[0031] [Figure 12] FIG. 12 is a flow chart illustrating a method for depositing a variable-thickness material according to an embodiment of the present invention.

[0032] [Figure 13]FIG. 13 is a flowchart illustrating a method for fabricating a master substrate according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0033] In some conventional diffraction-based waveguide optical displays, fixed-height nano / micro grating patterns are utilized. However, depending on the angle of incidence of the light as it interacts with the grating pattern, the light intensity that is coupled out toward the user's eye can vary significantly. For example, the light intensity can be non-uniform when measured across the user's field of view in near-field and / or far-field images.

[0034] Embodiments of the present invention improve the user experience by reducing non-uniformities in image intensity through the use of diffractive element depths (e.g., grating depths) that vary as a function of position, e.g., waveguide structures with gradually varying diffractive element depths. These nanoscale adjustments effectively tune the diffraction efficiency, providing a more uniform view for the user.

[0035] FIG. 1A schematically illustrates light paths in a viewing optical assembly (VOA) that may be used to present a digital or virtual image to a viewer, according to one embodiment. The VOA includes a projector 101 that may be worn by the viewer and an eyepiece 100. In some embodiments, projector 101 may include a group of red LEDs, a group of green LEDs, and a group of blue LEDs. For example, projector 101 may include two red LEDs, two green LEDs, and two blue LEDs. Eyepiece 100 may include one or more eyepiece layers. In one embodiment, eyepiece 100 includes three eyepiece layers, one for each of the three primary colors: red, green, and blue. In another embodiment, the eyepiece 100 may include six eyepiece layers, one set of eyepiece layers for each of the three primary colors configured to form a virtual image at one depth plane, and another set of eyepiece layers for each of the three primary colors configured to form a virtual image at another depth plane. In yet another embodiment, the eyepiece 100 may include three or more eyepiece layers, one for each of the three primary colors for three or more different depth planes. Each eyepiece layer includes a planar waveguide and includes an internal coupling grating (ICG) 107, an orthogonal pupil expander (OPE) region 108, and an exit pupil expander (EPE) region 109.

[0036] The projector 101 projects image light onto the ICG 107 in the eyepiece 100. The ICG 107 couples the image light from the projector 101 into a planar waveguide, which propagates it in a direction toward the OPE region 108. The waveguide propagates the image light horizontally by total internal reflection (TIR). The OPE region 108 includes diffractive elements that amplify the image light from the ICG 107 propagating in the waveguide and redirect it toward the EPE region 109. In other words, the OPE region 108 amplifies the beamlets in orthogonal directions to be delivered to different portions of the EPE. The EPE region 109 includes diffractive elements that outcouple a portion of the image light propagating in the waveguide and direct it toward the viewer's eye 102. In this manner, the image projected by the projector 101 can be viewed by the viewer's eye 102.

[0037] As described above, the image light generated by projector 101 may include three primary colors, namely, blue (B), green (G), and red (R) light. Such image light may be separated into its constituent colors so that the image light in each constituent color may be coupled into a respective waveguide within the eyepiece. Embodiments of the present disclosure are not limited to the use of the illustrated projector, and other types of projectors may also be utilized in various embodiments of the present disclosure.

[0038] Projector 101 includes an LED light source 103 and a liquid crystal on silicon (LCOS) spatial light modulator (SLM) 104, although embodiments of the present disclosure are not limited to this projector technology and can include other projector technologies including fiber scanning projectors, deformable mirror devices, micromechanical scanners, the use of laser light sources rather than LEDs, optics including front-lit designs, waveguides, and other arrangements of beam splitters, etc.

[0039] In some eyepiece layers, the grating structure can have spatially uniform diffraction efficiency and optical phase across the entire surface of the grating structure. In the case of an OPE such as that illustrated in FIG. 1A, this invariance can lead to coherent artifacts if the substrate is substantially flat. These coherent artifacts arise from interference effects associated with the multiple propagation paths light can take within the OPE grating region, which can manifest as strongly modulated output intensity at the exit of the grating region. These modulations often produce dark bands, referred to as "striations," in the output image produced by the eyepiece, which occur for different angles as the viewing position is changed. Additionally, the invariance in diffraction efficiency can result in an exponential decrease in output intensity as a function of propagation distance into the grating structure.

[0040] Thus, embodiments of the present invention reduce these and other image artifacts by varying the diffraction efficiency as a function of position, reducing or eliminating these interference effects. As described herein, variation in diffraction efficiency, also referred to as coupling efficiency, can be achieved by modifying the grating height as a function of position, resulting in the desired variation in diffraction efficiency. For example, a variable distribution of grating heights within the OPE will disrupt the optical phase, reducing coherence between all possible optical paths within the OPE and thus reducing interference-based image artifacts in the output image. Furthermore, gradual variation of the grating height within the EPE will result in increased brightness uniformity across the field of view and across different eye positions in the output image, thereby preventing the intensity drop-off as a function of position (which is characteristic of uniform diffraction efficiency structures) as light propagates through the grating.

[0041] FIG. 1B is a simplified cross-sectional view of a diffractive structure according to one embodiment of the present invention. As illustrated in FIG. 1B, the diffractive elements (i.e., grating teeth) are characterized by variable depth / height as a function of position measured along the x-axis. As discussed above, when a diffractive structure such as that illustrated in FIG. 1B is utilized in the EPE section of the eyepiece illustrated in FIG. 1A, the intensity of outcoupled light as a function of position, which may decrease with position when using conventional gratings, is characterized by increased uniformity, improving the user's experience.

[0042] If the grating structure is fabricated using a template, the grating depth gradation illustrated in FIG. 1B can be selectively adjusted in the template for the imprinted waveguide during the template etching process. If the grating structure is fabricated directly, the grating depth gradation can be selectively adjusted during fabrication (e.g., during etching) using the methods and systems described herein. In one embodiment, a structure with variable grating depth (i.e., depth is measured relative to the planar surface 160 of the substrate) is used as a master for imprinting copies that will include gratings with variable heights. Of course, other designs in which the master has variable grating heights (i.e., height is measured relative to a tilted plane aligned with the bottom of the grating teeth) can also be utilized.

[0043] Embodiments of the present invention overcome problems observed in conventional approaches that do not address both near-field and far-field image uniformity. Additionally, in conventional approaches, including multi-step etching or step-based (i.e., digital) variation approaches that can be fabricated using etching followed by electron beam lithography, the step-based variation introduces scattering as a result of digital steps and / or sharp boundaries between steps of different sizes. By using embodiments of the present invention, a predetermined analog variation of the depth of the diffractive element as a function of position is achieved, along with advantages not available from conventional step-based variation in grating depth. As explained above, the methods and systems described herein allow the grating coupling coefficient to vary as a function of position, improving brightness uniformity and user experience.

[0044] FIG. 2 is a simplified cross-sectional diagram illustrating variable depth etching using a shadow mask according to one embodiment of the present invention. In FIG. 2, a shadow mask 210 is placed proximate to a substrate 205, on which an etch mask 207 is patterned. The etch mask 207 may be referred to as a hard mask. In some embodiments, the plane of the shadow mask with its openings is positioned parallel to the top surface of the substrate, and the distance D between the shadow mask and the top surface of the etch mask can range from about 100 μm to several centimeters, e.g., about 1 mm. A plasma 220 passes through the shadow mask 210, and etching of the substrate 205 occurs in portions of the substrate not covered by the etch mask 207. It should be understood that the dimensions of the etch mask 207 will typically be on the order of submicron geometries. Therefore, FIG. 2 is not drawn to scale, as the center-to-center spacing between the openings and the opening dimensions are several orders of magnitude larger than the etch mask dimensions.

[0045] Shadow mask 210 is fabricated with openings 212, 214, 216, and 218 whose dimensions vary as a function of the x-direction. As illustrated in the example shown in FIG. 2, opening 212 has a width of 30 μm, opening 214 has a width of 60 μm, opening 216 has a width of 90 μm, and opening 218 has a width of 120 μm. In this example, the center-to-center spacing 215 of the openings is 150 μm, but this is not required by the present invention; other spacings between openings can be utilized. As a result of the variable opening dimensions, the plasma density reaching the substrate surface varies across the substrate, with higher plasma density adjacent the wider opening 218 and lower plasma density adjacent the narrower opening 212. The plasma density will be affected by a diffusion process. As a result, although the openings have different dimensions, the diffusion of plasma in the x- and y-directions will result in a nearly continuous variation in plasma density as a function of position across the substrate surface. While four apertures 212-218 are shown to illustrate variable-size apertures as a function of position, it will be apparent to those skilled in the art that the present invention is not limited to this small number of apertures. Rather, a larger number of apertures, each varying by a small amount, can be used to provide a series of apertures with increasing widths; for example, approximately 65 apertures on 150 μm centers can be used to vary the aperture widths from 30 μm to 120 μm, which can have an inter-aperture width increment of approximately one micron. In FIG. 2, the aperture size variation associated with mask periodicity is linear as a function of position (i.e., in the x-direction), but this is not required by the present invention. In other implementations, the variation with position is nonlinear. In addition to utilizing variable-size apertures with equal spacing between apertures, other embodiments can vary the spacing between apertures with equal-sized apertures, since the density of plasma at the etching surface is a function of the size and spacing of the apertures in the shadow mask. Furthermore, both aperture size and spacing can be varied to achieve a desired ratio of aperture to mask area and a resulting desired plasma density at the substrate surface. One of ordinary skill in the art would recognize many variations, modifications, and alternatives.

[0046] As will be apparent to those skilled in the art, the chemical / physical etch rate, which affects grating depth, is a function of the plasma density reaching the substrate surface. Thus, as illustrated in FIG. 2, the grating depth H2 adjacent the wider opening 218 is greater than the grating depth H1 adjacent the narrower opening 212. Thus, embodiments of the present invention utilize a shadow mask to generate a variable plasma density as a function of position, which results in a variation, which may be a step or analog variation, of the diffractive element depth / height as a function of position across the substrate. After etching the diffractive elements (e.g., grating teeth), the etch mask 207 can be removed.

[0047] While variation in a single direction (i.e., the x-direction) is illustrated in FIG. 2 , embodiments of the present invention are not limited to this example, and variation in two dimensions (e.g., both the x- and y-directions) is within the scope of the present invention. Furthermore, while FIG. 2 depicts the grating depth increasing approximately linearly from left to right, it is also possible for the grating depth to vary in another predetermined manner to achieve a desired depth profile. For example, the grating depth may vary linearly decreasing, linearly increasing, nonlinearly, sinusoidally, with a variable depth (e.g., a deep grating followed by a shallower grating followed by a deeper grating, or any particular pattern), etc. In some embodiments, these variable grating depth profiles may be controlled by the spacing of the openings in the shadow mask used to create the gratings and / or by the etch rate of the etchant used to create the gratings. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0048] Furthermore, while Figure 2 illustrates a structure in which a hard mask for the diffractive elements is patterned and then a shadow mask is utilized to form diffractive elements of variable depth, this is not required by the present invention. Other embodiments can utilize a process flow in which diffractive elements (e.g., a uniform grating pattern) are formed, the hard mask is removed, and a shadow mask is then used to selectively remove portions of the diffractive elements to form diffractive structures with variable heights. An etch mask can be utilized in embodiments in which variable depth is desired. Additionally, while a periodic grating structure is defined by etch mask 207 in Figure 2 as an illustrative example, this is not required by the present invention, and other nanostructures including posts or holes (of different cross-sectional shapes including circles, polygons, etc.), interrupted gratings, sinusoidal lines and spaces, line segments, etc., are also within the scope of the present invention.

[0049] Shadow masks can be fabricated using a variety of materials that are etch-resistant and provide mechanical rigidity in a plasma etching environment. For example, shadow masks can be fabricated using silicon wafers that are patterned using photolithography and then etched to provide variable-size openings across the wafer surface. Laser ablation of portions of a silicon wafer can also be used to fabricate shadow masks. Stainless steel mesh or anodized aluminum mesh can also be formed and used as a shadow mask. In addition, protective coatings including, for example, Al2O3, SiO2, ZnO, TiO2, Au, Ag, Cu, Pt, Ir, etc., can be formed on the mesh (e.g., silicon) to protect materials that are undesirable for etching.

[0050] Although some embodiments of the present invention are discussed with reference to their use in dry etching processes, embodiments of the present invention are applicable to a variety of semiconductor fabrication processes in which diffusion control of the etching / growth environment is utilized to vary the fabrication process as a function of position, including other plasma etching processes. As a further example, embodiments of the present invention are applicable to plasma deposition (e.g., sputtering, chemical vapor deposition (CVD), low-pressure CVD (LP-CVD), plasma-enhanced CVD (PECVD), etc.) that form features of variable height, wet etching processes including metal-assisted chemical etching, other fabrication processes in which precursor concentration or concentration of chemical species at the surface affects the deposition / etch rate, etc.

[0051] 3A is a simplified plan view of a substrate and exemplary shadow mask features according to one embodiment of the present invention. The exemplary shadow mask 310 illustrated in FIG. 3A illustrates how different aperture sizes as a function of x, y position can be used to vary etch depth, and the specific apertures illustrated are not limiting to embodiments of the present invention. In FIG. 3A, three different regions of the shadow mask 310 are illustrated: a first region 312 with small apertures, a second region 314 with medium apertures, and a third region 316 with large apertures.

[0052] In the exemplary shadow mask 310 illustrated in FIG. 3A, the center-to-center spacing between apertures, also referred to as aperture periodicity, is 150 μm, but this is merely exemplary. While the apertures in the shadow mask are illustrated as squares, this is merely exemplary; other aperture geometries, including rectangular, polygonal, circular, oval, etc., can also be utilized. The apertures can range in size from a few microns (e.g., 10 μm) to several millimeters, and the thickness of the shadow mask can range from 1 μm to several millimeters. Referring to FIG. 3A, the illustrated shadow mask is 80 μm thick and has a center-to-center spacing between apertures (i.e., aperture periodicity) of 150 μm and aperture dimensions ranging in size from 30 μm to 120 μm. Different regions of the shadow mask can be associated with different optical structures, including an orthogonal pupil expander 322 and an exit pupil expander 324. For example, one or more regions of the shadow mask can be associated with one or more optical structures.

[0053] Referring to FIG. 3A , each region of the shadow mask can be characterized by the ratio of aperture size to aperture periodicity. For apertures having equal dimensions in the x- and y-directions (i.e., square or circular apertures), the ratio of aperture size to aperture periodicity can be equal in both the x- and y-directions, but this is not required by the present invention, and the ratio can be different in different directions. Region 312 has a low ratio of aperture size to aperture periodicity due to its small aperture size, while region 316, in contrast, has a higher ratio of aperture size to aperture periodicity, assuming constant aperture periodicity (i.e., constant center-to-center spacing between apertures) between regions 312 and 316. While the ratio of aperture size to aperture periodicity is illustrated as uniform in each of regions 312, 314, and 316, it should be understood that this is merely exemplary and that the ratio can vary within and between each region, for example, as the size is gradually changed.

[0054] Figure 3B is an enlarged view of a portion of regions 312 and 316 according to one embodiment of the invention. In some embodiments, for example, as shown in Figure 3B, the aperture dimension in one region (e.g., x1 in region 316a) is larger than the aperture dimension in a second region (e.g., x2 in region 312a), and the center-to-center spacing of the apertures (e.g., p1 in regions 316a and 312a) is constant, thereby providing a ratio variation (e.g., R1 in region 316a and R2 in region 312a). Figure 3C is an enlarged view of a portion of regions 312 and 316 according to another embodiment of the invention. In the embodiment illustrated in FIG. 3C , the center-to-center spacing of apertures in a first region (e.g., p1 in region 316b) is smaller than the center-to-center spacing of apertures in a second region (e.g., p2 in region 312b), and the aperture dimensions in both regions (e.g., x1 in regions 316b and 312b) are constant, thereby providing a ratio variation (e.g., R1 in region 316b and R2 in region 312b). In different portions of the eyepiece, the ratio may vary as needed for a particular application. By way of example, in FIG. 3 , orthogonal pupil expander 322 has little or no variation in aperture size or periodicity, and thus has a constant ratio in the y-direction but an increasing ratio of aperture size to aperture spacing in the negative x-direction. Exit pupil expander 324, on the other hand, has a substantially linear variation in the y-direction but little or no variation in the x-direction.

[0055] While FIG. 3A illustrates the regions of the shadow mask as separate elements, it should be understood that these separate regions are used solely to illustrate a continuously varying shadow mask section having openings that gradually increase in size from region 312 to region 316. As the opening size increases, which is controlled during shadow mask fabrication, the plasma density at the substrate surface corresponding to region 312 of shadow mask 310 will be less than the plasma density at the substrate surface in the area corresponding to region 316 of shadow mask 310. Thus, the etch rate of the substrate area corresponding to region 316 will be higher than the etch rate of the substrate area corresponding to region 312, enabling the formation of diffractive elements with variable grating depth and variable coupling coefficient as a function of position. Thus, a shadow mask with variable-sized openings that allows the plasma density to be adjusted as a function of position results in adjustable etch depth as a function of position. Thus, the illustration in FIG. 3A of separate regions 312, 314, and 316 is merely to illustrate that the opening size varies (e.g., gradually) across the shadow mask, allowing for variable plasma density as a function of position, resulting in variations in etch rate and grating depth as a function of position on the substrate.

[0056] Figure 4A is an image showing a portion of an eyepiece including a variable height grating structure according to one embodiment of the present invention. As shown in Figure 4A, three measurement locations, 410, 412, and 414, are shown. Figures 4B-4D are illustrations of SEM cross sections of the grating teeth at the three measurement locations 410, 412, and 414, respectively, shown in Figure 4A. Figure 4E is a plot illustrating grating etch depth as a function of distance from the edge 405 of the eyepiece shown in Figure 4A.

[0057] Referring to Figures 4B-4D, the grating etch depth is measured at three locations across the eyepiece. Using the shadow mask technique described herein, an analog step gradient diffractive structure is fabricated. While the gradient in etch depth is illustrated in Figures 4A-4E for a silicon wafer with a one-dimensional gradient, it should be understood that this is merely an example and that other gradient structures, including two-dimensional structures, can be fabricated in other materials.

[0058] To fabricate the structure illustrated in Figures 4A-4D, a thermally grown SiO2 etch mask on a silicon wafer was first etched without a shadow mask to create the grating teeth. Then, a shadow mask with one-dimensional variation in aperture dimensions was positioned 2 mm from the silicon wafer surface, and the silicon wafer was etched. The grating etch depth was measured versus distance from the edge 405 of the eyepiece. At location 410, a grating depth d1 of approximately 60 nm was produced, as illustrated in Figure 4B; at location 412, a grating depth d2 of approximately 78 nm was produced, as illustrated in Figure 4C; and at location 414, a grating depth d1 of approximately 100 nm was produced, as illustrated in Figure 4D. The variation in grating etch depth with distance is illustrated in Figure 4E, where the grating depth ranges from approximately 60 nm to approximately 110 nm across the 35 mm size of the eyepiece. In contrast to conventional techniques that generally produce visible steps or lines across the eyepiece, embodiments of the present invention provide a substantially gradual grating depth variation with position, thus enabling smooth near-field and far-field images within the virtual image displayed to the user's eye.

[0059] In addition to the one-dimensional variation illustrated in FIG. 1 , embodiments of the present invention can utilize shadow masks with two-dimensional variation to fabricate structures characterized by a hybrid step change in grating depth / height for 2D contours. FIGS. 5A-5C are schematic plan views illustrating diffractive structures fabricated using conventional techniques. FIGS. 5D-5F are schematic plan views illustrating diffractive structures with one- or two-dimensional contours fabricated according to embodiments of the present invention. By controlling the size and duty cycle (i.e., pitch) of the openings in the shadow mask, either one- or two-dimensional patterns can be fabricated with a predetermined etch depth gradient. Compared to conventional techniques, such as one-dimensional step contours, two-dimensional contours that are difficult to achieve are made possible by embodiments of the present invention. Thus, the shadow mask techniques described herein not only allow for tailoring to specific designs based on two-dimensional contours or shadow mask opening distributions, but also allow for blending boundaries to achieve predetermined gradient profiles unavailable using conventional techniques.

[0060] 5A-5C, one-dimensional (FIG. 5A), two-dimensional (FIG. 5B), and circular (FIG. 5C) profiles fabricated using conventional techniques involving multi-step lithography / etching are illustrated. Darker gray areas are associated with deeper etch profiles, and lighter gray areas are associated with shallower etch profiles. The steps or boundaries between adjacent sections are clearly apparent. In use, diffractive structures with these distinct boundaries would produce scattering or variations in far-field and / or near-field image intensity at the boundaries, thereby adversely affecting image quality and user experience. In contrast to these conventional results, embodiments of the present invention provide one-dimensional (FIG. 5D), two-dimensional (FIG. 5E), and circular (FIG. 5F) profiles that vary in a stepwise manner and provide a gradient that appears continuous as a function of position.

[0061] Figure 6A is a simplified cross-sectional view of a diffractive structure with a grating height gradient according to an embodiment of the present invention, Figure 6B is a simplified plan view illustrating a digital dispense pattern with a variable dispense volume according to an embodiment of the present invention, and Figure 6C is a simplified cross-sectional view of the diffractive structure illustrated in Figure 6A with a blended interface region according to an embodiment of the present invention.

[0062] Referring to FIG. 6A, the diffractive structure includes grating teeth with variable depth, as described herein. This structure, which may also include variation in the second dimension, can be fabricated using the techniques described herein. In some embodiments, the diffractive structure is fabricated in silicon, SiO2, or the like and used as a template in a nanoimprint lithography process, such as a jet and flash imprint lithography (J-FIL) process. As illustrated in FIG. 6B, a resist pattern or other suitable liquid can be dispensed using a droplet pattern via a print head, which may utilize hundreds of nozzles. Thus, the dispense pattern can include droplets with an adjustable finite volume. However, in some implementations, the droplet volume cannot be continuously adjusted. In FIG. 6B, the liquid volume is dispensed in a digital dispense pattern, with each of sections 620, 622, 624, and 626 having a predetermined volume. For example, if the drop volume is 1 pL, section 620 will have 9 pL of liquid, section 622 will have 16 pL of liquid, section 624 will have 25 pL of liquid, and section 626 will have 36 pL of liquid. Thus, each section will have a predetermined residual layer thickness (RLT) that is a function of the drop volume and number of drops per section. After the initial dispense is completed, the different volumes within each section will result in a step 630 between adjacent sections.

[0063] Referring to FIG. 6C, a cross-section of a diffraction grating that can be formed by imprinting a patterned droplet volume such as that shown in FIG. 6B using an analog gradient template such as that shown in FIG. 6A is shown. During imprinting, as liquid diffuses into each section and traverses the sections by diffusion, the RLT near the section edges will vary smoothly, rather than intermittently, as shown in FIG. 6C. Thus, diffusion of liquid (e.g., resist) at the edges of section boundaries, such as boundary 632, will result in RLT hybridization at the interface region between the sections. As a result of the smooth transition at the section boundaries, optical scattering at the boundaries will be reduced compared to structures with discrete boundaries. The variation in grating tooth height from shallower to deeper, measured relative to the base of the substrate, will be related to the increased liquid volume from section 620 to section 626.

[0064] 7 illustrates a simplified process flow diagram illustrating a method of fabricating a shadow mask according to one embodiment of the present invention. The process begins with a silicon-on-insulator (SOI) substrate, onto which a chromium etch mask layer 738 is deposited (710). The SOI substrate includes a silicon handle wafer 730, a buried oxide layer 732, a single-crystal silicon layer 734, and a backside oxide layer 736. While a silicon shadow mask is illustrated in this process flow, this is not required by the present invention, and other materials, including SiO2, metals, other dielectric materials, etc., are also suitable for use as a shadow mask.

[0065] A patterning layer 740 is formed on the chrome layer 738, and the patterning layer (e.g., photoresist) is patterned with a predetermined opening size (712). As described below, openings that vary in size as a function of position are defined in this process step. Using the patterned patterning layer, the chrome layer 738 is etched (714), for example, using a dry etching process, to transfer the pattern in the patterning layer into the chrome layer, which will be used as a hard mask in a subsequent etching process.

[0066] For etching of the monocrystalline silicon layer 734, the patterned chrome layer 742 acts as a hard mask during, for example, plasma etching, and the pattern in the chrome layer will be transferred during etching into the monocrystalline silicon layer 716. As shown in process step 716, the silicon etching process is terminated once it reaches the buried oxide layer 732.

[0067] The backside oxide layer 736 is patterned (718) to define the lateral dimensions of the shadow mask. This opening is then used to etch the silicon handle wafer 730 using a deep etch (e.g., a dry etch) that terminates at the buried oxide layer 732 (720). The thickness of the silicon handle wafer (e.g., about 80 μm to 800 μm, e.g., 500 μm) provides mechanical strength and rigidity to the shadow mask. The buried oxide layer 732 is then removed (722), leaving a grid or mesh pattern in the single-crystal silicon layer 734. The fabrication process is completed by removing the chrome hard mask and the remaining portions of the patterning layer (724). Inset 750 illustrates, in plan view, the opening in the grid pattern illustrated in process step 724. As will be apparent to one skilled in the art, the dimensions of the openings vary across the shadow mask, and the uniformity of the opening sizes illustrated in inset 750 is merely to illustrate a portion of the shadow mask, as the opening sizes may, for example, vary slowly across the structure. Additionally, although an SOI substrate is used in the exemplary process flow illustrated in Figure 7, other materials and processes may also be used and are within the scope of the present invention.

[0068] For example, while dry etching is illustrated in FIG. 7, other implementations utilize a wet etching process in which silicon can be anisotropically etched using KOH to create holes on the wafer / sheet. Other metals and dielectric materials can also be wet etched, either anisotropically or isotropically. Furthermore, laser drilling / ablation can also be used; for example, a laser can be used to drill holes in silicon, metal, glass, or other dielectric materials with submicron precision suitable for openings on the order of a micron. In addition, a metal-assisted chemical wet etching (MaCe) process can also be used; for example, gold (Au) can be used to assist the silicon wet etching and produce anisotropic etching.

[0069] FIG. 8 is a simplified diagram illustrating the use of a shadow mask master to produce a substrate with variable-depth diffractive elements according to one embodiment of the present invention. In FIG. 8, a shadow mask master 805 with gradient duty cycle apertures is placed adjacent to a substrate 820. The shadow mask master 805, in this example, includes four regions 812, 814, 816, and 818, each of which has a variable aperture size to produce gradient duty cycle apertures, with lighter colors representing larger duty cycles similar to region 316 illustrated in FIG. 3A and darker colors representing smaller duty cycles similar to region 312 illustrated in FIG. 3A. In one example, four different eyepieces can be patterned using the shadow mask master.

[0070] Substrate 820 is etched to form master substrate 830. As shown in Figure 8, shadow mask master 805 is aligned adjacent to (e.g., on top of) substrate 820, which is generally protected with resist and exposed in four regions 822, 824, 826, and 828 corresponding to regions 812, 814, 816, and 818 in shadow mask master 805. In Figure 8, darker regions on master substrate 830 represent deeper etch depths. By tuning to a particular aperture size and varying the duty cycle on the shadow mask master, a target etch depth gradient can be etched into substrate 820 to form multi-field master substrate 830. The master substrate 830 can then be used as a master template for a contact-based nanolithography process (e.g., J-FIL) to imprint onto a high refractive index substrate, and thus can be used as a waveguide or as a shadow mask in directly etching a high refractive index layer or substrate to be used as an optical waveguide.

[0071] FIG. 9 is a simplified cross-sectional diagram illustrating a variable-thickness deposition layer according to one embodiment of the present invention. In FIG. 9, a shadow mask (not shown) is applied to a deposition process to produce a gradually varying deposition layer 910 with a thickness t1 of 1 in a first region and a thickness t2 of 2 in a second region. As illustrated in FIG. 9, layers deposited in regions adjacent to portions of the shadow mask with a higher ratio of aperture size to aperture periodicity have a greater thickness than layers deposited in regions adjacent to portions of the shadow mask with a lower ratio of aperture size to aperture periodicity. Thus, starting with uniform-height grating elements, the shadow mask techniques described herein are useful for forming diffractive elements characterized by a gradient depth of 1 in the first region and a gradient depth of 2 in the second region as a result of the variation in deposition layer thickness as a function of position. Furthermore, as illustrated in FIG. 9, the diffractive elements in the first region are characterized by a linewidth profile w1 measured in the x-direction that is wider than the linewidth profile w2 in the second region. As a result, diffractive elements provided by embodiments of the present invention can include variations in both depth and width as a function of position that are related to the ratio of aperture size to aperture periodicity of the shadow mask.

[0072] Numerous plasma-assisted deposition processes can be implemented using embodiments of the present invention, including conventional physical deposition processes such as atmospheric plasma-enhanced CVD (APPECVD), sputter deposition, and evaporation. The placement of a shadow mask adjacent to the substrate on which the deposited layer is formed results in variations in coating / deposition thickness, such that, for a similar aperture pitch, areas with larger openings are coated with a thicker coating, while areas with smaller openings are coated with a thinner coating. Thus, gradients in coating thickness are produced that are not easily produced using conventional deposition techniques. In Figure 9, a conformal coating produced using an APPECVD or sputter process is illustrated over a nanopattern, which may be a diffraction grating or other suitable diffractive structure.

[0073] FIG. 10 is a flowchart illustrating a method for fabricating a diffractive structure with variable diffractive element depth according to an embodiment of the present invention. The method includes, in step 1005, providing a shadow mask. The shadow mask may have a first region with a ratio of aperture size to a first aperture periodicity and a second region with a ratio of aperture size to a second aperture periodicity. The ratio of aperture size to the second aperture periodicity may be smaller than the ratio of aperture size to the first aperture periodicity. In some embodiments, the ratio of aperture size to the first aperture periodicity and the ratio of aperture size to the second aperture periodicity are defined by an aperture size of 1 in the first region that is larger than an aperture size of 2 in the second region and a constant center-to-center spacing of the apertures. In some embodiments, the ratio of aperture size to the first aperture periodicity and the ratio of aperture size to the second aperture periodicity are defined by a first center-to-center spacing of the apertures in the first region and a second center-to-center spacing of the apertures in the second region and a constant aperture size. As used herein, aperture periodicity may also be referred to as center-to-center aperture spacing.

[0074] The method further includes positioning a shadow mask adjacent to the substrate at step 1010. In some embodiments, the shadow mask is characterized by a ratio of opening size to opening periodicity that varies linearly across the shadow mask. The shadow mask may be fabricated using an etch-resistant material and may include a protective coating to protect materials that are undesirable to etching.

[0075] In some embodiments, the substrate may comprise a silicon substrate. In other embodiments, the substrate may comprise any suitable material that can be etched. The substrate may comprise an etch mask corresponding to the diffractive structures. The etch mask may also be referred to herein as a hard mask. In some embodiments, the diffractive structures may comprise a diffraction grating, and the diffractive elements comprise grating teeth.

[0076] The method further includes exposing the substrate to an etchant in step 1015. In some embodiments, exposing the substrate to the etchant may include performing a plasma etching process. For example, a plasma may pass through a shadow mask and etch the substrate in portions not covered by the etch mask. In other embodiments, any suitable etchant capable of etching the material selected for the substrate may be used.

[0077] The method further includes, in step 1020, etching the substrate adjacent the first region to form a diffractive element having a first depth. The method further includes, in step 1025, etching the substrate adjacent the second region to form a diffractive element having a second depth smaller than the first depth. In some embodiments, the diffractive element varies nonlinearly. In some embodiments, the first depth and the second depth may range from about 10 nm to about 150 nm. The first and second depths may vary due to variable-sized openings in the shadow mask, allowing a greater or lesser amount of etchant to contact the substrate.

[0078] In some embodiments, the method further includes removing the etch mask after etching the diffractive elements in the first region and etching the diffractive elements in the second region. In some embodiments, the method further includes performing a plasma-enhanced coating process on the substrate.

[0079] It should be understood that the specific steps illustrated in FIG. 10 provide a particular method for fabricating a diffractive structure with variable diffractive element depth according to another embodiment of the present invention. Other sequences of steps may be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Furthermore, individual steps illustrated in FIG. 10 may include multiple sub-steps that may be performed in various sequences appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular application. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0080] 11 is a flowchart illustrating a method for fabricating a master substrate according to an embodiment of the present invention. The method includes, in step 1105, providing a shadow mask having a first region and a second region. The first region may be characterized by a ratio of an aperture size to a first gradient aperture periodicity in at least a first direction. The second region may be characterized by a ratio of an aperture size to a second gradient aperture periodicity in at least a second direction. The shadow mask may comprise any suitable etch-resistant material. The aperture periodicity may also be referred to herein as the center-to-center aperture spacing.

[0081] In some embodiments, the ratio of aperture size to a first gradient aperture periodicity is equal to the ratio of aperture size to a second gradient aperture periodicity, and the first and second directions are the same. In some embodiments, the ratio of aperture size to a first gradient aperture periodicity can be defined by a constant center-to-center spacing of apertures and a graduated aperture size along the first direction, and the ratio of aperture size to a second gradient aperture periodicity can be defined by a constant center-to-center spacing of apertures and a graduated aperture size along the second direction. In some embodiments, the ratio of aperture size to a first gradient aperture periodicity can be defined by a constant aperture size and a graduated center-to-center spacing of apertures along the first direction, and the ratio of aperture size to a second gradient aperture periodicity can be defined by a constant aperture size and a graduated center-to-center spacing of apertures along the second direction.

[0082] The method further includes providing a substrate at step 1110. The substrate may have an etch mask characterized by diffractive features, a first exposed region, and a second exposed region. The etch mask may also be referred to herein as a hard mask. In some embodiments, the substrate may comprise a silicon substrate. In some embodiments, the substrate may comprise any suitable etchable material.

[0083] The method further includes positioning a shadow mask adjacent to the substrate in step 1115. The first region may be aligned with the first exposed region and the second region may be aligned with the second exposed region. The method further includes exposing the substrate to a plasma etching process in step 1120. This process may allow plasma to pass through the shadow mask in portions of the substrate not covered by the etch mask.

[0084] The method further includes, in step 1125, etching a first diffractive element adjacent to the first region. The first diffractive element may be characterized by a first gradient depth profile in at least a first direction. The method further includes, in step 1130, etching a second diffractive element adjacent to the second region. The second diffractive element may be characterized by a second gradient depth profile in at least a second direction. The different depth profiles between the first diffractive element and the second diffractive element may result from variable-sized openings in a shadow mask used in the plasma etching process. For example, a larger opening may allow more plasma to contact the substrate, while a smaller opening may allow less plasma to contact the substrate. This may result in variable plasma densities reaching the surface, creating diffractive elements of variable size and depth.

[0085] In some embodiments, the first diffractive element and the second diffractive element may comprise grating teeth. In some embodiments, the method further includes removing the etch mask after etching the first diffractive element adjacent to the first region and etching the second diffractive element adjacent to the second region.

[0086] In some embodiments, the first region may be further characterized by a ratio of aperture dimension to a third gradient aperture periodicity at least in a direction orthogonal to the first direction, the second region may be further characterized by a ratio of aperture dimension to a fourth gradient aperture periodicity at least in a direction orthogonal to the second direction, the first diffractive element is further characterized by a third gradient depth profile at least in a direction orthogonal to the first direction, and the second diffractive element is further characterized by a fourth gradient depth profile at least in a direction orthogonal to the second direction.

[0087] It should be understood that the specific steps illustrated in FIG. 11 provide a particular method of fabricating a master substrate according to another embodiment of the present invention. Other sequences of steps may be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Furthermore, individual steps illustrated in FIG. 11 may include multiple sub-steps that may be performed in various sequences appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular application. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0088] 12 is a flowchart illustrating a method of depositing a variable-thickness material according to an embodiment of the present invention. The method includes, in step 1205, providing a substrate. In some embodiments, the substrate may include a growth surface including a diffractive structure. The diffractive structure may include a diffraction grating.

[0089] In some embodiments, the substrate may comprise a uniform diffractive structure including a plurality of diffractive elements, and the variable thickness material may be characterized by a diffractive element depth of one in a first region and a second diffractive element depth that is less than the diffractive element depth of one in a second region. In some embodiments, the substrate may comprise a uniform diffractive structure including a plurality of diffractive elements, and the variable thickness material may be characterized by a diffractive element width of one in a first region and a second diffractive element width that is less than the diffractive element width of one in the second region.

[0090] The method further includes providing a shadow mask at step 1210. The shadow mask may have a first region with a ratio of aperture size to a first aperture periodicity and a second region with a ratio of aperture size to a second aperture periodicity that is smaller than the ratio of aperture size to the first aperture periodicity. In some embodiments, the shadow mask may be characterized by variable ratios of aperture size to aperture periodicity in two directions.

[0091] The method further includes positioning a shadow mask adjacent to the substrate at step 1215. The method further includes performing a plasma deposition process on the substrate to deposit a variable-thickness material at step 1220. The layer thickness adjacent to the first region can be greater than the layer thickness adjacent to the second region. In some embodiments, the variable-thickness material can comprise a conformal layer.

[0092] In some embodiments, the shadow mask may comprise a plurality of openings and a surface parallel to the plurality of openings. The substrate may comprise a deposition surface. In some embodiments, positioning the shadow mask adjacent to the substrate may include placing the surface of the shadow mask parallel to the deposition surface.

[0093] It should be understood that the specific steps illustrated in FIG. 12 provide a particular method of depositing a variable-thickness material according to another embodiment of the present invention. Other sequences of steps may be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Furthermore, the individual steps illustrated in FIG. 12 may include multiple sub-steps that may be performed in various sequences appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular application. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0094] 13 is a flowchart illustrating a method for fabricating a master substrate according to an embodiment of the present invention. The method includes, in step 1305, providing a shadow mask. The shadow mask may have a first region characterized by a ratio of an aperture size to a first gradient aperture periodicity in at least a first direction. The shadow mask may further have a second region characterized by a ratio of an aperture size to a second gradient aperture periodicity in at least a second direction.

[0095] The method further includes providing a substrate in step 1310. The substrate may have a mask characterized by diffractive features, a first exposed region, and a second exposed region. The method further includes positioning a shadow mask adjacent to the substrate in step 1315. The first region may be aligned with the first exposed region, and the second region may be aligned with the second exposed region. The method further includes exposing the substrate to at least one of a plasma coating or deposition process in step 1320.

[0096] The method further includes coating a first diffractive element adjacent to the first region in step 1325. The first diffractive element may be characterized by a first gradient depth and a first linewidth profile in at least a first direction. The method further includes coating a second diffractive element adjacent to the second region in step 1330. The second diffractive element may be characterized by a second gradient depth and a second linewidth profile in at least a second direction. In some embodiments, the first gradient depth may be greater than the second gradient depth. In some embodiments, the first linewidth profile may be characterized by a first width, and the second linewidth profile may be characterized by a second width that is smaller than the first width. Thus, diffractive elements provided by some embodiments of the present invention may include variations in both depth and width as a function of position related to the ratio of the aperture dimension to the aperture periodicity of the shadow mask.

[0097] It should be understood that the specific steps illustrated in FIG. 13 provide a particular method of fabricating a master substrate according to another embodiment of the present invention. Other sequences of steps may be performed according to alternative embodiments. For example, alternative embodiments of the present invention may perform the steps outlined above in a different order. Furthermore, individual steps illustrated in FIG. 13 may include multiple sub-steps that may be performed in various sequences appropriate to the individual step. Furthermore, additional steps may be added or removed depending on the particular application. Those skilled in the art will recognize many variations, modifications, and alternatives.

[0098] It should also be understood that the examples and embodiments described herein are for illustrative purposes only, and that various modifications or changes in light thereof will be suggested to those skilled in the art and are to be included within the spirit and scope of the present application and the appended claims.

Claims

1. 1. A method for depositing a variable thickness material, the method comprising: providing a substrate, the substrate comprising a uniform diffractive structure including a plurality of diffractive elements; providing a shadow mask having a first region having a first ratio of aperture size to center-to-center spacing of apertures and a second region having a second ratio of aperture size to center-to-center spacing of apertures, the second ratio being less than the first ratio; positioning the shadow mask adjacent to the substrate; depositing the variable-thickness material by performing a deposition process on the substrate, wherein a layer thickness adjacent the first region is greater than a layer thickness adjacent the second region; A method comprising:

2. The method of claim 1 , wherein the substrate is made of SiO 2 .

3. The method of claim 2 , wherein the uniform diffractive structure comprises a diffraction grating.

4. The method of claim 1 , wherein the variable-thickness material varies in thickness such that the layer thickness adjacent the first region is greater than the layer thickness adjacent the second region.

5. the shadow mask comprises a plurality of openings and a surface parallel to the shadow mask; the substrate having a substrate surface onto which the variable thickness material is deposited; The method of claim 1 , wherein positioning the shadow mask adjacent to the substrate comprises placing the surface of the shadow mask parallel to a surface of the substrate.

6. The method of claim 1 , wherein the shadow mask is characterized by a variable ratio of aperture size to center-to-center spacing of the apertures in two directions.

7. The method of claim 1 , wherein the variable-thickness material is characterized by a first diffractive element depth in the first region and a second diffractive element depth in the second region that is less than the first diffractive element depth.

8. The method of claim 1 , wherein the variable-thickness material is characterized by a first diffractive element width in the first region and a second diffractive element width in the second region that is smaller than the first diffractive element width.

9. The method of claim 1 , wherein performing the deposition process on the substrate comprises a plasma deposition process.

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