Patent Publication Number: US-2022212223-A1

Title: Edge blackening for optical devices

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. provisional patent application Ser. No. 63/117,569, filed Nov. 24, 2020, which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to optical devices. More specifically, embodiments described herein relate to an optical device coating assembly and method of coating the edge of optical devices with optically absorbent material. 
     Description of the Related Art 
     Virtual reality is generally considered to be a computer-generated simulated environment in which a user has an apparent physical presence. A virtual reality experience can be generated in 3D and viewed with a head-mounted display (HMD), such as glasses or other wearable display devices that have near-eye display panels as lenses to display a virtual reality environment that replaces an actual environment. 
     Augmented reality, however, enables an experience in which a user can still see through the display lenses of the glasses or other HMD device to view the surrounding environment, yet also see images of virtual objects that are generated for display and appear as part of the environment. Augmented reality can include any type of input, such as audio and haptic inputs, as well as virtual images, graphics, and video that enhances or augments the environment that the user experiences. As an emerging technology, there are many challenges and design constraints with augmented reality. 
     One such challenge is displaying a virtual image overlaid on an ambient environment. Optical devices including waveguide combiners, such as augmented reality waveguide combiners, and flat optical devices, such as metasurfaces, are used to assist in overlaying images. Generated light is propagated through an optical device until the light exits the optical device and is overlaid on the ambient environment. 
     Optical devices may require coating the edges of the optical devices with a coating of optically absorbent material. The coating of optically absorbent material improves the performance of the optical device. It is desirable for the coating to be uniform or substantially uniform across the entire edge of the optical device. Waveguide combiners generally have an irregular shape, which presents a challenge for providing a uniform coating on the edges of the waveguide combiner. 
     Accordingly, what is needed in the art are improved methods of coating the edges of optical devices with the optically absorbent material. 
     SUMMARY 
     In one embodiment, an optical device coating assembly is provided. The optical device coating assembly includes a substrate support operable to retain an optical device substrate. The coating assembly further includes a first actuator connected to the substrate support. The first actuator is configured to rotate the substrate support. The coating assembly includes a holder configured to hold a coating applicator against an edge of the optical device substrate when the optical device substrate is rotated on the substrate support and a second actuator operable to apply a force on the holder in a direction towards the substrate support. The second actuator is a constant force actuator. 
     In another embodiment, an optical device coating assembly is provided is provided. The coating assembly includes a substrate support; a first actuator connected to the substrate support, wherein the first actuator is configured to rotate the substrate support; a holder configured to hold a coating applicator against an edge of an optical device substrate that is rotated on the substrate support; and a controller in communication with the first actuator, the controller configured to cause the first actuator to adjust the rotational speed of the substrate support in order to cause different portions of an edge of a non-circular optical device substrate positioned on the substrate support to rotate against a coating applicator in the holder at a constant linear speed. 
     In another embodiment, a method of coating an optical device substrate is provided. The method includes positioning an optical device substrate on a substrate support, the optical device substrate including a bottom surface, a top surface, and one or more edges connecting the bottom surface with the top surface; rotating the substrate support with a first actuator to rotate the optical device substrate; and applying an optically absorbent coating to the one or more edges of the optical device substrate with a coating applicator positioned in a holder as the optical device substrate is rotated, wherein a second actuator applies a constant force to the holder during the applying of the optically absorbent coating. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments and are therefore not to be considered limiting of its scope, and may admit to other equally effective embodiments. 
         FIG. 1A  is a perspective view of a coating assembly, according to one embodiment. 
         FIG. 1  B is a perspective view of an alignment device, according to one embodiment. 
         FIG. 1C  is a bottom perspective view of the vacuum chuck, according to one embodiment. 
         FIG. 1  D is a top perspective view of the alignment device, according to one embodiment. 
         FIG. 2  is a process flow diagram of a method for aligning the substrate on the vacuum chuck by using the alignment device, according to one embodiment. 
         FIG. 3  is a process flow diagram of a method for coating the edges of the substrate with the coating system, according to one embodiment. 
         FIG. 4  is a perspective view of a coating assembly, according to one embodiment. 
     
    
    
     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. 
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure generally relate to optical devices. More specifically, embodiments described herein relate to equipment and methods for coating the edge(s) of optical devices (e.g., edges of a substrate to be used as a waveguide combiner or a flat optical device) with an optically absorbent material. 
     Although the following is largely described in reference to applying an optically absorbent coating to an optical device substrate that is to be used as a waveguide combiner, the advantages of the disclosure are applicable to applying a coating on the one or more edges of other substrates. Used herein, the term constant linear speed refers to a linear speed that is within 1% of a reference linear speed. Used herein, the term substantially constant linear speed refers to a linear speed that is within 5% of a reference linear speed. 
       FIG. 1A  is a perspective view of an optical device coating assembly  100 , according to one embodiment. The optical device coating assembly  100  is used to provide a coating on the edges  51  of an optical device substrate  50 . 
     The optical device substrate  50  includes the edges  51 , a top surface  52 , and a bottom surface  53 . The edges  51  connect the bottom surface  53  with the top surface  52 . The optical device coating assembly  100  can be used to coat the edges  51  of an optical device substrate  50  with a coating, such as a coating of an optically absorbent material. The optically absorbent material can darken (e.g., blacken) the edges of the optical device substrate  50 . In one embodiment, which can be combined with other embodiments described herein, the optical device substrate  50  includes, but is not limited to silicon (Si), silicon dioxide (SiO2), fused silica, quartz, glass, silicon carbide (SiC), germanium (Ge), silicon germanium (SiGe), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), or sapphire. In one embodiment, which can be combined with other embodiments described herein, the optical device substrate  50  is a waveguide combiner. In other embodiments, the optical device substrate can be a different type of device, such as a flat optical device, such as a flat optical device including a metasurface. In another embodiment, which can be combined with other embodiments described herein, the optical device substrate  50  can have optical device structures (e.g., structures having critical dimensions less than  1  micron) disposed on the optical device substrate  50 . 
     The optical device coating assembly  100  includes a substrate support  150 . The substrate support  150  can be used to retain and rotate the optical device substrate  50  such that the edges  51  of the optical device substrate  50  are coated. The optical device coating assembly  100  further includes a first actuator  110  that is used to rotate the substrate supporting surface of the substrate support  150 . The optical device coating assembly  100  further includes a holder  130  configured to hold an applicator  60  (e.g., a marker) that is used to coat the edges  51  of the optical device substrate  50 . The optical device coating assembly  100  further includes a pneumatic actuator  120  (also referred to as second actuator) that is used to move the holder  130  and the applicator  60 , for example in the X-direction, to enable the applicator  60  to contact and coat the edges  51  of the optical device substrate  50 . 
     The optical device coating assembly  100  further includes a platform  170  that is used to support and arrange the other components of the optical device coating assembly  100 . The platform  170  includes four legs  171  and a top  172  that is supported by the four legs  171 . The optical device coating assembly  100  further includes a mounting block  175  positioned on the top  172  of the platform  170 . In some embodiments, the mounting block  175  can include a track or guide (not shown) that can be used to ensure the holder  130  only moves in a specified direction (e.g., only in the X-direction). For example, the holder  130  could include a protrusion (not shown) extending from the bottom of the holder  130  to ensure the holder  130  moves along the track or guide in the mounting block  175  to provide for the movement that occurs only in the specified direction. 
     The substrate support  150  can include a vacuum chuck  151 . The optical device substrate  50  can be placed on the vacuum chuck  151 . The vacuum chuck  151  can be replaceable. The vacuum chuck  151  can grip defined zones of the optical device substrate  50 , such as the non-exclusion zones of the optical device substrate  50 . Although this disclosure describes the substrate being placed on  151  vacuum chuck, other types of substrate supporting surfaces can also be used. The vacuum chuck  151 can hold the optical device substrate  50  in place as the optical device substrate  50  is rotated. Additional information on the vacuum chuck is provided in the description of  FIGS. 1B-1D  below. The substrate support  150  can further include a rotary shaft  157  that is connected to the vacuum chuck  151 . The rotary shaft  157  is coupled to the first actuator  110  as described below enabling the first actuator  110  to control the rotation of the vacuum chuck  151 . The first actuator  110  and the vacuum chuck  151  may be configured to rotate at least  360  degrees enabling the coating applicator  60  to coat the edges  51  of the optical device substrate  50  along the rotation. 
     The first actuator  110  is used to rotate the substrate support  150 . The first actuator  110  can be an electric motor that is configured to rotate at different speeds, such as a servo or a motor connected to a variable frequency drive. The first actuator  110  can include an output shaft  113 . The output shaft  113  of the first actuator  110  can be coupled to another pulley (not shown). The optical device coating assembly  100  further includes a belt  115  and a pulley  116  to couple the rotational output of the first actuator  110  to the substrate support  150 . The pulley  116  can be coupled to the rotary shaft  157  of the substrate support  150 . The belt  115  and pulley  116  can be used to rotate the substrate support  150  at a different speed than the rotational speed of the first actuator  110 . The belts and pulleys described herein can be configured to be zero backlash belts (e.g., zero backlash timing belt) and pulleys. 
     The first actuator  110  can be suspended below the top  172  of the platform  170 . The first actuator  110  can be mounted to a plate  111  that is supported by supports  112  that are connected to the top  172  of the platform  170 . 
     The holder  130  is used to hold the coating applicator  60  against the edges  51  of the optical device substrate  50  as the edges  51  of the optical device substrate  50  are rotated past and against a tip  61  of the coating applicator  60 . The holder  130  includes a clamp  131 , a tilt arm  132 , and a base  133 . The base  133  can be a slidable base that can slide, for example towards or away from the substrate support  150 . The clamp  131  is used to hold the coating applicator  60 . The angle of the tilt arm  132  can be adjusted. For example, in one embodiment the angle of the tilt arm  132  can be adjusted in the XZ plane to orient the coating applicator  60  to be completely horizontal, completely vertical, or any angle in between. In some embodiments, the tilt arm  132  can also allow for angular adjustments in the XY plane, for example allowing the angle of coating applicator  60  to be adjusted plus or minus  15  degrees in the XY plane. 
     The coating applicator  60  includes the optically absorbent material to be coated on the edges  51  of the optical device substrate  50 . The coating applicator  60  is shown as a marker pen, but other types of coating applicators can be used. In various embodiments, which can be combined with other embodiments described herein, the applicator  60  can be a marker, pen, marker pen, a sponge applicator, a foam applicator, or a rubber wheel . The coating applicator  60  can apply optically absorbent coatings to the edges  51  of the optical device substrate  50 . Examples of optically absorbent coatings that can be used can include, but are not limited to, one or more pigment or die filled UV curable adhesives, one or more pigment or die filled heat curable adhesives, pigmented ink, or combinations thereof. 
     The base  133  can be coupled to the pneumatic actuator  120 . The base  133  can receive force from the pneumatic actuator  120  and apply that force to either move the holder  130  and/or to apply a corresponding force against the edges  51  of the optical device substrate  50  through the tip  61  of the coating applicator  60 . 
     As described in additional detail below in reference to  FIG. 1B , the optical device substrate  50  has an irregular shape (i.e., the top surface  52  of the optical device substrate  50  has an irregular shape). This irregular shape of the optical device substrate  50  makes it necessary for the tip  61  of the coating applicator  60  to move, for example in the X-direction, so that the tip  61  of coating applicator  60  can remain pressed against the edges  51  of the optical device substrate  50  while still allowing for the optical device substrate  50  to be smoothly rotated. 
     The pneumatic actuator  120  can include a base  121  and an extending portion  125 . The base  121  can receive pressurized air from a compressed air source (not shown) and apply force from the pressurized air to the extending portion  125 . The extending portion  125  can apply this force to press against the base  133  of the holder  130  to couple the force from the pneumatic actuator  120  to the holder  130 , so that the tip  61  of the coating applicator  60  can move in relation to the optical device substrate  50  and/or press against the edges  51  of the optical device substrate  50  as the substrate is rotated on the substrate support  150 . 
     The pneumatic actuator  120  can be a constant force actuator. The pneumatic actuator  120  can include a low-friction pneumatic cylinder. Using a constant force actuator as the pneumatic actuator  120  allows the pneumatic actuator  120  to apply a constant force to the holder  130  as the holder  130  moves, for example in the X-direction, to accommodate the irregular shape of the optical device substrate  50 . For example, the pneumatic actuator  120  can be configured to move the tip  61  of the coating applicator  60  in the holder  130  from a first position P 1  (i.e., the position in which the tip  61  is located in  FIG. 1A ) to a second position P 2  (e.g., a position that the tip  61  would move to when a portion of the edge  51  that extends out further than P 1  from a center of the substrate support  150  is rotated in front of the tip  61 ). As the portion of the edge  51  of the optical device substrate  50  that extends out to P 2  is rotated in front of the coating applicator  60 , the pneumatic actuator  120  allows movement of the holder  130  in the X-direction, so that the constant force of the tip  61  of the coating applicator  60  against the edge of the optical device substrate  50  can be maintained. 
     This constant force applied to the holder  130  allows for the tip  61  of the coating applicator  60  to remain pressed against the edges  51  of the optical device substrate  50  with a corresponding constant force for the entire  360  degrees rotation of the irregularly shaped optical device substrate  50 . For example, during rotation of the optical device substrate  50 , the pneumatic actuator  120  may be configured to move the extending portion  125 , so that the holder  130  moves, and the constant force can remain against the edge  51  of the optical device substrate  50 . This corresponding constant force of the tip  61  of the coating applicator  60  against the edges  51  of the optical device substrate  50  is one factor that enables a coating having a uniform thickness to be applied to the edges  51  of the optical device substrate  50 . 
     In some embodiments, the constant force applied by of the tip of the coating applicator  60  against the edge  51  of the optical device substrate  50  is from about 0.01 lbf to about 1.0 lbf, such as from about 0.05 lbf to about 0.5 lbf, such as about 0.25 lbf. In some embodiments, the uniform thickness of the coating applied by the coating applicator  60  to the edges  51  of the substrate is between about 100 μm and about 1000 μm, such as from about 300 μm to about 500 μm, such as about 400 μm. 
     In one embodiment, instead of moving the holder  130 , the angle of the tip  61  of the coating applicator  60  relative to the edge  51  of the optical device substrate  50  may change to accommodate the irregular shape of the optical device substrate  50 . For example, the coating applicator  60  could extend substantially parallel to the surface of the optical device substrate  50  when the portion of edge  51  closest to the center of the substrate support  150  is rotated past the tip  61 . The coating applicator  60  could then rotate in the XZ plane to being substantially perpendicular to the surface of the substrate when the portion of edge  51  furthest from the center of the substrate support  150  is rotated past the tip  61 . 
     The optical device coating assembly  100  further includes a controller  190  connected to the equipment shown in  FIG. 1A , such as the first actuator  110 , the pneumatic actuator  120 , and the vacuum chuck  151 . The controller  190  can be any type of controller used in an industrial setting, such as a programmable logic controller (PLC). Although the controller  190  is shown as a single component, this is not required. In some embodiments, the controller  190  can be distributed across multiple components of the optical device coating assembly  100 . For example, in some embodiments the first actuator  110  can be a “smart” motor that can be programmed to rotate according to a specific speed profile based on the shape of the top surface of the optical device substrate  50  and the positioning of the optical device substrate  50  on the substrate support  150 . The controller  190  includes a processor  192 , a memory  194 , and input/output (I/O) circuits  196 . The controller  190  can further include one or more of the following components (not shown), such as one or more power supplies, clocks, communication components (e.g., network interface card), and user interfaces typically found in controllers for equipment described herein. 
     The processor  192  is configured to execute various programs stored in the memory  194 , such as a program configured to execute the methods described below in reference to  FIGS. 2 and 3 . The memory  194  can further include various operational settings used to control the optical device coating assembly  100 . For example, the settings can include settings for controlling (1) the force applied by the pneumatic actuator  120  and (2) speed settings for the first actuator  110  based on the shape of the optical device substrate  50  being rotated by the substrate support  150  among various other settings. 
     The memory  194  can include non-transitory memory. The non-transitory memory can be used to store routines and settings, such as a routine and settings used to execute the methods described below in reference to  FIGS. 2 and 3 . The memory  194  can include one or more readily available types of memory, such as read only memory (ROM) (e.g., electrically erasable programmable read-only memory (EEPROM), flash memory, floppy disk, hard disk, or random access memory (RAM) (e.g., non-volatile random access memory (NVRAM). Routines for coating different optical device substrates  50  can be generally stored in the memory  194 . These routines can be executed by the processor  192  with signals being received from inputs (e.g., speed sensors, motor feedback sensors) and signals being transmitted to outputs (e.g., the first actuator  110  and the pneumatic actuator  120 ) through the I/O circuits  196 . In one embodiment, the optical device coating assembly  100  can include a solenoid valve (not shown) to supply the pressurized air to the pneumatic actuator  120 . 
     Although the coating system may include inputs and sensors, it is noteworthy that the optical device coating assembly  100  can be used to apply a uniform coating to the edges  51  of a optical device substrate  50  with very few inputs (e.g., stop/start controls) and in some embodiments zero sensors. This ability of the optical device coating assembly  100  to be fully functional without any sensors or other feedback offers a significant advantage relative to the complex designs included in the conventional techniques mentioned above that generally include multi-axis stages and/or robotic arms along with sensors for edge detection of the substrate and precision dispensers. 
     The memory  194  of the controller can include a program to adjust the speed of the first actuator  110  along a speed profile that is based on (1) the shape of the optical device substrate  50  (i.e., shape of the top surface  52 ) that is to be coated and (2) the positioning of the optical device substrate  50  on the substrate support  150 . This program to adjust the speed of the first actuator  110  based on the shape of the optical device substrate  50  is designed, so that the speed of the first actuator  110  is adjusted as the optical device substrate  50  is rotated in order to cause each portion of a same length of the edges  51  of the optical device substrate  50  to pass the tip  61  of the coating applicator  60  with a constant linear speed or a substantially constant linear speed. Having each portion of the edges  51  of the optical device substrate  50  pass the tip  61  of the coating applicator  60  with the same linear speed is another factor that enables a coating having a uniform thickness to be applied to the edges  51  of the optical device substrate  50 . In one embodiment, a distance from a center point of rotation on the optical device substrate  50  to the point on the edge  51  of the optical device substrate  50  is used to determine the rotational speed of the substrate support  150  when that point passes the coating applicator  60 . 
       FIG. 1B  is a perspective view of an alignment device  160  and the optical device substrate  50  aligned on the vacuum chuck  151 , according to one embodiment. The vacuum chuck  151  includes a body  152  having a top surface  154 . The optical device substrate  50  is positioned on the top surface  154  of the body  152  of the vacuum chuck  151 . The body  152  of the vacuum chuck  151  can include holes  153  in the top surface  154  for providing suction to keep the optical device substrate  50  properly positioned on the vacuum chuck  151 . 
     The optical device substrate  50  is described as being a substrate to be used as a waveguide combiner in an augmented reality device. Substrates, such as the optical device substrate  50  can often have a non-circular shape, such as an irregular shape. The shape referred to here is the shape of the top surface of the substrate, such as the top surface  52  of the optical device substrate  50 . Circular substrates are often used when the substrate being processed is rotated as it is generally easy to properly align circular substrates on the surface on which the substrate will be rotated. Rotating non-circular substrates (e.g., rectangular substrates) can be somewhat more challenging than rotating circular substrates and rotating irregularly shaped substrates (e.g., the optical device substrate  50 ) can be more challenging than rotating a rectangular substrate. An irregularly shaped substrate can be defined as a substrate that has no line of symmetry, which can bisect the top surface of the substrate into two identical portions. For example, the top surface  52  of the optical device substrate  50  cannot be bisected into two identical portions. A non-circular substrate can be defined as any substrate not having top surface with a circular shape and non-circular substrates include common shapes, such as substrates having a rectangular shape and also include irregularly shaped substrates. 
     Finding a proper center of rotation for an irregularly shaped substrate, such as the optical device substrate  50 , can be challenging. Properly aligning an irregularly shaped substrate (e.g., optical device substrate  50 ) on the rotational support (e.g., vacuum chuck  151 ) helps the irregularly shaped substrate stay balanced when rotated. To assist in aligning the irregularly shaped optical device substrate  50  over the vacuum chuck  151 , the alignment device  160  can be used. 
     The alignment device  160  can be placed under the body  152  of the vacuum chuck  151  as further described below in reference to  FIGS. 1C and 1D . The alignment device  160  can include a base  161 . The base  161  can include a top surface  164 . The alignment device  160  can further include a plurality of alignment pins  162  extending upward from the top surface  164  of the base  161 . The alignment device  160  can further include a registration pin  163  extending upward from the top surface  164  of the base  161 . The alignment pins  162  can extend a further distance (e.g., have a higher height) from the base  161  than the registration pin  163  extends from the base  161 . This height difference between the alignment pins  162  and the registration pin  162  allows the optical device substrate  50  to be placed on the vacuum chuck  151  without the registration pin  163  contacting the optical device substrate  50 . 
     The registration pin  163  can be used to align the alignment device  160  with the vacuum chuck  151 . In one embodiment, the registration pin  163  and the vacuum chuck  151  can include markings (e.g., matching lines or corresponding arrows) to assist with properly aligning the alignment device  160  with the vacuum chuck  151 . The alignment pins  162  can be used to align the optical device substrate  50  on the vacuum chuck  151  after the vacuum chuck  151  is aligned with the registration pin  163 . In some embodiments, the edges  51  of the optical device substrate  50  can contact the alignment pins  162 , for example as shown in  FIG. 1B . In some embodiments, the alignment pin  162  and the optical device substrate  50  can include markings (e.g., matching lines or corresponding arrows) to assist with properly aligning the optical device substrate  50  with the alignment pins  162 , so that the optical device substrate  50  is properly positioned on the vacuum chuck  151 . 
       FIG. 1C  is a bottom perspective view of the vacuum chuck  151 , according to one embodiment. The vacuum chuck  151  further includes a shaft  157  extending below the body  152  of the vacuum chuck  151 . The body  152  of the vacuum chuck includes a bottom surface  155 . Two magnets  158  are positioned on the bottom surface  155  of the body  152  of the vacuum chuck  151 . 
       FIG. 1D  is a top perspective view of the alignment device  160 , according to one embodiment. Two magnets  165  are positioned on the top surface  164  of the base  161  of the alignment device  160 . The magnets  165  can be positioned to align with the magnets  158  positioned on the bottom surface  155  of the body  152  of the vacuum chuck  151  to further assist in aligning the vacuum chuck  151  with the alignment device  160 . The alignment device  160  can further include a notch  166 . The notch  166  can allow the alignment device  160  to extend partially around the shaft  157  of the vacuum chuck  151  when the vacuum chuck  151  is aligned with the alignment device  160 , for example as shown in  FIG. 1B . The notch  166  also allows for removal of the alignment device  160  once the optical device substrate  50  is properly aligned on the vacuum chuck  151 , so that the edges  51  of the optical device substrate  50  can be coated by the optical device coating assembly  100 . 
       FIG. 2  is a process flow diagram of a method  2000  for aligning the optical device substrate  50  on the vacuum chuck  151  by using the alignment device  160 , according to one embodiment. The method begins at block  2002 . 
     At block  2002 , the alignment device  160  is aligned with the vacuum chuck  151 . In some embodiments, block  2002  can be performed prior to positioning the optical device substrate  50  on the vacuum chuck  151 . Aligning the vacuum chuck  151  with the alignment device  160  can include positioning the notch  166  ( FIG. 1D ) of the alignment device  160  around the shaft  157  ( FIG. 1C ) of the vacuum chuck  151 . This alignment can further include positioning the alignment device  160  to cause the magnets on the top surface  164  of the alignment device  160  to magnetically couple to the magnets  158  on the bottom surface  155  of the body  152  of the vacuum chuck  151 . This alignment can further include positioning alignment device  160  to cause the vacuum chuck  151  to contact the registration pin  163 , for example as shown in  FIG. 1B . In one embodiment, the registration pin  163  and the vacuum chuck  151  can include markings (e.g., matching lines) to assist with properly aligning the alignment device  160  with the vacuum chuck  151 . For example, in some embodiments these markings can be made to physically contact each other. 
     At block  2004 , the optical device substrate  50  is aligned with the alignment pins  162  on the alignment device  160 . For example, the edges  51  of the optical device substrate  50  can be positioned to contact the alignment pins  162 , for example as shown in  FIG. 1B . In some embodiments, the alignment pin  162  and the optical device substrate  50  can include markings (e.g., matching lines) to assist with properly aligning the optical device substrate  50  with the alignment pins  162 , so that the optical device substrate  50  is properly positioned on the vacuum chuck  151 . For example, in some embodiments these markings can be made to physically contact each other. Aligning the optical device substrate  50  with the alignment pins  162  allows a proper center of rotation to be established for the optical device substrate  50 , so that the optical device substrate  50  can remain balanced when rotated by the substrate support  150 . Furthermore, using the specified alignment resulting from use of the alignment pins  162  allows a clear starting point on the edges  51  of the optical device substrate  50  to be defined for coating the edges  51  of the optical device substrate  50 . With this clear starting point defined, the speed at which the substrate support  150  is rotated can then be properly adjusted throughout the rotation of the optical device substrate  50 , so that the linear speed of different portions of the edges  51  of the optical device substrate  50  remains constant as these different portions pass by and against the tip  61  of the coating applicator  60 . This constant linear speed helps achieve a uniform coating on the edges  51  of the optical device substrate  50 . 
     At block  2006 , suction is applied to the vacuum chuck  151  to clamp the optical device substrate  50 , so the optical device substrate  50  remains in the properly aligned position accomplished at block  2004 . For example, the suction can be applied to the holes  153  shown in  FIG. 1B  to clamp the optical device substrate  50  in the proper position. 
     At block  2008 , with the optical device substrate  50  clamped in the proper position, the alignment device  160  is removed. With the optical device substrate  50  in the proper position and the alignment device  160  removed, the edges  51  of the optical device substrate  50  can be coated by the optical device coating assembly  100 . After the optical device substrate  50  is coated, the method  2000  can then be repeated using the same alignment device  160  when a substrate that is the same as the optical device substrate  50  is to be coated. If a substrate having a different size or shape than the optical device substrate  50  is to be coated, then a different alignment device may be used. For example, a larger alignment device may be needed for a larger substrate that has the same shape as the optical device substrate  50 , so that the alignment pins can be placed further from the center of the vacuum chuck  151 . Having an alignment device for the substrates of the different sizes and shapes can significantly reduce the amount of time spent properly loading the different substrates on the substrate support  150 . 
       FIG. 3  is a process flow diagram of a method  3000  for coating the edges  51  of the optical device substrate  50  with the optical device coating assembly  100 , according to one embodiment. The method begins at block  3002 . 
     At block  3002 , the method  2000  is performed as described in reference to  FIG. 2 , so that the optical device substrate  50  is properly positioned on the vacuum chuck  151 . 
     At block  3004 , the coating applicator  60  is positioned in the holder  130 . This positioning can include positioning the coating applicator  60  positioned into the clamp  131  of the holder  130 . This positioning can further include adjusting the tilt arm  132  to ensure the tip  61  of the coating applicator  60  is at the proper height to contact the edges  51  of the optical device substrate  50  when the optical device substrate  50  is rotated on the vacuum chuck  151 . The coating applicator  60  includes the coating to be applied to the edges  51  of the optical device substrate  50 . In some embodiments, the coating can be an optically absorbent coating that can be used to darken (e.g., blacken) the edges  51  of the optical device substrate  50 . 
     At block  3006 , the first actuator  110  and the pneumatic actuator  120  are activated. Activating the first actuator  110  causes the substrate support  150  to begin to rotate, so that the optical device substrate  50  on the vacuum chuck  151  of the substrate support  150  begins to rotate. Activating the pneumatic actuator  120  causes the pneumatic actuator  120  to apply a force on the holder  130 . For example, the force from the pneumatic actuator  120  can move the holder  130  towards substrate support  150  (e.g., the X-direction in  FIG. 1A ). Because the coating applicator  60  was positioned at block  3004  to cause the tip  61  of the coating applicator  60  to be at the proper height to contact the edges  51  of the optical device substrate  50 , the force from the pneumatic actuator  120  also causes the tip  61  of the coating applicator  60  to contact a portion of an edge  51  of the optical device substrate  50 . 
     In some embodiments, the controller  190  can be used to activate the first actuator  110  and the pneumatic actuator  120 . In some embodiments, the actuators  110 ,  120  can be activated simultaneously. In other embodiments, the pneumatic actuator  120  can be activated and then after a short delay (e.g., 50 ms, 100 ms, 500 ms etc.), the first actuator  110  can be activated. The short delay can be timed to allow the tip  61  of the coating applicator  60  to briefly contact the starting point on the edge  51  of the optical device substrate  50  before the optical device substrate  50  begins to rotate, so that the starting point on the edge is coated similarly to the rest of the locations on the edges  51  of the substrate. 
     At block  3008 , the speed of the first actuator  110  is adjusted to vary the speed at which the vacuum chuck  151  on the substrate support  150  rotates in order to cause different portions of one or more edges  51  or all portions all of the edges  51  (i.e.,  360  degrees around the edges  51 ) of the optical device substrate  50  to pass by and against the tip  61  of the coating applicator  60  at a constant linear speed or a substantially constant linear speed. The controller  190  can be used to vary the speed of the first actuator  110  during block  3008 . The controller  190  can be configured to control this rotational speed to cause the constant or substantially constant linear speed of the edges  51  of the optical device substrate  50  by and against the coating applicator  60  for a non-circular substrate (e.g., a rectangular substrate) or for an irregularly shaped substrate (e.g., the optical device substrate  50 ). As mentioned above, although the controller  190  is shown as a single controller in  FIG. 1A , this is meant for ease of illustration, and control of the different components of the optical device coating assembly  100  can be distributed across multiple components. For example, in one embodiment, the first actuator  110  is a “smart” motor that can include its own controller to vary the speed at which the motor rotates in order to cause the constant or substantially constant linear speed of the edges  51  of the optical device substrate  50  past and against the coating applicator  60 . In another embodiment, the first actuator  110  is a servo that receives signals, for example from the controller  190 , that are used to control the speed of the first actuator  110 . 
     During blocks  3006  and  3008 , the vacuum chuck  151  is configured to hold the optical device substrate  50  positioned on the vacuum chuck  151  in place as the pneumatic actuator  120  applies a constant force on the edges  51  of the optical device substrate  50  during rotation of the vacuum chuck  151  for 360 degrees. 
     At block  3010 , the first actuator  110  and the pneumatic actuator  120  are deactivated. Deactivating the first actuator  110  causes the substrate support  150  to stop rotating. Deactivating the pneumatic actuator  120  can cause the holder  130  and the coating applicator  60  to move away from the substrate support  150 . In some embodiments, the pneumatic actuator  120  can be deactivated and then after a short delay the first actuator  110  can be deactivated. The pneumatic actuator  120  can be deactivated when a specified amount of rotation has occurred with the coating applicator  60  being pressed against the edges  51  of the optical device substrate  50 . In some embodiments, this specified amount of rotation can be when one or more full rotations of  360  degrees from the starting point (i.e., the first point on the edge  51  that is coated) are completed. The controller  190  can be used to deactivate the first actuator  110  and the pneumatic actuator  120 . 
     The optical device coating assembly  100  described above can also be used to provided different sections of uniform thickness around the edges of a substrate, such as the optical device substrate  50 . For example, if specifications call for a rectangular substrate to have a uniform coating of a first thickness on the two longer sides of the rectangular substrate and a uniform coating of a second thickness on the shorter two sides of the rectangular substrate, then optical device coating assembly  100  can adjust the rotational speed of the substrate support  150  by adjusting the speed of the first actuator  110  and/or adjust the amount of force applied by the pneumatic actuator  120  to generate the different thicknesses of uniform coatings. For example, to apply a thicker coating the rotational speed of the substrate support  150  can be slowed down by slowing down the first actuator  110 , or the amount of force applied by the pneumatic actuator  120  can be increased. Conversely, to apply a thinner coating the rotational speed of the substrate support  150  can be increased by speeding up the first actuator  110 , or the amount of force applied by the second actuator  120  can be decreased. The speed of the first actuator  110  and/or the force applied by the pneumatic actuator  120  can also be gradually changed to apply gradual changes in the thickness of the coating applied to the edges. 
       FIG. 4  is a perspective view of an optical device coating assembly  400 , according to one embodiment. The optical device coating assembly  400  is the same as the optical device coating assembly  100  described above in reference to  FIGS. 1A-1D  except that the pneumatic actuator  120  is replaced with a voice coil  420 . The voice coil  420  is a constant force actuator like the pneumatic actuator  120  described above. Thus, the voice coil  420  can provide any and all of the same functionality described above in reference to the pneumatic actuator  120 . While the pneumatic actuator  120  uses compressed air to apply a constant force on the holder  130  that holds the coating applicator against the optical device substrate  50 , the voice coil  420  uses electrical power to apply a constant force on the holder  130  that holds the coating applicator against the optical device substrate  50 . 
     The coating assemblies and related methods described above can be used for coating the edge(s) of optical device substrates (e.g., edges of a substrate to be used as a waveguide combiner or a flat optical device) with a uniform thickness of optically absorbent material. In one embodiment, the thickness of the coating is from about 10 μm to about 100 μm, with a surface encroachment width ranging from about 0 to about 1000 μm. This uniform thickness on the edge(s) of the optical devices is achieved by using the constant force actuator (e.g., pneumatic actuator or voice coil) to apply the coating on the edge of the substrate as the substrate is rotated past and against a coating applicator. In addition to the constant force applied by the actuator, the rotational speed of the substrate is adjusted throughout the rotation to cause edges of the substrate to move past and against the coating applicator at a constant linear speed, so that different segments of the edges spend the same time passing by the applicator. This constant force and constant linear speed results in a uniform coating on the edges of the substrate even when the substrate has an irregular shape, which is common for optical devices, such as waveguide combiners. Furthermore, the equipment and methods described herein are substantially less complex than conventional techniques used to apply a uniform coating on an edge of substrate, such as techniques involving multi-axis stages and/or robotic arms along with sensors for edge detection of the substrate and precision dispensers. While being substantially less complex than these conventional techniques, the equipment and methods described herein offers significantly improved uniformity results when compared to manually applying the coating to an edge of an optical device substrate. 
     While the foregoing is directed to examples of the present disclosure, other and further examples of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.