Patent Publication Number: US-2022221788-A1

Title: Duty cycle transition zone mask correction

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Application Ser. No. 63/136,956, filed Jan. 13, 2021, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to optical devices for augmented, virtual, and mixed reality. More specifically, embodiments described herein provide for methods of forming optical device structures having continuously increasing or decreasing duty cycles. 
     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 enhance or augment 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 device structures of the optical devices include multiple discrete zones. However, light is lost by the sudden transition between two discrete zones and therefore decreases the optical performance of the optical devices. 
     Accordingly, what is needed in the art are improved methods of forming optical device structures having continuously increasing or decreasing duty cycles. 
     SUMMARY 
     In one embodiment, a device is provided. The device includes a plurality of optical device structures. The plurality of optical device structures have a critical dimension across a length of each optical device structure of the plurality of optical device structures. Each optical device structure of the plurality of optical device structures includes a plurality of discrete zones. The device further includes a plurality of transition zones disposed between two discrete zones of the plurality of discrete zones. The critical dimension of each transition zone of the plurality of transition zones continuously increases or decreases across the length of each optical device structure of the plurality of optical device structures. 
     In another embodiment, a device is provided. The device includes a plurality of optical device structures. The plurality of optical device structures have a critical dimension across a length of each optical device structure of the plurality of optical device structures. Each optical device structure of the plurality of optical device structures includes a plurality of discrete zones. The device further includes a plurality of transition zones disposed between two discrete zones of the plurality of discrete zones. The critical dimension of each transition zone of the plurality of transition zones continuously increases or decreases across the length of each optical device structure of the plurality of optical device structures. Adjacent optical device structures of the plurality of optical device structures include a pitch. The device further includes a duty cycle defined as the critical dimension of the plurality of optical device structures divided by the pitch. The duty cycle continuously increases or decreases across the length of each optical device structure of the plurality of optical device structures. 
     In yet another embodiment, a method is provided. The method includes editing a design file. The design file corresponds to a mask. The mask includes a plurality of openings. The plurality of openings have a width that continuously increases or decreases across a length of the plurality of openings. The method further includes generating the mask. The mask is to be disposed over a photoresist disposed over a substrate. The method further includes passing light through the plurality of openings of the mask positioned over the photoresist. The method further includes developing the photoresist to form a patterned photoresist. The patterned photoresist corresponds to the plurality of openings of the mask. The method further includes patterning the substrate or a device material to form a plurality of optical device structures. The optical device structures correspond to the plurality of openings of the mask. The plurality of optical device structures have a critical dimension that continuously increases or decreases across a length of the plurality of optical device structures. 
    
    
     
       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. 1  is a perspective, frontal view of an optical device according to embodiments described herein. 
         FIGS. 2A and 2B  are schematic, top-views of a plurality of optical device structures according to embodiments described herein. 
         FIGS. 3A and 3B  are schematic, cross-sectional views of a substrate according to embodiments described herein. 
         FIG. 3C  is a schematic, top-view of a mask according to embodiments described herein. 
         FIG. 4  is a flow diagram of a method for forming a plurality of optical device structures according to embodiments described herein. 
     
    
    
     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 for augmented, virtual, and mixed reality. More specifically, embodiments described herein provide for methods of forming optical device structures having continuously increasing or decreasing duty cycles. In one embodiment, a device is provided. The device includes a plurality of optical device structures. The plurality of optical device structures have a critical dimension across a length of each optical device structure of the plurality of optical device structures. Each optical device structure of the plurality of optical device structures includes a plurality of discrete zones. The device further includes a plurality of transition zones disposed between two discrete zones of the plurality of discrete zones. The critical dimension of each transition zone of the plurality of transition zones continuously increases or decreases across the length of each optical device structure of the plurality of optical device structures. 
       FIG. 1  illustrates a perspective, frontal view of an optical device  100 . In one embodiment, which can be combined with other embodiments described herein, the optical device  100  is a waveguide combiner, such as an augmented reality waveguide combiner. In another embodiment, which can be combined with other embodiments described herein, the optical device  100  is a flat optical device, such as a metasurface. The optical device  100  includes a plurality of optical device structures  102  disposed in or on a substrate  101 . The optical device structures  102  may be nanostructures having sub-micron dimensions, e.g., nano-sized dimensions, such as critical dimensions less than 1 μm. In one embodiment, which can be combined with other embodiments described herein, regions of the plurality of optical device structures  102  correspond to one or more gratings  104 , such as a first grating  104   a,  a second grating  104   b,  and a third grating  104   c.  In one embodiment, which can be combined with other embodiments described herein, the optical device  100  is a waveguide combiner that includes at least the first grating  104   a  corresponding to an input coupling grating and the third grating  104   c  corresponding to an output coupling grating. The waveguide combiner according to the embodiment, which can be combined with other embodiments described herein, may include the second grating  104   b  corresponding to an intermediate grating. A portion  105  of the optical device  100  is further described in  FIG. 2B . 
     In one embodiment, which can be combined with other embodiments described herein, the optical device  100  is a master for an imprint process, such as a nano-imprint lithography process. In another embodiment, which can be combined with other embodiments described herein, the optical device  100  is patterned directly, such as via a direct etch process. As described herein, the optical device  100  is one of a master pattern of a master for an imprint process for optical device fabrication (e.g., nano-imprint lithography using a master to form an optical device) or an optical device (e.g., a waveguide combiner and flat optical device). As described herein, the optical device  100  is formed from at least one of a device material or substrate. 
     In some embodiments, which can be combined with other embodiments described herein, the plurality of optical device structures  102  can be formed from the substrate  101 . The substrate  101  may be formed from any suitable material, provided that the substrate  101  can adequately transmit light in a desired wavelength or wavelength range and can serve as an adequate support for the optical device  100 , described herein. In some embodiments, which can be combined with other embodiments described herein, the material of the substrate  101  has a refractive index that is relatively low, as compared to the refractive index of the plurality of optical device structures  102 . Substrate selection may include substrates of any suitable material, including, but not limited to, amorphous dielectrics, non-amorphous dielectrics, crystalline dielectrics, silicon oxide, polymers, and combinations thereof. In some embodiments, which may be combined with other embodiments described herein, the substrate  101  includes a transparent material. In one example, the substrate  101  includes silicon (Si), silicon dioxide (SiO 2 ), germanium (Ge), silicon germanium (SiGe), InP, GaAs, GaN, fused silica, quartz, sapphire, and high-index transparent materials such as high-refractive-index glass. 
     In some embodiments, which can be combined with other embodiments described herein, the plurality of optical device structures  102  can be formed from a device material. The device material is disposed over the substrate  101 . The device material includes, but is not limited to, one or more of silicon carbide (SiC), silicon oxycarbide (SiOC), titanium dioxide (TiO 2 ), silicon dioxide (SiO 2 ), vanadium (IV) oxide (VOx), aluminum oxide (Al 2 O 3 ), aluminum-doped zinc oxide (AZO), indium tin oxide (ITO), tin dioxide (SnO 2 ), zinc oxide (ZnO), tantalum pentoxide (Ta 2 O 5 ), silicon nitride (Si 3 N 4 ), zirconium dioxide (ZrO 2 ), niobium oxide (Nb 2 O 5 ), cadmium stannate (Cd 2 SnO 4 ), silicon carbon-nitride (SiCN) containing materials, combinations thereof, or other suitable materials. 
       FIG. 2A  is a schematic, top-view of a plurality of optical device structures  202  according to embodiments described herein. The plurality of optical device structures  202  include a plurality of discrete zones  204 . As shown in  FIG. 2A , the plurality of optical device structures  202  have a first discrete zone  204   a,  a second discrete zone  204   b,  and a third discrete zone  204   c.  Although only three discrete zones  204  of the plurality of discrete zones  204  are shown, more than three discrete zones  204  may be included in the plurality of optical device structures  202 . The plurality of optical device structures  202  may be parallel, normal, or angled with respect to the boundaries between adjacent discrete zones  204  of the plurality of discrete zones  204 . 
     The plurality of discrete zones  204  have a critical dimension  206 . The critical dimension  206  is the width of each of the plurality of optical device structures  202 . In one embodiment, which can be combined with other embodiments described herein, the first discrete zone  204   a  has a first critical dimension  206   a,  the second discrete zone  204   b  has a second critical dimension  206   b,  and the third discrete zone  204   c  has a third critical dimension  206   c.  In one embodiment, which can be combined with other embodiments described herein, the critical dimension  206  is between about 40 nm and about 380 nm. 
     Adjacent optical device structures  202  of the plurality of optical device structures  202  have a pitch  208 . The pitch  208  is the distance between leading edges of the adjacent optical device structures  202 . The pitch  208  between adjacent optical device structures  202  remains constant across a length L of the plurality of optical device structures  202 . In one embodiment, which can be combined with other embodiments described herein, the pitch  208  is between about 280 nm and about 450 nm. 
     Adjacent optical device structures  202  of the plurality of optical device structures  202  have a duty cycle. The duty cycle is determined by dividing the critical dimension  206  of the optical device structure  202  by the pitch  208 . Therefore, the plurality of discrete zones  204  have different duty cycles. The duty cycle changes across the length L of the plurality of optical device structures  202  as the critical dimension  206  is changed. In one embodiment, which can be combined with other embodiments described herein, the duty cycle of the plurality of discrete zones  204  is between about 0.2 to about 0.85. The difference in the duty cycle between adjacent discrete zones  204  of the plurality of discrete zones  204  is greater than 0.01. The critical dimension  206  of each discrete zone  204  of the plurality of discrete zones  204  is constant across the entire zone. The difference in the critical dimensions  206  and the duty cycle between the plurality of discrete zones  204  results in decreased optical device performance. 
       FIG. 2B  is a schematic, top-view of a plurality of optical device structures  102  of a portion  105  of an optical device  100 . In one embodiment, which can be combined with other embodiments described herein, the plurality of optical device structures  102  are of a flat optical device, such as a metasurface. In another embodiment, which can be combined with other embodiments described herein, the plurality of optical device structures  102  are of a waveguide combiner, such as an augmented reality waveguide combiner. The waveguide combiner according to the embodiment, which can be combined with other embodiments described herein, may include the plurality of optical device structures  102  in at least one of the gratings  104 . In yet another embodiment, which can be combined with other embodiments described herein, the plurality of optical device structures  102  correspond to the first grating  104   a,  the second grating  104   b,  or the third grating  104   c  of the optical device  100 . Although only four of the plurality of optical device structures  102  are shown, one or more of the plurality of optical device structures  102  may be disposed on the optical device  100 . 
     The plurality of optical device structures  102  have a critical dimension  206 . The critical dimension  206  is the width of each of the plurality of optical device structures  202 . The plurality of optical device structures  102  include a plurality of discrete zones  204 . As shown in  FIG. 2B , the plurality of optical device structures  102  have a first discrete zone  204   a,  a second discrete zone  204   b,  and a third discrete zone  204   c.  Although three discrete zones  204  are shown in  FIG. 2B , more than three discrete zones  204  may be included on the plurality of optical device structures  102 . The first discrete zone  204   a  has a first critical dimension  206   a,  the second discrete zone  204   b  has a second critical dimension  206   b,  and the third discrete zone  204   c  has a third critical dimension  206   c.  In one embodiment, which can be combined with other embodiments described herein, each discrete zone  204  of the plurality of discrete zones  204  has a critical dimension  206  that continuously increases or decreases across a length L of the plurality of optical device structures  102 . In one example of this embodiment, as shown in  FIG. 2B , the first critical dimension  206   a,  the second critical dimension  206   b,  and the third discrete zone  204   c  increase across the length L of the plurality of optical device structures  102 . 
     The plurality of optical device structures  102  further include a plurality of transition zones  210 . The plurality of transition zones  210  are disposed between two discrete zones  204  of the plurality of discrete zones  204 . In one embodiment, which can be combined with other embodiments described herein, a first transition zone  210   a  is disposed between the first discrete zone  204   a  and the second discrete zone  204   b.  A second transition zone  210   b  is disposed between the second discrete zone  204   b  and the third discrete zone  204   c.  Although two transition zones  210  are shown in  FIG. 2B , more than two transition zones  210  may be included on the plurality of optical device structures  102 . 
     In one embodiment, which can be combined with other embodiments described herein, the critical dimension  206  in the plurality of transition zones  210  continuously increases or decreases across the length L of the plurality of optical device structures  102 . For example, the critical dimension  206  continuously increases from the first critical dimension  206   a  to the second critical dimension  206   b  in the first transition zone  210   a.  Additionally, the critical dimension  206  continuously increases from the second critical dimension  206   b  to the third critical dimension  206   c  in the second transition zone  210   b.    
     Adjacent optical device structures  102  of the plurality of optical device structures  102  have a pitch  208 . The pitch  208  is the distance between leading edges of the adjacent optical device structures  102 . The pitch  208  between adjacent optical device structures  102  remains constant across the length L of the plurality of optical device structures  102 . In one embodiment, which can be combined with other embodiments described herein, the pitch  208  is between about 280 nm and about 450 nm. 
     Adjacent optical device structures  102  of the plurality of optical device structures  102  have a duty cycle. The duty cycle is determined by dividing the critical dimension  206  of the optical device structure  102  by the pitch  208 . The duty cycle increases or decreases across the length L of the plurality of optical device structures  102  as the critical dimension  206  is changed. Therefore, as shown in  FIG. 2B , the duty cycle will continuously increase or decrease across the length L. The critical dimension  206  of the plurality of optical device structures  102  continuously increasing or decreasing across the length L improves the optical performance of the optical device  100 . 
       FIGS. 3A and 3B  schematic, cross-sectional views of a substrate  101 .  FIG. 4  is a flow diagram of a method  400  for forming a plurality of optical device structures  102 . At operation  401 , as shown in  FIG. 3A , a photoresist  304  is deposited. The photoresist  304  is deposited over a hardmask  302 . The photoresist  304  is sensitive to electromagnetic radiation. The photoresist  304  can be a positive or negative photoresist. 
     A positive photoresist includes portions of the photoresist  304  that, when exposed to radiation, are respectively soluble to a photoresist developer applied to the photoresist  304  after the pattern is written into the photoresist  304  using the electromagnetic radiation. A negative photoresist includes portions of the photoresist  304  that, when exposed to radiation, will be respectively insoluble to the photoresist developer applied to the photoresist  304  after the pattern is written into the photoresist  304  using the electromagnetic radiation. The chemical composition of the photoresist  304  determines whether the photoresist is a positive photoresist or negative photoresist. Examples of the photoresist  304  include, but are not limited to, at least one of diazonaphthoquinone, a phenol formaldehyde resin, poly(methyl methacrylate), poly(methyl glutarimide), and SU-8. The hardmask  302  includes, but is not limited to, silicon, silicon nitride (SiN), SiO, low-k, SiC, SiOC, SiCONH, TaO, BPSG, PSG, dielectrics, spin on glass (SOG), or metallic alloys such as TiN. 
     At operation  402 , as shown in  FIG. 3B , the photoresist  304  is patterned. Prior to patterning the photoresist  304 , a design file is edited.  FIG. 3C  is a schematic, top-view of a mask  308 . As shown in  FIG. 3C , the design file corresponds to the mask  308  to be developed. The design file is edited such that the mask  308  will include a width  312  of a plurality of openings  310  that continuously increases or decreases across a length L. In one embodiment, which can be combined with other embodiments described herein, an optical proximity correction (OPC) step is performed on the design file. The OPC step is utilized to meet a critical dimension  206  of a plurality of discrete zones  204  and the critical dimension  206  of a plurality of transition zones  210 . 
     The mask  308  is generated. The mask  308  is generated with the plurality of openings  310  corresponding to the plurality of optical device structures  102  to be patterned. Light passes through the plurality of openings  310  in the mask  308  to expose the photoresist  304  to electromagnetic radiation. The plurality of openings  310  have identical or substantially identical shapes as the plurality of optical device structures  102  to be patterned. The width  312  corresponds to the critical dimension  206  of the plurality of optical device structures  102 . In one embodiment, which can be combined with other embodiments described herein, the width  312  of each opening  310  of the plurality of openings  310  is larger than the critical dimension  206  at the corresponding point along the length L. The width  312  of each opening  310  of the plurality of openings  310  is larger than the critical dimension  206  at the corresponding point along the length L to account for a decrease of the critical dimension  206  compared to the width  312  during the method  400 . For example, a patterned photoresist  306  to be formed after light is passed through the plurality of openings  310  will have smaller dimensions than the width  312  of each opening  310 . In another embodiment, which can be combined with other embodiments described herein, the width  312  of each opening  310  of the plurality of openings  310  is smaller than the critical dimension  206  at the corresponding point along the length L. 
     As shown in  FIG. 3B , after exposure of the photoresist  304  to the electromagnetic radiation, the photoresist  304  is developed to leave a patterned photoresist  306  on the hardmask  302 . At operation  403 , the hardmask  302  is etched. Using the patterned photoresist  306 , the hardmask  302  is pattern etched through the openings in the patterned photoresist  306 . The hardmask  302  includes, but is not limited to, silicon nitride (SiN), SiO, low-k, SiC, SiOC, SiCONH, TaO, BPSG, PSG, dielectrics, or metallic alloys such as TiN. 
     At operation  404 , the plurality of optical device structures  102  are formed. The plurality of optical device structures  102  are formed by etching the substrate  101 . In one embodiment, which can be combined with other embodiments described herein, the plurality of optical device structures  102  are formed by etching a device material disposed on the substrate  101 . The substrate  101  or the device material is pattern etched through the openings in the hardmask  302 . As shown in  FIG. 2B , the plurality of optical device structures  102  includes a plurality of discrete zones  204  and a plurality of transition zones  210 . In one embodiment, which can be combined with other embodiments described herein, a critical dimension  206  continuously increases or decreases across a length L of the plurality of optical device structures. Additionally, the duty cycle between the plurality of discrete zones  204  and the plurality of transition zones  210  continuously increases or decreases across the length L. 
     In summation, methods of forming optical device structures having continuously increasing or decreasing duty cycles are disclosed. The optical device structures each have a plurality of transition zones disposed between two discrete zones. The critical dimensions of each transition zone of the plurality of transition zones continuously increase or decrease across a length of each optical device structure. Adjacent optical device structures include a constant pitch. A duty cycle defined as the critical dimension of the plurality of optical device structures divided by the pitch continuously increases or decreases across the length of each optical device structure. The continuous increasing or decreasing across the length of each optical device structure improves the optical performance of the optical device structures. The performance is improved as the continuous transition between two discrete zones allows for the light to be propagated more efficiently in the optical devices. 
     While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.