Patent Publication Number: US-11662516-B2

Title: Method of direct etching fabrication of waveguide combiners

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. application Ser. No. 16/762,869, filed May 8, 2020, which is a 371 application of PCT/US2018/060651, filed Nov. 13, 2018, which claims benefit of U.S. Application No. 62/592,364, filed Nov. 29, 2017, all of which are herein incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to waveguides for augmented, virtual, and mixed reality. More specifically, embodiments described herein provide for methods of waveguide fabrication from substrates. 
     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 overlayed on an ambient environment. Waveguides are used to assist in overlaying images. Generated light is propagated through a waveguide until the light exits the waveguide and is overlayed on the ambient environment. Fabricating waveguides can be challenging as waveguides tend to have non-uniform properties. Accordingly, what is needed in the art are improved augmented waveguides and methods of fabrication. 
     SUMMARY 
     In one embodiment, a method for forming a waveguide structure is provided. The method includes imprinting a stamp into a resist to form a positive waveguide pattern. The resist is disposed on a hard mask formed on a surface of a portion of a substrate. The positive waveguide pattern includes a pattern including at least one of a first plurality of grating patterns, a waveguide pattern, and a second plurality of grating patterns. Each of the first plurality of grating patterns and the second plurality of grating patterns have a residual layer and top pattern surfaces. A curing process is performed to cure the positive waveguide pattern. The stamp is released. A first etching process is performed to remove the residual layer and the hard mask disposed under the residual layer and to expose the surface of the substrate. The substrate is masked to expose a first unprotected area of the surface of the substrate. A second etching process is performed for a first predetermined period of time to form a first plurality of gratings with first depths. The substrate is masked to expose a second unprotected area of the surface of the substrate. The second etching process is performed for a second predetermined period of time to form a second plurality of gratings with second depths. The top pattern surfaces, the waveguide pattern, and the hard mask disposed under the top pattern surfaces and the waveguide pattern are removed to form a waveguide structure including at least one of an input coupling region, a waveguide region, and an output coupling region. 
     In another embodiment, a method for forming a waveguide structure is provided. The method includes imprinting a stamp into a resist to form a positive waveguide pattern. The resist is disposed on a hard mask formed on a surface of a portion of a substrate. The positive waveguide pattern includes a pattern including at least one of a first plurality of grating patterns, a waveguide pattern, and a second plurality of grating patterns. Each of the first plurality of grating patterns and the second plurality of grating patterns have a residual layer, top pattern surfaces, and sidewall pattern surfaces slanted relative to the surface of the substrate. A curing process is performed to cure the positive waveguide pattern. The stamp is released. A first etching process is performed to remove the residual layer and the hard mask disposed under the residual layer and to expose the surface of the substrate. The substrate is masked to expose a first unprotected area of the surface of the substrate. Etching at a predetermined angle for a first predetermined period of time forms a first plurality of angled gratings with first depths. The substrate is masked to expose a second unprotected area of the surface of the substrate. Etching at the predetermined angle for a second predetermined period of time forms a second plurality of angled gratings with second depths; The top pattern surfaces, the waveguide pattern, and the hard mask disposed under the top pattern surfaces and the waveguide pattern are removed to form a waveguide structure including at least one of an input coupling region, a waveguide region, and an output coupling region. 
     In yet another embodiment, a method for forming a waveguide structure is provided. The method includes imprinting a stamp into a resist to form a positive waveguide pattern, the resist is disposed on a hard mask formed on a surface of a substrate, the positive waveguide pattern includes a pattern including at least one of a first plurality of grating patterns, a waveguide pattern, and a second plurality of grating patterns. Each of the first plurality of grating patterns and the second plurality of grating patterns have a residual layer, top pattern surfaces, and sidewall pattern surfaces slanted by relative to the surface of the substrate. The positive waveguide pattern is cured by electromagnetic radiation curing. The stamp is released. The residual layer is removed by plasma ashing. The hard mask disposed under the residual layer is reactive ion etched to expose the surface of the substrate. The substrate is masked to expose a first unprotected area of the surface of the substrate. Directional reactive ion etching (RIE) at a predetermined angle for a first predetermined period of time forms a first plurality of angled gratings with first depths. The substrate is masked to expose a second unprotected area of the surface of the substrate. Directional RIE at the predetermined angle for a second predetermined period of time forms a second plurality of angled gratings. The substrate is masked to expose a third unprotected area of the surface of the substrate. Directional RIE at the predetermined angle for a third predetermined period of time forms a third plurality of angled gratings with third depths. The top pattern surfaces and the waveguide pattern are removed by plasma ashing. Reactive ion etching the hard mask disposed under the top pattern surfaces and the waveguide pattern forms a waveguide structure including at least one of an input coupling region, a waveguide region, and an output coupling region 
    
    
     
       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, may admit to other equally effective embodiments. 
         FIG.  1    is a perspective, frontal view of a waveguide combiner according to an embodiment. 
         FIG.  2    is a flow chart illustrating operations of a method for forming a waveguide structure according to an embodiment. 
         FIGS.  3 A- 3 E  are schematic, cross-sectional views of a waveguide during fabrication according to an embodiment. 
         FIGS.  4 A- 4 E  are schematic, cross-sectional views of a waveguide during fabrication according to an embodiment. 
         FIG.  5    is a flow chart illustrating operations of a method for forming a waveguide structure according to an embodiment 
         FIGS.  6 A- 6 E  are schematic, cross-sectional views of a waveguide during fabrication according to an 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 described herein relate to methods for fabricating waveguide structures from substrates. The waveguide structures are formed having input coupling regions, waveguide regions, and output coupling regions formed from substrates. The regions are formed by imprinting stamps into resists disposed on hard masks formed on surfaces of the substrates to form positive waveguide patterns. Portions of the positive waveguide patterns and hard masks disposed under the portions are removed. The substrates are masked and etched to form gratings in the input coupling regions and the output coupling regions. Residual portions of the positive waveguide pattern and the hard masks disposed under the residual portions are removed to form waveguide structures having input coupling regions, waveguide regions, and output coupling regions formed from substrates. In one embodiment, the substrates include at least one of glass and plastic materials. In another embodiment, the substrates have a refractive index between about 1.5 and about 2.5. 
       FIG.  1    is a perspective, frontal view of a waveguide combiner  100 . It is to be understood that the waveguide combiner  100  described below is an exemplary waveguide combiner. The waveguide combiner  100  includes an input coupling region  102  defined by a plurality of gratings  108 , a waveguide region  104 , and an output coupling region  106  defined by a plurality of gratings  110 . 
     The input coupling region  102  receives incident beams of light (a virtual image) having an intensity from a microdisplay. Each grating of the plurality of gratings  108  splits the incident beams into a plurality of modes, each beam having a mode. Zero-order mode (T0) beams are refracted back or lost in the waveguide combiner  100 , positive first-order mode (T1) beams undergo total-internal-reflection (TIR) through the waveguide combiner  100  across the though the waveguide region  104  to the output coupling region  106 , and negative first-order mode (T−1) beams propagate in the waveguide combiner  100  a direction opposite to the T1 beams. The T1 beams undergo total-internal-reflection (TIR) through the waveguide combiner  100  until the T1 beams come in contact with the plurality of gratings  110  in the output coupling region  106 . The T1 beams contact a grating of the plurality of gratings  110  where the T1 beams are split into T0 beams refracted back or lost in the waveguide combiner  100 , T1 beams that undergo TIR in the output coupling region  106  until the T1 beams contact another grating of the plurality of gratings  110 , and T−1 beams coupled out of the waveguide combiner  100 . 
       FIG.  2    is a flow diagram illustrating operations of a method  200  for forming a waveguide structure. In one embodiment, waveguide structure  300  corresponds to at least one of the input coupling region  102 , the waveguide region  104 , and the output coupling region  106  of the waveguide combiner  100 . At operation  201 , a stamp having a negative waveguide pattern is imprinted into a resist that is disposed on a hard mask formed on a surface of a portion of a substrate to form a positive waveguide pattern. The positive waveguide pattern includes a pattern. The pattern includes at least one of an input coupling portion, a waveguide portion, and an output coupling portion to result in the formation of at least one of the input coupling region  102 , the waveguide region  104 , and the output coupling region  106  of the waveguide combiner  100 . The pattern includes a plurality of gratings patterns including a residual layer and top pattern surfaces. In one embodiment, the pattern includes plurality of gratings patterns and a waveguide pattern. 
     At operation  202 , a curing process is performed to cure the positive waveguide pattern. At operation  203 , the stamp is released from the resist. At operation  204 , the residual layer and the hard mask disposed under the residual layer are removed by performing a first etching process to expose the surface of the portion of the substrate. At operation  205 , the substrate is masked to expose a first unprotected area of the surface of the substrate. At operation  206 , a first plurality of gratings with first depths is formed by performing a second etching process for a first predetermined period of time. In one embodiment, the first plurality of gratings are a first portion of at least one of the plurality of gratings  108  of the input coupling region  102  and the plurality of gratings  110  of the output coupling region  106 . In another embodiment, the first plurality of gratings are at least one of the plurality of gratings  108  of the input coupling region  102  and the plurality of gratings  110  of the output coupling region  106 . 
     At operation  207 , the substrate is masked to expose a second unprotected area of the surface of the substrate. At operation  208 , a second plurality of gratings with second depths is formed by performing the second etching process for a second predetermined period of time. In one embodiment, the second plurality of gratings are a second portion of at least one of the plurality of gratings  108  of the input coupling region  102  and the plurality of gratings  110  of the output coupling region  106 . In another embodiment, the second plurality of gratings are at least one of the plurality of gratings  108  of the input coupling region  102  and the plurality of gratings  110  of the output coupling region  106 . At operation  209 , the top pattern surfaces, the waveguide pattern, and the hard mask disposed under the top pattern surfaces and the waveguide pattern are removed to form a waveguide structure. The waveguide structure includes at least one of the input coupling region  102 , the waveguide region  104 , and the output coupling region  106  of the waveguide combiner  100 . 
     Referring to  FIGS.  3 A- 3 E , imprinting the stamp, performing a curing process, releasing the stamp, performing the first etching process, performing the second etching process, and removing the top pattern surfaces, the waveguide pattern, and the hard mask will be described in more detail in regards to fabricating a waveguide structure  300 . 
     As shown in  FIG.  3 A , a stamp  336  is imprinted into a resist  325  disposed on a hard mask  303  formed on a surface  302  of a portion  330  substrate  301  to form a positive waveguide pattern  313 . The stamp  336  has a negative waveguide pattern  312  that includes an inverse pattern  331 . The inverse pattern  331  includes at least one of an input coupling portion  314 , an inverse waveguide portion  316 , and an inverse output coupling portion  318 . The stamp  336  is fabricated from a waveguide master having a master pattern. In one embodiment, the master pattern includes at least one of a master input coupling portion, a master waveguide portion, and a master output coupling portion. The stamp  336  is molded from the waveguide master. 
     The stamp  336  may include a semi-transparent material such as fused silica or polydimethylsiloxane (PDMS) to allow the positive waveguide pattern  313  to be cured by exposure to radiation, such as infrared (IR) radiation or ultraviolet (UV) radiation at operation  202 . In one embodiment, the resist  325  comprises a UV curable material (such as mr-N210 available from Micro Resist Technology) that is nano imprintable by the stamp  336  including PDMS. The positive waveguide pattern  313  may alternatively be thermally cured at operation  202 . In one embodiment, the surface  302  of the substrate  301  is prepared for spin coating of the UV curable material by UV ozone treatment, oxygen (O 2 ) plasma treatment, or by application of a primer (such as mr-APS1 available from Micro Resist Technology). In another embodiment, the resist  325  includes a thermally curable material that may be cured by a solvent evaporation curing process. The solvent evaporation curing process may include thermal heating or infrared illumination heating. The resist  325  may be disposed on the surface  302  using a liquid material pour casting process, a spin-on coating process, a liquid spray coating process, a dry powder coating process, a screen printing process, a doctor blading process, a physical vapor deposition (PVD) process, a chemical vapor deposition (CVD) process, a flowable CVD (FCVD) process, or an atomic layer deposition (ALD) process. 
     The positive waveguide pattern  313  includes a pattern  322 . The pattern  322  includes at least one of an input coupling portion  315 , a waveguide portion  317 , and an output coupling portion  319 . The pattern  322  includes at least one of a first plurality of grating patterns  305 , a waveguide pattern  333 , and a second plurality of grating patterns  327 . Each of the first plurality of grating patterns  305  and the second plurality of grating patterns  327  has a residual layer  311 , oftentimes referred to as bottom pattern surfaces, and top pattern surfaces  307  parallel to the surface  302  of the substrate  301  and sidewall pattern surfaces  309  oriented normal to the surface  302  of the substrate  301 . 
       FIG.  3 B  illustrates a schematic, cross-sectional view after the stamp  336  has been released at operation  203 . In one embodiment, the stamp  336  can be mechanically removed by a machine tool or by hand peeling as the stamp  336  may be coated with a mono-layer of anti-stick surface treatment coating, such as a fluorinated coating. In another embodiment, the stamp  336  may comprise a polyvinyl alcohol (PVA) material that is water soluble in order for the stamp  336  to be removed by dissolving the stamp  336  in water. In yet another embodiment, the stamp  326  comprises a rigid backing sheet, such as a sheet of glass, to add mechanical strength. 
       FIG.  3 C  is a schematic, cross-sectional view after the first etching process removes the residual layer  311  and the hard mask  303  disposed under the residual layer  311  and exposes the surface  302  of the substrate  301  at operation  204 . The residual layer  311  may be removed by plasma ashing, oftentimes referred to as plasma descumming, using an oxygen gas (O 2 ) containing plasma, a fluorine gas (F 2 ) containing plasma, a chlorine gas (Cl 2 ) containing plasma, and/or a methane (CH 4 ) containing plasma. In one embodiment, a first radio frequency (RF) power is applied to O 2  and an inert gas, such as argon (Ar) or nitrogen (N), until the residual layer  311  is removed. The hard mask  303  disposed under the residual layer  311  may be removed by ion etching, reactive ion etching (RIE), or highly selective wet chemical etching. For example, the hard mask  303  may have a first layer comprising a first material and a second layer comprising a second material, where the second layer is formed on the first layer disposed on the surface  302  of the substrate  301 . The second layer may be removed by utilizing a high etch selectivity of the second material over the first material and materials of the substrate  301 . The first layer may be removed by utilizing a high etch selectivity of the first material over the materials of the substrate  301 . In one embodiment, the hard mask  303  comprises a silicon containing layer (such as OptiStack® HM825-302.6 available from Brewer Science) disposed on a carbon containing layer (such as OptiStack® SOC110D available from Brewer Science). The silicon containing layer disposed under the residual layer  311  is removed by RIE using a F 2  containing plasma, and the carbon containing layer disposed under the residual layer  311  is removed by RIE using a O 2  containing plasma. In another embodiment, the hard mask  303  comprises a chromium containing layer disposed on a silicon dioxide (SiO 2 ) containing layer. The chromium containing layer disposed under the residual layer  311  is removed by RIE using a Cl 2  containing plasma, such as BCl 3 , and the silicon dioxide containing layer is removed by RIE using a F 2  containing plasma or CH 4  containing plasma. 
       FIG.  3 D  is a schematic, cross-sectional view after operations  205 - 208 . At operations  205 - 208 , the second etching process is performed for a first predetermined period of time after the substrate  301  is masked to expose the first unprotected area of the surface  302  of the substrate  301  and the second etching process is performed for a second predetermined period of time after the substrate  301  is masked to expose a second unprotected area of the surface  302  of the substrate  301 . In one embodiment, a chamber mounted etch end point detection system is used. Masking the substrate  301  may include placing a shadow mask in physical contact with the surface  302  of the substrate  301  or aligning a photomask over the substrate  301  to expose the first unprotected area. In one embodiment, the shadow mask is metal and may be used repeatedly. 
     In one embodiment, the first unprotected area corresponds to a first region and the second unprotected area corresponds to a second region of the resulting waveguide structure  300 . In another embodiment, the first unprotected area corresponds to the first region and the second region, and the second unprotected area corresponds to the second region. In one embodiment, as shown, the first region is the input coupling region  102  and the second region is the output coupling region  106 . In another embodiment, not shown, the first region and the second region are regions of the input coupling region  102  or the output coupling region  106 . The second etching process may include etching processes such as ion implantation, ion etching, RIE, directional RIE such as directed ribbon-beam ion etching, microblasting, waterjet cutting, and laser etching for the first predetermined period of time to form a first plurality of gratings  304  with first depths  321 . One example of an ion implantation apparatus is the Varian VIISTA® Trident, available from Applied Materials, Inc., Santa Clara, Calif. Ion etching may be in the presence of an etch gas to improve the ion etching rate 
     The first plurality of gratings  304  further includes top surfaces  306  and bottom surfaces  328  corresponding to the surface  302  of the substrate  301  and sidewall surfaces  308  oriented normal to the surface  302  of the substrate  301 . The shadow mask may be relocated or the photomask may be realigned to expose the second unprotected area. Performing the second etching process for a second predetermined period of time forms the second plurality of gratings  310  with second depths  323 . The second plurality of gratings  310  further includes the top surfaces  306 , the bottom surfaces  328 , and the sidewall surfaces  308 . 
       FIG.  3 E  is a schematic, cross-sectional view of the waveguide structure  300  formed after operation  209 . At operation  209 , the top pattern surfaces  307 , the waveguide pattern  333 , and the hard mask  303  disposed under the top pattern surfaces  307  and waveguide pattern  333  at operation  207  are removed to form the waveguide structure  300  including at least one of the input coupling region  102 , the waveguide region  104 , and the output coupling region  106  of the waveguide combiner  100 . The top pattern surfaces  307  and the waveguide pattern  333  may be removed by plasma ashing using the O 2  containing plasma, the F 2  containing plasma, the Cl 2  containing plasma and/or the CH 4  containing plasma. In one embodiment, a second radio frequency (RF) power is applied to O 2  and an inert gas, such as argon (Ar) or nitrogen (N), until the top pattern surfaces  307  and the waveguide pattern  333  are removed. The hard mask  303  disposed under the top pattern surfaces  307  and waveguide pattern  333  may be removed by ion etching, RIE, or highly selective wet chemical etching. In one embodiment, the hard mask  303  includes the silicon containing layer disposed on the carbon containing layer. The silicon containing layer is removed by RIE using the F 2  containing plasma, and the carbon containing layer is removed by RIE using the O 2  containing plasma. In another embodiment, the hard mask  303  comprises the chromium containing layer disposed on the SiO 2  containing layer. The chromium containing layer is removed by RIE using the Cl 2  containing plasma and the silicon dioxide containing layer is removed by RIE using the F 2  containing plasma or the CH 4  containing plasma. In another embodiment, the hard mask  303  is not removed and the hard mask  303  comprises materials, such as SiO 2  and titanium dioxide (TiO 2 ), with controlled refractive indices between about 1.5 and about 2.5. 
     The waveguide structure  300  resulting from the method  200  has a substantially uniform refractive index. Utilizing materials having a refractive index of between about 1.5 and about 2.5 for the substrate  301 , as compared to the refractive index of air (1.0), total internal reflection, or at least a high degree thereof, is achieved to facilitate light propagation through the waveguide structure  300 . 
     Referring to  FIGS.  4 A- 4 E , imprinting the stamp, performing a curing process, releasing the stamp, performing the first etching process, performing the second etching process, and removing the top pattern surfaces, the waveguide pattern, and the hard mask will be described in more detail in regards to fabricating a waveguide structure  400 . 
     At operation  201 , a stamp  426  is imprinted into a resist  425  disposed on a hard mask  403  formed on a surface  402  of a portion  430  of a substrate  401  to form a positive waveguide pattern  413 . As shown in  FIG.  4 A , the stamp  426  has a negative waveguide pattern  412  that includes an inverse pattern  431 . The inverse pattern  431  includes at least one of an inverse input coupling portion  414 , an inverse waveguide portion  416 , and an inverse output coupling portion  418 . At operation  202 , the positive waveguide pattern  413  is cured. 
     The positive waveguide pattern  413  includes a pattern  422 . The pattern  422  includes at least one of includes an input coupling portion  415 , a waveguide portion  417 , and an output coupling portion  419 . The pattern  422  includes at least one of a first plurality of grating patterns  405 , a waveguide pattern  433 , and a second plurality of grating patterns  427 . Each of the first plurality of grating patterns  405  and the second plurality of grating patterns  427  has a residual layer  411 , oftentimes referred to as bottom pattern surfaces, and top pattern surfaces  407  parallel to the surface  402  of the substrate  401  and sidewall pattern surfaces  409  slanted by an amount relative to the surface  402  of the substrate  401 . 
     At operation  203 , the stamp  426  is released from the resist  425 . At operation  204 , the residual layer  411  and the hard mask  403  disposed under the residual layer  411  are removed by performing a first etching process to expose the surface  402  of the substrate  401 . 
     At operation  205 , the substrate  401  is masked to expose a first unprotected area of the surface  402  of the substrate  401 . Masking the substrate  401  may include placing a shadow mask in physical contact with the surface  402  of the substrate  401  or aligning a photomask over the substrate  401  to expose the first unprotected area. In one embodiment, the first unprotected area corresponds to a first region and the second unprotected area corresponds to a second region of the resulting waveguide structure  400 . In another embodiment, the first unprotected area corresponds to the first region and the second region, and the second unprotected area corresponds to the second region. In one embodiment, as shown, the first region is the input coupling region  102  and the second region is the output coupling region  106 . In another embodiment, not shown, the first region and the second region are regions of the input coupling region  102  or the output coupling region  106 . At operation  206 , performing the second etching process at a predetermined angle for a first predetermined period of time of time forms a first plurality of gratings  404  with first depths  421  and sidewall surfaces  408  slanted by an amount relative to the surface  402  of the substrate  401 . The predetermined angle may be determined by computer simulation to maximize the light coupling efficiency of the waveguide structure  400  and the predetermined angle may range from about 15 degrees to about 75 degrees. The first plurality of gratings  404  further includes top surfaces  406  parallel to the surface and bottom surfaces  328  corresponding to the surface  302 . 
     Etching at the predetermined angle may include etching processes such as angled ion implantation, angled ion etching, and directional RIE such as directed ribbon-beam ion etching. Angled ion implantation includes accelerating ions towards the surface  402  of the substrate  401  at the predetermined angle relative to the surface  402  of the substrate  401  and bombarding the substrate  401  with ions at the predetermined angle to selectively remove material to form the first plurality of gratings  404 . In one embodiment, introduction of an etch gas close to an ion generation source will improve the ion etching rate. One example of an angled ion implantation apparatus is the Varian VIISTA® Trident, available from Applied Materials, Inc., Santa Clara, Calif. 
     At operation  207 , the substrate  401  is masked to expose a second unprotected area of the surface  402  of the substrate  401 . The shadow mask may be relocated or the photomask may be realigned to expose the second unprotected area. At operation  208 , a second plurality of angled gratings  410  with second depths  423  is formed by etching at the predetermined angle for a second predetermined period of time. The second plurality of angled gratings  410  further includes top surfaces  406  and bottom surfaces  428  and sidewall surfaces  408  slanted by an amount relative to the surface  402  of the substrate  401 . 
     At operation  209 , the top pattern surfaces  407 , the waveguide pattern  433 , and the hard mask  403  disposed under the top pattern surfaces  407  and the waveguide pattern  433  are removed to form the waveguide structure  400  having the input coupling region  102 , the waveguide region  104 , and the output coupling region  106 . The top pattern surfaces  407  and the waveguide pattern  433  may be removed to form the waveguide structure  400  including at least one of the input coupling region  102 , the waveguide region  104 , and the output coupling region  106  of the waveguide combiner  100 . In another embodiment, the hard mask  403  is not removed and the hard mask  403  comprises materials with controlled refractive indices between about 1.5 and about 2.5. The waveguide structure  400  resulting from the method  200  has substantially uniform refractive index. 
       FIG.  5    is a flow diagram illustrating operations of a method  500  for forming a waveguide structure  600  as shown in  FIGS.  6 A- 6 E . In one embodiment, waveguide structure  600  corresponds to at least one of the input coupling region  102 , the waveguide region  104 , and the output coupling region  106  of the waveguide combiner  100 . At operation  501 , a stamp  626  is imprinted into a resist  625  disposed on a hard mask  603  formed on a surface  602  of a portion  630  of a substrate  601  to form a positive waveguide pattern  613 . As shown in  FIG.  6 A , the stamp  626  has a negative waveguide pattern  612  that includes an inverse pattern  631 . The inverse pattern  631  includes at least one of an inverse input coupling portion  614 , an inverse waveguide portion  616 , and an inverse output coupling portion  618 . 
     The positive waveguide pattern  613  includes a pattern  622 . The pattern  622  includes at least one of an input coupling portion  615 , a waveguide portion  617 , and an output coupling portion  619 . The pattern  622  includes at least one of a first plurality of grating patterns  605 , a waveguide pattern  633 , and a second plurality of grating patterns  627 . Each of the first plurality of grating patterns  605  and the second plurality of grating patterns  627  has a residual layer  611 , oftentimes referred to as bottom pattern surfaces, and top pattern surfaces  607  parallel to the surface  602  of the substrate  601  and sidewall pattern surfaces  609  slanted by an amount relative to the surface  602  of the substrate  601 . 
     At operation  502 , the positive waveguide pattern  613  is cured by exposure to electromagnetic radiation, such as infrared (IR) radiation and ultraviolet (UV) radiation. At operation  503 , the stamp  626  is released from the resist  625 . At operation  504 , the residual layer  611  is removed by plasma ashing until the residual layer  611  is removed. At operation  505 , the hard mask  603  disposed under the residual layer  611  is removed by ion etching, reactive ion etching (RIE), and highly selective wet chemical etching. 
     At operation  506 , the substrate  601  is masked to expose a first unprotected area of the surface  602  of the substrate  601 . Masking the substrate  601  may include placing a shadow mask in physical contact with the surface  602  of the substrate  601  or aligning a photomask over the substrate  601  to expose the first unprotected area. In one embodiment, the first unprotected area corresponds to a first region, the second unprotected area corresponds to a second region, and a third unprotected area corresponds to a third region of the resulting waveguide structure  600 . In another embodiment, the first unprotected area corresponds to the first region, the second region, and the third region, the second unprotected area corresponds to the second region and the third region, and the third unprotected area corresponds to the third region. In one embodiment, as shown, the first region is the input coupling region  102 , and the second region and the third region are the output coupling region  106 . In another embodiment, not shown, the first region, the second region, and the third region are regions of the input coupling region  102  or the output coupling region  106 . 
     At operation  507 , in one embodiment, a first plurality of gratings  604  with first depths  621  is formed by directional reactive ion etching (RIE) at a predetermined angle for a first predetermined period of time to a first etch depth. The first plurality of gratings  604  further includes top surfaces  606  parallel to the surface  602  of the substrate  601 , bottom surfaces  628  corresponding to the surface  602  of the substrate  601 , sidewall surfaces  608  slanted by an amount relative to the surface  602  of the substrate  601 . 
     At operation  508 , the substrate  601  is masked to expose a second unprotected area of the surface  602  of the substrate  601 . The shadow mask may be relocated or the photomask may be realigned to expose the second unprotected area. In one embodiment, the second unprotected area corresponds to a first portion  634  of the output coupling region  106 . In another embodiment, the second unprotected are corresponds to the first portion  634  and a second portion  635  of the output coupling region  106 . In one embodiment, at operation  509 , a second plurality of angled gratings  610  with second depths  623  in the first portion  634  is formed by directional RIE at the predetermined angle for a second predetermined period of time. The second plurality of angled gratings  610  further includes top surfaces  606 , bottom surfaces  628 , and sidewall surfaces  608  slanted by an amount relative to the surface  602  of the substrate  601 . 
     At operation  510 , the substrate  601  is masked to expose a third unprotected area of the surface  602  of the substrate  601 . The shadow mask may be relocated or the photomask may be realigned to expose the third unprotected area. In one embodiment, the third unprotected area corresponds to the second portion  635  of the output coupling region  106 . At operation  511 , a third plurality of angled gratings  632  with third depths  629  is formed by directional RIE at the predetermined angle for a third predetermined period of time. The third plurality of angled gratings  632  further includes top surfaces  606  and bottom surfaces  628  parallel to the surface  602  of the substrate  601  and sidewall surfaces  608  slanted by an amount relative to the surface  602  of the substrate  601 . 
     At operation  512 , the top pattern surfaces  607 , the waveguide pattern  633 , and the hard mask  603  disposed under the top pattern surfaces  607  and the waveguide pattern  633  are removed by plasma ashing. At operation  513 , the hard mask  603  disposed under the top pattern surfaces  607  and waveguide pattern  633  is removed by ion etching, RIE, or highly selective wet chemical etching to form the waveguide structure  600  having the input coupling region  102 , the waveguide region  104 , and the output coupling region  106 . In another embodiment, the hard mask  603  is not removed and the hard mask  603  comprises materials with controlled refractive indices between about 1.5 and about 2.5. The waveguide structure  600  resulting from the method  500  has substantially uniform refractive index. 
     In summation, methods for fabricating waveguide structures utilizing substrates are described herein. The utilization of substrates provide for waveguide structures having input coupling regions, waveguide regions, and output coupling regions with substantially uniform refractive indices. Utilizing materials having a refractive index of between about 1.5 and about 2.5 for the substrates, as compared to the refractive index of air (1.0), total internal reflection, or at least a high degree thereof, is achieved to facilitate light propagation through the augmented structure. 
     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.