Patent Publication Number: US-11662524-B2

Title: Forming variable depth structures with laser ablation

Description:
BACKGROUND 
     Field 
     Embodiments of the present disclosure generally relate to optical devices for augmented, virtual, and mixed reality. More specifically, embodiments described herein provide forming depth-modulated device structures of optical devices. 
     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, or handheld device, to view the surrounding environment, yet also see images of virtual objects that are generated in the 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. Optical devices 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 optical devices can be challenging as optical devices tend to have non-uniform properties. Accordingly, improved methods of fabricating optical devices are needed in the art. 
     SUMMARY 
     The present disclosure generally relates to a method for forming a device structure for use in a display apparatus or in other applications. More specifically, the disclosure relates to a variable depth structure for use in the device structure using created using laser ablation. The method herein may also form a device structure that is used as a master for nano-imprint lithography. 
     In one embodiment, a method of forming a device structure is provided. The method includes forming a variable-depth structure in a device material layer using laser ablation. The method also includes forming a hardmask and a photoresist stack over the device material layer. The method further includes etching the photoresist stack. The method also includes forming a plurality of device structures in the device material layer. 
     In another embodiment, a method of forming a device structure is provided. The method includes forming a device material layer on a substrate and forming a variable-depth structure in the device material layer using laser ablation. The method also includes forming a hardmask and a photoresist stack over the device material layer. The method further includes etching the photoresist stack and forming a plurality of device structures in the device material layer. 
     In yet another embodiment, a method of forming a device structure is provided. The method includes forming a device material layer on a substrate and forming a sacrificial layer on the device material layer. The method further includes forming a variable-depth structure in the sacrificial layer using laser ablation. The method also includes forming a hardmask and a photoresist stack over the device material layer. The method further includes etching the photoresist stack and forming a plurality of device structures in the device material layer. 
    
    
     
       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 front view of an optical device according to an embodiment. 
         FIG.  2    is a flow diagram of a method for forming a device structure according to an embodiment. 
         FIGS.  3 A- 3 H  are schematic, cross-sectional views of a portion of a variable-depth structure according to an embodiment. 
         FIGS.  4 A- 4 C  are cross-sectional enlargements of examples of shapes of a variable-depth structure. 
         FIGS.  5 A- 5 C  are perspective views of examples of three dimensional shapes of a variable-depth structure. 
     
    
    
     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 of forming a device structure having variable-depth slanted device structures. To accomplish this, a method includes forming a variable-depth structure in a device material layer using laser ablation. A plurality of channels is formed in the variable-depth structure to define slanted device structures therein. The variable-depth structure is formed using laser ablation and the slanted device structures are formed using a selective etch process. The method described herein can also be used to create a device structure that functions as a master for nanoimprint lithography. 
       FIG.  1    is a front view of an optical device  100 . It is to be understood that the optical device  100  described below is an exemplary optical device. In one embodiment, the optical device  100  is a waveguide combiner, such as an augmented reality waveguide combiner. In another embodiment, the optical device  100  is a flat optical device, such as a metasurface. The optical device  100  includes a plurality of device structures  104 . The device structures  104  may be nanostructures having sub-micro dimensions, e.g., nano-sized dimensions, such as critical dimensions less than 1 μm. In one embodiment, regions of the device structures  104  correspond to one or more gratings  102 , such as the grating areas  102   a  and  102   b . In one embodiment, the optical device  100  includes a first grating area  102   a  and a second grating area  102   b  and each of the first grating area  102   a  and  102   b  each contain a plurality of device structures  104 . 
     The depth of the gratings  102  may vary across the grating areas  102   a  and  102   b  in embodiments described herein. In some embodiments, the depth of the gratings  102  may vary smoothly over the first grating area  102   a  and over the second grating area  102   b . In one example embodiment, the depth may range from about 10 nm to about 400 nm across one of the grating areas. The grating area  102   a , in an example embodiment, can range from approximately 20 mm to approximately 50 mm on a given side. Therefore, as one example, the angle of the change in the depth of the gratings  102  may be on the order of 0.0005 degrees. 
     In embodiments described herein, the device structures  104  may be created using laser ablation. Laser ablation, as used herein, is used to create three-dimensional microstructures in the device material, or optionally to create a variable-depth structure in a sacrificial layer overlaying the device material as part of a variable-depth structure process. Using laser ablation to create the optical structures  104  allows for fewer processing operations and higher variable-depth resolution than existing methods. 
       FIG.  2    is a flow diagram of a method  200  for forming a portion of optical device  300 , shown in  FIGS.  3 A- 3 H , having variable-depth structures, which corresponds with grating area  102   a  or  102   b . At operation  201 , a device material layer  306  is disposed over a surface of a substrate  302  as shown in  FIG.  3 A . The substrate  302  may be formed from any suitable material, provided that the substrate  302  can adequately transmit light in a desired wavelength or wavelength range and can serve as an adequate support for the portion of optical device  300 . In some embodiments, which can be combined with other embodiments described herein, the material of substrate  302 , includes, but is not limited to, one or more silicon (Si), silicon dioxide (SiO 2 ), or sapphire containing materials. In other embodiments, which can be combined with other embodiments described herein, the material of substrate  302  includes, but is not limited to, materials having a refractive index between about 1.7 and about 2.0. 
     The device material layer  306  may be disposed over the surface of the substrate  302  by one or more (PVD), chemical vapor deposition (CVD), plasma-enhanced (PECVD), flowable CVD (FCVD), atomic layer deposition (ALD), or spin-on processes. In one embodiment, which can be combined with other embodiments described herein, the device material of device material layer  306  is selected based on the modulated depth and slant angle of each of the plurality of device structures  104  of the portion of optical device  300  and the refractive index of the substrate  302 . In some embodiments, which can be combined with other embodiments described herein, the device material layer  306  includes, but is not limited to, one or more silicon nitride (SiN), silicon oxycarbide (SiOC), titanium dioxide (TiO 2 ), silicon dioxide (SiO 2 ), vanadium (IV) oxide (VOx), aluminum oxide (Al 2 O 3 ), indium tin oxide (ITO), zinc oxide (ZnO), tantalum pentoxide (Ta 2 O 5 ), silicon nitride (Si 3 N 4 ), zirconium dioxide (ZrO 2 ), or silicon carbon-nitride (SiCN) containing materials. In some embodiments, which can be combined with other embodiments described herein, the device material of the device material layer  306  may have a refractive index between about 1.5 and about 2.65. In other embodiments, which can be combined with other embodiments described herein, the device material of the device material layer  306  may have a refractive index between about 3.5 and about 4.0 
     In some embodiments, which can be combined with other embodiments described herein, an etch stop layer  304  may be optionally disposed on the surface of the substrate  302  between the substrate  302  and the device material layer  306 . The etch stop layer  304  may be disposed by one or more PVD, CVD, PECVD, FCVD, ALD, or spin-on processes. The etch stop layer  304  may be formed from any suitable material, for example titanium nitride (TiN) or tantalum nitride (TaN), among others, provided that the etch stop layer  304  is resistant to the etching processes described herein. In one embodiment, which can be combined with other embodiments described herein, the etch stop layer  304  is a non-transparent etch stop layer that is removed after the device structure  104  is formed. In another embodiment, the etch stop layer  304  is a transparent etch stop layer. 
     At operation  202 , a sacrificial layer  308  is formed over the device material layer  306 , as shown in  FIG.  3 B . In one embodiment, the sacrificial layer  308  is a SiN layer, SiOx layer or photoresist layer. In one embodiment, forming the sacrificial layer  308  includes disposing a resist material over the device material layer  306  and developing the resist material utilizing a lithography process. The resist material may include but is not limited to, light-sensitive polymer containing materials. Developing the resist material may include performing a lithography process, such as photolithography, digital lithography and/or laser ablation. In this embodiment, laser ablation is performed on the sacrificial layer  308  to create a shape of a variable-depth structure  301  within the sacrificial layer  308  over a length L with a depth of D on the left side and a depth of D′ on the right side. As described above, any desired one-, two-, or three-dimensional shape can be created in the sacrificial layer  308  using laser ablation. Laser ablation uses variable pulse repetition of a laser beam scanned across an area to be ablated. One benefit of laser ablation over other variable-depth process, such as gray-tone resist processes, is that laser ablation is a physical process as opposed to the chemical process using a gray-tone resist which can have a limited shelf life. Laser ablation also results in faster throughput and faster changes to the variable-depth structure without the need for masks. Laser ablation also results in increased spacial fidelity or resolution over typical etch processes. 
     In this embodiment, at operation  203 , a transfer etch process is then performed on the variable-depth structure  301  of the sacrificial layer  308  to form the variable-depth structure  301  within the device material layer  306 . The results of operation  203  are illustrated in  FIG.  3 C . In this embodiment, the transfer etch process removes the sacrificial layer  308  and etches the underlying device material layer  306  to produce the variable-depth structure  301  within the device material layer  306 . 
     The variable-depth structure  301  in this embodiment has a length L between a first end and a second end. The first end of the variable-depth structure  301  has a depth F and the second end has a depth F′. That is, the depth of the variable-depth structure  301  is minimal at the first end and maximum at the second end in this embodiment. The depth from F to F′ generally is within a range of about 0 nm to about 700 nm. In this embodiment, the length L is substantially large compared to the depths F and F′. For example, the length L may be about 25 mm while the depth F at the first end is about 0 nm to about 50 nm and the depth F′ at the second end is about 250 nm to about 700 nm. Accordingly, the variable-depth structure  301  has a substantially shallow slope. In this example, the angle of the slope is less than 1 degree, such as less than 0.1 degrees, like about 0.0005 degrees. The slope of the variable-depth structure  301  is illustrated herein with an exaggerated angle for clarity. 
     In one embodiment, which can combined with other embodiments, where the device design process does not require the deposition of a sacrificial layer  308  as described above, laser ablation may be performed directly on the device material layer  306  to form the variable-depth structure  301 . Laser ablation is performed to create the shape of the variable-depth structure  301  over the length L with a depth of F on the left side and a depth of F′ on the right side. In one embodiment, the shape of the variable-depth structure  301  over length L is in the shape of a wedge with varying levels of depth. The shape of the variable-depth structure  301  determines the modulation of the depth D of device structure  104  across the substrate  302 , as shown in  FIG.  3 H . 
     At operation  204 , a hardmask  312  is disposed over the device material layer  306  and variable-depth structure  301 . The results of operation  204  are illustrated in  FIG.  3 D . The hardmask  312  may be disposed over the device material layer  306  by one or more liquid material pour casting, spin-on coating, liquid spray coating, dry powder coating, screen printing, doctor blading, PVD, CVD, PECVD, FCVD, ALD, evaporation, or sputtering processes. In one embodiment, which can be combined with other embodiments described herein, the hardmask  312  is non-transparent and is removed after the portion of optical device  300  is formed. In another embodiment, the hardmask  312  is transparent. In some embodiments, which can be combined with other embodiments described herein, the hardmask  312  includes, but is limited to, chromium (Cr), silver (Ag), Si 3 N 4 , SiO 2 , TiN, or carbon (C) containing materials. The hardmask  312  can be deposited so that the thickness of the hardmask  312  is substantially uniform. In yet other embodiments, the hardmask  312  can be deposited so that the thickness varies from about 30 nm and about 50 nm at varying points on the device material layer  306 . The hardmask  312  is deposited in such a way that the slope of the hardmask  312  is similar to the slope of the variable-depth structure  301 . 
     At operation  205 , an organic planarization layer  314  is disposed over the hardmask  312 . The results of operation  205  are illustrated in  FIG.  3 E . The organic planarization layer  314  may include a photo-sensitive organic polymer comprising a light-sensitive material that, when exposed to electromagnetic (EM) radiation, is chemically altered and thus configured to be removed using a developing solvent. The organic planarization layer  314  may include any organic polymer and a photo-active compound having a molecular structure that can attach to the molecular structure of the organic polymer. In one embodiment, which can be combined with other embodiments described herein, the organic planarization layer  314  may be disposed using a spin-on coating process. In another embodiment, which can be combined with other embodiments described herein, the organic planarization layer  314  may include, but is not limited to, one or more of polyacrylate resin, epoxy resin, phenol resin, polyamide resin, polyimide resin, unsaturated polyester resin, polyphenylenether resin, polyphenylenesulfide resin, or benzocyclobutene (BCB). 
     As shown in  FIG.  3 E , the optical planarization layer  314  varies in thickness, such that a substantially planar top surface is formed. The optical planarization layer  314  varies in thickness, such that the space between the sloped conformal hardmask  312  and the substantially planar top surface of the optical planarization layer  314  is completely filled and has a varying thickness over the sloped wedge shaped-structure  301 . 
     Referring to  FIGS.  3 E- 3 H , at operation  206 , a patterned photoresist  316  is disposed over the organic planarization layer  314 . The patterned photoresist  316  is formed by disposing a photoresist material on the organic planarization layer  314  and developing the photoresist material. The patterned photoresist  316  defines a hardmask pattern  315 , shown in  FIG.  3 E  that corresponds to exposed segments  321  of the device material layer  306 , as shown in  FIG.  3 G . The hardmask pattern  315  functions as a pattern guide for formation of slanted device structures  104 . The exposed segments  321 , as shown in  FIG.  3 G , of the device material layer  306  to be etched correspond to gaps  324  between the device structures  104 , as shown in  FIG.  3 H . In one embodiment, which can be combined with other embodiments described herein, the photoresist material may be disposed on the organic planarization layer  314  using a spin-on coating process. In another embodiment, which can be combined with other embodiments described herein, the patterned photoresist  316  may include, but is not limited to, light-sensitive polymer containing materials. Developing the photoresist material may include performing a lithography process, such as photolithography and/or digital lithography. 
     At operation  207 , organic planarization layer portions  317  of the organic planarization layer  314  exposed by the resist pattern  315  are removed. Removing the organic planarization layer portions  317  exposes negative hardmask portions  319  of the hardmask pattern  315  that correspond to the gaps  324  between the device structures  104 . The organic planarization layer portions  317  may be removed by IBE, RIE, directional RIE, plasma etching, wet etching, and/or lithography. The results of operation  207  are shown in  FIG.  3 F . 
     At operation  208 , the negative hardmask portions  319  of the hardmask pattern  315  are etched. The results of operation  208  are shown in  FIG.  3 F . Etching the negative hardmask portions  319  exposes the exposed segments  321  of the device material layer  306  corresponding to the hardmask pattern  315 . In one embodiment, which can be combined with other embodiments described herein, etching the negative hardmask portions  319  may include, but is not limited to, at least one of IBE, RIE, directional RIE, or plasma etching. 
     At operation  209 , the patterned photoresist  316  and the organic planarization layer  314  are removed. The results of operation  208  are illustrated in  FIG.  3 G . Stripping the organic planarization layer  314  and the patterned photoresist layer  316  yields a set of negative hardmask portions  319 . 
     At operation  210 , an etch process is performed. In one embodiment, which can be combined with other embodiments described herein, an angled etching process is performed. The angled etch process may include, but is not limited to, at least one of IBE, RIE, or directional RIE. The ion beam generated by IBE may include, but is not limited to, at least one of a ribbon beam, a spot beam, or a full substrate-size beam. Performing the angled etch process etches the exposed segments  321  of the device material layer  306  to form a plurality of device structures  104 . As shown in  FIG.  3 H , the angled etching process forms the plurality of device structures  104  such that the device structures  104  have a slant angle ϑ relative to the surface of the substrate  302 . In one embodiment, which can be combined with other embodiments described herein, the slant angle ϑ of each of the device structures  104  is substantially the same. In another embodiment, which can be combined with other embodiments described herein, the slant angle ϑ of at least one device structure of the plurality of device structures  104  is different. 
     The device structure pattern  310  provides for a depth D of the device structures  104  to have gradient modulated across the substrate  302 . For example, as shown in  FIG.  3 H , the depth D of the device structures  104  decreases in the X-direction across the substrate  302 . In one embodiment, which can be combined with other embodiments described herein, the gradient of the depth D of the device structures  104  is continuous. In one embodiment, which can be combined with other embodiments described herein, the gradient of the depth D of the device structures  104  is step-wise. As described above, modulating the depth D of the device structures  104  provides for control of the in-coupling and out-coupling of light by the gratings  102  of the optical device  100 . 
     At operation  211 , an optional operation may be performed to strip the hardmask  312 . A wet clean may be performed in some embodiments. 
     The laser ablation process described herein advantageously allows the variable-depth structure to have a slope and/or curvature in one or more directions.  FIGS.  4 A- 4 C  illustrate other examples of shapes that can be used for the variable-depth structure.  FIG.  4 A  illustrates a variable-depth structure  420  in a device material layer  406  of a portion of optical device  400 . The variable-depth structure  420  has two planar sloped portions which extend from respective peripheral regions  420   a ,  420   b  towards a central region  420   c .  FIG.  4 B  illustrates a variable-depth structure  450  in a device material layer  436  of a portion of optical device  430 . The variable-depth structure  450  is a curved structure which has a shallow depth D at peripheral regions  450   a ,  450   b  and an increased depth at a central region  450   c . In one example, the variable-depth structure  450  has a parabolic shape. The depth D increases non-linearly from the peripheral regions  450   a ,  450   b  to the central region  450   c .  FIG.  4 C  illustrates a variable-depth structure  480  in a device material layer  466  of a portion of optical device  460 . The variable-depth structure  480  has a depth D that oscillates from a first end  480   a  to a second end  480   b  which forms a pattern of cyclical depths D for the variable-depth structure  480 . The variable-depth structure  480  is shown with linear, saw-tooth oscillations of the depth D. However, it is contemplated that the depth D can vary non-linearly so that the variable-depth structure has wave-like oscillations in the depth D. The depth D of a variable-depth structure, such as wedge-shapes structures  420 ,  450 ,  480  can change linearly or non-linearly across a length L thereof from a first end (i.e,  420   a ,  450   a ,  480   a ) to a second end (i.e.,  420   b ,  450   b ,  480   b ). Utilizing grayscale lithography, laser ablation and the techniques described herein, variable-depth structures of varying shapes can be patterned with a single pass instead of multiple operations as required by prior techniques. 
     In another example, the variable-depth structure has a three dimensional shape. That is, the depth changes in multiple directions (i.e., a first direction X and a second direction Y) as illustrated in the examples of  FIGS.  5 A- 5 C .  FIG.  5 A  illustrates a variable-depth structure  520  which has a saddle-point shaped curvature (i.e., hyperbolic paraboloid shape).  FIG.  5 B  illustrates a variable-depth structure  550  which has an elliptic paraboloid shape with positive curvature.  FIG.  5 C  illustrates a variable-depth structure  580  which has an elliptic paraboloid shape with negative curvature. The three dimensional shape of the variable-depth structure is not limited to the examples of  FIGS.  5 A- 5 C . Other desired shapes, for example a paraboloid in a square domain with positive curvature or negative curvature, an ellipsoid, and linear sloped shapes, among others, are also contemplated and can be used herewith. In these cases, the depth of the variable-depth structure changes in both the X and Y directions. Thus, upper surfaces of the slanted device structures are curved as defined by the shape of the curvature of the variable-depth structure. 
     In summation, methods for forming a device structure having variable-depth slanted device structures are described herein. The methods include forming a depth-modulated variable-depth structure in a device material layer using laser ablation. A plurality of device structures are formed in the variable-depth structure to define slanted device structures therein. The variable-depth structure is formed using laser ablation, and the slanted device structures are formed using an etch process. The method described herein can also be used to create a device structure that functions as a master for nanoimprint lithography.