MULTILAYER TRANSMISSION STRUCTURES FOR WAVEGUIDE DISPLAY

Embodiments of the present disclosure describe waveguides having device structures with multiple portions and methods of forming the waveguide having multiportion device structures. The plurality of device structures are formed having two or more portions. The materials of the plurality of portions are chosen such that impedance matching is enabled between the portions to reduce reflection of light from the optical device.

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 waveguides having device structures with multiple portions and methods of forming the waveguide having multi-portion device structures.

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 to 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 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. However, existing optical devices lack a desired level of coupling efficiency. Accordingly, what is needed in the art are optical devices with improved coupling efficiency.

SUMMARY

In one embodiment, a waveguide is provided. The waveguide includes an optical device substrate and at least one grating disposed over the optical device substrate. The at least one grating includes a plurality of device structures. Adjacent device structures of the plurality of device structures define a gap therebetween. The plurality of device structures include a device portion. The device portion includes a device material having a first refractive index of about 1.9 to about 4.0. The plurality of device structures include an impedance matching portion. The impedance matching portion includes a second refractive index of about 1.4 to about 2.0.

In another embodiment, a waveguide is provided. The waveguide includes an optical device substrate and at least one grating disposed over the optical device substrate. The at least one grating includes a plurality of device structures. Adjacent device structures of the plurality of device structures define a gap therebetween. The plurality of device structures include a device portion. The device portion includes a device material having a first refractive index of about 1.9 to about 4.0. The plurality of device structures include an impedance matching portion. The impedance matching portion includes a second refractive index of about 1.4 to about 2.0. The plurality of device structures include an anti-reflective portion. The anti-reflective portion includes an anti-reflective refractive index of about 1.4 to about 2.0. A difference between the first refractive index and at least one of the second refractive index or the anti-reflective refractive index is about 0.45 to about 1.15.

In yet another embodiment, a method is provided. The method includes disposing two or more layers of material on a surface of a substrate. The method further includes etching through the two or more layers of material to form a plurality of device structures having two or more portions. The two or more portions include a device portion having a first refractive index of between about 1.9 and about 4.0 and at least one of an impedance matching portion or an anti-reflective portion. The impedance matching portion or the anti-reflective portion include a second refractive index of about 1.4 to about 2.0. A difference between the first refractive index and the second refractive index is about 0.45 to about 1.15.

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 waveguides having device structures with multiple portions and methods of forming the waveguide having multi-portion device structures.

In one embodiment, a waveguide is provided. The waveguide includes an optical device substrate and at least one grating disposed over the optical device substrate. The at least one grating includes a plurality of device structures. Adjacent device structures of the plurality of device structures define a gap therebetween. The plurality of device structures include a device portion. The device portion includes a device material having a first refractive index of about 1.9 to about 4.0. The plurality of device structures include an impedance matching portion. The impedance matching portion includes a second refractive index of about 1.4 to about 2.0. The plurality of device structures include an anti-reflective portion. The anti-reflective portion includes an anti-reflective refractive index of about 1.4 to about 2.0. A difference between the first refractive index and at least one of the second refractive index or the anti-reflective refractive index is about 0.45 to about 1.15.

In another embodiment, a method is provided. The method includes disposing two or more layers of material on a surface of a substrate. The method further includes etching through the two or more layers of material to form a plurality of device structures having two or more portions. The two or more portions include a device portion having a first refractive index of between about 1.9 and about 4.0 and at least one of an impedance matching portion or an anti-reflective portion. The impedance matching portion or the anti-reflective portion include a second refractive index of about 1.4 to about 2.0. A difference between the first refractive index and the second refractive index is about 0.45 to about 1.15.

FIG.1is a schematic, top view of a waveguide100. It is to be understood that the waveguide100described below is an exemplary optical device. In one embodiment, which can be combined with other embodiments described herein, the waveguide100is a waveguide combiner, such as an augmented reality waveguide combiner. The waveguide100may additionally be a waveguide utilized for optical sensing (e.g., eye tracking capabilities).

The waveguide100includes a plurality of device structures102disposed on a top surface103of a substrate101. A portion105of the plurality of device structures102are shown inFIG.1. The device structures102may be nanostructures having sub-micron dimensions, e.g., nano-sized dimensions. In one embodiment, which can be combined with other embodiments described herein, regions of the device structures102correspond to one or more gratings104, such as a first grating104A, a second grating104B, and a third grating104C. In one embodiment, which can be combined with other embodiments described herein, the waveguide100is a waveguide combiner that includes at least the first grating104A corresponding to an input coupling grating and the third grating104C corresponding to an output coupling grating. The waveguide combiner, which can be combined with other embodiments described herein, includes the second grating104B corresponding to an intermediate grating. The substrate101may be formed from any suitable material, provided that the substrate101can adequately transmit light in a desired wavelength or wavelength range and can serve as an adequate support for the waveguide100, described herein. In one embodiment, which can be combined with other embodiments described herein, the wavelength range is between about 400 nm to about 2000 nm. Substrate selection may include substrates of any suitable material, including, but not limited to, silicon (Si), silicon dioxide (SiO2), doped SiO2, fused silica, quartz, silicon carbide (SiC), germanium (Ge), silicon germanium (SiGe), indium phosphide (InP), gallium arsenide (GaAs), gallium nitride (GaN), diamond, or sapphire containing materials.

FIGS.2A-2Care schematic, cross-sectional views of a portion105of a grating104of a waveguide100. The grating104includes a plurality of device structures102.FIGS.2A and2Care taken along section line1-1ofFIG.1, such that the portion105of the grating104corresponds to a first grating104A, e.g., an input coupling grating, of the waveguide100.FIG.2Bis taken along section line2-2ofFIG.1, such that the portion105corresponds to the third grating104C, i.e., an output coupling grating. AlthoughFIGS.2A and2Cshow the portion105corresponding to the first grating104a, andFIG.2Bshows the portion105corresponding to the third grating104c, the portion105ofFIGS.2A-2Care not limited to the grating104and may correspond to any of the first grating104a, the second grating104b, or the third grating104c. The plurality of device structures102are disposed on a top surface103of a substrate101. Each of the device structures102includes an upper surface210. The plurality of device structures102define a plurality of gaps220. Each gap of the plurality of gaps220is defined between adjacent device structures102.

Each device structure102of the plurality of device structures102has a structure width202. The structure width202is defined as the width of the device structure102closest to the top surface103of the substrate101. In one embodiment, which can be combined with other embodiments described herein, at least one structure width202may be different from another structure width202. In another embodiment, which can be combined with other embodiments described herein, each structure width202of the plurality of device structures102is substantially equal to each other structure width202. Each device structure102of the plurality of device structures102has a spacewidth204. The spacewidth204is defined as the distance between adjacent device structures102closest to the top surface103of the substrate101. In one embodiment, which can be combined with other embodiments described herein, at least one spacewidth204may be different from another spacewidth204. In another embodiment, which can be combined with other embodiments described herein, each spacewidth204of the plurality of device structures102is substantially equal to each other spacewidth204. A duty cycle of the waveguide100is defined as the ratio of the spacewidth204to the structure width202. In one embodiment, which can be combined with other embodiments described herein, the duty cycle is constant across the substrate101. In another embodiment, which can be combined with other embodiments described herein, the duty cycle varies across the substrate101. The duty cycle is between about 5% and about 95%.

A pitch206is defined as the summation of the spacewidth204and the structure width202for each device structure102. In one embodiment, which can be combined with other embodiments described herein, the pitch206is constant across the substrate101. In another embodiment, which can be combined with other embodiments described herein, the pitch206varies across the substrate101. The pitch206is between about 150 nm and about 1500 nm. A depth208is defined as the distance between the upper surfaces210of each device structure102to the top surface103of the substrate101. In one embodiment, which can be combined with other embodiments described herein, the depth208is constant across the substrate101. In another embodiment, which can be combined with other embodiments described herein, the depth208varies across the substrate101. The depth208of each device structure102is between about 10 nm and about 2000 nm.

The plurality of device structures102are formed at a device angle ϑ. The device angle ϑ is the angle between the surface103of the substrate101and a sidewall212of the device structure102. As shown inFIGS.2A and2C, the plurality of device structures102are angled relative to the top surface103of the substrate101. The device angle ϑ is between about 10 degrees and about 170 degrees, such as from about 40 degrees to about 140 degrees. For example, the device angle ϑ is from about 70 degrees to about 110 degrees. As shown inFIG.2B, the plurality of device structures102are vertical, i.e., the device angle ϑ is 90 degrees. In one embodiment, which can be combined with other embodiments described herein, each respective device angle ϑ for each device structure102is substantially equal. In another embodiment, which can be combined with other embodiments described herein, at least one respective device angle ϑ of the plurality of device structures102is different than another device angle ϑ of the plurality of device structures102.

The plurality of device structures102shown inFIGS.2A-2Cmay correspond to any one of the first grating104A, the second grating104B, or the third grating104C of the waveguide100, shown inFIG.1. As shown inFIG.2B, the plurality of device structures102may be disposed on both the top surface103of the substrate101and a bottom surface214of the substrate101. In one embodiment, which can be combined with other embodiments described herein, the plurality of device structures102are operable to couple light into the substrate101. In another embodiment, which can be combined with other embodiments described herein, the plurality of device structures102are operable to couple light out of the substrate101to a user. The light may be coupled out of the top surface103and/or the bottom surface214of the substrate101.

Although the plurality of device structures102are shown on the top surface103of the substrate101inFIGS.2A and2C, the plurality of device structures102may be disposed on the top surface103and the bottom surface214. Although the plurality of device structures102are shown on the top surface103and the bottom surface214inFIG.2B, the plurality of device structures102may be disposed only on one of the top surface103or the bottom surface214.

Each of the plurality of device structures102includes a plurality of portions216(i.e., portions216A-216C). For example, each device structure102may have two or more portions216. Each of the plurality of portions216includes a thickness218. The thickness218of the plurality of portions216is determined to improve the diffraction efficiency of the plurality of device structures102. For example, the thickness218of each of the plurality of portions216may be determined to enhance or lower the efficiency of the waveguide100for a certain wavelength coupled to the waveguide100. The ability to enhance or lower the efficiency of specific wavelengths of light coupled into the waveguide100(i.e., color balancing) is beneficial for waveguides100operable to couple more than one wavelength of light. Each portion216of the plurality of portions216may have a different thickness218or the same thickness218as adjacent portions216.

The plurality of device structures102having the plurality of portions216improves the light coupling efficiency and image quality of the waveguide100due to impedance matching between materials of the portions216. The plurality of portions216enable impedance matching between the portions216to adjust reflection of light from the waveguide100. For example, impedance matching the portions216may reduce the light reflection of one wavelength of light, but will intentionally increase the light reflection of another wavelength of light. The plurality of portions216increase the coupling efficiency of different wavelengths of light (e.g., colors of light) in a single waveguide100(where RGB colors are emitted from the same waveguide100).

The impedance matching allows for the device structures102to couple light from the substrate101to the surroundings or couple the incident light to the substrate101more efficiently, such that the incident light is angularly uniform, and/or spectrally uniform. The impedance matching between the portions216lowers reflections and back-diffracted light. When there is excessive reflections and back-diffraction, some of the reflected light can be coupled into the waveguide100, causing ghost imaging. Thus, the impedance matching between the portions216will reduce the occurrence of ghost imaging and stray light. Additionally, the plurality of portions216can improve optical efficiencies for large incident angles, such that the field-of-view FOV may be enlarged. Also, as the plurality of portions216are not disposed in the plurality of gaps220in-between the plurality of device structures102, a higher refractive index contrast may occur. The refractive index contrast between the device structures102and the plurality of gaps220provides a refractive index contrast. It is desirable to have a large contrast between refractive indices of the structure material of the plurality of device structures102and the materials surrounding the plurality of device structures102to improve the optical performance of the waveguide100.

In embodiments where the device structures102having the plurality of portions216are disposed on the third grating104C, as a result of impedance matching with the multiple portions216, see through transmission (or light from external world) can be improved. When multiple waveguides100are stacked, the plurality of portions216lower the occurrence of reflection at each waveguide, so that the user can see more light from outside the waveguide100. In addition to the benefit for enhancing the see through transmission, the device structures102on each waveguide100of the stacked waveguides100can allow higher light transmission from adjacent waveguides100to minimize stray light and improve the overall optical efficiency of the entire waveguide100.

In some embodiments, which can be combined with other embodiments described herein, the plurality of device structures102include two portions216. For example, the plurality of device structures102include a device portion216A and an impedance matching portion216B disposed over the device portion216A, as shown inFIGS.2A-2C. In other embodiments, which can be combined with other embodiments described herein, the plurality of device structures102include an anti-reflective portion216C disposed below the device portion216A, as shown inFIG.2B. Although only three portions216are shown inFIG.2B, the plurality of portions216may include more than three portions216.

The device portion216A is a high refractive index material. The refractive index of the device portion216A is between about 1.9 to about 4.0. The device portion216A includes, but is not limited to, materials such as or containing germanium, silicon, titanium oxide, niobium oxide, silicon nitride, hafnium oxide, tantalum oxide, scandium oxide, or combinations thereof. The impedance matching portion216B is a low refractive index material. The refractive index of the impedance matching portion216B is between about 1.4 and about 2.0. The impedance matching portion216B includes, but is not limited to, materials containing silicon nitride, silicon oxide, aluminum oxide, or combinations thereof. In one embodiment, which can be combined with other embodiments described herein, the device portion216A is silicon nitride and the impedance matching portion216B is silicon oxide. In some embodiments, the impedance matching portion216B corresponds to a hard mask layer utilized for waveguide fabrication. The materials of the hard mask layer are selected such that the patterned hard mask layer remains as the impedance matching portion216B. As different materials and combinations of material may be realized in the device structures, different designs of the waveguide100can be achieved.

In some embodiments, which can be combined with other embodiments described herein, the anti-reflective portion216C is an etch stop layer. The anti-reflective portion216C is an etch stop layer that remains after formation of the plurality of devices structures102. The anti-reflective portion216C is an etch stop layer when the anti-reflective portion216C is fully etched through or not etched at all. The refractive index of the anti-reflective portion216C is between about 1.4 and about 2.0. The anti-reflective portion216C includes, but is not limited to, materials containing silicon nitride, silicon oxide, aluminum oxide, or combinations thereof.

The ability to impedance match the impedance matching portion216B to the device portion216A as well as impedance match additional portions of the device structures102improves the anti-reflection capabilities of the plurality of device structures102. In some embodiments, to determine the refractive index of the impedance matching portion216B, a first impedance matching formula may be used. The first impedance matching formula is:

where N1 is the refractive index of the device portion216A, N2 is the refractive index of the impedance matching portion2166, and N3 is the refractive index of air (or another medium surrounding the waveguide100). In some embodiments, which can be combined with other embodiments described herein, the refractive index of the anti-reflective portion216C is determined with a second impedance matching formula. The second impedance matching formula is:

where N1 is the refractive index of the device portion216A, Nanti-reflectiveis the refractive index of the anti-reflective portion216C, and Nsubstrateis the refractive index is the substrate refractive index. Based on the impedance matching formulas, the materials of the plurality of portions216may be chosen to improve the impedance matching within the waveguide100.

FIGS.3A-3Care schematic, top-views of a portion105of a grating104of a waveguide100. The grating104includes a plurality of device structures102. The plurality of device structures102are disposed on a top surface103of a substrate101. Each of the device structures102includes an upper surface210. Each of the plurality of device structures102have a plurality of portions216(shown inFIGS.2A-2C).

As shown inFIG.3A, the plurality of device structures102are fin structures. The fin structures are disposed in parallel rows302. Although the plurality of device structures102inFIG.3Adepict a rectangular cross-section, the device structures102are not limited in the cross-section shape. As shown inFIG.3B, the plurality of device structures102may be discrete device structures102. Each device structure102is adjacent to other device structures102in both the first direction and the second direction, wherein the first direction is perpendicular to the second direction. For example, the plurality of device structures102are disposed along an x-direction and a y-direction, as illustrated inFIG.3B, such that the plurality of device structures102are each disposed only along the first direction and the second direction. Although the plurality of device structures102inFIG.3Bdepict an oval cross-section, the device structures102are not limited in the cross-section shape. As shown inFIG.3C, the plurality of device structures102may be discrete device structures102. Each device structure102is adjacent to other device structures102in both the first direction and the second direction, wherein the first direction is perpendicular to the second direction. The plurality of device structures102inFIG.3Care not limited to the cross-section shown inFIG.3C. For example, the cross-section of the plurality of device structures102may be any shape operable to support multiple layers of waveguides100formed thereon.

FIG.4is a flow diagram of a method for forming a waveguide having multiple portions of materials according to embodiments described herein. To facilitate explanation, the method400is explained with reference to the plurality of device structures102shown inFIGS.2A-2C, however it is contemplated that the method400may be performed to form any shaped device structure102.

At operation401, a plurality of layers of material are disposed over a substrate101. Each of the plurality of portions216of material are disposed 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 PVD process, a CVD process, a FCVD process, a PECVD process, magnetron sputtering, ion beam sputtering, electron beam evaporation, or an ALD process. Each layer of material is chosen such that impedance matching (e.g., anti-reflection) will occur between the layers of material to improve the ant-reflective properties of the device structures102to be formed. The layers of material include a device material and an impedance matching material.

Each of the layers has a thickness218chosen to improve the diffractive efficiency of the waveguide100to be formed. The material of each layer and the thickness218of each layer is determined via optical simulation to improve the diffractive efficiency and to improve anti-reflective capabilities. The optical simulation can be performed based on electromagnetic simulation approaches including, but not limited to, finite-difference time-domain (FDTD), finite-difference frequency-domain (FDFD), rigorous coupled-wave analysis (RCWA), or finite element analysis (FEM). In the optical simulations; the size and positioning of the plurality of device structures102as well as the refractive index of each of the portions216may be altered to improve the waveguide100performance.

At operation402, a plurality of device structures102are formed. Etching one or more of the plurality of layers forms the plurality of device structures102having a plurality of portions216of the layers. The plurality of device structures102are formed with one or more of a nanoimprint lithography, nanoimprint process, optical lithography, ion-beam etching, reactive ion etching, electron beam etching, or wet etching process, or combinations thereof. A device portion216A of a device material and an impedance matching portion216B of an impedance matching material may be formed.

In some embodiments, which can be combined with other embodiments described herein, the impedance matching portion216B of the plurality of portions216is a hard mask layer. For example, the operation402may include the impedance matching portion being partially etched to form a hard mask layer. The partially etched impedance matching portion216B can define the plurality of device structures102to be formed in a device portion216A and the impedance matching portion216B. The impedance matching portion216B remains one of the plurality of portions216.

In other embodiments, an anti-reflective portion216C is disposed between the substrate101and the device portion216A. The anti-reflective portion216C is an etch stop layer. For example, the operation402can include etching the etch stop layer through openings formed in the device portion after etching the device portion. The anti-reflective portion216C remains one of the plurality of portions216.

As the plurality of device structures102are formed with a single etch operation, additional operations, such as depositing an anti-reflective layer or a hard mask layer do not need to be completed as one or more of the portions216may serve as an anti-reflective layer or a hard mask layer. Thus, fabrication costs may be reduced.

In summation, waveguides having device structures with multiple portions and methods of forming the waveguide having multiportion device structures are described herein. The plurality of device structures are formed having two or more portions. The materials of the plurality of portions are chosen such that impedance matching is enabled between the portions to reduce reflection of light from the optical device. The impedance matching allows for the device structures to couple light more efficiently. As the material of the plurality of portions are not disposed in gaps in-between the plurality of device structures, the refractive index contrast between the materials of the device structures and the surrounding air is not affected. Forming the multi-portion device structures with a single etch step will reduce fabrication costs.