Variable-Etch-Depth Gratings for Near-Eye Displays
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Solution Overview
Problem
Existing techniques for fabricating gratings in waveguide-based near-eye display systems are unable to achieve a three-dimensional profile with non-uniform etch depth, slanted ridges, varying periods, and duty cycles, which are necessary for improved field of view, light transfer efficiency, and reduced display artifacts.
Innovation Solution
A method involving a patterned etch mask, UV-sensitive photoresist layer, and gray-scale photomask to etch a substrate at a slant angle, achieving variable etch depth, duty cycle, and period across the grating structure, using anti-reflective coatings to enhance resolution and uniformity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional grating fabrication techniques are used, then the manufacturing process is simple, but the grating structure cannot achieve non-uniform etch depth, slanted ridges, or variable periods
Solution Approach 1:
The fabrication process is segmented into multiple sequential steps: forming initial grating structures, depositing photoresist material, exposing to UV light through a transparency mask, developing to create variable depth profiles, and etching the substrate. Each step builds upon the previous one to achieve the complex 3D grating profile that cannot be obtained through conventional single-step fabrication methods.
Solution Approach 2:
The invention transitions from conventional 2D grating patterns to 3D grating structures with variable etch depths, slanted ridges, and non-uniform profiles. The transparency mask introduces a depth dimension by controlling UV light exposure intensity at different locations, creating photoresist relief structures that are then transferred to the substrate as three-dimensional grating features.
2Manufacturing precision
If a transparency mask with varying light transmission is used, then variable etch depth and 3D profiles can be achieved, but the fabrication process becomes more complex
Solution Approach 1:
The photoresist material serves as an intermediary layer between the transparency mask and the substrate. It absorbs UV light with intensity proportional to the mask's local transparency, converting the optical pattern into a physical relief structure with variable height. This photoresist intermediate is then etched away along with controlled substrate etching to transfer the 3D profile, simplifying the overall process compared to direct substrate patterning methods.
3Reliability
If slanted etching is performed to create slanted ridges, then light transfer efficiency improves, but the etching process requires precise angle control
Solution Approach 1:
The photoresist relief structure is formed in advance before substrate etching. This pre-formed 3D photoresist mask automatically defines the slant angle and ridge geometry through its relief profile. When the substrate is etched at an angle using this photoresist as the masking layer, the desired slanted ridge geometry is transferred with high precision without requiring complex real-time angle control during etching.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution allows for the creation of gratings with complex 3D profiles, improving light transfer efficiency, reducing artifacts, and enhancing the overall performance of waveguide-based near-eye display systems.
Implementation Method 1
depositing a photoresist material layer sensitive to ultra-violet (UV) light on the patterned etch mask, exposing the photoresist material layer to UV light through a variable transparency photomask
Implementation Method 2
etching the patterned photoresist layer and the substrate to obtain a grating having a non-uniform etch depth in the substrate
Data Source
AI summary
Disclosed herein are techniques for fabricating straight or slanted variable-etch-depth gratings. A photoresist material for fabricating a variable-etch-depth grating in a substrate is sensitive to light with a wavelength shorter than 300 nm and has an etch rate comparable to the etch rate of the substrate. A depth of an exposed portion of a photoresist material layer including the photoresist material correlates with the exposure dose. After exposure using a gray-scale mask and development, the photoresist material layer has a non-uniform thickness. The photoresist material layer with the non-uniform thickness and the underlying substrate are etched using a straight etching or slanted etching process to form the straight or slanted variable-etch-depth grating in the substrate. The variable-etch-depth grating is characterized by a non-uniform depth profile corresponding to the non-uniform thickness of the photoresist material layer before etching.