Variable Depth Gratings for Waveguide Displays
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Solution Overview
Problem
Conventional near-eye displays face challenges in achieving high out-coupling efficiency due to mismatched grating element sizes and shapes, and existing lithography methods cannot modulate the height of optical gratings, limiting the manufacturing of optical grating elements with variable depths and duty cycles.
Innovation Solution
A manufacturing system comprising a patterning, deposition, and etching system that uses gray-scale photoresists and masks to create optical gratings with variable etch depths and duty cycles by transferring patterns into nanoimprint molds and applying these to optical waveguides, enabling the formation of gratings with controlled heights and duty cycles.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional lithography methods are used to produce optical grating elements, then duty cycle can be varied, but the height of the optical grating cannot be modulated
Solution Approach 1:
The manufacturing process is divided into multiple sequential steps: first forming a grayscale mask with varying thickness, then using this mask to pattern photoresist at different exposure levels, and finally transferring these multi-level patterns to create gratings with variable heights. This segmentation allows conventional lithography to achieve height modulation through process decomposition.
Solution Approach 2:
A grayscale mask is fabricated in advance with spatially varying thickness or material composition, which then serves as a template to pre-determine the exposure levels received by different regions of the photoresist. This preliminary action enables subsequent single-step lithography to produce multi-height grating structures without requiring complex real-time height control.
2Manufacturing precision
If gray-scale photoresist lithography is performed to create variable heights, then manufacturing complexity increases
Solution Approach 1:
A grayscale mask serves as an intermediary element that translates desired grating height variations into corresponding photoresist exposure levels. This mask, with its spatially varying optical properties, mediates between the lithography system and the photoresist, enabling variable height patterning through a relatively simple single-step exposure process rather than complex multi-step direct patterning.
3Reliability
If optical gratings with variable depths are manufactured, then out-coupling efficiency improves, but manufacturing throughput decreases
Solution Approach 1:
The grayscale mask acts as a master template that can be used to pattern multiple photoresist layers or multiple substrates through replication processes. Once the grayscale mask is fabricated (which may be done once or infrequently), it serves as a reusable tool to efficiently produce many copies of variable-height grating structures, thereby decoupling the complexity of mask fabrication from the throughput of product manufacturing.
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 system effectively produces optical gratings with adjustable heights and duty cycles, enhancing the efficiency and uniformity of image light projection in near-eye displays by controlling diffraction orders and improving brightness and field-of-view.
Implementation Method 1
a first photoresist is lithographically patterned using a gray-scale mask to form a patterned photoresist including photoresist features of different heights
Implementation Method 2
A deposition system deposits a target material onto a substrate
Implementation Method 3
A first ion beam is used to erode a metal layer forming the gray-scale mask
Implementation Method 4
A pattern in the photoresist is transferred into the substrate to form an optical grating
Data Source
AI summary
A manufacturing system for fabricating self-aligned grating elements with a variable refractive index includes a patterning system, a deposition system, and an etching system. The manufacturing system performs a lithographic patterning of one or more photoresists to create a stack over a substrate. The manufacturing system performs a conformal deposition of a protective coating on the stack. The manufacturing system performs a deposition of a first photoresist of a first refractive index on the protective coating. The manufacturing system performs a removal of the first photoresist to achieve a threshold value of first thickness. The manufacturing system performs a deposition of a second photoresist of a second refractive index on the first photoresist. The second refractive index is greater than the first refractive index. The manufacturing system performs a removal of the second photoresist to achieve a threshold value of second thickness to form a portion of an optical grating.