Alignment Marks on Transparent Substrates for MEMS Mirror Projectors
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Solution Overview
Problem
The fabrication of double substrate spatial light modulators for MEMS devices is complicated and results in low yield due to pattern definition deviations caused by blind stepping during substrate alignment, which degrades the contrast ratio and gray scale of the mirror projector, and the alignment laser light penetration through deposition layers weakens reflective light intensity.
Innovation Solution
A method and system for accurately patterning alignment marks on an optically transparent substrate using sacrificial layers and photo lithography, plasma etching, and deposition techniques to create optically partially transparent alignment marks that enhance reflective light intensity and prevent pattern deviations during substrate joining.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If blind stepping is used during substrate alignment, then substrate joining is completed, but pattern definition deviation occurs on the optically transparent substrate
Solution Approach 1:
The patent applies preliminary action by pre-patterned alignment marks on the optically transparent substrate before the alignment and joining process. These alignment marks serve as reference features that enable precise alignment between substrates, preventing pattern definition deviation that would otherwise occur during blind stepping. The alignment marks are created in advance using a first deposition layer and first sacrificial layer, establishing a reference framework before the actual substrate joining takes place.
2Measurement precision
If alignment laser light passes through deposition layers, then alignment marking is possible, but reflective light intensity is weakened
Solution Approach 1:
The patent uses an intermediary approach by introducing a second deposition layer with different optical properties (lower transparency) over the alignment marks. This second layer acts as a mediator that allows the alignment laser to pass through during the alignment process (maintaining measurement precision) while subsequently providing sufficient reflective light intensity for the alignment detection system. The second sacrificial layer is used to selectively remove this intermediary layer after alignment, restoring full optical access when needed.
3Reliability
If tilt angle of reflective elements is increased, then contrast ratio and gray scale improve, but alignment precision deteriorates due to joining shift
Solution Approach 1:
The patent implements feedback by using the alignment marks and alignment detection system to monitor and control the relative position between substrates during the joining process. The system provides real-time feedback on alignment accuracy, allowing adjustments to be made before final bonding occurs. This feedback mechanism ensures that even when substrates are joined, minimal shift occurs, thereby maintaining the precise tilt angle of reflective elements needed for high contrast ratio and gray scale performance.
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 improves the alignment accuracy of substrates, reduces wafer rejection, and stabilizes the tilt angle of reflective elements, resulting in a significant improvement in contrast ratio and gray scale performance from 800:1 to at least 1000:1.
Implementation Method 1
when an alignment laser light passes through the deposition layer, such as silicon dioxide or indium tin oxide (ITO) of the transparent substrate, for patterning
Implementation Method 2
The reflective elements may be selectively deflected or twisted to spatially modulate light incident to the upper substrate (optically transparent substrate) to reflect light back to the upper substrate
Data Source
AI summary
Disclosed is a method and a system for forming alignment marks on a transparent substrate. A light reflective layer is deposited over an optically transparent substrate of a wafer. A region is defined around an alignment mark on the optically transparent substrate. The light reflective layer is removed from a substantial portion of the transparent substrate excluding the region. In addition, a micro electro-mechanical systems device is disclosed. The device comprises an optically transparent substrate, at least one optically partially transparent alignment mark on the optically transparent substrate, and a plurality of reflective elements or imaging pixels attached to the optically transparent substrate.