Stress-Decoupled MEMS Mirror Packaging for Ancillary Mode Stability
Solution Overview
Problem
MEMS scanning mirrors in imaging applications suffer from thermally unstable ancillary oscillation modes due to coefficient of thermal expansion (CTE) mismatches between the silicon MEMS die, die-bond adhesive, and PCB substrate, leading to temperature-dependent frequency variations and reduced image quality.
Innovation Solution
A cantilevered die package design is implemented, where one end of the MEMS die is partially fixedly-attached to the substrate while the other end is free, decoupling the effects of CTE mismatch and reducing temperature-dependent stress on flexure beams.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the MEMS die is fully die-attached to the substrate around the entire perimeter, then mechanical stability and positioning precision are improved, but thermally-induced stress from CTE mismatch increases causing frequency variation in ancillary modes
Solution Approach 1:
The die attachment is segmented into two distinct regions: a first region with strong die-attach material providing mechanical stability, and a second region with compliant material that accommodates thermal expansion differences. This segmentation allows each region to fulfill its specific function without compromising the other.
Solution Approach 2:
Different regions of the die substrate interface are given different mechanical properties. The first region uses rigid die-attach material for positioning precision, while the second region uses compliant material to reduce thermal stress. This local differentiation of material properties resolves the contradiction between stability and thermal sensitivity.
2Stability of the object's composition
If the die is rigidly fixed to the substrate, then mechanical stability is improved, but the impact of CTE mismatch and temperature variations on ancillary mode frequency increases
Solution Approach 1:
The attachment interface is divided into a first region for mechanical stability and a second region for thermal compliance. This allows the system to maintain mechanical stability while being less sensitive to temperature variations through the compliant second region.
Solution Approach 2:
The mechanical properties of the attachment material are changed across different regions. The first region uses material with high rigidity for stability, while the second region uses material with compliant properties that change with temperature to accommodate expansion without transmitting stress to the flexure beams.
3Ease of manufacture
If conventional full-perimeter die-attach is used, then manufacturing simplicity is maintained, but feedback loop control becomes difficult and may over-drive the MEMS mirror causing flexure beam breakage
Solution Approach 1:
The die attach pattern is segmented into functional regions that can be implemented through modified die-attach processes. While slightly more complex than full-perimeter attachment, the segmented approach provides inherent mechanical filtering of thermal stress that simplifies control by preventing ancillary mode excitation.
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 cantilevered die package stabilizes ancillary mode oscillations, improving image quality by limiting frequency variations and preventing mechanical failures across a wide temperature range.
Implementation Method 1
piezoelectric actuators to provide controllable beam steering of light
Implementation Method 2
thermally-induced stress experienced during scanner operations from mismatches in the coefficient of thermal expansion (CTE) of the die, substrate, and display module assembly housing
Data Source
AI summary
A torsional micro-electro-mechanical systems (MEMS) mirror module provides for thermally-stable ancillary modes of mirror oscillation (e.g., vertical, horizontal, and rocking) by utilizing MEMS die packaging techniques that implement a cantilevered (i.e., fixed-free) die package in which one end of the die is partially fixedly-attached to a die carrier substrate while the non-attached end of the die is free from the die carrier substrate and unsupported. The cantilevered die package with fixed-free architecture effectively decouples the effects of coefficient of thermal expansion (CTE) mismatch of the MEMS die, die-bond adhesive, and die carrier substrate on temperature-dependent MEMS flexure stress. Ancillary mode oscillation frequencies changes with temperatures are thus limited to a smaller range relative to those experienced with conventional package designs.