Black Silicon Surface via Cyclic Plasma Etching
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Solution Overview
Problem
Existing processes for creating black cavities in MEMS micromirrors are costly, complex, and prone to defects such as cracks and spurious reflectivity, which affect the performance of devices like picoprojectors and 3D gesture-recognition systems.
Innovation Solution
A process involving multiple cycles of depositing a non-planar polymeric layer and plasma etching to create a rough silicon surface, which reduces spurious reflectivity without deep structure formation, is used to achieve a black silicon surface with low reflectivity indices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional processes are used to create black cavities in MEMS micromirrors, then the cavities can be formed, but the processes become costly, complex, and prone to defects such as cracks and spurious reflectivity
Solution Approach 1:
The blackening process is divided into multiple sequential roughening cycles, each consisting of depositing a polymeric layer and performing plasma etching. This segmentation allows progressive surface modification without creating deep structures that cause defects, achieving a black surface with minimal cracks and spurious reflectivity.
Solution Approach 2:
The process controls parameters such as the number of roughening cycles (typically 5-20), polymeric layer thickness, and plasma etching conditions to optimize the blackening effect. By adjusting these parameters, the process achieves low reflectivity (below 2%) without forming deep cavity structures that lead to cracks.
2Object-affected harmful factors
If the cavity depth is increased to reduce reflectivity, then spurious reflectivity decreases, but the risk of cracks and structural defects increases
Solution Approach 1:
Instead of creating deep cavity structures, the process applies multiple partial roughening actions that progressively modify the surface. Each cycle removes only a thin layer of material, accumulating a rough surface topology that reduces reflectivity through multiple scattering events rather than deep structural modification.
Solution Approach 2:
A polymeric layer is deposited on the cavity surface before plasma etching to control the roughening process. This preliminary polymeric layer acts as a mask and etch-stop layer, preventing excessive etching that would create cracks, while still achieving sufficient surface roughness to reduce spurious reflectivity to below 2%.
3Object-affected harmful factors
If multiple roughening cycles are performed to achieve low reflectivity, then spurious reflectivity reduces to below 2%, but the manufacturing time increases
Solution Approach 1:
The roughening cycles are performed sequentially with each cycle building upon the previous one, continuously modifying the surface topology. The polymeric layer is deposited and etched in continuous cycles, with each cycle contributing incrementally to the final black surface, achieving reflectivity below 2% through sustained processing.
Solution Approach 2:
The blackening process uses periodic deposition and etching cycles, where a polymeric layer is deposited and then subjected to plasma etching, repeated for 5-20 cycles. This periodic action systematically builds surface roughness and reduces reflectivity through repeated scattering events, balancing manufacturing time with performance requirements.
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 process is simpler, less costly, and reduces spurious reflectivity to levels below 2%, enhancing the performance of MEMS devices by minimizing light disturbance and avoiding defects like cracks.
Implementation Method 1
plasma etching the polymeric layer and the area of the silicon body in a non-unidirectional way and removing, in a non-uniform way, a surface portion of the area of the silicon body
Data Source
AI summary
A roughened silicon surface is formed by a process including repetitively performed roughening cycles. Each roughening cycles including a step for depositing a non-planar polymeric layer over an area of a silicon body and a step for plasma etching the polymeric layer and the area of the silicon body etch in a non-unidirectional way. As a result, a surface portion of the silicon body is removed, in a non-uniform way, to a depth not greater than 10 nm.