Rough MEMS Surface Reduces Stiction via Oxide Transfer
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Solution Overview
Problem
MEMS devices face stiction issues due to molecular or Van Der Waal forces causing moving parts to stick together, especially when surfaces are smooth, leading to device malfunction.
Innovation Solution
A rough surface is formed by creating a roughened oxide layer through oxidation of an oxidizable material, followed by depositing a layer of silicon on this surface, which increases the surface roughness and reduces contact area, thereby preventing stiction. This is achieved by processes involving gaseous hydrochloric acid and silicon-containing gases in a deposition chamber, with specific temperature and gas flow ratios to achieve the desired surface morphology.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a smooth surface is used in MEMS devices, then manufacturing precision is improved, but stiction occurs between moving parts due to molecular or Van Der Waal forces
Solution Approach 1:
The patent applies local quality by creating a rough surface texture specifically at the contact regions of travel stops and moving parts, while maintaining smooth surfaces in non-contact areas. This localized roughness modification reduces stiction at critical contact points without compromising overall manufacturing precision or requiring complete surface roughening across the entire device.
Solution Approach 2:
The patent changes the surface roughness parameter from smooth to rough (Ra > 0.05 micrometers) at specific contact surfaces. This parameter change fundamentally alters the contact mechanics between moving parts and travel stops, reducing molecular and Van Der Waal forces that cause stiction, while maintaining compatibility with standard MEMS manufacturing processes.
2Reliability
If a rough surface is formed to prevent stiction, then reliability is improved, but manufacturing complexity increases due to additional oxidation and deposition steps
Solution Approach 1:
The patent merges the rough surface formation process with existing MEMS manufacturing sequences by integrating oxidation and polysilicon deposition steps into the standard fabrication flow. The rough surface is created as an inherent result of depositing polysilicon on an oxidized surface, combining multiple functions (surface preparation, material deposition, and roughness generation) into a unified process sequence.
Solution Approach 2:
The oxidation process naturally creates a roughened oxide surface that serves dual purposes: it provides a foundation for subsequent polysilicon deposition and simultaneously generates the desired surface roughness for stiction prevention. The process self-generates the functional rough surface without requiring separate roughening operations.
3Shape
If polysilicon is deposited on a rough oxide surface, then surface roughness is increased to reduce contact area, but deposition control becomes more difficult
Solution Approach 1:
The patent performs preliminary oxidation to create a roughened oxide surface before polysilicon deposition. This pre-prepared rough substrate provides a controlled foundation that guides the subsequent polysilicon deposition, ensuring that the roughness is transferred uniformly to the final polysilicon layer while maintaining deposition control through established process parameters.
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 rough surface effectively reduces the likelihood of stiction between moving parts and travel stops, ensuring the MEMS device operates freely by increasing the surface roughness and maintaining the polysilicon phase during deposition.
Implementation Method 1
oxidizing the layer of oxidizable material to form a roughened layer of oxide material
Implementation Method 2
forming a layer of polysilicon on the roughened oxide layer by reacting a silicon-containing gas and gaseous hydrochloric acid
Data Source
AI summary
A surface of a cavity of a MEMS device that is rough to reduce stiction. In some embodiments, the average roughness (Ra) of the surface is 5 nm or greater. In some embodiments, the rough surface is formed by forming one or more layers of a rough oxidizable material, then oxidizing the material to form an oxide layer with a rough surface. Another layer is formed over the oxide layer with the rough surface, wherein the roughness of the oxide layer is transferred to the another layer.