Permeable Polysilicon Layer for MEMS Cavity Pressure Control
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Solution Overview
Problem
Existing techniques for fabricating microelectromechanical systems (MEMS) inertial measuring units (IMUs) with multiple cavities at different pressures are complex and costly, particularly due to the need for getters or time-consuming sealing processes.
Innovation Solution
A method involving the formation of a permeable polysilicon layer, followed by sequential deposition and etching of layers to create sealed cavities at different pressures without the use of getters, by utilizing vapor etching and cap wafer bonding to establish and maintain pressure differentials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a getter layer is used to reduce pressure in the gyroscope cavity, then the desired operating pressure is achieved, but the device complexity increases
Solution Approach 1:
The patent removes the getter layer from the device structure entirely. Instead of using a getter to reduce pressure, the invention uses a through-substrate hole that allows the cavity to be sealed at the desired vacuum pressure during fabrication without requiring any pressure-reducing material inside the cavity.
Solution Approach 2:
The patent segments the pressure control function from the cavity structure by using a separate through-substrate hole pathway. This allows the cavity to be sealed independently at the desired pressure without needing a getter layer within the cavity itself.
2Reliability
If heat is applied to seal the access opening after pressure adjustment, then the cavity is sealed, but the process is time consuming and costly
Solution Approach 1:
The patent performs the sealing action during the fabrication process itself, before final device assembly. The cap wafer is bonded to the substrate while the through-substrate hole is still open, allowing pressure equalization and sealing to occur in a single integrated step rather than requiring separate heating and sealing operations afterward.
3Reliability
If individual die sealing is performed, then each cavity is sealed properly, but the cost increases
Solution Approach 1:
The patent merges the sealing operation with the cap wafer bonding step. By designing the cap wafer with integrated bonding structures that align with the through-substrate holes, multiple cavities can be sealed simultaneously in a single batch process rather than requiring individual handling and sealing of each die.
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
This approach simplifies the fabrication process, reduces complexity and cost, and allows for precise control of pressure in MEMS IMU cavities, enhancing the reliability and efficiency of MEMS IMU production.
Implementation Method 1
depositing a permeable layer on the oxide layer, the permeable layer being gas permeable but vapor etch resistant
Implementation Method 2
performing a first vapor etching through open portions of the structural layer to remove exposed portions of the sacrificial layer, the first vapor etching also passing through the permeable layer at the exposed portions thereof
Implementation Method 3
bonding a cap wafer to the structural layer at bonding points
Data Source
AI summary
Disclosed herein is a process flow for forming a MEMS IMU including an accelerometer and a gyroscope each located in a separate sealed cavity maintained at a different pressure. Formation of the MEMS IMU includes the use of a first vHF release to etch a sacrificial layer underneath a structural layer containing the accelerometer and gyroscope and capping the device under formation to set both cavities at a first pressure. The floor of one of the cavities is formed to including a gas permeable layer. Formation further includes forming a chimney underneath the gas permeable layer and then performing a second vHF release to etch through the gas permeable layer and expose the cavity containing the gas permeable layer so that its pressure may be set to be different than that of the other cavity when the chimney is sealed.