Polysilicon Pressure Sensor Electrodes Eliminate Hillock Effect
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Solution Overview
Problem
High-performance pressure sensors face instability issues due to large metal areas, which affect temperature coefficient and mechanical stability, leading to incorrect signal outputs and poor performance in extreme environments.
Innovation Solution
The use of polysilicon to form sensing membranes and electrodes, reducing the number of metal layers and eliminating the hillock effect, while providing better thermal stability and mechanical integrity through a process involving passivation layers, oxide layers, and fusion bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal layers are used for electrodes and routing, then electrical conductivity is achieved, but temperature coefficient matching deteriorates and thermal budget is limited
Solution Approach 1:
The patent changes the material parameter from metal to polysilicon for electrodes and routing. This material substitution improves temperature coefficient matching with the sensing membrane while allowing higher thermal budgets for vacuum bakeout processes, resolving the contradiction between reliability and device complexity.
Solution Approach 2:
The patent extracts and eliminates unnecessary metal layers from the structure. By using polysilicon for electrodes and routing instead of multiple metal layers, the design reduces the number of metal layers to only one for pad connection, simplifying the device structure while maintaining electrical functionality.
2Reliability
If metal layers are used for electrodes, then electrical connectivity is provided, but hillock effect occurs during high-temperature processing
Solution Approach 1:
The patent changes the material parameter from metal to polysilicon for electrodes. Polysilicon has a higher melting point and better thermal stability, which eliminates the hillock effect that occurs with metal during high-temperature vacuum bakeout processes, thereby improving electrode stability without the harmful hillock formation.
3Measurement precision
If large area membrane is used for pressure sensing, then pressure sensitivity is improved, but mechanical stability deteriorates due to gravity and stiction
Solution Approach 1:
The patent uses a thin polysilicon membrane for pressure sensing. The thin film structure provides adequate pressure sensitivity while reducing gravity effects and stiction forces, thereby improving mechanical stability. The polysilicon material offers both flexibility for pressure response and structural integrity for stability.
4Temperature
If multiple metal layers are used for electrodes and routing, then electrical functionality is achieved, but annealing temperature is limited due to metal melting
Solution Approach 1:
The patent changes the material parameter from metal to polysilicon for electrodes and routing. Polysilicon has a much higher melting point than metal, enabling higher annealing temperatures for vacuum bakeout processes to reach better vacuum levels without the risk of metal melting, while reducing the number of metal layers required.
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 enhances temperature coefficient matching, thermal budget, and mechanical stability, reducing gravity sensitivity and preventing membrane stiction, thereby improving the accuracy and reliability of pressure sensor outputs.
Implementation Method 1
depositing a passivation layer on a substrate
Implementation Method 2
depositing and patterning a first oxide layer on the first polysilicon layer
Implementation Method 3
fusion bonding another substrate to the second oxide layer to enclose the sensing cavity
Implementation Method 4
etching a portion of the second oxide layer corresponding to a sensing cavity, wherein the etching exposes the bottom electrode
Data Source
AI summary
A method includes depositing a passivation layer on a substrate; depositing and patterning a first polysilicon layer on the passivation layer; depositing and patterning a first oxide layer on the first polysilicon layer forming a patterned first oxide layer; depositing and patterning a second polysilicon layer on the patterned first oxide layer. A portion of the second polysilicon layer directly contacts a portion of the first polysilicon layer. A portion of the patterned second polysilicon layer corresponds to a bottom electrode. A second oxide layer is deposited on the patterned second polysilicon layer and on an exposed portion of the patterned first oxide layer. A portion of the second oxide layer corresponding to a sensing cavity is etched, exposing the bottom electrode. Another substrate is bonded to the second oxide layer enclosing the sensing cavity. A top electrode is disposed within the another substrate and positioned over the bottom electrode.