MEMS Microphone Sensitivity Trimming via Programmable Circuit
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Solution Overview
Problem
The mechanical properties of MEMS microphone diaphragms, manufactured by standard CMOS processes, are variable due to thin film thickness and residual stress variations, making it difficult to achieve stable sensitivity levels, which is costly to address through precise fabrication processes.
Innovation Solution
A programmable trimming circuit is implemented in the MEMS microphone, allowing for adjustment of the amplifier gain or capacitor biasing voltage to achieve target sensitivity levels, thereby compensating for diaphragm compliance variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If standard CMOS fabrication process is used for manufacturing MEMS diaphragm, then manufacturing cost is reduced and ease of manufacture is improved, but sensitivity stability deteriorates due to thin film thickness and residual stress variations
Solution Approach 1:
The patent replaces mechanical/physical fabrication control (controlling thin film thickness and residual stress during manufacturing) with an electronic trimming system. A trimming circuit is integrated into the MEMS device that can electronically adjust the diaphragm's mechanical properties after fabrication, substituting post-manufacturing electronic adjustment for pre-manufacturing mechanical precision control.
Solution Approach 2:
The patent changes the approach from controlling physical parameters during fabrication (film thickness, stress) to adjusting electrical parameters after fabrication. The trimming circuit modifies electrical parameters (voltage, current) to compensate for variations in the diaphragm's mechanical properties, thereby achieving sensitivity stabilization through parameter adjustment rather than precise manufacturing control.
2Reliability
If precise fabrication process is developed to control diaphragm mechanical properties, then sensitivity stability is improved, but manufacturing cost increases
Solution Approach 1:
The patent substitutes complex precise fabrication processes with a simpler standard CMOS process combined with an integrated trimming circuit. Instead of investing in expensive precision manufacturing equipment and processes, the solution uses standard manufacturing followed by electronic trimming, replacing mechanical precision requirements with electronic adjustment capabilities.
Solution Approach 2:
The trimming circuit acts as an intermediary between the standard fabrication process and the final performance requirement. It mediates the gap between the variability inherent in standard manufacturing and the stability required for sensitive operation, providing a cost-effective bridge without requiring expensive precision manufacturing.
3Reliability
If trimming circuit is added to adjust sensitivity, then sensitivity stability is improved, but device complexity increases
Solution Approach 1:
The patent merges the trimming circuit with the MEMS device structure, integrating the sensitivity adjustment functionality into the existing device architecture. The trimming circuit is combined with the diaphragm and capacitor structures, allowing sensitivity adjustment without requiring separate external adjustment mechanisms, thereby limiting the increase in overall device complexity.
Solution Approach 2:
The trimming circuit is designed to serve multiple functions within the MEMS device, including sensitivity adjustment and potential calibration functions. By creating a multi-functional component, the patent reduces the need for separate dedicated adjustment mechanisms, thereby managing device complexity while achieving sensitivity stability.
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 enables stable sensitivity adjustment without the need for costly fabrication process improvements, ensuring consistent performance across multiple MEMS devices.
Implementation Method 1
the diaphragm 100, as one electrode, is formed with a fixed electrode as a capacitor, the displacement of the diaphragm 100 change the capacitance followed the capacitance relation of C=∈ A/d
Implementation Method 2
the sensing diaphragm of the diaphragm capacitor 122 manufactured by standard CMOS process is variable due to the variance of thin film thickness and residual stress
Data Source
AI summary
A microelectromechanical system (MEMS) device includes a diaphragm capacitor, connected between a capacitor biasing voltage source and a ground. A source follower circuit is coupled to the diaphragm capacitor. An amplifier is coupled to the source follower circuit to amplify the voltage signal as an output voltage signal. A programmable trimming circuit is implemented with the amplifier to trim a gain or implemented with the capacitor biasing voltage source to trim voltage applied on the diaphragm capacitor. Whereby, the output voltage signal has a target sensitivity.