MEMS Force Sensor Etch Stop Layer Thickness Control
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Solution Overview
Problem
Existing MEMS force sensors face challenges in achieving precise membrane thickness control, leading to variations in sensitivity, typically in the range of 10% to 20%, which can be improved upon using grinding/polishing or other deposition methods.
Innovation Solution
The implementation of an etch stop layer before the cavity etch allows for precise membrane thickness control by absorbing etch non-uniformity, enabling the use of silicon as the membrane material and allowing for the formation of mesas for overload protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If membrane thickness is controlled through the etch process, then manufacturing simplicity is maintained, but manufacturing precision deteriorates with sensitivity variations in the range of 10% to 20%
Solution Approach 1:
An etch stop layer is introduced as an intermediary between the membrane and the cavity structure. This layer has a different etch rate than the membrane material, allowing it to absorb etch non-uniformity and protect the membrane thickness from variations during the cavity etching process, thereby achieving less than 5% sensitivity variation
Solution Approach 2:
The structure is segmented into distinct functional layers: the membrane layer for force sensing, the etch stop layer for thickness protection, and the cavity structure for mechanical amplification. This segmentation allows each layer to be optimized independently, with the etch stop layer specifically designed to have different etching characteristics to protect the membrane
2Measurement precision
If thinner membranes are used to increase sensitivity, then sensitivity increases, but manufacturing precision becomes more difficult to control
Solution Approach 1:
The etch stop layer is deposited beforehand on the membrane surface before cavity formation. This layer acts as a cushion that absorbs the variability of the etching process, ensuring that even when etching to create the cavity, the membrane thickness remains protected and controlled, enabling use of thinner membranes with better sensitivity
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 precise control of membrane thickness, thereby enhancing the sensitivity of MEMS force sensors and reducing sensitivity variations, achieving tolerances of less than 5%.
Implementation Method 1
an etch rate of the etch stop layer can be different than an etch rate of the first substrate. Optionally, an etch rate ratio between the etch rate of the first substrate and the etch rate of the etch stop layer can be between 50 and 150
Implementation Method 2
The sensing element can be configured to convert a strain on the surface of the membrane substrate to an analog electrical signal that is proportional to the strain
Implementation Method 3
The sensing element can be a piezoresistive or piezoelectric sensing element
Data Source
AI summary
An example microelectromechanical system (MEMS) force sensor is described herein. The MEMS force sensor can include a sensor die configured to receive an applied force. The sensor die can include a first substrate and a second substrate, where a cavity is formed in the first substrate and where at least a portion of the second substrate defines a deformable membrane. The MEMS force sensor can also include an etch stop layer arranged between the first substrate and the second substrate, and a sensing element arranged on a surface of the second substrate. The sensing element can be configured to convert a strain on the surface of the membrane substrate to an analog electrical signal that is proportional to the strain.