MEMS Capacitive Sensor Two-Layer Structure for Sensitivity
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Solution Overview
Problem
The existing semiconductor etch processes for forming MEMS devices are limited by the aspect ratio, which restricts the ability to achieve the desired gap spacing between capacitive plates, thereby limiting the sensitivity of the MEMS sensors such as gyroscopes and accelerometers.
Innovation Solution
The solution involves forming a second layer of semiconductor material with a thickness less than the first layer, allowing for a smaller gap distance, thereby increasing the sensitivity by forming a second capacitance that enhances the total change in capacitance when the moveable mass displaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a single thick layer of semiconductor material is used to form the proof mass, then the mass is sufficient for proper sensing, but the gap spacing between capacitive plates becomes too large, reducing sensitivity
Solution Approach 1:
The single thick semiconductor layer is divided into two separate layers: a first layer forming the proof mass and a second thinner layer forming the capacitive plates. This segmentation allows the gap spacing to be reduced in the second layer while maintaining sufficient mass in the first layer, thereby resolving the contradiction between adequate mass and small gap spacing for high sensitivity.
Solution Approach 2:
The solution transitions from a single-layer structure to a multi-layer structure, adding the dimension of layer stacking. The first layer provides mass while the second layer provides the capacitive sensing structure with reduced gap spacing, effectively solving the contradiction by utilizing vertical dimensionality rather than relying on a single thick layer.
2Measurement precision
If the gap spacing is reduced to increase sensitivity, then the capacitance change increases, but the etch process cannot form sufficiently small gaps in thick layers
Solution Approach 1:
The capacitive sensing structure is separated into a first layer for mass and a second thinner layer for capacitive plates. The etch process is applied separately to each layer, allowing the second layer to achieve smaller gap spacing that is manufacturable with standard etch processes, while the first layer maintains sufficient thickness for mass.
Solution Approach 2:
The thickness parameter of the semiconductor layer is changed by creating two distinct layers with different thicknesses. The second layer has a reduced thickness parameter that enables the etch process to form smaller gap spacings, thereby increasing capacitance change and sensitivity while remaining within manufacturing capabilities.
3Weight of moving object
If a thicker layer is used to provide sufficient mass, then the proof mass is adequate, but the aspect ratio of the etch process limits the minimum gap that can be formed
Solution Approach 1:
The structure is segmented into two layers where the first layer provides the required proof mass weight and the second layer provides the capacitive plates with reduced gap spacing. This segmentation decouples the requirements of sufficient mass from minimum gap formation, allowing each layer to be optimized independently.
Solution Approach 2:
By transitioning to a multi-layer vertical structure, the patent resolves the aspect ratio limitation. The first layer maintains sufficient thickness for mass while the second layer provides the reduced gap spacing, effectively bypassing the aspect ratio constraint that would prevent small gaps in a single thick layer.
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 significantly increases the sensitivity of the MEMS devices by allowing a smaller gap distance in the second layer, resulting in a greater change in capacitance for a given displacement, compared to traditional single-layer structures, while using standard materials and existing processes.
Implementation Method 1
The mass and the anchors, or sensing electrodes coupled to them, have portions that form plates of a variable capacitance. When the mass is displaced by motion, some of the capacitances increase and some decrease, depending on the direction of motion.
Data Source
AI summary
Methods and apparatus for forming MEMS devices. An apparatus includes at least a portion of a semiconductor substrate having a first thickness and patterned to form a moveable mass; a moving sense electrode forming the first plate of a first capacitance; at least one anchor patterned from the semiconductor substrate and having a portion that forms the second plate of the first capacitance and spaced by a first gap from the first plate; a layer of semiconductor material of a second thickness patterned to form a first electrode forming a first plate of a second capacitance and further patterned to form a second electrode overlying the at least one anchor and forming a second plate spaced by a second gap that is less than the first gap; wherein a total capacitance is formed that is the sum of the first capacitance and the second capacitance. Methods are disclosed.