MEMS Sensor Back Electrode Deformation-Free Design
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Solution Overview
Problem
MEMS sensors with capacitive mode of operation, such as inertia sensors and microphones, face sensitivity issues due to tensile stress from stoichiometric silicon nitride, leading to deformation and a variable distance between electrodes, making precise measurement impossible.
Innovation Solution
A MEMS sensor design featuring a patterned layer construction with a central cutout spanned by a covering layer, where the covering layer bears on the layer construction in a planar fashion, preventing deformation and allowing for the use of stoichiometric silicon nitride, which provides mechanical stability and stiffness even with thinner layers, and includes a multilayer construction with polysilicon and silicon nitride layers to compensate stresses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If stoichiometric silicon nitride is used for the mechanically stable layer, then mechanical stability and reproducible layer properties are improved, but tensile stress increases causing deformation of the three-dimensional construction and changing the distance between electrodes
Solution Approach 1:
The mechanically stable layer is segmented into a base portion and a covering layer portion. The covering layer is arranged to bear on the layer construction in planar fashion rather than forming a three-dimensional projection, which divides the structure to prevent stress-induced deformation while maintaining mechanical stability.
Solution Approach 2:
The harmful three-dimensional construction that projects above the base chip is extracted/removed. The covering layer is redesigned to lie planarly on the layer construction, eliminating the elevated structure that caused deformation under tensile stress from stoichiometric silicon nitride.
2Ease of manufacture
If stoichiometric silicon nitride is used for the covering layer, then production stability and uniform layer properties are improved, but tensile stress leads to deformation of the covering layer and back electrode
Solution Approach 1:
The covering layer is segmented into a central cutout region and a surrounding support region. The surrounding layer construction provides support to the covering layer, preventing deformation in the central region where the back electrode is located, while allowing the use of stoichiometric silicon nitride for production stability.
Solution Approach 2:
The surrounding layer construction acts as a counterweight or support structure that compensates for the tensile stress in the stoichiometric silicon nitride covering layer, preventing deformation and maintaining the planar shape.
3Strength
If the covering layer forms a three-dimensional construction projecting above the base chip, then mechanical stability is improved, but the tensile stress deforms the construction and alters the electrode distance
Solution Approach 1:
Instead of forming a three-dimensional construction that projects above the base chip, the covering layer is inverted to bear planarly on the layer construction. This inversion maintains mechanical stability through the surrounding support structure while preventing deformation that would alter electrode distance and affect measurement precision.
Solution Approach 2:
The covering layer transitions from a three-dimensional construction to a two-dimensional planar structure. This dimensionality change eliminates the vertical deformation component while maintaining mechanical stability through the surrounding support, preserving the defined distance between electrodes for accurate sensing.
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 design stabilizes the back electrode, maintains a constant distance between electrodes, and allows for precise measurement, enabling improved sensitivity and cost-effectiveness by using thicker, more stable silicon nitride layers while avoiding deformation and inhomogeneous material properties.
Implementation Method 1
the tensile stresses possibly produced during the production of the covering layer act only in the plane of the covering layer. At the same time, the covering layer is held by the layer construction surrounding the cutout, thereby preventing a deformation of the covering layer and in particular of the back electrode layer.
Implementation Method 2
MEMS sensors having a capacitive mode of operation such as can be used in particular as an inertia sensor, pressure sensor or microphone
Data Source
AI summary
An MEMS sensor constructed on a base chip and having a capacitive mode of operation is disclosed. The MEMS sensor has a patterned layer construction applied on the base chip. A cutout is produced in the layer construction, the moveable electrode, for example a membrane, being arranged in said cutout. The cutout is spanned by a covering layer, which bears on the layer construction around the cutout and comprises the back electrode.