Pressure Sensor Diaphragm Thermal Expansion Matching
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Solution Overview
Problem
Conventional pressure sensors face limitations in sensitivity and size reduction while maintaining temperature characteristics due to disturbance strain caused by thermal expansion differences between substrates and movable electrodes, leading to potential bending and decreased yield during manufacturing.
Innovation Solution
A physical quantity sensor design featuring substrates with different coefficients of thermal expansion, where the movable electrode's expansion is between those of the substrates, and anodic bonding with equalized electrical potentials to prevent electrostatic attraction, allowing for a thinned diaphragm and reduced disturbance strain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the movable electrode is thinned to achieve high sensitivity, then sensitivity is improved, but the movable electrode is easily drawn toward substrates by electrostatic force and receives disturbance strain from thermal expansion
Solution Approach 1:
The patent changes the coefficient of thermal expansion parameter of the substrate to match that of the movable electrode. By selecting a substrate with a coefficient of thermal expansion of 5-15×10^-6/K (matching single crystal silicon), the thermal expansion forces are balanced, preventing diaphragm distortion and eliminating the need for thick electrodes for thermal stability, thus achieving both high sensitivity and good temperature characteristics
Solution Approach 2:
The patent applies thermal expansion principles by carefully matching the thermal expansion coefficients of different components. The substrate is selected to have a coefficient of thermal expansion (5-15×10^-6/K) that matches the movable electrode material, ensuring that during temperature changes, both components expand and contract at the same rate, preventing relative displacement and distortion of the diaphragm structure
2Volume of moving object
If the size of the pressure sensor is reduced, then size is decreased, but the movable electrode needs to be thinned which increases susceptibility to electrostatic attraction and thermal strain
Solution Approach 1:
The patent changes the substrate's coefficient of thermal expansion parameter to match the movable electrode, eliminating thermal strain effects. This allows the movable electrode to be thinned for high sensitivity in small-sized sensors without suffering from thermal expansion mismatch, as the matched parameters ensure uniform thermal behavior across all components
Solution Approach 2:
The patent applies equipotentiality by setting the movable electrode and substrate at the same electrical potential during anodic bonding. This eliminates potential differences that would cause electrostatic attraction, allowing thinned movable electrodes in compact sensors to remain stable and prevent adhesion to the substrate
3Strength
If anodic bonding is performed at high temperature, then bonding strength is improved, but the movable electrode may be drawn toward substrates by electrostatic force or bent due to thermal expansion differences
Solution Approach 1:
The patent applies equipotentiality by connecting the movable electrode and substrate at the same electrical potential during the anodic bonding process. This eliminates potential differences that would generate electrostatic attraction forces, preventing the movable electrode from being drawn toward the substrate during high-temperature bonding, thus maintaining precise positioning and avoiding defects
Solution Approach 2:
The patent changes the coefficient of thermal expansion parameter of the substrate to match that of the movable electrode. During high-temperature bonding and subsequent cooling, both components expand and contract uniformly, eliminating differential thermal stress that would cause bending or distortion of the movable electrode, thus maintaining manufacturing precision
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 results in a highly sensitive, small-sized, and inexpensive pressure sensor with improved temperature stability by offsetting thermal expansion forces and preventing diaphragm bending, enhancing sensitivity and manufacturing efficiency.
Implementation Method 1
A known bonding method such as anodic bonding is used for bonding. In such a bonding, the movable electrode 20, the first substrate 30 and the second substrate 40 are heated to a high temperature (for example, approximately 400 degree C.).
Implementation Method 2
there is a possibility that the movable electrode 20 is drawn toward the first substrate 30 or the second substrate 40 by an electrostatic force and is bonded while the movable electrode 20 is bended.
Implementation Method 3
When the pressure sensor 10 heated by the bonding is cooled down to a room temperature, there is a possibility that the movable electrode 20 may be bended due to a difference in the coefficients of thermal expansion between the movable electrode 20 and the first and second substrates 30 and 40.
Data Source
AI summary
A physical quantity sensor includes two substrates and a movable electrode that is disposed between the two substrates and is bonded to the two substrates. In the physical quantity sensor, the movable electrode has an elastically deformable diaphragm and one of the two substrates is an electrode substrate having a detection electrode on a detection surface opposite to the diaphragm to detect capacitance between the diaphragm and the detection electrode. In the physical quantity sensor, in a range between a room temperature and a bonding temperature when the two substrates and the movable electrode are bonded, coefficients of thermal expansion of the two substrates are smaller than that of the movable electrode and in a temperature range where the physical quantity sensor is used, a coefficient of thermal expansion of the movable electrode is between a first and second substrates.