Single Silicon Wafer Thermal Conduction Sensor Design
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Solution Overview
Problem
Existing thermal conduction sensors face inefficiencies in fabrication processes due to the need for wafer-level bonding and potential natural convection issues, which complicate heat transfer analysis and increase production costs.
Innovation Solution
A single silicon wafer micromachined thermal conduction sensor with a heat transfer cavity and thin film bridge created in a silicon substrate, where the cavity has a Grashof number less than 1×10−2, ensuring no natural convection occurs, and the fabrication process eliminates the need for wafer-level bonding, allowing for simpler production and versatile use as both a vacuum and humidity sensor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If wafer-level bonding is used to assemble multiple silicon wafers for thermal conduction sensor fabrication, then the sensor can be constructed with separate heat source and heat sink wafers, but the fabrication process becomes complicated and production costs increase
Solution Approach 1:
The patent combines the heat source, heat sink, and gas cavity into a single silicon wafer structure. The heat source and heat sink are formed as integrated regions within the same wafer, eliminating the need for separate wafer bonding processes while maintaining the thermal conduction sensor functionality.
Solution Approach 2:
The single silicon wafer serves multiple functions simultaneously: it provides the substrate, contains the heat source region, forms the heat sink region, and defines the gas cavity. This multi-functional integration eliminates the need for multiple specialized components and assembly steps.
2Reliability
If the gas cavity is designed with larger dimensions to facilitate heat transfer, then heat transfer efficiency improves, but natural convection occurs which complicates heat transfer analysis
Solution Approach 1:
The patent changes the dimensional parameters of the gas cavity, specifically setting the gap height to be less than 50 micrometers. This parameter change suppresses natural convection by reducing the Rayleigh number below the critical threshold, ensuring that heat transfer occurs primarily through conduction rather than convection.
3Measurement precision
If chemical interactions are used between gas and sensor material to achieve high sensitivity, then gas detection sensitivity improves, but stability deteriorates due to chemical reactions over time
Solution Approach 1:
The patent replaces chemical interaction mechanisms with a physical thermal conduction mechanism. Instead of relying on chemical reactions between gas molecules and sensor materials, the sensor measures changes in thermal conductivity of the gas, which is a physical property that does not degrade over time through chemical changes.
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 simplifies the fabrication process, reduces production costs, and enables accurate one-dimensional heat transfer modeling, enhancing the sensor's ability to analyze gas mixtures without natural convection interference, making it suitable for applications like natural gas analysis.
Implementation Method 1
heat transfers from the heat source to the heat sink through the gas filled in the cavity
Implementation Method 2
The heated resistor is located on a thin film bridge and thus thermally insulated against a supporting substrate
Implementation Method 3
This change in temperature is measured with a temperature sensor, which is also located on the thin film bridge
Data Source
AI summary
A single silicon wafer micromachined thermal conduction sensor is described. The sensor consists of a heat transfer cavity with a flat bottom and an arbitrary plane shape, which is created in a silicon substrate. A heated resistor with a temperature dependence resistance is deposed on a thin film bridge, which is the top of the cavity. A heat sink is the flat bottom of the cavity and parallel to the bridge completely. The heat transfer from the heated resistor to the heat sink is modulated by the change of the thermal conductivity of the gas or gas mixture filled in the cavity. This change can be measured to determine the composition concentration of the gas mixture or the pressure of the air in a vacuum system.