Planar Thermopile Infrared Microsensor Thermal Isolation
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Solution Overview
Problem
Infrared temperature sensors, particularly those using silicon thermopiles, face challenges with low output signal levels, noise issues, and sensitivity to gas conduction and convection due to thermal dissimilarity between thermojunctions, leading to reliability and cost problems in packaging.
Innovation Solution
A planar IR microsensor design featuring a heat sink substrate with varying thermal conductivity, a thermocouple layer with hot and cold junctions on low thermal conductivity portions, a dielectric layer with a via to the hot junction, an IR reflector layer acting as a radiative heat pipe, and an integrated IR absorber, which eliminates the need for specific packaging and enhances sensitivity and immunity to convection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If micro-machined suspended structures with membranes or cantilevers are used to minimize conduction heat losses, then thermal resistance between hot and cold junctions is improved, but device complexity and packaging requirements increase
Solution Approach 1:
The patent merges the thermocouple structure directly with the silicon substrate by forming thermocouple legs in trenches etched into the substrate. This integration eliminates the need for separate suspended membrane structures and complex packaging, while maintaining high thermal resistance through the low thermal conductivity of the substrate in the trench regions.
Solution Approach 2:
The patent replaces the mechanical suspended membrane structure with an integrated substrate-based structure where thermocouple legs are formed directly in trenches. This substitution eliminates the need for complex suspension mechanics and packaging while achieving the desired thermal isolation through material property control in the substrate.
2Ease of manufacture
If polysilicon thermopile is used for integration on silicon wafer, then manufacturing is simplified, but sensitivity to gas conduction and convection increases due to thermal dissimilarity between thermojunctions
Solution Approach 1:
The patent applies local quality by creating regions of different thermal conductivity within the silicon substrate. The substrate has low thermal conductivity regions where thermocouple hot junctions are formed to minimize heat conduction to surrounding areas, while maintaining overall substrate integrity for manufacturing. This localized property variation reduces sensitivity to gas conduction and convection effects.
3Reliability
If encapsulated structures are used to protect against gas conduction and convection, then reliability is improved, but packaging cost and complexity increase
Solution Approach 1:
The patent implements self-service by making the silicon substrate itself provide the thermal isolation function that would otherwise require external encapsulation. The low thermal conductivity regions in the substrate automatically protect against gas conduction and convection effects, eliminating the need for separate encapsulated structures while maintaining reliability.
4Ease of manufacture
If silicon substrate is used for thermopile manufacturing, then cost is reduced, but thermal conductivity is too high leading to heat loss
Solution Approach 1:
The patent applies local quality by creating regions of different thermal conductivity within the silicon substrate. The substrate has low thermal conductivity regions where thermocouple hot junctions are formed to minimize heat conduction to surrounding areas, while maintaining overall substrate integrity for manufacturing. This localized property variation reduces sensitivity to gas conduction and convection effects.
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
The design achieves higher sensitivity, reduced convection losses, and cost-effective manufacturing by integrating the thermocouple layer and IR filter directly onto the substrate, providing precise temperature measurement with improved directivity and reduced non-linearity, suitable for automotive and imaging applications.
Implementation Method 1
a thermopile detector converts temperature differences into voltages by the Seebeck effect
Implementation Method 2
an IR absorber within the via
Implementation Method 3
the cold junction is thermally coupled to the silicon substrate
Implementation Method 4
The IR reflector layer implements a radiative heat pipe, which is patterned onto the sensor in place of the previously known planar radiative coating
Data Source
AI summary
An IR sensor comprises a heat sink substrate (10) having portions (12) of relatively high thermal conductivity and portions (14) of relatively low thermal conductivity and a planar thermocouple layer (16) having a hot junction (18) and a cold junction (20), with the hot junction (18) located on a portion (14) of the heat sink substrate with relatively low thermal conductivity. A low thermal conductivity dielectric layer (22) is provided over the thermocouple layer (16), and has a via (24) leading to the hot junction (18). An IR reflector layer (26) covers the low thermal conductivity dielectric layer (22) and the side walls of the via (24). An IR absorber (30; 30′) is within the via. This structure forms a planar IR microsensor which uses a structured substrate and a dielectric layer to avoid the need for any specific packaging. This design provides a higher sensitivity by providing a focus on the thermocouple, and also gives better immunity to gas conduction and convection.