Thermopile Infrared Sensor Vertical Wall Etching
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Solution Overview
Problem
Existing thermal infrared sensors face challenges in achieving high sensitivity and cost-effective mass production with standard CMOS processes, particularly due to limitations in thermal conductivity and vacuum requirements, which hinder the development of compact, high-resolution sensors suitable for normal or reduced pressure environments.
Innovation Solution
The design incorporates a thermopile sensor structure with long, narrow thermoelements arranged on connecting webs that separate the absorber region from the support body, utilizing a gas with low thermal conductivity and optimizing the membrane structure for vertical or nearly vertical walls to enhance thermal insulation and sensitivity, while maintaining a compact chip size and high filling factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If wet chemical etching process is used to create clearance in substrate, then manufacturing simplicity is improved, but thermal insulation performance deteriorates due to inclined walls reducing temperature differential
Solution Approach 1:
The invention changes the etching parameters and methodology from conventional wet chemical etching to a combination of deep reactive ion etching (DRIE) and isotropic etching. This allows achieving vertical walls through DRIE followed by rounding at the base through isotropic etching, thereby maintaining both manufacturing feasibility and optimal thermal insulation properties
Solution Approach 2:
The invention employs a composite etching approach combining anisotropic DRIE for vertical wall formation and isotropic etching for base rounding. This composite process methodology achieves the optimal wall geometry that balances manufacturing capability with thermal performance requirements
2Volume of moving object
If sensor structure size is reduced to achieve compact design, then device miniaturization is improved, but etching depth must be reduced which worsens thermal insulation
Solution Approach 1:
The invention transitions from surface micromachining to bulk micromachining, etching through the entire substrate thickness to create vertical walls. This dimensional approach allows maintaining adequate clearance depth even in compact sensors, ensuring optimal thermal insulation without compromising miniaturization goals
Solution Approach 2:
The invention changes the etching depth parameter to extend through the complete substrate thickness rather than partial depth. This full-depth etching creates vertical walls that maximize thermal insulation while allowing the sensor structure to be scaled down in lateral dimensions for compact design
3Measurement precision
If high vacuum housing is used to improve sensitivity, then thermal insulation is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The invention converts the potential harm of residual gas thermal conduction into a benefit by designing vertical walls that maximize clearance depth. This geometric optimization reduces the impact of gas thermal conduction, allowing operation in less stringent vacuum conditions while maintaining sensitivity, thereby simplifying housing requirements
Solution Approach 2:
The invention changes the operational parameter from requiring high vacuum to accepting reduced vacuum or fill gas environments. The vertical wall geometry compensates for increased gas thermal conduction, enabling sensor operation in simpler housing configurations without sacrificing measurement precision
4Ease of manufacture
If membrane is slitted to improve manufacturing, then ease of manufacture is improved, but thermal conditions are compromised due to reduced insulation effectiveness
Solution Approach 1:
The invention extracts the etching process from the membrane surface and relocates it to the substrate base. By etching vertical walls at the substrate level rather than creating slits in the membrane, the membrane remains intact and thermally effective while still achieving the necessary clearance and manufacturing feasibility
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 configuration results in a high signal sensitivity and response rate, enabling cost-effective mass production of thermal infrared sensors with improved thermal resolution and reduced manufacturing complexity, suitable for operation under normal or reduced pressure conditions.
Implementation Method 1
thermopile sensor structures on a membrane... The absorbed IR radiation produces a temperature differential... the sensitivity that the sensor cell can attain
Implementation Method 2
the thermal conductivity of the residual gas or the fill gas in the sensor housing reduces the temperature differential that can be attained between the absorber region... and the heat sink... The use of a fill gas is not described
Data Source
AI summary
A thermal infrared sensor is provided in a housing with optics and a chip with thermoelements on a membrane. The membrane spans a frame-shaped support body that is a good heat conductor, and the support body has vertical or approximately vertical walls. The thermopile sensor structure consists of a few long thermoelements per sensor cell. The thermoelements being arranged on connecting webs that connect together hot contacts on an absorber layer to cold contacts of the thermoelements. The membrane is suspended by one or more connecting webs and has, on both sides of the long thermoelements, narrow slits that separate the connecting webs from both the central region and also the support body. At least the central region is covered by the absorber layer.