Microbolometer Raised Strut Design for Thermal Conductance Reduction
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Solution Overview
Problem
Current microbolometers operating in MWIR and LWIR wavelengths face limitations in sensitivity due to high thermal conductance, which affects their responsivity, detectivity, and noise equivalent temperature difference (NETD), making them less effective compared to cooled quantum detectors.
Innovation Solution
A microbolometer design featuring a cavity layer and mesh support structure with nano-meter size studs to minimize thermal conductance, utilizing a substrate with an insulating layer, support layers fabricated using e-beam lithography, and a sensing layer with high TCR materials, along with a meandered electrode structure to reduce heat conduction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If serpentine structured electrode arms are used for suspension, then the device can be manufactured using surface micromachining technique, but the thermal conductance is high (minimum 1×10−7 W/K) which reduces detection sensitivity
Solution Approach 1:
The support structure is segmented into multiple discrete nano-meter size studs instead of continuous serpentine arms. This segmentation reduces the thermal conduction path while maintaining mechanical support, achieving thermal conductance reduction by at least an order of magnitude below 5×10−9 W/K
Solution Approach 2:
The thermal conductance parameter is changed by reducing the cross-sectional area of the support structure from micrometer-scale serpentine arms to nano-meter size studs. This parameter change in the support structure geometry directly reduces thermal conductance while maintaining ease of manufacture through standard lithography techniques
2Measurement precision
If thermal conductance is reduced to improve detection sensitivity, then figures of merits such as responsivity and detectivity improve, but the device becomes more sensitive to environmental thermal fluctuations
Solution Approach 1:
A mesh support structure acts as an intermediary between the sensing layer and the substrate. This mesh structure provides both mechanical support and thermal isolation, allowing the sensing layer to be thermally isolated for high detectivity while the substrate provides environmental stability. The mesh structure mediates between these two requirements
3Measurement precision
If nano-meter size studs are used to reduce thermal conductance, then thermal conductance is reduced by at least an order of magnitude, but the fabrication complexity increases requiring e-beam lithography
Solution Approach 1:
The fabrication process substitutes conventional photolithography with electron beam lithography to achieve the required nano-meter scale precision. This replacement of the lithography method enables the creation of nano-meter size studs with precise dimensional control, achieving the required thermal conductance reduction
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 a significant reduction in thermal conductance by at least an order of magnitude, enhancing the microbolometer's sensitivity and figures of merit, allowing it to operate effectively without external cooling.
Implementation Method 1
Thermal IR detectors are heated by the incident IR radiation and provide detection through the change in a measurable parameter
Implementation Method 2
provide detection through the change in a measurable parameter... change in resistance on the sensing material due to absorption of heat flux
Implementation Method 3
Using a cavity layer followed by a mesh support structure minimizes the thermal conductance of the device
Implementation Method 4
minimizes the thermal conductance... effective thermal conductance to be governed by the radiation over the detector
Data Source
AI summary
A semiconducting microbolometer sensor for detecting electromagnetic waves in the medium wavelength infrared (MWIR) and long-wavelength infrared (LWIR) is provided. A preferred embodiment provides a substrate layer, a bottom and top support structure with a strut-based mesh design, a meandered electrode layer that follows the top support structure design, a bolometer sensing material with a high TCR, and a disk-shaped absorber on top of the sensing material to maximize the heat flux absorption on the sensor. The bottom support of the sensor suspends the top support mesh, creating an air cavity. This air cavity along with the strut based mesh design and optimized thickness, dimension and shape of the layers contributed towards minimizing the thermal conductance of microbolometer and hence improved the figures of merits—responsivity, detectivity, noise equivalent power and noise equivalent temperature difference of microbolometer.