Thermally Isolated MOS Thermal Sensor with Suspended Spring Structure
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Solution Overview
Problem
Conventional thermal sensing devices, such as thermocouples and infrared image sensors, fail to meet requirements for thickness, area, yield, and bidirectional illumination, necessitating further development for specific applications.
Innovation Solution
A method for forming thermally isolated MOS sensors (TMOS) involving patterning an active layer on a handle substrate, bonding it to an integrated circuit substrate, and sealing with a top cap to create a vacuum-sealed cavity with thermally isolated TMOS transistors suspended by springs, allowing for electrical contact and reduced reflection of infrared radiation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional thermal sensing devices (thermocouples or infrared image sensors) are used, then thermal sensing function is achieved, but the device thickness, area, and manufacturing yield requirements are not met
Solution Approach 1:
The device is divided into separate functional components: the active layer with TMOS sensors is fabricated on a handle substrate, then bonded to an integrated circuit substrate, and finally enclosed by a top cap. This segmentation allows each component to be optimized independently, improving overall manufacturing precision and yield while meeting thickness requirements.
Solution Approach 2:
The patent transitions from planar thermal sensors to a three-dimensional suspended structure where the active layer is released and suspended above the integrated circuit substrate. This vertical arrangement reduces the effective area footprint and allows bidirectional illumination from both front and back sides, resolving the contradiction between area constraints and sensing functionality.
2Length of moving object
If the sensor structure is made thinner to meet thickness requirements, then thickness specification is satisfied, but mechanical strength and structural stability deteriorate
Solution Approach 1:
The active layer is released from the handle substrate and suspended by thin spring structures, forming a flexible membrane configuration. This allows the sensor to achieve thin overall thickness while the spring-supported membrane maintains mechanical strength and stability during operation.
Solution Approach 2:
The sensor structure incorporates spring elements that provide dynamic mechanical support to the suspended active layer. These springs allow the thin membrane to flex and respond to thermal changes while maintaining structural integrity, resolving the contradiction between thinness and mechanical strength.
3Ease of manufacture
If the sensor is designed for single-side illumination, then manufacturing simplicity is maintained, but the capability to be illuminated from both front and back sides is lost
Solution Approach 1:
By suspending the active layer in three-dimensional space above the integrated circuit substrate, the sensor structure allows infrared radiation to reach the sensing elements from both the front side (through the top cap) and the back side (through the integrated circuit substrate), enabling bidirectional illumination without complicating the manufacturing process.
4Measurement precision
If thermal isolation structures are added to improve sensor sensitivity, then measurement precision improves, but device complexity and manufacturing difficulty increase
Solution Approach 1:
The active layer is extracted from the handle substrate and suspended in free space, removing thermal conduction paths to the substrate. This thermal isolation improves temperature sensing precision by preventing heat leakage to the mounting structure, while the suspension mechanism itself serves as the isolation structure rather than adding separate complex insulation layers.
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 solution results in a thinner, more robust thermal sensor with improved yield and reduced mechanical stresses, capable of sensing ambient and external temperatures with enhanced sensitivity and reliability.
Implementation Method 1
a spring structure connecting the mass to the frame while thermally isolating the mass from the frame
Implementation Method 2
The bonding of the frame to the pads may be performed via metal bonding to thereby make electrical contact from the pads to the at least one TMOS sensor through a non-metal conductive path extending from the frame, through the spring, to the at least one TMOS sensor within the mass
Implementation Method 3
The bonding of the top cap to the first face of the integrated circuit substrate may be performed by glass frit bonding a sidewall of the top cap to the first face of the integrated circuit substrate. The glass frit bonding may result in glass material expanding into a trench defined in an interior surface of the top cap.
Implementation Method 4
The bonding of the top cap to the first face of the integrated circuit substrate may be performed in a vacuum such that a pressure within the chamber is substantially at vacuum.
Implementation Method 5
Prior to bonding the top cap to the first face of the integrated circuit substrate, the method may include forming a getter layer on an interior surface of the top cap about a perimeter thereof, wherein the getter layer maintains the vacuum within the chamber during operation.
Implementation Method 6
The method may further include forming an anti-reflective layer on a second face of the top cap, the anti-reflective layer reducing reflection of incident infrared radiation.
Implementation Method 7
Thermal sensors are used in a variety of devices to determine the temperature of desired targets and function by sensing infrared radiation
Data Source
AI summary
Disclosed herein is a method of forming a thermal sensor, including patterning an active layer on a first face of a handle substrate to form a frame, a mass carrying at least one thermally isolated MOS (TMOS) transistor, and a spring structure connecting the mass to the frame while thermally isolating the mass from the frame. The frame is then bonded to pads on a first face of an integrated circuit substrate. The handle substrate is removed, and a top cap is bonded to the first face of the integrated circuit substrate to enclose at least the mass and spring within the sealed cavity.