Heated Air Mass WCSP Accelerometer
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Solution Overview
Problem
Existing inertial sensors face challenges with stiction in closely spaced conductive fingers under extreme acceleration, high cost, and require costly MEMS fabrication techniques, limiting their accuracy and cost-effectiveness for widespread applications.
Innovation Solution
A differential thermocouple sensor design using a heated air mass and a common poly silicon element, which generates a differential voltage based on the relative location of the heated gas mass to thermocouple junctions, allowing for lower-cost production using conventional integrated circuit processing without additional MEMS techniques.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If closely spaced conductive fingers are used to measure acceleration, then the sensor can detect acceleration, but stiction occurs under extreme acceleration
Solution Approach 1:
The patent uses a heated air mass instead of solid conductive fingers. The air mass is heated to create temperature differences that drive thermal convection currents, which move the air mass in response to acceleration. This pneumatic approach eliminates mechanical contact and stiction while maintaining acceleration sensing capability through thermal field interactions.
Solution Approach 2:
The patent replaces the mechanical capacitor finger system with a thermal field-based system. Instead of measuring capacitance changes from mechanical displacement, the system measures temperature differences created by the movement of heated air mass. This substitution eliminates mechanical stiction by using thermal convection and diffusion processes.
2Reliability
If conventional heated air mass accelerometer design is used, then stiction is avoided, but manufacturing cost and complexity increase due to MEMS fabrication requirements
Solution Approach 1:
The patent makes the poly silicon element serve dual functions: as the common electrode for the differential thermocouple and as the heater element. This multi-functionality reduces the number of separate components and simplifies the fabrication process, eliminating the need for additional MEMS techniques while maintaining the heated air mass mechanism.
Solution Approach 2:
The patent merges the thermocouple electrode structure with the heater element into a single poly silicon component. The common poly silicon element acts as both the reference electrode for temperature measurement and the heating element, consolidating multiple functions into one structure and simplifying manufacturing.
3Measurement precision
If traditional inertial sensor design is used, then acceleration can be measured, but chip area and device footprint are large
Solution Approach 1:
The patent divides the sensor into a differential thermocouple configuration with two separate measurement channels (positive and negative thermocouple junctions). This segmentation allows for compact integration of the sensing elements and heater within a small chip area while maintaining measurement precision through differential temperature sensing.
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 provides a lower-cost, more accurate inertial sensor with improved signal-to-noise ratio and reduced chip area requirements, enabling broader applications such as in buildings and consumer devices without the need for expensive MEMS processing.
Implementation Method 1
A heater (8) is aligned with respect to the first (+) and second (−) thermopile junctions. A region (9,12) proximate to the heater (8) and the poly trace (6) encloses a mass of gas (10) that is heated by the heater (8)
Implementation Method 2
A differential thermocouple (13) includes first (4A) and second (4B) metal traces, a poly trace (6) with a first end connected to a first end of the first metal trace to form a first (−) thermocouple junction and a second end connected to a first end of the second metal trace to form a second (+) thermocouple junction
Data Source
AI summary
An inertial sensor (16) includes a differential thermocouple (13) including first (4A) and second (4B) metal traces, a poly trace (6) with a first end connected to a first end of the first metal trace to form a first (−) thermocouple junction and a second end connected to a first end of the second metal trace to form a second (+) thermocouple junction. A gas mass (10) located symmetrically with respect to the thermocouple junctions is heated by a heater (8). Acceleration or tilting of the sensor shifts the relative location of the gas mass relative to the thermocouple junctions, causing differential heating thereof and generation of a corresponding thermocouple output signal.