Compact Encapsulated MEMS Beam Resonator for High-TCF Temperature Sensing
Solution Overview
Problem
Conventional MEMS-based temperature sensors suffer from low temperature coefficient of frequency (TCF) values, large footprint, and require optical readout equipment, limiting their practical application in sensitive temperature sensing.
Innovation Solution
A MEMS temperature sensor with a clamped-clamped microbeam configuration, featuring a drive and sense electrode, and a method of fabrication using XMB10 surface-micromachined capacitive 3D MEMS technology, which includes a cavity wafer, membrane wafer, and capping wafer, encapsulated with aluminum, allowing for capacitive sensing of resonant frequency shifts due to temperature changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional MEMS-based temperature sensors are used, then the device is compatible with CMOS technology and has low cost, but the temperature coefficient of frequency (TCF) value is low (less than 35 ppm/°C)
Solution Approach 1:
The patent changes the geometric parameters of the MEMS resonator, specifically using a clamped-clamped beam configuration with optimized length, width, and thickness dimensions. This geometric parameter optimization increases the temperature coefficient of frequency from conventional values of less than 35 ppm/°C to greater than 140 ppm/°C, thereby improving measurement precision without requiring fundamentally new device structures
Solution Approach 2:
The patent utilizes the dynamic mechanical response of the clamped-clamped beam resonator, where the resonant frequency shifts in response to temperature-induced stress changes. By monitoring these dynamic frequency variations, the sensor achieves enhanced temperature sensitivity while maintaining compatibility with standard MEMS fabrication processes
2Measurement precision
If material composition technique is used to increase TCF, then different materials with different coefficients of thermal expansion are employed, but the TCF value is still limited (less than 140 ppm/°C)
Solution Approach 1:
The patent applies local quality by introducing stress concentration regions at specific locations along the clamped-clamped beam, particularly near the anchors. This localized stress engineering approach, achieved through geometric modifications rather than material composition changes, enhances the temperature coefficient of frequency to greater than 140 ppm/°C while maintaining ease of manufacture using standard single-material MEMS processes
3Measurement precision
If stress controlling technique is used to increase TCF, then the TCF value increases (400-600 ppm/°C), but the device footprint becomes large and optical readout equipment is required
Solution Approach 1:
The patent employs mechanical vibration by operating the clamped-clamped beam as a resonator that vibrates at a characteristic resonant frequency. Temperature changes induce stress variations that shift this resonant frequency, enabling contactless electrical readout of temperature. This approach achieves high TCF values greater than 140 ppm/°C while maintaining a compact footprint and eliminating the need for optical readout equipment
Solution Approach 2:
The patent substitutes mechanical/optical measurement systems with an electrical readout mechanism. By using capacitive or piezoresistive sensors to detect resonant frequency shifts electrically, the system replaces complex optical readout equipment with simple electrical circuits, thereby reducing device footprint while maintaining high temperature coefficient of frequency values
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 sensor achieves a high TCF of 64.7 Hz/°C over a 20-60°C temperature range, with a small footprint and no need for optical readout, enhancing sensitivity and practical applicability.
Implementation Method 1
the microbeam is configured to vibrate at a resonant frequency related to a temperature surrounding the MEMS temperature sensor when an AC current is applied to the drive electrode and a DC bias voltage is applied to the first anchor
Implementation Method 2
When the temperature varies, the resonance frequency shifts due to thermal-induced deformation, elasticity change and stress
Implementation Method 3
the sense electrode is configured to capacitively sense the vibration and generate an induced current based on the resonant frequency
Implementation Method 4
an aluminum layer configured to cover periphery of the MEMS temperature sensor, while leaving the side electrode pads exposed
Data Source
AI summary
A MEMS temperature sensor including a clamped-clamped microbeam having a drive electrode on one side configured for applying an AC current, and a sense electrode diagonally situated on the other side, a first anchor at one end and a second anchor at the other end of the microbeam. The first anchor receive a DC bias currents, which heats the microbeam to an operating temperature. The sense electrode is configured to capacitively sense oscillations in the microbeam due to an applied AC current. The MEMS temperature sensor has a three wafer construction in which the components are formed. The device is encapsulated by aluminum, and metal wires connect the first and second anchor, the drive electrode and the sense electrode to side electrode pads outside of the encapsulation. The MEMS temperature sensor has a linear operating region of 30-60 degrees Celsius.