Temperature Distribution And Stress Adjustable Microhotplate Gas Sensors

Microhotplate gas sensor has gained widely study for its low power consumption, small volume, easy integration etc. Temperature has a great influence on the sensitivity of micro gas sensor; residual stress and thermal stress determine the reliability of the microhotplate gas sensor. So a microhotplate has three thermal resistors was designed and fabricated to gain a uniform temperature distribution, and also the temperature gradient can be adjusted on the microhotplate. Then a stress modulation system for the microhotplate gas sensor was designed, the device was based on comb electrostatic actuator, and it can be used to adjust the stress distribution in the sensing film without change the temperature distribution, then to study how the stress influence the properties of the microhotplate gas sensor.First, in order to study how the temperature gradient influences the MHP gas sensor, a microhotplate with three thermal resistors instead of just one resistor is designed. The temperature distribution on the microhotplate is simulated by the FEM software CMOSOL Multiphysics when it heated by different thermal resistors. Simulation results illustrate that the both vertical and transverse temperature gradient can be significant adjusted. And the microhotplate gas sensor is fabricated by standard CMOS processes and post-CMOS processes.Secondly, the properties of the microhotplate gas sensor are measured. The results show that as the highest temperature on the MHP is 300℃, when it heated by double side resistors, a uniform temperature distribution can be attained, and the measured temperature gradient is 0.18℃/μm, the power consumption is 19mW. When it heated by middle resistor, the temperature gradient is 0.57℃/μm, the power consumption will be lower to 15.5mW. When it heated by one side resistor, the temperature gradient will high to 1.57℃/μm, and the power consumption is 14.7mW. At the uniform temperature, the deformation of the MHP is 1.5μm when it heated up to 500℃. And sensing response test results show that the sensing response to ethanol of the MHP sensor, mainly influenced by the temperature on the active region, the temperature gradient changes in a small range on that region, the sensing response change is little. The resistance drift of the resistors at ultra-high temperature is also tested, when the operating temperature high than 700℃, the resistance drift will occur, and high than 900℃, the MHP will be burned down.Then, we designed a stress modulation system based on electrostatic actuator for the microhotplate gas sensor. Thermoelectric mechanical properties of the device were simulated by FEM. When the MHP operating at 200～50O℃, and the actuate voltage of the electrostatic actuator between 0-250 V, as the voltage increasing, the bending degree of the MHP increase and the stress in the sensing film decrease. The reduction of the stress among 5%-10%and related to the operating temperature, the device can be used to adjust the stress in sensing film.Finally, the fabrication processes and results of the device were provided, and the power consumption, temperature distribution, and response to ethanol used as gas sensor were measured. The experiment result shows, the MHP and the electrostatic actuator can be fabricated simultaneously by CMOS and post-CMOS processes. And the power consumption is 21mW when the device is heated up to 300℃, and it shows a good sensitivity to ethanol when the operating temperature between 300～400℃. And the break down voltage of the comb actuator is about 270V, it can work at 0-250V voltage range.