Analytical Research of Temperature Distribution and Thermal Stresses within Circular Micro-hotplates

Micro-hotplate （MHP） technology is one key part in the manufacturing of gas sensors. The pursuit of analytical solutions for the temperature distribution and also thermal stresses within the MHP is of intrinsic scientific interest. In this study, analytical solutions for the temperature field, and both radial and tangential stresses and van Mises stress for circular MHP were obtained. Two geometries were considered： one had a circular heater at the center and the other had a circular heater at the center and an annular heater within the membrane part. Internal heat generation was incorporated in the energy equation for the MHP and different values of convection heat transfer coefficient were used at the upper and lower surfaces of the MHP. It has been shown that the MHP with two heaters can provide more uniform temperature field compared with the MHP with one heater. The main objective of this work is to provide an exact analytical solution for thermal stresses within the circular micro-hcater with a simple geometry as a benchmark, from mathematical point of view, against which the accuracy of new numerical schemes can be checked. To make sure that the analytical procedure is correct, the analytical results are checked against numerical solutions derived from finite element simulation. Since the analytical models for the temperature field and especially for the thermal stresses of MHP ace seldom investigated in the literature, the obtained results are believed to facilitate the design and performance evaluation of MHPs as well.

A NOVEL MICRO-PELLISTOR BASED ON NANOPOROUS ALUMINA BEAM SUPPORT

The traditional design of the catalytic combustion gas sensor in Micro ElectroMechanical Systems (MEMS) micro-hotplate employs a Pt resistive track as the micro-heater. The realized structure and fabrication are the key elements of the micro-hotplate. Directly fabrication of micro-pellsitor catalyst is very difficult because of the small dimensions of the active area. In this paper, a novel micro-pellistor was designed by combining micro fabrication technique and nano technology. The supporting beams and micro-hotplate of the micro-pellistor were made of nonoporous alumina film. The active area of the designed and fabricated micro-pellistors ranges from 200×200 μm2 to 450×300 μm2. The micro- pellistor was heated by platinum thin film heater and the Pd catalyst was deposited by dipping the PdCl2 solution on the detecting element. The lowest power consumption is 50 mW at 500 °C and the maximum temperature can reach 900 °C before rupture. The response of the devices to methane is also tested. The new design provides a new way to fabricate micro-pellistor.

System on chip thermal vacuum sensor based on standard CMOS process

An on-chip microelectromechanical system was fabricated in a 0.5μm standard CMOS process for gas pressure detection. The sensor was based on a micro-hotplate （MHP） and had been integrated with a rail to rail operational amplifier and an 8-bit successive approximation register （SAR） A/D converter. A tungsten resistor was manufactured on the MHP as the sensing element, and the sacrificial layer of the sensor was made from polysilicon and etched by surface-micromachining technology. The operational amplifier was configured to make the sensor operate in constant current mode. A digital bit stream was provided as the system output. The measurement results demonstrate that the gas pressure sensitive range of the vacuum sensor extends from 1 to 105 Pa. In the gas pressure range from 1 to 100 Pa, the sensitivity of the sensor is 0.23 mV/Pa, the linearity is 4.95%, and the hysteresis is 8.69%. The operational amplifier can drive 200 Ω resistors distortionlessly, and the SAR A/D converter achieves a resolution of 7.4 bit with 100 kHz sample rate. The performance of the operational amplifier and the SAR A/D converter meets the requirements of the sensor system.