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High TemperaturSiC Micro Hotplate for Gas Sensors

 

Contact Person : Dr.-Ing. Ha-Duong Ngo

Introduction

Within the IESSICA (Industrial gas-sensor (expert) system (in traffic and environement) on silicon carbide basis) – project, being funded by the Federal Department for Education and Research (BMBF), the Microsensor & Actuator Technology group is developing mechanical and electrical stable high temperature silicone carbide based micro hotplates. These hotplates will be the basis for gas sensitive metal oxide layers (e.g. In2O3 and MoO3) which will also be optimized. The need for gas detection in harsh and high temperature surroundings calls for new technological and material approaches. Standard micro hotplates are based on membranes made of silicon nitride and oxide therefore, the operating temperature is limited to a maximum of about 550°C. The demand for high operating temperatures of up to 800°C restricts choice of materials that can be used as a membrane material or as a heater structure. Silicon carbide is the material of choice for its mechanical and thermal stability as well as its chemical resistance even at high temperature. As high temperature metallization platinum is the next on hand, but also combinations of titanium and tungsten are promising. For reasons of efficiency it was decided to use a single platinum metallization layer. Out of this layer the heater structure, a temperature sensor, and the sensing electrodes are structured in a single step. This approach limits the available sensitive area on the membrane and makes high demands on the design of the heater structure to achieve maximal temperature homogeneity over the sensitive area. To achieve optimal homogeneity extensive FEM – analyses have been conducted, covering electro-thermal as well as thermo-mechanical analyses.

 

Figure. 1: left: Electro-thermal FE-analysis of the membrane and Pt-heater. (Values are given in [K])
right: Thermo-mechanical FE-analysis.(Values are given in [µm])

 

The results of the analyses are very promising. The gas sensitive area covers 0.7*0.7 mm2 of the 1*1 mm2 membrane. The maximum temperature difference within the sensitive are is 22 K resulting in a temperature gradient of 0.029 K/µm. These results were used to calculate the power consumption to be around 300 mW at a membrane temperature of 1150 K.

Based on FE-analyses the fabrication process as well as the device design could be optimized and hotplates were successfully fabricated.

  

Figure 2: left: Microscope image of a hotplate based on 600 nmthick SiC membrane.
right: SEM- image of the membrane

 

Figure 3:left: Setup for Thermo graphic measurement (Two hotplates mounted)
right: Thermo graphic image of the hotplate