Design considerations for monolithic integration of a micro hotplate temperature controller in a MEMS gas sensor

A control system to regulate the temperature of the micro hotplate in a MEMS gas sensor is presented. The controlelement, called micro hotplate, is comprised of a micro heater and a temperature sensor, both made with polysilicon, located near each other. This material has a Temperature Coefficient of Resistance (TCR) that is the basis for the design of the temperature controller of the gas sensor system. A high temperature between 250 and 400°C is needed to produce a chemical reaction between the gas and the sensing film, hence a reliable temperature control for the micro hotplate is desired. Thermal insulation of the circuitry from the heating element, having a monolithic sensor system, and low power consumption, are the main specifications for the system. This is obtained by means of a micro pit realized with MEMS micromachining processes. The analysis of the circuit proposed to fulfill these characteristics is presented, for its future integration with a standard CMOS technology. A trade off is established between the sensor structure parameters and the circuit design.