STABILIZATION OF POWER CONSUMPTION OF THE HEATER OF A MICROMACHINED SILICON GAS FLOW SENSOR A. A. Nassiopoulos 1 , G. Kaltsas 2 and A. G. Nassiopoulou 2 1 Technological Educational Institution of Athens,Department of Electronics, 12210 Egaleo, Athens, Greece. 2 IMEL/NCSR Demokritos, P.O. box 60228, 15310 Aghia Paraskevi, Athens, Greece. ABSTRACT In this paper, an electronic circuit designed to compensate changes in power consumption of the heater of a micromachined silicon gas flow sensor is presented. The sensor is of the thermal type and it uses a polycrystalline silicon resistor on top of a porous silicon area locally formed on the silicon substrate and two thermopiles integrated on both sides of the resistor. Under gas flow the resistance of the heater undergoes small changes, which have to be compensated by a change in the current in order to keep the power constant. 1. INTRODUCTION Micromachined silicon thermal sensors are successfully used in gas flow sensing [1-3]. Their main advantages are their small size, high sensitivity and rapid response. In this work we refer to a silicon thermal sensor based on a heater and integrated thermopiles on both sides of this heater, which has been described in detail elsewhere [4,5]. The heater and the hot contacts of the thermopiles lie on a thick porous silicon layer, which serves for thermal isolation [6], while their cold contacts lie on bulk crystalline silicon. The heater is kept at constant temperature, so as the gas flow on top of it induces temperature changes, sensed by the thermocouples. Since the temperature of the heater is slightly influenced by the gas flow, thus causing a change in its resistivity, an electronic feedback is needed in order to stabilize power consumption and to keep this temperature stable. The proposed electronic circuit in this work assures this function. Other solutions proposed in the literature stabilize the applied current or voltage [7,8], but we think that the stabilization of the power is more appropriate for the sensor applications. 2. DESIGN, REALIZATION AND TESTING OF THE ELECTRONIC CIRCUIT Figure 1 : Top view of the flow sensor The flow sensor has been described in detail elsewhere [4]. The porous silicon film is formed locally on the silicon wafer by electrochemical dissolution of bulk crystalline silicon. A thin film of silicon oxide or silicon nitride is then deposited and on top of it the heater and thermocouples are deposited and patterned. Two thermopiles, one on each side of the heater are formed, as in fig. 1. Laminar gas flow is in a direction parallel to sensor surface and perpendicular to the resistor. A special package is used, which was designed to assure laminar flow. The heater is a polycrystalline silicon resistor with a typical total resistance of the order of 2.7 K. By heating this resistor at constant temperature, gas flow causes lowering of the temperature Direction of gas flow Cold thermopile contacts Hot thermopile contacts Heating resistor Al Pads Bulk Silicon Porous silicon area 121 0-7803-7057-0/01/$10.00 ©2001 IEEE.