Technological processes for microheater and micro-hot-plate in the implementation of a MEM gas sensor

In this work a microsystem fabrication process for a microheater (MH) and a microhotplate (MHP), as well as their electrical behavior, are reported. This microstructure consists of a SiO₂ membrane supported by four cross shaped beams, and two layers of polysilicon (poly1 and poly2); poly1 forms the microheater and the poly2 layer works as a micro hot plate (MHP), whose function is to achieve a homogeneous temperature distribution all over the microsystem. Both poly1 and poly2 are separated by a SiO₂ layer, isolating them electrically, and were patterned by lift off process. The microstructure or membrane is located over a micropit (IMP) that was made by etching the Si substrate with KOH for seven hours; a Si₃N₄ sacrifice layer was used for this purpose. Electrical contact for the layers was made evaporating aluminum and patterned by lift off also. Electrical measurements were made by applying a ramp from -1.5 V through 1.5 V and measuring the current through poly1 and poly2, with temperature as a parameter, changing temperature with 25 °C steps, from room temperature up to 300°C, heating and cooling. The temperature coefficient of resistance (TCR) for poly1 and poly2 was calculated.