Influence of oxide thickness on an integrated electrothermal filter

An electrothermal filter (ETF) is an AC driven device consisting of a heater and thermopile implemented in the same substrate. A crucial characteristic is the frequency f_90 where the phase shift between the heater drive signal and the thermopile`s response is 90°.Theoretically, for a given layout, f_90 solely depends on the thermal parameters of the substrate. These are essentially process independent and have a very well defined temperature dependence (especially for silicon). By consequence the ETF is very suitable for use in a temperature to frequency convertor. This provides an attractive way to build a highly reproducible integrated temperature sensor, whose accuracy will only be limited by lithography.Earlier, an integrated ETF was modelled, fabricated and characterised. We noticed the predicted (130kHz) and measured (100kHz) values for f_90 were quite different. In this paper we will demonstrate the deviation can be mainly attributed to the presence of a thin (typically 3-4 microns in the process used) layer of silicon oxide on the substrate. The analysis is carried out by means of numerical 3D- and 2D-simulations. A clear influence of the oxide thickness on the phase shift is observed, which lowers f_90 to 112kHz. The remaining difference between this result and the experimental value can be explained by the non-uniform doping of the substrate. Counter-intuitive effects are noticed, such as smaller phase shifts for thicker oxides, and oscillations in the phase shift versus oxide thickness plot. These phenomena are confirmed by a simple analytical 1D-model. This finally leads to the formulation of some design guidelines, describing the minimal drive frequency or oxide thickness required for reproducible operation of an integrated ETF