Thermal analysis of surface micromachined porous silicon membranes using the 3? method: Implications for thermal sensing

Investigations into porous silicon thermo-resistive type thermal sensors and their advantages in speed-sensitivity trade-off relative to other comparable thermal materials are presented in this work. Porous silicon films were suspended above a silicon substrate using successive patterning and micromachining steps. The 3w method was used in both supported and suspended configurations, allowing the analysis of both cross-plane and in-plane thermal properties of the micromachined films. By utilising a low noise, broad-frequency measurement of the in-phase and out-of-phase temperature components, thermal conductivity and thermal diffusivity were simul-taneously determined. The measurements showed isotropic thermal conductivity of 0.38 +/- 0.02 W/m K and 0.12 +/- 0.01 W/m K, and thermal diffusivity of 0.39 +/- 0.04 mm2/s and 0.23 +/- 0.02 mm2/s, for the surface micro -machined released porous silicon films at 50 % and 77 % porosity, respectively. The implemented suspended 3w method leveraged a thermal model that accounted for the finite length of the heater (180 mu m) and membranes (400 mu m x 400 mu m), eliminating the millimetre scale dimensional constraints in previous works. Use of a thermal passivation technique rendered the thermal properties of the films robust against successive photolithography and micromachining. The obtained thermal properties were utilised in finite element modelling of a thermo-resistive thermal sensor with 50 mu m x 50 mu m x 100 nm dimensions. The modelling results suggest that a thermal time constant of 5 ms could be achieved, comparable to that of amorphous silicon based thermal sensors but with 2-4 times larger temperature sensitivity due to smaller intrinsic thermal conductivity and heat capacity in the porous silicon films, thus achieving a significant improvement in overall speed-sensitivity trade-off.