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.

Automated summary

This work presents investigations into porous silicon thermo-resistive type thermal sensors and their advantages in speed-sensitivity trade-off compared to other similar thermal materials. Porous silicon films were suspended above a silicon substrate and their thermal properties were analyzed using the 3w method. The measurements showed isotropic thermal conductivity and thermal diffusivity for different porosity levels of the films. Finite element modeling using these thermal properties suggested that a thermal time constant of 5 ms could be achieved with significant improvement in overall speed-sensitivity trade-off.