Highly insulating, fully porous silicon substrates for high temperature micro-hotplates

As alternative to established thermal substrates and thin membranes, we have investigated fully porous silicon substrates as highly insulating material for thermal devices. Exhibiting a thermal conductivity similar to silica glass and considerably lower than silicon nitride due to increased phonon scattering, thick mesoporous silicon also offers improved thermal and mechanical stability. Our work has focused on full wafer thickness porosification as a not extensively documented use of porous silicon and its application to thermal devices. Here we present measurement and finite element simulation results for our latest generation thin film microheaters on fully porous silicon substrates as proof of concept devices. Porosity, mass density, and specific heat capacity of porous silicon are deduced from fabrication parameters, thermal conductivity is determined by the so-called 3ω-measurement method, and all material properties are validated by fitting measurement data to our finite element models. For thick fully porous domains we estimated a thermal conductivity of ≈0.9 W/m/K, as well as a density of ≈1200 kg/m3, a specific heat capacity of ≈780 J/kg/K and a corresponding volumetric porosity of ≈50%. Thin film fabrication of nitride passivation and molybdenum meander microheaters on fully porous domains allowed characterization of thermal performance and insulation. For 10 mm2 microheaters we measured a power efficiency of 0.40 K/mW stable up to a maximum temperature of 475 °C, compared to 0.37 K/mW stable up to 440 °C on silica glass. Both static and dynamic heater measurements show superior performance of fully porous silicon substrates compared to reference samples on thin silica glass substrates.