Porous Silicon as an Effective Material for Thermal Isolation on Bulk Crystalline Silicon

Abstract

Highly porous silicon has been successfully used to create thermally isolated local areas on bulk crystalline silicon, for the fabrication of silicon integrated sensors. Heat distribution and heat transfer have been simulated by using finite element analysis with the ANSYS software. Three cases were compared: the case of bulk crystalline silicon, the case of porous silicon and that of free standing polycrystalline silicon membranes. A flow sensor of the thermal type using porous silicon isolation has been designed and fabricated, showing high sensitivity and rapid response. Its operation principle and main characteristics will be presented.Introduction Highly porous silicon shows a thermal conductivity of the order of 1.2 W/mK [1], which is 120 times lower than that of bulk crystalline silicon. This gives wide possibilities for applications in silicon integrated sensors and microsystems. Thermally isolated areas may be locally created on a silicon wafer by selective electrochemical dissolution of bulk silicon through a patterned masking layer [2,3]. The most effective masking material in this respect is undoped polycrystalline silicon, which shows very high etching resistance to the chemical solution used for porous silicon (PS) formation. It can be used to grow thick porous silicon layers, providing good thermal isolation compared to bulk crystalline silicon.In this paper, the thermal isolation approach mentioned above is used to design and fabricate a miniaturized silicon flow sensor of the thermal type.

Silicon Porosification: State of the Art

“…Passive photonic devices designed in silicon such as waveguides, couplers and splitters have been reported. 185 It has been shown that micro-and meso-PSi layers are excellent materials for local thermal isolation on bulk silicon 106,186 and for fabrication of micro-hotplates for low-power sensors. [187][188][189] Micro-and meso-PSi could also be used as isolation material for radio-frequency (rf) applications because of its high resistivity and compatibility with modern very-largescale-integration IC technology.…”

Characteristic length of phonon transport within periodic nanoporous thin films and two-dimensional materials

“…[19][20][21] For thermoelectrics, the effectiveness of a material is evaluated by its thermoelectric figure of merit (ZT), defined as ZT ¼ S 2 rT/k, where S, r, k, and T represent Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. 22 In semiconducting thermoelectric materials, k can be further split into the lattice contribution k L and electronic contribution k E .…”

Porous Silicon Characterization and Application: General View

“…It has been shown that micro-and meso-PSi layers are excellent materials for local thermal isolation on bulk silicon (Nassiopoulou and Kaltsas 2000;Lysenko et al 2002) and for fabrication of micro-hotplates for low-power sensors (Maccagnani et al 1998(Maccagnani et al , 1999Tsamis et al 2003). Experiment has shown that the technology of silicon porosification is alternative for silicon on insulator (SOI) and silicon on sapphire (SOS) technologies.…”