Results of four interrelated studies leading to surface micromachining and integrated sensor applications of porous silicon are reported.
1. Aspects of the structural, chemical, and electrical properties of porous silicon are investigated and characterized. Etching, silver decoration, SEM, and TEM are used to examine pore structure. Transmission FTIR, Auger spectroscopy, and thermal desorption are employed to investigate the chemistry of the porous-silicon surface. Results of capacitance and current voltage measurements are reported.
2. Resistance to room-temperature oxidation and control over wetting properties are achieved through chemical modification of the porous-silicon surface. Surface-modification methods investigated include: a.) vapor-phase silation using either hexamethyldisilazane (HMDS) or trimethylchlorosilane (TMCS), and b.) rapid thermal annealing (RTA) in nitrogen, ammonia, or argon ambients. The surface chemistry is monitored using transmission FTIR. Development of these surface-modification techniques shows promise of enabling applications of porous silicon as a high surface-area material for integrated sensors.
3. A new surface-micromachining technique is explored in which porous silicon is grown laterally in a thin film of polycrystalline silicon constrained between two insulating layers of silicon nitride. Voltage and electrolyte composition are used to control the etch rate and regime: either porous or uniform attack. Direct observation of the etch front and pore structure are demonstrated. Uniform attack is confirmed using profilometry and SEM. Special structures (chambers surrounded by porous plugs) are formed by alternating between pore formation and uniform attack; methods for sealing these chambers by closing the porous plugs are proposed and evaluated. Finally, the feasibility of forming a porous layer over a suspended silicon nitride membrane is demonstrated.
4. A 440 percent increase in capacitance in response to a humidity change from 0 to 100% has been measured in a Schottky-bamer RH sensor made using an aluminum contact to porous silicon. This high sensitivity can be accounted for by an increased permittivity in the space-charge region owing to condensation within the pores. Transient behavior is characterized in detail. Surface-modification techniques are employed to improve the stability of the RH sensor.
April 30, 1990
Anderson, R. C. (1991). Formation, Properties, and Applications of Porous Silicon. United States: University of California, Berkeley.