Engineering 1/f noise in porous silicon thin films for thermal sensing applications

Abstract

The sensitivity of present-day thermal sensors operating in the long-wave infrared region, a critical range for night vision and other reconnaissance applications, is limited by the 1/ f noise in the thermistor material, while their widespread applicability is restricted due to microfabrication incompatibilities with the standard processing for silicon-based microelectronics. Porous silicon films have a high temperature coefficient of resistance ( TCR ~2–10%K −1 ) and are compatible with microfabrication processes, making them suitable candidates for use in thermal sensors. In this work, the 1 /f noise in porous silicon films in conjunction with TCR , both key parameters for high sensitivity thermal sensors are characterized. The inherent 1 /f noise in these as-fabricated films is very high and increases significantly after exposure to atmosphere (oxidation). However, surface passivation at 600 °C, stabilises the film against atmospheric oxidation and reduces the 1 /f noise level by orders of magnitude. Further, decreasing the porosity of passivated films from 90 to 40% decreased the 1 /f noise by an order of magnitude. High TCR ~6–7%K −1 along with low 1/ f noise constant, K~ 10 - 12 are obtainable with an optimised porosity~60–75% for films surface passivated at 600 °C. These results confirm great promise of porous silicon for low manufacturing cost, and higher sensitivity of the thermal sensors. • As-fabricated porous silicon films have very high 1/ f noise that further increases after exposure to atmosphere due to oxidation. • Surface passivation of porous silicon films reduces 1/ f noise by three orders of magnitude. • 1/ f noise can be further tuned by controlling the porosity: decreasing the porosity from 90 to 40% reduced the 1/ f noise constant by an order. • Higher temperature coefficient of resistance ( TCR ) and lower 1/ f noise constant ( K) important to realize high sensitivity based thermal sensors , can be simultaneously obtained by controlling the fabrication parameters and surface passivating the porous silicon films.