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

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.