Development of ultra-low-power consumption MOX sensors with ppb-level VOC detection capabilities for emerging applications

This paper deals with the development of state-of-the-art metal oxide semiconductor (MOX) gas sensors based on ultra-low-power (ULP) consumption micro-machined hotplates and targeted to VOC detection at ppb-level. A very simple, single metal, front-side silicon bulk micromachining fabrication technology was conceived and proposed. Several types of ULP devices, differing in shape and size, have been designed and fabricated to assess both the most efficient layout geometry and optimal fabrication process parameters. The ULP hotplates functional behavior was thoroughly investigated, and typical results on measurements of the hotplate temperature vs. applied power are reported. A very satisfactory value of 8.9 mW at 400 °C can be highlighted, for a device featuring an innovative self-insulated layout between heater and sensing layer. Transient temperature responses and evaluation of the hotplate thermal time constant were also carried out. On the center of the suspended hotplate structures a thin film of tin oxide has been deposited by means of modified rheotaxial growth and thermal oxidation (M-RGTO) process. A very controlled amount of gold nanoparticles is finally sputtered onto the sensing layer to enhance its response to VOC by catalytic effect. As expected and confirmed by morphological characterization the tin oxide film is structured in nanoclusters very uniform in size. Results of functional characterizations towards different gases under different working conditions are reported and prove the capability of detecting volatile organic compounds (VOC) down to few parts a per billion (ppb).