Nano- and microsized metal oxide thin film gas sensors

Functional micro- and nanosized metal oxide thin film structures are very promising candidate for future gas-sensors. Their reduced size offers an increased surface to volume ratio thus improving sensitivity and sensor performance. Whilst most experimental nanostructures are produced using a bottom-up approach, a top-down sputtering technique for structuring nano-sized gas sensitive metal oxide areas is presented in this letter. Oxidised silicon wafers were used as substrates. The silicon dioxide film of 1 μm thickness was prepared by thermal oxidation in order to insulate the gas sensing elements from the substrate. The sensor chips had an overall size of (1.5 × 1.5) mm2 onto which a Ta/Pt film (20/200 nm thickness) was deposited and patterned to act as electrodes, heater and temperature sensor. In a second step micro-scaled tin dioxide layers (60 nm thick, 5 μm width) were deposited by sputtering techniques and photolithographical patterning between the platinum micro-electrodes (4 μm gap). Finally, the width of the stripes was reduced using focused ion beam technology to obtain the desired size and structure. This enables the control of the dimensions of the structures down to the resolution limit of the FIB-system which is about 10 nm. The structural and electrical characterisation of the sensors and their responses during exposure to several test gases including O2, CO, NO2 and H2O are presented as well.