Comparison of ion beam and magnetron sputtered vanadium oxide thin films for uncooled IR imaging

Uncooled Infrared (IR) focal plane arrays are an enabling technology for both military and commercial high sensitivity night vision cameras. IR imaging is accomplished using MEMS microbolometers fabricated on read-out integrated circuits and depends critically on the material used to absorb the incoming IR radiation. Suitable detector materials must exhibit a large temperature coefficient of resistance (TCR) and low noise characteristics to efficiently detect IR photons while also maintaining compatibility with standard integrated circuit (IC) processing. The most commonly used material in uncooled infrared imaging detectors is vanadium oxide deposited by reactive ion beam sputtering. Here we present a comparison of vanadium oxide thin films grown via commercial reactive ion beam sputtering to films grown using reactive pulsed DC magnetron sputtering. Films deposited using both methods were optically and structurally characterized using Raman spectroscopy, transmission electron microscopy, atomic force microscopy and grazing incidence X-ray diffraction. The measured electrical properties of the films were found to be very sensitive to the deposition conditions used. The ion beam sputtered films contained twinned FCC VOx nanocrystals with sub-nanometer twin spacing, in the form of large 10-20 nm wide columnar/conical grains. In contrast, the un-biased magnetron sputtered films consisted of equiax grains of FCC VOx (5-10 nm) encapsulated in an amorphous matrix. However, applying an RF bias to the sample substrate during the magnetron sputtering process, resulted in films that are similar in structure to ion beam deposited VOx. These differences in microstructure and composition were then correlated to the measured resistivities and TCRs of the films.