Vanadium oxide (VOx) and hydrogenated silicon germanium (SixGe1-x) are the two predominant thin film material systems used as the active layer in resistive infrared imaging. Thin films of VOx used in microbolometers have a resistivity typically between 0.1 and 1 Ω-cm with a temperature coefficient of resistance, |TCR| between 1.4%/K to 2.4%/K, while SixGe1-x:H thin films have a resistivity between 200-4,000 Ω-cm with a |TCR| between 2.9%/K to 3.9%/K. Future devices may require higher TCR materials, however, higher TCR is loosely associated with higher resistivity and therefore also with high noise. This work compares 1/f noise of high resistivity VOx and Ge:H thin films having |TCR| < 3.6%/K. The high TCR thin films of VOx were found to be amorphous while, depending on the deposition conditions, the Ge:H thin films were either amorphous or mixed phase of amorphous + nanocrystalline. Evaluation of these VOx and Ge:H thin films indicates a prospects for a superior process-property relation of 1/f noise in Ge:H thin films in comparison with thin films of VOx.
Vanadium oxide (VOx) thin films have been intensively studied as an imaging material for uncooled microbolometers due to their low resistivity, high temperature coefficient of resistivity (TCR), and low 1/f noise. Our group has studied pulsed DC reactive sputtered VOx thin films while reactive ion beam sputtering has been exclusively used to fabricate the VOx thin films for commercial thermal imaging cameras. The typical resistivity of imaging-grade VOx thin films is in the range of 0.1 to 10 ohm-cm with a TCR from -2%/K to -3%/K. In this work, we report for the first time the use of a new biased target ion beam deposition tool to prepare vanadium oxide thin films. In this BTIBD system, ions with energy lower than 25ev are generated remotely and vanadium targets are negatively biased independently for sputtering. High TCR (<-4.5%/K) VOx thin films have been reproducibly prepared in the resistivity range of 103-104 ohm-cm by controlling the oxygen partial pressure using real-time control with a residual gas analyzer. These high resistivity films may be useful in next generation uncooled focal plane arrays for through film rather than lateral thermal resistors. This will improve the sensitivity through the higher TCR without increasing noise accompanied by higher resistance. We report on the processing parameters necessary to produce these films as well as details on how this novel deposition tool operates. We also report on controlled addition of alloy materials and their effects on VOx thin films’ electrical properties.
Reactive pulsed DC sputtering was used to grow a systematic series of films with resistivity ranging from 1 × 10-3 to 6.8
× 104 Ohm cm and TCR varying from 0 to -4% K-1. Throughout the parameter space studied a transition from
amorphous to nano-crystalline growth was observed. Films in the resistivity range of interest for microbolometers
contained a FCC VOx (0.8 < x < 1.3) phase. Altering the sputtering energetics via substrate biasing resulted in highlycolumnar,
nano-twinned grains of FCC VOx, providing a microstructure reminiscent of ion beam sputtered bolometer
material. Electron diffraction in the TEM confirmed the presence of a secondary, oxygen-rich amorphous phase. Micro-
Raman spectroscopy, which was also found to be sensitive to the secondary amorphous phase, was used to probe the
chemical composition and morphology of VOx thin films. Raman spectra from high resistivity amorphous films show a
broad feature around ~890 cm-1, while spectra from lower resistivity nano-crystalline films exhibit this same amorphous
feature and a second broad feature at ~320 cm-1. The resulting microstructure can be described as a nano-composite
material composed of a low-resistivity crystalline phase embedded in a high-resistivity amorphous matrix. Our results
suggest that both phases are required to achieve a high TCR, low resistivity material.
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.