EFFECT OF RF SUBSTRATE BIAS ON VANADIUM OXIDE THIN FILMS DURING REACTIVE PULSED DC MAGNETRON SPUTTER DEPOSITION

Open Access
Author:
BASANTANI, Hitesh Arjun
Graduate Program:
Engineering Science
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
June 27, 2011
Committee Members:
  • Mark William Horn, Thesis Advisor
Keywords:
  • Low Noise
  • Sputtering
  • Substrate Bias
  • Microtwins
  • Infrared Imaging
  • High TCR
  • Bolometer
  • Vanadium Oxide
Abstract:
High resolution, portable night vision cameras rely on uncooled infrared focal plane arrays to detect wavelengths in the 8-14 μm region. These arrays consist of MEMS microbolometer pixels as small as 17 μm × 17 μm that are fabricated on CMOS read-out circuitry (ROIC). Vanadium oxide thin films are often used as the imaging layer in the microbolometers because they have a high temperature coefficient of resistance (TCR), low noise characteristics and low resistivity. These films have been commercially made for over two decades using ion beam sputtering. This work focuses on fabrication of VOx layers via pulsed dc reactive magnetron sputtering of a vanadium metal target. In particular, this work discusses the effect of rf substrate bias on the structure and electrical properties of vanadium oxide (VOx) thin films processed via pulsed dc sputter deposition. The VOx films were deposited using pulsed DC (20 kHz, 90% duty cycle) magnetron reactive sputtering with various Ar:O ratios at pressures between 1 and 5 mTorr. The substrate rf bias was varied between 0-40 Watts (corresponding to dc self-bias of -50 volts to -340 volts) during deposition of the VOx thin films. Earlier work on pulse dc magnetron sputtering indicated that VOx films processed without substrate bias have resistivity 1 Ω.cm and consist of fcc rocksalt type VOx nanocrystallites in an amorphous matrix. On the other hand, cross-section transmission electron microscopic studies of films fabricated with an rf substrate bias indicated formation of a nanocrystalline columnar microstructure with micro-twins embedded in them. Electrical properties such as resistivity, temperature coefficient of resistivity, and 1/f noise of VOx films are compared as functions of substrate bias conditions. Films processed with rf substrate bias exhibit lower resistivity and higher TCR values than unbiased samples deposited under identical conditions (Ex: 0.05 Ω.cm resistivity sample exhibited TCR values ≈ -2.2%/◦K with rf substrate bias as opposed to 0.1 Ω.cm with TCR of ≈ -1.7%/◦K). The rf bias during the film growth dramatically changes the structure and thereby changes the electrical properties. In conclusion it is demonstrated that it is possible to deposit VOx thin films for μ-bolometer applications using pulsed dc reactive sputtering with rf substrate bias that have electrical prop-erties that are equal to or surpass films currently fabricated via ion beam sputter deposition.