REACTIVE SPUTTER DEPOSITION OF VANADIUM, NICKEL, AND MOLYBDENUM OXIDE THIN FILMS FOR USE IN UNCOOLED INFRARED IMAGING

Open Access
Author:
Jin, Yao
Graduate Program:
Engineering Science
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 21, 2014
Committee Members:
  • Mark William Horn, Dissertation Advisor
  • Thomas Nelson Jackson, Committee Member
  • S Ashok, Committee Member
  • Michael T Lanagan, Committee Member
  • Mark William Horn, Committee Chair
Keywords:
  • uncooled infrared imaging
  • microbolometer
  • vanadium oxide
  • nickel oxide
  • molybdenum oxide
  • reactive sputtering deposition
Abstract:
A vanadium oxide (VO x) thin film is the most common imaging layer used in commercial uncooled focal plane arrays for infrared cameras. These VOx thin films have an x value ranging from 1.3 to 2 and have low resistivity (0.1 to 10 Ω•cm), high temperature coefficient of resistance (TCR) (−2 to −3 %/K), and low 1/f noise. Reactive ion beam sputtering is typically used to deposit these VOx thin films for commercial thermal imaging cameras. However, the reactive ion beam deposition system for the VOx is reported to have less than desirable throughput and a narrow process window. In this work, the potential for reactive pulsed-dc magnetron sputtering of nanocomposite VOx thin films for microbolometer applications was investigated. VOx thin films with resistivity from 10-4 to 105 Ω•cm with a TCR from 0 to -4.3 %/K were deposited by reactive sputtering from a metallic vanadium target in argon/oxygen mixtures with substrate bias. Magnetron sputtered VOx shows bolometric properties comparable to those of commercial-grade IBD prepared VOx. Important limitations for manufacturing implementation of reactive magnetron sputtering such as hysteresis oxidation and non-uniform oxidation of the vanadium target surface were evaluated. The VOx film deposition rate, resistivity, and temperature coefficient of resistance were correlated to oxygen to argon ratio, processing pressure, target-to-substrate distance, and oxygen inlet positions. To deposit VOx in the resistivity range of 0.1–10 Ω•cm with good uniformity and process control, it was found that a lower processing pressure, larger target-to-substrate distance, and an oxygen inlet near the substrate are useful. Other processing methods employing magnetron sputtering were investigated such as co-sputtering of V and V2O5 target, sputtering from a VC target, a V2O5 target, and a V2Ox target but initial investigation of these methods did not yield a superior process to the simple sputtering of a pure metallic vanadium target. Another technique, biased target ion beam deposition (BTIBD), was investigated for deposition VOx thin films with potential alloy additions. In this BTIBD system, ions with energy lower than 25 eV were 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 Ω•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 architecture could improve the sensitivity through the higher TCR without increasing noise normally accompanied by higher resistance. Processing parameters necessary to produce high TCR VOx films and details on how this novel deposition tool operates are discussed. Addition of molybdenum and its effects on the VOx thin films’ electrical properties were also studied. Using the BTIBD system, VOx films in the resistivity range of 0.1-10 Ω•cm desired for current microbolometer application were difficult to produce. Pure molybdenum oxide (MoOx) and nickel oxide (NiOx) thin films were deposited by reactive biased target ion beam deposition and evaluated in a search for materials with a larger process latitude. MoOx thin films were deposited with resistivity from 3 to 2000 Ω•cm and with TCR from -1.7 to -3.2 %/K. NiOx thin film were deposited with resistivity from 1 to 300 Ω•cm and with TCR from -2.2 to -3.3 %/K. The thermal stability of these films was also investigated. It was found that biased target ion beam deposited high TCR MoOx and NiOx thin films are polycrystalline semiconductors and have good stability in air. Compared to commonly used VOx thin films, MoOx or NiOx thin films may offer improved process control for resistive temperature sensors and a superior deposition rate. However, preliminary experiments indicate that these films might have relatively higher 1/f noise.