Thin Film Materials and Devices for Resistive Temperature Sensing Applications

Open Access
Author:
BASANTANI, Hitesh Arjun
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
August 29, 2014
Committee Members:
  • Mark William Horn, Dissertation Advisor
  • Thomas Nelson Jackson, Committee Member
  • Susan E Trolier Mckinstry, Committee Member
  • S Ashok, Committee Member
  • Michael T Lanagan, Committee Member
Keywords:
  • Microbolometer
  • High TCR
  • 1/f Noise
  • Infrared Imaging
  • Hydrogenated Germanium
Abstract:
Thin films of vanadium oxide (VOx) and hydrogenated amorphous silicon (a-Si:H) are the two dominant material systems used in resistive infrared radiation detectors (microbolometers) for sensing long wave infrared (LWIR) wavelengths in the 8-14 μm range. Typical thin films of VOx (x < 2) currently used in the bolometer industry have a magnitude of temperature coefficient of resistance (TCR) between 2%/K - 3%/K. In contrast, thin films of hydrogenated germanium (SiGe:H) have |TCR| between 3%/K to 4%/K. Devices made from either of these materials have resulted in similar device performance with NETD ≈ 25 mK. The performance of the microbolometers is limited by the electronic noise, especially 1/ƒ noise. Therefore, regardless of the choice of bolometer sensing material and read out circuitry, manufacturers are constantly striving to reduce 1/f noise while simultaneously increasing TCR to give better signal to noise ratios in their bolometers and ultimately, better image quality with more thermal information to the end user. In this work, thin films of VOx and hydrogenated germanium (Ge:H), having TCR values > 4 %/K are investigated as potential candidates for higher sensitivity next generation of microbolometers. Thin films of VOx were deposited by Biased Target Ion Beam Deposition (BTIBD) (~85 nm thick). Electrical characterization of lateral resistor structures showed resistivity ranging from 104 Ω-cm to 2.1 x 10^4 Ω^cm, TCR varying from -4%/K to -5%/K, normalized Hooge parameter (αH/n) of 5 x 10-21 to 5 x 10-18 cm^3. Thin films of Ge:H were deposited by plasma enhanced chemical vapor deposition (PECVD) by incorporating an increasing amount of crystal fraction in the growing thin films. Thin films of Ge:H having a mixed phase, amorphous + nanocrystalline, having a |TCR| > 6 %/K were deposited with resistivity < 2,300 Ω-cm and a normalized Hooge’s parameter ‘αH/n’ < 2 x 10-20 cm3. Higher TCR materials are desired, however, such materials have higher resistivity and therefore unacceptable large electrical resistance in a lateral resistor configuration. This work looks at an alternate bolometer device design which incorporates higher TCR materials in a vertically integrated configuration. Thin films of high TCR hydrogenated germanium (Ge:H, |TCR| > 6%/K) and vanadium oxide (VOx , TCR > 5%/K) were integrated in lateral and through film configuration. The electrical performance of the vertically integrated devices is compared with lateral resistance structures. It was confirmed experimentally that the device impedance was significantly lowered while maintaining the signal to noise ratio of the lateral resistor configuration. The vertically integrated devices allow higher device currents without any increase in self heating. These structures may help reduce integration time and may result in higher frame rate. Finally, one dimensional arrays were fabricated using both lateral and vertically integrated configurations and their performance was evaluated. It was found that the performance of the lateral devices was limited by noise floor of the measurement setup used. However, due to the lower impedance of the vertically integrated resistors, a higher signal and therefore higher signal to noise ratio could be obtained. These vertically integrated devices exhibited low RMS noise values of 12 mK.