Regular Article

Reduction of residual stress in polymorphous silicon germanium films and their evaluation in microbolometers^★

¹ National Institute of Astrophysics, Optics and Electronics, INAOE, Electronics Group, Tonantzintla, Puebla 72840, México Puebla 72000, Mexico
² Meritorious Autonomous University of Puebla, BUAP, Puebla 72000, Mexico
³ Electronics department of the Popular Autonomous University of Puebla, UPAEP, Puebla 72000, Mexico
⁴ Department of Physics and Astronomy, University of Texas at San Antonio, One U TSA Circle, San Antonio, TX 78249, USA

^* e-mail: rjzavala@inaoep.mx

Received: 25 August 2019
Received in final form: 15 January 2020
Accepted: 16 March 2020
Published online: 23 April 2020

Abstract

Hydrogenated polymorphous silicon germanium (pm-Si_xGe_1–x:H) thin films were deposited by the PECVD technique at 200 °C. Three compositions were investigated by changing the silane/germane gas mixture. It was found that the temperature coefficient of resistance (TCR) varies from 2.25% K⁻¹ to 4.26% K⁻¹ while the electrical conductivity ranges from 9.1 × 10⁻⁶ S cm⁻¹ to 3.7 × 10⁻³ S cm⁻¹. On the other hand, the residual stress of as-deposited films was highly compressive reaching values of nearly 700 MPa. After a thermal annealing of 3 hours, it was observed an acceptable reduction and a slight change towards tensile stress. A thin film with low residual stress and high TCR was chosen to manufacture test microbolometers in order to assess if the thermosensing properties of pm-Si_xGe_1–x:H were not affected. After fabricating the microbolometers, their structural conditions were evaluated by scanning electron microscopy and it was found that the reduction of stress significantly improved their mechanical stability and reduced the warping of the membranes. Finally, test structures were characterized at a chopper frequency of 30 Hz, with a DC current of 2.5 μA in a vacuum environment of 20 mTorr. Voltage responsivity of 1.9 × 10⁶ V/W, detectivity of 4.4 × 10⁸ cm ∙ Hz^1/2/W, NEP of 1 × 10⁻¹¹ W/Hz^1/2, NETD of 18 mK and 2 ms of thermal response time were measured. In summary, we have studied different process conditions to obtain better pm-Si_xGe_1–x:H films in terms of their electrical and mechanical properties. In this sense, the results obtained with microbolometers show that pm-Si_xGe_1–x:H is a very attractive material to develop infrared vision systems with high sensitivity.