Development of a nanomechanical membrane resonator for infrared and terahertz radiation detection

Abstract

The present paper deals with the topic of sensitive light sensing especially concerning the frequency spectrum reaching from infrared to terahertz. Within this electromagnetic spectrum a wide range of applications are of particular interest including research, medical imaging, astronomy, pharmaceutics and security. The objective of this work lies in the development and improvement of a detector capable of measuring sensitivities in the picowatt region at room temperature. The designed detector consists of a nanomechanical membrane resonator made of silicon nitride with a titanium nitride absorption layer on top of it. Due to the detector’s underlying thermomechanical working principle and thus its ability to measure high sensitivity, it can also be applied to detect visible light e.g. red light at 633 nm. In addition to the successful fabrication of a detector chip, it could be determined in an infrared measurement that the chip is able to measure a sensitivity of 8.6 pW/Hz1/2. This high sensitivity was reached due to a local photothermal expansion of an absorbing layer on a membrane resonator. Further improvements regarding the responsivity of the detector were achieved by stress tuning of the silicon nitride membrane. Investigations on stress tuning led to the discovery that an additional thermal treatment can reverse the stress reduction of tensile stressed membranes due to an oxygen plasma post-treatment. Although the membranes might have exhibited a compressive level of tensile stress due to the performed oxygen plasma treatment, by means of thermal post-treatment tensile stress on membranes could be reinstated and made functional for measurement again. Based on the developed nanomechanical detector and the described post-treatment methods, the foundation for further research was laid in order to increase the detectors stability and responsivity or to reach even higher sensitivities in the femtowatt regime. By implementing these improvements the detector can compete easily with cyrogenic detection technologies at room temperature and thereby aims to open new possibilities in the application field.