Electron Beam Deposited Chalcogenide Glass Thin Films for Chemical Sensor Applications

Abstract

Infrared absorption spectroscopy plays a significant role in real-time continuous high resolution medical, environmental and manufacturing sensors, since most molecules and all gases have a signature absorption spectrum in the infrared region of the electromagnetic spectrum. Mature CMOS line materials for integrated photonics devices are limited in sensitivity due to the wavelength limitations at long midinfrared region. Additionally, these materials require high temperature processing and are restricted to the substrate selectivity for lab-on-chip integration. Chalcogenide glasses (ChG) have a much wider window of infrared transparency than CMOS line materials and oxide based glasses. The amorphous nature and low glass transition temperature of ChG enables a variety of substrates and low-cost CMOS fabrication methods. As ChG are new materials for integrated photonic devices, the main challenge is developing scalable fabrication process of low-loss structures at CMOS safe temperatures. Low-temperature depositions would enable greater integration with other photonic and even CMOS electronics. The objective of this study was to develop scalable low-temperature deposition process for low-loss integrated photonic devices by optimizing the optical properties of deposited ChG thin films and examining the impact of stoichiometric and index of refraction changes. Materials used in this work were: Arsenic Sulfide (As2S3), Germanium Arsenic Selenium (GeAsSe) and Gallium Lanthanum Sulfide (GLS). The fabrication approach consisted of: using electron beam(e-beam) evaporation in order to facilitate lift-off structures; heating at low temperatures the substrate during deposition to optimize film quality; and varying deposition time to examine stoichiometric repeatability. Characterization of thin films entailed: AFM for roughness measurements, XRD to identify any crystallinity (which would lead to optical loss); Spectrophotmeter and spectroscopic ellipsometry for optical characteristics like refractive index and absorption coefficient; Energy dispersive spectroscopy (EDS) for stoichiometric measurements. The results are competitive. The e-beam evaporated films had very smooth surface morphology without any pinholes or cracks. The surface RMS values were found to be comparable with published results. The effect of substrate heating showed no significant difference on the optical properties of films such as refractive index but offered lower surface RMS roughness values. The stoichiometry of the films were close (within 2%) to that of the parent bulk glass for binary As2S3 and GeAsSe glasses. For GLS, the stoichiometry of the thin films was found to change over time due to the greater difference in vapor pressures of the 3-4 elements than the other deposited glasses. In summation, we have developed a low-cost deposition method for ChG at CMOS safe temperatures for making low-loss photonics devices, which is significant for infrared sensors.

Date of Award	Jun 2013
Original language	American English
Supervisor	Clara Dimas (Supervisor)