277 Estimation of Optical Constants and Thicknesses of E-Beam Deposited TiO 2 Thin Films by Envelope Method Vemuri SRS Praveen Kumar a , Parinam Sunita a , Mohit Saraf b , Mukesh Kumar a,b , Parinam K Rao b , Neelam Kumari a,b , Vinod Karar a,b and Amit L. Sharma a,b* a Academy of Scientific & Innovative Research (AcSIR), CSIR-CSIO, Chandigarh b CSIR-Central Scientific Instruments Organisation, Chandigarh, India, 160030 *corresponding author: Amit L. Sharma, Email: amitsharma_csio@yahoo.co.in, Mobile No. +91 9417850180 Abstract Titanium Dioxide films are one of the most widely used thin films catering to the needs of various sensing and optical filter applications owing to their excellent environmental durability, best reproducibility, lower thermal stress as well as widely tuneable refractive index properties. In this study, TiO2 films were grown on glass substrates by using electron-beam deposition technique under reactive oxygenated environment and the thickness of the film was monitored using quartz crystal in-situ thickness monitoring. This study focuses on estimation of film thicknesses and optical constants by envelope method to verify and validate the data received through in-situ thickness monitoring. This technique was applied to the transmission spectra of three samples which were deposited under identical conditions but in different batches and having different thicknesses. The calculated thicknesses of these samples were found to be almost in accordance with the in-situ thickness monitoring data and can be extended to other films for estimation of optical constants. The thickness computed through this method was used further for estimating other important optical constants like absorption coefficient and extinction coefficient for the samples under study. Keywords TiO2 thin films; E-Beam deposition; envelope method 1. Introduction Titanium dioxide (TiO2) is one of the most studied transition material oxide due to its remarkable chemical, optical and electrical properties that make it suitable for a variety of applications like sensors, optical coatings, self-cleaning/ antifogging surface coatings [1] and photovoltaic applications [2] etc. TiO2 in combination with other dielectric materials are widely used for anti-reflection coating as well as band pass, band stop and reflective notch filter applications [3, 4]. It has a wide application in optical thin film coatings due to its widely tuneable refractive index properties and high stability. The properties and structures of deposited TiO2 films are highly sensitive to variation in deposition parameters such as O2 partial pressure, substrate temperature and vacuum levels during deposition. The TiO2 thin films can be deposited by various techniques such as spin coating, electron-beam evaporation [5- 8], reactive evaporation, ion-assisted deposition and sputtering. The deposited TiO2 thin film thickness and refractive index can be determined accurately by spectrophotometry in the visible region, but for weak absorbing material calculation is somewhat challenging. Various computational techniques involving reflection/transmission measurements as well as interferometric methods have been suggested to accurately estimate thicknesses and optical constants of different films [9-11]. In this study, TiO2 films were grown on glass substrates by using Electron-beam deposition technique under reactive oxygenated environment and the thickness of the film was monitored using quartz crystal in situ thickness monitoring. The film thicknesses and optical constants were estimated by applying envelope method [8-13] to the transmission spectra of these films to verify and validate the data received through in situ thickness monitoring. The optical properties include refractive index (n), absorption coefficient (α) and extinction coefficient (k). 2. Materials and methods The TiO2 thin films for the present studies were grown by Electron-beam gun evaporation technique (PLS 570 coating system, Pfeiffer Vacuum) on optical grade glass substrate (BK7, n=1.52) of 70 mm x 70 mm size. The substrates were cleaned ex-situ prior to mounting for deposition by ultrasonication in acetone medium and then dried by hot drier. The substrates were further cleaned in-situ by glow discharge for 4-5 minutes in the presence of argon gas to make substrate free of any contamination and ensure high quality of deposited films. The deposition system consists of one e-beam source and four liners (for materials) for evaporation, a substrate holder, glow discharge, quartz crystal and optical thickness monitor to control evaporation. Figure 1 shows the schematic diagram of the vacuum thin film deposition system used for this study. A thermocouple was used to measure the temperature of the substrate holder. The film thickness, the rate of deposition and the cut point deposition were controlled using quartz crystal thickness (Intellemetrics, IL820) monitor. Table 1 lists the deposition conditions parameters maintained during deposition. Figure 1. Schematic diagram of a vacuum thin film deposition system used for this study Before deposition, the chamber was initially evacuated to the base pressure of 2x10 -5 mbar and the Argon was flushed in a controlled manner to clean the substrate by glow discharge. A plasma discharge was generated for 3-4 minutes with a discharge current 100-200 mA. Thereafter, the films were deposited under 2x10 -4 mbar partial pressure at 250 ̊ C substrate temperature. Stoichiometric growth during the deposition process was ensured by a controlled oxygen flow into the deposition chamber. The flow of high purity oxygen was regulated by EB