Measurement of Residual Stress In TiO 2 Sol-Gel Thin Films Using Raman Spectroscopy
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NANCY J. HESS AND GREGORY J. EXARHOS Pacific Northwest Laboratory, Richland WA 99352
ABSTRACT The deposition of thin films can result in residual stresses that affect film properties such as optical properties, phase stability, chemical durability, and film adhesion. There are many components to the total residual stress measured in a film. These include interfacial stress, volume changes associated with phase transitions, film thickness, and film microstructure. For sol-gel films, some contributors to residual stress can be controlled by such variables as the precursor chemistry and gelation time or heat-treatment following deposition. Measurements of the state of stress during crystallization or as a function of processing conditions and sol-gel chemistry allows optimization of film deposition parameters that promote minimal residual stress. We have constructed a threedimensional surface in Raman shift-pressure-temperature space from spectroscopic measurements on single crystal anatase at high pressure and temperature. Location of the Raman shift of the two E, modes of anatase on this surface the state of stress (pressure) and temperature of the film can be determined. For materials with large Raman scattering cross-sections, such as anatase, timeresolved crystallization experiments on films as thin as 100 nm are possible. Using this technique, the evolution of the state of stress during isothermal thermal and laser-induced crystallization processes can be monitored in real time and compared to the residual stress upon cooling.
INTRODUCTION Sol-gel processing typically involves the dissolution of a metal alkoxide in a reagent where the hydrolysis and condensation reactions can be controlled to develop the oxide phase and microstructure desired. Pre-treated substrates can be dipped or spin-cast with the precursor solution immediately following preparation or after aging. The deposition of titania thin films by sol-gel processing produces amorphous, chemically homogeneous films with uniform optical properties and thickness. However when subjected to mild heat-treatments or laser irradiation, the amorphous films begin to densify and, at temperatures greater than 300 C, nucleate and grow crystalline phases. The densification and crystallization process is accompanied by increasing residual stress in the film as a result of mismatch of the film-substrate thermal properties, grain growth, and non-uniform microstructure. All these factors influence the optical properties, chemical durability, and phase stability of the film. In initial work we demonstrated the use of Raman spectroscopy to measure the magnitude of residual stress and identify the crystalline phase of isothermally heat-treated titania thin films. The acidity of the precursor solution and the gelation time strongly influenced the crystalline phase and the magnitude of residual stress in the film. [1,21 Using time-resolved Raman spectroscopy, we followed the growth of the anatase phase of TiO2 during isothermal crystallization by monitoring the intensity
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