Effects of the catalysts on the evolution of in-plane stress in alkoxide-derived silica gel coatings during heating
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Tetsuya Iwase Department of Materials Science and Engineering, Faculty of Engineering, Kansai University, Suita, Osaka-fu 564-8680, Japan (Received 19 November 2008; accepted 13 May 2009)
The effects of the catalysts on the evolution of in-plane stress during heating were studied for silica gel coatings prepared from alkoxide solutions. Tetramethylorthosilicate was hydrolyzed in the presence of nitric acid, acetic acid, and ammonia as catalysts. Gel films were deposited on Si(100) wafers by spin coating, and heated at a constant rate of 5 C/min up to 500 C. During heating, in situ measurement of the in-plane stress was conducted by measuring the radius of curvature of the substrate. In-plane, tensile stress increased up to 560 and 370 MPa in the films prepared with HNO3 and CH3COOH, respectively. However, the stress was much smaller at 30–40 MPa, which remained almost constant during heating, for the films prepared with NH3. The much smaller stress resulted from the much lower degrees of the progress of densification during heating, which was revealed in the changes in thickness and infrared absorption spectra during heating. The low degrees of the progress of densification were caused by the colloidal nature of the gel films prepared with NH3.
I. INTRODUCTION
Stress evolution in sol-gel-derived ceramic or glass coatings is a fundamental issue to be clarified in all fields of applications, because it causes cracking,1,2 affects the properties of coatings, for instance, ferroelectricity,3 and accompanies substrate bending.4 Sol-gel deposition comprises the hydrolysis and condensation of alkoxies, the deposition of gel films, and the heat treatment at temperatures normally over 500 C. The gel-to-ceramic or gelto-glass conversion is achieved in the heating-up stage, where in-plane, tensile stress on the order of 102 MPa is generated in the films.1,2 The stress evolution in the heating-up stage results from solvent evaporation, which generates capillary pressure, and from densification or shrinkage of gel films that are constrained on the substrate.1,2 In other words, the difference in thermal expansion coefficient between the film and the substrate is not the primary origin of the stress; i.e., the stress generated in the heating-up stage is not basically the thermal stress. When the fired films are cooled to room temperature (RT), on the other hand, no more densification occurs. However, due to the difference in thermal expansion coefficient between the fired film (ceramic or glass film) a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0307 J. Mater. Res., Vol. 24, No. 8, Aug 2009
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and the substrate, the thermal stress is generated in the cooling-down stage.1,2 The residual stress measured at RT is the sum of the stress generated in the heating-up and cooling-down stages. Therefore, to study the stress evolution during heating, in situ stress measurement is needed while the residual stress measurem
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