Heat treatment of spun-on acid-catalyzed sol-gel silica films
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SiO2 films, formed by spin coating acid-catalyzed TEOS-based sol-gel on Si substrates, were annealed at 300-1000 °C and analyzed using ellipsometry, FTIR, and in situ stress measurements. Film porosity ranged from an average of 28% before annealing to 7% after annealing 3 h at 1000 °C. Below ^800 °C, water and silanol removal caused a decrease in refractive index and increase in the in-plane tensile stress. Infrared spectra indicated compressive strain normal to the plane, however. Above =800 °C, further densification and structural relaxation occurred. Exposure to H 2 O also caused relaxation after annealing, as the most compressed Si-O-Si units reacted preferentially with moisture.
I. INTRODUCTION The sol-gel technique is an alternative method for producing thin SiO2 films which may be of use in integrated circuits. In this process, a silicon alkoxide precursor such as tetraethylorthosilicate (TEOS) is hydrolyzed under acidic or basic conditions.1'2 The monomeric =Si-OH units condense to form oligomers in solution, which can be used to produce thin films on substrates via spin or dip coating.3 After suitable annealing, SiO2 films produced from acid-catalyzed sol-gels have been shown4"6 to exhibit promising electronic characteristics, including interface state densities comparable to thermally grown oxide.6 In addition, the sol-gel process offers the possibility of lowering the thermal budget necessary to produce oxide films.4 For real applications in electronics, however, critical issues such as porosity, moisture sensitivity, and film stress must be managed. In this work, silica films formed by spin coating acid-catalyzed TEOS sol-gel solutions onto Si substrates are examined after annealing at temperatures ranging from 300 to 1000 °C. Ellipsometry and transmission Fourier transform infrared (FTIR) spectroscopy are used to identify changes in film porosity and Si-O-Si bonding arrangements both after annealing and subsequent exposure to H 2 O. Results are interpreted using reference data taken from thermal oxides. In addition, film stress is measured in situ during annealing by monitoring changes in wafer curvature. This study is a follow-up of previous work7 in which film characteristics were studied immediately after deposition, that is, prior to annealing. The ultimate goal is a better understanding of
^Present address: Texas Instruments, Semiconductor 13353 Floyd Road, MS 374, Dallas, Texas 75243.
Group,
J. Mater. Res., Vol. 9, No. 3, Mar 1994
http://journals.cambridge.org
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film structure leading to improved preparation methods and new applications for sol-gels. II. EXPERIMENTAL Sols were prepared in polyethylene bottles by mixing =45 g of TEOS (electronic grade, Olin Hunt Specialty Products, Inc.) with absolute ethyl alcohol (200 proof, Quantum Chemical Corp.), «-butanol (certified A. C. S., Fisher Scientific), H 2 O (18 Mfi), and HNO 3 (electronic grade, J. T. Baker, Inc.). H 2 O: HNO 3 (100:1 molar ratio) was added to the TEOS/solvent mixture while stirring. The H 2 O : TEOS m
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