Optical and physical properties of sputtered Si:Al:O:N films
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C. H. Henager, Jr. Battelle, Pacific Northwest Laboratories, Richland, Washington 99352 (Received 8 March 1991; accepted 19 December 1991)
Mechanical and optical properties and structural characteristics are described for Si: N films with Al and O additions ( S ^ A l ^ O ^ N ^ ) deposited by reactive RF diode sputtering on Si and SiO2 substrates. The thermal and intrinsic stress components, elastic stiffness, coefficient of thermal expansion (CTE), and refractive index were measured for films ranging in thickness from ~ 2 /v,m to 50 /v,m. Some structural and microstructural data were obtained using x-ray diffraction, optical and scanning-electron microscopy, and surface profilometry. Alloying Si: N with Al to form Si: Al: N greatly reduced the compressive intrinsic and total stress found in pure Si: N films on Si. Addition of O to the Si: Al: N moderately increased the intrinsic stress, decreased the elastic stiffness, and produced a smoother, more glassy (amorphous) film.
I. INTRODUCTION This work was motivated by the need to develop transparent, hard, mechanically stable (low stress) films, ranging in thickness up to ^100 fim, for a particular optical device known as a buried grating. Reactively sputter-deposited Si: N films were investigated initially for this purpose, and their properties appeared desirable except for high compressive stresses in the Si: N films. High stress, especially in thick films, destroys the film through cracking or debonding, and can damage the substrate. Previous measurements of stresses in optical thin films at this laboratory1 indicated that the addition of Al: N to Si: N films deposited on SiO2 reduced the film stresses. In the course of the present work, the Si: Al: N system was investigated more fully, and Si: Al: N films with metal ratios Si7oAl3o : N and Si 60 Al 40 : N were selected for further study. Smooth, transparent, low stress films thicker than 50 /y,m were deposited on polished Si substrates. Properties of these Si: Al: N materials were further modified by the addition of O to form Si7oAl3o: O: N and Si60Al40 : 0 : N films. Film properties measured directly or indirectly included total stress and its intrinsic and thermal components, coefficient of thermal expansion (CTE), density, and the index of refraction and optical absorption coefficient at 750 nm wavelength. Effects of the addition of Al and O on the properties of Si: N films are described and discussed in this paper. Many studies have been conducted on the deposition and characterization of thin (2 sputtering gas ratio = 2 : 1 . f N 2 / O 2 gas ratio = 1:2. 8 Typical standard deviation of mean, including variation due to thickness. b
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J. Mater. Res., Vol. 7, No. 5, May 1992 Downloaded: 16 Mar 2015
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R.W. Knoll and C. H. Henager, Jr.: Optical and physical properties of sputtered S i : A I : O : N films
0.6 A1 2 O 3
Increasing Oxide
Si 3 N 4 Si 7 0 Al 3 0 :N
A1N Si 6 0 Al 4 0 :N
Increasing Aluminum
FIG. 1. Comparison between film and bulk the
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