Structure of Al-N Films Deposited by a Quantitative Dual Ion Beam Process
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STRUCTURE OF Al-N FILMS DEPOSITED BY A QUANTITATIVE DUAL ION BEAM PROCESS H.T.G. HENTZELL*,J.M.E. HARPERandJ.J. CUOMO IBM Thomas J. Watson Research Ctr. P.O. Box218, YorktownHeights,NY 10598 ABSTRACT We describe the structure of Al-N films deposited with N/Al ratios varying from 0 to 1. A dual ion beam system supplies the Al flux by inert ion sputtering, and the N flux by low energy (100-500 eV) N2+ ion bombardment of the growing film. For N/Al < 1, a cermet structure forms with large Al grains mixed with AlN. Above the composition N/Al = 1, excess N is rejected from the AlN. The AlN films show a pronounced change in preferred orientation from c-axis perpendicular to the film surface, to c-axis parallel to the filmsurfacewith increasingN2+ energy. INTRODUCTION Thin film nitrides have exceptional electrical properties and high temperature and mechanical stability. AlN is interesting because of its wide band gap (6.3 eV), high decomposition temperature (2490'C) and chemical stability. AlN is also one of the promising thin film piezoelectric materials owing to its high ultrasonic velocity and fairly high piezoelectric coupling factor.. In this work, thin films of AlN were deposited at room temperature in a dual ion beam system (Fig. 1) where the Al atom flux (deposition rate = 2.5A/s) is supplied by inert ion beam sputtering (1500 eV Ar+, PAR =2 x 10-4 Torr) and the reactive flux (PN = 1x10-4Torr) is supplied by a low energy (100-500 eV) ion beam directed at the growing film. Insitu monitors of atom and ion flux enable the fundamental deposition parameters of ion and atom arrival rates, ion energy and direction to be measured and controlled [1,2]. The accumulated flux, i.e. the composition, is analyzed by Rutherford Backscattering (RBS). In the present paper, which is an extended abstract of Ref. 3, the deposition parameters are related to the microstructure of AlN films across the substrate holder. We alsoreportonthe influenceof theN2 + ionbeam energy on the microstructure of AlN films. The crystallographic structure and microstructure of Hat. Res.
Soc. Symp. Proc. Vol.
27 (1984) (Elsevier
Science Publishing Co.,
Inc.
520
FIG. 1 Diagramof dual ionbeamdeposition system. the films were analyzed as deposited, on Si 3N4 windows and after floating off NaCl, in a TEM. The size and shapes of the grains were measured from bright field images for the Al-AIN films and from high magnification dark field images for the AlN films. Inaccuracies in measurements of grain size are small for Al-AlN films but may be as high as 25% in the AlN films. DEPENDENCE OF STRUCTURE ON COMPOSITION In this section we describe the changes in crystallographic structure and microstructure as the arrival flux of 500 eVN 2+ ions onto the filmsurface increases. Fig. 2a shows a TEMmicrograph of a pure Al film, ion beam sputter deposited with Ar. The microstructure consists of small equiaxed grains. The diffraction pattern indicates that the filmhas grownwithoutanytexture. In filmsdeposited in the presence of 1 x 10-4 Torr N2 but no N + ion beam, n
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