Thermal Annealing Investigation of the Optical Properties of Si 1-x N x Films Fabricated by Ion Beam Assisted Deposition
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THERMAL ANNEALING INVESTIGATION OF THE OPTICAL PROPERTIES OF SIIxNx FILMS FABRICATED BY ION BEAM ASSISTED DEPOSITION E.P. Donovan, C.A. Carosella, D. Van Vechten* Naval Research Laboratory, Code 4671, Washington, DC 20375-5000 *Sachs-Freeman Associate at NRL
ABSTRACT The annealing behavior of the optical properties of silicon nitride films (Sil-xNx) is described for films fabricated by ion beam assisted deposition. The data are needed for the precise manufacture of optical filters, where the index of refraction must be predicted from deposition parameters and film annealing history. The reflection of homogeneous, amorphous samples deposited on (100) silicon substrates was measured from 500 to 3120 nm. Fits to the interference spectra were obtained over the range 1000 to 3120 nm to obtain the index of refraction vs wavelength as a function of film nitrogen content. Nitrogen atom fraction was varied from .2 to .58 by variation of the incident relative fluxes of nitrogen ion beam current to evaporant silicon flux. The films were annealed in argon at 450 C, 600 C, 750 C, and 1100 C and the measurements repeated. The systematic shifts in index of refraction with annealing temperature are described. INTRODUCTION The relaxation of simple covalent amorphous systems fabricated by ion beam techniques has been a subject of great interest in recent years [1-4]. Infrared reflection measurements, for example, have been used to correlate the heat of relaxation of ion implantation amorphized germanium layers with changes in the index of refraction upon temperature ramp annealing [4]. Among ion beam fabrication techniques, ion beam assisted deposition is of increasing importance [5]. Ion beam nitrogen and evaporant silicon deposition is useful for the fabrication of complex inhomogeneous infrared optical filters [6], but the performance changes upon annealing due to refractive index changes have not been well understood. Therefore, the purpose of this work is to measure the index of refraction changes which occur in homogeneous silicon nitride films which span the composition range needed for the filter fabrication, as a function of annealing temperature. EXPERIMENTAL A schematic of the ion beam assisted deposition system used to fabricate Base pressure of the chamber is 2*10-7 the films is shown in Figure 1. Torr and operating pressure with the nitrogen feed gas in the chamber is 2*10.4 Torr. Silicon (100) substrates were polished on one side and roughened on the back with silicon carbide grit, then cleaned with ethanol prior to insertion in the vacuum chamber. The substrates were sputter cleaned in situ prior to deposition. The substrate was mounted directly over the 40 cc hearth electron beam evaporation source, at an angle of 200 to the evaporant stream. The substrate surface normal points directly at the center of the extraction grids of a 3 cm Kaufman ion source, which is normally operated at 500 eV. Three apertures for the voltage suppressed Faraday cups are mounted symmetrically about the substrate to measure the charged f
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