Substrate Temperature Effects on Properties of Silicon Nitride Films Deposited by Ion Beam Assisted Deposition
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SUBSTRATE TEMPERATURE EFFECTS ON PROPERTIES OF SILICON NITRIDE FILMS DEPOSITED BY ION BEAM ASSISTED DEPOSITION.
E.P. Donovan, C.A. Carosella, K.S. Grabowski and W.D. Coleman* Naval Research Laboratory, Code 4671, Washington, DC * Sachs Freeman Associates, Washington, DC.
ABSTRACT Silicon nitride films (Sil.xNx) have been deposited on silicon by simultaneous evaporation of silicon and bombardment of nitrogen ions. Films approximately I pin thick were deposited in an ambient nitrogen pressure of 50 pTorr. The substrate temperature (Tsua) ranged from nominally room temperature to 950' C for films with X between 0 and 0.6. Nitrogen atom fraction, X, was measured with Rutherford backscattering spectrometry (RBS). Refractive index was measured with near-IR reflection spectroscopy. Differences in film structure were measured by FTIR on the Si-N bond bending absorption mode, and by x-ray diffraction (XRD). X was found to depend upon the incident flux ratio of energetic nitrogen atoms to vapor silicon, and upon Tsu. Refractive index depends upon X and Tsm. XRD found evidence of the presence of amorphous structure, poly-crystalline silicon and (101) oriented P3-Si3N 4 depending on X and Tsu,. The Si-N absorption signal increases with X and shows some structure at high Ts..
INTRODUCTION The composition and optical properties of thin silicon nitride films (Si,.xNx, X < 4/7) deposited by IBAD at 100 pTorr ambient pressure on nominally room temperature substrates have been studied for inhomnogeneous optical filter applications I.]. Perfomiance of the filters altered under thermal stress due to changes in refractive index [2]. The crystalline silicon nitride ot phase was found to form for post-deposition anneal temperatures >1 100" C for nitrogen atom fractions between 0.3 and 0.57 13]. To study temperature effects, we have measured the asdeposited values of refractive index, infrared absorption, and x-ray diffraction patterns of IBAD silicon nitride samples fabricated at a substrate temperatures Tsu up to 950' C. 5 EXPERIMENTAL PROCEDURES A schematic of the ion beam assisted deposition (IBAD) system was published previously [1]. Electron beam evaporation of 1 pm of Si occurred simultaneously with bombardment of 500 eV nitrogen from a Kaufman ion source. Base pressure of the cryopumped chamber was 0.2 pTorr, and chamber pressure was 50 pTorr during ion source operation. Si(100) substrates, sized lxlxO.074 cm 3 , polished on both sides, were de-greased in ethanol, then N-sputter cleaned in situ prior to deposition. An electron impact emission spectrometer (EIES, Inficon Sentinel 111) was first calibrated with a quartz crystal oscillator (4.3 cm from the substrate) at the operating pressure, then used to monitor the evaporation rate. Both the EIES and quartz monitor were shielded from the ion beam. The ionic portion of the ion beam was measured with three suppressed Faraday cups symmetrically surrounding the substrate. Substrates were 20 cm from the ion source extraction grids with the beam at an angle of 12" from substrate norm
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