In-situ Ellipsometry Study of Ion Bombardment Effects on Low Temperature Si Epitaxy by dc Magnetron Sputtering

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ABSTRACT We deposit Si films on Si(100) substrates at temperatures of 300 - 350 *C using dc magnetron sputtering, and characterize the structure by in-situ spectroscopic ellipsometry. Changes in the ion or electron bombardment, produced by biasing the sample with respect to the floating potential, are found to exert a strong effect on the kinetics of the crystalline (epitaxial) to amorphous transition for films deposited just below the apparent minimum temperature (350 'C) for sustained epitaxy. At 320*C, the best results are found at the floating potential, which is 25 V below the plasma potential and produces an ion flux equal to the depositing Si flux on the substrate. At +14 V above the floating potential, the volume fraction of crystalline Si decreases exponentially with thickness, and the characteristic decay length is a function of substrate temperature. At -14 V below the floating potential, the deposited film is amorphous with a large void content. These observations demonstrate the subtle tradeoff between enhanced surface mobility and defect creation by low energy ion bombardment.

INTRODUCTION Sustained homoepitaxial silicon growth at low substrate temperature has been achieved recently by several vapor deposition processes, including remote plasma-enhanced chemical vapor deposition (PECVD), electron-cyclotron-resonance PECVD, ion beam deposition, and reactive magnetron sputtering (RMS). 1-5 Compared to molecular beam epitaxy, the reduction in the thermal energy requirement for epitaxial growth in these processes comes from energetic deposition of species or plasma ions. The energetic bombardment continually restructures the growing surface and promotes low-temperature epitaxy. 6 Ohmi et al. have successfully deposited device-grade epitaxial silicon films around 300 *C by low-energy bias sputtering. 5 Spectroscopic Ellipsometry (SE) is a proven structure characterization tool for thin film microstructure and crystallinity.4. 8-10 In this study, we will examine the role of energetic neutrals vs ion flux on low temperature Si epitaxy in dc magnetron sputtering system. Magnetron sputtering is known for producing an energetic sputtered neutral flux from the target. Myers et al. estimate the average energy of neutral Si flux to be -13 eV at the substrate. 7 Changing 75 Mat. Res. Soc. Symp. Proc. Vol. 354 01995 Materials Research Society

substrate bias can change the ion bombardment energy without effecting this neutral flux impinging on the growing surface. We deposit Si films on Si(100) substrates at temperatures between 230 - 350 "C, and characterize the structural transition from crystalline (epitaxial) to amorphous just below the apparent minimum temperature for sustained epitaxy by in-situ spectroscopic ellipsometry under different substrate biases.

EXPERIMENTAL DETAILS We deposit epitaxial Si films by dc magnetron sputtering of a 2" diameter c-Si target in a UHV system. The working gas is 2 mTorr of Ar. The cathode current is regulated to a constant

value of 80 mA (- 40 mA / cm2 over the erosion are