A mass spectroscopy study of palladium silicide formation
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A Mass Spectroscopy Study of Palladium Silicide Formation
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B. S. LIM and J. P. STARK
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Many of the thin film silicides grow at very low temperatures, and grain boundary diffusion is a reasonable basis to understand such a reaction event. Of these reactions, the formation of PdzSi has been found to occur in the vicinity of room temperature and yet the compound melts at 1330 ~ Studies of the reaction, Pd2Si, conclude that the reaction layer thickness follows a (time) 1/2 dependence with an activation energy of 1.3 to 1.5 ev. 1-7 Such a time dependence is consistent with a bulk diffusion mechanism, but a low reaction temperature and activation energy suggest either interstitial diffusion of the mobile species or grain boundary diffusion. Marker studies 7 have confirmed that both Pd and Si are mobile in the reaction. Furthermore, the formation of Pd2Si in [111] silicon has been observed to be epitaxial L2'3 with the [001] of the Pd2Si. Similar marker studies 8 have been performed on Mg2Si and have shown that only magnesium is mobile in that system. To assist in the understanding of this reaction, we have followed the reaction using AES and SIMS to determine if isotope separation occurs during the reaction. [111] p-type silicon single crystalline wafers were degreased in organic solvents, etched in an HF solution, sputter etched, and sputter deposited at 20 m torr of argon pressure with Pd in layers of 200 to 1000 nm. Samples were vacuum encapsulated at 10 -6 tort and annealed at 300 ~ from 5 to 35 minutes. Following the anneal X-ray diffraction confirmed the formation of epitaxial Pd2Si. Samples were sputter sectioned while simultaneously determining the Auger electron spectra (AES) and the secondary ion mass spectra (SIMS) of the reaction products. The AES was capable of determining the location of the compound interfaces while the SIMS determined the extent to which isotope separation occurred. Figure 1 presents the combined results of the AES and SIMS analysis of a Pd=Si sample run at 300 ~ The AES can determine the location of the metal-silicide and siliconsilicide interfaces while the SIMS yields the isotopic distribution. The interface location is shown in Figure 1 as the vertical dashed lines. Between these lines, we observe a separation of the isotopes of silicon as noted by the increase in the ratio of the peak heights for the isotopic masses 29 and 28. Since the lines represent the interface location, these data are consistent with a separation of these isotopes during the growth of palladium silicide (Pd:Si). It is notable that the ratio of the isotopic concentration is consistent with the lighter isotope being enriched at greater distances from the Si/Pd2Si interface and hence the silicon source. For reference, we also present a graph of the silicon isotope data B.S. LIM is Assistant Professor, Mechanical Engineering, with The Sung Kyun Kwan University, Cheoncheon-Dong, Suwon 170, Korea. J.P. STARK is Professor, Mechanical
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