Thermal atomic layer deposition of Sn metal using SnCl4 and a vapor phase silyl dihydropyrazine reducing agent

This work explores a novel, thermal atomic layer deposition (ALD) process to deposit tin metal at a low temperature. The authors employ 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (DHP) to reduce SnCl4 on silicon substrates. The authors explored a range of temperatures between 130 and 210 degrees C to determine the ALD window, which was found to be 170-210 degrees C. The authors show that this process yields a growth rate of similar to 0.3 angstrom per cycle at 190 degrees C. Furthermore, X-ray photoelectron spectroscopy results showed that the film impurities are reduced for depositions within the ALD window. The reaction mechanism was explored using in situ mass spectrometry and in situ quartz crystal microbalance (QCM). Within the ALD temperature window, the QCM results showed a saturated mass gain during the SnCl4 exposure and a net mass loss during the DHP dose. Consistent with the QCM results, in situ mass spectroscopy data indicate that the DHP exposure step removes surface Cl via formation of volatile trimethylsilyl chloride and pyrazine by-products, effectively reducing the oxidation state of surface-bound Sn. This work is the first thermal Sn metal ALD process to be reported in literature and the oxidation/reduction chemistry presented here may be applied to other metal precursors, increasing the applicability of metal ALD use in industry. Published by the AVS.