Thermal ALD of Sn Metal Using a Silyl Dihydropyrazine Reducing Agent

  • Authors:
    Eric Stevens (NC State), Moataz Bellah M. Mousa (NC State), Gregory Parsons (NC State)
    Publication ID:
    Publication Type:
    Received Date:
    Last Edit Date:
    2401.001 (North Carolina State University)


Current deposition techniques are usually limiting due to poor conformality, high deposition temperatures, and/or poor film quality. This work explores a novel, thermal ALD process to deposit tin metal at low temperature. We employ 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (DHP) as a strong reducing agent, to reduce SnCl4 on a silicon substrate. We explored a range of temperatures between 130 and 210oC to determine the ALD window, which was found to be 170-210oC. We show that this process yields a growth rate of ~0.28 angstroms per cycle at 190oC and shows good conformality. Furthermore, XPS results showed that the film impurity content is significantly reduced by operating at elevated temperatures (e.g., 190 vs 130 C). The reaction mechanism was explored using in situ mass spec and in situ FTIR. Analog scans of the unreacted DHP molecule revealed that the expected reaction byproducts (trimethylsilyl chloride and pyrazine) share many of the same peaks as unreacted DHP. Additionally, no byproducts were observed during the SnCl4 dose, which is in good agreement with the proposed mechanism. Lastly, in situ FTIR spectra showed C and N bonds present for Sn films deposited at 130oC; these bonds were not clearly visible at 190oC. Further in situ FTIR studies are needed to determine the effect of temperature, reactant exposure, and purge time on the reaction chemistry. This work is the first thermal Sn metal ALD process to be reported in literature; it is anticipated to expand the applicability of metal ALD to new metals (e.g. Al, Zn) and substrates (polymers, complex geometries, high surface area) that may not have been possible previously.

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