Stability of Alkanethiol Self-Assembled Monolayers of Varied Chain Lengths for Area-Selective Atomic Layer Deposition

Alkanethiol (CH3(CH2)n-1SH) self-assembled monolayers (SAMs) are a common blocking layer in area-selective atomic layer deposition (AS-ALD), yet the stability and selectivity of this class of SAMs under different deposition conditions have not been fully explored. Here, alkanethiol SAMs of varying carbon chain lengths (n = 6, 12, and 18) were tested for their ability to prevent ALD of alumina and hafnia on copper surfaces using trimethylaluminum (TMA) and tetrakis(dimethylamino)hafnium (TDMAH) as metal precursors at temperatures from 100 to 180 °C. Additionally, monolayer stability was evaluated by exposing the SAMs to isolated ALD processing variables─elevated temperature, metal precursor vapor exposure, and water vapor exposure. Results showed that selectivity was higher for hafnia growth and increased with chain length, though no clear trend as a function of chain length was observed for alumina deposition. SAMs of all chain lengths showed clear degradation via loss of sulfur with TMA exposure, suggesting that drops in selectivity are due to penetration of the smaller, more reactive TMA molecule through the SAM to reach surface-bound sulfur, resulting in SAM desorption. In general, alkanethiol SAMs display unique blocking behavior that is dependent on carbon chain length, temperature, and metal precursor size and reactivity, with the highest selectivity displayed by a long, ordered SAM (n = 18) against a larger, less reactive precursor (TDMAH).