Understanding inherent substrate selectivity during atomic layer deposition: Effect of surface preparation, hydroxyl...

Area-selective thin film deposition is expected to be important for advanced sub-10 nanometer semiconductor devices, enabling feature patterning, alignment to underlying structures, and edge definition. Several atomic layer deposition (ALD) processes show inherent propensity for substrate-dependent nucleation. This includes tungsten ALD (W-ALD) which is more energetically favorable on Si than on SiO₂. However, the selectivity is often lost after several ALD cycles. We investigated the causes of tungsten nucleation on SiO₂ and other "non-growth" surfaces during the WF₆/SiH₄ W-ALD process to determine how to expand the "selectivity window." We propose that hydroxyls, generated during the piranha clean, act as nucleation sites for non-selective deposition and show that by excluding the piranha clean or heating the samples, following the piranha clean, extends the tungsten selectivity window. We also assessed how the W-ALD precursors interact with different oxide substrates though individual WF₆ and SiH₄ pre-exposures prior to W-ALD deposition. We conclude that repeated SiH₄ pre-exposures reduce the tungsten nucleation delay, which is attributed to SiH₄ adsorption on hydroxyl sites. In addition, oxide surfaces were repeatedly exposed to WF₆, which appears to form metal fluoride species. We attribute the different tungsten nucleation delay on Al₂O₃ and TiO₂ to the formation of nonvolatile and volatile metal fluoride species, respectively. Through this study, we have increased the understanding of ALD nucleation and substrate selectivity, which are pivotal to improving the selectivity window for W-ALD and other ALD processes.