(45h) Anomalous ALD Growth per Cycle Under Precursor-Limited Conditions

Atomic layer deposition (ALD) is a thin film deposition process used to create highly conformal films with precise control of thickness and composition. ALD utilizes a cycle of sequential, self-limiting surface reactions to deposit the desired film one monolayer or sub-monolayer during each cycle. Typically, a binary sequence of gaseous precursors is used, with purge periods in between to prevent gas phase reactions. The self-limiting nature of the deposition half-reactions arises from surface saturation due to a finite density of surface reaction sites, or steric hindrance when precursor ligands remain after the chemisorption reactions.

Alumina ALD using trimethylaluminum and water as precursors is among the most widely studied ALD processes [1]. Under nominal deposition conditions, a growth-per-cycle (gpc) of 0.11 nm/cycle is typically found under self-saturating conditions. As one reduces either or both precursor dosages, one would expect to find that the gpc ultimately begins to drop from the value corresponding to the self-saturating plateau to a value of zero as the dosage drops to the same. Henn-Lecordier and coworkers [2], however, found anomalously high gpc values in the precursor-starved region of operation. In this paper, we will report similar findings in which gpc values that are approximately double that of the nominal value of 0.11 nm/cycle as the TMA does is reduced while holding the water dose constant. Furthermore, we find that film thickness remains spatially uniform in this region, up to the point where precursor depletion becomes important. Potential reaction mechanisms for the observed behavior will be presented.

[1] Puurunen, R. L., Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process, Appl. Phys. Rev. 97 121301:52 (2005).

[2] Henn-Lecordier, L., M. Anderle, E. Robinson, and G. W. Rubloff, J. Vac. Sci. Technol. A, 29 051509-1 (2011).