(Invited) Vacuum Ultraviolet Photochemical Atomic Layer Deposition of Alumina and Titania Films

Conventional atomic layer deposition (ALD) is a thermo-chemical process where co-reagents are sequentially pulsed in cycles onto a heated substrate. As an alternative to substrate heating, various forms of other “energy – enhanced” ALD processes are being investigated. Herein, the photochemical atomic layer deposition of Al₂O₃ and TiO₂thin films at 60°C is reported using a shuttered vacuum ultraviolet (VUV) light source to excite molecular oxygen as a co-reagent with the metal precursors. The growth mechanisms using trimethyl aluminium (TMA) and titanium isopropoxide precursors, are investigated using in-situ quartz crystal microbalance (QCM) and post-deposition ellipsometric measurements.

The figure below illustrates the mass gain / loss processes during a series of ALD cycles of TMA with O₂/ UV excitation from a VUV lamp with peak emissions at 125 nm (9.9 eV) and 160 nm (7.7 eV). Further, the figure shows the growth per cycle (GPC) derived from the QCM data and also separately from single wavelength (633 nm) ellipsometry measurements made on silicon (100) samples exposed to the same growth conditions. Both data sets indicate that as the VUV illumination time is extended beyond 5 s to 10 s the overall mass gain per cycle saturates and becomes approximately constant, which is consistent with other self limiting ALD processes. The photochemical process is exploited to achieve area-selective film deposition. Selective area deposition was achieved by projecting the VUV light through a metalized magnesium fluoride photolithographic mask. The selectivity of deposition is a function of the UV exposure time and the mask – substrate separation.

The photochemical atomic layer deposition of TiO₂ thin films from titanium isopropoxide exhibits similar growth characteristics to the Al₂O₃ photochemical process, however subtle differences in the growth mechanisms are attributed to the different reactions of the excited oxygen species (O(³P)) with the Al-CH₃ and Ti-OCH(CH₃)₂ bonds.