Time-resolved ambient pressure x-ray photoelectron spectroscopy: Advancing the operando study of ALD chemistry

Today, atomic layer deposition (ALD) has become a firm corner stone of thin film deposition technology. The microelectronics industry, an early adopter of ALD, imposes stringent requirements on ALD to produce films with highly defined physical and chemical properties, which becomes even more important as device and component dimensions decrease. This, in turn, means that our understanding of the chemical processes underlying ALD needs to increase exponentially. Here, we show that one can use synchrotron-based time-resolved ambient pressure x-ray photoelectron spectroscopy (APXPS) to obtain highly detailed operando information on the surface chemistry of ALD, not only, as proven earlier, during the initial ALD cycles, but also for the steady-growth regime reached during the later stages of deposition. Using event averaging and Fourier-transform methods, we show that the ALD of TiO2 from titanium tetraisopropoxide (TTIP) and water precursors in the steady-growth regime follows the suggested ligand-exchange reaction mechanism, with no sign of oxygen transport between the deposited layers and the bulk of the film, as has been observed for other materials systems. Hence, the TiO2 ALD from TTIP and water constitutes a textbook example of metal oxide ALD, as expected for this well-known ALD process. The detailed insight is made possible by computerised control of the precursor pulses that enable the recording of long data sets, which comprise many ALD cycles at highly regular intervals, in combination with an advanced data analysis that allows us to pick out signals undetectable in the raw data. The analysis method also allows to separate oscillating contributions to the signals induced by the ALD pulsing from the overwhelming bulk signal.