Quadrupole mass spectrometry for atomic layer depositon

Quadrupole mass spectrometry (QMS) can be a very valuable in-situ tool to monitor and study atomic layer deposition (ALD) processes. In scan mode, QMS can be used to determine the cracking pattern of precursor molecules, revealing information on chemical structure, and it can be used to detect the formation of molecules during plasma assisted ALD. Using multiple ion detection mode (MID), the ALD process can be monitored in realtime, tracking the precursor and reactant gasses as well as the formation of reaction products. One of the challenges that need to be addressed when using QMS on an ALD reactor without adequate pressure control system is the pressure artifact, which causes the QMS signal to depend on the total pressure inside the reactor.
Another issue that arises during any QMS measurement for ALD is the possible overlap of mass to charge ratios of different species in the reactor. Both these issues can be addressed by performing precursor only and oxygen only cycles that can be used as a reference for the full cycle data. To optimize the information that can be extracted from the QMS data, a systematic approach was developed. This included optimization of the ALD cycle itself, the data acquisition, and the data processing. A program was written in Matlab to automatically align the QMS data for several cycles and calculate the average. This greatly improved the signal-to-noise ratio and allowed for a higher sample frequency. After optimization the QMS data provided detailed real-time information on the amount of reaction product formation and even reaction kinetics.