Photons can be utilized to create new ALD chemistry or enhance existing ALD processes. Photo-enhancement provides many potential advantages such as lower deposition temperature, novel growth chemistries, selective area deposition, and improvement of film properties. Despite these attractive advantages, there are not many reports available on the topic, materials deposited being mostly oxides, e.g. ZnO, Al₂O₃ and Ta₂O₅.[1-3] The main challenges are the reactor design and the complexity of photochemistry in the heterogeneous system. The reactor must ensure intensive light of appropriate wavelength on the substrate while avoiding film growth on the optical components. On the other hand, optical properties of the adsorbed precursor, the substrate, and the film can all contribute to the deposition photochemistry.

We have been studying and developing photo-assisted ALD processes for oxide and metal thin films. A customized Picosun™ R-200 ALD reactor with two alternative UV sources has been employed. A single-source approach with metal alkoxides has proved itself an effective method for depositing Ti, Zr, Hf, Nb, and Ta oxides.[4] Area-selectivity of the photo-assisted Ta₂O₅ ALD was demonstrated by depositing a University of Helsinki emblem with two aluminum shadow masks (Fig.1). Resolution does not allow device level patterning but can be exploited to protect contact areas, for example. The photo-assisted ALD of metals is fundamentally different from the case of oxides in that the deposited material is optically absorbing and electrically conductive. Furthermore, the metal typically needs to be reduced during the ALD cycle. In this presentation, results on process development and the material properties of photo-assisted ALD of metal oxides, and Ru and Cu will be discussed, together with potential applications in sight. Focused ion beam patterning, wet etching and pre-patterned substrates can be used to demonstrate the capabilities of photo-ALD films.

The Semiconductor Research Corporation (SRC) and Finnish Centre of Excellence in Atomic Layer Deposition (ALDCoE) are acknowledged for funding. Picosun provided the customized reactor for the project.

References

[1] Yamada, A.; Sang, B.; Konagai, M. Appl. Surf. Sci. 1997, 112,216.

[2] Lee, Y.-H.; Kwak, J.-C.; Gang, B.-S.; Kim H.-C.; Choi, B.-H.; Jeong, B.-K.; Park, S.-H.; Lee, K.-H. J. Electrochem. Soc. 2004, 151, C52.

[3] Chalker, P. R.; Marshall, P. A.; Dawson, K.; Brunell, I. F.; Sutcliffe, C. J.; Potter, R. J. AIP Advances 2015, 5, 017115.

[4] Miikkulainen, V.; Väyrynen, K.; Kilpi, V.; Mizohata, K.; Räisänen, J; Ritala, M., manuscript.