Plasma-assisted atomic layer deposition : an in situ diagnostic study

Abstract

The method of atomic layer deposition (ALD) is considered one of the primary candidates for the uniform and conformal deposition of ultrathin films vital for the continuous miniaturization in the semiconductor industry and related high-technology markets. By the virtue of two selflimiting surface reactions, the ALD technique yields an ultimate control of film growth in the sense that a submonolayer of material is deposited per so-called ALD cycle. With established materials being at the verge of industrial implementation, efforts are continuously undertaken to optimize and develop new ALD configurations and processes. So far, the main emphasis within the field of ALD has been on the materials characterization of the films by means of ex situ analysis. The research described in this thesis aims at the development of the relatively new configuration of plasma-assisted ALD and at in situ diagnostics studies of the (plasmaassisted) ALD processes. In plasma-assisted ALD, a plasma is used to activate the reactants in the gas phase in order to supply additional reactivity to the ALD chemistry. Plasma-assisted ALD is researched to provide benefits in the development of new ALD processes and materials. In particular, the opportunities to improve and tune the film properties, and to deposit films at reduced substrate temperatures have been addressed in this thesis. This work has been accompanied by studies using various in situ diagnostics, from which fundamental insight into the reaction mechanisms governing the ALD processes can be obtained. Moreover, in situ techniques provide the opportunity to monitor, optimize, and control the ALD process. In this work the use of in situ spectroscopic ellipsometry, transmission infrared spectroscopy, mass spectrometry, and optical emission spectroscopy has been demonstrated in studies of the plasma-assisted ALD processes of metal nitrides and metal oxides. The results of the film characterization obtained by these techniques have been corroborated and complemented by extensive ex situ analysis. In particular, the combination of in situ spectroscopic ellipsometry and the layer-by-layer ALD growth has been explored comprehensively. The merits of this in situ technique during ALD have been demonstrated by addressing various aspects relevant to ALD processes and materials. A large part of the work has concentrated on the plasma-assisted ALD process of the metal nitrides TiN and TaN. The merits of plasma-assisted ALD were observed in the deposition TiN films with excellent conductivity and low impurity content, even at low deposition temperatures. Furthermore, it was shown that by variation of the plasma condition in the ALD process of TaN, the film properties could be tailored from conductive, cubic TaNx;x??1 to semiconductive, amorphous Ta3N5. These aspects were clearly demonstrated by in situ spectroscopic ellipsometry, where the transition in TaNx phase could be distinguished by monitoring the energy dispersion in the optical constants. For the conductive films, the light absorption by free conduction electrons could be probed and that enabled extraction of the electrical film properties from the ellipsometry data. The latter was valuable to demonstrate electron-impurity scattering and finite size effects in TiN films. Furthermore, fundamental insight into the reaction mechanisms of plasma-assisted ALD process of TaN was obtained by detection of the volatile reaction by-products by mass spectrometry and optical emission spectroscopy. The possibilities for plasma-assisted ALD to improve the material properties and to deposit at reduced temperatures have been demonstrated for the process of Al2O3. The Al2O3 films were deposited at substrate temperatures down to room temperature and these films yielded good moisture permeation barrier properties as relevant for encapsulation purposes. The fundamental reaction mechanisms of this plasma-assisted ALD process were elucidated by transmission infrared spectroscopy in order to understand and further improve the film properties obtained at these reduced deposition temperatures. It was established that the surface chemistry is ruled by –CH3 and –OH surface groups created by the Al(CH3)3 precursor adsorption and the combustionlike reactions during the O2 plasma step, respectively. Moreover, infrared spectroscopy provided insight into the influence of deposition temperature on the material properties. It was shown that by prolonging the plasma exposure, i.e., by supplying more plasma reactivity to the ALD process, the surface chemistry at low temperatures was enhanced and the impurity content in the Al2O3 was reduced. In conclusion, the knowledge gained through the in situ diagnostic studies in this work is relevant to further develop the ALD technique. The insight obtained into the reaction mechanisms and the material properties of the ALD films in this work are particularly useful to further exploit the possibilities and opportunities of the plasma-assisted ALD technique in the synthesis of novel (complex) materials.