Plasma-assisted atomic layer deposition of metal oxides and nitrides

Abstract

The atomic layer deposition (ALD) technique has recently gained considerable interest as a suitable method for the fabrication of nanoscale thin films. The virtue of this technique is that the deposition is controlled at the atomic level by self-limiting surface reactions through the alternate exposure of the substrate surface to different gaseous precursors. ALD provides ultimate control of film thickness and has the potential to achieve uniform film properties over the entire substrate surface, even in high aspect ratio structures. Over the past years it has been proven that with ALD high quality, atomically smooth, and conformal thin films of a wide variety of materials can obtained. Currently, several ALD processes are on the verge of being incorporated into the production of devices, such as CMOS transistors and DRAM memory. A recent development to broaden the applicability of ALD is the use of a plasma as an alternative reactant source. Because the activation of the reactive species already takes place in the gas phase, this so-called plasma-assisted ALD, can provide certain benefits. In this thesis work, the plasma-assisted ALD of the metal oxides (Al2O3, Ta2O5 and HfO2) and metal nitrides (TiN) were investigated. For this purpose, a versatile plasma-assisted ALD reactor based on a remote plasma configuration was constructed. To study the plasma-assisted ALD processes, in situ diagnostics were employed. New in this respect was the use of spectroscopic ellipsometry (SE) to measure film thickness in situ and optical emission spectroscopy (OES) to study the electronically excited reaction products in the plasma. Furthermore, also a quartz crystal microbalance and quadrupole mass spectrometer were employed to monitor the mass uptake per half-cycle and the reaction products created, respectively. The composition, microstructure, and electrical properties of the films were determined by ex situ techniques. During the project, the collaboration with Oxford Instruments contributed to the design of one of the first commercially available R&D tools for plasma-assisted ALD, the FlexAL. Currently, a beta version of the FlexAL tool is installed at our university and the first results on the deposition of TiN and HfO2 on 200 mm wafers were reported in this thesis work. The merits of plasma-assisted ALD have been studied and made apparent for the materials investigated. The improvement of material properties by the plasma-based process was demonstrated for the case of TiN. Good material properties in terms of impurity content and electrical resistivity were obtained for TiN, also at a deposition temperature as low as 100 °C. The feasibility of depositing good quality Al2O3, Ta2O5, and HfO2 films by plasma-assisted ALD at low substrate temperatures was also demonstrated, even down to room temperature for the case of Al2O3. The reaction mechanisms of plasma-assisted ALD have been studied for the deposition of Al2O3 and Ta2O5 from metal-organic precursors in which an O2 plasma is used as oxidant source. Two different types of metal-organic precursors, a metal-alkyl (Al(CH3)3) and metal-alkylamide (Ta[N(CH3)2]5), were used. During the O2 plasma exposure, the presence of CO, CO2 and H2O was detected in both processes. Furthermore, the depletion of the O2 source gas indicated the consumption of O radicals. These observations demonstrated that combustion-like reactions in which the surface groups are converted by O radicals into combustion products occur at the surface. Secondly, in the Al2O3 process the detection of CH4 during the O2 plasma exposure indicated that the production of H2O has an effect on the surface chemistry during the Al2O3 deposition process. The produced H2O apparently forms an alternative thermal ALD-like reaction pathway in concurrence with the combustion-like reactions. The possible existence of more reaction pathways is suggested by the presence of C2Hx and CN species during the O2 plasma exposure in the Al2O3 and Ta2O5 deposition, respectively. Additionally, the dissociation and excitation of molecules are other reactions that can take place in the plasma. This was demonstrated by the light emission coming from the plasma during Al2O3 and Ta2O5 processing, which changes in the presence of reaction products released from the surface.