Expanding Thermal Plasma Deposition of Silicon Dioxide-Like Films for Microelectronic Devices
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Expanding thermal plasma deposition of silicon dioxide-like films for microelectronic devices M. Creatore, M.F.A.M. van Hest, J. Benedikt, M.C.M. van de Sanden Department of Applied Physics, Equilibrium and Transport in Plasmas Eindhoven University of Technology, P.O.Box 513, 5600 MB Eindhoven, The Netherlands ABSTRACT In this paper we report on the use of the expanding thermal plasma (ETP) technique for the deposition of carbon-free SiO2 films by means of hexamethyldisiloxane (HMDSO)/oxygen mixtures, at a growth rate of 8-10 nm/s. Information concerning the film chemical properties and refractive index/growth rate have been obtained by means of FTIR measurements and in situ single wavelength ellipsometry, respectively. Because of its geometry, the ETP configuration has proven its suitability for studies concerning the fragmentation paths of the HMDSO molecule and the reactions occurring in the plasma phase. In this framework, very recent results obtained by coupling the SiO2 film-deposition set up with a very sensitive, high spectral resolutionabsorption technique, Cavity Ring Down Spectroscopy, are presented. INTRODUCTION Silicon dioxide (SiO2) film is used as insulator between metal layers in multilevel metal systems, gate oxide and capacitor dielectric in MOS (metal oxide semiconductor) devices, mask against diffusion and implantation, passivator, and sacrificial film in MEMS. In recent years the research related to SiO2 deposited by hexamethyldisiloxane (HMDSO)/O2 plasmas has been mainly focused on the optimisation of the coating chemistry, in terms of Ccontent and SiOH functionalities reduction or total removal. RF discharges and ECR sources have successfully provided silica films, at fairly high deposition rates [1,2]. Ion bombardment has also been applied to promote the condensation of silanol groups (SiOSi bridge formation); however, not much attention has been paid to possible surface damage induced by this “physical approach”. The complexity of the plasma phase, on the other hand, has not allowed yet to understand the HMDSO dissociation paths and to unambiguously identify the deposition precursors. Therefore, a correlation between the species densities in the plasma phase and the chemical composition of the film can be hardly disclosed, which leads, in conclusion, to a difficult control of the final performance of the SiO2 film and to a not immediate process scaling up to industrial reactors. Mainly, the literature reports on the use of Optical Emission Spectroscopy and Infrared Absorption Spectroscopy by FTIR [3,4]. The first pictures the less representative part of plasma population and, without a valid actinometric approach, shows its limit in the correlation between the excited and fundamental states of the species. Moreover, high molecular weight fragments show low emission. FTIR, though useful for depletion measurements and identification of stable molecules and cluster formation, lacks sensitivity and spectral resolution for identifying the radicals, which are primarily responsible for the depositi
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