Stress Modification in Tungsten Films Deposited by Ion-Assisted Evaporation
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STRESS MODIFICATION IN TUNGSTEN FILMS DEPOSITED BY ION-ASSISTED EVAPORATION R. A. ROY, R. PETKIE, D. S. YEE, J. KARASINSKI, A. BOULDING, IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598 ABSTRACT The modification of film stress in evaporated tungsten was studied as a function of deposition environment. Using concurrent ion bombardment of the growing film, the stress was seen to vary systematically with ion energy, ion flux, and substrate temperature. The qualitative behavior fits the model of stress modification developed for niobium films. X-ray diffraction was used to study the structure of the films, and a clear correlation between crystallographic texture and film stress is found. The original structure/impurity model for film stress modification due to ion bombardment has been modified to account for the relationship between film stress and texture. INTRODUCTION Modification of film properties by ion bombardment during film growth has long been a recognized phenomenon that can be used to minimize stress in vapor-deposited films (1-10). In refractory metals with bcc crystal structure, we have previously shown that film stress can be modified by a combination of substrate temperature and argon ion bombardment during film growth (7,8). A stress behavior of niobium films attributed to the presence of two ion bombardment mechanisms which had opposite effects on film stress. The first mechanism was impurity removal, which tended to increase tensile stress; the second was termed "structure modification" and was believed to involve changes in the microstructure of the films associated with the trend toward compressive stress at high ion flux. In the current study, we have adopted an experimental methodology similar to the niobium study using tungsten metal. We have expanded the matrix of experimental parameters to include a range of ion flux, ion energy, and substrate temperature to verify whether increasing ion energy has an effect similar to increasing substrate temperature. 'We have used x-ray diffraction to examine the nature of film structural changes that were believed to occur at high ion flux where film stress became compressive in previous studies of Nb and Cr. We will show that the stress behavior is very similar to niobium, and that changes in stress are closely associated with changes in film texture. EXPERIMENTS AND ANALYSIS A schematic diagram of the vacuum system is shown in Fig. 1. The chamber is pumped with a 10 in. diameter cryopump. The base pressure was typically 3x10-7 Torr after elevated temperature conditions were attained through use of radiant quartz lamps. The electron beam heated W source and the 6.0 cm. Ar ion source were outgassed and stabilized prior to deposition. During film deposition the chamber pressure resulting from Ar admitted through the ion source was about Ix 10- Torr. The angle of the Ar ion beam incident on the substrate plate was at about 17' from the normal. A I cml Faraday cup was used to measure the ion flux as a function of position on the s
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