Properties and microstructure of tungsten films deposited by ion-assisted evaporation
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I. INTRODUCTION
Refractory metals such as tungsten have several features that make them attractive candidates for metallization in VLSI devices, including resistance to interdiffusion and electromigration. Other desirable features for metallization include low stress and relatively low resistivity. However, physical vapor deposition processes tend to produce high stress in tungsten. Film stress in general depends on the amount of substrate ion bombardment during film growth.1"5 In refractory metals with bcc crystal structure, we have previously shown that film stress can be minimized by control of substrate temperature and concurrent argon ion bombardment during film growth.5"7 However, in tungsten films, changes in resistivity, which will be examined in this paper, do not parallel stress changes that were reported previously.7 This presents a challenge to minimize both resistivity and stress using concurrent ion bombardment. In this paper we shall document effects of ion bombardment on film microstructure to elucidate the factors that determine resistivity changes. Additional stress data are presented, and changes in resistivity, grain size, and impurity content caused by concurrent ion bombardment are examined. The goal is to determine the aspects of microstructure and composition that contribute to resistivity and stress, so that deposition parameters can be tailored to optimize film properties. II. EXPERIMENTS AND ANALYSIS
A schematic diagram of the vacuum system is shown in Fig. 1. The chamber is pumped with a 10 in. 80
J. Mater. Res., Vol. 6, No. 1, Jan 1991
diameter cryopump. Although the base pressure was in the low 10~8 Torr range before each deposition, the pressure was typically 3 x 10~7 Torr after elevated substrate temperature conditions were attained through use of radiant quartz lamps. The electron beam heated tungsten 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 1 x 10~4 Torr. The angle of the Ar ion beam incident on the substrate plate was about 17° from the normal. All substrates used in the current study were thermally oxidized silicon wafers cut into 2.5 x 0.3 cm strips. These were arranged so that their long axes were mutually parallel and perpendicular to the direction of travel of a 1 cm2 Faraday cup which was used to measure the ion flux as a function of position on the substrate plate. Once the ion beam stability was established, the tungsten source was stabilized at an evaporation rate of 5 A/s and the samples were exposed to the incident fluxes. After deposition the ion flux was measured again. The ion current density profile of each run was evaluated by averaging these two profiles, and the current density for each sample was determined by interpolation of the flux versus position curve. The film stress was evaluated from the change in curvature of the strips after deposition using an x-ray technique.8 Film thickness was measured with a pr
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