Microstructure and Stress Analyses of Copper Films Deposited on Biased Substrates by Microwave Plasma-Assisted Sputterin
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Microstructure and Stress Analyses of Copper Films Deposited on Biased Substrates by Microwave Plasma-Assisted Sputtering François Thièry, Yves Pauleau, Yves Arnal and Jacques Pelletier, CNRS-LEMD, Grenoble, France Luc Ortega, CNRS-Cristallographie, Grenoble, France ABSTRACT Pure copper films have been deposited on Si substrates either at the floating potential or biased to various dc voltages ranging from 0 to - 125 V. Argon ions from the discharge produced in a distributed electron cyclotron resonance microwave plasma reactor were used for sputtering of a copper target biased to various dc voltages. For sputter deposition, the Si substrates placed on a water-cooled substrate holder were maintained at ambient temperature. The argon pressure was 0.13 Pa and the dc target voltage was fixed at - 600 V. The deposition rate of films was investigated as a function of the substrate bias voltage. The crystallographic structure of films and size of copper crystallites were determined by x-ray diffraction analyses. The surface morphology of films was examined by atomic force microscopy. The electrical resistivity of films was deduced from the thickness and sheet resistance of films determined by profilometry and four point probe measurements, respectively. The magnitude of residual stresses in copper films was calculated from the radius of curvature of Cu/Si samples deduced from profilometry measurements. The evolution of the microstructure, surface morphology and electrical resistivity of films as well as the magnitude of residual stresses developed in these films were studied as functions of the substrate bias voltage. These major characteristics of films were found to be dependent on the energy of argon ions impinging on the surface of films grown on biased substrates. The effect of the ion energy on the physical features of films is analyzed and discussed in this paper. INTRODUCTION With the increasing use of copper instead of aluminum as interconnect materials in integrated circuits for microelectronics, it has become important to control the microstructure of copper thin films, particularly in terms of grain size and residual stresses. These features may have a great influence on the electrical conductivity of films and resistance to corrosion [1-3]. Recently, the stress-driven nucleation of the oriented grains was observed for pure copper films produced by direct current magnetron sputtering [1]. The microstructure characterizations indicated that a film of uniform texture first was deposited, followed by the formation of oriented hillocks at boundaries of textured grains. The oriented grain size increased as the films became thicker. The purpose of the present work was to investigate the effect of low-energy ion-bombardment on the microstructure, morphology and residual stresses for copper thin films produced by direct current sputtering. The deposition equipment used was a DECR microwave plasma reactor, which offers a greater homogeneity of ion density than magnetron discharge [4]. EXPERIMENTAL SETUP Copper
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