Quantitative analysis of strengthening mechanisms in thin Cu films: Effects of film thickness, grain size, and passivati
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Quantitative analysis of strengthening mechanisms in thin Cu films: Effects of film thickness, grain size, and passivation R-M. Keller, S. P. Baker, and E. Arzt Max-Planck-Institut f¨ur Metallforschung, and Institut f¨ur Metallkunde, University of Stuttgart, Seestrasse 71, D-70174 Stuttgart, Germany (Received 27 February 1997; accepted 6 August 1997)
Thermal stresses in thin Cu films on silicon substrates were examined as a function of film thickness and presence of a silicon nitride passivation layer. At room temperature, tensile stresses increased with decreasing film thickness in qualitative agreement with a dislocation constraint model. However, in order to predict the stress levels, grain-size strengthening, which is shown to follow a Hall–Petch relation, must be superimposed. An alternative explanation is strain-hardening due to the increase in dislocation density, which was measured by x-ray diffraction. At 600 ±C, the passivation increases the stress by an order of magnitude; this leads to a substantially different shape of the stress-temperature curves, which now resemble those of aluminum with only a native oxide layer. The effect of passivation is shown to be very sensitive to the deposition and test conditions.
I. INTRODUCTION
In the last few years, research on integrated circuit metallizations has focused on copper as a possible alternative to aluminum both because of its higher electrical and thermal conductivity, and because of its higher melting temperature and the correspondingly lower rates at which diffusive processes such as electromigration take place at the same operating temperature. However, copper is much stiffer than aluminum, which leads, for Cu films on Si substrates, to significantly larger thermal stresses for a given temperature change. Furthermore, copper diffuses rapidly into Si and SiO2 , and does not develop a native passivating oxide. Thus, Cu metallizations will always require encapsulation by diffusion barrier and passivation layers. To avoid potential reliability problems, it is necessary to understand the mechanical properties of Cu metallizations and how these are affected by barrier and passivation layers. The mechanical properties and microstructural evolution of continuous copper films with different barrier layers have been investigated previously.1–3 In particular, Vinci2 has shown that the stress-temperature behavior changes significantly when copper films are passivated and that the observed behavior is independent of the passivation layer thickness. However, a detailed report of the influence of a passivation on the stress evolution in Cu films as a function of film thickness has not been presented. It is well known that the mechanical behavior of thin films can be very different from that of bulk samples. It is found, for example, that thin films support very high stresses at room temperature.4 This can be explained on the basis of both dimensional5 and microstructural6 J. Mater. Res., Vol. 13, No. 5, May 1998
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