Characterization of Thin Films by Internal Friction Measurements
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CHARACTERIZATION OF THIN FILMS BY INTERNAL FRICTION MEASUREMENTS H.G. BOHN AND C.M. SU**
Institut fiir Festk6rperforschung, Forschungszentrum Jfilich, Postfach 1913, 5170 MJilich 1, Germany *On leave of absence from Institute of Solid State Physics, Academia Sinica. tPresent address: University of Maryland, College Park MD 20742.
ABSTRACT
Internal friction has been employed to characterize various properties of thin Al and Al-alloy films. The grain boundary relaxation peak was used a) to determine the activation energies for grain boundary diffusion in the alloy films, b) to investigate the influence of impurities on the grain boundary diffusion in Al at concentration levels as low as 300 ppm, and c) to get information about the adhesion strength between the film and the substrate.
INTRODUCTION Internal friction (IF) has been widely applied to the investigation of defects like dislocations, grain boundaries (GB), point defects, etc. in bulk materials [1,2]. Due to the advances of the experimental technique it became possible to also apply it to the investigation of thin films on substrates where the ratio of the film thickness to that of the substrate typically is of the order of 1/100 [3]. Thin Al or Al-alloy films on Si-substrates are of particular interest as these materials are commonly used in the metallization of microelectronic devices. Here mechanisms like electro- and stressmigration have been identified as the main source for device failure. They are primarily controlled by GB diffusion. In their pioneering work Berry and Pritchet observed the mechanical relaxation in pure Al-films deposited on silica substrates [4]. From their experiments they derived an activation energy of 0.55 eV which is equal to that for GB diffusion. They concluded that the relaxation occurs via a GB sliding mechanism. Thus it seemed to be promising to apply IF as a tool to characterize thin films with respect to properties which are governed by GB processes. In this contribution we will demonstrate how the technique of IF is advanced to be applicable to any coated substrate, in particular to industry processed wafers, and how it is applied to the determination of GB diffusion activation energies of Al-alloy films and the study of thin film reactions. We will also show that it has the potential to non-destructively characterize the adhesion of the film to the substrate.
INTERNAL FRICTION IF basically measures the dissipation of elastic energy in an oscillating sample due to the stress induced motion of defects in solids. 1 In our experimental setup the sample is driven to a resonant vibration and the damping Q- is obtained from the time constant T1 of the free decay of the vibrational amplitude after switching off the excitation. For sufficiently low damping (Q- 1 4. 1) and exponentially decaying signals it holds that Q-_
1
7TfT
1 1
2ff
AW
W
Mat. Res. Soc. Symp. Proc. Vol. 239. 01992 Materials Research Society
216
where f is the frequency of the sample and AW/W the elastic energy dissipated per cycle normalized to th
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