Yield Stress in Single Crystal and Polycrystalline Al Films
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ABSTRACT The stresses in single crystal and polycrystalline Al films were determined by wafer curvature measurements during temperature cycling. Both elastic and plastic behavior was observed. The single crystal film exhibited anisotropic yield stresses indicating that dislocation glide is the dominant deformation mechanism. The polycrystalline film exhibited similar behavior, although without the anisotropy, indicating that it too deforms by dislocation glide. Unlike the single crystal film, the yield stress in the polycrystalline film varied depending on thermal and deformation history. These effects are attributed to the presence of grains and grain boundaries in the polycrystalline film.
INTRODUCTION During processing, microelectronic devices undergo temperature excursions of several hundred degrees. Because of the difference between the thermal expansion coefficients of the metallization and surrounding materials, these temperature changes generate large stresses in the metal interconnect structures, which can lead to reliability problems such as electrical shorts and opens. The details of these problems depend on the elastic and plastic properties of the metallization. In order to understand these properties, the stresses in thin polycrystalline Al films have been studied extensively and have been found to depend strongly on the film thickness and grain size'. The purpose of this study is to compare the stresses in single crystal and polycrystalline Al films, that were deposited and measured under similar conditions, in order to assess the effect of polycrystallinity on the mechanical properties.
EXPERIMENTAL DETAILS The single crystal Al films (0.35gm thick) were deposited by evaporation from a pure Al source onto a (110) oriented NaCl single crystal held at 350 °C 2. After deposition, the NaCl was dissolved and the films rinsed and transferred to an oxidized Si substrate. Finally, they were annealed at 300 'C in air for two weeks to promote the adhesion of the Al to the SiO 2. The polycrystalline Al films ( 0.44gm thick) were deposited in the same system by evaporation from a pure Al source directly onto an unheated, oxidized Si substrate. Rutherford Backscattering Spectrometry, with 2 MeV He ions, was used to measure the film thicknesses and composition; no impurities with an atomic weight larger than that of Al were detected. X-ray diffraction showed the single crystal film was (110) oriented and of good quality, since no other crystallographic orientations were found and the full width of the (220) reflection at half the maximum value (FWHM) was only 0.50. X-ray diffraction measurements of the polycrystalline film revealed a strong. (111) texture with a FWHM of 14'. In addition, Scanning Electron Microscopy (SEM) at 5 kV and Focused Ion Beam measurements with a 30 79 Mat. Res. Soc. Symp. Proc. Vol. 391 01995 Materials Research Society
keV Ga÷ beam were used to study microstructural features of the samples before and after temperature cycling, the results of which will be discussed later. In a thin film
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