Experimental Determination of the Strain Transfer Across a Flexible Intermediate Layer in Thin Film Structures as a Func
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EXPERIMENTAL DETERMINATION OF THE STRAIN TRANSFER ACROSS A FLEXIBLE INTERMEDIATE LAYER IN THIN FILM STRUCTURES AS A FUNCTION OF FLEXIBLE LAYER THICKNESS L.S. SCHADLER AND I.C. NOYAN IBM Reseurch Division, T. J. Watson Research Center, PO Box 218, Yorktown Heights, NY 10598 ABSTRACT The tensile deformation behavior of nickel/ polyimide-thin-film / copper-thin-film structures is observed using an in situ x-ray stress analysis technique. The x-ray stress is measured in the nickel and the copper simultaneously. From the analysis, the stress/strain transfer from the nickel through the polyimide to the copper can be measured. The effect of polyimide thickness (in the range from no polyimide to 33gtm of polyimnide) on the stress/strain transfer behavior is reported here. INTRODUCTION In this paper the transfer of stress/strain across compliant polymer passivation layers is analyzed theoretically and experimentally. This is an industrially important issue because the electronics industry utilizes such compliant polymers as passivation between levels of metallization. Thus, in order to design interconnects with good thermal and mechanical stability, an understanding of the mechanical interaction between the layers is important. As a result of the interest in metal polyimide layers, there have been many studies on the residual stress and properties of polyimide (Pt) thin films [1-4], metal thin films [5-7], the chemical interactions between PI and metal thin films [8,9], and metal / PI adhesion [10]. Stresses have also been measured in multi-layer structures [11-15] and modeled using continuum models [16-18] and finite element analysis [19-20]. The first in situ direct measurement of the strain transfer through an intermediate P! layer was reported recently [21]; the stress transfer was measured in a Ni / PI thin film / Cu thin film structure using an x-ray tensile testing method 1_22]. The sample (shown in figure I-a) was stressed in situ on a diffractometer; the applied stress and strain as well as the Cu and Ni x-ray stresses were measured simultaneously. Because the applied stress and strain are known, as are the separate Ni and Cu stresses, the stress transfer behavior could be studied directly without assuming a perfect interface. The conclusions were: 1) the stress in the Cu did not equal the value predicted by continuum theory indicating that the interface region is not ideal, and 2) the stress in the Cu was the same in a two layer Ni/Cu sample as it was in a 3 layer Ni/PI/Cu sample indicating that the PI had no effect on the stress transfer behavior. In this paper the continuum model developed by Chen and Nelson [19] has been modified to fit the specific geometry and loading situation. The model predicts that the PI will have no effect on the stress transfer as was observed in the previous report. In order to further test this result the stress transfer behavior has been studied as a function of PI thickness for the range of thickness from 0 to 33ttm.
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