Effects of Stress on the Adhesion of Metals to Polyimides
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EFFECTS OF STRESS ON THE ADHESION OF METALS TO POLYIMIDES
K. SESHAN AND R. H. LACOMBE International Business Machines Corporation
East Fishkill Facility, Route 52 Hopewell Junction, New York 12533 J. B. WAGNER, JR. Center for Solid State Science Arizona State University Tempe, Arizona 85251
ABSTRACT This paper describes the effects of ambient, thermal and mechanical stress on the adhesion of titanium (Ti) to polyimide (PI). Pull testing on the Ti/PI system shows that metal/polyimide bonding degrades when the composite is thermally cycled. A thermochemical mechanism is proposed that accounts for the interface degradation. We do not treat the adhesive interface as a discrete layer, but rather as a gradual "transition zone" between metal and polymer -- a zone that may grow, and/or change in composition and stress state, thus altering the adhesive properties. The mechanism predicts discontinuities that may develop in the transition zone. Through the use of finite element techniques, it is demonstrated that when loads and displacements are imposed on the composite containing interface discontinuities, large localized stresses develop. Such stresses could explain the experimentally observed low strength failures. INTRODUCTION It is critical in various applications in the electronics industry that there is maximal adherence of metals to polyimides. To ensure good bonding, an adhesion layer is included in a multilayered thin film structure. Systems of primary importance, listed by Mattox [1], are:
TABLE I MULILAYER SYSTEMS 2 LAYER Au/Ti Cu/Cr Au/Mo Au/Nb
3 LAYER Au/Pd/Ti Au/Pt/Ti Pd/Cu/Ti Au/Mo/Ti Au/W/Ti
Mat. Res. Soc. Symp. Proc. Vol. 130. c1989 Materials Research Society
4 LAYER Au/Rh/Pt/Ti Au/Ni/Cu/Ti Etc.
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These films follow a pattern: an adhesion layer -- usually a transition metal -- followed by functional layers, and capped by passivatlng layers. (Fig. 1).
".s.vl .. i0 ........ ........... 3 - PASSIVATION ......... ........... .2 - FUNCTION ......... .............. 1
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ADHESION
...........................
PT SUBSTRATE Fig.l. Typical metal multilayer structure, deposited on polyimide (PI) on substrate, usually silicon or ceramic.
Polymides used in this study are of the type PMDA-ODA made by E. I. DuPont de Nemours Inc., and by Hitachi Corp. under various trade names [2]. (PMDA =pyromellitic di anhydride; ODA = oxydianiline). Bond energy calculations by Ho et al. [3] and Johnson et al. [4] show that bonding occurs chiefly via the oxygen atoms of the polyimide and the metal atoms. The adhesion of the metal layers are typically in the 30-50 gm/mm range as deposited [5,6]. It is evident that the adhesion will change upon heating and exposure to ambient gases. Such changes are discussed in the next section. EXPERIMENTAL RESULTS Results of thermal cycling on the failure modes of titanium and chromium adhesion layers deposited on PT [7,81 are summarized in Figs. 2-5.
These findings provide the experimental basis for the proposed thermochemical mechanism. These references reflect a change from a cohesive
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