Temperature Dependence of Stress Relaxation at the Copper/Polyimide Interface
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TEMPERATURE DEPENDENCE OF STRESS RELAXATION AT THE COPPER/POLYIMIDE INTERFACE S.T. CHEN, C.H. YANG, H.M. TONG and P.S. HO IBM Thomas J. Watson Research Center P.O. Box 218, Yorktown Heights, New York 10598 ABSTRACT A Cu/polyimide thin film couple prepared on a thin quartz reed has been used to study interfacial stress relaxation during thermal cycling between room temperature and 400 *C. The polyimide thickness varies from 0 (no polyimide at all) to 10.51Mm while the Cu thickness was fixed at 0.53j•m. The average copper film stress has been calculated from the curvature of the quartz reed. The information clarifies the relation between the polyimide thickness and the average Cu film stress. The Cu/polyimide interfacial morphology after thermal cycling has also been examined using the cross sectional TEM technique. The results suggest that the interfacial stress is partially released through the deformation of polyimide near the Cu/polyimide interface. INTRODUCTION The Cu/polyimide thin film couple has been widely used both in high end and low end electronic packaging areas. Copper has the advantage of high conductivity while polyimide has the advantages of low dielectric constant and low processing temperature which make it well suited to form multilayer structure for packaging applications. The thermal cycling during the fabrication process can generate a high level of stress at the interface due to the thermal mismatch between metal and polymer. The high level residual stress generated during the cooling period has been observed to delaminate or crack the metal film. It is, therefore, important to understand the mechanical properties of the metal/polyimide interface, particularly the nature of stress relaxation at the interface. This paper reports a measurement of the stress generated at Cu/polyimide interface during thermal cycling. Cross-sectional transmission electron microscropy has been empolyed to examine the change in the structure and morphology accompanied the stress cycling. These studies aim at exploring the nature of the stress state and its relaxation mechanism. SAMPLE PREPARATION The substrate used for stress sample is fused quartz strip (1.5"x 0.123"x 0.005"). The fused quartz has good elastic properties in the temperature range of the experiment, in addition polyimide adheres well on fused quartz substrate. The polymer used is a polyamic acid solution similar to DuPont PI-2545. The solution was spun on the substrate. The thickness of the polymer was controlled by the spinning speed and the concentration of the solution. After spinning, the polymer sample was baked at 85°C for 10 minutes for solvent removal and then cured for 30 minutes each at 1500C, 230°C, 300°C and 400'C in nitrogen to form the polyimide. The polyimide film stress between 20"C and 450 'C was measured before copper deposition. The 0.531m copper was e-beam evaporated after a 80'C bake in vacuum. The copper film on all substrates are deposited at the same time to assure that the Cu thickness is the same for all samples.
Mat. Res.
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