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ABSTRACT Polyimide thin films and coatings (< 20ýtm) for electronic applications have been fully characterized. Analysis of residual stresses and determination of all 21 anisotropic constants are necessary for a better understanding of these coatings in complex geometries. The residual stresses have been measured by a real-time vibrational holographic interferometry technique. The number of elastic constants is reduced to 9 independent constants by finding the orthotropic axes. All the 9 orthotropic constants - 3 normal compliances, 3 shear compliances and 3 independent Poisson's ratios - have been determined. Moisture diffusion constants have also been measured.

INTRODUCTION Polyin-ildes are widely used in electrical, electronic and aerospace applications due to their excellent mechanical and electrical properties, solvent resistance and thermal stability1 ,2. In recent years, polyimides have become commonly used insulators for microelectronics devices including advanced semiconductor chips and high density packaging. They have the advantages of low dielectric constants, low dielectric losses and low processing cost. Typical polyimides are used in a thickness range from 1-20 gtm. It is important to understand the thermal, hygroscopic and

mechanical properties of a microelectronics material as these affect the device structural integrity. Residual stresses built up from processing can lead to problems like delamination and cracking. These stresses result from shrinkage due to solvent loss and mismatch in the coefficients of thermal expansion (CTEs) of the polyimide and the substrate3. Polyimides having low CTEs, similar to that of the substrate, are hence preferred to lower the residual stresses. Some fluorinated polyimides have also been developed as they have lower dielectric constants and lower moisture absorption 4. These low CTE polyimides are usually anisotropic materials with different in-plane and out-of-plane properties. In applications such as multichip modules (MCMs), it is not feasible to measure the actual state of stress in complex geometries such as near holes, copper vias, walls, etc. Consequently, finite element methods are applied for reliability analyses of these structures 5. This necessitates the determination of far field (residual) stresses as the initial boundary conditions. An evaluation of the material elastic constants is also essential for mechanical and thermal stress modeling studies. A real-time vibrational holographic interferometry technique for the measurement of residual stresses has been developed 6 . This has even been used to determine the moisture diffusion constants 7. The orthotropic axes for these anisotropic materials can also be found by this technique. 529 Mat. Res. Soc. Symp. Proc. Vol. 356 @1995 Materials Research Society

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An anisotropic material has 21 independent elastic constants. Once the orthotropic axes of symmetiy are known, the number of elastic constants needed to fully characterize the material reduces to 9 independent constants for an orthot