Finite Element Analysis of the Mechanical Performance of Benzocyclobutene Structures in Multichip Modules
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FINITE ELEMENT ANALYSIS OF THE MECHANICAL PERFORMANCE OF BENZOCYCLOBUTENE STRUCTURES IN MULTICHIP MODULES. E. 0. SHAFFER II, P. H. TOWNSEND, M. J. RADLER, AND C. J. CARRIERE The Dow Chemical Company, M. E. Pruitt Research Center 1702 Building, Midland, MI 48674.
ABSTRACT Polymeric materials are used as interlayer dielectrics for multichip modules. One of the central challenges for these structures is management of the large stresses induced by the differences in the thermal properties of the inorganic component structures and the organic film during processing. Finite element analysis has been used to model the stresses in the dielectric material in practical structures used in multilayer microelectronic architecture, such as interlayer vias and the associated internal metal patterns. Calculations have been performed on the stress field at the corner of square vias in benzocyclobutene based interlayer dielectrics and related polymeric materials. The effects of the thermal and mechanical properties of the polymeric coating on the stress fields in metal patterns have been calculated and provide insight for design and process optimization.
INTRODUCTION Recently, great interest has developed in the electronics industry in the use of benzocyclobutene (BCB) thermosets as interlayer dielectrics for multichip modules. This interest is due to the advantages of BCB based dielectrics. These advantages include a lower dielectric constant, smoother planarization, higher temperature stability, and lower moisture adsorption. However, BCB thermoset systems suffer the same disadvantage as all polymeric coatings in that they have a higher coefficient of thermal expansion than both internal metal structures and substrate materials. This expansion difference results in thermal residual stresses in both the polymeric coating and internal metal structures during fabrication. Many techniques are available for measuring average stresses in the film and metal structures. These methods include curvature techniques for measuring biaxial far-field stresses [1] in the polymeric coating and x-ray diffraction methods for measuring triaxial stresses [2] in metal structures. However, both techniques are limited to measuring only average stresses and cannot provide information on stress contours about complex geometries of internal metal structures. Finite element methods can be used for calculating stress contours about internal structures. In this paper, we use finite element methods for calculating the stresses for internal metal structures coated by Dow's divinyl-siloxane bisbenzocyclobutene (DVS-BCB, CAS 117732-87-3) thermoset resin as the part is cooled from the curing temperature of the resin. Two typical metal internal structures are evaluated. First, we model the stresses in a metal line on a silicon substrate coated with DVS-BCB after thermal processing. To establish the model's validity, we compare the calculated stresses to x-ray diffraction data of the triaxial stresses in the metal line and to curvature measurements of the far-field
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