Stress in Thermally Annealed Parylene Films
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STRESS IN THERMALLY ANNEALED PARYLENE FILMS S. Dabral, J. Van Etten, C. Apblett, G. R. Yang, P. Ficalora, and J.F. McDonald Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180. Abstract Stress generated by thermal annealing of thick parylene-n (PA-n) films (4-7 pm) has been studied. We examined the behavior of stress for as-deposited films and rapidly thermally annealed films. The annealing was done in vacuum and the stress measured after the anneal. In another experiment the stress was also studied in-situ with thermal annealing. The as-deposited films are under compressive stress of about 18 MPa. When the backside film was removed by plasma etching, the stress changed from compressive to tensile. As the films are annealed, the stress decreases and then becomes tensile, peaking at about 60 MPa. The film crystallinity and stress are correlated for PA-n films. It was observed that stress is reduced during the crystal phase transformations at around 2200C and 2700C. This property can be used to relieve stress in multilevel structures. It was also found that slower heating and cooling led to lowered stress. The stress has been compared to reported values from other sources. Stress in PA-c (chlorinated) and co-polymerized PA-n and -e (esterfied) films has also been reported. I. Introduction Parylene is being investigated as an alternative to polyimides being currently employed for interconnection applications in computer hardware. Thick films, together with low dielectric constant, are necessary for fast interconnection applications. The stress build up in these layers can have a deleterious effect on device and interconnect performaniceu1,21. The lower dielectric constant (2.36-2.65) and vapor deposition capability make it a highly suitable candidate for fast interconnection systems [3,4]. There is no thermal curing required for parylene, as is needed for polyimides. The stress in these filns is investigated and reported here. Some early studies of stress in parylene films were clone by Beach [5] and Bachmann [6], who found the stress to be compressive IS MPa. The stress was invariant with thickness. However, effects of thermal cycling have not been studied. The thermal effects on stress are important because these are normally experienced during fabrication and chip joining processes. II. Experiment Early in the investigation it was realized that there was a loss of parylene when the annealing was carried out in air with temperatures in excess of 300 0 C. The oxygen attacks the polymer at elevated temperatures causing it to dissociate slowly. Lowering of the partial pressure of oxygen helps in reducing such losses. For this reason, all further studies were carried out in vacuum of 50-200 mTorr. In one set a nitrogen ambient was provided to further reduce such filn loss. The stress was measured by measuring the changes in radius of curvature. To measure this curvature changes in the separation of two parallel laser beam spots which reflected off the sample were recorded. This setup has
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