Physical Properties and Stress Analysis of Low Dielectric Polyimide Films Containing Adamantane Pendant Group

Abstract

ABSTRACT:Low dielectric polyimides were synthesized from a new diamine, bis(4-aminophenyl)-1-adamantyl phosphine oxide (DAAPO) containing adamantane cage structured group in the pendant position. Thermomechanical properties were investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and thermomechanical analyzer (TMA). The thickness dependence of dielectric constant was measured using a LCR meter, and the results were compared with those of other polyimide systems reported. Variation of stress during film coating and annealing processes was analyzed using a bending beam curvature measurement system complemented with an in-situ film thickness measurement system, the interferometer. Stress was developed as solvent evaporated during film baking process, and the residual stress was obtained when the coated films were prepared. The stress behavior was significantly affected by ramping and annealing schedules.KEY WORDS Polyimide / Dielectric Constant / Adamantane / Cage Structure / Stress / Aromatic polyimides have been used as dielectric, coating, and adhesive layers in microelectronics fabrications, because they have excellent electrical, chemical, thermal, and mechanical properties compared to other organic polymers. [1][2][3][4][5] In order to fabricate a high performance chip, however, solutions to some problems should be addressed. One is about the dielectric performance. As dielectric constants of commercial polyimides (PIs) are reported to be higher than 3, it should be lowered to transport more signals through more integrated circuits. The residual stress is another problem, especially in the thin polymer film application for chip and wire packaging. Stress is usually developed by thermal expansion difference between the film and substrate. Organic/inorganic pairs like PI/silicon wafer imposed high stress due to relatively high thermal expansion difference. When it is considerable in multiplayer fabrication process, it sometimes results in cracking, delamination, or bending of films.Among many trials to reduce the dielectric constant and residual stress, one has recently proposed introduction of nano or molecular foams in PI matrix. This was based on the idea that the dielectric constant of air is close to 1 and that the stress is significantly affected by relative density of polymer foams, ρ/ρ 0 ; moduli of foams are known to be proportional to (ρ/ρ 0 ), 2, 6 under the circumstances that thermal expansion coefficients of bulk and foam polymers are almost the same. More than 10 year trials for insertion of nano or molecular foams in PIs are summarized by two approaches: i) the phase separation by heat and non-solvent during preparation of PI systems, 7 and ii) the synthesis of block copolymer composed of polyimide and thermally degradable polymer. 8 Thermal degradation of the second component leaves nano-scaled pores behind it. Not many of these, however, were successfully industrialized, because their preparation process was too complicated and the resulting material pr...

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