Effect of polyimide processing on multichip glass ceramic module fabrications

This paper discusses the yield analysis of the thin film wiring layers fabricated on the 127 mm multichip glass ceramic modules (MCM-D), currently used on the IBM Enterprise System/9000 family of computer processors. To select a suitable polyimide (PI) for the thin film wiring layer, modules were fabricated with either the 3,3´,4,4´-benzophenone tetracarboxylic dianhydride-1,3-bis(3-aminophenoxy)benzene (BTDA-APB) or the pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide. By keeping all other processing parameters and structures the same, the wiring layers fabricated with the PMDA-ODA polyimide exhibited significantly better yield than those made of the BTDA-APB PI. The yield loss in the modules fabricated with the BTDA-APB PI occurred during thermal processing, where some of the transmission lines fabricated atop the PI were found cracked. Further investigations indicate that, during lift-off processing, the presence of a small contaminant, such as a fiber, metal flake, a particle, or polishing scratches can expose the PI to the hot N-methylpyrollidinone (NMP) solvent. Depending on the polyimide used for thin film processing, the diffusion and swelling of the PI by the low molecular weight organic solvent can potentially produce significant damage to the polyimide/Cu wiring structure and the consequent yield loss of the modules due to line opens. The magnitude of the damage was found to depend on the rate of solvent diffusion, process temperature, lift-off time and the amount of PI swelling during processing. To understand these important process parameters in detail, in-plan solvent diffusion and PI swelling in the PMDA-ODA and BTDA-APB films were quantitatively measured. It was found that the rate of in-plane (X-Y plane) solvent diffusion in BTDA-APB is D=0.0617 ×ex-(-9.4 Kcal/RT) cm²/s versus D=0.00155×exp(-10.24 Kcal/RT) cm²/s for the PMDA-ODA film. Both types of PI films were fully imidized at 375°C in nitrogen for 1 h. Thus, the rate of lateral diffusion of NMP in BTDA-APB is approximately 150 times faster than that of the PMDA-ODA film at 85°C. Furthermore, the swelling of PMDA-ODA PI in the 85°C NMP bath is 45% versus 82% for the BTDA-APB PI. The poor elongation property of the BTDA-APB film (strain at break ~4-7% versus 90-130% for the PMDA-ODA PI) further aggravated the solvent problem, BTDA-APB was always found cracked or crazed after being exposed to the NMP solvent, making it very vulnerable to the lift-off process. Consequently, by selecting the PMDA-ODA polyimide for module manufacturing, along with an improved clean room manufacturing facility, the glass ceramic MCM-D has become an extremely reliable product with excellent manufacturing yield