Solutions to catastrophic yield problems in MCM-D interconnection production

Yield issues are a constant concern during production of an MCM circuit with 1.0 mil lines, 3 aluminum layers separated by polyimide insulator, 250 traces per square inch, and a top layer of aluminum and gold bond pads. The complexity of the circuit creates numerous defect opportunities, but two failure modes are catastrophic. These include megohm shorts and selective gold/aluminum layer defects such as blistering gold, voids and discoloration of aluminum bond pads. Designed experiments are performed to elucidate the root causes of the problems and determine and implement corrective actions. Megohm short testing includes process flow evaluations (lift-off vs. etchback), metal wet etches, and TiW oxidation. Results indicate that megohm shorts are eliminated by changing from a lift-off to an etchback process. Experimental parameters for selective gold/aluminum layer defects include metal deposition, TiW etch, resist strip, and polyimide etch. Significant interactions are observed between the parameters studied and test part analysis shows a gold/aluminum displacement reaction on the aluminum pads. The design of experiments, results, and corrective actions are presented. Tests showed that the aluminum deposition was the root cause of the problem. Discontinuation of use of the metal deposition machine resulted in increased yields.