Processing thick multilevel polyimide films for 3-D stacked memory

This paper discusses thick polyimide film processing for a three-dimensional (3-D) semiconductor chip-stacking application. The formation of a complex, multilevel via structure is demonstrated. The issues that arise in forming these vias relate to apply, develop, profile modification, and integration. Apply issues include "outgassing" defects, edge-bead effects, as well as the planarity and leveling of both resist and polyimide over deep-via structures. Develop issues pertain to the implementation of a thick resist process that increases the structural integrity of the resist and controls its breakage, and to a vacuum bake before applying resist, which reduces solvent absorption into the resist. Profile modification issues include rounding via edges while minimizing bulk polyimide loss and maintaining image-size control. Developer attack of the metal pads during wet processing is discussed and a solution is proposed. Finally, additional process-integration issues relating to polyimide-to-metal adhesion and composite stress levels of the multilayer thick films are presented.