High frequency electrical properties of copper interconnects patterned by resolution enhanced lithography

Abstract

Signal integrity problems associated with on-chip interconnects have become very significant with increase in device integration and circuit frequency. Copper and low-κ dielectric materials are used to improve electrical performance of interconnects for integrated circuits. At radio and microwave frequencies, the signal propagation behaviour of on-chip interconnects are complex to analyze and predict especially for lossy lowresistivity silicon substrates. Interconnects behave as transmission lines, and signal delay, transients, crosstalk and power dissipation become critical. In addition, integration of mesoporous dielectrics and scaling of feature size has made the patterning and processing of damascene interconnects far more challenging than initially anticipated. The fabrication, high-frequency characterization and modeling of copper interconnects are the objectives of this thesis. This work is divided into two parts. First, the fabrication of deep submicron (100-250 nm) copper interconnects with porous ultra low-κ dielectrics in a damascene process is addressed. Resolution enhanced optical lithography is used to pattern copper interconnects in a step and scan system with a 248 nm excimer laser source. A mask with alternating phase shifted sub-resolution assist features is designed to enhance the resolution and process latitude of the lithography process. The spurious reflections associated with deep ultraviolet exposure of transparent film stacks and resist poisoning effects are systematically studied and solutions demonstrated. In addition, the effects of plasma processing on the structure, composition and electrical properties of an ultra low-κ dielectric and the microstructure of barrier layer deposited on it are investigated.