Ph.D. Thesis Defense by Mitul Modi
Fri day, April 4, 2003
(Dr. Suresh Sitaraman, advisor)
"Fracture in Stress-Engineered, High Density, Thin Film Interconnects"
With the continued reduction in feature size and with the increased demand for better performance and lower cost in microelectronics, there is a need for an innovative, yet reliable, chip-to-next level interconnect technology to meet the International Technology Roadmap for Semiconductor (ITRS) requirements for year 2016 and beyond. In this work a novel, highly-compliant, cantilevered interconnect structures is being developed in order to meet the future industry requirements. The new technology, called micro-contact springs, is a cost competitive, highly compliant cantilever plate fabricated through common thin film processing and IC fabrication techniques. Micro-contact springs are sputter deposited thin films with a engineered stress gradient through the thickness of the films – relaxation of the intrinsic stress gradient causes the spring layers to curl up, forming the spring interconnects. When fabricated on a wafer these interconnects can be transitioned to WLP (wafer level packaging) or FCOB (flip chip on board) needs. When fabricated on a substrate, these interconnects become probes for WLP test and burn-in applications.
The high stress gradient placed in the springs and the large stress concentration at the spring/substrate interface introduces a good candidate for cohesive and interfacial fracture. Complexities of large displacement and curvature bifurcation, anisotropies due to physical vapor deposition, local plastic deformation, and other nonlinearities suggest the need to understand how to prevent fracture in micro-contact springs. Analytical methods to predict delamination during fabrication, numerical models to predict crack growth during complex loading situations in packaging and probing applications, and an experimental method, based on a novel modification to the dechosion test, to efficiently measure the mode mixity dependent interfacial fracture toughness in thin film interfaces, are discussed. General governing design guidelines to enhance the performance of the micro-contact springs, while not compromising its reliability with respect to fracture, are also addressed.