Study of Mechanical Behavior of Compliant Micro-Springs for Next Generation Probing Applications

Abstract

Advances in integrated circuit fabrication have created a need for an innovative, inexpensive, yet reliable probing technology with ultra-fine pitch capability. Research teams at Georgia Tech, Xerox PARC, and NanoNexus, Inc. are developing flexible micro-spring structures that can far exceed the probing needs of the next-generation microelectronic devices. Highly compliant cantilevered springs have been fabricated at pitches as small as 6 μm. These micro-springs are designed to accommodate topological variation in probing surfaces while flexing within the elastic regime. To be able to use the micro-springs for probing applications, several design challenges must be addressed. When the probe head is brought into contact with the bonding pads, the micro-springs will slide across the surface of the bonding pad and establish contact. The bonding pads typically have surface oxides. Thus, from a mechanical standpoint, it is important to ensure that the springs would apply enough force to break through the surface oxides and establish good electrical contact. The damage done to the pads in the process has to be minimal. It is also important that the distance through which the springs will slide across the bonding pad surface does not exceed the pad dimensions. From a mechanical fatigue standpoint, the stress amplitude that the springs will be subjected to, needs to be within the elastic limit of the spring material. This will enhance the life of the micro-spring probes. Typical probing devices are expected to last about half a million touchdowns. Numerical models and sub-models have been developed to simulate the mechanical contact between a single spring and the bonding pad, and to determine the probing force. The model simulates the establishment of contact, sliding, and indentation resulting in plastic deformation of the pad. The length of the scrub mark and the indentation depth are validated with experimental measurements using focused ion beam. The spring geometry parameters are varied and their influence on the penetration depth studied. Finally, the variation of contact resistance with probing force is outlined.