Coupled thermal electric-modeling of flexible nanospring interconnects for high-performance probing

Abstract

Advances in integrated circuit fabrication have given rise to 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 nanospring 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 /spl mu/m. These nanosprings are designed to accommodate topological variation in probing surfaces while flexing within the elastic regime. For both probing and packaging applications, these nanosprings will service as current carriers and will heat up as a result of Joule heating. In probing applications, a current of about 200 mA is passed through these probes. Due to their extremely small dimensions, the electrical resistance of-these nanosprings will be higher than in conventional probes. Moreover, the existence of a sharp tip implies very high current densities at the spring tip. This, in turn, could result in higher temperatures at the spring tip. These higher temperatures could be detrimental to the reliability of the wafer that is being probed. It is important to determine the maximum temperature reached during probing, and how that, maximum temperature could be reduced. Coupled thermal-electric finite element models have been developed to understand the thermal contours developed across these springs, and to determine the maximum temperature reached. The current density through the nanospring is determined through an electrical model, and temperature contours are determined from the coupled electro-thermal model. The magnitude and the location of the maximum temperature depends on the spring geometry and the spring tip-bonding pad interface. Optimization of the spring geometry to reduce this maximum temperature is outlined. The role of depth of penetration in the bonding pad, on the maximum temperature and the electrical resistance is explained. Temperature dependent variation of resistivity of the spring material is also studied, and incorporated in the model. In addition, the role of scale effects on the thermal conductivity of the spring material is studied and incorporated in the model to study their impact on the temperature profile and overall resistance. Thermoelectric effects are also studied, as related to the nanosprings. Finally, transient coupled thermal electric analysis is performed to determine the time it takes for the temperature and voltage to reach steady state.