X-ray micro-beam diffraction measurement of the effect of thermal cycling on stress in Cu TSV: A comparative study

Abstract

Microelectronic devices are subjected to constantly varying temperature conditions during their operational lifetime, which can lead to their failure. In this study, we examined the impact of thermal cycling on the evolution of stresses in Cu TSVs using synchrotron-based X-ray microdiffraction. Two test conditions were analyzed: as-received and 1000 cycled samples. The principal and shear stresses in the 1000 cycled sample were five times greater than in the asreceived sample. This was attributed to the increased strain hardening upon thermal cycling. The variation in stresses with thermal cycling is a clear indication that the impact of Cu TSV proximity on front-end-of-line (FEOL) device performance will fluctuate throughout the lifetime of the 3D stacked dies, and thus should be accounted for during FEOL keep-out-zone design rule development. INTRODUCTIONStress-related reliability challenges are known to be one of the main causes of failure in electronic devices. This failure type arises due to the mismatch in the mechanical properties of the constituent materials. One of the causes of stressrelated failures is the continuous fluctuation of temperature in microelectronic devices during their in-service usage [1]. This leads to thermal fatigue occurrence, and subsequently to failure.The emergence of three-dimensional stacked integrated circuits further increases the risk of thermal fatigue failures. This is because the electrical connection between the stacked dies are achieved using mainly Cu through-silicon vias (TSVs), which are fabricated through the active silicon. The large dissimilarity in the material properties of the Cu TSV and the surrounding Si can result in huge stresses.In order to understand and mitigate thermal fatigue concerns in Cu TSV interconnects, many studies have been reported that have assessed the thermal cycling effect on their reliability performance [2], [3], [4], [5]. These reported studies were focused on understanding how thermal cycling impacts the electrical characteristics of Cu TSVs, as well as the use of focused ion beam (FIB) and scanning electron microscopy (SEM) failure analysis tools to identify damage growth and propagation. However, no reported study has been able to experimentally relate the underlying root cause of the changes in the Cu TSV, being the buildup of stresses, with the witnessed number of thermal cycles.

Experimentally, how does Cu TSV diameter influence its stress state?

“…The X-ray microdiffraction measurements were performed at sector 34-ID-E, of the Advanced Photon Source (APS), Argonne National Laboratory. The experimental procedure used is similar to those previously reported in [10], [11], [12], [13], [14]. The procedure includes the use of both polychromatic and monochromatic X-rays, three area detectors and a depth profiler.…”