High resistance failures in P+ and N+ contact chains were traced to contacts partially filled with silicon dioxide (SiO 2 ) instead of the intended tungsten. Investigation revealed that oxygen (O 2 ) entered the deposition chamber through a faulty valve during silane gas (SiH 4 ) flow for tungsten seed deposition. This contamination triggered a gas-phase reaction producing SiO 2 particles that partially filled the contacts. Analysis of reaction kinetics explained the predominance of SiO 2 formation over tungsten deposition: the bond dissociation energy for SiO 2 formation is lower than that for tungsten, and SiO 2 -producing molecular collisions occur more frequently than tungsten-producing ones. The issue was resolved by replacing the leaking valve.

Failure Analysis labs involved in customer returns always face a greater challenge, demand from customer for a faster turnaround time to identify the root cause of the failure. Unfortunately, root cause identification in failure analysis is often performed incompletely or rushing into destructive techniques, leading to poor understanding of the failure mechanism and root-cause, customer dissatisfaction. Scanning Acoustic Tomography (SAT), also called Scanning Acoustic Microscope (SAM) has been adopted by several Failure Analysis labs because it provides reliable non-destructive imaging of package cracks and delamination. The SAM is a vital tool in the effort to analyze molded packages. This paper provides a review of non-destructive testing method used to evaluate Integrated Circuit (IC) package. The case studies discussed in this paper identifies different types of defects and the capabilities of B-Scan (cross-sectional tomography) method employed for defect detection beyond delamination.

Many semiconductor products are manufactured with mature technologies involving the uses of aluminum (Al) lines and tungsten (W) vias. High resistances of the vias were sometimes observed only after electrical or thermal stress. A layer of Ti oxide was found on such a via. In the wafer processing, the post W chemical mechanical planarization (WCMP) cleaning left residual W oxide on the W plugs. Ti from the overlaying metal line spontaneously reduced the W oxide, through which Ti oxide formed. Compared with W oxide, the Ti oxide has a larger formation enthalpy, and the valence electrons of Ti are more tightly bound to the O ion cores. As a result, the Ti oxide is more resistive than the W oxide. Consequently, the die functioned well in the first test in the fab, but the via resistance increased significantly after a thermal stress, which led to device failure in the second test. The NH4OH concentration was therefore increased to more effectively remove residual W oxide, which solved the problem. The thermal stress had prevented the latent issue from becoming a more costly field failure.

Coating of the Cu bond wire with Pd has been a rather widely accepted method in semiconductor packaging to improve the wire bonding reliability. Based on comparison of a Cu bond wire and a Pd-coated Cu bond wire on AlCu pads that had passed HAST, new insight into the mechanism of the reliability improvement is gained. Our analysis showed the dominant Cu-rich intermetallics (IMC) were Cu3Al2 for the Cu wire, and (CuPdx)Al for the Pd-coated wire. The results have verified the Cu-rich IMC being suppressed by the Pd-coating, which has been extensively reported in literature. Binary phase diagrams of Al, Cu, and Pd indicate that the addition of Pd elevates the melting point and bond strength of (CuPdx)Al compared with CuAl that formed with the bare Cu wire. The improvements are expected to decrease the kinetics of phase transformation toward the more Cu-rich IMC. With the suppression of the Cu-rich IMC, the corrosion resistance of the wire bonding is enhanced and the wire bonding reliability improved. We find that Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to improve the corrosion resistance.

Cu wires were bonded to AlSi (1%) pads, subsequently encapsulated and subjected to uHAST (un-biased Highly Accelerated Stress Test, 130 °C and 85% relative humidity). After the test, a pair of bonding interfaces associated with a failing contact resistance and a passing contact resistance were analyzed and compared, with transmission electron microscopy (TEM), electron diffraction, and energy-dispersive spectroscopy (EDS). The data suggested the corrosion rates were higher for the more Cu-rich Cu-Al intermetallics (IMC) in the failing sample. The corrosion was investigated with factors including electromotive force (EMF), self-passivation of Al, thickness and homogeneity of the Al-oxide on the IMC, ratio of the Cu-to-Al surface areas exposed to the electrolyte for an IMC taken into account. The preferential corrosion observed for the Cu-rich IMC is attributed to the high ratios of the surface areas of the cathode and anode that were exposed to the electrolyte, and the passivation oxide of Al with the lower homogeneity. The corrosion of the Cu-Al IMC is just a manifestation of the well-known phenomenon of dealloying. With the understanding of the corrosion mechanisms, prohibiting the formation of Cu-rich IMCs is expected be an approach to improve the corrosion resistance of the wire bonding.