Corrosion of the Cu/Al interface in Cu-Wire-bonded integrated circuits

Abstract

A model for corrosion induced failure in wire bond devices made with either Cu or Au wire was developed. The model is based on detailed analysis of the chemical composition, crystallography, and microstructure of the corrosion induced failure sites. The detailed analysis was enabled by both a scanning electron microscope (SEM) and a transmission electron microscope (TEM) equipped with the appropriate analytical detectors.The combined characterization results were used to develop a detailed failure mechanism model which explains not only the overall chemical reactions but also the resultant two phase microstructures of the corrosion product observed in both metal systems. In addition the model helps to explain why Cu wire bonded devices are more susceptible to corrosion than Au wire bonded devices. In both systems corrosion occurs in the IMC not the end components pure metals. It is proposed this is due to the oxide on the surface of the IMCs being less resistant to pitting corrosion than that for Al. Once IMC passivity is broken down, corrosion of the IMC proceeds via selective oxidation of Al. This leads to the formation of a corroded region which is composed of a two phase microstructure, crystalline -Al2O3 with embedded crystalline Au and Cu metal particles. The resulting oxidized interface is highly susceptible to fracture, which is the ultimate reason for device failure.Although similar there are differences in the Cu/Al and Au/Al systems. There is a notable difference in the size and distribution of Cu particles in the aluminum oxide corrosion product of the Cu-Al system as compared to the Au particles in the aluminum oxide corrosion product of the Au-Al system. In particular, the Cu particles appear to be more uniformly distributed as compared to their Au counterparts. This difference is likely related to the crystal structure of the IMC from which the corrosion product was formed. It is proposed that this difference is related to the presence and probability of Al to Al bonding in the IMC phase/s. Furthermore, the IMC structures are also suspected to be responsible for the better immunity to contamination (specifically Halide) induced corrosion of Au relative to Cu.

The corrosion performance of Cu alloy wire bond on Al pad in molding compounds of various chlorine contents under biased-HAST

“…This is because CuAl 2 was corroded by Chlorine from molding compound resulting in Cu reduction. [13] However, no obvious damage was found at the interface between Cu and Al after 1056h of biased-HAST as shown in Figure 7(b). Figure 7(d) is the enlarged image of the selected area of Figure 7(b).…”

“…Presumably the CuAl 2 should be corroded during biased-HAST testing. [8,13] The corrosion behavior will discuss later. Figure 7 represents the cross section images of failed bare Cu wire and Pd/Cu wire with high Cl -molding compound after 1056h of biased-HAST.…”

Review on silver wire bonding

“…Furthermore, its higher modulus reduces wire sweep concern for fine pitch wire bonding. However, there are some drawbacks of Cu wire in terms of material properties which raise reliability concern and decrease productivity: (a) Cu is much harder than Au, no matter wire or FAB, the structureunderneathbondpad of the IC chip is more susceptible to be damaged, which narrows the wire bonding process window [2]; (b) Cu wire bonding makes serious "Al splash" compared to Au wire bonding, which hinder finer bond pad pitch design [2]; (c) CuAl IMC (intermetallic compound) is reported to have shorter HAST life than AuAl IMC's, which causes reliability concern [3,4]; (d) Cu wire bonding is challenging to apply to stackeddie configuration and dietodie wire bonding, due to its high hardness and stiffness; (e) Cu is susceptible to be oxidized, the bondability of second bond is impacted, although Pd coated or Pd/Aucoated Cu wire can alleviate this problem, however material cost increasing as a penalty; finally, (f) Cu wire bonding has lower production throughput as well as process yield, in general, this is due to its high hardness (requiring longer bonding time and larger bonding power) and narrower process window (implying lower yield).…”

Corrosion-induced fracture of Cu–Al microelectronics interconnects

“…Experiments by Osenbach et al [11] show selective oxidation of the Al in Cu 9 Al 4 that results in the formation of a two-phase microstructure composed of γ-Al 2 O 3 with embedded crystalline Cu metal particles. We use a simplified model for the corrosion reaction following [11]:…”

“…Even though Al is the most electrochemically active, Al passivation oxides prevent the corrosion of Al and Alrich IMCs. Passivation oxide pitting is observed primarily in the Cu 9 Al 4 and therefore, this is the first phase to be corroded [9][10][11][12][13]. Boettcher et al [13] observed the resulting oxidized interface which is highly susceptible to fracture and therefore, the ultimate reason for failure.…”