Corrosion study at Cu–Al interface in microelectronics packaging

Abstract

The interfacial shear (IS) force of copper ball onto aluminium-based bond pad in microelectronics packaging depends on the formation and growth of Cu–Al intermetallic. This paper reports the study on the behaviour of the IS force of the Cu–Al bonds that were subjected to pressure cooker test up to 576 h. Initially, the IS force increases with test readout point until 192 h, due to Cu–Al intermetallic growth that has strengthened the bonding. However, IS force decreases significantly from 157.4 gf at 192 h to 97.6 gf at final test readout point, 576 h. The number of shear-induced cratering shows similar reduction trend after 288 h. Result of scanning electron microscopy (SEM) on bonding morphology shows the evidence of crack at the aluminium bond periphery or outer Cu–Al interface and the cracking trend continues at higher time point. Surface analysis of ball-peeled bond pad using X-ray photoelectron spectroscopy (XPS) indicated that the cracks were due to stress corrosion cracking at aluminium that has been stimulated by copper. The concentration of CuO at the surface bonded area was found to be increased at higher readout point and reached 100% at 576 h. These results indicated that the Cu–Al bond had been weakened by stress corrosion cracking at outer bond interface and reduced the IS force.

Introduction

Products of increasingly higher quality are being made in an environment of continually lowering costs in the consumer electronics industry. In microelectronics packaging, gold wire is the most successful interconnect materials used in thermosonic wire bonding technique. However, there are numerous studies found that the reliability degradation of gold wire was associated with bond interface voids formation [1], [2], [3]. The demands in enhancing reliability raise the attention on the focus on copper ball bonding. Copper has rapidly established itself as one of the main materials for the wire bonding in microelectronic packaging. This is due to its low cost and high electrical conductivity compared to gold or aluminium. The advantages of copper over gold have been extensively reported [4], [5], [6], [7], [8]; copper has high thermal conductivity, low electrical resistance and higher breaking load allowing for reduced wire diameter, which leads to reduced pad size and pad pitch. These are critical for packaging process development where manufacturers always go for the smaller size package and lower cost to win the competitive market.

However, complexity is also increasing such as bondability, reliability, electrical performance, etc. For that reason, chances of problems during the life of the products are rising. The application of copper wire bonding has been limited by:

• Corrosion or oxidation on the bare copper wire, especially at elevated bonding temperature [9], [10].

• Silicon cratering/invisible initial crack increases the electrical resistivity or reduces the electrical performance [5], [6], [7], [11], [12].

Identification of corrosion sites and corrosion products on electronic components and devices is essential for ensuring a high degree of reliability of the electronic systems where such components or devices are installed [13]. According to Engel et al. [10], Au–Al contact represents a galvanic couple in the presence of moisture. Thomas and Berg [9] reported that aluminium metallisation films with copper additions are found to exhibit highly localised pitting in the presence of moisture due to galvanic action of aluminium surrounding Al₂Cu theta phase particles.

This type of problem is especially troublesome to handle since its solutions are difficult to carry out and it can undermine consumer confidence in the manufacturer. For these reasons, it is desirable to use accelerated life and other environmental stress testing to detect potential corrosion problems prior to the marketing of a product [14].

The susceptibility of a material to stress-corrosion cracking is affected by its chemical composition, preferential orientation of grains, composition and distribution of precipitates, dislocation interactions, and progress of the phase transformation. These factors further interact with the environmental composition and stress to affect time of cracking. Plastic deformation of an alloy can occur in the region immediately preceding the crack tip because of high stresses.

Mold compound used in this package will form a strong binding of Si–O–Cu with lead frame, thus molded unit could last until 480 h in autoclave test [4]. This paper reports the performance of unencapsulated thermosonic copper wire bonded devices that were loaded into a pressure cooker apparatus, baked under high pressure with moisture. The concentrations of corrosion by-products were studied by X-ray photoelectron spectroscopy (XPS), and the morphology was studied by scanning electron microscopy (SEM). The effect of corrosion to ball shear force has also been reported.