Bond Pad Surface Research Articles

Study of Mechanical Behavior of Compliant Micro-Springs for Next Generation Probing Applications

Advances in integrated circuit fabrication have created 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 micro-spring 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 μm. These micro-springs are designed to accommodate topological variation in probing surfaces while flexing within the elastic regime. To be able to use the micro-springs for probing applications, several design challenges must be addressed. When the probe head is brought into contact with the bonding pads, the micro-springs will slide across the surface of the bonding pad and establish contact. The bonding pads typically have surface oxides. Thus, from a mechanical standpoint, it is important to ensure that the springs would apply enough force to break through the surface oxides and establish good electrical contact. The damage done to the pads in the process has to be minimal. It is also important that the distance through which the springs will slide across the bonding pad surface does not exceed the pad dimensions. From a mechanical fatigue standpoint, the stress amplitude that the springs will be subjected to, needs to be within the elastic limit of the spring material. This will enhance the life of the micro-spring probes. Typical probing devices are expected to last about half a million touchdowns. Numerical models and sub-models have been developed to simulate the mechanical contact between a single spring and the bonding pad, and to determine the probing force. The model simulates the establishment of contact, sliding, and indentation resulting in plastic deformation of the pad. The length of the scrub mark and the indentation depth are validated with experimental measurements using focused ion beam. The spring geometry parameters are varied and their influence on the penetration depth studied. Finally, the variation of contact resistance with probing force is outlined.

Chemical and morphological studies of plasma-treated integrated circuit bond pads

The effect of plasma treatment on the surface chemistry and morphology of Al-1%Si bond pads has been studied by a battery of analytical techniques. The ball shear strengths of the bond pads increased by ∼17% after CF4 plasma treatment. The improvement in wire bondability after plasma treatment is largely attributed to a decrease in the oxide thickness and a reduction in the height of the crystallites on the bond pad surface. However, micro-pits were formed after CF4 plasma treatment. The micro-pits become initiation sites of the first stage of a fluorine-induced corrosion where breakdown of the native passivating oxide occurs. In the second stage of corrosion, fresh Al beneath the oxide is exposed to the attack from the fluoride ions in the presence of water.

An investigation on the plasma treatment of integrated circuit bond pads

Integrated circuit (IC) bond pads play an important role in the wire bond reliability of the microelectronic devices. Being the device’s only electrical connection to the package and electronic systems, it is mandatory that the bond pads are free of contaminants and possess excellent bonding characteristics. Contaminants such as oxides and organic residues impair the bondability to a considerable extent and are very resistant to conventional wet cleaning methods. In this paper, we report the effects of an Ar/H 2 plasma treatment on the surface chemistry and morphology of IC bond pads. Surface and sub-surface chemical analyses have been conducted using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Results reveal that the oxygen level of the bond pad surface has decreased significantly after the plasma treatment. Although the treatment has successfully removed the surface crystallites on the bond pads on prolong etching; however, the aggressive process has also damaged the passivation layers that surround the pad areas.