The Use of Secondary Ion Mass Spectrometry to Investigate Wire Bonding Yield Problems on Gold Contacts
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ABSTRACT With ongoing demand for high density wiring and high I/O on VLSI chips, the requirement of high wire bond yield is a challenge to achieve low cost, high performance and reliable products. Secondary Ion Mass Spectrometry (SIMS) was used to investigate the metallurgical contaminants on the gold wire bond pads and their impact on wire bond yields. SIMS depth profile studies showed that copper and nickel in concentrations greater than 1 wt% caused poor wire bondability, while copper concentration at less than 0.1 wt% resulted in good bondability of Al ultrasonic wire bonded to the gold pads. INTRODUCTION High wire bond yields on the chip and package are highly desirable and challenging as the lead counts increase. The demands for high yield production and low cost are the major concerns in the microelectronic packaging manufacturing process. Rework of the defect sites can be difficult and expensive. Wire bond yields are affected by a number of factors, all of which must be addressed in order to achieve the high production yield [1]. These factors include: choice of metallization on the chip and on the package, the choice of wire, cleaning procedures prior to bonding, the setting of bonder parameters, and the chemistry and structure of the wire and bonding pads. The objective of this study was to investigate the chemistry of the surface and near surface region of electrolytic plated gold pads used for ultrasonic aluminum wire bond. Gold plated nickel and/or copper contacts are routinely used as wire bonding surfaces in many electronic packaging applications. High wire bond yields require a gold surface free of contamination. These contaminants, often in low concentration, are identified from two sources: (i) process-induced contaminants in the form of thin layers of organic (flux) residue and metal oxide/salts deposited on the gold surfaces as a result of "Chip-On-Board" (COB) manufacturing processes, and (ii) metallurgical contaminants within the gold layer due to plating impurities, defects in the gold layer and diffusion effects.
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Mat. Res. Soc. Symp. Proc. Vol. 390 01995 Materials Research Society
Work done prior to the SIMS analysis showed that the effects of the process-induced contamination could be controlled by proper cleaning techniques [2]. Wire bond yield improvements could be made by reducing the level of organic residues such as carbonaceous and sulphur-bearing compounds. Similar findings have been reported by other workers for aluminum bond pads [3]. The initial investigations of metallurgical contaminants were carried out using Xray Photoelectron Spectroscopy (XPS) and Scanning Auger Spectroscopy (AES). The sensitivity of both techniques is limited to approximately 1 at.%. The low detection limits of SIMS (approximately I ppm for most of the elements), provides the ability to detect low concentrations of impurities within the gold layer. In particular, the ability of SIMS to measure the distribution as well as the concentration of contaminants as a function of depth into the gold layer, g
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