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Selective Electroless Nickel and Gold Plating of Individual Integrated Circuits for Thermocompression Gold Stud Bump Flip-Chip Attachment
Tuesday, March 02, 2010 | David M. Lee, Eldwin L. Dodson and Guy V. Clatterbaugh, Johns Hopkins University Applied Physics Laboratory

Abstract

Flip chip bonding is the most desirable direct chip attachment approach for minimizing electronic assembly size as well as improving device performance. For most prototyping applications it is not cost-effective to purchase individual integrated circuits (ICs) that are solder-bumped as this typically requires the purchase of entire wafer. Also, many unpackaged IC's in die form are not available for purchase as an entire wafer for subsequent solder bumping. As an alternative to solder bumping, manufacturers of wire bond equipment have developed the gold stud bump process which allows single IC's to be automatically bumped using 1-mil gold wire. However, the rapid formation of brittle aluminum-gold (Al-Au) intermetallics at elevated temperatures (>200°C) precludes the use of thermocompression flip chip bonding due to the unreliability of the bond at the IC pad interface.

To overcome the intermetallic problem at the IC's aluminum-metallized bonding pads, an electroless nickel and gold plating process was developed for making a gold-bondable diffusion barrier for use on individual, unpackaged silicon ICs. This process provided an electroless gold layer suitable for accepting the gold wire stud bumps as well as providing the necessary barrier to Al-Au intermetallic formation. A number of experiments were conducted using electroless nickel of various phosphorus contents to determine which would provide an optimal diffusion layer. Data will be presented comparing immersion and autocatalytic gold plating processes. Test wafers were stud-bumped and exposed to accelerated temperatures then shear tested.  Electroless nickel, immersion and autocatalytic gold plating process parameters were optimized to provide high reliability interconnections when using the high temperature thermocompression flip-chip bonding die-attach method. 

Introduction

The gold stud bump flip chip process creates gold bumps on integrated circuit (IC) die bond pads. The die is then directly connected to a circuit board or substrate. Gold stud bumps are formed by a wire bonder utilizing a modified wire bonding technique. This process allows individual, off the shelf die to be gold stud bumped. This process is ideal for product development and prototyping.

Aluminum is generally the metallization used for IC bond pads flip chip bonding of gold stud-bumped chips can be accomplished using an anisotropic adhesive or by thermocompression bonding. Thermocompression bonding is a much faster than waiting for anisotropic adhesive to cure. However, bonds made with stud bumps that are made directly on aluminum bond pads are unreliable due to the aluminum-gold intermetallics that form readily at thermocompression bonding temperatures (~300°C). It's well known that the formation of aluminum-gold (Al-Au) intermetallics at elevated temperatures (>200°C) degrades and compromises the bond at the gold bump and aluminum pad interface.

A number of Under Bump Metallization (UBM) processes have been developed to overcome the intermetallic formation. UBM may be vacuum deposited by evaporation and sputtering techniques or electroless and immersion plating processes. However, vacuum deposition would is not suitable for metalizing individual die because of the number of masking steps they require. Electroless plating is an ideal alternative to vacuum deposition. This paper discusses an electroless nickel, immersion and autocatalytic gold plating process that was developed to plate aluminum bond pads on individual ICs.

Background

When electroless nickel and gold (ENIG) plating aluminum, a zincation process is used to activate the exposed aluminum before the nickel is deposited. The aluminum oxide layer is removed and the surface is activated through zinc displacement plating using a zincate solution. A double-zincation process is used to assure maximum adhesion.  Electroless nickel (EN) is then deposited from a hypophosphate-based nickel bath. The EN is an autocatalytic process which chemically deposits a nickel-phosphorus alloy onto the zincated aluminum. The phosphorus content of EN baths can range from 3% up to 15%. Immersion gold is then deposited thru a galvanic reaction with the electroless nickel to deposit a thin layer of gold onto the nickel. For applications that require a thick gold layer such as stud bumping and wire bonding, autocatalytic electroless gold is then plated over the immersion gold layer.


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