Enhancing bondability with coated copper bonding wire

Abstract

There is growing interest in Cu wire bonding for LSI interconnection due to the cost savings and better electrical and mechanical properties. However, the scope of use for Cu bonding wires is generally severely limited compared to Au wires; e.g. for wire oxidation, lower bondability, forming gas of N₂ + 5%H₂, and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application.

A coated Cu wire (EX1) has thus been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Its stitch strength was far superior under fresh conditions and remained constant without any deterioration after being stored in air for a prolonged period. EX1 had a lifetime of over 90 days in air, as compared to just 7 days for bare Cu wire. Spherical balls were formed with pure N₂ (hydrogen-free), whereas the bare Cu produced off-center balls. Finally, the cost-effective and secure gas, pure N₂ was only available for EX1. The excellent performance of the EX1 coated Cu wire is comparable to that of Au wires, making it suitable for LSI packaging.

Introduction

In semiconductor packaging, wire bonding is the main technology used to form electrical connections between chips and substrate. Gold is the most popular interconnection material in wire bonding. Gold prices have risen significantly over the last few years, hence the demand for lower material cost has increased. There is also growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties [1], [2], [3].

Cu wires with large diameters (>38 μm) have been used for discrete and low-I/O power devices for many years. However, there are difficulties when utilizing Cu wires in advanced LSI packaging such as fine pitch bonding [1], [2]. A lower yield than the gold wire bonding process and long-term reliability are vital. A decreased yield would eventually offset the cost saving for Cu wires. The yield when using Cu wire bonding must be equivalent or superior to that for the current gold wire bonding process. In addition, to be applicable, high-volume production requires various long-term reliability tests.

In general, conventional Cu bonding wires have severely limited applications compared to Au wires, typically due to the following: (1) oxidation on the wire surface, (2) storage lifetime before bonding, (3) stitch bondability, and (4) the running cost issue due to gas formation. These inherent problems limit the usage of thin Cu wires for LSI packaging.

• The major reason for the hindrance in the usage of Cu wire is the fact that Cu readily oxidizes when exposed to air. Cu oxide deteriorates the bondability at the first bond (ball bond) and second bond (stitch bond), which lowers the manufacturability for Cu wires.

• Oxidation progresses further during the wire bonding process, leading to non-stick-on-lead (NSOL) failure. In order to maintain a fresh surface, the winding length of the Cu wire is limited to less than 500 m, which is much shorter than that for Au wires. As the life expectancy of Cu wire is limited before and after unpacking, the issue of scrap occurs when the wire is unexpectedly stored for longer periods. Prolonging the life expectancy of Cu wires is thus strongly demanded with the manufacturing process in mind.

• Stitch bonding is a great challenge for Cu wire bonding. Advanced machine conditions and capillary design should be specifically optimized for Cu wires [2], which can sometimes achieve good results at the development stage or during small-volume pre-production. There are hurdles to overcome when shifting to high-volume production. These are mainly due to the narrower process parameter windows at the stitch bonding. A high performance Cu wire must be developed to improve the stitch bonding.

• The formation of spherical and high quality free-air-balls (FABs) is a critical requirement for Cu wires. A forming gas is necessary to provide an oxygen-free environment in ball formation. N₂ + 5%H₂ mixture is a standard forming gas for Cu wires. H₂ has the advantage that the FAB shape is readily stabilized [1], [4]. N₂ + 5%H₂ also has several disadvantages: higher running costs, initial investment involved in setting up the specified pipes, and safety issues concerning flammable gas. Pure N₂ gas is preferable since the lack of hydrogen reduces costs and makes it more secure to manufacture. Some papers have reported that pure N₂ is not allowed for Cu wires [1], [4], [5]. If pure N₂ were available, it would be significantly beneficial and provide a good manufacturing process control for packaging companies.

Cu wire material has been developed to improve the bonding properties and soft Cu wire reportedly improves the bondability at first and second bonds [2]. Softness mainly depends on the purity of the Cu material. As the Cu purity increases from 4 N to 6 N, however, its cost increases dramatically and the softening effect would be limited even with higher purity [6]. Current commercial Cu wires are mostly of 4 N purity to balance out the overall bonding performances.

The conventional product of Cu bonding wire, so-called “bare Cu wire”, is covered by copper itself at the wire surface. Preventing surface oxidation and improving bondability are trade-offs for Cu wire bonding. It is difficult for bare Cu wires to achieve the higher targets of LSI application.

The industry demands an advanced Cu bonding wire for LSI packaging. Metal-coated bonding wires, for example, are expected to enhance the surface performance [7]. However, they have not been utilized to date because it is difficult for them to have the appropriate high quality and stable performance to meet all the requirements.

We developed a surface-enhanced copper bonding wire, namely EX1 wire, which is a Pd-coated Cu wire [8]. The desirable characteristics of Pd-coating are oxidation-free, good adhesion to Cu wire, and good bondability. The high quality of the Pd-coated Cu wire allows for consistent productivity and high yield.

In this study, the basic bonding performance of the Pd-coated Cu and bare Cu wires are compared, with the investigation focusing on stitch bondability and FAB formation with pure N₂. The mechanism of enhancing stitch bondability and the relation between arc discharge and ball shape are also discussed.