Gold bumps dominate wafer-level packaging (WLP) market

Dr. Robert Castellano, The Information Network

This is an extract of an article written for my Column “The Business of Doing Business” in the July issue of Wafer & Device Packaging and Interconnect magazine.

NEW TRIPOLI, USA: Wafer-level packaging (WLP), the fabrication of the package directly on the wafer, is experiencing exceptional growth and stands out as one of the bright growth areas in electronics today. WLP offers lower cost, a smaller package, higher performance and added functionality compared to older methods.

WLP is increasingly being viewed as a subset of flip-chip technology. The industry is seeing increased utilization of flip-chip, where historically it’s been used for very high-I/O devices with thousands of I/O, and it’s starting to migrate down to the hundreds of I/O (600-700) range.

The Under Bump Metallization (UBM) provides the critical interface between the metal pad of the integrated circuit and the Au bump used for the flip chip interconnect to the substrate.

There are four major bumps in production: Tin-lead (solder), Copper, Tin-silver, and Gold.

Copper pillar bumps were introduced in 2006 by Intel in their 65-nm “Yonah” microprocessor. Copper pillars offer advantages over solder bumps such as higher interconnect densities, higher reliability, improved electrical and thermal performance, and reduction or elimination of lead. When final bump composition is gold instead of solder, a gold seed is used, and gold electrochemical etching is often used in place of wet etching to remove the seed layer. This is then followed by wet etching of the barrier layer

Currently, the industry is using a wide variety of metal layers in various UBM schemes, leading to some difficulty in generating a standard process for UBM etching. Most barrier layers combine barrier and adhesive properties by mixing materials.

For example, tungsten – a good barrier – mixed with 10 percent titanium improves adhesion. Barrier layers include chrome, titanium, tantalum, titanium-tungsten-nitride (TiWN), and nickel-vanadium (NiV). These are usually thin layers (250Å–2,500Å). Deposition is generally by sputtering. The seed layer is usually 5,000Å of copper deposited by physical vapor deposition.

Critical for equipment and materials suppliers to the WLP market is the type of bump, as the UBM varies with bump material, and hence the methods of deposition and etching. As shown below, in 2009, 5 million 300mm equivalent wafers were bumped using these technologies. The majority was gold. Despite the record high price of gold,

In 2011, the number of bumps will increase more than 40 percent, and gold will still dominate the market.