03/01/1999

"Gold-free" processing is RF`s entry to advanced fabs

Adapting aluminum-based technology from its CMOS fabs, engineers at Motorola are applying a proprietary new "hot-metal" process to RF MOS devices in production. It replaces conventional gold interconnects on RF devices and achieves 10 times better mean time between failure (MTBF) reliability. This process is the first time gold metallization has been replaced on RF devices in almost 30 years, and offers more possibilities for the future of advanced, high-performance RF circuits.

The patented sputter-deposition process produces a 4-?m aluminum, copper, tungsten multilayer metal stack over a titanium-nitride barrier layer. The layers are deposited hot to achieve large grain boundaries, which are directly related to high MTBFs. George Anderson, technology manager in Motorola`s Wireless Infrastructure Division (WISD), says, "Particularly with RF transistor designs, we can achieve grain boundaries that are as large as the interconnect `finger` widths, thereby optimizing metal reliability."

Although the introduction of a gold-free top metal for high power RF MOS transistors is new, Motorola has a wealth of experience in developing top metal systems for RF applications. It has shipped more than 15 million RF MOS transistors using gold-free top metal for applications such as consumer wireless phones and land mobile radios.

The "hot metal" system makes possible the use of metal designs that have a much lower current density than equivalent gold designs and exhibit much lower electromigration effect over time.

The Motorola "hot metal" was used to fabricate NIST electromigration test structures in ceramic packages that were evaluated at Sandia National Labs using 250? and 1.6 MA/cm current density stress conditions. Sandia engineers concluded that the electromigration characteristics of this metallization show superior reliability for industry standards. Failure distributions were lognormal with high sigmas that Sandia is not used to observing. While Sandia recommended further testing, including wafer level tests, the extrapolated median time to failure (MTTF) was in excess of 400 years, more than 10 times the MTTF of traditional gold-top metal systems.

With gold gone, the new process is aimed at advanced VLSI designs and is capable of being processed in Motorola`s "newest high-definition MOS wafer fabs that would never have allowed gold due to the severe contamination risk," says Anderson.

The new process makes possible the formation of thicker metal films on chips, enabling the design of higher power RF systems with enhanced reliability. It also allows for greater flexibility in values of on-board inductors, making possible advanced designs for everything from single high-power discrete devices to complex power ICs; for example, RF ICs with up to 10 W of output power. - P.B.