A tungsten coat for silicon.

Tungsten is hot. During the last few years, dozens of microelectronics companies have formed research teams to study this tough, hard metal, which is more commonly associated with light-bulb filaments. They are interested in using it as a replacement for the thin, conducting threads of aluminum or polycrystalline silicon that connect transistors and other devices laid down on integrated circuit chips.

At the Sandia National Laboratories in Albuquerque, N.M., where tungsten research has been going on for two years, a group of researchers recently announced several advances in techniques for depositing tungsten on silicon surfaces. Their results may be particularly useful in the manufacture of the newest generation of electronic circuits--"very large-scale integrated" (VLSI) chips.

"The circuits are getting smaller and smaller, and the features on them are shrinking correspondingly," says Sandia's Robert S. Blewer. "The use of tungsten solves several problems."

In VLSI circuits, even a small current coursing through a thin wire, which may be less than one-hundredth the thickness of a human hair, generates a high current density. In the case of aluminum, this is high enough to bump aluminum atoms from one part of the wire to another, eventually thinning the material enough in some places to break the circuit. "Tungsten is very resistant to this kind of behavior," says Blewer. It is also a better electrical conductor than doped polycrystalline silicon, an alternative wiring material.

The Sandia work shows that thick, as well as thin, tungsten films can be chemically deposited exactly where needed on silicon surfaces. This requires careful cleaning of the silicon wafers on which circuits are built, along with precise control of the temperature, pressure and chemical environment in a special chamber where the process, called "chemical vapor deposition," takes place.

In the past, the two chemical reactions that produce tungsten films were allowed to occur at the same time. In one process, gaseous tungsten hexafluoride reacts with silicon to deposit tungsten and produce a gaseous byproduct. A second reaction involves hydrogen gas and leads to the deposition of more tungsten atoms. The Sandia researchers discovered that doing the two reactions in sequence rather than together produced much better results.