Organometallic chemists are
finding new ways to make gold molecules that "fly,"
and to control where they land. The result is a new way to
make thin gold films for the electronics industry.
Gold is very important in
microelectronics, especially for connecting individual
components and forming switches. it is preferred over other
materials like copper and aluminum because gold conducts
electricity very well, and is not corroded by the oxygen and
water in the atmosphere. But gold is expensive, so making a
thin film of it saves money as well as space.
Thin films can now be made
through a process called "Chemical Vapour
Deposition," or CVD, as Iona as the coating material is
part of a compound that can be easily vaporized. For
instance, a thin silicon coating-the kind used in some solar
cells-can be obtained when silane, SiH4(g), is passed over a
hot object. It decomposes into two molecules of hydrogen that
are carried off in the gas stream, and one atom of silicon,
which stays on the surface.
There are two special
problems involved in making gold films by CVD, but recent
research has solved them both. First, for a heavy element
like gold, it is hard to find a molecule that is volatile
enough to "fly."
Now Richard Puddephatt at
the University of Western Ontario in London, Ont. has found
one-the alkylgold compound CF3-Au-Cº
N-CH3 works especially well. It is a solid but it vaporizes
easily and, on contact with hot objects, it decomposes into a
very pure gold coating.
The second problem is that
microelectronics often require the gold to be deposited as
very thin lines that connect individual components on a
silicon wafer. Obviously, the CVD process will coat more than
just those lines if the whole wafer is heated.
The solution is to focus a
laser beam to create intense heat onlv where the coating is
desired. Then, as the alkylgold compound flies over the
wafer, the gold film forms only where the laser beam is
focused. The resulting lines can be made just a few
manometers thick.
The entire gold circuit is
traced out with 'laser writing' — moving the laser beam
around on the wafer. That kind of writing makes a valuable
point!
