| The
Randex Model 2400-8SA Sputtering System is designed to deposit
a wide variety of materials onto substrates such as ceramics,
metals, plastics, glass, and semiconductors; the resulting thin
films can range in thickness from a few Angstroms up to a fraction
of a millimeter. This sputtering system can deposit in sequence
up to three different materials onto a single substrate, thereby
creating a sandwich-structured film such as multiplayer optical
interference filters. The system can also be used for ionetching,
a process in which material is removed from, rather than deposited
on, the substrate.
The Sputtering Process
Sputtering is a momentum transfer process in which atoms from
a cathode/target are driven off or sputtered by bombarding ions.
In this process the momentum of bombarding ions is more important
than their energy. For example, a hydrogen or helium ion accelerated
to 3,000 eV will cause very little sputtering compared to an ion
of argon (which is chemically inert) with the same 3,000 eV energy,
simply because the lighter hydrogen or helium ion has much less
momentum. Sputtered atoms travel until they strike a substrate,
where they deposit to form the desired layer. As individual atoms
they are chemically active and readily form compounds with the
ions and atoms of the bombarding gas. For this reason inert argon
is used as the bombarding gas. In some applications however, a
reactive gas is purposely added to argon so that the deposited
film is a chemical compound, and not the elemental target materials.
When argon ions strike the target their electrical charge is neutralized
and they return to the process as atoms. If the target is an insulator,
the neutralization process results in a positive charge on the
target surface. This charge may reach the level where bombarding
ions are repelled and the sputtering process stops. To continue
the process the polarity must be reversed to attract enough electrons
from the discharge to eliminate surface charge. This periodic
reversal of polarity is done automatically by applying RF voltage
onto the target assembly (hence the term RF sputtering). Of interest
here is the diode rectifier-like behavior of the target and discharge
system. This is caused by the vast difference in mobilities of
ions and electrons; the electrons, which are much lighter, are
attracted in much greater numbers to the target during their positive
half-period of RF voltage than are the heavy ions during their
half-period. |
