Sputtering is a momentum transfer
process in which atoms from a cathode/target are driven off (or
sputtering) by bombarding ions. In this process the momentum of
the bombarding particles is more important than the 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 much higher hydrogen or helium ion has much less momentum.
Sputtered atoms travel until they strike a substrate , where they
are deposited to form the desired thin film. As individual atoms,
they can be chemically active and form compounds with the ions
and atoms of the bombarding gas. For this reason, inert argon
typically is used as the bombarding gas. In some applications,
however, a reactive gas is intentionally added to the argon to
alter the chemical composition of the deposited film (e.g., nitrogen
gas in combination with tantalum sputtering to form tantalum nitride,
When argon atoms 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. The charge may grow so large that
the bombarding ions are repelled and the sputtering process stopped.
To allow the process to continue, polarity of the target must
be reversed, attracting enough electrons from the discharge to
eliminate the surface charge. This periodic reversal of polarity
is accomplished automatically by applying RF voltage to the target
assembly ( hence the term “RF” sputtering).
Of interest here is the diode rectifier-like behavior of the target
and discharge systems. This results from the vast difference in
mobility of ions and electrons. Electrons , being so much fast,
are attracted in greater numbers to the target during the positive
half-period of the RF voltage than are ions during the negative
half-period. Thus, the target develops a negative DC bias.
Perkin-Elmer 4400-Series Production Sputtering Systems perform
a number of sputtering process, each of which is ideal for a different
RF Diode Sputter Deposition
When the vacuum set point is reached, sputtering gas (typically
argon) is introduced in the process chamber at a pre-selected
rate (typically 40 sccm). A plasma, or self-sustaining glow charge
, initiated by an automatic plasma igniter, appears when RF power
is applied between the target and electrical ground, ionizing
the argon gas.
A negative (-) potential applied to the target, as a result of
the applied RF power, attracts the ionized argon at a momentum
determined by a) the magnitude of applied potential and b) the
mass of the ion. The momentum of the incoming ion is transferred
to the target material, causing surface atoms or molecules of
target material to be ejected (sputtered). These sputtered atoms
travel across the gap separating the target (cathode) and substrate
table (anode), and are deposited on the substrate (wafers) which
are arranged on the substrate pallet.
RF Magnetron Sputter Deposition
RF magnetron and RF diode sputtering are very similar, except
that during RF magnetron sputtering a magnetic field deflects
the secondary electrons (which are produced during normal sputtering
operation) away from the substrates. The sputtering process, which
is cooler than RF diode sputtering, permits materials to be sputter
deposited on substrate at lower temperatures and greatly reduces
the chance of radiation damage to delicate substrates.
Because the impedence of a magnetron is lower, higher power densities
are possible at lower potentials, effecting high sputter rates.
DC Magnetron Sputter Deposition
DC magnetron targets enhance the plasma density and increase the
sputtering rate, by trapping electrons in an electromagnetic “envelope”.
This “envelope” is formed when lines of the magnetic field enter
and exit the target face and when the loci of maximum transverse
magnetic fields form a closed figure. Because the currents involved
are very large, a separate, positively-biased anode (a dark space
shield) is used to collect the electrons. A similar dark space
shield is used in RF diode and RF magnetron deposition, This dark
space shield prevents the sides of the target and target backing
plate from sputtering.
Essentially the reverse process of RF diode sputter deposition,
in which the substrate table becomes the cathode (negative pole)
and the target assembly becomes the anode (positive pole). Under
these circumstances, surface material from the substrates is ejected.
Surface impurities are ejected along with substrates material,
making this process useful for pre-cleaning substrate prior to
sputter deposition. In order to prevent ejected material from
contaminating the target, s shutter is positioned between target
Bias Sputter Deposition
Bias sputtering combines the DC or RF sputtering and the RF etching
operations. While DC or RF power is applied to the target, a small
amount of RF is also applied to the substrate table. As a result,
the substrate and target are both bombarded by ions ( the substrate
to a lesser extent than the target ). In many applications this
process yields superior quality films than can be attained using
DC or RF sputtering with grounded substrates. Bias sputtering
influences the crystal structure, and tends to re-sputter trapped
argon from the growing film during deposition and rearrange individual
atoms of the sputtered material; this improves stoichiometry and
step coverage. Bias sputtering can be used to adjust film resistivity
and film stress to desired levels.
Some metals, such as nitrides and oxides, are best deposited by
this method: the target is the parent metal and a small amount
of nitrogen or oxygen is introduced into the process chamber along
with the argon sputtering gas. Because ionized gases are typically
highly reactive, a film deposited in a mixture of argon and a
reactive gas will often form a compound with the reactive gas
(e.g., a nitride or an oxide).
Sometimes called co-sputtering or dual deposition, co-deposition
is identical in principle and practice to other types of sputter
deposition, except that two targets (typically of different materials)
are simultaneously activated. Substrates passing sequentially
and repeatedly beneath the targets are coated with alternating,
very thin films of two materials. Under certain circumstances,
the resultant film can be equivalent to or better than one formed
using a composite target. During co-deposition, both targets may
be RF, both DC, or one RF and one DC.
Elmer Systems Spare Parts