Plasma Sputter Coating
In conventional SEM sputter coating a gold (gold-palladium, or platinum) target isbombarded with heavy gas atoms (usually argon but air is a fair substitute). Metal
atoms ejected from the target by the ionized gas cross the plasma to deposit onto theany surface within the coating unit including the specimen. A low vacuum
environment is used (0.1 to 0.05 mbar), which with one of the modern low voltage
sputter coaters, enables metal to be deposited at up to 1nm/s1.
Sputtered metals are deposited in the form of islands, NOT a continuous coating.
Although the coating will be relatively random, a coating unit should be tuned inorder to obtain the optimum grain size, coating penetration and coating thickness, forthe task in hand.
Operation
The conventional sputter chamber is pumped with a rotary pump until the vacuum
level is beyond half scale on whatever gauge the manufacturer supplies (do not worry
about the vacuum units). Leak argon gas into the chamber to flood the chamber,allow the vacuum to recover. Apply the lowest voltage that will allow a plasma tostrike, determined by the tests below.

Tuning a Sputter Coater
For fairly smooth surfaces deposition rate tests may be conducted using a glass slide
or a piece of white typing paper. The variables are, gas pressure, specimen to targetdistance, discharge current and operating voltage. A grey coating on the white paper
should be sufficient for most work. Adjust the coating unit to obtain this level ofcoating in the shortest time, but over the widest possible area. Take care, as it ispossible for the coater to deposit at different rates in different areas of the specimen
stage! Typical values could be on a variable voltage system: - 0.8kV, 20mA, 5cm
working distance, 0.1 mbar, for 30 seconds, to achieve a 2nm coating.

Deposition Rate and Film Thickness
Coating speed is depended on target material and sputtering current and coating time.
The following formula can be used to estimated gold and platinum film thickness
D=KIT
Here:
 D= film thickness (A) K = material constant , for gold material in Ar gas, K ~ 0.17, and for Pt, K ~ 0.9 I = Sputtering current (mA) , which is adjusted by Ar gas partial pressure t = time (S)For example, for gold film under Ar partial pressure with current 8 mA for sputtering
100 s, film thickness is about 0.17x8x100 = 136A. You may repeat coating to obtain
thick film.
For different materials, you shall practice by yourselves to achieve best parametersand experience.

Modern Low Voltage systems use a variable frequency rather than a variable voltage
to change the deposition rate. These instruments run between 400 and 600 volts andusually require very low currents for a suitable coating e.g. 10mA for 30 seconds.
To determine the optimum conditions for penetration into rough surfaces you requirea set of nuts (as in nuts and bolts) varying in height between 2 and 5mm and varying
in hole diameter between 1 mm and 3mm. Once again vary the operating conditions,but this time the object is to deposit metal onto the paper but within the nuts. A goodcoater, well set up, should be able to deposit metal down a hole 5mm deep by 2mm
diameter, without making the surround any more than a pink colour. You will findthat the better vacuum level the easier it will be to deposit metal deep within
structures, in other words turn off the gas inlet and give the system plenty of time topump down.
In the case of small particles, the shadow created in the coating by the particle, may
be sufficient to prevent a continuous conducting layer. Even a very thick coating will
rarely overcome this problem. The ideal solution is to coat the specimen tilted at 45
degrees to the target and then repeat with the specimen tilted in the opposite direction.

Tooth brush shaped specimens may also need a "straight line" coat by running thesystem with the gas turned off. Leakage is usually enough to enable you to strike aplasma but a higher than normal voltage will be required.