Sputter coating allows uniform coating on the specimen even on the parts that are not directly facing the metal to be evaporated. Hence it is the most acceptable technique.
· Specimens are placed on a cooled base plate which functions as an anode.
· The cathode, which is the metal (e.g., gold) to be evaporated, is positioned above the specimen.
· The chamber, in which the cathode and anode are located, is evacuated.
· Inert gas such as argon is flooded into the chamber and is again evacuated.
· A negative high voltage is applied to the cathode. This results in glow discharge and argon gas molecules gets ionized.
· The positively charged ion move towards the cathode and strikes on it, thereby releasing Au atoms.
· The dislodged metal atoms due to numerous collisions with gas molecules get deflected several times and hit the specimen at different angles. Thus a uniform thin layer is deposited on the specimen.
The thickness of coating depends upon the current applied, anode to cathode distance and sputtering time. With Balzers SCD 020 Sputter coater, using gold as the target (cathode), a coating of about 35 nm thickness can be obtained under the following conditions:swiss replica watches
Current - 21.5 mA
Pressure - 0.05 mbar
Gas - Argon
Cathode to anode distance - 30 mm
Sputtering time - 1 minute
After metal coating specimen are ready for observation in SEM.