Magnetron sputtering is based on the generation of a magnetically enhanced glow discharge in vacuum. When a sufficiently high DC voltage is applied between two electrodes in a confined area at a reduced gas pressure, the gas can be ionised and an electrical discharge can form. The term “glow” refers to the light emission that can be observed in such discharges. The discharge is maintained because positive discharge gas ions are accelerated towards the cathode. The resulting ion bombardment leads to the emission of secondary electrons from the cathode. The emitted electrons will be accelerated into the discharge. Hence, they can ionise gas atoms leading to the generation of new ions, thus sustaining the discharge.
In a sputter magnetron, the ionisation degree of the generated glow discharge is increased by placing magnets behind the cathode.
The placing of the magnets results in an elongation of the electron path towards the anode. Hence, the ionisation probability is increased. The “magnetically enhanced glow discharge” is generally referred to as the “magnetron discharge”. When this type of discharge is used for thin film deposition, the process is referred to as magnetron sputter deposition
When ions strike the cathode or the target not only electrons are emitted. Indeed, the energetic ion bombardment results in the ejection of target atoms or molecules. This process is called sputtering, a word which finds its origin in the Dutch language, i.e. “sputteren” or “to spit out in small particles and with a characteristic explosive sound”, says the dictonary.
Simulation of sputtering (Taken from the work of Urbassek)
Thin Film Growth
When the sputtered atoms arrive at the substrate, a thin film is formed. The thickness of these films vary between 2 nm to 1000 nm or 1µm. The morphology, microstructure and crystallographic orientation of the thin films depend on the growth conditions, which is sometimes depicted in a so-called structure zone model.
Structure Zone Model by Thornton
To grow a compound layer when using a metallic target, a reactive gas (e.g. oxygen or nitrogen) is added to the plasma. The reaction between the reactive gas and sputtered atoms on the substrate results in the formation of the compound layer. Compound formation can also occur on the target which changes the overall deposition process and results in complex behaviour of the deposition parameters as a function of the reactive gas flow.
Hysteresis behaviour of the discharge voltage as a function of the reactive gas flow.
Some history ...
Sputtering was discovered by W. Grove in 1852 while investigating discharge tubes using a silver needle cathode. He observed film deposition on the anode plate. Because of his poor vacuum conditions, he probably deposited silver oxide. By reversing the voltage, he sputtered the deposit off again.
W. Grove and the set-up he used.