What is balanced/unbalanced?
Magnetrons are commonly classified as 'balanced' or 'unbalanced'. Conventional magnetrons are usually of the balanced type. The ability of the electrons to escape from the magnetic trap is determined by the position of the null point in the plasma above the target. If the null point is high above the target, there is little chance of electrons escaping and the magnetron is balanced. In this case, there is low ion bombardment on the substrate (assuming the substrate is positioned above the null point). If the null point is close to the target surface, the electrons can escape more easily and the magnetron is unbalanced. Unbalanced designs can produce high ion bombardment of the thin film at the same time as deposition.
Window and Savvides were the first people to formally recognise and classify unbalanced magnetrons, see figure below. Type 1 is a balanced design and is characterised by the open field lines extending over and beyond the target surface. Type 2 is an unbalanced design and is characterised by the closed field lines that are confined above the target surface.
Source: B Window and N. Savvides, J. Vac. Sci. Technol. A4(2), 196 (1986)
Magnetron classification
Gencoa use a simple method to determine the degree of unbalance and to classify magnetrons into 6 groups according to the value of g, which is the ratio, ZBz=0:W1/2, Z is the distance to the null point and W is the target width, according to the following figure and table:
| Group Number |
Group Description |
K factor (typical) |
g=ZBz=0/W1/2 |
| I |
Extremely balanced |
200 |
g≥2.00 |
| II |
Very balanced |
150 |
1.75≤ g <2.00
|
| III |
Middle balanced |
125 |
1.50≤ g <1.75 |
| IV |
Unbalanced |
100 |
1.25≤ g <1.50 |
| V |
Very unbalanced |
60 |
1.00≤ g <1.25 |
| VI |
Extremeley unbalanced |
40 |
g<1.00 |
K Factor
The electron retention factor or 'K factor', can also be used to classify the degree of unbalance. It varies linearly with the level of ionisation at the substrate, hence enabling a chosen ionisation level to be defined. The absolute value of the K factor varies for each source since the size and geometry of the magnetron must be considered. However, the linear relationship with ionisation remains the same. (insert graphs of escape probability and retention factor varying with ion current)
From the table above, it is possible to determine the optimum level of ion bombardment for a particular process by using the K factor. By varying this, levels of ion assistance from 0 to 10mA/cm2 can be achieved for the coating process. This additional energy improves the coating structure, adhesion and physical properties. Reactive processes benefit in terms of the additional plasma energy at the susbtrate that encourage reactions at the surface and also in the plasma.
Graph showing Ion current versus Retention Factor for a Gencoa V-tech magnetron
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