Power and Cooling
Power and cooling requirements are critical considerations in magnetron operation, and the two are inextricably intertwined.
The power level directly affects the deposition rate. But approximately 80% of the power in the plasma is converted to heat, which must be removed by the water-cooling system. Otherwise, it may damage the target, the magnets, and perhaps the substrate as well.
We provide two types of magnetron cooling designs:
- Direct Cooling
In this approach, the cooling water is brought into direct contact with the target (if it is a metal), or with the target's backing plate (when an insulator or a "tiled" target must be bonded to one). This is the most efficient method of cooling, but it demands that target changes be done very carefully, to avoid allowing excess water into the chamber.
- Indirect Cooling
In this approach, the cooling water is not brought into direct contact with the target itself, but with the cathode body, to which it is clamped. Cooling is thereby achieved via conduction through the cathode body. Because this is less efficient than direct cooling, an indirectly cooled magnetron has a lower maximum power limit, and hence a lower maximum sputtering rate.
The following table is a useful guide to maximum power limits, and selection of the appropriate power supply. Note that the power limit for indirectly cooled magnetrons can be increased by about 15% by using a thermally conductive paste to enhance heat transfer.
|Max. DC Power (watts/ in2)||Max. DC Power (watts/ cm2)|
- Pulsed or Medium Frequency -MF- maximum power is the same as the DC level.
- Maximum RF power is 1/3 the values in the above table.
We strongly suggest that the magnetron power supply be interlocked with a fluid flow switch placed in the exit line of the magnetron. That way, power cannot be applied to a magnetron unless water is flowing through the cathode. The water should be supplied at or slightly below room temperature, and the flow rate should be .25 gal/min (1L/min) for every kilowatt of applied DC power. The inlet pressure should be maintained at less than 60 psi (4 bar), to prevent leakage or distortion of the cathode body. Outlet pressure should be 0 psi or into an open drain. Open Drain Diagram Operation manual will have actual specifications.
.25 gallon per minute (1 liter per minute)/1 kW Power.
68°F (20°C ) inlet recommended. Range 55 - 75°F (13 - 24°C).
Range 6 to 8.
Unless otherwise specified, 60 PSI (4 bar) maximum inlet.
Greater than 100k Ohms (relative to ground).
Less than 10 microSiemens/cm.
Water should be filtered to less than 50 microns,especially for magnetic particles, which can build up on magnets and impede cooling water flow.
The chart below represents the maximum flow rate possible when using that sized tubing on a magnetron. For example this means that the max flow rate through any cathode with ¼" convoluted tubing will be no more than 1 gallon/minute. All of these tests were done with a water inlet pressure of 60 psig (4 bar) and an outlet to am atmospheric drain.
|3/16" O.D. x .040 Wall||.75||N/A||-|
|1/4" O.D. x .040 Wall||1.25||1/4" O.D. x .030 Wall||1|
|3/8" O.D. x .060 Wall||3||3/8" O.D. x .030 Wall||2.8|
|1/2" O.D. x .060 Wall||6.4||1/2" O.D. x .030 Wall||5.7|