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Power Density Limits and Target Conditioning

Plasmaterials

Plasmaterials, Inc. is a leader in providing high purity materials for all types of thin film applications. We produce and market a full range of products for R&D and full scale production.

I really think that we have beaten this issue to death however, almost weekly, we get a customer request asking us to explain how best to condition or “Break In” a new sputtering target and what is the maximum power level that they could apply to it. Perhaps once and for all I can combine these two very basic, yet very important, aspects of process technology for sputter deposition applications.

Recommending a maximum operational power level that a given sputtering target can sustain in a plasma under equilibrium conditions over an extended period of time and for a conditioning cycle to break in a new target of specific composition is always very difficult. These two procedural conditions are highly dependent on the individual manufacturer’s design of the magnetically enhanced cathode assembly being utilized in a particular tool. We always recommend that our customers first check with the Operations Manual that came with either their system or cathode assembly itself before beginning to sputter with any given material. If one is not available, it is still probably best to contact the cathode manufacturer directly since each has their own way of implementing critical design features such as water cooling channels, magnet composition, magnetic shielding, magnetic positioning, positioning of dark space shields, etc. All of which affect the pattern and distribution of the power density (E) when measured within the erosion profile (as a function of both magnetic strength “M” and electrical field “B” as measured at the target surface in the mathematical relationship of E=MxB). Cathode manufacturers that utilize high strength rare earth magnets in their design that are shunted to provide a wide erosion profile offer both technical and product throughput advantages, but it may require scaling back a bit on the electrical power component. Certain magnetically enhanced cathode manufacturers; such as Angstrom Sciences, Materials Science, Sierra Applied Science, etc. utilize rare earth magnets in their cathode assembly designs. These state-of-the-art cathode designs pretty much optimize the magnetic component of the MxB power density associated with the plasma. So take it easy, at least initially, and be conservative when igniting the plasma and ramping up the power density to the sputtering target.

So after saying all that, we would typically recommend about 30 watts/sq.in. for a typical indirect water cooled cathode assembly when depositing with a ceramic or powder metallurgiclly produced target and around 65 watts/sq.in. for a directly water cooled cathode assembly configuration. When depositing with an rf generator with an associated matching (tuning) network, care should be taken to ensure that the reflective power is at absolute zero, at all times, independent of the forward power level. All reflective power turns to heat due to secondary electron bombardment. Heat that is generated during the sputter deposition process, especially that which is initiated at the target surface and the associated heat dissipation through the target to the water cooled backing plate, is an enemy to the sputtering target and the metallic bond. In all cases it will be necessary to pre-condition targets prior to use, especially conditioning a new target for the first time. For applications involving sputter applications with more highly conductive metallic targets the general rule of thumb is to restrict the maximum power density to around 250 watts/sq. in. with direct water cooling cathode assemblies and closer to around 100 watts/in. sq. for indirect cooled cathodes.

There are a number of processing operations involved when producing (melting or consolidating) a sputtering target and the associated bonding operations that apply various internal stresses within the target. These stresses need to be thermally relieved before normal thin film deposition processing can be carried out with a new target. This must be done without thermally shocking the target.

For the initial target conditioning step be sure to begin with a shutter positioned between the cathode and any substrates that may be in the system. Any resultant films produced from the initial conditioning run are not likely to be optimal.

To initially condition a new target, start, or ignite, your plasma at the lowest possible power setting that you can sustain the plasma at. If you do not utilize a plasma ignition spark in your tools and are having a difficult time igniting the plasma, manually raise the partial pressure of argon slowly until the plasma ignites. This may be as high as 50-100 microns or so. For rf applications the reflected power may be a bit high at this point but the gas pressure won’t be at this elevated level very long. There also may be some arcing in the plasma but this is only a result of the increased conductivity of the plasma at these higher partial pressures. Essentially ignore any arcing as well – but only momentarily.

After the plasma has been ignited, slowly lower the argon flow rate until you reach your normal operating pressure, say somewhere around 2-3 microns or so for r.f. This should be around 5-7 microns for DC applications. Simultaneously keep lowering the electrical power input such that you can still see (or maintain a few milliamps on the current meter if you don’t have a viewport) a dull plasma hovering above the target surface. Once the plasma system has been stabilized, hold everything at these levels for about five minutes or so to allow everything to equilibrate. It may be necessary to slightly tweak the gas flow or power level settings as things begin to stabilize and maybe even adjust the impedance on your matching network for rf applications to maintain a zero reflective power level. That is if you do not have an automatic tuning device in the r.f. matching network with your system.

After this initial “Soak” cycle, slowly “Raise” the power level another 25 to 50 watts or so and hold it at that level for about another five minutes, again stabilizing the plasma and allowing the target to reach a thermodynamic equilibrium. Continue with these rise and soak cycles until you have reached the power density that you wish to operate at during your normal deposition cycle. Hold the settings at this point for an additional 15 minutes and then add one more rise and soak cycle of 25 to 50 watts above the level that you intend to operate at. This procedure will allow the internal stresses within the target to self relieve and help to avoid any unnecessary cracking or de-bonding of the target from the backing plate in subsequent runs.

It should not be necessary to perform this type of run-up procedure after the target has been initially conditioned but care should always be taken not to thermally shock the target. Always be sure to slowly apply the power level when starting the deposition process and watch for any arcing or rise in reflected power for rf applications – both of which will create heat and may damage the target.

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