The issue that I am trying to address here is a customer’s request for information on the proper procedure on how to precondition and operate an IGZO i.e. In2O3/Ga2O3/ZnO 1:1:1: At % sputtering target configuration with a standard (extended) Semicore Equipment, Inc’s. 4.75″ By 22″ magnetically enhanced cathode assembly.
For most metal oxide, i.e. non-conductive, sputtering targets, we typically recommend sputtering in a plasma with a maximum power density of around 65 watts/sq. in. of target surface area for direct water cooling configurations and around 30 watts/sq. in. with indirect cooling cathodes. Since this Semicore design is an indirect water cooling system with a target surface area of around 105 sq. in., the maximum power level should be kept below 3KW or so for long term depositions under equilibrium conditions.
The real issue here however, as far as any recommendations are concerned, is not simply related to the target material itself but more importantly in the design of the cathode assembly as it relates to the magnetic enhancement characteristics and water cooling efficiency. The width of the erosion profile and magnetic shunts, the configuration and location of the dark space shields, the distance that the magnets are placed from the cathode surface and the specific composition (rare earth magnets vs. aluminum nickel alloys) which determines the strength of the magnetic field, all play a very important role in both the strength and size (surface pattern) of the magnetic field in the MxB relationship of the magnetic and electrical components associated with the power density on a given cathode assembly. Additionally, the specific type of power supply being utilized in the system to generate the plasma is also critical as it relates to the electrical/thermal conductivity of the sputtering target. An rf power supply with an associated matching network must be tuned very specifically to the system impedance to eliminate any possible reflected power, since secondary electrons result in heat generation on the target surface; AC Vs. DC power supplies are different as to the coupling into the network of the cathode; mid-frequency range pulsed generated power supplies (such as the Advanced Energy Pinnacle Plus unit) add an additional component and flexibility for use with poorly conductive metal oxide target materials.
It may be better to consult the actual deposition system (Tool) manufacturer or cathode design engineer to ask for any specific advice as to maximum power density constraints for any given material classification rather than asking us target producers, since the issue is more process dependant than material oriented. In this case, the people at Semicore are very competent and very customer oriented when it comes to sharing equipment and process information.
As always, especially with any new ceramic based sputtering target, care should be taken to avoid thermally shocking the target material; especially with metallically bonded target assemblies. The initial plasma should be initiated at the lowest possible power level that will sustain a plasma and then hold that level until equilibrium conditions are established, i.e. any arcing is suppressed and there are no fluctuations in the current or voltage readings in the power supply. This may take up to five minutes or more with a new target. After this initial soak time, the power level should be slowly increased in 50-100 watt increment rise cycles and then soaked each for increment until equilibrium conditions are once again established. These rise level and soak times should be repeated until the desired operating parameters are established but not above the maximum operating level for any given target material.