When I co-sputtering Al target and Cr target using DC magnetron sputtering, the voltage of Al target keeps increasing at a rate up to 30V/min for around 20 minutes and stabilize at some 700V (Power is around 80W). However when I sputtering Al target only, the voltage is stable from the beginning at around 500V (same power). Why I am having this increasing voltage of Al when I am co-sputtering Al and Cr targets, but not when single sputtering Al. The Cr voltage is very steady when co-sputtering, also when single sputtering.
The vacuum of my chamber is pretty good, during the deposition, the partial pressure of oxygen, nitrogen, and H2O are all below 10^(-9) Torr, so I don't think it is the poisoning thing, also I did EDS on the film, and almost no O nor N.
My 2 inches Al target and Cr target are sort of closed to each other, they are around 6 inches separated from the center. so I was wondering that is it anything related to the interaction of plasma from two different targets or what could cause the increasing voltage?
The reason that I am worrying about the increasing voltage is that I won't be able to have a high Al current (thus high Al deposition rate) when I co-sputtering, because I don't want my power to go above 150V due to the cooling issue.
No, the voltage of Cr target is very steady both when co-sputtering and single sputtering
Try to ensure the
1) ground shields, and
2) shielding rings over each one of the circular shielding rings around the target
3) and the contact of the ground shield (of each target) with the base platearound each one of the targets to be absolutely clean.
Take a soft emery paper paper or cloth, and clean up all the surrounding areas.
Often a ceramic/teflon spacer ring exists around the target, it becomes dirty due to earlier sputtering, see that is is absolutely clean.
the target (Al , or Cr) must be well isolated, with the insulator spacer ring from the ground. Many a times if the insulating spacer is dirty, it creates all kinds instability, such as arcing, and voltage stability.
K. Sreenivas
Both Al and Cr have roughly same ist ionization potential (Al-~ 6ev, Cr ~ 6.8 eV). When operated for Al target using Ar gas the discharge voltage is (~ 500 V) lower. This means your plasma density is higher. When operated using both Al and Cr, the discharge voltage is higher (~700 V) due to lower in the discharge current. If you measure the deposition rate the you will find that Al (single) sputtering will have higher value than that of co-sputtering.
It is nothing about the Al or Cr, it is about the role of plasma that adjusts both discharge voltage and current as long as there is no arcing in the taget (with the ground shielding).
Further in co-sputtering you are putting the same power. SO power is spent for both AL and Cr sputtering.
Do not worry. Please change the operating pressure and increase the power so that there will be reduction in voltage by higher plasma density (ionization).
Good luck!