The present invention relates to a method for determining a critical size for a diameter of an Al.sub.2O.sub.3 inclusion (38) in an Al or Al alloy sputter target (42) to prevent arcing during sputtering thereof. This method includes providing a sputtering apparatus having an argon plasma. The plasma has a plasma sheath of a known thickness during sputtering under a selected sputtering environment of an Al or Al alloy sputter target having an Al.sub.2O.sub.3 inclusion-free sputtering surface. When the thickness of the sheath is known for a selected sputtering environment, the critical size of an Al.sub.2O.sub.3 inclusion (38) can be determined based upon the thickness of the sheath. More specifically, the diameter of an Al.sub.2O.sub.3 inclusion (38) in an Al or Al alloy sputter target (42) must be less than the thickness of the plasma sheath during sputtering under the selected sputtering environment to inhibit arcing.