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AC sputtering is a discharge system often used for high-resistance targets. It has a benefit of suppressing charge-up damage of the target surface, as two targets are discharged alternately.
AC (alternating current) is an electronic current whose direction changes periodically, and the number of times it changes direction per second is called frequency. AC discharge is a system of electric discharge in which two targets are arranged so that positive and negative electrodes can be alternately inverted using low-frequency alternating current.