Aluminum oxide films were etched using low energy argon ions generated by a microwave electron cyclotron resonance (ECR) source argon plasma. The argon ion energies were controlled by biasing substrates placed on a 13.56 MHz capacitively coupled electrode. Reactively sputtered aluminum oxide films were used to study the relationship between the dc bias applied to these substrates and the etch rate of their films.Insitux‐ray photoemission spectra of the Al 2pand O 1stransitions showed that the ECR plasma was effective in completely removing native aluminum oxide and adventitious hydrocarbon in 1 min at ion energies as low as 100 eV. This preclean technology did not change the dielectric breakdown distribution of antenna structures with 12‐nm‐thick gate oxide capacitors.