Turret-type electron gun for EBM-8000

To reduce down time associated with routine cathode replacement we developed a turret-type electron gun for our electron beam mask writer, EBM-8000. This enables us to exchange cathodes without venting the gun to atmosphere, thereby reducing the downtime for cathode replacement by 80% compared to conventional single-cathode guns. Two key elements were considered in developing the turret-type electron gun: reducing fluctuation in beam current, and eliminating discharge sources. Reducing fluctuation in beam current is one of the most important elements because it has an impact to critical CD accuracy. In EBM-8000, the current fluctuation must be 0.1% (3s) or less to attain the CD accuracy specification. We will explain how the beam stability is realized. Special treatment to eliminate discharge sources is necessary to realize long-term stability of the electron gun. We devised a conditioning sequence which repeatedly increases and decreases the voltage applied to the barrel in N2 atmosphere. This conditioning sequence allowed us to dramatically decrease the probability of discharges, allowing us to achieve long-term stability operation.