A thermal heated field emitter gun for E-beam exposure

For several years tungsten field emitter (FE) sources have been used with scanning electron microscopy. Also the FE cathodes have been used in electron beam microfabrication systems. The reasons for this are the high brightness and the low energy spread in FE-sources compared with conventional thermal heated cathodes. The obtainable current from a field emitter at room temperature shows great fluctuations and it is not useful in electron beam lithography systems without some form of automatic dosage control. The authors' experience is that one is able to condition the field emitter for stable operation within 30 min. Beam current fluctuation after the initial period of operation lies within +- 5% and the current will stay stable up to 10 h. One easily obtains a current level of 5-10 nA in the system which is equipped with a 100 μm diameter limiting aperture, giving a beam half angle α of 2.0 mrad. With the small semiaperture angle, α, obtainable and the high brightness in the TFE source it is possible to obtain a current of 100 nA in a spot size of about 0.1 μm diameter. In direct beam writing the figures given above imply a steprate of 10 MHz (dose 1 X 10-4 As cm-2). If 10% of a 4 inch wafer is exposed the exposure time will be 3 h for one layer in a process sequence. (Auth.)