A Field Emitter Electron Beam Exposure System

Abstract

An electron beam exposure system is built around a field emission scanning electron microscope. To the basic microscope a set of post-lens deflection coils has been added. These coils are optimized for low deflection astigmatism. The coils and the high brightness field emission gun allow a beam with a diameter of less than 50nm to be deflected over an area of 4 × 4 mm2. The beam current is 3-4 nA. Dose variations due to current instabilities in a beam emitted from a field emission cathode is reduced by measuring the beam current continuously and adjusting the scan speed accordingly. Dose variations are thus reduced to a few percent.The system is controlled from a minicomputer with a magnetic disc for pattern and program storage. The deflection system consists of two parallel digital to analogue converters for each deflection coordinate. One converter has a high accuracy and is used for placing pattern elements in the deflection field. The other has a high speed and is used for the fill-in scan for each element. The converter outputs are added together at the input of the main deflection amplifier. This deflection system allows both a high scanspeed and a high positioning stability (300 ns per 125 nm step/± 32 nm in a 2 × 2 mm2 scanfield).