Beam current instability analysis with FFT

In shaped-beam mask writing tools, beam current is fairly small, often under 1 muA, with required stability of Lt1%. Beam is originated at the cathode, and passes thru multitude of electron-optical components before reaching target plane. Hence, beam current stability is affected by multiple factors, like power supplies, spurious electromagnetic fields, temperature, pressure, and the emitterpsilas own instability. We report results of analysis which allows us to uncover and separate environment-related beam current instabilities from emitter-related ones.