NPGS : Magnetic Loops
Note that this device was written on a cold cathode FE SEM with no beam blanker.
Cold cathode FE SEMs inherently have an unstable beam current, where the fluctuations and drift depend primarily on the quality of the vacuum in the gun chamber.
Linewidths as small as 30 nm have been obtained on this system, although the instability of the beam current is an issue when the total exposure time is greater than a few minutes. Specifically, for 50 nm linewidths, yields on the order of 80% for exposure times of ~5 minutes have been obtained, but the yield drops to about 40% for total exposure times of ~20 minutes.
Historically, cold cathode FE SEMs have given the highest imaging resolution, however, modern thermal (Schottky) FE SEMs are clearly preferable for SEM lithography, since they typically have a beam stability on the order of 1% per 10 hours.