LIMITS OF LOW-ENERGY-ELECTRON OPTICS

High-resolution beams of low-energy (20-2000 eV) electrons are obvious ly attractive because of the very compact volume of interaction betwee n the beam and sample, and because the associated high secondary emiss ion coefficient minimizes charging of insulating samples. Previous wor k showed that values of aberration coefficients can be scaled down wit h voltage and minimum values are achieved by maximizing the focusing f ield. Here we derive some very simple expressions for the minimum beam diameter and show some experimental results. For a magnetic lens with a constant field of 1 T operating at a large demagnification, the lim iting beam diameter is set by chromatic aberration (with an energy spr ead of 1 eV) and diffraction, and is approximately d(m) = 126 V-1/2 nm , where V is the electron energy in eV; the dependence on magnetic fie ld strength B is B-1/2. For a retarding field lens operating at a larg e demagnification and with the final landing voltage V much less than the accelerating potential, the limiting value of beam diameter is giv en by d(e) = 17 V-1/4 nm; the dependence on electric field strength E is E-1/2 . Experimentally, preliminary results were obtained with a sm all, (permanent) magnetic lens that approximates the constant field ca se and can be inserted into the work chamber of a conventional scannin g electron microscope (SEM). This added immersion lens effectively ext ends the range of operation of an ordinary SEM down to low energy, and has so far achieved 40 nm resolution at 300 V.