For SEM operating at very low current, the spot size is now limited by diffraction (Rd, same expression as in optical microscope) and the spherical aberration which Is expressed as: R, = Ecso where Cs IS the spherical aberration coefficient and a is the half-angle of the cone of beam from the focus spot to the objective aperture. The combined spot size is the quadratic sum of the diffraction resolution and spherical aberration resolution, given by the expression: (R2 = Rd2+ R52). (a) Find out the optimum a to achieve the minimum R, and the corresponding R. (b) Now use your result to estimate lthe optimum value of o: and spot size R at 10W and 200 W acceleration voltages. Cs~1mm for this electron microscope. (c) Clearly, even without correcting spherical aberration, the spot size can be much smaller than 1 nm. Should we make an SEM operating at 200 W