Simulation of an improved magnetic–electrostatic detector objective lens for LVSEM

The simulations of the imaging properties of several magnetic–electrostatic detector objective lenses are presented. We have assumed that the magnetic circuit has a radially arranged pole-piece gap. By using this snorkel lens design the specimen is immersed in a strong magnetic field. The calculations show that the chromatic aberration coefficient, which essentially determines the resolution in low-voltage scanning electron microscopy, only decreases with increasing immersion ratio if the radius of the inner pole piece is not too small. Furthermore we determined the collection efficiency of secondary electrons for a lens variant which is optimized with respect to primary electron optics.