A new focusing system was designed by optimizing its optical properties in different beam currents and energies. Damped least square method was used to design a single lens and lens system.The single lens was designed through combination of separating and assembling

and the ratio of aberration coefficient and focus length at infinite magnification was chosen as the objective. Both optical properties and practicability were considered when choosing the working modes between lenses. Collimated mode was used in the larger beam current while crossover mode was in the smaller one. Axial aberration and magnification were the objective in bigger beam current and in the smaller one respectively when optimizing the two lenses. The results show that the resolution is 31.52 nm (the magnification is -0.539 095 5 and the aberration is 16.33 nm) and 4.73 nm (the magnification is 0.084 359 9 and the aberration is 2.15 nm) under beam current of 2 nA and 2.5 pA. The lens system can work under different beam energies when adjusting the potential on the second plane and the space between the second plane and the third plane of the lens 2

which will enlarge its applications

and the etching

deposition

implantation and imaging can be realized in the same system.