Active shielding suitable for electron-beam lithography systems

This paper describes suitable active shielding (AS) for electron-beam lithography systems (EBLSs). The optimal position of a reference sensor (RS), of which the signal is used to feed a current into a canceling coil (CC), is discussed. Preliminary experiments using an EBLS verification model and three-dimensional (3-D) magnetic field computations were carried out to find the optimal position of RS outside EBLS, which can attenuate magnetic disturbances inside EBLS effectively. As ferromagnetic metal walls mainly composed of permalloy are installed near EB orbits, it is considerably difficult to estimate their effects on magnetic disturbances. Transfer functions G_c and G_e were computed, which were defined as a ratio of the flux density outside EBLS to the average flux density inside EBLS in the cases when magnetic fields were generated by CC and the exciting coil (EC, simulating the magnetic disturbances), respectively. A region with G_c/G_e≈1, which corresponds to the optimal position of RS for noises perpendicular to EB orbits, is located above an electron optical column. This tendency coincided with that obtained by preliminary experiments using an EBLS verification model. It is confirmed by using a commercial-based EBLS. The degree of disturbance of EB in the case using AS with the optimal position of RS can be reduced to a quarter as compared with that in the case without AS.