a device for influencing an electron beam , especially a deflector unit for an electron beam lithography machine , comprises a plurality of coil formers each with a bore defining a passage for the beam and each carrying coils operable to generate magnetic fields for deflecting the path of the beam when passing through the passage . each former is made of a high - strength ceramic material having a high thermal conductivity and low coefficient of thermal expansion so that , with respect to a given output of heat by the associated coils during quasi - continuous operation for repeated beam deflection during pattern writing , the heat is dissipated at such a rate as to preclude thermal expansion of the coils and thus avoid distortion of the magnetic fields generated by the coils .