Optical design of a combined ion and electron beam system for nanotechnology

Abstract

Nanometer sized structures can be fabricated with an ion beam in several ways, but they would be destroyed if ions were used for observation of the newly made structures. Therefore, they must be observed with an electron beam or scanning probe microscope. A system is developed based on a scanning transmission electron microscope in which both an ion beam and an electron beam can be focused on the specimen. Several design concepts are discussed. In the chosen design, the ion beam is brought on the optical axis of the microscope by means of a 90° deflector which is also used as part of an energy filter. The particles are focused on the specimen by a combined electrostatic and magnetic objective lens. The smallest structure size that can be achieved, is determined by the aberrations of the lenses in the system and the interactions of the ions with each other in the beam (Coulomb interactions) and with the specimen (proximity effect). The system is optimized for these effects resulting in a design which should allow a smallest structure size of 7 nm when using Ar ion sputtering. The spatial resolution when using the electron beam to observe these structures is designed to be 0.6 nm.