A design for a subminiature, low energy scanning electron microscope with atomic resolution

Abstract

We describe a type of scanning electron microscope that works by directly imaging the electron field-emission sites on a nanotip. Electrons are extracted from the nanotip through a nanoscale aperture, accelerated in a high electric field, and focused to a spot using a microscale Einzel lens. If the whole microscope ͑accelerating section and lens͒ and the focal length are both restricted in size to below 10 m, then computer simulations show that the effects of aberration are extremely small and it is possible to have a system with approximately unit magnification at electron energies as low as 300 eV. Thus a typical emission site of 1 nm diameter will produce an image of the same size, and an atomic emission site will give a resolution of 0.1-0.2 nm ͑1-2 Å͒. Also, because the beam is not allowed to expand beyond 100 nm in diameter, the depth of field is large and the contribution to the beam spot size from chromatic aberrations is less than 0.02 nm ͑0.2 Å͒ for 500 eV electrons. Since it is now entirely possible to make stable atomic sized emitters ͑nanopyramids͒, it is expected that this instrument will have atomic resolution. Furthermore the brightness of the beam is determined only by the field emission and can be up to 1 ϫ 10 6 times larger than in a typical ͑high energy͒ electron microscope. The advantages of this low energy, bright-beam electron microscope with atomic resolution are described and include the possibility of it being used to rapidly sequence the human genome from a single strand of DNA as well as being able to identify atomic species directly from the elastic scattering of electrons.

Fabrication of nanoscale electrostatic lenses

“…The simulated properties of our 500 nm electrostatic lens system were reported in a previous paper [11], where the first electrode is used to accelerate the electrons, and the rest are used to create an Einzel lens (figure 1). In these simulations, it was shown that a focused beam spot size of 1.24 nm is possible to achieve at a working distance of 4.9 μm with a unit beam magnification, while the dominant chromatic aberrations were estimated to be as low as 0.02 nm.…”

“…This makes it possible to obtain a bright electron beam with a high resolution at low voltages, where even the dominant chromatic aberrations can be significantly reduced [8,9]. Based on this idea of down-scaling, extensive work has been done to develop practical low-energy electron-beam systems as early as 1976 [10], and progress in nanofabrication and metrology has generated a renewed interest in such devices [7,9,[11][12][13][14][15][16][17]. Consequently, complex electron-beam systems with column dimensions in the millimeter range have been developed [14,15].…”