November 19, 2008, Wednesday, at 15:30
T. L. Kirk, Laboratory for Solid State Physics, ETH Zurich
Near Field Emission Scanning Electron Microscopy

In conventional scanning electron microscopy (SEM) the lateral resolution is limited by the electron beam diameter impinging on the specimen surface and interaction volume of the electron collisions. This limit is also critical for the subsequent analysis of the resultant electrons, e.g. spin polarization (SEMPA), electron energy loss spectroscopy (EELS), and auger electron spectroscopy (AES). The close proximity between the probe and sample surface of a scanning tunneling microscope (STM) operating in field emission (FE) mode provides a means of overcoming this limit. In this talk, I present a simple "near field emission scanning electron microscope" (NFESEM) capable of imaging conducting surfaces with high spatial resolution. This microscope is also refined to overcome the problems associated with the prior art, while introducing a means of comparative surface imaging using both the variations in electron intensity and the FE current. Variable current imaging generated by an STM operated in constant height (CH) mode has often been used to produce high resolution images of the surface. Moreover, STM FE mode operation was also used for mapping the FE sites on surfaces. It is our intention to employ such well-known imaging techniques as a comparison to our electron intensity images, which are the product of the primary beam of field-emitted electrons from an STM tip.

I will report on the first topographic electron intensity image of terraces and mono-atomic steps on a single crystal substrate, not yet attained with a remote electron gun in conventional SEM. In addition the simultaneously recorded FE current surface mapping, limited only by the incident beam girth, closely resembles the topography of the electron intensity. This indicates that the maximum resolution, in accordance with the tip-sample geometry, has been reached. High spatial resolution was achieved by adhering to established theoretical models relating to the beam width of field-emitted electrons from a sharp Tungsten (W)-tip , which was shown to mainly depend on the emitter radius and the tip-sample separation gap. Complimentary STM imaging, directly following NFESEM measurements, is feasible and can easily be performed. We assert that additional analysis of the secondary electrons will also exhibit a comparable resolution. This is ongoing work in collaboration with Dr. Urs Ramsperger and Prof. Dr. Danilo Pescia.