We have fabricated an instrument designed to couple both the imaging process in scanning electron microscopy and the precise positioning of scanning probe microscopy. This microscope is comprised of a metallic field emitter that generates an electron beam with a minimal impingement diameter of approximately one nanometer. In particular, the sensitivity of the field emission current to local topographic variations on the specimen surface enables the microscope to achieve atomic vertical resolution.

Project Description
Our technique, which we call Near Field Emission Scanning Electron Microscopy (NFESEM) [1, 2], produces two simultaneously-generated images by raster-scanning the field emitter, placed at a distance of some ten nanometers above a sample surface via a piezoelectric device.
The preferred scanning mode consists of keeping the emitter-to-surface distance approximately constant (i.e. constant height mode) and allows simultaneous detection of the field emission (FE) current, in accordance with the surface topography, and the secondary electron (SE) yield. Our usage of the term SEs refer to electrons, which are ejected from the sample as a consequence of the beam of field-emitted electrons impinging on the surface. Both images, one from the FE current mapping and one from the SE current mapping, closely resemble each other indicating that the main contrast mechanism is the FE current variations due to the local surface topography. It is important to note that the FE current (I_FE) and the primary electron beam energy (E_P) are analogous to the tunneling current (I_T) and bias voltage (V_B) used in scanning tunneling microscopy (STM). The aim of this project is to demonstrate the feasibility of imaging complex surfaces, such as those obtained from molecular beam epitaxy (MBE) growth of monolayers (ML)s Fe on W (110), which display controlled roughness on a variety of lateral and vertical scales. Ultimately, we plan to use NFESEM for magnetic contrast imaging of thin films and surfaces [3].

Sample Preparation
The W (110) substrate was prepared using the following method: First the substrate was annealed at 1000°C for 30 min..

Next oxygen was introduced into the vacuum chamber at a pressure of 4.0 x 10^-8 mbar for 30 min., in order to bind with the omnipresent carbon that migrates to the surface from the bulk.
This was followed by an additional annealing in an oxygen free atmosphere for 30 min.. The substrate was then flashed at a series of temperatures up to ~2000°C, in order to remove residual CO and CO₂ [4], before finally being flashed at 1700°C and cooled. Clean surfaces are very important for providing well-defined substrates for epitaxial growth of high quality overlayers.
Ultrathin films of iron have been subsequently grown by UHV MBE. Fig. 1 shows the topography of two epitaxial layers with different thicknesses, recorded by standard STM. The Fe grows, in this thickness regime (1.8 MLs in fig.1a, 1.1 MLs in Fig.1b), approximately layer-by-layer, meaning that the next layer does not start to grow before the previous one is completed. The layer-by-layer growth is only approximate: more than one layer is observed in a single image, as shown in figure 1, note the patches with various brightnesses.

Results and Comparison with STM
After growth, the samples were transported to the scanning Kerr microscope chamber for magnetic measurements. Longitudinal magneto-optical Kerr effect (MOKE) detected a square hysteresis loop for a magnetic field applied along the easy in-plane direction. No hysteresis was detected for samples with a thickness less than 2 MLs.
Both NFESEM and STM were performed on selected Fe coated samples, see e.g. figures 2 and 3. Although major features of the surface can be observed in the raw NFESEM images, the images were enhanced to reveal more contrast by correcting for the tilt of the sample [3]. The samples shown in figure 2 and 3 consist of 1.8 and 3.2 MLs of iron, respectively. In both images, we recognize at least four levels of contrast, indicating the presence of three Fe layers on top of a, still visible, W (110) substrate (the darkest regions). This can clearly be observed by the color-coded contour mapping of the topography in fig. 3b, depicted in fig. 3c. Here, the color spectrum ranges from "dark blue," indicating the W (110) substrate, to "dark red," which is the highest point on the surface (~ 3.2 MLs).
figure 4 shows the high spatial resolution imaging capabilities of NFESEM, as it is compared to a subsequent STM image performed in the same scan area. There are two large scratches, most-likely due to the substrate polishing procedure, which help to identify the same locations in all three images. The sample consists of 1.1 MLs of Fe -MBE grown- on a W (110) substrate, in accordance with the aforementioned procedure. The arrow indicates the location of some iron islands on a terrace. Most importantly, the precision of the subsequent scans show that drift does not significantly alter the scan area, when converting from NFESEM to STM.