Energy spectra of field emission electrons from some materials at different temperatures are reviewed in the practical points of views; they are conventional refractory metals, multi-walled carbon nano-tubes, Nb superconductors and single-atom electron sources. While the spectra of conventional refractory metals are explained well on the basis of Folwer-Nordheim theory, those of the other systems are not; the main peak with a different shape appears in the spectra of CNT, the extra peaks overlap in addition to the main peak of CNT and the single-atom electron sources. An extra sharp peak appears at the Fermi level of the spectra of a Nb superconductor.
A single-atom electron source was applied to a low-voltage scanning electron microscope. We developed a mounting technique of an electron emitter by using X-ray diffraction method, and a mounting stage which enables us the mechanical adjustment of the emitter's tilt angle as well as its XY positions. It was made possible to align the emission direction precise enough for the electron optics of the microscope. In the trial operation, a spatial resolution of 1.7 nm was achieved at an acceleration voltage of 5 kV by using a ZrO/W Schottky electron emitter. At present, the mounting of a single-atom electron source is in progress.
A compact review of our research activities to improve various beam performances of spin polarized electron source, such as, 1) spin polarization, 2) quantum efficiency, 3) peak current density, 4) brightness, 5) beam energy resolution, 6) response time, and 7) NEA surface life time is given. In particular, our efforts toward higher brightness and high spin polarization electron source or LEEM using transmission photocathode are described in detail. The achievements are summarized as follows: A 20-keV gun was constructed with a transmission photocathode including an active layer of a GaAs-GaAsP superlattice. The photocathode illuminated by laser light with a spot diameter as small as 1.3 μm and a brightness of ∼2×107 A·cm−2·sr−1 (a reduced brightness of ∼1.0×107 A·cm−2·sr−1·V−1) and a peak polarization of ∼90% were achieved at the same time for an extracted current of 5.3 μA. This brightness is still one order magnitude smaller than those of the W-field-emitters, but one order higher than those of LaB6 emitters and three orders than those of conventional polarized electron sources.
A high resolution X-ray microscope equipped with a multi-walled carbon nanotube cathode was developed. This system has ordinary scanning electron microscopy functions available for the precise adjustment of focusing conditions including astigmatism and an alignment of an electron beam. By the adoption of a transmission type, X-ray images with high magnification can be easily obtained. The diameter of an electron probe, which is one of the factors limiting a spatial resolution of X-ray microscopes, was estimated to be about 270 nm from a resolution of obtained SEM images. Clear X-ray images with the resolution higher than 400 nm were successfully obtained.
An extreme high vacuum field emission microscope (XHV-FEM) was constructed for the study of inherent fluctuations of field emission (FE) current. Damping and fluctuation of FE from clean W(111) tips at 90 K were observed using the XHV-FEM. Semilogarithmic damping curves of FE currents were linear in our thoroughly degassed XHV-FE system. The slope of semilogarithmic damping curves was linearly proportional to the operation pressure, suggesting a method for measuring pressure in an XHV range. The noise of FE currents ranging from 10 pA to 100 μA was measured under ∼7×10−10 Pa. The lowest frequency measurement of shot noise was recorded even at 4 Hz.
A simple and repeatable modification technique for single-crystal tungsten <111> oriented tips is reported. The modification technique was based on field-assisted oxygen etching of the peripheral tungsten atoms of the tip apex. Field-ion microscopy (FIM) was used to etch and to visualize the real-time etching events, and field emission patterns were observed. During modification via controlled etching of the tungsten tip, the FIM bias typically decreased from 4.4 kV to 1.6 kV. This bias change corresponded to a reduction of the radius of curvature from 10 nm to 3 nm. The shape of the etched tip was evaluated by field evaporation and ball models. The sharpened tips emitted electrons at low-bias voltages with good geometrical confinement. The features of field emission were evaluated by Fowler-Nordheim plots. We are also developing a new method to obtain low-energy electron diffraction (LEED) patterns from surfaces using field emission beams. LEED patterns of the Cu(001) clean surface was observed using field-emitted electrons from tungsten tips. The typical emission current, bias voltage and estimated probing diameter for the observed diffraction patterns were 0.15 nA, 75 V, and 4 μm, respectively.
We proposed a novel CdTe X-ray image sensor, which was driven by the FEA, to obtain high spatial resolution X-ray images and have demonstrated the principle operation by using the CdTe image sensor with one pixel. We have also fabricated a FEA matrix with 12×12 pixels to obtain X-ray images. For the further improvement of spatial resolution in the CdTe image sensor, we have proposed a novel double-gated FEA with a focusing lens, which was fabricated by using the etch-back method. The double-gated FEA showed a good focusing characteristic without significant decrease of the emission current, when the height of the focus electrode was optimized. The CdTe image sensor driven by the double-gated FEA is promising for an ultra-high-resolution X-ray image sensor.
A 640×480 pixel image sensor, which consisted of an integrated field emitter array (FEA) equipped with an active drive circuit and a high-gain avalanche rushing amorphous photoconductor (HARP) film, in close proximity to each other, was fabricated and tested as a step toward the development of ultrahigh-sensitivity compact image sensors. Our experimental results revealed that the prototype sensor provided both high sensitivity due to the avalanche multiplication effect of the HARP film and sufficient resolution as a VGA image sensor.
We have fabricated a manipulation system with a force detection, which uses a self-sensing cantilever, in a chamber of a scanning electron microscope. This system can simultaneously manipulate a nanoparticle and detect a force needed to move it, a detection resolution of which is approximately 1 nN. In this work, nanoscale peeling processes of a multi-walled carbon nanotube (MWCNT) on the graphite substrate have been studied.