A Single-Atom Electron Source in Practical Gun of an Extreme High Vacuum

There exists a long history of developing electron sources relating to improvement of performance of electron microscopes. A century ago, thermionic electron sources were developed by Richardson, and cold field emission (CFE) electron sources were developed by Crew a half century ago. Recently, novel single-atom electron sources have attracted large attention as the novel electron source with high brightness in high-performance electron microscopes because of their excellent emission characteristics in practice [1–5]. The single-atom electron sources are prepared by heating noble-metal-covered W<111> tips in ultra high vacuum (UHV). Since the surface energy of the {211} faces are extremely decreased by monolayer coverage of noble metals, the anisotropy of surface energy generates spontaneously the faceting structure at elevated temperatures; the areas of the stable {211} faces extend on the W surface, and nano-pyramids with three {211} faceting faces grew on the (111) faces [6–8]. If the flat area of the (111) face is narrow, only one nanopyramid is generated [1, 2]. Recently, Field Ion Microscopy (FIM) observations showed that the top of the nano-pyramids was always terminated by a single atom, and the top single atom was stable against heating [1, 3]. In addition, Field Electron Microscopy (FEM) and Field Electron Spectroscopy (FES) studies showed that the electron beam emitted from the single atom of the nano-pyramid was collimated, and the energy width was the same as those of conventional CFE


I. INTRODUCTION
There exists a long history of developing electron sources relating to improvement of performance of electron microscopes.A century ago, thermionic electron sources were developed by Richardson, and cold field emission (CFE) electron sources were developed by Crew a half century ago.Recently, novel single-atom electron sources have attracted large attention as the novel electron source with high brightness in high-performance electron microscopes because of their excellent emission characteristics in practice [1][2][3][4][5].The single-atom electron sources are prepared by heating noble-metal-covered W<111> tips in ultra high vacuum (UHV).Since the surface energy of the {211} faces are extremely decreased by monolayer coverage of noble metals, the anisotropy of surface energy generates spontaneously the faceting structure at elevated temperatures; the areas of the stable {211} faces extend on the W surface, and nano-pyramids with three {211} faceting faces grew on the (111) faces [6][7][8].If the flat area of the (111) face is narrow, only one nanopyramid is generated [1,2].
Recently, Field Ion Microscopy (FIM) observations showed that the top of the nano-pyramids was always terminated by a single atom, and the top single atom was stable against heating [1,3].In addition, Field Electron Microscopy (FEM) and Field Electron Spectroscopy (FES) studies showed that the electron beam emitted from the single atom of the nano-pyramid was collimated, and the energy width was the same as those of conventional CFE electron sources, ∼0.2 eV.Because a collimated beam was emitted from the single atom, the beam brightness was extremely high, 2 × 10 10 A/cm 2 sr (2 keV), which is about two orders of magnitudes as high as those of conventional CFE emission sources [3][4][5].Furthermore, the single-atom electron sources have demountable and repairing functions [3,4].Even if the top of them was contaminated with various gases by air exposure or destroyed by ion bombardment, the nano-pyramids terminated with a single atom could be regenerated repeatedly by heating in UHV.These peculiar properties discussed above indicate clearly that they were suitable for an electron source in many kinds of electron microscopes.
In this work, for developing scanning electron microscope (SEM) mounted with the single-atom electron source, we constructed a practical XHV chamber, and demonstrated the excellent characteristics of the emission beams from the single atom electron source in this chamber.

II. EXPERIMENTAL A. Gun chamber and evacuation system
In general, the higher vacuum is better for both stability of the emission and lifetime of FE electron sources [9].Because of the narrow emission area, single-atom area, and bombardment of ionized gases give fatal damage to nano-pyramids of the single-atom electron sources, we tried to reduce the operating pressure.Figure 1 shows a schematic diagram of vacuum evacuation system.The gun chamber of 6.9 in volume was made of stainless steel SUS316L.To decrease the outgas from both the electrodes and the gun chamber walls, we polished all the components inside of gun chamber including inner walls of the chamber to mirror finish by electromechanical polishing, and heated them up to 350 faces [10].The large outgas of the first baking was evacuated by a diffusion pump charged with Santovac5 (600 /s vs. N 2 ) with a cold trap cooled by Lq.N 2 .The high vacuum was maintained by an ion pump (125 /s vs. N 2 ) with a NEG pump (St707, 400 /s vs. H 2 , SAES Getters), because NEG pump can evacuate hydrogen gases even in XHV.The pressure was measured by extractor gauge (IM540, Leybold ).

B. Preparation method of single-atom electron sources
The single-atom electron sources used in this experiment were prepared as follows.At first, we sharpened one end of a single-crystalline <111>-oriented W wire with a diameter of 0.127 mm by electrochemical etching with KOH.In UHV, we cleaned up the tip surface by heating and/or field evaporation, and deposited palladium (Pd) film in a few-atomic layer on the tip surface.The Pdcovered tip was heated at 700 • C in UHV for manufacturing of the single-atom electron source [1], which was confirmed by using FEM and Fowler-Nordheim plots measurement [3].Since the single-atom electron sources have a demountable function, they were transferred through air atmosphere to the gun chamber constructed in this work.In the gun chamber, we investigated emission characteristics.Figure 2 illustrates a schematic diagram of electron optics used in this experiment.It is a part of the electron optics in a high resolution SEM, which constitutes an electron gun, a condenser lens, and a Faraday cup with a screen.
Because the tip alignment was not perfect in this experiment, we used the condenser lens to measure the probe current incoming to the Faraday cup I F .The total emission current I T and the probe current I F were measured as a function of operation time in the various stages of the tip preparation.

III. RESULTS AND DISCUSSIONS
Figure 3 shows the evacuation characteristics of the gun chamber constructed in this work.We baked uniformly all the vacuum walls at 150 • C for three days during the evacuation by the diffusion pump; in baking, the temperature differences at various points of the chamber walls were kept to be within 5 degree.When the temperature decreased to a room temperature and the cold trap was filled with Lq.N 2 , the pressure reached 3×10 −9 Pa.Subsequently, we degassed the NEG pump and activated it by heating at about 430 • C for a one day.As a result, the base pressure attained 1 × 10 −9 Pa.
The single-atom electron source was mounted in the XHV chamber and then heated for fabricating nanopyramid.Since there are no ways to observe its emission pattern in this chamber, we measured the ratio of I F to I T , I F /I T , as an indication of the beam collimation due to the pyramid formation.The large ratio means the high collimation of the emission beam.Figure 4 shows change in the ratio of I F to I T vs. the number of heat treatments.In the early stage of the heating, the ratios were extremely low, which means that no confinement of the emission area occurred.With repeating the heat treatments, faceting structure spontaneously grew, resulting in the large increase in the ratio and finally, saturating the ratios.The final ratios were about 0.8.In this experiment, the beam was not perfectly aligned as described above; the ratio should increase furthermore, if one can adjust more precisely the beam direction.
Figure 5 shows typical emission characteristics as a function of operating time at the condition of the saturated ratio; the left vertical axis indicates the emission currents, I F and I T .The right vertical axis means the ratio of I F to I T .As shown in Fig. 5, changes in I F and I T were synchronized each other, which means that both the two currents were emitted from the same area.It was consistent with the fact that the ratio was about 0.8: namely, most of the emitted electrons could reach the Faraday cup through the electron optics.The ratio was about two orders of magnitude as high as those of conventional CFE electron sources.The data demonstrated clearly the following peculiar characters of the novel single-atom electron sources.
1.The first one is the demountable character; the single-atom electron source was prepared in the separated vacuum chamber by deposition of Pd metals and heating in UHV.The source was re-mounted in the gun chamber.The collimated beam was reproduced in the different vacuum chamber.
2. The second one is the high efficient character, which means that the collimated beam was realized in the practical electron optics.
3. The third one is the stable emission.The fluctuation of total emission current of ∼1 nA was about 0.1 %.However, it is not high enough to generate the secondary electrons for SEM image.With increasing the emission current, in general, the ionization probabilities of residual gases increase.As a result, the probability of ion bombardment increases and the emission current tends to be unstable.
The sufficient total current for the electron optics of the practical SEM is estimated to be 20 nA in the present electron source.Therefore, we measured the stability of  a total emission current at 20 nA.As shown in Fig. 6, the excellent stability was realized: The fluctuation of emission current was about 0.8 %.Hence, one can expect that the single-atom electron sources can work excellently in practical SEM.Main reasons for the observed stable emission originated from the collimated beam in XHV.The observed emission angle of the electron beam was about ±1 • (FWHM), which is highly collimated as compared with conventional ones.The fact together with operation in XHV resulted in the extremely low ionization probability of the residual gases around the electron source as compared with those of the conventional ones.

IV. CONCLUSION
We summarized this work as follows: 1. We constructed a XHV chamber of an electron gun.
All the inner wall of the chamber and the electrode surfaces of the electron gun were polished to mirror surface.All the components were baked at 350 3.In the XHV, the emission current of the single-atom electron source was highly stable, even at 20 nA of the total current, which is enough for operation of SEM function.
Observation of SEM image by using single-atom electron source is now in progress.

FIG. 2 :FIG. 3 :
FIG.2: Schematics of electron gun and electron optics.Here, IT and IF represent the total emission current and the incoming current to Faraday cup, respectively.

FIG. 4 :
FIG.4: Change in the ratio of IF to IT vs. the number of heating treatments.

8 FIG. 5 :
FIG. 5: Total emission current IT and incoming current to faraday cup IF, and the ratio of IF to IT as a function of operating time.

FIG. 6 :
FIG.6: Fluctuation of the total emission current at 20 nA vs. operating time.
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