Conference-ISSS-7-Time of Flight Analysis of Field-Ionized He in Mixtures with Ne or Ar

An enhancement of current of field-ionized He from a W⟨111⟩ gas field ion emitter in mixtures with Ne or Ar and its origin were investigated by a field ion microscope equipped with a time of flight (ToF) spectrometer. He ion current was increased up to three times by He-Ne mixture and two times by He-Ar mixture, respectively. Mass to charge ratio spectra show that only He atoms were field-ionized at a field of operating condition of a He gas field ion source. In both gas mixture conditions, a peak of singly charged He in ToF spectra shifted after the mixture of He and Ne or Ar, which corresponds to that He was field-ionized at lower potential energy. These observations indicate that He atoms field-adsorbed on a tungsten surface are exchanged for Ne or Ar atoms and the exchange of species of field-adsorbed atom is also related to the enhancement of He ion current. [DOI: 10.1380/ejssnt.2016.23]


INTRODUCTION
Recently, investigations of gas field ion sources (GFISs), whose principal is based on field ionization, have been expected that its applications are wider range of technical applications, e.g.photomask repairing for semiconductor devices, microfabrication for micro electro mechanical systems and sample preparation for transmission electron microscopy [1,2].In 2006, a helium ion microscope (HIM) commercialized by ALIS inc.performed to image with high spatial resolution, minimal charging and surface sensitivity [3].He ion beam generated in the HIM could be focused into a sub-nm probe size on a sample and resulted in possibility of milling and observing fine structures without ionic contamination problems caused by Ga ions [4,5].Since an angular current density of GFIS is several orders of magnitude lower than that of a gallium liquid metal ion source (Ga-LMIS), which is equipped with an existing focused ion beam (FIB) system.Therefore GFISs with higher ion intensity are required [2].
Over a few decades from an invention of a field ion microscope (FIM) by E. W. Müller in 1951 [6], many researchers [7][8][9] had attempted to improve contrast, intensity and resolution of FIM images.K. D. Rendulic reported that a use of mixture of noble gases as an imaging gas is one of attempts for intensifying image brightness [8].By using mixed noble gases, the intensity of FIM image became larger than sum of the intensities produced by the individual noble gas in the FIM.In He-Ne mixture, especially, it has been reported that a quality of FIM image is improved by higher thermal accommodation due to an exchange of field-adsorbed He for Ne atom [9].But there are no experimental evidences for the exchange of field-adsorbed atoms and field ion species in gas mixtures.
In this study, for increasing an ion current, we mixed He with Ne or Ar under operating conditions of He-GFIS.We evaluated ratios of secondary gaseous ions to He ion and effects on He ion by using a FIM equipped with a time of flight (ToF) spectrometer.

II. EXPERIMENTAL
A schematic diagram of our experimental system which consists of a gas chamber, a field emission microscope (FEM) and a FIM chamber and the ToF analyzer chamber is shown in Fig. 1.The FEM/FIM and ToF chambers are evacuated differentially by turbo molecular pumps.Ultimate pressures of the gas chamber, the FEM/FIM chamber and the ToF chamber are 1.0 × 10 −6 Pa, 2.0 × 10 −8 Pa and 1.0 × 10 −7 Pa, respectively.
An emitter was spot welded on a tip of a tungsten hairpin filament.The temperature of the emitter can be cooled down to 50 ± 0.05 K with a He-cryostat.By rotating the emitter by 90 degrees, the emitter surface can be evaluated using the FEM, the FIM and the ToF analyzer.To avoid effects of impurity gases on emission properties, high purity He, Ne, Ar gases (99.9995% up) were used and passed through a NEG pump in the gas chamber and the FEM/FIM chamber, respectively.To observe FIM images, the positive acceleration and negative extraction voltages were applied to the emitter and an extractor, which was placed at 3 mm apart from the emitter, respectively, and field ions from the emitter were accelerated toward a micro channel plate (MCP) placed at 50 mm from the emitter.For measurements of the ion current, specific ion emission site was adjusted by a gimbal system toward a probe hole with a diameter of 3 mm at a center of the MCP and ions passing through the probe hole were corrected by a retractable Faraday cup placed just behind the MCP, and the ion current was measured by a pico-ammeter (Keithley type-6485).
For ToF analysis, an ion beam passing through the A ⟨111⟩-oriented single-crystalline tungsten wire with 0.127 mm in diameter, which was used as the emitter material, was sharpened by electrochemically etching with 5 mol/l NaOH.The prepared tungsten emitter was introduced to the experimental system, and its surface was cleaned in-situ by a resistive heating of about 1000 K under a UHV. Figure 2 (a) shows a FIM image of a clean surface of the W⟨111⟩ emitter after field evaporation at 14.87 kV.After the cleaning process, a field of about 8 V/nm was applied to the emitter apex and the emitter temperature was raised to about 1500 K and thereby the apex was terminated in several atoms, as shown in Fig. 2(b).The FIM image shown in Fig. 2(b) indicates that a nano triangular pyramidal structure was fabricated on the emitter apex which cases a confinement of emission site and the enhancement of angular density [2].
Over all experiments, the emitter temperature was fixed at 50 K.The experiment was performed as following step.He ion current was measured as a function of ap-FIG.3. Applied voltage dependence of He-FIM images of the W⟨111⟩ emitter as initially formed by field evaporation at 50 K under He pressure of 6.7 × 10 −5 Pa.(a) 5.9 kV, (b) 6.1 kV, (c) 6.5 kV and (d) 6.9 kV.The resolution of FIM image of trimer is highest at the voltage of 6.9 kV which corresponds to the best image field.
plied voltage just after introducing He gas.After mixing 1.5 × 10 −4 Pa of He-2%Ne or 8.0 × 10 −4 Pa of He-18%Ar, ToF measurements were conducted for about 10-15 minutes repeatedly over a period of 75-90 minutes.In the ToF measurements, the acceleration voltage was fixed at 5.93 kV and an electric field was controlled by applying the extraction voltage.After ToF measurements, I-V characteristics of He-GFIS operated in gas mixture conditions were measured.

III. RESULTS AND DISCUSSIONS
To estimate a field enhancement factor of the emitter, FIM images were observed with respect to the applied voltage.Figure 3  of the emitter and k is a field enhancement factor defined by a local geometry of emitter [10].By using this equation and the best image field of 44 V/nm for He-W system, the field enhancement factor can be estimated.The best resolution was obtained from a FIM image taken at 6.9 kV which corrensponds to the best image voltage (BIV).In terms of the estimated field enhancement factor, a local electric field was calculated in the present experiments.
An enhancement of current of field-ionized He from a W⟨111⟩ gas field ion emitter in mixtures with Ne or Ar were investigated.Figure 4 shows FIM images and I-V characteristics before and after mixture of He and Ne.In FIM images, ion currents emitted from encircled emission sites were measured.For He-Ne mixture, the ion current was enhanced over the measured range.For He and He-Ne mixture, the maximum ion currents were obtained at 5.93 kV and 6.13 kV, respectively, which are defined as the best source voltage (BSV).Comparing the ion currents at BSV, He ion current was increased by a factor of three for He-Ne mixture.Figure 5 shows FIM images and I-V characteristics before and after mixture of He and Ar.In FIM images, ion currents emitted from encircled emission sites were measured.For He-Ar mixture, the ion current was enhanced above the BSV of 5.25 kV for He-Ar mixture.Comparing the ion currents at He-BIV of 5.68 kV, He ion current was increased by a factor of two for He-Ar mixture.
Ion species and charge states of the ion beam under gas mixture conditions were analyzed by ToF spectrometer.Figure 6 shows mass to charge spectra of detected ion beams under He-Ne and He-Ar mixture conditions.For He-Ne mixture, the He-BSF of 40 V/ nm was applied.For He-Ar mixture, 20% above the He-BIF was applied.In both He-Ne and He-Ar mixture conditions, only a peak of singly charged He was observed.Although Ne and Ar can be field ionized far away from the emitter where the fields were ionization field of 37 and 22 V/nm for Ne and Ar, respectively, these ionization rate was undetectable level in the present experimental conditions.
More detailed investigation of the peak of singly charged He in ToF spectra was performed.Figure 7 shows ToF spectra of He ions under He-Ne and He-Ar mixture conditions.In each spectrum, signals were accumulated for 10 or 15 minutes after introducing Ne or Ar gas, respectively.For He-Ne mixture, an abrupt peak shift of 2.4 ns was observed after 45 minutes from introducing of Ne gas.For He-Ar mixture, a gradual peak shift of 1.1 ns was observed after 15 minutes from introducing of Ar gas.After 75 minutes for He-Ne mixture and 90 minutes for He-Ar mixture, additional peak shifts could not be observed.
To clarify origin of the peak shifts, we calculated as follows; Relation between the potential energy of n charged ion and the kinetic energy is given by, neV = 1 2 m where e is an electric charge, V the acceleration voltage, m an atomic mass, L a flight length and t a flight time.Firstly, we estimated the flight length, L, in terms of a first arrival time of He ions and the acceleration voltage when no other gas was introduced.Secondary, a calculation of a difference of an ionization position was performed with an electrical potential after the peak shift, which was obtained by substituting L and the arrival time after the peak shift, and the applied field which is assume to be FIG.9. Experimental results for each field in gas mixture at 50 K.A current enhancement factor is defined as a value that maximum current for gas mixture divided by maximum of current for pure He.The distance difference is defined as the value of ∆x.
uniform near the emitter surface.Under this consideration, distance differences of the ionization position can be estimated to be 3 Å for He-Ne mixture, and 1 Å for He-Ar mixture, respectively.In a diagram of a model for origin of the peak shift as shown in Fig. 8, X He and X Ne or Ar are distances of the ionization position from the tungsten surface for He adatom and Ne or Ar adatom, respectively, and the difference between X He and X Ne or Ar , ∆x, is estimated by the calculation of times of peak shifts.Assuming that the field is uniform near the emitter surface and a critical distance Xc is independent of adatoms and applied field, for He-Ar mixture, it is considered that the peak shift was caused by He ion ionized above field-adsorbed Ar atoms since the distance difference from the surface of adsorbed He or Ar was van der Waals radius of about 1 Å as listed in Table I.Similarly, for He-Ne mixture, there are some possibilities that the distance difference of  by He ion ionized above Ne dimer field-adsorbed perpendicular to the surface which was pointed out by T. Tessner et al. [11].
Experimental results for each field in gas mixture were performed as shown in Fig. 9. Adjusting conditions of applied fields, a severalfold increase of He ion current was confirmed in all combinations.In addition, it is considered that these experimental results suggest that the ionization position depends on the local electric fields at an atomic site in both He-Ne and He-Ar mixture condi-tions.A field dependence confirmatory experiment is in progress.

IV. CONCLUSION
We performed the experiments for increasing the fieldionized He in mixtures with Ne or Ar and evaluated ratios of secondary gaseous ions to He and effects on He ion.As a result, He ion current was increased by a factor of three at the BSV for He-Ne mixture, and two at the BIV for He-Ar mixture.In addition, in both gas mixture conditions, at field ranging from 40 to 53 V/nm corresponding to field between BSF and BIF+20%, only singly charged He was detected beyond background level which was confirmed by ToF spectrometer.From a set of experimental results for the peak shifts, it was interpreted that the field ionization occurred on the field-adsorbed atom [12], substitution of field-adsorbed atoms occurred in gas mixture condition, and the exchange of species of field-adsorbed atom is also related to the enhancement of He ion current.
FIG. 4. FIM images and I-V characteristics at the He pressure of 1.5×10 −4 Pa and mixing with 3.4×10 −6 Pa Ne at 50 K.FIM images of the W⟨111⟩ emitter observed at 5.93 kV (a) before and (b) after introducing Ne gas.(c) I-V characteristics before and after introducing Ne gas.

FIG. 6 .
FIG. 6. Mass to charge spectra of detected ion beams under He-Ne and He-Ar mixture conditions at 50 K.(a) 1.5×10 −4 Pa of He-2%Ne was mixed and the applied field was 40 V/nm (BSF).(b) 8.0 × 10 −4Pa of He-18%Ar was mixed and the applied field was 53 V/nm (BIF+20%).In both gas mixture conditions, at field ranging from 40 to 53 V/nm, only singly charged He was detected beyond background level and the peak of Ne or Ar was less than lower detection limit.
FIG. 7. ToF spectra of He ions under He-Ne and He-Ar mixture conditions at 50 K.(a) 1.5 × 10 −4 Pa of He-2%Ne was mixed and the applied field was 40 V/nm (BSF).The abrupt peak shift of 2.4 ns was observed after 45 minutes from introducing of Ne gas.(b) 8.0 × 10 −4 Pa of He-18%Ar was mixed and the applied field was 53 V/nm (BIF+20%).The gradual peak shift of 1.1 ns was observed after 15 minutes from introducing of Ar gas.