Improvement of Mass Resolution in Wide-Angle Laser-Assisted Atom Probe by Flight Path Compensation

A wide-angle laser-assisted three-dimensional atom probe (3DAP) was developed, with a wide angle (0.75 sr) and short flight distance (110 mm) adopted to enlarge the analysis area of the tip. However, the short flight distance of ions resulted in the degradation of mass resolution. Therefore, flight path compensation was examined to improve the mass resolution in the 3DAP with a flight distance of 110 mm. With geometric compensation based on a simple concentric sphere model, the detected ions could be identified. By the compensation of the flight path for a small inclination on the microchannel plate (MCP) in the direction normal to the tip axis, the isotopes of detected ions were identified and high mass resolution was attained for a very short flight distance. [DOI: 10.1380/ejssnt.2009.35]


I. INTRODUCTION
The three-dimensional atom probe (3DAP) is one method for the three-dimensional imaging of materials on the atomic scale [1].The atoms are field-evaporated from the tip apex as ions when high voltages and high voltage pulses are applied to the tip.In this case, a high voltage pulse works as a trigger for the field evaporation of atoms.The elemental identity of ions is determined by the time-of-flight (TOF).The two-dimensional position of each detected ion is determined with a position sensitive detector (PSD) and the depth positions are inferred from the sequences of detected ions.An atomic scale image of the tip is reconstructed from these data in three-dimensional virtual space.In recent studies, laser pulses instead of voltage pulses have been used in 3DAP, which has enabled high mass resolution and the ability to analyze a large area of the tip [2].
If the diameter of the microchannel plate (MCP) is almost equal to the distance from the tip to the MCP, the flight path of ions hitting the outermost part of the MCP is longer than that for ions hitting the center by almost 10% or more.Therefore, the arrival time of ions with the same mass-to-charge ratio is dependent on the detected positions on the MCP.This causes a broadening of the TOF.When the first imaging atom probe was invented by Panitz, a spherically curved chevron MCP and phosphor screen were employed as an ion detector to eliminate broadening of the TOF [3].However, a modern wideangle 3DAP is equipped with a flat MCP accompanied with a computerized PSD; therefore, the flight path difference could be compensated from the detected positions of ions on the MCP [1,4].
We have developed a wide-angle laser-assisted 3DAP [5,6] with a MCP (120 mm diameter) set up at a distance of * Corresponding author: mayama@iis.u-tokyo.ac.jp 110 mm in front of the tip.The wide-angle (0.75 sr) atom probe is equipped with a femtosecond (180 fs) laser and a high-resolution (25 ps) time-to-digital converter (TDC) system, so that the mass resolution is dependent on the flight path difference of detected ions.Use of a delay-line PSD with a high spatial resolution (<0.1 mm) has made it possible to accurately perform flight path compensation.In this paper, we report the method of flight path compensation in a wide-angle 3DAP with a short flight distance of ions.

II. EXPERIMENT A. Instrumentation
A schematic diagram of the 3DAP instrument is shown in Fig. 1.A local electrode with a hole of typically 100 µm in diameter was adopted and the distance from the tip to the local electrode is almost the same as the diameter of the hole in the local electrode [7].MCP assemblies with a diameter of 120 mm and the delay-line PSD (RoentDek DLD120) were set up at a distance of 110 mm in front of the tip.The signal output from each end of the two wire pairs on the PSD is amplified and then digitized by the TDC.A photodiode accurately detects the moment at which the pulsed laser is sent to the tip, and this signal is also digitized by the TDC.The TOF of ions can be calculated from the difference between the signals from the photodiode and the PSD.Details of the system have been given in our previous papers [5,6].
The resolution of TOF is expected to be less than nanoseconds, due to the very short duration of the laser pulse (180 fs) and the high-resolution (25 ps) TDC system.Typical TOF of detected ions are in the order of microseconds.Therefore, the resolution of mass spectra calculated from the TOF depends only on the difference of the flight path of the detected ions.
where m is the mass of an ion, n is the charge state, e is the elementary charge, V is the applied voltage, t is the TOF and L 0 is the distance from the tip to the MCP.An ion hitting the point (x, y) on the MCP, where x and y are coordinates provided from the delay-line PSD, takes a longer flight path, L, than L 0 .Because L 0 is very short and a local electrode is adopted in the 3DAP, the ion trajectory can be approximated to be linear; therefore, L is calculated by t in Eq. ( 1) should be converted to t c , which corresponds to the time required for the flight path, L, by Then, the mass-to-charge ratio of an ion hitting the point (x, y) on the MCP can be written by If the MCP is inclined in the direction normal to the tip axis, so that the X-and Y -axes are at angles α and β, respectively, as shown in Fig. 2, then L is obtained from From Eq. ( 5), t c is calculated by The mass-to-charge ratio of an ion hitting the point (x, y) on the MCP can be rewritten by and then C. Tip preparation Pure tungsten wire (0.1 mm diameter) was prepared for examination of the flight path compensation.Tungsten is generally used for evaluation of atom probe instrumental performance.One end of the wire was fixed in a Cu-tube and another end was fabricated by electro-polishing [1].Furthermore, the tip apex of the tungsten was fabricated http://www.sssj.org/ejssnt(J-Stage: http://www.jstage.jst.go.jp/browse/ejssnt/) e-Journal of Surface Science and Nanotechnology by focused ion beam (FIB) (SIINT SMI3050) etching and the end radius of the tip was less than 100 nm.The tip may include a small trace of gallium due to implantation by the gallium ion beam.

D. Experimental procedure
The ultra high vacuum in the main chamber was 2.8 × 10 −8 Pa.The TOF of field-evaporated ions was measured for the tungsten tip at room temperature.The local electrode and the input face of the MCP were set to ground potential.Field emission was observed when the tip was negatively biased.The applied voltage of field evaporation was estimated from the field emission voltage.For field evaporation, the tip was connected to a positive high-voltage power supply.The applied voltage was increased gradually and the tip was irradiated by laser pulses with repetition frequency and laser power controlled at 5 kHz and 10 nJ/pulse, respectively.

III. RESULTS AND DISCUSSIONS
The effectiveness of flight path compensation was examined for the tungsten tip by measurement of TOF as a function of coordinates on the MCP.Figures 3(a) and 3(b) show the TOF distribution along the X-and Y -axes on the MCP without flight path compensation, and Fig. 3(c) shows the mass spectrum calculated from these TOF.In Fig. 3(c), two broad peaks were identified for W 2+ and W 3+ .Figures 4(a tained using Eq. ( 4).Comparing the mass spectra shown in Figs.3(c) and 4(c), W 2+ and W 3+ could be clearly identified after flight path compensation.In Fig. 4(c), Ga 2+ was also identified, although no Ga ion peak was observed in Fig. 3(c).The mass resolution, m/∆m, deduced from the mass spectrum after flight path compensation, was less than 100 (FWHM, full width at half maximum), because the isotopes of detected ions could not be identified.
In Figs.4(a) and 4(b), small and systematic deviations of the TOF along the coordinates were present.It was considered that these small slopes were due to the inclination of the MCP in the direction normal to the tip axis.The inclinations (α and β for the X-and Yaxes, respectively) of the MCP could be obtained from the TOF with the same mass-to-charge ratio in Figs.4(a) and 4(b).From the TOF (t c in Eq. ( 3)) at the outermost x-coordinate, t was calculated by Eq. ( 3).The inclination for the X-axis (α) was calculated using Eq. ( 6) from t and the TOF (t c in Eq. ( 6)) at the point (0, 0) on the MCP.As a result, it was found that the X-axis of the MCP was inclined −0.4 • in the direction normal to the tip axis.In the same way, β was calculated to be −0.6 • .Figures 5(a  spectrum calculated from Eq. ( 7), and Fig. 5(d) shows the mass spectrum around the mass-to-charge ratio of W 2+ .All major isotopes of tungsten could be observed and were in agreement with the natural abundances.Furthermore, the minor isotope, 180 W 2+ , (natural abundance approximately 0.1%) was also observed in Fig. 5(d).Thus, it is clear that flight path compensation for slight inclination of the MCP resulted in significant improvement of the mass resolution to approximately 300 (FWHM).Stender et al. [4] reported a mass resolution of approximately 300 (FWHM) in their 3DAP instrument with a distance of 160 mm from the tip apex to the MCP.Although the distance in our instrument was shorter, almost the same resolution was obtained by flight path compensation for slight inclination of the MCP.

IV. CONCLUSION
A method of flight path compensation was proposed for a wide-angle 3DAP with a short flight distance of ions.A mass resolution of approximately 300 (FWHM) was achieved for the 3DAP instrument.

FIG. 3 :
FIG. 3: Time-of-flight distributions and mass spectra for a W tip. (a) and (b) Time-of-flight distributions along the Xand Y -axes on the MCP without flight path compensation, respectively.(c) Mass spectrum obtained from these data.
FIG. 4: Time-of-flight distributions and mass spectra for a W tip after flight path compensation using Eq.(3).(a) and (b) Time-of-flight distributions along the X-and Y -axes on the MCP after compensation, respectively.(c) Mass spectrum obtained from these data.
) and 5(b) show the TOF after compensation using Eq.(6) along the X-and Y -axes on the MCP.Increase of the TOF at the outermost part of the x-coordinate is shown in Fig.5(b).A similar phenomenon was observed for the image response of the MCP[8].The anomaly at the outermost parts on the MCP may result from an imperfect wire array and limitations in the detection area of the delay-line PSD.In Figs.5(a) and 5(b), the TOF was almost independent of the detected position, which suggests that the slight inclination of the MCP affected the TOF of the ions.
FIG.5: Time-of-flight distributions and mass spectra for a W tip after flight path compensation using Eq.(6).(a) and (b) Time-of-flight distributions along the X-and Y -axes on the MCP after compensation for inclination of the MCP, respectively.(c) Mass spectrum obtained from these data, and (d) enlarged view of the mass spectrum for W 2+ .