Development of Mass Spectrometer Using Two Rotating Electric Fields for Separation of High-Mass Ions

We developed a mass spectrometer with a novel mass-separation mechanism using two rotating electric fields (REFs). This mass spectrometer realizes a wide mass range with the continuous separation of ion beams. In principle, it has no limitation on the mass range. It can be operated stably for the detection of high-mass ions. To estimate the mass-separation ability, we directly introduced the mass spectrometer, which consists of two REFs, to a Ga focused ion beam column. The mass spectra of Ga isotopes were obtained by sweeping the frequencies of the REFs. The peaks of the Ga isotopes were clearly separated on the mass spectra. [DOI: 10.1380/ejssnt.2016.161]


I. INTRODUCTION
Recently, Ar gas cluster ion beams (GCIBs) have been used as the primary ion beam in secondary ion mass spectrometry (SIMS). The irradiation of cluster ions increases the secondary ion yields of molecular ions and typical fragment ions [1]. Cluster ions decompose at the sample surface and distribute their own kinetic energies depending on the incident cluster sizes. Accordingly, fragmentation patterns, which depend on the irradiation energy, can be controlled by cluster-size filtering [2,3]. However, existing mass filters are not suitable for cluster-size filtering, because of their focusing properties and mass resolution.
In this study, we developed a novel mass spectrometer that realizes a wide mass range with the continuous separation of ion beams. In principle, this mass spectrometer has no limitation on the mass range. It can be operated stably for the detection of high-mass ions. * This paper was presented at the 10th International Symposium  Therefore, it is applicable to mass filters for massive cluster ion beams such as Ar-GCIB and water cluster ion beams. The mass spectrometer selects and separates a specific ion from other ions using two rotating electric fields (REFs) aligned on the same axis [4]. The phases of each REF are opposite for the selected ion. We control the cycle of the REF to be the travel time of the We connected this mass spectrometer to a Ga focused ion beam (FIB) column and separated the Ga isotopes contained in the liquid metal ion source in order to estimate the mass-separation ability.

A. Operation method of REFs
In this mass spectrometer, two REFs are aligned on the same axis. We arrange cylindrical electrodes divided into eight pieces at symmetric positions. Figure 2 indicates that the cylindrical electrodes on the concentric circle generate a uniform REF inside the electrodes [5][6][7]. These electrodes are applied sinusoidal waves corresponding to each position. The sinusoidal waves ϕ i are as follows: where ω is the angular velocity of the sinusoidal waves, V is the amplitude of the sinusoidal waves, and t is time.
The sinusoidal waves generate REFs with the same cycles. Therefore, we can control the cycles of the REFs by varying the cycles of the sinusoidal waves. We control the cycle of the REF to be the travel time of the selected ion in each REF.
Assuming that the energy distribution of the selected ions is negligible, the cycle of the REF, τ , is given as where f is the frequency of the REF, L is the length of the REF, V acc is the accelerating voltage of the selected ion, m is the mass weight of the ion for measurement, and q is the charge of the selected ion. m/z is given as where z is the valence number of the ion, and e is the quantum of electricity. If we define α as the incidentangle distribution of the ions, m/z is given as

B. Ion trajectory in REFs
We define z as the direction of the ions. Assuming that the field strength of the REF is constant, without dependence on the position of the selected ion, the motion equations for the selected ion in the upstream REF are as follows: where E is the field strength of the REF. Integration of these motion equations gives the velocity and the position of the selected ion, as follows: Here, we assume that there are no velocity components of the ion in the x-y plane at t = 0. Equations (11) and (12) indicate that the ion trajectory resembles a cycloid in the x-y plane. Substituting ωt = 2π in Eqs.
Integration of these motion equations gives the velocity and position of the selected ion, as follows: The ion trajectory resembles a cycloid rotated 180  distance between the upstream REF unit and the downstream REF unit was 70 mm. An aperture unit, a Faraday cup, and a fluorescent screen were located behind the mass spectrometer. We used a fluorescent screen made of a glass substrates coated with Y 2 O 2 S : Tb 3+ [8]. The aperture unit and Faraday cup were movable. We switched between the image-observation mode on the fluorescent screen and the ion-beam current detection mode.
We introduced a direct digital synthesizer (DDS) to generate the sinusoidal waves for the REFs. The DDS output the sinusoidal waves with 16 different-phase channels. Each output sinusoidal wave was amplified by a high-frequency, high-voltage power amplifier and applied to an electrode of the REF unit.
The accelerating voltage of the Ga-FIB was 10 kV. The potential of each sinusoidal wave was 230 V (peak to peak). First, we set the image-observation mode and optimized the phase contrast between the upstream REF and downstream REF to make the phases of each REF mutually opposite for the selected ion. The phase contrast depended on the travel time of the selected ion between the upstream REF and the downstream REF. The selected ions converged to the center axis because of the optimized phase contrast. Then, we set the movable aperture unit and measured the ion-beam current. The diameter of the aperture was 50 µm. The mass spectra of Ga isotopes were obtained by sweeping the frequencies of the REFs from 1,120 to 1,090 kHz in steps of 1 kHz. Figure 4(a) shows the mass spectrum assuming the lengths of the REFs to be 150 mm. The peaks of 69 Ga + and 71 Ga + were clearly separated; however, the peaks were shifted to the high-mass region. This peak shift was apparently caused by the fringing fields near the opening ends of the cylindrical electrodes. Figure 5 shows the numerical solution of the electric field near the opening end in the x-z plane. The figure indicates that the effective lengths of the REFs were greater than the actual lengths of the cylindrical electrodes because of the fringing fields. Accordingly, we calibrated the effective lengths of the REFs to the precise positions of the peaks. Figure 4(b) shows the mass spectrum assuming the effective lengths of the REFs to be 150.34 mm. The peak area ratio of 69 Ga + : 71 Ga + was 0.595 : 0.405. The relative error between this value and the natural abundance is