We have developed a new nano-beam time-of-flight secondary neutral mass spectrometry system: laser ionization mass nanoscope or LIMAS. The primary ion beam column was equipped with a Ga liquid metal ion source and aberration correction optics. The primary ion beam was down to 40 nm in diameter under a current of 100 pA with an energy of 20 keV. The sputtered particles were post-ionized under non-resonance mode by a femtosecond laser. The post-ionized ions were introduced into a multi-turn mass spectrometer. A mass resolution of up to 40 000 was achieved. The vacuum of the sample chamber was maintained under an ultrahigh vacuum of 2 Â 10 À8 Pa. This instrument would be effective for ultrahigh sensitive analysis of nanosized particles such as return samples from asteroids, comets, and planets.

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Laser ionization mass nanoscope is a time-of-flight sputtered neutral mass spectrometer associated with laser post-ionization by tunneling effect. A spherical and chromatic aberration corrector is installed in the primary ion column. The lateral spatial resolving power of He imaging of solid surface has been evaluated by scanning image using a probe diameter of 90 nm from crater edge slope of a He ion-implanted Si substrate. Helium distribution from the scanning image is quantitatively equivalent with depth profiling analysis from surface of the same substrate, indicating that spatial resolving power of 20 nm for depth resolution has been achieved on the He scanning image through use of oblique incident effect of the primary beam.

We have evaluated the performance of a multi-turn time-of-flight mass spectrometer (MULTUM II) equipped with the ion injection optics of a laser ionization mass nanoscope (LIMAS). We surveyed the optimal parameters for the ion injection optics, which consist of ion extraction from a sample surface and ion introduction into MULTUM II. We developed mass calibration methods for correcting the modulation of load voltage for MULTUM II and injection timing for the ion injection optics. As a result, the mass-resolving power of LIMAS increased linearly with increasing the flight path length, and reached 6.2 × 10 5 (full width at half maximum) at 1000 multi-turn cycles of MULTUM II (flight path length: 1.3 km). The transmittance of LIMAS decreased to 60-70% after 20 multi-turn cycles of MULTUM II, compared with the linear mode transmittance. The transmittance per multi-turn cycle became constant (99.96%) after 20 multi-turn cycles. A useful yield of 3 × 10 À3 for Si ions was obtained for LIMAS at 30 multiturn cycles of MULTUM II.

Helium has the largest ionization potential of all elements; thus, it is difficult to ionization for measurement by mass spectrometry. In order to analyze He, a tunnel-ionization time-of-flight sputtered neutral mass spectrometry system (called LIMAS) has recently been developed. LIMAS uses a femto-second laser technique, and can ionize He to achieve ~10% ionization yields [1]. We quantified the effectiveness of this method for He analysis from a 2.5 × 4 µm 2 area of He-implanted silicon. The amount of He in a implant was quantified by measuring the ion-current, giving a nominal implant fluence per unit area. Thus, the fraction of total He measured by LIMAS during depth profiling could be quantified by comparison with the He-concentration of the reference implant. The He + intensities normalized by host ions of Si linearly correlated with the known He concentrations with a reproducibility of 10% at concentrations less than 10 21 cm -3 . The detection limit was down to 10 18 He cm -3 (20 ppm). For concentrations exceeding 10 21 cm -3 , the He intensities are smaller than those expected from the lower concentration range. This non-linearity may reflect the limit of retention of He in the Si lattice, since He is chemically inert.2