Quantitative analysis of helium by post‐ionization method using femtosecond laser technique

Abstract

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

High spatial resolution imaging of helium isotope by TOF‐SNMS

“…The data points from the measured profiles in deeper layer are higher than that of TRIM. This is due to very small ion count for each data point and residual He in the sample chamber . At least we were able to improve the lateral resolution for He analysis.…”

“…LIMAS can examine microdistribution of He in the SW implants. An actual useful yield of the He may be higher because the implanted He would be partially escaped from the substrate . The detection limit corresponds to the background level of the depth profiling (Fig.…”

“…The background level of the He imaging is similar to that of the depth profiling. The background of He would be derived from residual He gas in the sample chamber of which vacuum was 7 × 10 −8 Pa.…”

Quantifying low fluence ion implants in diamond-like carbon film by secondary ion mass spectrometry by understanding matrix effects

“…This remaining 24 Mg depth prole was integrated [i.e., ( Ð 24 Mg/ 12 C) STD dx] to calculate the ion-mixed surface contamination. Then the integrand was multiplied by the terrestrial 25 Mg/ 24 Mg and 26 Mg/ 24 Mg ratios 18 to give the 25 Mg and 26 Mg contamination, respectively. The 25 Mg and 26 Mg contamination was then subtracted from the integrals of the 25 Mg and 26 Mg implants.…”

Aberration-corrected focused ion beam for time-of-flight secondary neutral mass spectrometry

“…[12][13][14][15] A laser ionization mass nanoscope (LIMAS) ionized sputtered secondary neutrals ∼100% (except He which yielded ∼70%) by irradiation of high power femto-second laser. 13,16,17) Consequently, a useful yield improves to 0.2% at mass resolution (M/ΔM) of 600 000 [full width at half maximum (FWHM)] for Si. 18) This yield is almost comparable to conventional SIMS of 0.7% under M/ΔM = 500 19) and the low secondary ion yield from the Ga primary beam has not caused weakness in SNMS imaging.…”