Recent progress of SIMS technique is reported in conjunction with novel primary ion beams and advanced mass spectrometers. Numerous applications of SIMS have been proposed and demonstrated in last decade, covering from organic semiconductors to cells and tissues. One of the biggest problems was “Static-Limit” for analysis of organic molecules, which is very fragile for ion bombardment. After innovation of massive cluster ion beams, the issue of “Static-Limit” has been overcame. This opens a new possibility for SIMS to analyze organic materials. Ultra-high mass resolution mass spectrometer, MS/MS technique, cationization enhancement and future prospects of SIMS are discussed.
Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) with low-energy bismuth cluster ion beam was investigated to achieve fragment-free ionization and high secondary ion yields of molecular ions. TOF-SIMS spectra taken by 2 keV Bi3＋ were compared with those taken by 54 keV Bi32＋. 2 keV Bi3＋ spectra showed much less fragmentation of organic molecules and there were few atomic secondary ions of such as 1H＋ and 12C＋. Moreover, secondary ions of intact molecules were clearly observed in 2 keV Bi3＋ spectra for some organic materials. Results suggest that low-energy bismuth primary ion beams are potential candidate for molecular ion detection in TOF-SIMS.
We have developed a vacuum-type electrospray droplet ion (V-EDI) gun as a novel massive cluster beam for surface analysis instruments in which the beam source is a vacuum electrospray of aqueous solution. The prototype of the V-EDI gun was connected to a time-of-flight analyzer. Secondary ion spectra produced by the V-EDI beams were measured using a sample-bias pulsing method, because a short pulse of the beam could not be obtained. The secondary ion yields of biomolecular samples produced by the V-EDI and Bi cluster ion beams were compared, and the secondary ion yields produced with the V-EDI beams were found to be much higher than those produced with the Bi cluster ion beams.
This article provides an overview of the authors' activity on organic secondary ion mass spectrometry (organic SIMS) using ionic liquids. Ionic liquids, i.e., molten salts with a melting point less than 100℃, have negligible vapor pressures, so that they are compatible with ultra-high vacuum. They are divided into two groups : aprotic ionic liquids and protic ionic liquids. Both ionic liquids were tested as liquid matrices and as primary ion beams in organic SIMS. Protic ionic liquids proved to be effective to enhance molecular secondary ion intensities, whereas aprotic ionic liquids were not useful. Among ionic liquids, alkylammonium titrates such as propylammonium nitrate will be most promising for organic SIMS.
Recently, water and some other molecules are applied to the massive cluster projectiles for SIMS to improve the secondary ion yields by chemical effects. The ionization probability should be influenced by the properties of the primary ion beams as well as the chemical nature of the sample surface. However, the mechanisms of ionization by the large cluster collision is still unclear. In this study, we have generated water, methanol and methane molecular cluster ion beams and investigated the secondary ion emission from amino acid thin films bombarded by the molecular cluster ion projectiles.
Thanks to the development of a variety of huge cluster ion sources, time-of-flight secondary ion mass spectrometry (TOF-SIMS) has been applied to more fields. By means of gas cluster ion beams (GCIB) such as Ar cluster ions, 3D analysis of soft materials including biological cells and tissues is feasible. Moreover, the sputtering by GCIB is useful for removing damage caused by a conventional primary ion beam such as a Bi cluster ion beam. In addition, recently instrumental issues regarding mass spectrometers (MS) have been improved by introducing tandem MS and orbital ion trap MS to TOF-SIMS, which made TOF-SIMS more powerful for bio sample analysis. On the other hand, data analysis techniques are getting more important to interpret such TOF-SIMS data.
The international standards for SIMS are developed in ISO TC 201 SC 6. Seven standards for dynamic SIMS and three standards for TOF-SIMS have been developed so far. And, two standards are being developed now. In this paper, two standards which are related to detector saturation are explained. Then the trend of ISO on SIMS is also introduced.
To improve the surface flatness, impact and corrosion resistance of magnetic recording media, low–pressure (0.6 Pa) deposition of CoPtCr–SiO2 granular films by Ar＋O2 reactive sputtering using a Co65Pt14Cr9Si12 target was investigated. It was found that as the fraction of oxygen in the sputtering gas flow (FO2) was increased from 0 to 20%, the deposition rate increased from 2.8 to 4.3 nm/s, and then decreased to 1.1 nm/s as FO2 was further increased. For FO2＝10–12%, the designed composition with the desired oxygen content (Co74Pt16Cr10–8 mol%SiO2) could be achieved. For FO2＝8%, while a flat surface granular film with c-plane oriented CoPtCr grains was realized, a slight amount of Si remaining in magnetic grains which indicated the insufficient segregation of SiO2 into the grain boundaries was observed.