The invention concerns a mass spectrometer for analysis of secondary ions and post-ionized neutral secondary particles with an ion source for creating a primary ion beam to irradiate a sample, and to produce secondary particles. The source possesses a heatable ion emitter that is coated in the area exposed to the field with a liquid-metal layer that contains an ionizable metal that is emitted and ionized as the primary ion beam. The primary ion beam contains metal ions with various stages of ionization and cluster statuses. In particular, the invention concerns a spectrometer unit for mass analysis of the secondary particles as well as the ion source of such a mass spectrometer.
In the description below, the conventional designation will be used for ions in clusters related to their mass and charge status, thus:Binp+wherein n=the quantity of atoms in a cluster, and p+=charge status.
It is known to use liquid metal sources in secondary-ion mass spectroscopy in particular when operated as time-of-flight secondary-ion mass spectroscopy (TOF-SIMS). Applicants have proposed a liquid metal gold-cluster ion source for a spectrometer (see prospectus: Liquid Metal Gold Cluster Ion Gun for Improved Molecular Spectroscopy and Imaging, published 2002) that represents the state of the art for the overall TOF-SIMS concept.
The efficiency of TOF-SIMS measurements with respect to primary ion beams from mono-atomic Gallium ions could be significantly increased using Gold Primary Clusters, e.g., of type Au3+. The disadvantage of the use of Gold as the material for the primary ion beam is that when Gold ions are created, those of type Au1+ predominate, while cluster formats such as Au2+ or Au3+ provide only low components of the overall ion current.
Bismuth has been used successfully during the intensive search for additional cluster-forming substances, containing only one natural isotope for secondary-ion mass spectroscopy. Bismuth is an an-isotopic element with a melting point of 271.3° C. Additionally, Bismuth alloys such as Bi+Pb, Bi+Sn, and Bi+Zn are known that possess lower melting points (46° C.-140° C.) than pure Bismuth. Pure Bismuth, however, is given preference for a liquid metal ion source.
In U.S. Pat. No. 6,002,128 it is noted that Bismuth is suited for the creation of charged particles. However, neither cluster formation nor the option of a liquid metal ion source with Bismuth is described. Also, the Japanese Patent No. 03-084435 proposes a calibration alloy for a secondary-ion mass spectrometer with which mass spectra with high resolution may be obtained. For this, the elements V, Ge, Cd, Os, and Bi are named as elements with high negative secondary ionization. The isotope patterns for the above-mentioned elements provide characteristic, repeatable spectra. However, this document does not mention cluster formation of a liquid metal ion source. Also, it is not indicated that Bismuth is well suited for cluster formation.