1. Field of the Invention
The present invention relates to a secondary ion mass spectrometer, and more specifically, to a secondary ion mass analyzer for analyzing a sample by irradiating the sample with a primary beam, such as a high speed atom beam, and simultaneously separating and detecting both positively and negatively charged secondary ions emitted from the sample.
2. Description of the Prior Art
FIG. 1 schematically illustrates the structure of a secondary ion mass spectrometer of the prior art. In this figure, reference numeral 1 designates a high speed beam source for emitting a high speed atom beam, 2 an analyzing tube, 3 a sample, 4 a quadrupole mass spectrometer, 5 a secondary electron multiplier, 8 an amplifier, 7 a recorder, 8 a vacuum pump, 9 a high speed atom beam, and 10 secondary ions generated when the sample is irradiated with the high speed atom beam.
This secondary ion mass spectrometer of the prior art operates as follows. An analyzing tube 2 and a quadrupole mass spectrometer 4 are sufficiently evacuated with a vacuum pump 8. A beam source 1 emits a high speed atom beam 9 to irradiate a sample 8. Secondary ions 10 are emitted from the sample 8 which is irradiated and bombed by the high speed beam 9 and these ions are discriminated by the quadrupole mass spectrometer 4, whereby only the secondary ions having a particular mass are selected and enter a secondary electron multiplier 5. Secondary ions 10 are converted into electrons equivalent to the input secondary ions in the secondary electron multiplier 5 and the output is fed through an amplifier 8 and finally recorded by a recorder 7.
Such a secondary ion mass spectrometer is used for mass analysis of secondary ions generated from a solid surface of a sample irradiated with a high speed beam. This analyzing method provides extremely high sensitivity in comparison with other surface analyzing methods such as Auger electron spectroscopy and X-ray electron spectroscopy, and is characterized by its ability to analyze all of the elements of the periodic table and isotopes. Particularly, a high speed atom beam having energy of several hundred electron volts to several kiloelectron volts is suitable for mass analysis because it is electrically neutral and therefore is not influenced by a charged insulator, and the range of the orbit of the atom beam remains constant as the atom beam is not influenced by space charges.
However, a secondary ion mass spectrometer of the prior art does not separate both positively and negatively charged secondary ions, although both positively and negatively charged secondary ions are simultaneously emitted from a sample. Therefore, positively charged secondary ions cannot be detected when negatively charged secondary ions are detected, and vice versa. Accordingly, when it is required to obtain a mass spectra of secondary ions of different polarities from one sample, analysis must be conducted twice, whereby the operation is complex and cannot be carried out swiftly.