Radiant energy – Ion generation – Field ionization type
Patent
1997-08-07
1999-02-02
Nguyen, Kiet T.
Radiant energy
Ion generation
Field ionization type
B01D 5944
Patent
active
058669094
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to ion beam generators of the type in which a beam of mixed ions initially is produced and the beam of mixed ions is subjected to the action of a magnetic field so as to select one or more specific types of ions.
Ion beam generators which utilise the principle of an Aston mass spectrometer to produce beams of selected ions are well known and have been available for many years. A typical such ion beam generator is shown in FIG. 1 of the accompanying drawings and consists of an ion source 1 including an ion production chamber 2 which has an exit aperture 3 and an ion extraction/acceleration electrode system 4. A beam of ions 5 produced by the ion source 1 passes through an entrance aperture 6 in a diaphragm 7 into an ion flight tube 8 which has an exit aperture 9 in a second diaphragm 10. The ion flight tube 8 has a first straight section 11, an arcuate section 12 and a second straight section 13. The arcuate section 12 of the ion flight tube 8 is situated within the pole pieces 14 of an electromagnet 15 which is arranged to produce a magnetic field B perpendicular to the plane of the ion flight tube 8. In the example shown in FIG. 1 the magnetic field is homogeneous and the sector angle of the arcuate section 12 of the ion flight tube 8 is 60.degree.. The pole pieces 14 of the electromagnet 15 have rotatable portions 16 and 17 where the ion beam 5 enters and leaves the magnetic field B, respectively. The rotatable portions 16 and 17 of the pole pieces 14 of the electromagnet 15 enable the configuration of the ion beam 5 to be varied if desired. Two ports 18 and 19 enable the ion flight tube 8 to be evacuated to a dynamic vacuum of about 10.sup.-5 torr. The ion source 1 also has a port 18' by means of which the same level of vacuum can be maintained in it.
As can be seen from FIG. 1, the ion beam 5 is divergent as it enters the ion flight tube 8 and continues to be so throughout its passage through the first straight section 11 and the arcuate section 12 of the ion flight tube 8. A typical angle of divergence is 5 degrees. This angle of divergence may not seem to be large, but, in practice, in cases where a number of ions are present in the beam of ions 5, a considerable portion of the heavier ions will impinge on the walls of the arcuate section 12 and the first part of the second straight section 13 of the ion flight tube 8. FIG. 1 shows the position in the case where boron trifluoride is used with the object of providing .sup.11 B.sup.+ ions for use in the production, by ion implantation of semiconductor devices. In this case, ions of .sup.10 BF.sup.+.sub.2 ; .sup.11 BF.sup.+.sub.2 ; .sup.10 BF.sup.+ ; .sup.11 BF.sup.+ ; F.sup.+ ; .sup.10 B.sup.+ and .sup.11 B.sup.+ all are present in the ion beam 5. If the energy of the ion beam 5 is sufficient, this can lead to appreciable sputtering of material from a relatively large area of the said portions of the ion flight tube 8. Most of this sputtered material having randomly orientated flight paths and relatively low energy will be re-deposited on the walls of the ion flight tube or be extracted by the vacuum system. For many purposes, what remains, and which becomes entrained in the desired ion beam or otherwise finds its way through the exit aperture 9 in the diaphragm 10 is relatively unimportant, but if the final ion beam 5' produced by the ion beam generator is to be used in the production of semiconductor devices, then such sputtered impurities can be of great importance, particularly in the case of the production of very large scale integrated circuit devices. At present, to reduce the level of such contamination of the final ion beam 5', the relevant parts of the ion flight tube 8 may be lined with a material, such as silicon, which has no effect on the semiconductor wafer which is being treated. Also, additional pumping and beam deflection systems at the output end of the ion flight tube 8 may be employed, which adds both to the size and cost of ion implanters for the production of semiconductor devices which employ such i
REFERENCES:
patent: 4016421 (1977-04-01), Hull et al.
AEA Technology PLC
Hinds William R.
Holt William H.
Nguyen Kiet T.
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