Patent Number: 
Section: description

Referring to FIG. 1, a semiconductor wafer 10, which constitutes a substrate for ion implantation, is shown mounted on a holder 11 in the process chamber of an ion implantation apparatus. A section 12 of the wall of the evacuated housing of the process chamber is shown for illustrative purposes only. In accordance with normal practice which will be understood by those familiar with the technology, the implantation apparatus comprises a source 13 of ions to be implanted. The source 13 is shown out of scale and schematically only in FIG. 1. Ions from the source 13, pass through a mass selection arrangement to select only those ions which it is desired to implant in the wafer 10, and are then directed into the process chamber as a beam 14, travelling in the direction of the arrow 15. Further details of a typical form of ion implantation apparatus can be seen in GB-A-2307096. Referring again to FIG. 1, as the beam 14 of positive ions approaches the wafer 10, the beam enters a conductive tube 16 having an axis aligned with the beam direction. A Plasma Flood System 17 is arranged to generate a supply of low energy electrons and flood these electrons into the interior of the conductive tube 16. An example of Plasma Flood System is described in the aforementioned U.S. Pat. No. 5,399,871. Any suitable flood system may be used provided the electrons entering the region of the beam in front of the wafer 10 from the flood system have very low energies, typically not greater than 5 eV. The purpose of the electron flood system is to ensure a supply of these low energy electrons in the beam region in front of the substrate, so that any positive charge build up on the substrate or wafer 10 will be discharged by the attraction to the wafer of low energy electrons. In the described example, the conductive tube 16 serves as an electron confinement tube, with the tube 16 typically at a low negative potential relative to the potential of the wafer 10. At the end of the conductive tube 16 immediately adjacent the wafer 10 on the holder 11, a magnetic filtering field is produced, by opposed magnetic poles 18 and 19 located on opposite sides of the tube 16. The poles 18 and 19 produce a magnetic field, represented by the field lines 20 in FIG. 1, extending transversely right across the beam region in front of the wafer 10. Referring now to FIG. 2, the effect of the magnetic field produced by the poles 18 and 19 can be explained in more detail. FIG. 2 is a partial cross-section through the confinement tube 16 viewed in the direction of arrow X in FIG. 1 so that only the North pole 18 can be seen in FIG. 2 located behind the end of the tube 16. The magnetic field 20 is illustrated coming directly out of the paper using the conventional symbols. Importantly, the conductive tube 16 is made from non-ferromagnetic material so that the distribution of the magnetic field is dependent on the shape and positions of the pole pieces 18 and 19. The magnetic field 20 from the magnetic poles 18 and 19 has sufficient strength to deflect relatively higher energy secondary electrons, such as illustrated by the dotted lines 21, out of the region containing the ion beam 14, so as to impact and be absorbed on the inner surface of the conductive tube 16. The giro radius (Larmour radius) of movement of an electron in a magnetic field is proportional to the energy of the electron. In the present arrangement, a magnetic field is selected to provide a relatively large giro radius for secondary electrons having relatively higher energies, so that these secondary electrons will tend to strike the inner surface of the conductive tube 16. However, lower energy electrons, including those provided by the flood system 17 have lower giro radiuses so that they can diffuse through the magnetic field region without striking the conductive tube 16. In this way, the magnetic field provides a filtering effect, preventing high energy electrons from passing across the field, without hindering the passage of low energy electrons. Preferably, the magnetic field is set to prevent the passage of secondary electrons from the wafer 10 having energies about 10 eV, and even more preferably to prevent the passage of electrons having energies greater than about 5 eV. Then, only those plasma flood electrons having energies below 5 eV are able to pass through the filter towards the substrate to take part in neutralising any positive charge build up on the substrate. In the absence of the magnetic field 20, the population density of higher energy secondary electrons in the region within the tube 16 falls in an exponential fashion with distance from a maximum immediately in front of the substrate. The presence of the magnetic field 20 has the effect of greatly reducing the population density of these electrons in the region of or just in front of the field 20. This reduction in density a short distance in front of the substrate results in a reduction also in the population density immediately at the substrate surface. To maximise the reduction where required, at the substrate surface, the magnetic field should be as close as possible to the substrate. FIG. 3 is a view in cross-section along the line IIIxe2x80x94III of FIG. 1, and illustrates a pair of permanent magnets 24 and 25, providing the magnetic field 20 across the end of the conductive tube 16 in front of the wafer 10. The permanent magnet 24 has a North pole 18 on a long face of the magnet directed towards the conductive tube 16, and a South pole on the opposite long face directed away from the tube 16. Magnet 25 has a corresponding South pole 19 on a long face directed towards the tube on the opposite side of the tube 16 from magnet 24, and has a North pole on the opposite long face of the magnet 25 directed away from the tube 16. There is thus produced a dipole magnetic field 20 extending transversely across the end of the tube 16 completely over the region in front of the wafer 10 containing the ion beam. Instead of a pair of permanent magnets 24 and 25, FIG. 4 illustrates an alternative construction in which a single horseshoe magnet has opposed North and South poles 28 and 29 extending on opposite sides of the conductive tube 16. The pole arms 28 and 29 are interconnected by a yoke section 30. The horseshoe magnet illustrated may be formed of a first bar magnet 31 having a North pole to the left and a South pole to the right as illustrated by the dotted N and S in FIG. 4, and a second bar magnet 32 beneath the tube 16, having a South pole to the left and a North pole to the right. The right hand ends of the two bar magnets 31 and 32 are then interconnected by a soft ion yoke 30 to produce in effect the horseshoe magnet with the upper North pole 28 and lower South pole 29. With this construction, the strength of the magnetic field between the upper and lower poles 28 and 29 can be adjusted by adjusting the spacing along the yoke 30 of the two bar magnets 31 and 32. A reduced spacing, to the position illustrated in dotted outline in FIG. 4, produces an increased magnetic field. In this way, the magnetic field can be xe2x80x9ctunedxe2x80x9d to filter out only secondary electrons of energies above a desired value, and to ensure adequate diffusion of lower energy electrons through a magnetic field, as necessary to provide adequate neutralising of positive charge build up on the wafer. Instead of using permanent magnets, the magnetic filter can be formed by an electromagnet, comprising a soft ion U shaped member of the form shown in FIG. 4, with an energising coil 35 wound around the yoke portion 30 and energised with a DC current from a supply 36. With an electromagnet, the magnitude of the magnetic field produced can be adjusted by varying the energising current through the coil 35 with a variable supply 36. In the examples described above, dipole magnetic fields are shown. However other magnetic field structures may be employed provided they ensure sufficient field strength completely across the beam region so that secondary electrons emitted from any part of the substrate can be deflected to the conductive tube 16 for absorption. Although the illustrated construction shows a conductive tube surrounding the beam in front of the wafer 10, it should be understood that any conductive element may be employed to collect high energy secondary electrons deflected by the magnetic field out of the beam region. For example, if most of such secondary electrons are deflected to one side, e.g. upwards with a field as illustrated in FIG. 2, a collecting plate may be provided only at this one location to collect the deflected secondary electrons.