Patent Number: 
Section: description

A block diagram of an embodiment of an ion implanter is shown in FIG. 1. An ion source 10 generates ions and supplies an ion beam 12. Ion source 10 may include an ion chamber and a gas box containing a gas to be ionized. The gas is supplied to the ion chamber where it is ionized. The ions thus formed are extracted from the ion chamber to form ion beam 12. Ion beam 12 is directed between the poles of a resolving magnet 32. A first power supply 14 is connected to an extraction electrode of ion source 10 and provides a positive first voltage V0. First voltage V0 may be adjustable, for example, from about 0.2 to a 80 kV. Thus, ions from ion source 10 are accelerated to energies of about 0.2 to 80 KeV by the first voltage V0. Ion beam 12 passes through a suppression electrode 20 and a ground electrode 22 to a mass analyzer 30. The mass analyzer 30 includes resolving magnet 32 and a masking electrode 34 having a resolving aperture 36. Resolving magnet 32 deflects ions in ion beam 12 such that ions of a desired ion species pass through resolving aperture 36 and undesired ion species do not pass through resolving aperture 36 but are blocked by the masking electrode 34. In one embodiment, resolving magnet 32 deflects ions of the desired species by 90xc2x0. Ions of the desired ion species pass through resolving aperture 36 to a first deceleration stage 50 positioned downstream of mass analyzer 30. Deceleration stage 50 may include an upstream electrode 52, a suppression electrode 54 and a downstream electrode 56. Ions in the ion beam arc decelerated by deceleration stage 50 and then pass through an angle corrector magnet 60. Angle corrector magnet 60 deflects ions of the desired ion species and converts the ion beam from a diverging ion beam to a ribbon ion beam 62 having substantially parallel ion trajectories. In one embodiment, angle corrector magnet 60 deflects ions of the desired ion species by 70xc2x0. An end station 70 supports one or more semiconductor wafers, such as wafer 72, in the path of ribbon ion beam 62 such that ions of the desired species are implanted into the semiconductor wafer. The end station 70 may include a cooled electrostatic platen and a scanner (not shown) for moving wafer 72 perpendicular to the long dimension of the ribbon ion beam 62 cross-section, so as to distribute ions over the surface of wafer 72. The ribbon ion beam may be at least as wide as wafer 72. The ion implanter may include a second deceleration stage 80 positioned downstream of angle corrector magnet 60. Deceleration stage 80 may include an upstream electrode 82, a suppression electrode 84 and a downstream electrode 86. The ion implanter may include additional components known to those skilled in the art. For example, end station 70 typically includes automated wafer handling equipment for introducing wafers into the ion implanter and for removing wafers after ion implantation. End station 70 may also include a dose measuring system, an electron flood gun and other known components. It will be understood that the entire path traversed by the ion beam is evacuated during ion implantation. The ion implanter of FIG. 1 may operate in one of several modes. In a first operating mode, known as the drift mode, deceleration stages 50 and 80 are connected to ground, and the ion beam 12 is transported through the beamline at the final beam energy established after extraction from ion source 10. In a second operating mode, known as the enhanced drift mode, the ion beam 12 is accelerated to an intermediate energy at electrode 22 before passing through mass analyzer 30 and then is decelerated to the final beam energy by first deceleration stage 50. In a third operating mode, known as the double deceleration mode, the ion beam is accelerated to a first intermediate energy at electrode 22 before passing through mass analyzer 30, is decelerated by first deceleration stage 50 to a second intermediate energy as it passes through angle corrector 60 and then is decelerated to the final beam energy by second deceleration stage 80. A fourth operating mode transports the beam at the intermediate energy through to the second deceleration stage 80, and the gap at the first deceleration stage 50 is operated with a short circuit shunt. By transporting the ion beam through part of the beamline at higher energy, space charge expansion can be reduced in comparison with the drift mode for a given final beam energy. As noted above, space charge expansion of low energy ion beams is problematic in magnets, because the gap between magnet polepieces is typically small and the beam path through the magnet is typically long. Thus, in the ion implanter of FIG. 1, ion beam 12 may undergo space charge expansion as it is transported through resolving magnet 32 and angle corrector magnet 60. Space charge expansion causes the ion beam to strike beamline components and results in reduced beam current delivered to the wafer. A magnet assembly in accordance with a first embodiment of the invention is illustrated in FIG. 2. FIG. 2 is a side view of the magnet assembly. The magnet assembly may be incorporated into the ion implanter of FIG. 1 as either the resolving magnet 32 or the angle corrector magnet 60, or both, or into any other ion implanter which requires a magnet for deflecting ions. Basic components of the magnet assembly include a magnet and one or more electron sources. The magnet includes polepieces 100 and 102, magnet coils 110 and 112 disposed around polepieces 100 and 102, respectively, and a magnet power supply 120 coupled to magnet coils 110 and 112. Ion beam 12 is transported through a magnet gap 124 between polepieces 100 and 102. When magnet power supply 120 is energized, magnetic fields 130 are produced in magnet gap 124. The magnetic fields 130 are perpendicular to the direction of ion beam transport and deflect ion beam 12 as known in the art. The magnet assembly further includes one or more electron sources disposed on or in proximity to at least one of polepieces 100 and 102 for producing low energy electrons in magnet gap 124. A purpose of the electron sources is to produce low energy electrons which neutralize the positive space charge of the ion beam 12, at least partially, and which thereby limit space charge expansion of ion beam 12. Because the low energy electrons are supplied to ion beam 12 in magnet gap 124, the ion beam space charge is neutralized as the beam is transported through magnet gap 124. In the embodiment of FIG. 2, electron sources 140, 142, 144, 146 and 148 are disposed on polepiece 100. Each of the electron sources 140, 142, 144, 146 and 148 produces low energy electrons which are directed toward ion beam 12. The electrons follow spiral electron trajectories 150 because of the magnetic fields 130 in magnet gap 124. The electrons neutralize the space charge of ion beam 12 and limit space charge expansion of ion beam 12 in magnet gap 124. The effectiveness of ion beam space charge neutralization depends on the quantity and energy of electrons produced in relation to the quantity of positive ions in the ion beam. The ion beam 12 includes positively charged ions and, without additional charged particles, produces a very positive electric potential in the space through which it is transported. This xe2x80x9cspace chargexe2x80x9d causes the ions to be expelled from the regions where it is highest, i.e. the beam core. The presence of electrons in the region can help reduce the space charge potential. Some electrons are normally produced by collisions with the residual gas. Electron trajectories 150 are caused to spiral by magnetic fields 130 and are attracted to the positive potential of the ion beam. If the electrons lose enough energy by collisions with atoms in the residual gas, the electrons may be captured in the potential well of the beam, oscillating across the beam path until they are lost by atomic capture, well leakage or crossed magnetic and electric field (ExB) transport. The population of electrons in the potential well effectively neutralizes a portion of the space charge, and the ion beam is transported with less space charge expansion as a result. Ideally, it is desirable to achieve 100% compensation for the positive beam potential, but losses and low electron production makes this objective difficult to achieve. The magnet assembly may include one or more electron sources. The electron sources may be mounted on, near, or recessed within, one or both of polepieces 100 and 102. The effectiveness of the electron sources in reducing space charge expansion of ion beam 12 is a function of the number of electron sources, the number and energy of the electrons produced by the electron sources, and the distribution of the electron sources relative to ion beam 12. In the case of a ribbon ion beam having a ribbon beam width, the electron sources may be distributed between the magnet polepieces across the ribbon beam width. Also, the electron sources may be distributed between the magnet polepieces along the direction of beam transport. The electron sources may be implemented as a one or two-dimensional array of individual electron emitters, one or more linear electron emitters, or an area electron emitter, for example. Any electron source of suitable size, shape and electron generating capability may be utilized. Examples of suitable electron sources are described below. Preferably, the electrons have energies of about 10 electron volts (eV) or less. A magnet assembly in accordance with a second embodiment of the invention is shown in FIG. 3. FIG. 3 is a top view of the magnet assembly. Like elements in FIGS. 2 and 3 have the same reference numerals. The magnet assembly of FIG. 3 may have the same magnet structure as the magnet assembly of FIG. 2. In the embodiment of FIG. 3, the electron source includes a field emitter array 200 disposed on a surface of polepiece 100 facing magnet gap 124. The field emitter array 200 may include multiple field emitters 210 formed on a substrate 212. Techniques for fabricating field emitter arrays are described by D. Kim et al. in xe2x80x9cFabrication of Silicon Field Emitters by Forming Porous Silicon,xe2x80x9d J. Vac. Sci. Tech. B14(3), May/June 1996, pages 1906-1909 and by I. Kim et al. in xe2x80x9cFabrication of Metal Field Emitter Arrays on Polycrystalline Silicon,xe2x80x9d J. Vac. Sci. Tech. B15(2), March/April 1997, pages 468-471. A silicon field emitter array may provide reduced risk of contamination when a silicon semiconductor wafer is being implanted. It will be understood that the field emitter array 200 may have any desired configuration. For example, the field emitters 210 may be arranged in a pattern of rows and columns. The field emitter array of FIG. 3 has three columns and six rows. A practical embodiment may include a larger number of rows and columns. In other embodiments, the field emitter array may utilize one or more linear arrays that are perpendicular to the trajectories of ion beam 12 as the beam is deflected by the magnet. Thus, for example, a plurality of linear field emitter arrays may diverge from a central point. The field emitter array 200 may cover all or a selected portion of the surface of polepiece 100 and/or polepiece 102. The spacing between field emitters in the array depends on the desired electron current to be delivered to ion beam 12. The field emitter array 200 may extend across the full width of ion beam 12 to ensure relatively uniform neutralization of ion beam 12. The field emitter array 200 is electrically connected to an electron source power supply 220. Power supply 220 provides to field emitter array 200 an appropriate voltage and current for emitting low energy electrons. A magnet assembly in accordance with a third embodiment of the invention is shown in FIG. 4. FIG. 4 is a top view of the magnet assembly. Like elements in FIGS. 2-4 have the same reference numerals. The magnet assembly of FIG. 4 may have the same magnet structure as the magnet assembly of FIG. 2. In the embodiment of FIG. 4, the electron source includes one or more electron-emitting wires 300, 302, 304, etc. The electron-emitting wires 300, 302, 304, etc. may be mounted on or near polepiece 100 and/or polepiece 102. The electron-emitting wires 300, 302, 304, etc. are connected to electron source power supply 220. Power supply 220 provides an appropriate voltage and current to heat electron-emitting wires 300, 302, 304, etc. to an electron emission temperature. Electron-emitting wires 300, 302, 304, etc. may be a refractory metal, such as tungsten or molybdenum. Electron-emitting wires 300, 302, 304 may be disposed perpendicular to or parallel to the ion trajectories of ion beam 12. In other embodiments, electron-emitting wires 300, 302, 304, etc. do not have a specified relation to the ion trajectories of ion beam 12. A first configuration for mounting electron-emitting wires 300, 302, 304, etc. is shown in FIG. 5. FIG. 5 is an enlarged partial cross-sectional view of polepiece 100. Polepiece liners, which may be fabricated of graphite, are typically utilized to prevent ion beam 12 from striking polepiece 100. As shown in FIG. 5, a polepiece liner 350 is provided with grooves 360, 362, 364 for receiving electron-emitting wires 300, 302, 304, respectively. Grooves 360, 362, 364 may have rectangular cross-sections and may have sufficient depth to prevent ion beam 12 from being incident on electron-emitting wires 300, 302, 304. A second configuration for mounting electron-emitting wires 300, 302, 304, etc. is shown in FIG. 6. FIG. 6 is an enlarged cross-sectional view of polepiece 100. In the configuration of FIG. 6, polepiece liner 350 is provided with V-shaped grooves 370, 372, 374 for receiving electron-emitting wires 300, 302, 304, respectively. Insulators 380, 382, 384 may be disposed in grooves 370, 372, 374, respectively, behind the electron-emitting wires. The surfaces of the insulators 380, 382, 384 may build up electrical charge which reflects electrons emitted by electron-emitting wires 300, 302, 304 toward ion beam 12. A third configuration for mounting electron-emitting wires 300, 302, 304, etc. is shown in FIG. 7. FIG. 7 is an enlarged cross-sectional view of polepiece 100. In the configuration of FIG. 7, polepiece liner 350 is provided with V-shaped grooves 370, 372, 374 for receiving electron-emitting wires 300, 302, 304, respectively. Electrically isolated conductors 390, 392, 394 are disposed in grooves 370, 372, 374, respectively, behind the electron-emitting wires. Conductors 390, 392, 394 are connected to a bias power supply 396. The conductors 390, 392, 394 are biased to reflect electrons emitted by electron-emitting wires 300, 302, 304 toward ion beam 12. Bias power supply 396 may be adjusted to control the efficiency of electron reflection. Electron-emitting wires 300, 302, 304, etc. may have different configurations. In one configuration, straight wires are stretched across polepiece 100 and are affixed to appropriate mounting elements. In another configuration, wires 300, 302, 304, etc. are supported by insulating elements along their length. For example, wires 300, 302, 304, etc. may be wound around ceramic supports. A variety of electron-emitting wire configurations may be utilized within the scope of the invention. As shown in FIGS. 5 and 6, the electron-emitting wires may be recessed in a polepiece liner to avoid contact with ion beam 12. In another configuration, electron-emitting wires may be recessed into the polepiece itself. The ion implanter shown in FIG. 1 and described above delivers a ribbon ion beam to the semiconductor wafer or other workpiece. Other known ion implanter configurations utilize a scanned ion beam. For example, the ion beam may be scanned in one direction, and the wafer may be mechanically translated in an orthogonal direction to distribute the ion beam over the wafer. The present invention may be utilized in ion implanters which employ a scanned ion beam. One or more electron sources may be located on or near one or both polepieces of a magnet, and the electron sources may be distributed across the scan width. Thus, the invention may be utilized with a stationary ion beam or a scanned ion beam. The ion implanter shown in FIG. 1 and described above includes resolving magnet 32 and angle corrector magnet 60. One or both of the magnets may include an electron source disposed on or in proximity to at least one polepiece for producing low energy electrons as described above. Other ion implanter architectures may include one or more magnets. In such ion implanter architectures, one or more of the magnets may include an electron source disposed on or in proximity to at least one of the polepieces for producing low energy electrons as described above. While there have been shown and described what are at present considered the preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.