Increased ion beam throughput with reduced beam divergence in a dipole magnet

Spreading of an ion beam when passing through a dipole magnet is reduced or suppressed by electrostatic ion beam confinement which supplements magnetic confinement which may be provided. The magnetic confinement is enhanced by the provision of a magnetic mirror through concentration and localized increase of the dipole field with a concave profile of the pole pieces faces and/or provision of permanent magnets or localized regions of material of increased permeability to form magnetic cusps. Pitch and geometry of convex portions of the pole piece faces are adjusted to increase the mirror ratio and the location of the maximum mirror field relative to the thickness of a graphite or insulating liner which may be employed. Electrostatic confinement elements in the form of negatively charged electrodes and/or electrically isolated electrodes or insulators which assume a negative charge. Ionization of plasma between the pole pieces may be enhanced by application of a VHF/UHF field having a frequency of about 40 MHz to 100 MHZ or higher.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention generally relates to apparatus employing charged
 particle beams and, more particularly, to ion beam apparatus employing
 magnets to manipulate the ion beam.
 2. Description of the Prior Art
 The art of semiconductor electronic device manufacture has become highly
 sophisticated in recent years to provide a wide range of electrical
 properties of the devices, often at very high integration density. The
 capability to determine the electrical properties with high reliability,
 consistency and manufacturing yield is often limited, as a practical
 matter, by the tools used for processing the semiconductor material,
 usually in the form of a wafer. Such tools are often complex and of high
 precision. Therefore such tools are generally expensive to build and
 maintain. The principal expense of modern semiconductor devices is thus a
 portion of the cost of the tools used to produce them and, therefore,
 varies inversely with tool throughput.
 As is generally known, pure semiconductor materials are poor conductors of
 electricity but, as such, the electrical properties of semiconductor
 materials can be altered radically by impurities and/or electrical fields
 established therein; the latter being generally used for control of the
 device while the former is generally used for establishing device
 specifications. Impurities can be introduced into semiconductor materials
 either during growth or deposition or by implantation. Implantation is
 often preferred for high precision of placement of the impurities and
 process simplicity. That is, implantation of particles in the form of ions
 can accurately place impurities at a desired depth within an existing
 structure in accordance with the energy (or, more accurately, the
 distribution of energies) imparted to the particles to be implanted and
 the nature of the material in which the particles are to be implanted. By
 the same token, impurities may be implanted into an existing structure in
 a single process whereas at least two growth or deposition processes would
 be required to form a buried layer having impurities therein.
 It should be appreciated that the distribution of energies of the
 particles, often referred to simply as beam energy, is dictated by the
 device design (e.g. where the impurities are to be placed) as is the
 concentration of impurities to be achieved. The desired concentration of
 impurities is determined as a function of charged particle flux at the
 surface of the semiconductor material or target and the duration of the
 implantation process. It follows that the energy of the beam cannot be
 altered to increase particle flux and thereby reduce the duration of the
 implantation process. Accordingly, the desired concentrations of
 impurities may require substantial time to achieve; thereby reducing tool
 throughput and increasing expense.
 Unfortunately, several physical mechanisms of ion beams tend to
 substantially reduce ion flux. Specifically, it is common practice to use
 a magnetic field to control or manipulate the ion beam. One particular
 such manipulation is referred to as mass analysis. In the mass analysis
 process, ions will have the same charge and their motion along the beam
 path represents a current. Therefore, when such charged particles pass
 through a magnetic dipole, a force is exerted on each ion perpendicular to
 both. the direction of the beam and the direction of the magnetic field.
 Due to this force, the trajectory of each ion is altered to a degree
 inversely proportional to the square root of its mass. This effect allows
 removal of ions from the beam which are not of the desired material and
 the remainder of the beam will be limited to ions of a particular mass.
 This type of structure is routinely included in ion beam tools for that
 reason and the reduction of ion flux in the beam by removal of ions of
 undesired materials is not of concern.
 However, in the magnetic dipole gap of the mass analysis magnet or any
 other magnet in the tool, the ion beam tends to diverge significantly in a
 manner similar to effects of Coulomb interactions between ions (sometimes
 referred to as "space charge blow up") even when the ion beam energy is
 sufficiently great to create a plasma within the magnet. (Presence of a
 plasma including free electrons tends to reduce the repulsion forces
 between ions in the beam, sometimes referred to as space charge
 neutralization.) Even though the mechanism of beam divergence may not be
 fully or accurately understood, beam divergence within the magnet is known
 to be significantly greater than in a comparable length of unmagnetized
 beam line (provided there are no electrodes along that length of beam line
 that would destroy the beam plasma). It is generally believed, however,
 that the increase in beam divergence is due to an increase in electron
 temperature within the beam plasma in the magnet relative to plasma
 electron temperature outside the magnet.
 The divergence of the beam within the magnet is also principally in the
 direction of the magnetic field (e.g. across the gap between the pole
 pieces) and, at the same time, the transverse size of the beam is limited
 by the size of the pole gap thus reducing flux by the truncation of the
 edges of the beam as ions impinge upon the pole pieces. The beam
 divergence increases with increased ion beam current and decreased beam
 energy. At low beam energies, the ion beam is less effective to produce
 ionization which would, in turn, produce a beam plasma that partially
 compensates for the space charge of the ion beam. Therefore, it can be
 seen that seeking to increase tool throughput by increasing beam current
 provides only marginal, if any, advantage since increased beam current
 increases beam divergence and loss of ion flux in the magnet and at the
 target which largely counteracts the increase of beam current. Further,
 the spreading effect is aggravated at low beam energies and particularly
 at high currents.
 In a plasma outside of a dipole magnet, it is well-known to confine the
 plasma and reduce the electron temperature by confining the electrons
 magnetically with a multi-pole magnetic structure. In such a structure,
 the electrons are confined by a large mirror ratio at magnetic cusps.
 However, when a multi-pole field is combined with a dipole field as
 disclosed, for example, in U.S. Pat. No. 5,206,516, either the electrons
 are confined by a cusp at one side of the dipole field but the field is
 decreased at the other side of the dipole field or, if the electrons are
 confined by cusps at both sides of the dipole field, the region of the
 cusps will be followed, along the beam line, by a region of reduced
 magnetic field. Thus, very little net confinement is achieved in either
 case.
 Accordingly, it is seen that there has been, prior to the present
 invention, no known technique for increasing ion beam tool throughput
 since neither increase of ion beam current nor magnetic confinement with a
 multi-pole structure provides a significant increase in ion beam flux at
 the target.
 SUMMARY OF THE INVENTION
 It is therefore an object of the present invention to provide a technique
 for reducing ion beam divergence in dipole magnet structures.
 It is another object of the invention to provide increased ion beam
 currents in ion beam tools.
 It is a further object of the invention to provide dipole magnet structures
 with much reduced ion beam divergence and improved plasma and/or electron
 confinement.
 It is another further object of the invention to provide a dipole magnet
 structure through which increased ion beam currents can be passed.
 It is yet another object of the invention to provide additional ionization
 in the beam plasma for low energy beams and low beam pressures.
 In order to accomplish these and other objects of the invention, an
 apparatus utilizing an ion beam and including a dipole magnet for
 manipulating the ion beam is provided wherein the dipole magnet includes a
 gap between pole pieces of the dipole magnet through which the ion beam
 passes, and a plurality of electrodes or insulators for maintaining a
 negative charge disposed within the gap between the pole pieces of the
 dipole magnet. The negative charge thus confines electrons in the plasma
 to neutralize the space charge of the ion beam and thus reduces beam
 spreading and loss of ions to the pole pieces.
 In accordance with another aspect of the invention, an ion beam apparatus
 having a dipole magnet and a beam plasma inside a gap of the dipole magnet
 is provided, comprising an arrangement for increasing magnetic fields near
 the pole faces of the dipole magnet, and an arrangement for
 electrostatically confining said ion beam at a plurality of locations
 along said pole faces in a direction of ion beam travel to provide
 additional electrostatic confinement of electrons, particularly where
 magnetic fields are reduced.
 In accordance with a further aspect of the invention, a method for reducing
 ion beam divergence in a gap of a dipole maget is provided comprising the
 steps of increasing a magnetic field of the dipole magnet in a region near
 its pole faces and electrostatically confining a beam plasma at a location
 along the pole faces in a direction of ion beam travel where the magnetic
 field is reduced.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION
 Referring now to the drawings, and more particularly to FIGS. 1 and 1A,
 there is shown, in cross section, the gap of a dipole magnet such as may
 be used for mass analysis or any other manipulation of the ion beam in
 accordance with one exemplary embodiment of the invention. The magnetic
 field pattern is depicted in FIG. 1A to aid in visualization of the
 principles of the invention but omitted from FIG. 1 in the interest of
 clarity.
 It should be understood that in an ion beam transport system in which the
 beam is travelling near the local ground potential or where there are no
 electrodes to draw off electrons, the ion beam will produce a beam plasma
 by ionization of the background neutral gas. The number of ions and
 electrons produced to form the plasma will depend on the pressure of the
 background gas and the beam energy. In general, the beam plasma will
 partially fill the beam line or beam raceway which is provided to surround
 the beam. It will thus be in the region of the beam and serves to reduce
 the effects of the space charge of the ion beam which cause the beam to
 diverge.
 In FIGS. 1 and 1A, a part of a dipole magnet 10 is shown in cross-section
 with ion beam 11 passing between the pole pieces 1. The pole pieces 1 are
 formed with convex regions 14 and concave regions 16. These formations
 serve to concentrate the magnetic field at regions 14 to form cusps, as
 may be observed from the depiction of magnetic field lines 3 converging at
 convex regions 14 in FIG. 1A. A similar effect or enhancement thereof can
 be produced by the placement of magnetic material (e.g. permanently
 magnetized or higher permeability material such as samarium or cobalt) in
 similar locations on the pole piece faces. These cusps serve to
 magnetically confine electrons in the beam plasma by virtue of the
 increased magnetic field near the beam raceway surface relative to the
 field at the axis of the beam which tends to reflect electrons having a
 trajectory component toward the cusps and away from the beam.
 Concave regions 16 are preferably provided since their complementarity to
 convex regions 14 increases the mirror ratio of the magnetic confinement
 and the recess provided by the concave profile accommodates the electrodes
 or insulators which will be used for electrostatic confinement in
 accordance with the invention without reduction of the width of the beam
 raceway. Further, the recess provided by the concavity allows the
 potential sheaths of electrodes and/or insulators 6, 7 to be completely
 outside of the ion beam.
 In the embodiment of FIGS. 1 and 1A, convex portions 14 of the pole piece
 faces 1 are located opposite concave regions 16 across the raceway so that
 the magnetic confinement is only on one side of the beam plasma at any
 given location along the beam path. Opposite convex regions 14 and at
 concave regions 16 the electrons are electrostatically confined in
 accordance with the invention.
 The electrostatic confinement is accomplished by negative electrodes 6 or
 isolated electrodes or insulators 7. If isolated electrodes and/or
 insulators are used, the greater diffusivity of the electrons in the
 plasma will cause the isolated electrodes and/or insulators 7 to charge
 negatively as electrons from the plasma are incident thereon and
 accumulated. Therefore a negative charge on either electrodes 6 or
 isolated electrodes and/or insulators 7 will cause electrons in the plasma
 to be repelled and thus confined electrostatically at location where the
 electrons are not confined magnetically by (and opposite to) the cusps and
 transverse field lines of the magnetic field across the dipole gap. That
 is, in the embodiment of FIGS. 1 and 1A, the electrons will be confined in
 one direction by the mirror fields 4 and, in the opposite direction, the
 electrons will be confined electrically (e.g. electrostatically). These
 two directions and modes of confinement will alternate along the faces of
 the pole pieces in the direction of the beam and motion of ions therein.
 As a perfecting feature of the invention but not necessary to the
 successful practice thereof, an optional raceway liner 22, preferably of
 graphite, can be provided to reduce contamination due to the ion beam
 sputtering the pole faces 1. The raceway liner 22 should have open regions
 adjacent regions where electrical confinement of electrons in the beam
 plasma is performed. The raceway liner 22 is connected to a local ground
 to avoid charging by accumulation of electrons thereon.
 As another perfecting feature of the invention which extends the
 applicability and performance of the invention but is not necessary to its
 practice in accordance with its basic principles, the negative voltage
 applied to electrodes 6 can be modulated with a high frequency (e.g. radio
 frequency (RF), preferably in the 40 MHz-100 MHz range or possibly higher)
 voltage, as depicted at 40 of FIG. 1, or a high frequency-electric field
 similarly superimposed thereon by electrodes 6' in addition to negatively
 charged electrodes 6 and/or isolated electrodes and/or insulators 7 to
 increase ionization and plasma particle density. That is, for conditions
 of low ion beam energy and/or low pressure of background gas, additional
 ionization beyond that provided by the ion beam, itself, can be developed
 by the provision of a radio frequency electric field. The additional
 ionization reduces beam divergence, as alluded to above, under conditions
 such as low beam energy where beam divergence would otherwise be
 relatively more severe.
 A further perfecting feature shown in FIGS. 1 and 1A is the optional
 inclusion of field clamps 33 that preferably extend within the dipole
 magnet coil 34. The field clamp 33, if employed, is essentially a magnetic
 shield for reducing the magnetic field outside of pole faces 1, assisting
 in the reduction of electron temperature outside the dipole gap.
 It should be appreciated that any or all of the perfecting features
 discussed above can be employed, as desired, in any or all of the
 disclosed exemplary embodiments of the invention described below and
 illustrated in FIGS. 2 and 4-6 as well as all variations thereof.
 Accordingly, discussion of these features in regard to the following
 exemplary embodiments is unnecessary.
 Referring now to FIGS. 2 and 2A, a second embodiment in accordance with the
 invention is shown in cross-sectional views similar to those of FIGS. 1
 and 1A, respectively. (Reference numerals used in FIGS. 1 and 1A will be
 used to identify corresponding structure in other Figures.) This
 embodiment differs from the embodiment of FIGS. 1 and 1A principally by
 the convex regions 14 and concave regions 16 being directly opposed across
 the magnet gap and ion beam raceway. In this case, each of the magnetic
 cusps which provides magnetic mirror containment of the plasma electrons
 is directly opposite another cusp and each pair of cusps serves to contain
 the plasma electrons from both sides of the beam in regions 13. Between
 pairs of cusps, the electrons are confined electrostatically by
 electrodes, isolated electrodes or insulators 6, 7, from both sides of the
 beam in regions 23.
 It should be appreciated in this embodiment that the electrons in region 23
 are also confined from regions 13 by magnetic field lines 3. However, low
 energy electrons (which have many more collisions than hotter electrons)
 can move (e.g. diffuse) from region 23 to region 13. (The hot electrons
 set up a high frequency field that allows cold electrons but not hot
 electrons to diffuse across the boundary of these regions.) Thus regions
 13 will have colder electrons than in regions 23 and, consequently,
 reduced beam spreading.
 FIG. 3 is a graph of the mirror ratio developed by the cusps of the
 embodiments of FIG. 2A as a function of the magnet gap to cusp spacing or
 pitch (e.g. the ratio of the "vertical" spacing of regions 14 to the
 center-to-center "horizontal" spacing of regions 14 across concave regions
 16) at a particular distance from the face of the pole pieces. The mirror
 ratio is the ratio of the magnetic field strength at the inner surface of
 the beam raceway (e.g. 22) to the magnetic field strength at the center of
 the beam raceway. The mirror ratio is a measure or figure of merit
 indicating the quality or effectiveness of magnetic plasma electron
 confinement.
 The curve of FIG. 3 illustrates that the smaller the cusp pitch, the larger
 the mirror ratio but that the distance above the faces of the pole pieces
 where the mirror ratio is maximized also becomes closer to the pole piece
 faces and may actually occur within the thickness of the liner, if used,
 resulting in the reduction of the mirror ratio at the surface of a liner
 of given thickness as the pitch is reduced beyond a dimension
 corresponding to location of the maximum mirror field at the liner
 surface.
 Further, if the length of concave region 16 (e.g. the portion of the pitch
 of regions 14, which is the sum of the "horizontal lengths of a region 14
 and a region 16) is increased relative to the ("horizontal") length of
 each convex region 14, the mirror ratio can be increased because the
 magnetic field near the pole piece faces will become more concentrated as
 regions 14 become more narrow. At same time, however, depending on the
 depth of concave regions 16, the average gap length (and magnetic circuit
 reluctance) will be increased, decreasing field strength in the gap for a
 given excitation of coil 34. This effect and exploitation thereof in
 preferred embodiments of the invention will be discussed in regard to the
 embodiments of FIGS. 4 and 5 which derive increased mirror ratios by
 variation of cusp geometry and pitch.
 Specifically, in FIGS. 4 and 4A, the regions 16 are three times as long as
 the regions 14 while other dimensions are maintained the same as in the
 embodiments of FIGS. 1 and 2. For this geometry and pole piece profile,
 the mirror ratio has been increased to 1.57 while the magnetic field
 strength on axis (in the center of the gap) is reduced by only 12.5%
 relative to a known flat pole piece. Perhaps more importantly, the
 magnetic field at the surface of liner 22 above the convex regions 14 of
 the pole pieces 1 are substantially constant; thus deriving a nearly
 constant mirror ratio at the liner 22. Accordingly, this embodiment is
 preferred over the embodiments of FIGS. 1 and 2 for a liner 22 of the same
 thickness. The flat parts of the convex regions 14 are preferably
 protected by liner 22 while the tapered and flat parts of concave regions
 16 are protected by electrodes or insulators 6, 7.
 The ratio of magnet gap to pitch of regions 14 of the embodiment of FIG. 4
 is 1.25. This ratio can be increased essentially at will (to lessen the
 reduction of on-axis field strength) by reducing the pitch of the cusps.
 For example, the embodiment shown in FIGS. 5 and 5A, is preferred to the
 embodiment of FIG. 4 but requires a thinner liner 17 as compared with
 liner 22. Liner 17 may be conveniently formed of Teflon (a trademark for
 polytetrafluoroethylene) tape or other thin insulating material or a thin
 insulator covered by stripes 16 of isolated conductors, such as graphite
 or metal tape, may be used. Negative electrodes 6 may also, optionally, be
 used.
 For the structure shown in FIGS. 5 and 5A, the mirror ratio is 1.75 and the
 gap/pitch ratio is 3.0 which, again, gives a substantially constant
 magnetic field at the surface of liner 17. In this configuration, the
 average field in the center of the gap is reduced by only 4.5% compared to
 a flat pole face with the same minimum gap.
 The limiting case having a pitch equal to the pole face length by virtue of
 omission of concave regions 16 is shown in cross-section in FIG. 6.
 Accordingly, there is only a single cusp corresponding to the entire face
 of the pole pieces. This embodiment is not preferred as a matter of ion
 beam containment performance but may be preferred for retrofitting the
 invention to existing dipole magnets or in other applications where the
 preferred pole piece face profile and cusp geometry cannot be practically
 provided. Nevertheless, a substantially improved degree of beam
 confinement is provided by their provision of additional electrostatic
 confinement even in this limiting case of only a single magnetic cusp
 coextensive with the pole piece face.
 In this case, the liner 16 must be an insulator with short sections of
 electrode 6 or isolated electrode 7. Here, the insulator protects the flat
 pole pieces and the electrode 6 or isolated electrode 7 provides
 additional confinement for the plasma electrons even though there is only
 a single cusp and magnetic confinement is effectively not enhanced. The
 electrode or insulator 7 will be more positive at the beginning and end of
 the pole piece along the ion beam path where the mirror ratio is higher
 and will be more negative in the falt center of the pole piece.
 To generalize the embodiments of FIGS. 1, 2 and 4-6, the continuity
 equations for the creation of electrons and non-beam ions in the beam
 plasma combined with the momentum equations for the electrons and non-beam
 ions demonstrate that for low pressures of about 2.times.10.sup.-7 to
 2.times.10.sup.-5 Torr which are typical for mass analysis and ion beam
 tools used in semiconductor manufacture, the electrons are confined
 principally by the inertia of the ions and the magnetic mirror
 confinement. Further, the total current of electrons to the walls is equal
 to the total current of ions to the walls.
 Where the electrons are confined by the magnetic mirror fields at the
 magnetic cusps, the ions are not pulled out of the beam or plasma to the
 beam liner wall. The ions are dominantly pulled out at the negative
 electrodes where the electrons are more effectively confined
 electrostatically. The invention thus effectively reduces the area to
 which ions can be lost and, in turn, reduces the electron temperature and,
 further, reduces the number of ions that can be pulled out of the beam in
 those areas by confinement of the plasma electrons. By the same token, the
 plasma density is increased, thus further lowering the electron
 temperature. This confinement and reduced electron temperature thus
 produced in accordance with the invention reduces the electric fields
 which are set up in the direction of the pole faces and reduces the
 divergence of the ion beam.
 While the invention has been described in terms of four preferred
 embodiments (two being particularly preferred) and a limiting case
 embodiment, those skilled in the art will recognize that the invention can
 be practiced with modification, particularly over a continuum of pole pies
 geometries among the disclosed embodiments within the spirit and scope of
 the appended claims.
 For example, the profile of a concave region 16 and a convex region 14 are
 depicted as a pair of flat regions connected by tapered regions, forming
 four faces or facets for simplicity of illustration. It should be apparent
 that the principles of the invention are fully applicable to the use of
 more than four faces or, in a limiting case, a smooth curve. For another
 example, the concave sections may be proportionately much deeper or wider
 that illustrated in the exemplary embodiments discussed above.
 Likewise, a particular shape of such a continuous curve or the angles of
 the particular faces or facets may be preferred for particular
 applications of the principles of the invention which differ from the
 exemplary embodiments specifically disclosed. All of these variations of
 the invention should be considered as being schematically depicted by the
 faces or facets illustrated in FIGS. 1, 2, and 4-6. Similarly, it should
 be understood that the various perfecting features such as the field clamp
 33 and the use of a RF field to enhance ionization as well as various
 forms of liner 22 described above can be included singly or in combination
 with any of the embodiments disclosed.