Ion implantation device arranged to select neutral ions from the ion beam

An ion implantation device includes at least two successive deceleration stages the first deceleration stage, looking in the downstream direction, being arranged to decelerate the ion beam, to deflect the ion beam, and to form an intermediate crossover, whereas the second deceleration stage is arranged to decelerate the ion beam further and to subject the beam to a converging effect.

BACKGROUND OF THE INVENTION
 The invention relates to an ion implantation device which includes: an ion
 source for producing a beam of ions to be implanted into a substrate, an
 acceleration electrode for accelerating the ion beam emanating from the
 ion source, ion-optical elements which are arranged downstream from the
 acceleration electrode in order to influence the direction of the ions in
 the ion beam, and a deceleration device which is arranged downstream from
 said ion-optical elements in order to decelerate the ion beam.
 The invention also relates to a method of implanting ions in a substrate.
 A device of the kind set forth is known from the abstract in English of
 Japanese patent application No. 3-47123, filed on Mar. 13, 1991 and
 published under publication No. 4-284343 on Oct. 8, 1992.
 Ion implantation is commonly used in the manufacture of integrated circuits
 in order to form specified doping profiles, for example a specified doped
 ion concentration as a function of the depth in the substrate. The ion
 beam required for this purpose is produced in known manner by an ion
 source, after which the beam is accelerated to a desired velocity by an
 (electrostatic) acceleration electrode which directly succeeds the ion
 source. For the further influencing of the ion beam such a device may also
 be provided with ion-optical elements, such as a deflection device for
 scanning the beam across the substrate to be doped and charged particle
 lenses for focusing or otherwise converging or diverging the ion beam.
 Subsequent to the acceleration electrode said known device is provided with
 an ion-optical element in the form of a mass separation unit for
 separating ions having an undesirable mass from the ion beam, so that the
 ion beam thus produced consists of one type of ion only. From the
 particle-optical technique it is generally known that in order to achieve
 suitable and controlled influencing of the ion beam by the ion-optical
 elements is desirable that the ion beam has a sufficiently high velocity,
 for example a velocity which corresponds to a kinetic energy of the order
 of magnitude of from tens to hundreds of keV. A typical value in this
 respect is 30 keV, thus corresponding to a voltage of 30 kV on the
 acceleration electrode (the acceleration voltage). This is because when
 the energy of the bam is too low (for example, 1 keV), the beam becomes
 highly susceptible to disturbing influences from inside and outside the
 apparatus and to undesirable expansion of the beam due to space charging
 in the beam.
 Said specified doping profiles often require the ions to be implanted only
 in a zone up to a specified depth in the substrate to be doped. To this
 end, the ions may be incident on the substrate only with a given,
 specified velocity, i.e. energy. This specified energy may typically be of
 the order of magnitude of 1 keV. In order to conduct the ion beam through
 the ion-optical elements with a sufficiently high energy and nevertheless
 make the beam land on the substrate with the specified energy, downstream
 from said ion-optical elements there is arranged in known manner a
 deceleration device for decelerating the ion beam to the desired energy.
 The described processes take place in an evacuated space. The vacuum of
 this space is often of poor quality because gases are released during
 irradiation of the substrate by means of ions (notably from the residual
 material on the substrate), which ions are spread through the vacuum
 space. During traveling of the path from the ion source to the
 deceleration device, interaction with the released gases and the residual
 gases always present in the apparatus neutralizes a part of the ions in
 the beam. These neutralized ions (i.e. atoms) are no longer sensitive to
 influencing by the ion optical elements and the deceleration device, so
 that these atoms strike the substrate with the full energy of, for example
 30 keV and hence penetrate therein to a depth which is much greater than
 the depth corresponding to the specified doping profile. Moreover, such
 atoms are not sensitive to fields applied for scanning the beam across the
 substrate to be treated, so that these atoms form a stationary dot "spot"
 at the center of the substrate region to be doped, thus locally causing an
 inadmissibly high concentration of the relevant element in the substrate.
 In order to counteract the problems concerning the neutralized ions, the
 deceleration device for decelerating the ion beam in the known ions
 implantation device is also arranged to deflect the ion beam. The
 neutralized ions (i.e. the atoms) which are not sensitive to
 electromagnetic deflection then continue their travel in the original
 direction and hence can be separated from the deflected ion beam.
 In these known devices a problem is encountered in that the deceleration
 device consists of an assembly of three electrodes which together
 constitute an electrostatic lens. The first electrode of this lens carries
 a potential which amounts to a fraction of the acceleration voltage (thus,
 this first electrode is actually formed by the boundary of the drift space
 carrying said potential); the third electrode of this lens carries ground
 potential (the third electrode is actually formed by the entrance of the
 treatment space of the substrate which carries ground potential), whereas
 the central electrode carries a potential which lies between said two
 potentials. In particle optics it is generally known that electrostatic
 deceleration is ineviatably accompanied by a lens effect exerted by the
 decelerating field. Due to this lens effect, the ion beam is subjected to
 a diverging or a converging action. For said order of magnitude of the
 acceleration voltage and the ultimate speed of landing of the ion beam,
 the ions in the beam are given an inadmissibly large velocity component
 transversely of the beam axis due to said diverging or converging effect.
 Consequently, a significant part of the ions would not reach the
 substrate, because they would be intercepted by beam limiters between the
 deceleration electrodes and the substrate. Moreover, a large angular
 spread of the ions in the beam could cause a shading effect on the
 substrate to be doped. This means that the ion beam which apparently
 emanates from one point fans out in a conical manner, so that regions on
 the substrate which directly adjoin an edge of a region with a given
 difference in height with respect to the remainder of the substrate are
 situated in the shade of said edge and hence receive fewer ions than the
 regions which are not situated in the shade. For these two reasons the
 angular spread of the ion beam incident on the substrate must be small.
 SUMMARY OF THE INVENTION
 It is an object of the invention to provide an ion implantation device of
 the kind set forth in which neutralized ions can be separated from the ion
 beam and in which the ion beam is decelerated without imparting an
 inadmissibly large angular spread to the ion beam.
 To achieve this, the device according to the invention is characterized in
 that the deceleration device includes at least two successive deceleration
 stages the first one of the two deceleration stages, viewed in the
 downstream direction, being arranged to decelerate the ion beam, to
 deflect the ion beam, and to form an intermediate crossover, said second
 deceleration stage being arranged to decelerate the ion beam further and
 to subject the beam to a converging effect.
 The first deceleration stage is constructed in such a manner that the beam
 is subjected to a first deceleration therein. This stage can be
 proportioned in such a manner that an intermediate crossover of the ion
 beam is formed by the lens effect associated with this deceleration, i.e.
 a crossover which is situated in the region between the exit of the first
 deceleration stage and the entrance of the second deceleration stage, that
 is to say in a position such that the lens effect of the decelerating
 field of the second stage converges the beam in such a manner that the
 beam exhibits the required small angular spread on the substrate. The
 neutralized ions are separated from the ion beam by the deflecting effect
 of the first stage.
 In a preferred embodiment of the device according to the invention a
 selection gap is provided between the first and the second deceleration
 stage, the direction of said gap extending transversely of the plane in
 which the beam is deflected by the first deceleration stage. The neutral
 ions can thus be readily separated from the ion beam, and at the same time
 energy selection can also be performed in the ion beam by a suitable
 choice of the gap width. The selection gap is advantageously provided in
 the final electrode of the first deceleration stage, so that in that case
 it is not necessary to mount separate elements in the vacuum housing of
 the apparatus.
 In a further embodiment of the device according to the invention, the
 second deceleration stage is succeeded by a further selection gap whose
 direction extends transversely of the plane in which the beam is deflected
 by the first deceleration stage. This is because it may occur that
 neutralization of ions in the ion beam still takes place in the region in
 the first deceleration stage in which complete or partial deflection of
 the beam has already taken place. The ions produced at that area are
 situated in the direct vicinity of the intermediate crossover, so that a
 very large part thereof has a significant transverse component in the
 speed, because the beam is strongly diverging in the vicinity of the
 intermediate crossover. These neutral particles can then pass the
 selection gap, but are not subjected to the converging effect of the
 second acceleration stage. Thus, beyond this second stage such an
 undesirable neutral particles will be separated from the ion beam by means
 of a further selection gap. Like in the first deceleration stage, the
 further selection gap is advantageously provided in the final electrode of
 the associated deceleration stage, so that it again will not be necessary
 to mount separate elements in the vacuum housing of the apparatus.

DETAILED DESCRIPTION OF THE INVENTION
 FIG. 1 shows diagrammatically a part of an ion implantation device which is
 of relevance to the invention. An ion source in the device produces an ion
 beam in a manner which is not shown, said beam being accelerated to an
 energy of 30 keV directly behind the ion source. The production and
 acceleration of such an ion beam is generally known per se and need not be
 elucidated in the context of the present invention. In order to influence
 the ion beam further the device is also provided with ion optical elements
 (not shown in the Figure), such as particle lenses for focusing or
 otherwise diverging or converging the ion beam. The production and
 acceleration of such an ion beam and the focusing thereof are generally
 known per se and need not be further elucidated in the context of the
 present invention.
 After having traversed the above-mentioned elements, the ion beam 2 reaches
 the part of the device shown in FIG. 1. This part consists of a first
 deceleration stage 4 which is succeeded by a second deceleration stage 6.
 The first deceleration stage 4 consists of a first electrode 8, a second
 electrode 10, a deflection system 12 which consists of two deflection
 plates 12-1 and 12-2, and a third electrode 14 which also constitutes the
 final electrode of the first deceleration stage. The second electrode 14
 has a tubular shape with a rectangular cross-section. The acceleration of
 the (positively charged) ions immediately behind the ion source is
 realized in that the ion source carries a potential of +30 kV relative to
 the environment. The acceleration electrode used for the acceleration of
 the ions and the subsequent particle-optical elements then carry ground
 potential relative to the environment. Consequently, the ions in the beam
 have a kinetic energy of 30 keV when they reach the electrode 8. The
 second electrode 10 carries a potential +22 kV, so that the ions are
 decelerated to a kinetic energy of 8 keV. The deflection system 12 carries
 a mean potential of +22 kV, the deflection plate 12-1 carrying a potential
 which is 800 V higher than said mean value whereas the deflection plate
 12-2 carries a potential which is 800 V lower than said mean value. Thus,
 a voltage difference amounting to 1.6 kV exists between these two plates.
 Finally, the third electrode 14 carries a potential of 22 kV again, so
 that the ions leave the first deceleration stage with a kinetic energy of
 8 keV. The third electrode 14 is provided with a selection gap 16 whose
 longitudinal direction extends perpendicularly to the plane of drawing.
 The decelerating electrostatic field between the first electrode 8 on the
 one side and the electrodes 10, 12 and 14 on the other side focuses the
 ion beam in the plane of the selection gap 16 and the deflection field
 between the deflection plates 12-1 and 12-2 moreover, deflects the ion
 beam to the selection gap 16. The selection gap 16 thus extends
 perpendicularly to the plane in which the ion beam 2 is deflected by the
 first deceleration stage.
 When the distance between the electrodes 8 and 14 amounts to 87 mm, the
 length of the electrode 10 amounts to 30 mm and the length and the plate
 spacing of the deflection system 12 amount to 45 mm and 30 mm, and using
 the said voltages, the incident ion beam 2 is deflected to an angle of
 7.degree. (0.122 rad). It will be evident that in the case of a gap width
 of 4 mm for the selection gap 16, practically all neutralized ions are
 intercepted by the electrode 14.
 After having traversed through the first deceleration stage 4, the
 resultant focused and deflected ion beam 2 reaches the second deceleration
 stage 6. The second deceleration stage 6 consists of a tubular fourth
 electrode 18 which has a rectangular cross-section, and a fifth electrode
 20 which constitutes a final electrode of the second deceleration stage.
 The fifth electrode 20 is provided with a further selection gap 22.
 The fourth electrode 18 carries a potential of 29 kV, so that the ion beam
 is further decelerated from a kinetic energy of 8 keV to 1 keV between the
 electrode 16 and the electrode 18, and the fifth electrode 20 carries a
 potential of 28 kV so that the ions are locally accelerated between the
 electrode 18 and the electrode 20, but nevertheless in total are subjected
 to a deceleration from 8 keV to 2 keV. Because of the decelerating
 electrostatic field in the second deceleration stage 6, the ion beam is
 subjected to a converging effect in this trap. Because the first crossover
 of the ion beam is situated ahead of the second deceleration stage, the
 beam enters the deceleration stage 6 in a diverging fashion. The focusing
 effect of the deceleration stage 6 ensures that the ion beam is given the
 desired, small angular spread at the area of the substrate 24.
 Because of the presence of the further selection gap 22, neutralized ions
 are substantially intercepted in the intermediate space between the second
 electrode 10 and the fourth electrode 20. This is because it may happen
 that neutralization of ions in the ion beam still takes place In the
 region in the first deceleration stage where complete or partial
 deflection of the beam has already taken place. The neutralized ions
 arriving at that area are situated in the direct vicinity of the
 intermediate crossover, so that a very large part thereof has a
 significant transverse component in the velocity, because the beam is
 strongly diverging in the vicinity of the intermediate crossover. Such
 neutral particles can now have the first selection gap 16, but are not
 subjected to the converging effect of the second deceleration stage 6.
 Subsequent to the second stage 6, the undesirable neutral particles can
 thus be separated from the ion beam by means of the further selection gap
 22. Like the electrode 10, the substrate 24 carries a potential of 28 kV,
 so that no electric field is present in the state between the final
 electrode 20 and the substrate 24; consequently, the ions of the ion beam
 2 land on the substrate 24 to be doped with an energy of 2 keV.
 In order to realize a . . . implantation of ions in the substrate 24, it is
 necessary to displace the ion beam relative to the substrate. This
 displacement is performed by . . . the beam in a first direction
 perpendicular to the plane of drawing, using an electrostatic or magnetic
 . . . device which is not shown in the Figure. The . . . device is
 preferably situated upstream from the electrode 8. The desired
 displacement of the beam relative to the substrate in a second direction
 perpendicularly to the first direction is performed by displacing the
 substrate in the vertical direction of FIG. 1, for example from the bottom
 upwards in the plane of drawing.
 FIG. 2 shows the path of the ion trajectory in the ion beam in the part of
 the ion implantation device shown in FIG. 1. The ion trajectory shown in
 this Figure has been obtained in relation by means of a computer program.
 For this purpose values were assumed which are all the same as those
 stated in the description of FIG. 1. The various relevant dimensions are
 given in FIG. 2.
 FIG. 2 clearly shows that in the region between the electrodes 8 and 14 the
 parallel incident beam is focused as well as deflected. If desirable, the
 gap width 16 could be chosen to be substantially smaller, without impeding
 the focused beam. Furthermore, this figure clearly shows that the angular
 spread of the beam which is strongly diverging at the area of the
 selection gap 16 is significantly reduced by the converging effect of the
 second deceleration stage 6, so that this beam is incident on the
 substrate 24 in a substantially parallel fashion.