Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
2000-12-15
2002-12-24
Nguyen, Kiet T. (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML
Reexamination Certificate
active
06498348
ABSTRACT:
The present invention relates to apparatus and methods for acting on charged particles. The invention relates in particular, but not exclusively, to a charged particle focusing system using magnetic fields to achieve mass (and energy) dependent focusing of a charged particle beam, or series of charged particle beams, so that very high beam currents of mass analysed charged particles, typically positive ions, can be extracted from a multiple slot source and transmitted without any substantial change in particle current distribution in a plane containing the nominal beam direction and at right angles to the plane of mass dispersion. More specifically, a series of uniform ribbon ion beams can be extracted from a multiple slot ion source and mass analysed to achieve high beam purity without disturbing the uniformity or geometry of the ribbon beam. In other aspects the invention relates to means for using elements analogous to optical elements as part of a system to achieve optimum performance for particular requirements and circumstances; and to means for removing the particle masses not transmitted from the system where their continuous build up might lead to undesirable consequences; and to means for preventing the high currents produced from causing surface charging problems on semiconductor wafers or flat panel display substrates; and to means for achieving the required ion source and extraction conditions necessary for the successful application of this invention.
At the end of this specification there is set out a list of references which will be referred to in this specification to assist understanding of the invention, the contents being incorporated herein by reference. Ions are extracted from ion sources [1] and magnetically analysed to achieve mass separation [2] in order to produce a high purity, directed beam of ions (usually positive ions) which can be used to implant into various substrates, of which semiconductor wafers, solar cells and flat panel displays are important commercial examples. Existing technology predominantly uses a system of analysis which will be referred to as ‘conventional mass separator’ optics [3] which restricts the system to the production and analysis of a single ion beam with a significant constraint on the size (and beam current) of that single ion beam.
In U.S. Pat. No. 4,578,589 there is described firstly a conventional apparatus for producing a mass analysed ribbon beam of charged particles in which the ribbon beam is analysed by dispersion in a plane perpendicular to the slot producing the ribbon beam. This is followed by a description of the invention of that prior patent, in which a ribbon beam is mass analysed by dispersion in a plane parallel to the slot producing the ribbon beam. The first known system will now be described briefly with reference to
FIGS. 1
a
to
1
c
of the present specification, followed by a description of the second form of known apparatus, described with reference to
FIGS. 1
d
to
1
e
of the present specification.
FIG. 1
a
shows the dispersion plane (the plane in which there is dispersion of the ion beam into many directions according to their (mass)×(energy) product and charge state) of a conventional mass separator. The ion beam can be extracted from the ion source [4] as a circular beam, but where a high beam current is required it is usual to extract from a long slot (long in this context being typically a 10:1 aspect ratio or more). The ion source
11
produces ions which are extracted from the ion source aperture
12
(circular or long slot) using electrically biased extraction electrodes
13
to form an ion beam
14
(with an energy determined by the extraction voltage) which typically diverges from the ion source extraction region. The ion beam is then passed between the poles of an analysing magnet
15
as also shown in a side view in
FIG. 1
b,
the beam in this case being a parallel ribbon beam
14
A. This magnet has two functions, one being to achieve mass dispersion and the other being to focus the beam so that mass analysis can be achieved at the resolving slit
16
. It is necessary to focus through a resolving slit so that slightly lower ion masses (deflected through a larger angle) or slightly higher masses (deflected through a smaller angle) are not transmitted. This analysis technique does not allow the use of multiple ion beams (as viewed in the dispersion plane of
FIG. 1
a
) and the size of the beam in the extraction slot plane is limited by size of the magnet pole gap. The size and therefore the cost of the magnet, and its power consumption (for an electromagnet) are important commercial considerations. One technique that has been used to improve this situation is shown in
FIG. 1
c.
The use of a curved extraction geometry [5] to produce a converging ribbon beam in the plane containing the axis of the long extraction slot (referred to as the ‘ribbon plane’) with a beam crossover in the magnet pole gap, increases the size of the beam which can be transmitted through a particular size pole gap. If, as is usually the case, a parallel beam is required, then an optical element
17
focusing in the ribbon plane is required to produce a parallel beam (such as a curved electrode acceleration system [5]) is required after the resolving slit. Referring again to
FIG. 1
a,
the divergence in the dispersion plane, which is normally small (typically a half-angle of 1-3°), may be acceptable for ion implantation; if it is not then an optical element
18
in the dispersion plane can be used to create a parallel beam before arrival at the target
19
. The optical components
17
and
18
may be separate or achieved in a single optical element.
In order to overcome the current limitations imposed by conventional mass separator optics, the previous invention [6] by the present inventor (in U.S. Pat. No.4,578,589) achieved improvement by placing the long ion source slot in (or parallel to) the dispersion plane. This removed the practical correlation between the length of the slot and the pole gap required in the analysing magnet and made it possible to analyse beams from a series of long slots.
FIG. 1
d
shows the dispersion plane of such a system with a parallel beam
24
P (in the dispersion plane) leaving the ion source
21
, from a long slot
22
and extraction electrodes
23
and entering the analysing magnet
25
, the apparent object position being at infinity. The length of the slot is limited only by the acceptable divergence from the resolving slit
26
(from the point of view of the angular acceptance of the rest of the ion beam system) and the maximum acceptable length from the magnet exit to the resolving slit. Multiple slots, one above the other, see the same geometry in the dispersion plane.
FIG. 1
e
shows a side view along the axis of the long slot, the beamline being unfolded into a single plane for convenience of illustration. A divergent beam
24
D is shown leaving the ion source
21
, its outlet aperture
22
and the beam forming extraction electrodes
23
. The beam enters an angled entry analysing magnet field which (for this particular angle of entry) produces a convergent lens
27
A (a well known technique for achieving useful converging or diverging focusing [7]), which significantly reduces the divergence of the beam, and then is focused more at the angled exit region
25
B and
27
B, ideally producing a near parallel beam.
FIG. 1
f
shows a source-to-magnet view of a multiple (three) beamlet beam
24
M from three outlet apertures
22
. The direction of each of the beamlets in the extraction region is chosen for optimum transmission through the analysing magnet.
The two mass analysis techniques described above represent the known relevant prior art with regard to overall system design. Other relevant prior art includes the angled entry focusing [7] already mentioned, and magnetic multipole focusing.
Magnetic multipole focusing is commonly used in accelerator beamlines [8&rs
Nguyen Kiet T.
Pillsbury & Winthrop LLP
Superion Limited
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