U.S. patent number 3,689,766 [Application Number 05/065,941] was granted by the patent office on 1972-09-05 for apparatus for bombarding a target with ions.
This patent grant is currently assigned to GB Atomic Energy Authority, London, GB2. Invention is credited to James Harry Freeman.
United States Patent |
3,689,766 |
|
September 5, 1972 |
APPARATUS FOR BOMBARDING A TARGET WITH IONS
Abstract
An electromagnetic separator adapted for ion implantation on an
industrial production scale has its beam current stabilized and a
mechanism within the target chamber for automatically moving
targets through the ion beam according to a predetermined scanning
pattern.
Inventors: |
James Harry Freeman (Abingdon,
GB2) |
Assignee: |
GB Atomic Energy Authority, London,
GB2 (N/A)
|
Family
ID: |
10432104 |
Appl.
No.: |
05/065,941 |
Filed: |
August 21, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Sep 5, 1969 [GB3] |
|
|
44,171/69 |
|
Current U.S.
Class: |
250/400; 438/514;
438/961; 313/361.1; 250/325 |
Current CPC
Class: |
B01D
59/48 (20130101); H01J 49/022 (20130101); H01J
49/025 (20130101); Y10S 438/961 (20130101) |
Current International
Class: |
H01J
49/02 (20060101); B01D 59/48 (20060101); B01D
59/00 (20060101); H01j 037/00 (); G01n
023/00 () |
Field of
Search: |
;250/49.5R,49.5TE,49.5T,49.5P,49.5A ;148/1.5 ;219/121EB
;118/49.1,49.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: William F. Lindquist
Attorney, Agent or Firm: Larson, Taylor & Hinds
Claims
1. Apparatus for bombarding a target with ions, which apparatus
comprises an ion beam source, a window, a magnet for deflecting the
ion beam towards the window, means for modulating the energy of the
ion beam so that the deflection produced by the magnet varies in
correspondence with the modulation to thereby cause the deflection
ion beam to sweep back and forth over the window, detector means
for detecting the ion beam current passing through the window, ion
beam control means responsive to the output of the detector means
for controlling the ion beam energy modulating means so as to tend
to maintain constant the intensity of the ion beam current passing
through the window, target holder means for supporting a target
behind the window, and drive means for imparting controlled
movement to the target holder means to scan the target through
2. Apparatus as claimed in claim 1, wherein the window is at least
as wide as the target to be bombarded and the ion beam control
means control the ion beam to provide a substantially uniform ion
beam intensity over the
3. Apparatus as claimed in claim 2, wherein the target holder
locates the target behind the window with the width of the target
in register with the width of ion beam passing through the window,
and the said drive means move the supported target lengthwise past
the window at controlled speed.
4. Apparatus as claimed in claim 3, wherein the target holder
carries a plurality of targets in a row and, when moved as
aforesaid, moves the
5. Apparatus as claimed in claim 4, wherein the said drive means
reciprocates the target holder, whereby the targets are moved
repeatedly past the window, the total ion dose being determined by
the number of
6. Apparatus as claimed in claim 5, wherein the target holder is
adapted to carry a plurality of rows of targets and the drive means
includes means for indexing the target holder to bring another row
of targets into
7. Apparatus as claimed in claim 6, wherein the target holder
comprises a cylinder with a plurality of racks arranged around the
periphery, each rack carrying a row of targets extending parallel
to the axis of the
8. Apparatus as claimed in claim 1, wherein the target holder
comprises a plate for supporting one large target or a plurality of
smaller targets, and a support for the plate, which support is
moved by the said drive
9. Apparatus as claimed in claim 8, wherein the drive means
comprise an X-Y
10. Apparatus as claimed in claim 9, wherein the said support is
adapted to release a plate it is carrying automatically at the end
of a complete scanning operation and to return to the starting
position for a further
11. Apparatus as claimed in claim 10, wherein means are provided
for mounting a plurality of plates loaded with targets at a loading
station having means for automatically loading a plate onto the
said support when
12. Apparatus as claimed in claim 1, wherein the ion beam is
produced by an
13. Apparatus as claimed in claim 1, wherein the means for
modulating the energy of the ion beam comprises an electrical
source connected to superimpose an alternating voltage upon a high
direct voltage supplied to the accelerating electrode of the ion
beam source.
Description
The invention relates to apparatus for bombarding a target with
ions, for example to implant ions into the target.
The invention provides apparatus for bombarding a target with ions,
which apparatus comprises an ion beam source, a magnet for
deflecting the ion beam towards a window, means for modulating the
energy of the ion beam so that the deflection produced by the
magnet varies in correspondence with the modulation to thereby
cause the deflected ion beam to sweep back and forth over the
window, detector means for detecting the ion beam current passing
through the window, ion beam control means responsive to the output
of the detector means for controlling the ion beam energy
modulating means so as to tend to maintain constant the intensity
of the ion beam current passing through the window, target holder
means for supporting a target behind the window, and drive means
for imparting controlled movement to the target holder to scan the
target through the ion beam according to a predetermined scan
pattern.
For certain applications, especially for example where the ions
comprise boron ions which can be readily separated from other ions,
the window can be as wide as the target to be bombarded and the
means for stabilizing the ion beam control the ion beam to provide
a substantially uniform ion beam intensity over the area of the
window.
In this case, in a preferred arrangement according to the invention
the target holder locates the target behind the window with the
width of the target in register with the width of ion beam passing
through the window, and the said drive means move the supported
target lengthwise past the window at controlled speed.
Preferably the target holder is adapted to carry a plurality of
targets in a row and, when moved as aforesaid, moves the targets in
succession at controlled speed past the window.
Preferably the said drive means reciprocates the target holder,
whereby the targets are moved repeatedly past the window, the total
ion dose being determined by the number of reciprocations.
Preferably the target holder is adapted to carry a plurality of
rows of targets and the drive means includes means for indexing the
target holder to bring another row of targets into register with
the window.
Preferably the target holder comprises a cylinder with a plurality
of racks arranged around the periphery, each rack being adapted to
carry a row of targets extending parallel to the axis of the
cylinder.
Where a narrower window is required for separating the wanted ions
from unwanted ions, an alternative arrangement is preferred wherein
the target holder comprises a plate capable of supporting one large
target or a plurality of smaller targets, and a support for the
plate, which support is moved by the said drive means.
With this arrangement, the drive means preferably comprise an X-Y
drive.
Preferably the said support is adapted to release a plate it is
carrying automatically at the end of a complete scanning operation
and to return to the starting position for a further scanning
operation.
Preferably means are provided for mounting a plurality of plates
loaded with targets at a loading station having means for
automatically loading a plate onto the said support when it returns
to the starting position. perpendicularly ions
In a preferred arrangement according to the invention, the ion beam
is produced by an ion beam separator in which the magnet is an
electromagnet. In ion beam separators, a beam containing ions of a
plurality of isotopes is caused to pass through a strong magnetic
field which extends substantially perpendicularly to the ion beam
path. The magnetic field deflects the ions into a curved path, the
magnitude of the deflection being dependent, inter alia, upon the
mass and the energy of the ions. Thus, ions of different isotopes
may be separated into diverging beams, and, by appropriate
geometrical arrangement of the components of the separator and
location of the window, ions of a selected isotope may be focused
upon the target. In this case, it is important that the sweep
imparted to the ion beam should not be such as to cause unwanted
ions of a mass different from that selected for bombarding the
target to pass through the window.
Preferably the means for modulating the energy of the ion beam
comprises an electrical source connected to superimpose an
alternating voltage upon a high direct voltage supplied to the
accelerating electrode of the ion beam source.
Specific constructions of apparatus embodying the invention will
now be described by way of example and with reference to the
accompanying drawings, in which:
FIG. 1 is a diagrammatic plan view of the apparatus,
FIG. 2 is a sectional view of part of the apparatus on the line
C--C of FIG. 3,
FIG. 3 is a section on the line A--A of FIG. 2,
FIG. 4 is a fragmentary sectional view on the line B--B of FIG.
2,
FIG. 5 is a view, partly in section, of another part of the
apparatus,
FIG. 6 is a diagrammatic perspective view, partly cut away, of part
of modified apparatus, and
FIGS. 7 and 8 are like views of a detail of the part of the
apparatus shown in FIG. 5, illustrating different stages in the
operation thereof.
DESCRIPTION OF PREFERRED EMBODIMENTS
In these examples, the apparatus comprises an electromagnetic ion
beam separator of the type described in U.S. Patent Application
Ser. Nos. 3,959 and 3,958 both filed on Jan. 19, 1970 . FIG. 1
illustrates the separator diagrammatically and shows an
accelerating electrode 11 of an ion source which is as described in
Patent Application Ser. No. 3,958 mentioned above.
A diverging beam 12 of ions emerges from the ion source through the
accelerating electrode 11 and passes between pole pieces, of which
one pole piece 13 is shown, of an electromagnet. The magnetic field
between the pole pieces is perpendicular to the direction of travel
of the ion beam and perpendicular to the plane in which the ion
beam is to be deflected. That is, as seen in FIG. 1, the magnetic
field lines between the pole pieces are perpendicular to the plane
of the paper.
The ion beam is deflected by the magnetic field towards a window 14
provided by an aperture in a plate 15. Ions in the beam passing
through the window 14 impinge upon a target 16. A detector probe 17
projects a small amount into the path of the ion beam passing
through the window 14 and intercepts a small fraction of the ion
beam for providing an indication of the ion current passing through
the window 14. A high, stabilized, direct voltage applied to the
accelerating electrode 11 is provided by a stabilized E.H.T.
("extra high tension") power supply indicated at 18. Superimposed
upon this direct voltage applied to the accelerating electrode 11
is an alternating voltage generated by a sweep voltage generator
19. The amplitude of the sweep voltage is controlled by a sweep
control indicated at 21 which, in turn, is controlled by an
electrical signal indicative of the ion beam current passing
through the window 14 and derived from the probe 17 via amplifier
22.
FIG. 1 illustrates diagrammatically the separation of two ions of
differing mass. The solid lines indicate the ion beam of ions of
one mass with the sweep effect indicated by arrow 23. The dotted
lines indicate the ion beam path of the ions of the other mass with
the sweep effect indicated by arrow 24. It will be appreciated that
the deflections indicated on the scale of FIG. 1 are greatly
exaggerated. However, the amplitude of the sweep should be such
that unwanted ions do not pass through the window at any position
of the sweep.
In operation, the apparatus is set up so that the window 14
intercepts the desired ions and the direct voltage applied to the
accelerating electrode 11 is adjusted, as a coarse setting, to give
the desired ion beam intensity at the target 16. A fine setting of
this intensity is achieved by adjustment of the sweep voltage
amplitude. Once the ion beam intensity through the window 14 has
been set up, feedback from the probe 17 operates via the sweep
control 21 to control the amplitude of the sweep in such a way as
to tend to reduce variations in the ion beam intensity.
It is an important feature of the arrangement that the sweep effect
is generated by the magnetic field simply by varying the energy of
the ion beam.
Although the magnetic field provides the principal focusing of the
emergent ion beams, provision is required for fine focusing of the
desired ions onto the target.
This is provided, together with the window 14, by a lens assembly
shown in FIGS. 2, 3 and 4.
Referring to FIG. 2, a vacuum enclosure is provided by an
electrically insulating glass cylinder 23 clamped at each end
respectively to flanges 24 and 25 with interposing sealing rings
26, 27.
The flange 24 is sealingly coupled to the vacuum system of the
electromagnetic separator. The flange 25 is sealingly coupled to an
evacuated target chamber hereinafter more fully described with
reference to FIG. 5.
Three apertured metal plates comprising respectively a
mass-defining slit 28, a suppressor electrode 31, and a probe 29
are mounted so as to be electrically insulated from one another
upon a tubular support 32 within the glass cylinder 23.
The tubular support 32 in addition to supporting the three metal
plates 28, 29 and 31 also supports a further earth electrode plate
34 and, in between this earth electrode plate 34 and the suppressor
electrode plate 31, a beam stop device 35.
The fine focusing of the ion beam is produced in the lens assembly
by an electrostatic field generated between the earth electrode
plate 34 and an electrode plate 36 spaced from the earth electrode
plate 34 and mounted upon the flange 25. In this example, the
electrode plate 36 is held at a positive potential of 140 kV. It
will be appreciated that both the the earth electrode plate 34 and
the electrode plate 36 are centrally apertured to permit passage of
the ion beam therethrough.
Adjustment of the focusing effect is produced by adjusting the
separation of the earth electrode plate 34 and the electrode plate
36. For this, the support tube 32 is telescopically mounted upon a
tube 33 fixed to the flange 24. A rigid tube 37, fixed at 38 to the
tubular support 32, extends through a slot 39 in the tube 33 to a
drive mechanism 41 (see FIG. 4) adapted to move the tube 37 back or
forth longitudinally with appropriate rotation of a drive pinion
42.
The inside of the tube 37 provides a convenient passage for the
cable for making the various electrical connections to the
components carried on the tubular support 32.
As may be seen from FIG. 2, the wires in the cable fan out to a
connector 43 from which connections to the various components are
made.
Mounted upon the tubular support 32 and encompassing the region of
the electrostatic lens is an X-ray shield 44.
The beam stopping device 35 comprises a metal plate 45 mounted
within and electrically insulated from a faraday cup 46 which
encompasses the metal plate 45 except for the aperture facing the
direction of the incident ion beam and through which the ions pass
onto the beam stopping plate 45. The beam stopping device 35 as a
whole is rotatable about a pivot 47. Rotation of the device 35 into
and out of the beam stopping position is effected by an actuating
solenoid 48 via coupling 49 and rotatable rod 51.
The mass-defining slit 28 has an area of 1 in.sup.2 (6.4 cm.sup.2)
and, in operation, the electromagnetic separator is set up so that
the ions of the desired mass focus upon the mass-defining slit and
are swept over an area of approximately 11/4 in. .times. 11/4 in.
(3.2 cm. .times. 3.2 cm.).
The probe plate 29 has its aperture arranged so that a small known
area of the plate intercepts the ion beam passing through the
mass-defining slit 28. The suppressor electrode 31 is connected to
a positive electrical potential in order to attract and remove
secondary electrons that may be produced by the bombardment by the
ion beam of the probe 29 or the plate 28 with the mass-defining
slit.
The faraday cup 46 is similarly held at a positive potential to
perform a similar function in relation to the beam stopping plate
45 when this is in the beam stopping position.
It will be appreciated that it is important to prevent secondary
electrons from penetrating back into the separator as they may be
accelerated to high energies, travelling in the opposite direction
to the positive ion beam, and produce X-radiation at some
unspecified location in the electromagnetic separator.
It will be appreciated that the mass-defining slit provided by the
plate 28 as described with reference to FIG. 2 comprises the window
14 as described with reference to FIG. 1, and the probe plate 29 as
described with reference to FIG. 2 comprises the probe 17 as
described with reference to FIG. 1.
The ion beam passing through the mass-defining slit is focused by
the electrostatic lens onto a target mounted within the target
chamber coupled to the flange 25.
FIG. 5 is a view of one form of target chamber from behind looking
towards the oncoming ion beam.
In this example, provision is made for moving targets
systematically so that they may be scanned through the ion beam in
such a way as to achieve a uniform dose of ions applied to the
target.
One example of the mechanism for moving the targets is shown in
FIG. 5, in which outer cylinder 51a comprises the vacuum enclosure
of the target chamber. This mechanism is particularly appropriate
where it is necessary to use a narrow mass-defining slit for
selecting the desired ions.
The mechanism is intended for automatic processing of a large
number of slices of substrate, for example for the manufacture of
semiconductors by implanting doped ions into substrate slices.
The substrate slices are carried on rectangular supporting plates
(not shown) approximately 5 in. .times. 4 in. (12.7 cm. .times.
10.2 cm). Generally, several semiconductor slices will be mounted
on each single support plate, but where a particularly large
substrate surface is to be implanted, a support plate may support
but a single semiconductor slice.
The rectangular support plates are provided with a pair of
laterally projecting ears at the opposed top edges.
A batch of plates carrying substrate slices is supported within the
target chamber upon two spaced horizontal chain runs (not shown),
the ears of the support plates resting on the chains. The chains
are driven forward in steps synchronized with the drive to the
remainder of the mechanism to bring the plates forward in
succession to a loading station located at the top left-hand corner
of the mechanism as seen in FIG. 5.
At this loading station, a plate is loaded onto a carrier 52 in a
manner described hereinafter.
The carrier 52 is mounted on an X-Y drive mechanism. The Y movement
is provided by movement of the carrier 52 itself upon supporting
rods 53, 54. The X movement is provided by lateral movement of the
rods 53 and 54 themselves upon horizontal shafts 55, 56.
Y drive is derived from a stepping motor which drives shaft 57
coupled to the carriage 52 via a wire and pulley drive mechanism
(not shown). The X drive, also derived from a stepping motor, is
applied to a tube 59 supported for lateral movement upon a spider
58 carried by bearings 61 upon shafts 62.
The operation of the mechanism will be described starting from the
moment when the carriage 52 is at the top left-hand corner of the
framework as seen in FIG. 5 and about to start to scan the plate,
which has been automatically loaded onto the carrier 52, through
the ion beam. The X drive carries the carrier 52 across at a
uniform speed to the right-hand side of the framework and one step
of Y drive is applied to make an incremental movement of the
carriage 52 downwards. The X drive then drives the carriage back
across to the left-hand side as seen in FIG. 5 and a further
incremental Y step downwards occurs before X drive back to the
right-hand side. This sequence is repeated until the whole area of
the plate mounted on the carrier 52 has been scanned through the
ion beam. The arrangement is such that the plate is clear of the
ion beam before reaching either end of the X scan so that there is
no dwell effect at the ends of the traverse.
At the completion of the scanning operation, the carriage 52 is
moved downwards in the bottom left-hand corner as seen in FIG. 5 by
the Y drive, whereupon two downwardly projecting sliders 63, 64
engage upon fixed studs 112 (See FIGS. 7 and 8) in the framework.
Referring to FIGS. 7 and 8, the upper ends of these sliders 63 and
64 have inclined surfaces (surface 63a being shown in FIGS. 7 and
8) which engage with a ears (one of which, denoted 111, is shown in
FIGS. 7 and 8) of the plate 110 mounted on the carrier 52, as the
carriage 52 continues to move downwards, and the sliders 63 and 64
lift the plates 110 by their ears 111 out of U-shaped slots 52a in
the carriage 52 in which they are resting. As the ears 111 clear
the U-shaped slots 52a, the plates 110 slide away from the carriage
52 on the inclined surfaces of the sliders 63 and 64 onto a fixed
inclined trackway 113 appropriately located adjacent the
framework.
The Y drive then reverses and drives the empty carriage 52 up
towards the top left-hand corner (as seen in FIG. 5). During this Y
travel, a rack (not shown) on the side of the carriage 52 is
knocked into an operative position by a cam so that the rack
engages gear wheel 65. The rotation of gear wheel 65 is transmitted
via gear wheel 66 to fingers 67 and 68. The length of the rack is
such that one complete revolution is imparted to these fingers 67
and 68 which are so positioned as to engage the top of the plate
which has just been released from the carriage 52 and move it along
the trackway into a storage position. When the rack on the carriage
52 has completed its actuation of gear wheel 65, it is moved by a
further cam surface into an inoperative position.
As the plate is released from the carriage 52 in the unloading
operation, the chains carrying the plates awaiting exposure to the
ion beam are driven to move the leading plate up to a loading
station on tracks close to the starting position at the top
left-hand corner of the mechanism as seen in FIG. 5.
As the carriage 52 is moved upwards by the Y drive into the loading
position, a rack 69 on the carriage engages with a pinion 71 and
causes two fingers 72 and 73 to rotate. These fingers engage behind
the top of the plate which has moved into the loading station and
push the plate forward so that it drops onto the carriage 52 with
its ears engaging in the U-shaped recesses provided on the carriage
52. The Y drive then reverses and moves the carriage 52 down until
the rack 69 is clear of the pinion 71 and carriage 52 is in the
starting position for the X-Y scan. The return movement of the rack
69 over the pinion 71 causes a harmless reverse revolution of the
fingers 72 and 73.
It will be seen that, with the apparatus of this example, a high
uniformity and reproducibility of doping of the target substrates
is achieved by the combination of two important features, namely,
beam sweeping with feedback stabilization to give constant ion beam
intensity and X-Y scanning of the substrates in front of the
stationary ion beam. The mechanical system for scanning the targets
through the beam is completely separate from the beam control
equipment so that any coupling between the X and Y scanning
frequencies and modulation of the ion beam (arising for example
from ripple on the ion source supplies) is avoided. This is
important because, if any such coupling does occur, standing waves
will result and give non-uniform doping.
Further, the beam shape is unimportant since summation of repeated
implants leads to smoothing out of non-uniformities.
To avoid faulty doping of targets, mechanism is provided for
actuating the beam stop device 35 automatically in the event that
the ion beam should go out of control. Provision is made for
stopping the X-Y drive mechanism when the beam stop device 35 is
actuated and for starting the X-Y drive mechanism again when the
beam stop device 35 is removed after the correct ion beam has been
restored. In this example, to simplify the control mechanism, the
X-Y drive is not stopped until the end of the X scan in which the
beam stop device 35 is actuated. However, provision may be made for
both X and Y drives to stop and start automatically simultaneously
with corresponding operation of the beam stop device 35.
The arrangement of this example has a number of advantages for the
ion implantation of substrates. In particular, a large number of
target substrates can be processed in a single run so that the
through-put of the apparatus is up to industrial production
quantities whilst, at the same time, the problems of uniformity of
dose and beam heating of the substrates have been solved.
The use of a large plate for supporting the substrate targets
reduces the beam heating effect without reducing implantation rate
because individual slices can cool during periods out of the beam.
The large plate also permits implantation of any size sample up to
the plate dimensions. This is important for example when doping
large area (e.g., 3 in. diameter) nuclear detectors.
Automatic programme control of the X-Y movements allows (a)
repeated scans if required for very high doping levels, or, for
example, for successive implantation with different dopants, and
(b) control over speed of scanning and number of X scans per unit
distance of Y movement allows through-put of the target chamber to
be adjusted. For example, the through-put can be increased if the
loss of uniformity is acceptable. Automatic programme control of
the X-Y movement further permits control over the size of the
scanning area so that individual small samples can be rapidly
implanted for experimental requirements.
By use of an electrically isolated target chamber, the ion beam can
be accelerated (up to 180 keV total) or decelerated to give precise
energy control of the implantation over a wide range. If necessary,
the target chamber voltage can be programmed to give an implanted
profile control without the need to adjust the electromagnet of the
separator. For this, however, it is important that there is no
coupling between the slow scan frequency and the fast frequency of
the voltage programme.
This facility for programming the target chamber voltage has
imposed special requirements upon the design of the lens assembly
(a) to minimize loss of beam angular definition, and (b) to ensure
that in spite of large voltage changes all of the beam transmitted
through the beam defining and monitoring slits passes through a 1
in. .times. 1 in. aperture in the target chamber.
FIG. 6 illustrates an alternative form of mechanism for moving
targets past the window. This mechanism is particularly appropriate
for special implantations where separation of the desired ions from
unwanted ions is comparatively easy and a wider mass-defining slit
is acceptable.
Thus, for example, boron ions and, to a somewhat lesser extent,
phosphorus ions are quite widely separated in mass from impurity
ions commonly produced by ion sources in association with boron or
phosphorus ions. For implantation of such ions it is consequently
possible to arrange for the mass-defining slit to be as wide as the
width of the target substrate to be implanted.
Referring to FIG. 6, a target chamber comprises a cylindrical
enclosure 81 with a pipe 82 for connection to a vacuum pump. A
tubular extension 83 is adapted for connection to the vacuum system
of the electromagnetic separator and houses a lens assembly, which
may be similar to that shown in FIG. 2, but with a wider window or
mass-defining slit.
Extending along the axis of the target chamber enclosure 81 is a
lead screw 84, driven by a stepping motor 85. The drive program for
the stepping motor is controlled by an electronic programmer
86.
Within the target chamber enclosure 81 is a multi-sided target
holder 87 of generally cylindrical form. The target holder 87 is
co-axial with the enclosure 81 and is driven by the lead screw 84.
Rotation of the target holder 87 is normally resisted by a torque
resisting tube 88, to which the target holder 87 is keyed. The
target holder 87 is, however, free to slide up and down as
indicated by arrow B.
Each side of the multi-sided target holder 87 is in the form of a
rack adapted to support a row of targets which, in this example,
are two inch diameter wafers 89. For simplicity of drawing, only
one rack is shown filled with wafers.
The arrangement is such that one row of targets supported on the
holder 87 is in register with the window. The ion beam from the
separator is illustrated diagrammatically at 91. Arrows C indicate
the sweep, which is large enough, in this example, for a
substantially uniform intensity ion beam to pass through the window
and fall upon the full 2 inch width of the wafer immediately behind
the window. Ion implantation to a uniform dose of the whole row of
eight wafers can thus be secured by a simple Y-scan. In practice
the holder 87 is reciprocated several times in one angular position
so that the implantation dose is built up with greater net
uniformity and also to avoid undue heating of the wafers. Dwell
effects are avoided by arranging the holder 87 to carry the wafers
clear of the ion beam before reversal.
After the desired implantation of one row of wafers has been
carried out, the holder 87 is indexed by one step angularly to
bring the next row of wafers into register with the ion beam.
The indexing mechanism comprises fingers 92 and 93 with
co-operating cam surfaces 94, 95 fixed to the target chamber
enclosure 81.
During reciprocation of the holder 87 for an implanting operation,
the stepping motor 85 is programmed to reverse before an indexing
finger 92 or 93 engages the cam surface 94 or 95. At the end of an
implantation operation, the holder 87 is driven up or down (as the
case may be) until a finger 92 or 93 engages the cam surface 94 or
95. Means, not shown, is provided for releasing the holder for
rotation at this moment and the effect of the cam surface upon the
finger is to rotate the holder 87 until the next adjacent finger is
positively stopped against shoulder 96 or 97 at the high end of the
cam surfaces 94 and 95.
With this particular indexing arrangement, the fingers 92 at one
end have to be slightly displaced relative to those (93) at the
other end. The cam surfaces 94 and 95 have to be correspondingly
relatively displaced. Further, each successive indexing step has to
be effected at opposite ends of travel of the holder 87, that is,
the holder must execute an odd number of reciprocations between
each indexing operation. It will be appreciated that other
techniques for indexing may be employed if desired, but that
described has the virtue of mechanical simplicity.
Assuming an ion beam current of 1 mA, corresponding to
approximately 5 .times. 10.sup.15 ions/sec, a two inch wafer will
receive a very heavy dose of 5 .times. 10.sup.15 ions/cm.sup.2 in
about 25 seconds exposure.
Assuming maximum utilization of current, a row of eight wafers will
thus take about 4 to 5 minutes to implant up to this high dose. A
typical scanning rate would be 5 inches in 1 second leading to
several reciprocations required to implant each row.
The invention is not restricted to the details of the foregoing
example. For example, provision may be made for heating the samples
by radiation using large area heaters facing the sample plates.
Alternatively, the samples may be cooled by positioning in front of
them large plates cooled with liquid nitrogen.
* * * * *