Ion implantation system

A plurality of beam generating units 76, 78, 80 and 82 each produces separated rectangular ion beams for implantation onto a targets 52 and 54 rotatively moving therepast. Each rectangular footprint is long in the direction of motion and is scanned transversely to the direction of motion. A plurality of beam generating units can be positioned adjacent to each other to multiply implant targets because of the compact structure of the separated ion source.

BACKGROUND OF THE INVENTION 
This invention is directed to a compact ion implantation system which has a 
separated ribbon beam ion source which has geometry which permits beam 
traverse across the wafer with minimum waste beam for maximum productivity 
and employs the same magnetic field for both the ion source and separator. 
The separated ribbon beam ion source is sufficiently compact that a 
plurality of such sources can be placed adjacent to each other for 
successive ion beam impingement on the same target or adjacent targets. 
Prior ion implantation beams sources were comprised of separate functional 
components which were connected together to form the ion beam line. An ion 
source was used and it had its own magnetic field structure, if such was 
required, for the production of the ion beam. Ion separation downstream 
from the ion source required additional separation components. Due to the 
separate-element approach to the problems, the beam line is unnecessarily 
long and complex. These disadvantages are particularly difficult in the 
case of high current low energy beams because severe space charge 
expansion occurs in the region between the ion source and separator. 
Attempts to locate the separator just downstream of the ion source were 
unsuccessful because the magnetic fields interfered. That is, the axial 
magnetic field in the ion source was disturbed by the transverse magnetic 
field of the separator. 
With the present close coupled, compact ion source, a small target chamber 
can be employed with several of the separated ion sources working on the 
same target or the same target wheel to provide higher ion dosage 
capability. 
A suitable ion source is shown in U.S. Pat. No. 4,163,151. 
SUMMARY 
In order to aid in the understanding of this invention it can be stated in 
essentially summary form that it is directed to an ion implantation system 
wherein a plurality of compact separated ribbon beam ion sources, which 
are sufficiently compact that they can be placed adjacent to each other, 
direct ions at a target so that multiple beams impinge upon the same 
target. The rectangular cross-section of the beam produced by the ion 
source also reduces the effects of space-charge and enables mass 
separation to occur in a relatively weak magnetic field. 
It is thus an object of this invention to provide an ion implantation 
system which comprises a plurality of separated ion sources which have 
both the ion production structure and the ion separation structure therein 
so that the plurality of sources form an ion implantation system wherein a 
plurality of beams can be directed against the same target to increase the 
implantation dosage or area implanted. 
It is a further object to provide an ion implantation system wherein the 
beam line is short to provide a compact system which is sufficiently small 
that a plurality of ion sources can be incorporated together into the 
system for increased implantation. 
Other objects and advantages of this invention will become apparent from a 
study of the following portion of the specification, the claims and the 
attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A single compact separated ion source is generally indicated at 10 and 
FIGS. 1, 4 and 5. It comprises ion source 12, which is seen in FIGS. 2, 3, 
and 4 and ion separator 14 which is seen in FIGS. 1 and 4. Magnets 16 and 
18, see FIG. 1, connected to pole pieces 20 and 22, provide a magnetic 
field across both the ion source and the ion separator. 
Ion source 12 has a Penning discharge type configuration. The cathode 24 
has an interior cylindrical cathode surface. Anode 26 is positioned within 
the cathode and is electrically insulated therefrom by means of insulator 
27 in the top cover 28. Anode 26 extends through the top-cover insulator 
27 to provide a terminal 29 to which the anode voltage is applied. Hot 
filament 31 is positioned adjacent the anode and has terminals 33 for hot 
filament energization which also extend from the cathode space through the 
top cover. The gas to be ionized is introduced into the discharge chamber 
25 by means of a feed tube 35. The radii of the anode and cathode 
surfaces, the gas pressure and the magnetic field strength in the 
interelectrode spaces are such that a Penning low pressure glow discharge 
is formed. The Penning discharge causes ionization and it is from this 
discharge plasma that the ions are extracted. 
The magnetic field is supplied in a direction parallel to the anode and has 
a value of about 1100 gauss at the center of the ion source. In the 
presence of the gas to be ionized, with a pressure in the discharge 
chamber in the order of about 75 pascal, a Penning discharge is sustained 
in discharge chamber 25 between the anode and cathode with a voltage of 
100 to 150 volts. Electrons, which ionize the gas molecules, are provided 
by hot filament 31. The magnetic field causes the path length of electrons 
traveling between the cathode and the anode to be much greater than their 
separation. This increases the probability of ionization and in turn 
results in a gas consumption which is less than one tenth the gas 
consumption required without a magnetic field. After ignition, the 
discharge in the plasma operates at a discharge current of 100 to 2000 
milliamperes. The discharge is very stable and since the voltage is lower 
than for most types of cold cathode discharge, sputtering is less of a 
problem. 
Ions are extracted from the discharge through a slit 30 in the discharge 
chamber 25. The slit 30 has a high aspect ratio rectangular cross section 
whose long dimension is shown in FIG. 3 and whose narrow dimension is 
shown in FIG. 4. The extraction geometry is based on the design criteria 
developed by J. R. Pierce. Electrode 32 and 34 adjacent the slit are 
Pierce-type electrodes to prevent space charge effects from spreading the 
beam. Furthermore, the strip beam geometry of high aspect ratio which is 
typically 50 times higher than its width, reduces space charge effects and 
it is compatible with the separator geometry. Accelerator electrode 36 is 
positioned adjacent the ion extraction opening and accelerates the ion 
beam into the ion separator. 
Ion separator 14 comprises separator plates 38 and 40 which are positioned 
in the magnetic field and oriented parallel to the ribbon beam. A power 
supply applies a voltage of 1500 volts across the separator plates to 
provide an electric field at right angles to or crosswise to the magnetic 
field which is perpendicular to the paper in FIG. 4. Thus, the magnetic 
field and the electric field define the E.times.B ion separator. The same 
permanent magnet is used to provide the magnetic field in the ion source 
and thus it is possible to reduce the system length as compared to other 
designs. Furthermore, in the present design, the permanent magnet is 
within the vacuum envelope 102. Thus, the permanent magnetic field can be 
provided at very much lower cost than the customary external 
electro-magnet. The magnetic field in the separator region has a value of 
1100 gauss. 
Another suitable E.times.B separator is shown in patent application Ser. 
No. 151,009 filed May 19, 1980 by Richard Vahrenkamp where focusing plates 
are used in the analyzer. 
The main beam is indicated at 42 in FIG. 4. This beam has been analyzed, 
with impurity beams 41 and 43 being separated therefrom. Aperture plate 44 
has analyzing opening 46 therein which permits the main beam to pass 
through the target holder 48. The target holder is preferably movable into 
the path of the beam, and may have an opening herein so that when the 
opening in the target holder is moved in line with the beam, the beam can 
pass through to Faraday cup 50. By use of the cup, beam data can be 
obtained. 
In a particular embodiment, the slit 30 through which the beam is extracted 
measures 1 inch by 0.020 inches. When the source is operating on argon of 
BF.sub.3, with a ten kV extraction voltage, the total current is 2.5 
milliamperes. When the gas is BF.sub.3 operating under these conditions, 
150 microamperes of boron is delivered through analyzing opening 46 to the 
target. 
The ribbon shape of the beam is critical to this invention. In a ribbon 
shaped beam, a high beam current can be achieved with a narrow beam. The 
narrow beam configuration very much reduces the spreading of the beam due 
to space charge effects, as compared to a circular beam of the same 
current. Furthermore, in analyzing the beam, the impurities can be 
laterally deflected from the ribbon beam, but need only be deflected a 
small angle because the main beam can pass through an analyzing opening 
which is in the shape of an elongated slot, having generally the same 
proportions as the extraction slot at which the ions are extracted from 
the plasma. Thus, by using a beam which is taller than it is thick, and 
causing analyzing deflections of the impurities in the direction of the 
thickness direction, a high brightness, high flux density ion beam can be 
achieved by minimum spreading due to space charge effects and maximized 
analyzing. 
An embodiment of the ion implantation system of this invention which 
incorporates the separated ion source 10 is generally indicated at 100 in 
FIG. 5. System 100 has vacuum envelope 102 which houses the ion source 10. 
Ion source 10 is oriented in such a direction that its ribbon beam is 
horizontally directed toward the front right in FIG. 5 with the height of 
the beam, upright in FIG. 5, being the longer transverse section through 
the beam. Envelope 102 is pumped by vacuum pump 104 through vacuum 
connection 106. The right end of envelope 102 is substantially closed by 
cover 108 which is half broken away in FIG. 5. Cover 108 only has slot 110 
therethrough. Sliding gate 112 can be slid from a position where it covers 
slot 110 to a position where it uncovers slot 110. Sliding gate 112 is 
sealed when closed so that envelope 102 can be maintained at vacuum while 
the target chamber volume on this slide of cover 108 is open. Gate 
operator 114 extends to the outside of the vacuum enclosure so that the 
gate can be externally operated. 
Flange 116 is mounted behind cover 108. Target chamber cover 118 is pivoted 
to the bottom of flange 116 and can be swung to the open position shown in 
FIG. 5. Stop 120 holds cover 118 in a horizontal position when open. When 
raised against flange 116, a target chamber cover 118 can be sealed 
thereto to form an enclosed target chamber which can be evacuated. Vacuum 
pump 122 is connected through vacuum connection 124 to the target chamber 
to permit the target chamber to be evacuated. By closure of sliding gate 
118, the target chamber can be opened by lowering cover 118, while 
maintaining the vacuum in vacuum envelope 102. The maintenance of the 
vacuum on the ion source is helpful to the ion source and the reduction in 
volume which must be pumped down after a change in targets decreases the 
down time between the implantation on one set of wafers and the next set 
of wafers. 
Wafer wheel 126 is mounted for rotation on a wheel shaft. Wafer wheel drive 
motor 130 drives female spline 132 which in turn drives male spline 134 
which is connected through a bevel gear box to drive wafer wheel 126. A 
plurality of wafers is mounted on the front of wafer wheel 126. These 
wafers are of semiconductor material and are masked to receive ions from 
the ion beam 162, see FIG. 6, for implantation therein. The masking 
controls the implantation pattern. FIG. 6 illustrates wafers 164, 166, and 
168 secured to the wafer wheel. During the implantation process the wheel 
126 is rotated so that the implantation is evenly distributed. For proper 
dosage accuracy, the beam flux onto the wafer must be uniform within 1 
percent over the wafer area. To accomplish this, the wafer wheel is 
rotated around its axis to rotate the wheel and the wafer is carried 
thereby past the impingement pattern of beam 162. As is seen in FIG. 6, 
beam 162 is a ribbon beam and its height is perpendicular to the radius of 
the wafer wheel. With rotation of the wafer wheel the wafers move in an 
accurate direction, but at the ribbon beam they are generally moving 
parallel to the height cross-section of the beam. The wafer wheel is 
rotated and traversed so that the wafers pass under the beam pattern on 
the wheel. In addition to the rotary motion of the wheel, it is moved in 
the direction parallel to male spline shaft 134 which is a direciton 
perpendicular to the longer cross-section. It is the left to right 
direction in FIG. 6. The importance of this direction of transverse is 
that scanning must occur from a point where the beam pattern is on one 
side of the wafer at a starting point, with motion of the wheel 
perpendicular to its rotational axis while the pattern scans across the 
wafer areas, to a point where the pattern is outside of the wafer areas, 
as shown in dotted line FIG. 6. In this way scanning of the wafer wheel 
cross-wise of the beam from a point where the pattern is on one side of 
the wafers to a point where the pattern is on the other side of the wafers 
can be accomplished with less traverse of the wafer wheel because of the 
narrowness of the ribbon beam in the direction of traverse. In this way, 
the use of a ribbon beam improves productivity of the system. 
The compact ion beam source which produces an analyzed rectangular high 
current beam permits combining of several compact analyzed ion beam 
sources or beam generating units so that they can cooperatively implant 
the same target. In FIG. 7a, targets 52 and 54 are mounted on target drum 
56 which moves the targets past the footprints of a plurality of ion 
sources. In In FIG. 7a the targets 52 and 54 are discs, such as 
semiconductor wafers, although they can be of different shape. FIG. 7a 
also shows footprints 58, 60 and 62 which are successively scanned over 
the targets on the target drum. The target drum corresponds to wafer wheel 
48 in the single implantation embodiment of FIGS. 5 and 6. FIG. 7a 
illustrates that the overscan of one full width of the rectangular ion 
beam covers a minimum overscan width to minimize lost implantation time. 
This minimum overscan is accomplished by means of the rectangular ion 
beam. Such overscan of one beam width is required for a uniform 
implantation. The rectangular ion beams, with their footprints shown in 
FIG. 7a, can be compared to the usual circular ion beam illustrated in 
FIG. 7b. In FIG. 7b the target 64 is scanned by a single large high 
current beam having a round footprint 66. The amount of overscan, equal to 
one beam diameter on each side of the target, requires excessive 
nonproductive time, and for this reason the round beam is undesirable, as 
has also been shown with respect to FIG. 6. 
In FIG. 8 the advantages of compact beam generating units used in multiple 
can be seen for implanting a large number of wafers in short time in one 
machine cycle. As is seen in FIG. 8, drum 56 has four circumferential rows 
68, 70, 72 and 74 of wafers arranged around the drum and the wafers in the 
rows may be arranged in longitudinal lines. Target wafers 52 and 54 are in 
the first row 68. 
Circumferentialy arranged around drum 56 is a plurality of beam generating 
units. Sixteen beam generating units are shown in FIG. 8, eight in a near 
and eight in a far row, each arranged about 45.degree. apart. Two units 
are broken away in each of the two rows to show the drum structure in FIG. 
8. The geometry of the previously described compact beam generating units 
permits a plurality of them to be closely positioned with respect to each 
other. Beam generating units 76, 78, 80 and 96 are particularly indicated 
in FIG. 8, and comprise half of the beam generating units in the near row 
in FIG. 8. A similar circular assemblage of eight such beam generating 
units is also indicated at the far end of FIG. 8. In view of the fact 
there are two rows of beam generating units circumferentialy arranged in 
the implantation device of FIG. 8 and there are four rows of wafers, the 
beam generating units are arranged so that half of them, preferrably 
alternate beam generating units in the same row, be deflected to different 
target rows. For example, beams generating units 76 and 80 may be 
deflected to the right to implant row 70, while beam generating units 78 
and 82 are deflected to the left to implant row 68. This deflection is in 
addition to the axial scan of the rectangular ion beams. By this 
construction, each of the rows 68, 70, 72 and 74 of target wafers has four 
beam generating units providing ion beams for implanting therein. 
Beam generating unit 82 is broken away in FIG. 8 to show its C-shaped 
magnet 84, ion source 86, and analyzer plates 88 and 90 which control the 
path of the desired ion species and deflect the path of the undesired 
species. This structure is the structure shown in FIG. 4. In addition, 
deflection plates 92 and 94 are provided to provide the deflection bias 
necessary to cover the correct row and scan the beam footprint in the 
axial direction of drum 56 while the drum rotates around its axis to 
provide for uniform impingement. The housings, for example housing for 
unit 96, are preferrably vacuum type housings with a beam outlet gate such 
as gate 112 illustrated in FIG. 5 so that the beam generating unit may be 
maintained in vacuum while the target drum is changed. Furthermore, 
housing 96 may contain beam accelerator electrodes positioned in 
accelerator portion 97 either upstream or downstream from the deflection 
plates. The implantation system 98 illustrated in FIG. 8 thus provides 
implantation by four beams onto each target wafer in each row 68, 70, 72 
and 74 as the target drum 56 rotates and as the beam footprints are 
deflected in the axial direction of the drum and width-wise of the 
rectangular beam footprint. The small size of each beam generating unit 
thus permits multiple application of beam generating units and increases 
target implantation rate. The use of plurality rectangular ion beams is 
superior to using one larger, usually round ion beam, because the space 
charge effect within the large ion beam would cause excessive beam 
spreading and so much time would be lost during implantation, as 
illustrated in FIG. 7b. 
The structure of FIG. 8 represents a low energy ion implantation system, 
for example in the energy range of 5-100 kev. Very high current can be 
achieved, in the order of 20-50 milliamperes, depending upon the ion 
species. This total current is the total of four beam generating units 
successively operating on one set of targets, as illustrated in FIG. 8. 
FIG. 9 shows an ion implantation system 140 which is comprised of a 
plurality of beam generating units, of which three are shown at 142, 144 
and 146. These beam generating units are very similar to the beam 
generating units of FIG. 8, and the particulars of unit 82 are shown in 
more detail. The same detail is seen with respect to beam generating unit 
142, but note that the beam footprint is horizontal and deflection plates 
150 and 152 are arranged for up and down deflection of the beam footprint 
156. This arrangement is made because target 154 is a strip being 
longitudinally advanced across the front of the beam generating units, and 
the beams are deflected up and down. The footprint 156 of beam generating 
unit 142 is seen in FIG. 9. It is deflected up and down by the deflection 
plates 150 and 152, and is arranged to deflect in a crosswise direction 
with respect to the rectangular shape of the footprint. The structure of 
FIG. 9 is particularly useful for implanting strips, such as long strips 
of solar-electric material, surface alloy of metals, or the like. By using 
a plurality of beam generating units, a sufficiently high total ion 
current is available so that these procedures can be accomplished. High 
total dosages of ions can be implanted when a plurality of beam generating 
units is arranged in parallel. This parallel arrangement can be achieved 
only with a compact analyzed ion beam source, of the type illustrated. All 
referenced are incorporated herein in their entirety. 
This invention has been described in its presently contemplated best mode 
and it is clear that it is susceptible to numerous modifications, modes 
and embodiments within the ability of those skilled in the art and without 
the exercise of the inventive faculty. Accordingly, the scope of the 
invention is defined by the scope of the following claims.