Note: Descriptions are shown in the official language in which they were submitted.
~2Z~139
METHOD AND APPARATUS FOR GENERATING ION BEAMS
_
FIELD OF THE INVENTION
The present invention relates to an ion beam
source and, moxie particularly, to a hot-cathode reflex arc
(duoPI~atron) ion beam source wherein electrons from the
hot cathode are confined in a reflex region radially by a
magnetic field and axially by electrostatic mirrors.
These electrons which reflect back and forth between the
mirrors ionize a gas contained in an arc chamber.
BACKGROUND OF THE INVENTION
In an article entitled "Ion Implantation of
Surfaces" by S. Thomas Picraux and Paul S. Percy in the
March, 1985 issue of Scientific American (at page 102),
the authors outline the importance of ion beam
implantation technology in the manufacture of Integrated
Circuits and in ion-beam modification of metal surfaces.
The latter is an emerging technology, while the former is
a maturing technology now at the stage of Very Large Scale
Integration (VLSI), where, according to the authors,
sharply focused ion beams offer much higher resolution
than electron beams and visible light. Such sharply
focused ion beams would permit defining doped features,
without intervening steps of masking, of less than a
micrometer across, whose electrical activity might be
controlled by as few as 100 Dupont atoms.
There are three well defined physical indicia of
the source ion plasma and the therefrom derived ion beam
that affect controlled implantation and sharp focusing.
They are:
- beam ion temperature
- plasma potential; and
- plasma fluctuations.
One beam characteristic which sums up the effect
of the above three indicia is beam remittance.
Beam remittance is a difficult concept to grasp.
It is a measure of the uniformity of beam divergence in a
~227289
chosen cross-sectional plane. Accordingly, it is measured
by scanning the beam plane with a small slit and plotting
the divergence of the emerging beam let in milli-radians
against the radial displacement in centimeters of the slit
from the central beam axis. The generated plot is called
a phase space diagram, the area of which, in units of
cm-mrads, is the beam remittance. The smaller the
remittance, the more orderly is the ion beam. A related
beam characteristic is brightness, which is defined as the
beam current divided my the square of remittance. This
definition expresses the difficulty of generating
powerful, high current, continuous do beams, that are
well ordered in phase space.
Beam ion temperature is a measure of beam
disorder and directly affects the ability to focus it
narrowly. It is a measure of the random kinetic energy of
ions in the cross-sectional plane. A high temperature
beam is a fuzzy one. Beam ion temperature is usually given in
electron volt (eve) units, 1 eve being equal to 11,600K.
Plasma potential is the electrostatic potential
of the ion plasma in the space charge region, ahead of
beam extraction apertures, with respect to the surrounding
reference potential. Low plasma potential reduces erosion
and sputtering of the apparatus. It also results in lower
beam contamination and lower variation in ion energy. The
latter directly affects the ion implantation-depth
definition, which is important in semi-conductor
processing.
Plasma fluctuations, sometimes referred to as
"noise", are rapid variations in the density of the plasma
from which the ions are extracted. For best beam quality,
the plasma dynast must be matched to the strength of the
electric field which extracts the ions from the plasma.
Plasma fluctuations maws it impossible to properly match
the density and electric field at all times and thus lead
~LZ;~7~
to a loss of beam quality and an increase in the
time-averaged remittance.
Ion sources of the duoPIGatron type may be
conceptually partitioned into three regions: electron beam
generation, plasma generation and ion beam extraction.
The present invention concentrates on the region between
the electron-emitting hot cathode and the ion beam
extraction apertures. Apparently minor design changes in
that region directly affect the ion plasma prior to beam
extraction. Ion beam quality, as expressed by low beam
remittance and low beam temperature, is itself a result of
the special and physical homogeneity and uniformity of the
source ion plasma.
PRIOR ART OF THE INVENTION
United States Patent No. 3,238,414, issued on
March 1, 1966, (Kelly et at), discloses a high output
duoplasmatron-type ion source. In their ion source, an
arc between a hot cathode and an anode generates an
electron beam in the path of which a feed gas to be
ionized is located. The electron beam ionizes the feed
gas and the ionized feed gas (plasma) is drawn through an
aperture in the anode to an expansion region defined by a
plasma expansion cup extension. The use of the plasma
expansion cup extension increases the quantity and quality
of the extracted ion beam. however, the expansion cup
extension is a simple cylinder. Moreover, the
duoplasmatron of Kelly is not a reflex arc source.
United States Patent No. 3,924,134, issued on
December 2, 1975, human et at), teaches another type of
ion source. This ion source has a cathode filament
chamber and an ionization chamber in which two uncoupled
discharges are maintained whose characteristics can be
controlled independently. Electrons generated by the
discharge it the cathode filament chamber are used to
sustain the discharge in the ionization chamber. This
. ..
~22728~
-- 4
double chamber configuration permits the use of an inert
gas in the cathode filament chamber and a feed gas in the
ionization chamber. A low voltage axe discharge in the
inert gas atmosphere, in the cathode filament chamber,
minimizes sputtering and prolongs the filament lifetime.
Moreover, the entire source is immersed in an axial
magnetic field parallel to a line connecting the filament,
the aperture between the top chambers and the ion beam
extraction orifice. As will be shown later, significant
improvement to the stability and uniformity of the ion
plasma will be achieved by providing a contoured, plasma
confining, magnetic field.
DuoPIGatron-type ion source was proposed in the
present inventor's earlier publication, "High Current DC
Ion source Development at CRANIAL", IEEE Trans. on Null.
Sat., Vol., NS-26, No. 3, June 1979, pp. 3065-3067, and
was described further in articles "A High-Current DC Heavy
Ion Source" by MAR. Shabbily, Inst. of Physics Con. Ser.
No. 54, Chapter 7, 1980, pp. 333-338, and "sigh Current Do
Ion Beams" by MAR. Shabbily et at, IEEE Trans. on Null.
Sat., Vol. NS-30, No. 2, April 1983, pp. 1399-1401.
The ion source described in the above articles is
called duoPIGatron and includes a hot cathode and an
intermediate electrode defining a cathode chamber in which
the hot cathode is positioned in an inert gas atmosphere.
The ion source further has a reflex arc chamber which is
formed with the intermediate electrode at one end, a
plasma aperture plate at the other, and two anodes between
the intermediate electrode and the plasma aperture plate.
An arc produced between the cathode and the first anode
generates a beam of electrons which is led into the reflex
arc chamber containing a feed gas to be ionized. The
electrons are bounced back and forth between the
intermediate electrode and the plasma aperture plate and
collide with the feed gas to ionize it. 'rho ionized gas
is extracted through holes in the plasma aperture plate.
- 5 - ~227~
Even though the ion source of duoPIGatron type
described above has improved performances over the earlier
ion sources of the US. Patents referred to above, with
the ion beam source of the present invention, a wider
variety of gases can be ionized, gases such as phosphine
PHI, Arizona Assay, boron trifluroide BF3, oxygen
2' all of which have not previously been reported.
More importantly, however, while the concave
shaped iron nose piece in the intermediate electrode
disclosed in MAR. Chablis 19~0 paper led to stable
operation at a current 50% higher than previously
possible, that improvement in the nose piece combined with
two other improvements to the intermediate electrode canal
and to the plasma confining magnetic field results in
doubling of the current accompanied by an improvement in
beam remittance, beam temperature and lower plasma
potential. The higher output current and lower remittance
synergistically produce a significant increase in
brightness of the ion beam source.
normalized brightness is a useful parameter for
comparing the various ion beam sources; it is the ion beam
current divided by the normalized remittance squared and is
measured in units of Amperes divided by millimeters-
milliradians squared. Normalized remittance is the
remittance area per unit of arc multiplied by a
relativistic factor, which for small ion velocity
approximately equals its quotient by the velocity of
light. For ion velocities approaching the speed of light,
the factor tends toward infinity; Accordingly, it is
important to increase ion beam current as the penetrating
power of an ion beam, i.e. ion velocity, increases, in
order to maintain brightness of the source.
For a state of the art review of ion sources,
reference is made to thy paper by Rod Erich Keller entitled
"Ion Sources and Low Energy Beam Transport", published in
Proceedings of the 1984 Linear Accelerator Conference,
May 7-11, 1984, Report # GSI-84-11
., .
- 6 - ~2~7~9
United States Patent lo. 3,546,513, issued
December 8, 1970 (Henning), discloses a "High-Yield Ion
source". The patent states under the heading "BACKGROUND
OF THE INVENTION":
Magnetic fields have been used with ion
sources for various purposes. One of these is
to constrain electrons to paths along magnetic
lines which results in greater ion efficiency.
In these systems, focusing and shaping of the
plasma front from which ions are extracted has
been accomplished by the use of shaped extraction
electrodes made of materials with various
permeabilities. With these devices the
extraction geometries are fixed.
It was thus recognized early on, that plasma
shaping is important. The means to achieve this via
electrodes made of materials with various permeabilities,
however, does not appear to be the optimal solution. The
contribution of the Henning patent itself was to add a
second shaping magnet, which permits the extraction front
geometry to be changed so that optimum yields and focusing
may be obtained.
SUMMARY OF THE INVENTION
As in the prior art, it is also an object of the
present invention to provide an improved ion source which
has a large area of uniform plasma (i.e. high current
output) in a compact and simple design and gives
reproducible and consistent results both over time and0 from source to source.
The present invention has several features that
may be combined to provide an improved ion beam source.
In the course of experimenting with the present
ion beam source, it was found that the uniformity and5 stability of source plasma are enhanced by shaping the
I`
Jo
7- ~.22~289
electron constraining magnetic field strength to have,
near the ion extraction front, volcano-shaped topology.
As a result, the bulk of the plasma volume near the
extraction front coincides with a valley in the magnetic
field stretch and is surrounded by rotationally
symmetrical higher field strength.
An advantage of the just mentioned feature is a
more uniform, and hence stable, plasma front with small
fluctuations of c. I or less. A further advantage also
is that the better confinement of the ionizing electrons
results in lower plasma potential. With the design of the
preferred embodiment, plasma potential of between 10-20
volts has been possible.
Low plasma potential reduces erosion of
components due to sputtering, reduces plasma contamination
and reduces variation in ion energy.
Another feature of the present invention is that
the intermediate electrode canal, through which the
ionizing primary electrons pass into the reflex arc
chamber, has a smaller cross-section on the side of the
electron emitting cathode. It is, however, not fully
understood how such a feature results in improved
performance of the apparatus as a whole. One possibility
is that it results in better electron beam confinement,
permitting a narrower anode aperture, which in turn lowers
gas migration towards the cathode away from the region of
interest and lowers gas flow requirements. Another
possibility is that, since the intermediate electrode is
made of magnetic mild steel, the shaping of the canal,
together with the concave nose piece shape known from the
prior art, improves the magnetic field strength topology
at the extraction front. One limitation on how
constricted the canal may become is that too small a
cross-section would make arc transfer too difficult and
unreliable.
- 8 - ~Z7~9
et another feature of the present invention is
the provision of an anode (or interchangeable anode
insert) having a particularly narrow entrance aperture
that is as small or slightly larger than the exit aperture
of the intermediate electrode canal. Furthermore, the
anode aperture flares conically into the reflex arc
chamber in order to match the gas flow to the expanded
electron beam. The flaring cross-section is profiled to
intercept the constant flux of the magnetic field
developed by the intermediate electrode. The flaring
probably also helps to prevent turbulent gas flow and thus
aids in the formation of uniform interaction between the
ionizing electron beam and the injected gas.
An improvement in the anode or anode insert that
is at an early stage of development is the use of
precisely positioned injection slots to introduce the feed
gas into the region of the reflex discharge where it will
be most effectively utilized. In such embodiment, the gas
is injected downwards through slots at the lower periphery
of the flared anode cone. This provides increased
ionization efficiency, reduced gas flow, and more stable
; operation of the ion beam source. It also reduces
undesired gas migration towards the cathode and, thus,
damage to the cathode.
The above features may be used singly or in
combinations. In the preferred embodiment all features
have, in fact, been implemented to advantage (except for
the last mentioned feature) in an experimental ion
implanted favorably reported on in the issue of
30 Electronic Design dated December 27, 1984 at page 37.
According to the method aspect of the present
invention an improved method for operating an ion beam
source is provided comprising:
(a) generating an ionizing electron beam;
(b) generating ionizable atoms; and
I
- 9
(c) colliding the electron beam with the atoms
in an ionizing chamber to produce an ion
plasma in an ion beam extraction region;
CHARACTERIZED By:
(d) generating a confining magnetic field, for
both said electron beam and said plasma,
having a central region of low field
strength substantially coextensive with said
plasma at said ion beam extraction region,
and having a peripheral region of higher
field strength surrounding said central
region, thereby aiding stability and
uniformity of said ion plasma in the
extraction region.
An ion beam source apparatus according to the
present invention comprises cathode means in a cathode
chamber for generating an electron beam, means for
collimating said electron beam, aperture means in said
cathode chamber for admitting the collimated electron beam
into an ionizing chamber for containing an ion plasma
anode means at one end of said ionizing chamber adjacent
said aperture means and ion beam extraction means at the
opposite end of said ionizing chamber, CHARACTERIZE IN
THAT said aperture means is a bore in said cathode chamber
having a smaller cross-section adjacent said cathode
means, said smaller cross-section being sufficient for
electron beam passage there through, and said aperture
means including a predetermined length thereof having a
predetermined cross-section larger than said smaller
cross-section.
In a preferred embodiment, the aperture in the
cathode chamber comprises first, second and third
sections: the first section, adjacent the cathode means,
having the smallest cross-section, and the second,
intermediate, section having an intermediate cross-section
~2~2~9
between the smallest cross-section and a larger
corss-section of the third section.'
In a narrower aspect, the first and second
sections of the aperture are cylindrical, while the third
section is conical.
From experiments it became apparent that the
second, intermediate, section was critical in respect of
its length and cross-section, which is slightly larger
than that of the first, narrowest section. The length of
the latter was not as critical. The third, conical,
section is, of course, known from the prior art. As is
also well known, the cathode chamber, generally termed the
intermediate electrode in the art, is preferably made of
magnetic mild steel and is strongly magnetized by means of
a surrounding coil powered by a direct current of several
amperes.
Adjacent the bore in the cathode chamber is an
anode, or an interchangeable anode insert, separated from
the cathode chamber by a very small distance. In order to
make the anode aperture as small as possible, its
position is chosen to be where the waist of the electron
beam is. The narrow aperture is followed by one or more
flared (conical) sections. In the preferred embodiment
the flaring it accomplished in two sections with two
slightly different conical angles in order to facilitate
machining. As mentioned earlier, the flaring serves to
match the gas flow to the (expanding) electron beam size.
Accordingly, the anode electrode or anode insert
of the present apparatus comprises at least two sections,
a narrow aperture comparable in cross-section to that of
the bore in the cathode chamber, followed by a
substantially longer, flared, section of increasing
cross-section.
In a narrow aspect of the present invention, the
; 35 ion beam source comprises a hot cathode for generating
I I
electrons. The hot cathode is positioned in a cathode
chamber defined by a generally cylindrical, partly
; conical, intermediate electrode, to which chamber primary
gas supply means supply a gas. The intermediate electrode
is made of a magnetic material and has a canal at the apex
of the conical part of the electrode for admitting the
electrons into a generally cylindrical reflex arc chamber
via an intermediate region. The reflex arc chamber is
axially aligned with, but separated from, the intermediate
electrode, and defines the said intermediate region
between them. The reflex arc chamber has the intermediate
electrode at one end, a plasma aperture plate at the other
end and first and second anodes at locations between the
ends. The first anode is provided with a hole therein
which is axially aligned with the intermediate electrode
canal and through which the electrons are admitted into
the reflex arc chamber. Secondary gas supply means supply
a gas (a feed gas) to be ionized into the reflex arc
chamber. The plasma aperture plate has one or more
apertures therein through which the ionized gas emerges
from the reflex arc chamber. Extraction means are
positioned near the plasma aperture plate and comprise
accelerating and decelerating electrodes to extract and
accelerate the ions emerging from the reflex arc chamber.
The intermediate electrode canal is in a stepped
configuration at one end nearest to the hot cathode and
has a concave opening at the other end facing the
intermediate region. The intermediate electrode ring made
of a non-magnetic material is located about and connected
electrically to the intermediate electrode. There are
further provided a compressor coil on the intermediate
electrode to create an electron confining magnetic field
in the reflex arc chamber and the intermediate region, and
power supply means for supplying electric potentials to
the cathode, the intermediate electrode, the first and
- 12 - lZ27289
second anodes and the plasma aperture plate so that the
electrons admitted into the reflex arc chamber bounce back
and forth between the intermediate electrode and the
plasma aperture plate and in so doing, ionize the iced gas
by colliding with its atoms or molecules.
BRIEF DESCRIPTION OF THE DRAWINGS
The preferred embodiment of the present invention
will now be described in detail in conjunction with the
annexed drawings, in which:
Figure 1 is a sectional view of the ion beam source
according to the present invention:
Figure 2 is an enlarged sectional view of a part of the
ion beam source shown in Figure l;
Figure 3 shows dimensions of the intermediate electrode
canal shown in Figure 2;
Figure 4 compares the magnetic field strength pattern
according to the present invention to other
patterns;
Figure 5 illustrates plasma flute instability due to
non-optimal plasma containment;
Figure 6 is a perspective view of an anode insert
according to the present invention;
Figure 7 shows a dimensioned section of the anode insert
shown in Figure 6; and
Figure 8 shows a section of an alternative design of the
anode insert.
Through the drawings, a like numeral designates a
like-component of the ion beam source.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to figures l and 2 of the drawings,
there is shown a hot cathode filament l held on a filament
holder 2 through which power (typically 2-10 V and 25-100
A) is supplied to the filament. The hot cathode l is
located in a cathode chamber which is defined by a
generally cylindrical, partly conical, intermediate
- 13 -
electrode 5 made of a magnetic material, e.g. mild steel.
A primary gas supply inlet 30 feeds into the cathode
chamber a gas which permits the operation of the
filament. Such gas as xenon or argon is mainly used for
this purpose. The intermediate electrode holds a
compressor coil 4 thereon which provides an excitation of
1800-9000 ampere-turns. At the apex of the conical
portion of the intermediate electrode 5, there is provided
an intermediate electrode canal having three sections; an
intermediate section 6, an entrance section 7 and an exit
section 8. The preferred dimensions of the intermediate
electrode canal sections 6, 7 and 8 in this embodiment as
shown in Figure 3 are as follows:
A = 5.5 mm
B = 6.0 mm
C = 12.0 mm
D = 6.3 mm
E = 1.6 mm
X = 26
The dimensions of the intermediate section 6 are
the most critical and should be determined
experimentally. Its diameter B or cross-sectional area is
important for reliable arc transfer. In the preferred
embodiment it has been found that the minimum diameter B
of the entrance section 7 may not be smaller than 5.8 mm,
which permits arc transfer with a probability of 0.9.
Increasing B to 6.0 mm permits starting with complete
reliability. The reason for keeping the canal diameter as
small as possible is that for stable operation and high
output current of the source, a higher pressure in the
cathode chamber than in the ionizing reflex arc chamber is
required.
The length of the entrance section 7 is not as
critical and is dictated by the other dimensions.
Tune concave or conical exit section 8 is quite
- 14 1 2 72 8 9
shallow and is known from the prior art publication (1980)
by the present inventor.
An intermediate electrode ring 9 made of
non-magnetic material, e.g. copper, surrounds and supports
the intermediate electrode 5 about its apex. The
intermediate electrode ring 9 has a rounded surface 10 and
is electrically connected to the intermediate electrode 5.
A generally cylindrical reflex arc chamber 11 is
disposed axially and aligned with the intermediate
electrode 5. One end of the reflex arc chamber 11 is
defined by the tip of the intermediate electrode 5
followed by a first anode 12 and the other end by a plasma
aperture plate 13. A ring-shaped second anode 14 is
located at approximately a mid-point between the first
anode 12 and the plasma aperture plate I
The intermediate electrode 5, being made of
magnetic mild steel and having a powerful magnetization
induced therein, serves to collimate the electrons emitted
by the cathode 1 so that they may pass through as narrow
as possible a canal aperture. The profiling of the canal
into the three sections 6, 7 and 8, in addition to
improving electron beam constriction, appears to improve
ion plasma confinement near tune plasma aperture plate OWE
Figure 4 shows the magnetic field strength pattern in that
region as curve I, which, of course, exhibits rotational
symmetry about the central axis. Ideally, the plasma
confining field would have the cup-shape of curve Y,
whereby the ions would be well confined within the central
region. Such field is of course impossible to obtain and
curve X is the practical alternative. A field strength
pattern with a maximum along the central axis would not
offer such stable confinement and often causes what is
known in the art as flute instability of plasma,
illustrated in Figure 5. Also, a peaked magnetic field
pattern creates a sharply peaked plasma density profile,
122~2~39
- 15 -
reducing the usable area of the discharge. In the
vicinity of the plasma aperture plate 13, the relative
field strength minimum should be only a small percentage P
below the field maximum; here P I
Turning now to Figures 2 and 6, the first anode
12 has a hole therein in which an anode insert 15 is
rotatable fitted. As shown in the figures a concave
surface 16 of the anode insert is shaped to give a
predetermined clearance from the front surface 8 of the
intermediate electrode 5. The first anode 12, together
with the anode insert 15, is aligned with but separated
from the intermediate electrode 5 and the intermediate
electrode ring 9 to form an intermediate region 17.
The anode insert 15 is shown perspectively in
; 15 Figure 3, in which radial ports I and 19 are clearly seen
milled in a flange 20. The flange is adapted to secure
the anode insert 15 against the first anode 12. The port
18 is located in the surface of the flange facing the
intermediate electrode and the port 19 is in the other
surface of the flange. Either the port 18 or the port 19
can be aligned with a secondary gas passage 21 connected
to a secondary gas inlet 22 by turning the anode insert 15
so that a gas can be fed either directly into the reflex
arc chamber, as shown in Figure 2, or indirectly through
the intermediate region 17 and then through a bore 23 in
the anode insert 15. The bore is located coccal with
the intermediate electrode canal and is flared toward the
reflex arc chamber, as shown by numeral 24 in the
drawings. The flare cross-section is chosen to intercept
a constant flux of the magnetic field emanating from the
intermediate electrode 5. It therefore matches the gas
flow to the electron beam size.
The dimensions of the anode insert 15 are shown
in Figure 7 and are as follows:
G = 6.4 mm
- 16 - ~2~9
H = 17.8 mm
I = 1.9 mm
J = 1.3 mm
K = 12.7 mm
L = 12.7 mm
M = 17.5 mm
N = 26.9 mm
O = 30
The flaring of the anode insert 15 is accomplished in two
segments, or ease of machining. The flaring of the anode
insert 15 is not critical and the use of a single flaring
angle would not affect operation to any significant degree.
As is immediately apparent, the bore 23 in the
anode insert 15 is only slightly larger than the
intermediate section 6 in the intermediate electrode 5.
This is advantageous in that it further restricts the
migration of the ionizable gases toward the cathode inside
the intermediate electrode S, and permits operation with
lower gas flows. Thus consumption of expensive, toxic or
corrosive gases is reduced, vacuum pumping is reduced in
the system using the ion source, and the production of the
desirable atomic, as opposed to molecular, ions is
increased. The narrow bore I in the anode insert 15 has
been made possible by the good confinement of electrons
passing through the intermediate electrode canal. The
bore 23 is positioned advantageously at the waist of the
electron beam.
In an alternative design of the anode insert,
shown in Figure 8, the ionizable gas is injected through
slots 31 to 38, of which only slots 31 to 35 are seen in
figure 8, in the lower periphery of the skirt of the anode
insert 15. This increases the efficiency and improves the
stability of the arc discharge. Since the gas is injected
into the most favorable region for ionization, the
necessary gas flow is reduced.
- 17 1~27~9
Operation
Referring to figure 1, the first anode 12, a
second anode 14 and the plasma aperture plate 13, all
being made of a non-magnetic material, are stacked
together with insulators 25 between them. Clamp rods 26
clamp them together to form the major part of the reflex
arc chamber 11. The plasma aperture plate 13 is provided
with a plurality of apertures 27. There are three
apertures in one preferred embodiment of the present
invention, and in another preferred embodiment seven
apertures are provided of which six are located in a
hexagonal array and the seventh in the center thereof. An
accelerating electrode 28 and a decelerating electrode 29
disposed adjacent to the plasma aperture plate 13 have
also a corresponding number of apertures which are all
aligned with the apertures 27 of the plasma aperture
plate. It is of course possible to use in other
embodiments one or more apertures.
Appropriate power supplies are shown in Figure l
and suitable coolant passages are also provided in various
elements to maintain properly the operating temperature of
the ion beam source. However, only a few of the passages
are shown in the drawings.
In the operation of the duoPIGatron of the
present invention, a protective cover gas, e.g. argon or
xenon, is introduced into a cathode chamber through the
primary gas inlet 30 and the cathode filament is heated to
produce electrons for discharge. The electrical discharge
is caused between the cathode (negative) and the anode
(positive). Inside the mild steel intermediate electrode
5, there is no magnetic field from the compressor coil 4.
However, when the electrons exit this region through the
intermediate electrode canal, they are in a strong
magnetic field It should be noted that the intermediate
electrode ring 9, the anodes 12 and I and the plasma
- 18 -
~2;~7~89
aperture plate 13 are made of a non-magnetic material. By
the strong magnetic field, the electrons are constrained
to spiral along the magnetic field lines forming tight
helical paths. These field lines do not intersect the
anodes so that the electrons cannot go directly to them.
The plasma aperture plate is at a negative potential
relative to the two anodes to reflect back the electrons
flowing along the field lines towards the intermediate
electrode 5, which is also kept at a negative potential.
The electrons thus bounce back and forth (or reflex)
between the plasma aperture plate 13 and the intermediate
electrode 5. Meanwhile, the reflex arc chamber is fed
with a feed gas to be-ionized through the secondary gas
inlet I and the secondary gas passage 21. In the anode
15 insert 15 of figure 6, the ports 18 and 19 permit the feed
gas to be fed into the reflex arc chamber either directly
or through the bore 23 via the intermediate region 17.
The type of gas used as the feed gas dictates the choice
of port to obtain the optimum performance. The reflexing
electrons collide with the feed gas atoms or molecules and
ionize them. The efficiency of the duoPIGatron comes from
this containment of the electrons. The electrons are used
many times and not lost after one transit of the ion
source.
The accelerating electrode 28 and decelerating
electrode 29 form an extraction column and function to
pull positive ions from the plasma which exists in the
reflex arc chamber through a plurality of apertures in the
plasma aperture plate I and to form the ions into a beam
with a desired energy. Typically, a potential of thirty
to fifty thousand volts is applied between the plasma
aperture plate and the accelerating electrode. The ions
from the plasma pass through the apertures in the plasma
aperture plate and are accelerated toward the accelerating
electrode. The apertures in the plasma aperture plate are
- 19 _ ~L227~39
contoured to control the shape and uniformity of the
extracted ion beam. The accelerating electrode it kept at
a small negative potential (typically three thousand
volts) with respect to the decelerating electrode 29 which
is at the ground potential. This forms a potential
barrier which prevents electrons formed below the
extraction column from being accelerated back towards the
reflex arc chamber, producing high X-ray fields and
causing sparking.
It has been described that a gas, e.g. argon or
xenon, is supplied in the cathode chamber 3 to protect the
filament l from being damaged by the feed gas which is
supplied by the secondary gas supply means. however, if
argon, xenon, nitrogen, hydrogen and neon, or other gases
which do not damage the filament in operation, is the gas
to be ionized, it can be introduced through the primary
gas inlet without the use of the secondary gas inlet which
is to be closed by a valve (not shown). Such gas flows
into the reflex arc chamber from the cathode chamber
through the electrode canal and the bore in the anode
insert.
The diameter and length of the reflex arc chamber
are chosen to give a large uniform area of plasma and to
provide stable arc operation. If the diameter of the
chamber is made smaller or the length shorter, the usable
extraction area is decreased. Extending the length past
the values of the present embodiment leads to unstable
operation. In the present embodiment, they are 57 mm in
diameter and 79 mm in length.
The main application of the present invention
would be in semi-conductor implanters. These devices are
used to implant desired do pants into silicon wafers to
fabricate the integrated circuits that are used in a wide
range of computer and other electronic systems. The ion
sources presently used in these implanters are limited to
~ZZ~2~
- 20 -
currents of =12.5 ma of phosphorous and arsenic and =5 ma
of boron. The boron is especially a limitation since
wafer cooling is adequate for currents of 15 ma at 100 key
and 30 ma at 50 key. Table 1 gives some typical output
currents from the duoPIGatron of the present invention,
running on Arizona, phosphine and boron trifluoride. These
initial measurements were made with only three apertures
(I mm dia.) open. Values are given for the useful species
from three apertures (as measured) or from seven apertures
(as would most likely be used). The current extracted
depends on the open area of the plasma plate within a
circle of approximately 2 cm radius.
Another related application is the formation of
buried oxide layers in silicon wafers. This requires a
high current of oxygen as one is forming significant
quantities of Sue. Presently used sources provide
approximately 4 ma of Ox ions and lead to implant times
of up to eight hours. Approximately sixty percent of
atomic oxygen ions (O ) were available with 100 ma total
beam current from three apertures. Under the same
operating conditions, a total beam current of 250 ma was
extracted from seven aperture. Therefore, as shown in
Table 1, up to 140 ma of O is available from the
source, leading to implant times of the order of 20
minutes. This is a factor of 25 improvement in
throughput, assuming that the wafer cooling and handling
does not limit the usable current.
Some more future applications are nitrogen ,
implantation into steel for wear improvement, and the use
of ion beams to control and modify the properties of
materials being built up by evaporation or other
processes. Both of these, as with the oxygen application
require high currents.
- 21 - ,7~39
in ox In I Jo + + a
In X X I
OLD 5
. .
I I) N Ed N
It I O Lowe En I
"!~ + us ++ + O O
Us U O
O h
to
I I: H
I I h I h
m Lo o 3 3 clue 3 3 pa
o o o o o o
En 4 I ~1~1::1 I I ~-r1
O
_
` h-- O O O 111 *
a) a:) o ox aye o us
m
In 5
U-- .
I - O O QJ
'I-- OX a I
I`
X O
+ + X pa
I ,
.+
Al or O a
s
1 a) o o
ray us us o w o o
Jo O o I Al JO 'd O U
O S ~~10 I X Al
my En o z $ I I æ
