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Claims  |
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What is claimed is:
1. An ion source including a microwave discharge chamber adapted to have a
plasma therein generated in a feed gas introduce therein, a gas inlet for
introducing said feed gas into said discharge chamber, and an ion beam
exit slit for extracting ions from said plasma generated in the feed gas
introduced into said discharge chamber, wherein the ion beam exit slit is
located on an end surface of the discharge chamber, and wherein the
distance d between the end surface of said discharge chamber on which said
ion beam exit slit is disposed and said gas inlet, and the distance d'
between the wall surface of the portion of said discharge chamber on which
said gas inlet is disposed and a wall surface facing the former, satisfy
the relation d.ltoreq.2d', so as to reduce deposition onto the ion beam
exit slit as compared to the deposition thereon when the distances d and
d' do not satisfy said relation.
2. The ion source according to claim 1 wherein said d and said d' satisfy
the relation d.ltoreq.d'.
3. The ion source according to claim 1, wherein another gas inlet for
feeding gas to the discharge chamber is disposed at a portion of the
chamber other than close to said ion beam exit slit to match a state of
deposit at the ion beam exit slit and on a surface of the discharge
chamber other than close to said ion beam exit slit.
4. The ion source according to claim 3, further including a gas
introduction pipe communicating with said gas inlet and another gas
introduction pipe communicating with said another gas inlet, and in which
a gas flow rate control valve is disposed in each of said gas introduction
pipes.
5. The ion source according to claim 1, wherein another gas inlet for
feeding gas to the discharge chamber is disposed at a portion of the
chamber other than close to said ion beam exit slit, and wherein said gas
inlet is adapted to feed at least one halide gas selected from the group
consisting of boron halide gases to the discharge chamber, and said
another gas inlet is adapted to feed at least one gas which reacts with
boron and forms a boron compound, the ion source further including means,
associated with the ion beam exit slit, for extracting a B.sup.+ ion beam.
6. The ion source according to claim, wherein said ion beam exit slit is
divided into a plurality of portions, and the portions close to the
aperture formed by the slit are thermally isolated.
7. The ion source according to claim 6, wherein electrically conductive
boron nitride is used for at least a part of a portion constituting said
ion beam exit slit.
8. The ion source according to claim 1, which further includes a magnetic
field generator and a microwave generator in order to generate said
plasma.
9. The ion source according to claim 8, wherein said gas inlet is adapted
to feed a feed gas that is a mixed gas prepared by mixing at least one
halide gas selected from the group consisting of boron halide gases with
at least one gas which reacts with boron and forms a boron compound, the
ion source further including means, associated with the ion beam exit
slit, for extracting a B.sup.+ ion beam.
10. The ion source according to claim 9, wherein said boron halide gas is
at least one member selected from the group consisting of boron
trifluoride and boron trichloride, and said at least one gas which reacts
with boron and forms a boron compound is at least one member selected from
the group consisting of oxygen gas, hydrogen gas, and gas of an
oxygen-containing compound and their mixtures.
11. The ion source according to claim 10, wherein said gas of an
oxygen-containing compound is CO, CO.sub.2, NO, N.sub.2 O, SO.sub.2 or
H.sub.2 O.
12. The ion source according to claim 10, wherein said at least one gas
which reacts with boron and forms a boron compound is oxygen.
13. The ion source according to claim 10, wherein said at least one gas
which reacts with boron and forms a boron compound is hydrogen.
14. The ion source according to claim 9, further comprising means for
mixing said halide gas and said at least one gas which reacts with boron
and forms a boron compound, and means for feeding the gas mixture to the
gas inlet, and wherein the means for mixing is adapted to provide a gas
mixture wherein the quantity of said gas which reacts with boron is at
least 0.1% of said halide gas in terms of pressure ratio.
15. An ion source including a microwave discharge chamber adapted to have a
pasma therein generated in a feed gas introduced therein, an ion beam exit
slit for extracting ions from said plasma generated in the feed gas
introduced into said discharge chamber, and feed means for feeding at
least said feed gas to the discharge chamber, wherein said feed means
includes means for feeding a boron halide gas and means for feeding a gas
which reacts with boron and forms a boron compound, so as to provide boron
ions while preventing deposition of boron compounds on the surface of the
discharge chamber and on the ion beam exit slit.
16. The ion source according to claim 15, further comprising a source of
said boron halide gas and a source of said gas which reacts with boron,
said source of said born halide gas being in flow communication with said
means for feeding the boron halide gas, and the source of the gas which
reacts with boron being in flow communication with the means for feeding
the gas which reacts with boron.
17. A method of forming an ion beam comprising the steps of passing a feed
gas to a microwave discharge chamber, forming a plasma from said feed gas
in said chamber, and extracting ions, in an ion beam, from said plasma,
wherein said feed gas includes a boron halide gas and a gas which reacts
with boron and forms a boron compound, so as to provide boron ions while
preventing deposition of boron compounds on the surface of the discharge
chamber and on the ion beam exit slit.
18. The method of forming an ion beam according to claim 17, wherein the
boron halide gas is selected from the group consisting of boron
trifluoride and boron trichloride.
19. The method of forming an ion beam according to claim 18, wherein the
gas which reacts with boron and forms a boron compound is selected from
the group consisting of oxygen gas, hydrogen gas, a gas of an
oxygen-containing compound, and mixtures thereof.
20. The method of forming an ion beam according to claim 19, wherein said
gas on an oxygen-containing compound is selected from the group consisting
of CO, CO.sub.2, NO, N.sub.2 O, SO.sub.2 and H.sub.2 O.
21. The method of forming an ion beam according to claim 17, wherein the
quantity of the gas which reacts with boron fed to the discharge chamber
is at least 0.1% of the boron halide gas in a pressure ratio.
22. A method of forming an ion beam comprising the steps of passing feed
gas to a microwave discharge chamber, forming a plasma from said feed gas
in said chamber, and extracting ions, in an ion beam, from said plasma,
said ions being extracted from the discharge chamber through an ion beam
exit slit, wherein the ion beam exit slit is located on an end surface of
the discharge chamber, and wherein the distance d between the end surface
of said discharge chamber on which said ion beam exit slit is disposed and
the location where the feed gas is passed into the discharge chamber, and
the distance d' between the wall surface at the location where the feed
gas is passed into the discharge chamber and a wall surface facing the
former, satisfy the relation d.ltoreq.2d' so as to reduce deposition onto
the ion beam exit slit as compared to the deposition thereon when the
distances d and d' do not satisfy said relation.
23. The method of forming an ion beam according to claim 22, wherein said
feed gas includes a boron halide gas, and wherein a gas which reacts with
boron and forms a boron compound is also passed into the discharge
chamber.
24. The method of forming an ion beam according to claim 23, wherein the
feed gas, fed to the discharge chamber, is a mixture of said boron halide
gas and said gas which reacts with boron, said mixture being fed at a
location so as to satisfy said relation.
25. The method of forming an ion beam according to claim 23, wherein the
feed gas passed so as to satisfy said relation is said boron halide gas,
and wherein said gas which reacts with boron is passed into the discharge
chamber further from the exit slit than the location at which feed gas fed
at a location so as to satisfy said relation is passed into the discharge
chamber.
26. The method of forming an ion beam according to claim 23, wherein the
feed gas is fed at a first inlet so as to satisfy said relation and at a
second inlet spaced further than said first inlet from said exit slit,
with the feed gas fed at the first and second inlets being fed at
independently controlled rates to match a state of deposit at the ion beam
exit slit and on a surface of the discharge chamber other than close to
said ion beam exit slit.
27. The method of forming an ion beam according to claim 22, wherein said
feed gas is passed into the discharge chamber sufficiently close to the
exit slit such that deposits are not formed on the exit slit.
28. The ion source according to claim 1, further comprising a microwave
generator for generating the plasma.
29. The ion source according to claim 15, further comprising a microwave
generator for generating the plasma.
30. The method of forming an ion beam according to claim 17, wherein said
plasma is formed using microwaves transmitted to the discharge chamber.
31. The method of forming an ion beam according to claim 22, wherein said
plasma is formed using microwaves transmitted to the discharge chamber.
32. The ion source according to claim 15, wherein said means for feeding a
gas which reacts with boron and forms a boron compound is a means for
feeding said gas in an amount of at least 0.1% of the boron halide in a
pressure ratio.
33. The method of forming an ion beam according to claim 17, wherein said
gas which reacts with boron and forms a boron compound is hydrogen. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
This invention relates to an ion source for an ion implanter, an ion
milling machine or the like, and more particularly to an ion source
suitable for obtaining a stable ion beam for an extended period of time,
and further particularly to a microwave ion source suitable for obtaining
a B.sup.+ ion beam.
FIG. 1 is a schematic illustration of the structure of a conventional
microwave ion source.
The microwave ion source consists of rectangular waveguides 2a and 2b as
the waveguide for propagating a microwave, discharge electrodes 4
constituting a ridged waveguide structure, a discharge chamber 5 made of
boron nitride and disposed between the ridged waveguides, and extraction
electrodes 8a, 8b and 8c which extract the ion beam 21. An axial magnetic
field generated by the excitation of a coil 13 is applied to the discharge
chamber 5, and a feed gas for discharge is introduced into the discharge
chamber 5 through a gas introduction pipe 6.
FIG. 2 shows in detail the discharge chamber and portions near the chamber.
FIG. 3 is a sectional view of the discharge chamber 5 and portions near
the chamber, and is useful for explaining the discharge chamber 5, a gas
inlet 10 and an ion beam exit slit 7. In the drawings, reference numeral 1
represents a microwave generator; 3 is a microwave introducing flange; 5a
is a lining of the discharge chamber 5; 7a is a portion near the ion beam
exit slit; 11 is a dielectric filler; and 12 is an insulator.
PH.sub.3 (phosphine), AsH.sub.3 (arsine) or the like as a hydride is used
as the feed gas in order to obtain the beam of P.sup.+ (phosphorus) ion,
As.sup.+ (arsenic) ion beam or the like that is used for the ion
implantation for a semiconductor in the ion source shown in FIG. 1. In
this case, the P.sup.+ or As.sup.+ ion beam can be extracted stably for an
extended period of time. If a BF.sub.3 gas is introduced in order to
obtain B.sup.+ ion beam necessary for the ion implantation for a
semiconductor, however, two problems develop, which have made it difficult
so far to obtain stably a high current ion beam for an extended period of
time:
(1) deposit at the ion beam extraction opening portion (i.e., ion beam exit
slit 7); and
(2) deposit inside the discharge chamber 5.
If the deposit of the item (1) occurs, the opening area is reduced so that
the extracted beam current drops. In a conventional ion source, the gas
inlet 10 is positioned near the center of the discharge chamber 5.
Therefore, if plasma is generated by a halide gas such as BF.sub.3, boron
nitride (BN) constituting the lining 5a of the discharge chamber 5,
particularly its portion near the gas inlet 10, is etched, and an etching
product (most of which is BN) precipitates at the other portions,
particularly at the ion beam exit slit 7. In consequence, the slit width
is reduced, and the current drops eventually. According to an experiment
which uses BF.sub.3 gas as the feed gas, the area of the opening portion
is reduced almost by half in the course of the operation of the ion source
for about four hours. When the deposit is hit by the plasma or ion beam,
it is peeled off from the ion beam exit slit 7 and flies sometimes into
the space to which an electric field for extracting the ions is applied.
The peeled matter strikes the electrode 8b and generates secondary
electron emission, which in turn generates an abnormal discharge between
the electrode 8a to which a high positive voltage is applied and the
electrode 8b to which a high negative voltage is applied. This results in
instability of the ion source.
The deposit is often peeled off in the case of (2), too, and the plasma
state becomes unstable; and, in addition, an abnormal discharge develops
between the electrodes in the same way as in the case of (1), and reduces
the stability of the ion source.
If the plasma is generated in a conventional ion source with a halide gas
such as BF.sub.3, the extracted ion beam current decreases and the
stability of the ion source drops, as described above.
It is believed that the reason why the deposit occurs when, e.g., BF.sub.3
or BCl.sub.3 gas is used is that since the fluorine or chlorine atom
generated by microwave discharge is extremely active chemically, it
corrodes and dissociates boron nitride (BN) that constitutes the lining 5a
of the discharge chamber 5. As a matter of fact, when the deposit is
physically analyzed, it is determined to be boron nitride. In order to
prevent the occurrence of such a deposit, it is effective to constitute
the discharge chamber in a thermally isolated structure, and to raise its
operation temperature so as to thermally dissociate or evaporate the
deposit. However, the temperature cannot be raised beyond a certain limit
due to the structural limitations of the discharge chamber (generally, the
approximate upper limit is 800.degree. to 900.degree. C.), and it has been
difficult in practice in the past to restrict the quantity of the deposit
to a level which presents no practical problems.
As a prior art reference disclosing the state of the art in this field,
mention can be made of Japanese Patent Laid-Open No. 132754/1981.
SUMMARY OF THE INVENTION
The present invention is directed to provide an ion source, particularly a
microwave ion source, and method of operating such ion source to produce
an ion beam, which can eliminate the problems with the prior art described
above, which does not reduce the width of an ion beam exit slit even when
a halide gas such as BF.sub.3 or BCl.sub.3 is used as a feed gas to be
introduced into a discharge chamber, and which makes it possible to stably
extract an intended ion beam for an extended period of time.
To accomplish these objects, the ion source in accordance with the present
invention is equipped with a gas inlet (for the feed gas) in close
proximity of an ion beam exit slit for extracting ions from the plasma of
the feed gas introduced into a discharge chamber. The effect of the
present invention can be further enhanced by providing, as the feed gas
utilized in the discharge chamber, a mixture of gases which includes at
least one gas selected from the above-mentioned boron halide gases and at
least one gas which reacts with boron to form a boron compound, such as
oxygen, hydrogen or an oxygen-containing compound gas.
If d is the distance between the gas inlet (that is, the middle of the
aperture of the gas inlet) and the end surface of the discharge chamber on
which the ion beam exit slit is disposed, d should be up to 2d', and
preferably is up to d', d' being the distance between the wall of the
portion of the discharge chamber at which the gas inlet is disposed and
the wall opposing the former; and if the gas inlet is positioned closer to
the ion beam exit slit, a better result can be obtained
In the structure described above in which the gas inlet is disposed in the
proximity of the ion beam exit slit, the gas pressure is elevated in the
proximity of the ion beam exit slit, and the portion of the exit slit is
etched in the same way as the walls inside the discharge chamber, thereby
making it possible to reduce the deposit onto the exit slit portion.
The quantity of the gas which reacts with boron to form a boron compound
and which is to be mixed with the halide gas is at least 0.1% of the
halide gas in a pressure ratio, and the effect of the invention increases
remarkably as reactive gas quantity increases. If the gas quantity is
below 0.1%, the effect of the invention cannot be observed. As the
quantity of the mixing gas such as oxygen increases, the quantity of the
intended ions contained in the extracted ion beam decreases in proportion
to the mixing quantity, so that the upper limit of the mixing is
determined in accordance with the object of use of the ion source. When
the mixing gas is hydrogen, its mixing quantity is the same as for oxygen
described above. Examples of the oxygen-containing compounds described
already include CO, CO.sub.2, NO, N.sub.2 O, SO.sub.2 and H.sub.2 O. The
mixing gas may be any one of the above-mentioned gases of hydrogen, oxygen
and various compounds containing oxygen, but two or more of these gases
may be mixed and be used as the mixing gas. Addition of the mixing gas
such as oxygen to the gas to be utilized in the discharge chamber provides
naturally the effect of the invention in combination with disposition of
the gas inlet in the proximity of the ion beam exit slit, but when it is
used alone, addition of the mixing gas stabilizes ion beam extraction, and
thereby provides beneficial results by itself.
Microwaves in a magnetic field are generally used as a plasma generation
means, and can also be used for the ion source of the present invention.
Besides application to ion source plasmas, the field of utilization of the
plasma generated by microwave discharge in the magnetic field includes
etching of Si by plasma. It is generally known that the etching rate drops
due to the mixture of oxygen depending on the microwave plasma etching. It
is known that when the B.sup.+ beam is extracted by introducing BF.sub.3
into the microwave ion source, relatively great quantities of ions of
oxygen-containing compounds such as BO.sup.+, BOF.sup.+ or the like are
detected as being generated if H.sub.2 O as the residual gas inside a
vacuum chamber is great. It can be understood from the above that if
O.sub.2 is positively mixed with the feed gas for the microwave ion
source, deposition can be prevented because etching of BN as the material
of the discharge chamber can be restricted, and at the same time the
dissociated BN molecules change to BO.sup.+, BOF.sup.+, and the like.
Even if deposition occurs, it escapes in the form of BO.sup.+, BOF.sup.+,
or the like, so that a remarkable reduction of the deposition rate can be
expected.
As an ion source for obtaining B.sup.+ ion beam of a mA class, an ion
source utilizing low voltage arc discharge with a hot filament is known.
BF.sub.3 is also used for such an ion source. Since the hot filament is
rapidly corroded by oxygen, operation of the ion source for an extended
period cannot be expected even if oxygen (O.sub.2) is introduced in order
to obtain a stable B.sup.+ beam. In this sense, introduction of O.sub.2 is
a method which can be characterizingly used for a microwave ion source not
containing a hot filament.
In the description above, oxygen is used as a typical example, but the
effect of reduction of deposition can be obtained with those gases which
react with boron and which readily form a boron compound, such as H.sub.2
gas.
Heretofore known techniques or knowledge in the field of ion sources can be
used for the ion source of the present invention for those matters which
are not particularly described in this specification.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration showing the structure of a conventional
microwave ion source;
FIG. 2 is a perspective view showing a discharge chamber and portions
nearby;
FIG. 3 is a sectional view showing the discharge chamber and portions
nearby;
FIG. 4 is a sectional view showing the discharge chamber and portions
nearby in a microwave ion source in accordance with one embodiment of the
present invention;
FIG. 5 is a sectional view taken along line A--A of FIG. 4;
FIG. 6 is a schematic illustration showing the overall structure of the
microwave ion source in accordance with one embodiment of the present
invention;
FIG. 7 is a diagram showing the relation between the position of gas inlets
and the deposition rate at an ion beam exit slit;
FIGS. 8a and 8b are sectional and plan views showing the discharge chamber
and portions nearby in the microwave ion source in accordance with another
embodiment of the present invention, respectively;
FIG. 9 is a sectional view showing the discharge chamber and portions
nearby in the microwave ion source in accordance with still another
embodiment of the present invention; and
FIGS. 10 and 11 are schematic illustrations showing the overall structure
of the microwave ion source in accordance with other embodiments of the
present invention, respectively.
DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiment 1:
FIGS. 4 and 5 are sectional views, each showing the structure of a
discharge chamber and portions nearby in a microwave ion source in
accordance with a first embodiment of the present invention, and FIG. 5 is
a sectional view taken along line A--A in FIG. 4.
This embodiment uses microwave discharge in a magnetic field as means for
generating plasma. The microwave is introduced into the discharge chamber
5 from the left side of FIG. 4 through the space between the discharge
electrodes 4. The magnetic field is applied in a direction crossing an
electric field by the microwave, and the interaction of these fields
generates the plasma of the feed gas inside the discharge chamber 5. Ion
beam 21 is extracted from an ion beam exit slit 7 which is disposed on one
end surface of the discharge chamber. The feed gas is introduced into the
discharge chamber 5 through a plurality of gas inlets 110 (five inlets are
shown in FIG. 5) that are disposed in the proximity of the ion beam exit
slit. As described earlier, the distance d between these gas inlets 110
and the end surface 120 of the discharge chamber on which the exit slit 7
is disposed should be smaller than 2d', and is preferably smaller than d',
which is the distance between the wall surface on which the gas inlets are
disposed and the wall surface of the discharge chamber 5 facing the
former. When a halide gas such as BF.sub.3 is used as the feed gas in this
embodiment, the deposit at the ion beam exit slit 7 can be reduced because
the inner walls of the discharge chamber 5 close to the gas inlets 110 and
the surface exposed to the plasma at the ion beam exit slit 7 are also
etched.
As an example, the diameter of the gas inlet 110 is 2 mm, the length of the
ion beam exit slit 7 is substantially the same as that of the discharge
chamber, the thickness of the slit 7 is about 1 mm, and the discharge
chamber 5 is 8 mm wide, 20 mm deep and 46 mm long. In this example, d is 2
mm and d' is 8 mm. Thus, in this example, d is 1/4 d'. The same material
is used for each portion as used in the prior art device. That is, the
discharge electrode 4 is made of stainless steel, the dielectric filler 11
is made of BN, the lining 5a of the discharge chamber is made of BN, and
carbon is used for the portion 7a in the proximity of the ion beam exit
slit 7.
FIG. 6 is a schematic illustration showing the overall structure of the
microwave ion source of this embodiment.
Next, the disposition of the gas inlets in this embodiment will be examined
in detail.
FIG. 7 is a diagram showing the relation between the position of the gas
inlets 110 and the deposition quantity of the deposit at the ion beam exit
slit. The abscissa represents d/d' ratio (R) wherein d' is the distance
between the gas inlet and the inner wall of the discharge chamber facing
the gas inlet and d is the distance between the end surface of the
discharge chamber on which the ion beam exit slit is disposed and the gas
inlet (that is, the center of the aperture thereof), and the ordinate
represents the deposition rate (mm/hr). The ion beam exit slit and the
discharge chamber have the same structure as described already. The
microwave power to be charged into the plasma is 800 W, the intensity of
magnetic field is 500 Gauss, and the feed gas quantity expressed in terms
of pressure is 1.times.10.sup.-1 Pa, and these values are kept constant. A
negative deposition rate indicates that the ion beam exit slit has been
etched. As can be seen clearly from FIG. 7, undesirable deposition occurs
greatly in the region where R>2. Within the range of 0.ltoreq.R.ltoreq.2,
the effect of the present invention can be observed, and the smaller the
value R, the smaller the deposition quantity. Particularly in the region
where R.ltoreq.1, an operating life sufficient for practical use as an ion
source for an ion implanter can be obtained. In other words, (implant
current).times.(implant time)=40 mA.hr can be achieved. In accordance with
the prior art, the operating life is at most 10 mA.multidot.hr. According
to an experiment, life has been found to be more than 120 mA.hr (the ion
source can be used for more than 30 hours at 4 mA) when R=0.25.
Embodiment 2:
FIGS. 8a and 8b are sectional and plan views showing a discharge chamber
and portions nearby in a microwave ion source in accordance with a second
embodiment of the present invention. The feed gas is introduced into the
discharge chamber in the same way as in the first embodiment shown in FIG.
4. In this embodiment, the ion beam exit slit is divided into a plurality
of units in a thermally isolated state so that the temperature of the
portions of the ion beam exit slit 7 which are exposed to plasma (e.g.,
the portions adjacent the aperture) can be further raised. The ion beam
exit slit 7 in this embodiment consists of three members 7a, 7b and 7c,
whereby, as an example, 7a is made of stainless steel, 7b is made of an
electrically conductive composite material of electrically conductive BN
and Ti, and 7c is made of carbon. By employing this split structure, the
ion beam exit slit 7 is thermally isolated and consists of a plurality of
units as shown in FIG. 8b. This split structure raises the temperature of
BN by thermally isolating the slit 7, and makes the slit 7 more readily
etchable when the discharge gas passes therethrough.
This embodiment can further enhance the etching effect of the ion beam exit
slit 7 by the feed gas, and can eliminate the deposit on the ion beam exit
slit 7.
Embodiment 3:
FIG. 9 is a sectional view showing the structure of the discharge chamber
of the microwave ion source and portions nearby in accordance with a third
embodiment of the present invention. The feed gas is introduced into the
discharge chamber through the gas inlet 110 disposed close to the ion beam
exit slit 7 and through a gas inlet 110a which is additionally disposed on
the surface at the depth of the discharge chamber 5 (e.g., near the end of
chamber 5 opposite to the end thereof having the exit slit 7--the chamber
surface at the left in FIG. 9). The gas introduction pipes 6 and 6a
communicating with these two gas inlets 110 and 110a have separate gas
flow rate control valves 9a and 9b, respectively, and can independently
control the gas flow rate. This embodiment can uniformly reduce the
deposit inside the discharge chamber 5 by adjusting the respective gas
flow rates to match the state of deposit at the ion beam exit slit 7 and
on the deep surface of the discharge chamber 5.
Embodiment 4:
This embodiment is the same as the ion source shown in the first embodiment
except that leak valves 14 and 14' are fitted to the gas introduction pipe
6 as shown in FIG. 10, and a gas mixture, boron trifluoride (BF.sub.3) and
oxygen (O.sub.2), is introduced.
When the mixing quantity of O.sub.2 is changed to 5%, 10% and 20%, of
BF.sub.3 in pressure ratio, the proportion of B.sup.+ contained in the
extracted ion beam tends to decrease somewhat in proportion to the
concentration of O.sub.2, but no deposit is seen occurring in any case
inside the discharge chamber 5 and at the portion of the ion beam exit
slit 7. As a result, a B.sup.+ beam of at least 60 keV and at least 4 mA
can be stably obtained for more than four hours by mass-separating the ion
beam extracted from the ion source with a sector magnet mass separator.
The effect of reducing the deposit can be seen occurring from the 0.1%
O.sub.2 concentration with respect to the pressure of the BF.sub.3, and
can also be seen increasing with an increasing O.sub.2 concentration.
Besides the generation of various atom and molecule ions, large quantities
of chemically active, neutral atoms and molecules are generated in the
plasma. Their quantities and component ratios vary in a complicated manner
with microwave power, temperature, gas pressure, and so forth. For this
reason, chemical analysis in further detail will be necessary in order to
clarify the reason why discharge free from the deposit can be maintained
by introducing O.sub.2.
A similar effect can be obtained when boron trichloride (BCl.sub.3) is used
in place of BF.sub.3.
Embodiment 5:
The microwave ion source in accordance with a fifth embodiment of the
invention is the same as the ion source of the fourth embodiment except
that BF.sub.3 and O.sub.2 are introduced into the discharge chamber 5
through separate gas introduction pipes 6 and 6', as shown in FIG. 11. A
stable B.sup.+ beam free from deposition can be obtained in this
embodiment in the same way as in the fourth embodiment shown in FIG. 10.
Reference numerals 15 and 15' in FIG. 11 represent the leak valves,
respectively.
Embodiment 6:
The fourth and fifth embodiments of the invention shown in FIGS. 10 and 11,
respectively, mix the gases with the two needle valves. However, if a gas
tank storing therein a mixed gas is used, the mixed gas can be introduced
into the discharge chamber through one needle valve as shown in FIG. 6,
and the same effect as those of the fourth and fifth embodiments can be
obtained.
Embodiment 7:
When an oxygen-containing gas such as CO.sub.2 is used as the gas to be
mixed with BF.sub.3 or BCl.sub.3 in place of oxygen used in the fourth,
fifth and sixth embodiments, the same effect as when oxygen is introduced
can be obtained. A stable B.sup.+ beam can be obtained similarly when two
or more kinds of oxygen-containing gases are mixed.
Embodiment 8:
When about 10% H.sub.2 is mixed in place of O.sub.2 with BF.sub.3 in the
fourth, fifth and sixth embodiments of the invention, a stable B.sup.+
beam can be extracted without causing any deposition.
It is known that among boron compounds, hydrogen compounds such as B.sub.2
H.sub.6 are relatively stable. It is therefore believed that when H.sub.2
is mixed, the reactive hydrogen radical particles react with the deposit
or the like, thereby exhibiting the effect of reducing deposition.
As described above, even when a halide such as BF.sub.3 or BCl.sub.3 is
used as the feed gas, the present invention can prevent the deposition on
the ion beam exit slit, and hence can extract ions stably for an extended
period of time. For example, a B.sup.+ ion beam of at least 4 mA, which
has not so far been accomplished by the prior art ion source, can now be
obtained for 4 or more hours stably. In view of the fact that an ion
implant current of a semiconductor ion implanter on a production line
remains at about 2 mA at present, the present invention makes it possible
for the first time to carry out high current B.sup.+ ion implantation at a
practical level, and provides a great effect in practice.
Obviously many modifications and variations of the present invention are
possible in light of the above teachings. It is therefore to be understood
that within the scope of the appended claims, the invention may be
practised otherwise than as specifically described herein.
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