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FIELD OF THE INVENTION
This invention relates to electron-bombardment ion beam sources, and more
particularly, to a multipole plasma containment structure as an element
thereof.
BACKGROUND OF THE INVENTION
Electron-bombardment ion beam sources have been employed in sputter
machining to selectively remove material from non-protected portions of a
target substrate and in sputter deposition wherein portions of a substrate
are masked to selectively deposit sputter material by ion beam bombardment
in accordance with a predetermined pattern. Further, such ion beam sources
have been employed in the implantation or doping of ions into a
semiconductor material. In the case of the latter, higher ion energy is a
requisite if useful penetration depths are achieved for the doping
material.
Basically, all electron-bombardment ion beam sources require a chamber into
which an ionizable material (generally in vapor form) such as argon,
arsenic etc. is introduced. The chamber bears both an anode and a cathode,
with the anode attracting high velocity electrons from the cathode. The
impingement of electrons upon atoms (molecules) of the introduced vapor
results in the ionization of the atoms (molecules). Typically at one end
of the chamber, there is provided an apertured electrode followed by an
apertured extraction electrode and a potential is impressed upon the
latter electrode which accelerates the ions out of the chamber through the
apertures in both electrodes.
Further, the interior of the chamber is subjected to a magnetic field to
effect gyration of the electrons in their travel towards the anode, thus
greatly increasing the chance of an ionizing collision between any given
electron and one of the source material atoms. This results in an
increased efficiency in ionization.
Typically, ion sources for isotope separators and implantation systems have
used solenoid magnetic fields for increasing the plasma density and gas
efficiency by increasing the path length of the ionizing electrons between
the cathode and anode. However, as this solenoid field is increased, the
plasma constricts, becomes noisy, and the helical instability limits the
regime of effectiveness of the magnetic field.
Recent efforts have shown that a quiescent plasma may be obtained by using
a multipole containment of the ionizing electrons. U.S. Pat. No. 3,969,646
to Reader et al., issued July 13, 1976, teaches the use of a multipole
configuration wherein a plurality of successively-spaced segments of
electrically-conductive magnetic material are distributed within the
chamber, the segments being interconnected with a potential impressing
means so that the segments collectively constitute the anode. Further,
individually adjacent segments are respectively polarized oppositely in a
magnetic sense so that segments collectively establish the magnetic field.
In one specific structural assembly, each of the anode segments is
composed of a strip of magnetizable material and each successive pair of
such strips are spaced apart by respective individual magnets.
Ion sources used for high current in implantation and isotope separation
generally operate at higher current densities, higher source temperatures
(.about.1000.degree. C.) and higher extraction voltages than existing
steady state sources using multipole configurations. For operation at high
voltage, a source should operate at high efficiency to reduce the pressure
in the extraction region which may lead to voltage breakdown. In
particular, isotope separation requires high efficiency to minimize
material losses to the vacuum system. Higher efficiencies are usually
obtained by operation at higher plasma densities and hence higher source
temperatures. Higher plasma densities can be achieved more easily in a
small source geometry for a given cathode emission. Furthermore, a small
source geometry is desired to achieve adequate high voltage isolation
without extensive consumption of space.
Such multipole sources as found in the prior art are generally large area
multi-aperture sources in which the ratio of the low (<100 Gauss)
intensity magnetic field region to high intensity magnetic field region is
reasonably large. The region of low magnetic field intensity corresponds
to the region of quiescent plasma formation during source operation. When
constrained to small volumes, commercially available sources do not assume
multipole configurations due to the difficulty in achieving a reasonable
region of low magnetic field intensity and also in extracting an ion beam
in the presence of the high magnetic fields desired at the walls.
Furthermore, the high source temperatures resulting from operation at high
plasma densities are not compatible with most permanent magnets which tend
to lose field strength when heated.
Nevertheless, applicants have determined that a multipole source
configuration has certain properties which are advantageous for the high
voltage, high efficiency operation of ion implantation or isotope
separation sources. Multipole configurations reduce the ionizing electron
losses which improves the source efficiency for a given cathode emission
level, which would lead to longer cathode lifetimes. Also, since it is
extracted from a quiescent plasma, the ion beam from a multipole source
will be less noisy and less likely to strike electrodes, producing
secondary electrons which can lead to arcing. Furthermore, it has been
determined that the presence of magnetic fields in the extraction gap
(such as occurs in standard solenoidal field configurations) leads to
increased probability of ionization of residual gases by secondary
electrons, which enhances conditions for voltage breakdown. The multipole
configuration minimizes the level of magnetic fields in the extraction
region.
Typically, multipole sources are large broad beam, multi-aperture sources.
However, in ion implantation and isotope separation, one wants to achieve
both high beam current density and reasonable (50 hours or more) filament
lifetime. Of necessity, such a source is one in which the plasma volume is
considerably smaller than prior art multipole sources and one which
operates in the 1000.degree. C. temperature range.
Accordingly, it is an object of the present invention to provide an ion
implantation and isotope separation ion source which will operate at high
temperature and high plasma density while maintaining 50 hours or more
filament lifetime.
It is a further object of the invention to provide a quiescent plasma ion
source compatable with high extraction voltages (80 kilovolts or more).
SUMMARY OF THE INVENTION
The present invention is directed to a highly effective multipole type
implantation-isotope separation ion beam source of the multipole plasma
containment type and comprises a high temperature plasma confining
structure having a plurality of magnetic poles arranged about the external
walls of the confining structure. The present invention provides a small
structure for ion implantation having high voltage compatibility (i.e. no
strong external magnetic field to increase electron path leading to
ionization and breakdown). Further, a plurality of permanent magnets are
provided which individually separate pairs of the plurality of magnetic
poles or pole pieces with the magnetic poles being separated from one
another and the confining structure by a distance so as to produce a
minimum field toward the center of the confining structure and with an
effective containing field around the periphery of the confining
structure. Further, means are provided to cool the magnetic poles and
permanent magnets. Preferably, the plurality of magnetic poles constitutes
an odd number to create a virtual pole (for the missing even pole) at the
extraction slit to enhance plasma presence at extraction slit.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-section of one embodiment of the improved multipole ion
beam source of the present invention.
FIG. 2 is an exploded, perspective view of a second embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In the illustrated embodiments of the invention, like elements bear like
numerical designations. Referring to FIG. 1, one embodiment of the
multipole containment ion beam source is indicated generally at 10 and
takes the form of a rectangular box assembly including an outer box
indicated generally at 12, being open ended and formed of copper sheet
material including a back wall 12a and laterally opposed side walls 12b.
The back wall 12a may include a liquid refrigerant circulation loop
connected to a standard refrigeration circuit (not shown) such that an
appropriate refrigerant such as "Freon 13" may be circulated through the
back 12a of the block to remove the source heat. The ends of copper side
walls 12b abut opposite ends of the back wall 12a and readily transmit
heat by conduction to the back wall 12a where it is removed by the
refrigerant. Within the outer box 12 is disposed an inner, open ended,
heat shield box indicated generally at 14, defining internally, an arc
chamber 24. As illustrated, in this embodiment, the heat shield box 14
comprises relatively thin sheets of molybdenum as at 14a and bearing on
their internal faces relatively thick layers of graphite as at 14b. Heat
shield box 14 comprises a high temperature plasma confining structure
formed of one or more materials such as graphite, tungsten, molybdenum,
tantalum, etc. The heat shield box 14 is physically mounted to the outer
box 12 by way of spacers 36 across which spans a mounting plate 38 to
which a molybdenum sheet 14a is affixed; the spacers 36 being mounted to
the outer box 14 by way of bolts or screws 40 which pass through soft iron
yokes 42 to each side of the assembly. The soft iron yokes 42 form part of
the magnetic circuit structure. The graphite layers 14b may alternatively
be formed of tantalum.
Surrounding the inner box 14 and confined by the outer box 12 is a
multipole magnet assembly indicated generally at 16 and comprised of a
plurality of spaced, open framed poles or pole pieces 20, between which
are sandwiched a plurality of aligned permanent magnets 22 separated by
the pole pieces and additionally by the soft ion yokes 42 at the internal
corners of the box assembly. The pole pieces 20 constitutes an odd number
in the instant embodiment; three to each side and three to the back of the
magnet assembly 16. The magnetic poles are preferably of an odd number,
nine being shown in the embodiment of FIG. 1, although the number of poles
could be five or three, for example. These odd number poles complete the
field around the wall of the arc chamber 24 formed by the heat shield box
14. In a typical manner, a loop filament 28, positioned within chamber 24
and borne by boron nitride insulators 28a, functions as the cathode, the
graphite layer 14b functioning as the anode and the anode and cathode
being subjected to a particular electric potential difference. In that
respect, the cathode is connected by way of leads which project through
the graphite layer 14b of the heat shield box 14. A plurality of spaced
molybdenum heat shield foils 15 are employed within vacuum space 25
through which project the pole pieces 20. The alternating pole arrangement
for the permanent magnets 22 is as shown. The pole pieces 20 may be formed
of Ni steel for example.
To complete the assembly and for ion implantation application, an ion beam
is extracted from the source via an aperture or slit 32 within a cover
plate 34, also formed of graphite and overlying the open end of the heat
shield box 14. Boron nitride insulators 44 function to electrically
isolate the graphite cover plate bearing the extraction slit 32 from the
rest of the assembly and particularly to permit the cover plate 34 to be
maintained at an electrical potential which is more negative than that of
the anode and preferably at least as negative as that of the cathode
during operation of the source. In that respect, schematically, the
filament 28 is shown being electrically connected via conductor 46 to the
graphite cover plate 34. As may be appreciated, the assembly is completed
by top and bottom walls corresponding to the illustrated portion of the
structure of the outer box 12 and inner box 14a, respectively. If the
multipole magnetic structure does not extend around the top and bottom
surfaces, then these surfaces should be insulated or be isolated from the
side walls and biased at or near cathode potential.
Tests have shown that the confinement time for an ion beam source, such as
that at 10 in the drawing, is greater than that observed in a solenoid
type source. This is contrary to simple plasma theory where one would
expect the confinement time to be longer in the solenoid type source.
However, such solenoid type source becomes noisy at low pressure and high
magnetic field, and the noise in turn causes enhanced diffusion of the
electrons to the arc chamber walls.
Applicants have determined that in order to cause the ions to flow to the
extractor in such a source, while the slit 32 should appear to be one of
the magnet poles, since a pole face cannot be placed at that location, the
source is required to have an odd number of poles (ferromagnetic pole
faces) with the missing pole appearing to be at the slit. The slit region
constitutes a virtual pole but of lower magnetic field strength in the
embodiment illustrated in FIG. 1.
The magnetic field distribution M closely hugs the interior graphite layers
14b, that is, the interior wall of the heat shield box 14 for effective
operation in the manner described previously.
While the source temperature increases and thus there is an increase in the
percentage of As.sub.2 versus As.sub.4 vapour in the source 10 with
increased efficiency in the production of the As.sup.+ beam, an aspect of
the present invention lies in the source 10 being provided with a heat
shield inner box 14 forming the chamber 24 of high temperature material
such as graphite or tantalum, this being required to adequately protect
the permanent magnets 22. Thus, the design of the magnetic poles, their
orientation and the cooling thereof is critical, all of these aspects
being provided for within the embodiments of the present invention.
The ion beam sources are particularly applicable to mass separation and
implantation, highly useful in the semiconductor industry. Where the ion
beam source runs on arsenic, the arsenic load is deemed to last longer,
the beam line and beam line components remain cleaner, and the pumping
requirements of the ion beam source are effectively reduced. Through the
utilization of longer confinement times and path lengths of the ionizing
electrons with the multipole field source as illustrated, as compared to a
conventional solenoid field type, this permits the cathode 28 to operate
at a lower discharge current and results in longer filament lifetime.
Further, it is believed that the ion beam source 10 permits an ease of
obtaining boron from boron trifluoride when such is used as the plasma
base.
FIG. 2 shows such a source configuration 10' which is preferred for the
operation with BF.sub.3 for producing B.sup.+. Reference to FIG. 2 shows a
source configuration wherein source 10' is in many ways similar in
construction and operation to that of the first embodiment. The principal
difference resides in the utilization of two filaments as at 28' which
consist of two straight rods oriented parallel to the aperture slit 32 in
cover plate 34 which overlies the open end of the inner shield box 14.
This configuration produces less stress related area over that of the
embodiment illustrated in FIG. 1 wherein the filament is essentially a
short loop filament projecting internally of the arc chamber 24.
In other respects, the multipole containment ion beam source 10'
constitutes a rectangular box assembly including in addition to the inner,
open ended heat shield box 14, the outer box 12. The heat shield box 14 is
constructed similarly to that of the first embodiment and preferably
comprises an inner layer of graphite or tantalum. Additionally and in the
manner of the prior embodiment, surrounding the inner box 14 and confined
by the outer box 12 is a multipole magnet assembly indicated generally at
16. Spaced, open frame pole pieces 18, 19 and 21 support a plurality of
bar magnets 22 of rectangular cross-section. Additionally, three pole
pieces as at 26 are separated by additional bar magnets, at the rear or
back of the assembly. Thus, as may be appreciated, on each side of the
chamber 24 are provided three pole pieces (18, 19, 21) and at the rear an
array of three pole pieces 26 of appropriate magnetic polarization as
shown, forming an odd number of magnetic poles, with the virtual pole
corresponding to slit 32 in the manner of the prior described embodiment.
In the exploded, perspective view of FIG. 2, the nature of the electrical
insulation of the graphite cover plate 34 and the electrical connection
between filaments 28' and that cover plate is as shown, and may be
accomplished in the manner of the embodiment of FIG. 1. Again, the
graphite layer for the inner heat shield box 14 may preferably form the
anode for the source 10'.
As may be appreciated, the pole pieces 18, 19 and 21 are of the so-called
"picture frame" type formed of a magnetic material with bar magnets 23 and
27 also being of open frame, rectangular form, and being comprised of
vertical and horizontal sections. In that respect, bar magnet 27 carries
holes as at 29 drilled therein through which pass leads 31 connected to
the rod filaments 28', the leads 31 being appropriately electrically
insulated from the bar magnet 27. In the embodiment of FIG. 1, bar magnets
and magnetic pole pieces may extend across the bottom of inner box 14
while in FIG. 2 the bar magnets and their magnetic field pole structure
completely surrounds all of the walls of the inner box 14, with the
exception of cover 34, wherein the virtual pole is formed. As may be
appreciated, structural aspects found in the embodiment of FIG. 2 may be
incorporated within the structure of FIG. 1 and vice versa. For instance,
the back wall 12a of the outer box 12 may be cooled in the same manner as
that described in conjunction with source 10, FIG. 1. Further, while top
and bottom walls 12c and 12d are partially illustrated in FIG. 2 with
respect to the outer box 12 in that embodiment, these walls are not shown
in the embodiment of FIG. 1, although they may be incorporated therein as
desired.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood by
those skilled in the art that various changes in form and details may be
made therein without departing from the spirit and scope of the invention.
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