U.S. patent application number 11/502695 was filed with the patent office on 2006-12-07 for method and apparatus for extracting ions from an ion source for use in ion implantation.
Invention is credited to Thomas N. Horsky, Dale Conrad Jacobson, Wade Allen Krull, Robert W. III Milgate, George P. JR. Sacco.
Application Number | 20060272776 11/502695 |
Document ID | / |
Family ID | 38834246 |
Filed Date | 2006-12-07 |
United States Patent
Application |
20060272776 |
Kind Code |
A1 |
Horsky; Thomas N. ; et
al. |
December 7, 2006 |
Method and apparatus for extracting ions from an ion source for use
in ion implantation
Abstract
Thermal control is provided for an extraction electrode of an
ion-beam producing system that prevents formation of deposits and
unstable operation and enables use with ions produced from
condensable vapors and with ion sources capable of cold and hot
operation. Electrical heating of the extraction electrode is
employed for extracting decaborane or octadecaborane ions. Active
cooling during use with a hot ion source prevents electrode
destruction, permitting the extraction electrode to be of
heat-conductive and fluorine-resistant aluminum composition. The
service lifetime of the system is enhanced by provisions for
in-situ etch cleaning of the ion source and extraction electrode,
using reactive halogen gases, and by having features that extend
the service duration between cleanings, including accurate vapor
flow control and accurate focusing of the ion beam optics. A remote
plasma source delivers F or Cl ions to the de-energized ion source
for the purpose of cleaning deposits in the ion source and the
extraction electrode. These techniques enable long equipment uptime
when running condensable feed gases such as sublimated vapors, and
are particularly applicable for use with so-called cold ion sources
and universal ion sources. Methods and apparatus are described
which enable long equipment uptime when decaborane and
octadecaborane are used as feed materials, as well as when
vaporized elemental arsenic and phosphorus are used, and which
serve to enhance beam stability during ion implantation.
Inventors: |
Horsky; Thomas N.;
(Boxborough, MA) ; Milgate; Robert W. III;
(Gloucester, MA) ; Sacco; George P. JR.;
(Topsfield, MA) ; Jacobson; Dale Conrad; (Salem,
NH) ; Krull; Wade Allen; (Marblehead, MA) |
Correspondence
Address: |
PATENT ADMINISTRATOR;KATTEN MUCHIN ROSENMAN LLP
1025 THOMAS JEFFERSON STREET, N.W.
EAST LOBBY: SUITE 700
WASHINGTON
DC
20007-5201
US
|
Family ID: |
38834246 |
Appl. No.: |
11/502695 |
Filed: |
August 11, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11452003 |
Jun 12, 2006 |
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11502695 |
Aug 11, 2006 |
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PCT/US04/41525 |
Dec 9, 2004 |
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11452003 |
Jun 12, 2006 |
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60529343 |
Dec 12, 2003 |
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Current U.S.
Class: |
156/345.37 |
Current CPC
Class: |
H01J 37/08 20130101;
H01J 2237/0812 20130101; C23C 14/48 20130101; H01J 27/024 20130101;
H01J 2237/022 20130101; H01J 9/38 20130101; C23C 14/564 20130101;
H01L 21/265 20130101; H01J 2237/006 20130101; H01J 2237/083
20130101; H01J 2237/31701 20130101; H01J 2209/017 20130101; H01J
37/3171 20130101 |
Class at
Publication: |
156/345.37 |
International
Class: |
H01L 21/306 20060101
H01L021/306 |
Claims
1-35. (canceled)
36. A heated cathode assembly for use in an ion source, the heated
cathode assembly comprising: a suppression cathode; a ground
cathode disposed adjacent to said suppression cathode and
electrically insulated therefrom forming an extraction cathode; and
a heater for heating said extraction cathode.
37. The heated cathode assembly as recited in claim 36, wherein at
least one of said suppression cathode and said ground cathode is
formed from a fluorine resistant material.
38. The heated cathode assembly as recited in claim 37, wherein
said fluorine resistant material is aluminum.
39. The heated cathode assembly as recited in claim 37, wherein
said fluorine resistant material is a refractory material.
40. The heated cathode assembly as recited in claim 39, wherein
said refractory material is molybdenum
41. The heated cathode assembly as recited in claim 36, wherein
said heater is disposed in contact with said ground cathode.
42. The heated cathode assembly as recited in claim 36, wherein
said assembly is configured so that said suppression cathode is
heated by radiant heat energy.
43. The heated cathode assembly as recited in claim 36, further
including a temperature sensing device for sensing the temperature
of the extraction cathode.
44. The heated cathode assembly as recited in claim 43, wherein
said temperature sensing device is a thermocouple.
45. The heated cathode assembly as recited in claim 44, further
including a temperature controller for controlling the temperature
of the cathode assembly.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation-in part of International
Patent Application No. PCT/US2004/041525, filed on Dec. 9, 2004,
which, in turn, claims priority to and claims the benefit of U.S.
Patent Application No. 60/529,343, filed on Dec. 12, 2003.
FIELD OF THE INVENTION
[0002] The present invention relates to producing ion beams in
which one or more gaseous or vaporized feed materials is ionized in
an ion source from which the ions are extracted by an extraction
electrode. It also relates to a method and apparatus for operating
an ion source and extraction electrode to produce an ion beam for
ion implantation of semiconductor substrates and substrates for
flat panel displays. In particular the invention concerns extension
of the productive time (i.e. the "uptime") of systems that produce
ion beams and to maintaining stable ion-extraction conditions
during the productive time.
BACKGROUND
[0003] Ion beams are produced from ions extracted from an ion
source. An ion source typically employs an ionization chamber
connected to a high voltage power supply. The ionization chamber is
associated with a source of ionizing energy, such as an arc
discharge, energetic electrons from an electron-emitting cathode,
or a radio frequency or microwave antenna, for example. A source of
desired ion species is introduced into the ionization chamber as a
feed material in gaseous or vaporized form where it is exposed to
the ionizing energy. Extraction of resultant ions from the chamber
through an extraction aperture is based on the electric charge of
the ions. An extraction electrode is situated outside of the
ionization chamber, aligned with the extraction aperture, and at a
voltage below that of the ionization chamber. The electrode draws
the ions out, typically forming an ion beam. Depending upon desired
use, the beam of ions may be mass analyzed for establishing mass
and energy purity, accelerated, focused and subjected to scanning
forces. The beam is then transported to its point of use, for
example into a processing chamber. As the result of the precise
energy qualities of the ion beam, its ions may be implanted with
high accuracy at desired depth into semiconductor substrates.
[0004] The precise qualities of the ion beam can be severely
affected by condensation and deposit of the feed material or of its
decomposition products on surfaces of the ion beam-producing
system, and in particular surfaces that affect ionization, ion
extraction and acceleration.
The Ion Implantation Process
[0005] The conventional method of introducing a dopant element into
a semiconductor wafer is by introduction of a controlled energy ion
beam for ion implantation. This introduces desired impurity species
into the material of the semiconductor substrate to form doped (or
"impurity") regions at desired depth. The impurity elements are
selected to bond with the semiconductor material to create
electrical carriers, thus altering the electrical conductivity of
the semiconductor material. The electrical carriers can either be
electrons (generated by N-type dopants) or "holes" (i.e., the
absence of an electron), generated by P-type dopants. The
concentration of dopant impurities so introduced determines the
electrical conductivity of the doped region. Many such N-- and
P-type impurity regions must-be created to form transistor
structures, isolation structures and other such electronic
structures, which collectively function as a semiconductor
device.
[0006] To produce an ion beam for ion implantation, a gas or vapor
feed material is selected to contain the desired dopant element.
The gas or vapor is introduced into the evacuated high voltage
ionization chamber while energy is introduced to ionize it. This
creates ions which contain the dopant element (for example, in
silicon the elements As, P, and Sb are donors or N-type dopants,
while B and In are acceptors or P-type dopants). An accelerating
electric field is provided by the extraction electrode to extract
and accelerate the typically positively charged ions out of the
ionization chamber, creating the desired ion beam. When high purity
is required, the beam is transported through mass analysis to
select the species to be implanted, as is known in the art. The ion
beam is ultimately transported to a processing chamber for
implantation into the semiconductor wafer.
[0007] Similar technology is used in the fabrication of flat-panel
displays (FPD's) which incorporate on-substrate driver circuitry to
operate the thin-film transistors which populate the displays. The
substrate in this case is a transparent panel such as glass to
which a semiconductor layer has been applied. Ion sources used in
the manufacturing of FPD's are typically physically large, to
create large-area ion beams of boron, phosphorus and
arsenic-containing materials, for example, which are directed into
a chamber containing the substrate to be implanted. Most FPD
implanters do not mass-analyze the ion beam prior to its reaching
the substrate.
Ion Contamination
[0008] In general, ion beams of N-type dopants such as P or As
should not contain any significant portion of P-type dopant ions,
and ion beams of P-type dopants such as B or In should not contain
any significant portion of N-type dopant ions. Such a condition is
called "cross-contamination" and is undesirable.
Cross-contamination can occur when source feed materials accumulate
in the ion source, and the source feed material is then changed,
for example, when first running elemental phosphorus feed material
to generate an N-type P.sup.+ beam, and then switching to BF.sub.3
gas to generate a P-type BF.sub.2.sup.+ beam.
[0009] A serious contamination effect occurs when feed materials
accumulate within the ion source so that they interfere with the
successful operation of the source. Such a condition invariably has
called for removal of the ion source and the extraction electrode
for cleaning or replacement, resulting in an extended "down" time
of the entire ion implantation system, and consequent loss of
productivity.
[0010] Many ion sources used in ion implanters for device wafer
manufacturing are "hot" sources, that is, they operate by
sustaining an arc discharge and generating a dense plasma; the
ionization chamber of such a "hot" source can reach an operating
temperature of 800 C. or higher, in many cases substantially
reducing the accumulation of solid deposits. In addition, the use
of BF.sub.3 in such sources to generate boron-containing ion beams
further reduces deposits, since in the generation of a BF.sub.3
plasma, copious amounts of fluorine ions are generated; fluorine
can etch the walls of the ion source, and in particular, recover
deposited boron through the chemical production of gaseous
BF.sub.3. With other feed materials, however, detrimental deposits
have formed in hot ion sources. Examples include antimony (Sb)
metal, and solid indium (In), the ions of which are used for doping
silicon substrates.
[0011] Cold ion sources, for example the RF bucket-type ion source
which uses an immersed RF antenna to excite the source plasma (see,
for example, Leung et al., U.S. Pat. No. 6,094,012, herein
incorporated by reference), are used in applications where either
the design of the ion source includes permanent magnets which must
be kept below their Curie temperature, or the ion source is
designed to use thermally-sensitive feed materials which break down
if exposed to hot surfaces, or where both of these conditions
exist. Cold ion sources suffer more from the deposition of feed
materials than do hot sources. The use of halogenated feed
materials for producing dopants may help reduce deposits to some
extent, however, in certain cases, non-halogen feed materials such
as hydrides are preferred over halogenated compounds. For
non-halogen applications, ion source feed materials such as gaseous
B.sub.2H.sub.6, AsH.sub.3, and PH.sub.3 are used. In some cases,
elemental As and P are used, in vaporized form. The use of these
gases and vapors in cold ion sources has resulted in significant
materials deposition and has required the ion source to be removed
and cleaned, sometimes frequently. Cold ion sources which use
B.sub.2H.sub.6 and PH.sub.3 are in common use today in FPD
implantation tools. These ion sources suffer from
cross-contamination (between N-- and P-type dopants) and also from
particle formation due to the presence of deposits. When
transported to the substrate, particles negatively impact yield.
Cross-contamination effects have historically forced FPD
manufacturers to use dedicated ion implanters, one for N-type ions,
and one for P-type ions, which has severely affected cost of
ownership.
Borohydrides
[0012] Borohydride materials such as B.sub.10H.sub.14 (decaborane)
and B.sub.18H.sub.22 (octadecaborane) have attracted interest as
ion implantation source materials. Under the right conditions,
these materials form the ions B.sub.10H.sub.x.sup.+,
B.sub.10H.sub.x.sup.-, B.sub.18H.sub.x.sup.+, and
B.sub.18H.sub.x.sup.-. When implanted, these ions enable very
shallow, high dose P-type implants for shallow junction formation
in CMOS manufacturing. Since these materials are solid at room
temperature, they must be vaporized and the vapor introduced into
the ion source for ionization. They are low-temperature materials
(e.g., decaborane melts at 100 C., and has a vapor pressure of
approximately 0.2 Torr at room temperature; also, decaborane
dissociates above 350 C.), and hence must be used in a cold ion
source. They are fragile molecules which are easily dissociated,
for example, in hot plasma sources.
Contamination Issues of Borohydrides
[0013] Boron hydrides such as decaborane and octadecaborane present
a severe deposition problem when used to produce ion beams, due to
their propensity for readily dissociating within the ion source.
Use of these materials in Bernas-style arc discharge ion sources
and also in electron-impact ("soft") ionization sources, have
confirmed that boron-containing deposits accumulate within the ion
sources at a substantial rate. Indeed, up to half of the
borohydride vapor introduced into the source may stay in the ion
source as dissociated, condensed material. Eventually, depending on
the design of the ion source, the buildup of condensed material
interferes with the operation of the source and necessitates
removal and cleaning of the ion source.
[0014] Contamination of the extraction electrode has also been a
problem when using these materials. Both direct ion beam strike and
condensed vapor can form layers that degrade operation of the ion
beam formation optics, since these boron-containing layers appear
to be electrically insulating. Once an electrically insulating
layer is deposited, it accumulates electrical charge and creates
vacuum discharges, or so-called "glitches", upon breakdown. Such
instabilities affect the precision quality of the ion beam and can
contribute to the creation of contaminating particles.
SUMMARY OF THE INVENTION
[0015] Objects of this invention are to provide a method and
apparatus for producing ions beams without disturbance in the
stability of the ion beam by electric discharges at the extraction
electrode and to provide a method and apparatus for producing an
ion beam which increases service lifetime and reduces equipment
down time.
[0016] The invention features an extraction electrode for
extracting ions from the ion source in which the electrode includes
an active thermal control system.
[0017] The invention also features in-situ cleaning procedures and
apparatus for an ion source and associated extraction electrodes
and similar components of the ion-beam producing system, which
periodically chemically remove deposits, increasing service
lifetime and performance, without the need to disassemble the
system.
[0018] The invention also features an actively heated ion
extraction electrode which consists of a material which reduces the
frequency and occurrence of electrical discharges, preferably this
material being a metal.
[0019] Another feature is, in general, heating an extraction
electrode above the condensation temperature of feed materials to
an ion source, in preferred cases the electrode being comprised of
metal, preferably aluminum or molybdenum.
[0020] The invention also features an ion extraction electrode
comprised of aluminum, suitable for in situ reactive gas cleaning.
Preferred embodiments include provisions for active temperature
control of the extraction electrode adapted to the type of ion
source with which the electrode is constructed to operate.
Embodiments feature active heating of the extraction electrode for
operation with cool-operating ion sources, active cooling of the
extraction electrode for operation with hot-operating ion sources,
and both active heating and cooling of the extraction electrode for
selective operation with cool and hot-operating ion sources.
[0021] These and other innovations of the invention may include one
or more of the following features:
[0022] A supply of a reactive gas is provided and introduced into
the ion source, and the ion source and extraction electrode are
cleaned in situ through exposure to reactive products from that
supply such as atomic fluorine, F, or molecular fluorine, F.sub.2;
the atomic or molecular fluorine is injected into the ion source
from a remote plasma source; the ion source and extraction
electrode are cleaned through exposure to gaseous ClF.sub.3 flowing
from a remote supply; reactive components of the ionization
apparatus are shielded from reactive gas during the cleaning phase
of operation; the ion source is fabricated of aluminum; the
extraction electrode is fabricated of aluminum; the front face of
the extraction electrode is devoid of sharp or rough features; the
plates of the extraction electrode are actively temperature
controlled; the plates of the extraction electrode are actively
heated; heating of the extraction electrode is radiative or is
resistive; the plates of the extraction electrode in other
situations are actively cooled.
[0023] Another feature is the use of the features described with
apparatus suitable to form "cluster" or "molecular" ion beams, of
feed material that is particularly subject to thermal breakdown and
deposit.
[0024] While most ion implantation experts would agree that the use
of borohydrides to form "cluster" ion beams such as
B.sub.10H.sub.x.sup.+ and B.sub.18H.sub.x.sup.+ is very attractive
for shallow junction formation, means to ionize and transport these
large molecules have presented problems. For example, U.S. Pat.
Nos. 6,288,403 and 6,452,338 describe ion sources which have
successfully produced decaborane ion beams. However, such
decaborane ion sources have been found to exhibit particularly
short service life as compared to other commercial ion sources used
in ion implantation. This short service life has been primarily due
to the accumulation of boron-containing deposits within the ion
source, and the deposition of insulating coatings on the ion
extraction electrode, which has lead to beam instabilities
requiring implanter shut down and maintenance.
[0025] According to another feature, means are provided to
substantially reduce the deposition of such deposits in the
borohydride ion source and on the ion extraction electrode, and
means are provided to clean deposits on these components without
removing them from the ion implanter, i.e., in-situ. This invention
enables the commercial use of borohydride cluster beams in
semiconductor manufacturing with long service lifetime.
[0026] A particular aspect of the invention is a system for
generating an ion beam comprising an ion source in combination with
an actively temperature-controlled extraction electrode and a
reactive gas cleaning system, the ion source comprising an
ionization chamber connected to a high voltage power supply and
having an inlet for gaseous or vaporized feed materials, an
energizeable ionizing system for ionizing the feed material within
the ionization chamber and an extraction aperture that communicates
with a vacuum housing, the vacuum housing evacuated by a vacuum
pumping system, the extraction electrode disposed in the vacuum
housing outside of the ionization chamber, aligned with the
extraction aperture of the ionization chamber and adapted to be
maintained at a voltage below that of the ionization chamber to
extract ions through the aperture from within the ionization
chamber, and the reactive gas cleaning system operable when the
ionization chamber and ionizing system are de-energized to provide
a flow of reactive gas through the ionization chamber and through
the ion extraction aperture to react with and remove deposits on at
least some of the surfaces of the ion generating system.
[0027] Preferred embodiments of this aspect have one or more of the
following features.
[0028] The system is constructed for use in implanting ions in
semiconductor wafers, the ionization chamber having a volume less
than about 100 ml and an internal surface area of less than about
200 cm.sup.2.
[0029] The system is constructed to produce a flow of the reactive
gas into the ionization chamber at a flow rate of less than about 2
Standard Liters Per Minute.
[0030] The extraction electrode is constructed to produce a beam of
accelerated ions suitable for transport to a point of
utilization.
[0031] The extraction electrode is located within a path of
reactive gas moving from the extraction aperture to the vacuum
pumping system so that the extraction electrode is cleaned by the
reactive gas.
[0032] The extraction electrode is associated with a heater to
maintain the electrode at elevated temperature during extraction by
the extraction electrode of ions produced in the ionization
chamber, e.g. above the condensation temperature, below the
disassociation temperature, of solid-derived, thermally sensitive
vapors.
[0033] The extraction electrode is associated with a cooling
device, e.g. when the electrode is formed of thermally sensitive
material and is used with a hot ion source.
[0034] The extraction electrode has a smooth, featureless
aspect.
[0035] The reactive gas cleaning system comprises a plasma chamber,
the plasma chamber arranged to receive a feed gas capable of being
disassociated by plasma to produce a flow of reactive gas through a
chamber outlet, and a conduit for transporting the reactive gas to
the ionization chamber.
[0036] The plasma chamber is constructed and arranged to receive
and disassociate a compound capable of being disassociated to
atomic fluorine, for instance NF.sub.3, C.sub.3F.sub.8 or
CF.sub.4.
[0037] The reactive gas cleaning system is constructed and arranged
to share a service facility associated with the ion source.
[0038] The system is constructed to direct an ion beam through a
mass analyzer, in which the reactive gas cleaning system is
constructed and arranged to share a service facility with the mass
analyzer.
[0039] The reactive gas cleaning system comprises a conduit from a
container of pressurized reactive gas, for instance C1F.sub.3.
[0040] The system is in combination with an end-point detection
system adapted to at least assist in detecting substantial
completion of reaction of the reactive gas with contamination on a
surface of the system for generating an ion beam.
[0041] The end point detection system comprises an analysis system
for the chemical makeup of gas that has been exposed to the surface
during operation of the reactive gas cleaning system.
[0042] A temperature detector is arranged to detect substantial
termination of an exothermic reaction of the reactive gas with
contamination on a surface of the system.
[0043] The energizeable ionizing system includes a component within
or in communication with the ionization chamber that is susceptible
to harm by the reactive gas and means are provided to shield the
component from reactive gas flowing through the system.
[0044] The means to shield the component comprises an arrangement
for producing a flow of inert gas, such as argon, past the
component.
[0045] The means for shielding a component comprises a shield
member that is impermeable to the reactive gas.
[0046] The system is constructed to operate with reactive halogen
gas as the reactive gas and the extraction electrode and associated
parts comprise aluminum (Al) or alumina (Al.sub.2O.sub.3).
[0047] The ion source is constructed to produce ions within the
ionization chamber via an arc-discharge, an RF field, a microwave
field or an electron beam.
[0048] The system is associated with a vaporizer of condensable
solid feed material for producing feed vapor to the ionization
chamber.
[0049] The ion source is constructed to vaporize feed material
capable of producing cluster or molecular ions, and the ionization
system is constructed to ionize the material to form cluster or
molecular ions for implantation.
[0050] The vacuum housing of the system is associated with a
pumping system comprising a high vacuum pump capable of producing
high vacuum and a backing pump capable of producing vacuum, the
high vacuum pump operable during operation of the ion source, and
being capable of being isolated from the vacuum housing during
operation of the reactive cleaning system, the backing pump
operable during operation of the reactive gas cleaning system.
[0051] The system is associated with an ion implantation apparatus,
the apparatus constructed to transport ions following the
extraction electrode implantation station within a vacuum chamber.
In preferred embodiments an isolation valve is included for
isolating the implantation station from the ionization chamber and
the extraction electrode during operation of the reactive gas
cleaning system.
[0052] The ion source is constructed and adapted to generate dopant
ions for semiconductor processing, and the reactive gas cleaning
system is adapted to deliver fluorine, F, or chlorine, Cl, ions to
the ionization chamber or the extraction electrode for cleaning
deposits from a surface.
[0053] The ion source is adapted to be temperature-controlled to a
given temperature.
[0054] The ion source is adapted to generate a boron-containing ion
beam; in preferred embodiments the boron-containing ion beam is
generated by feeding vaporized borohydride material into the ion
source, especially either decaborane, B.sub.10H.sub.14 or
octadecaborane, B.sub.18H.sub.22.
[0055] The ion source is adapted to generate arsenic-containing ion
beams.
[0056] The ion source is adapted to generate phosphorus-containing
ion beams.
[0057] The ionization chamber of the ion source comprises
aluminum.
[0058] The ionization chamber of the ion source or the extraction
electrode comprises a material resistant to attack by halogen gases
such as fluorine, F.
[0059] Another particular aspect of the invention is a method of
in-situ cleaning using the system of any of the foregoing
description, or of an ion source and temperature-controlled ion
extraction electrode associated with an ion implanter, in which
reactive halogen gas is flowed into an ion source while the ion
source and ion extraction electrode are de-energized and under
vacuum.
[0060] Embodiments of this aspect have one or more of the following
features.
[0061] The reactive halogen gas is fluorine, F.
[0062] The reactive halogen gas is chlorine, Cl.
[0063] The fluorine gas is introduced into the ion source from a
remote plasma source.
[0064] The fluorine gas is produced in the remote plasma source by
an NF.sub.3 plasma.
[0065] The fluorine gas is produced in the remote plasma source by
a C.sub.3F.sub.8 or CF.sub.4 plasma.
[0066] The reactive halogen gas is ClF.sub.3.
[0067] The cleaning procedure is conducted to remove deposits after
the ion source has ionized decaborane, B.sub.10H.sub.14.
[0068] The cleaning procedure is conducted to remove deposits after
the ion source has ionized octadecaborane, B.sub.18H.sub.22.
[0069] The cleaning procedure is conducted to remove deposits after
the ion source has ionized arsenic-containing compounds, such as
arsine, AsH.sub.3, or elemental arsenic, As.
[0070] The cleaning procedure is conducted to remove deposits after
the ion source has ionized phosphorus-containing compounds, such as
elemental phosphorus, P, or phosphine, PH.sub.3.
[0071] The cleaning procedure is conducted to remove deposits after
the ion source has ionized antimony-containing compounds, such as
trimethylantimony, Sb(CH.sub.4).sub.3, or antimony pentaflouride,
SbF.sub.5.
[0072] The cleaning procedure is conducted for an ion source in
situ in an ion implanter between changing ion source feed materials
in order to implant a different ion species.
[0073] Another particular aspect of the invention is an ion
implantation system having an ion source and an extraction
electrode for extracting ions from the ion source, in which the
extraction electrode includes a heater constructed to maintain the
electrode at an elevated temperature sufficient to substantially
reduce condensation on the electrode of gases or vapors being
ionized and products produced therefrom. Another aspect is such an
extraction electrode, per se, useful in such system.
[0074] Embodiments of these aspects have one or more of the
following features.
[0075] The electrode comprises aluminum.
[0076] The electrode comprises molybdenum.
[0077] The electrode is heated by radiative heating.
[0078] The electrode is heated by a resistive heating element).
[0079] The temperature of the electrode is controlled to a desired
temperature; in embodiments the temperature is between 150 C. and
250 C.
[0080] The electrode is periodically cleaned in situ by exposure to
reactive halogen-containing gas.
[0081] The electrode comprises at least two electrode elements
constructed and arranged in close succession along a beam path from
the ion source, the electrode elements having elongated, slot-form
beam apertures through which a ribbon-like ion beam passes, the
heater including heater portions disposed on each of the long sides
of the slot-form apertures. In some preferred forms at least one
electrode element comprises an inner portion defining its beam
aperture and an outer portion in heat conductive relation to the
inner portion, the outer portion defining a heat receptor face for
absorbing radiated heat. In some preferred forms, at least one of
the electrode elements comprises a portion that defines its beam
aperture, this portion being exposed to be a receptor for absorbing
radiated heat.
[0082] The heater comprises a continuous electrical resistance
heating element. In preferred forms this heating element is
arranged to heat multiple electrode elements by radiative heating.
Preferably the heating element is sealed within a protective tube
to form a tubular heater, the tube constructed to be heated
internally by the heating element and the tube exposed to heat the
electrode elements by radiative heating, and preferably the tube
being of circular configuration and disposed to surround beam-path
defining portions of the electrode elements.
[0083] In preferred forms an electrode element comprises an inner
portion defining its beam aperture and an outer portion in heat
conductive relation to the inner portion, the outer portion
defining a heat receptor face for absorbing radiated heat, a
tubular heater surrounding the beam path and opposed to the
receptor face in radiant heating relationship. In one preferred
form, a pair of electrode elements of the extraction electrode each
comprises an inner portion defining its beam aperture and an outer
portion in heat conductive relation to the inner portion, the outer
portions defining heat receptor faces for absorbing radiated heat,
the tubular heater disposed between, and in radiant heating
relationship to the receptor faces of these two electrode elements.
This arrangement may be employed in a two-electrode element
configuration or in a configuration having more than two electrode
elements. In one preferred form, besides the pair of electrode
elements there is at least a third electrode element disposed
between the pair, the third electrode element comprising a portion
that defines its beam aperture, this portion exposed to be a heat
receptor for heat radiating radially inwardly from the surrounding
tubular heater.
[0084] Another aspect of the invention is a method of in-situ
cleaning of an ion extraction electrode of any of the systems
described or a temperature-controlled ion extraction electrode
which is associated with an ion implanter, in which reactive
halogen gas is flowed over the ion extraction electrode while the
electrode is in situ and under vacuum. Another aspect is a
temperature-controlled ion extraction electrode constructed for in
situ cleaning by such gas.
[0085] Embodiments of this aspect have one ore more of the
following features.
[0086] The reactive halogen gas is fluorine, F or chlorine, Cl.
[0087] Fluorine gas is introduced from a remote plasma source into
a vacuum housing in which the extraction electrode resides.
[0088] Fluorine gas is produced in the remote plasma source by a
NF.sub.3 plasma.
[0089] Fluorine gas is produced in the remote plasma source by a
C.sub.3F.sub.8 or CF.sub.4 plasma.
[0090] The reactive gas is ClF.sub.3.
[0091] The cleaning procedure is conducted to remove deposits after
the ion source has ionized decaborane, B.sub.10H.sub.14.
[0092] The cleaning procedure is conducted to remove deposits after
the ion source has ionized octadecaborane, B.sub.18H.sub.22.
[0093] The cleaning procedure is conducted to remove deposits after
the ion source has ionized arsenic-containing compounds, such as
arsine, AsH.sub.3, or elemental arsenic, As.
[0094] The cleaning procedure is conducted to remove deposits after
the ion source has ionized phosphorus-containing compounds, such as
elemental phosphorus, P, or phosphine, PH.sub.3.
[0095] The cleaning procedure is conducted between changing ion
source feed materials in order to implant a different ion
species.
Ion Source and Ion Extraction Electrode Provided With In-situ Etch
Cleaning
[0096] According to a preferred embodiment, the in situ chemical
cleaning process utilizes atomic F gas, to effectively clean
deposits from the ion source and from the ion extraction electrode,
while the ion source and extraction electrode remain installed in
the ion beam-producing system. In a preferred embodiment an
electron impact ion source with cooled chamber walls is employed.
Preferably, the ionization chamber and source block and the
extraction electrode, comprise aluminum, i.e. are fabricated of
aluminum or of an aluminum containing alloy, enabling aluminum
fluoride to be created on the aluminum surfaces to act as a
passivating layer, that prevents further chemical attack by F.
Insulators of the assembly are preferably formed of alumina
(Al.sub.2 O.sub.3) which is also resistant to attack by F.
[0097] One embodiment of this feature uses the outlet of a remote
reactive gas source directly coupled to an inlet to the ion
source.
[0098] In a preferred embodiment the reactive gas source is a
plasma source which introduces an etch feed gas, such as NF.sub.3
or C.sub.3F.sub.8, into a supplemental ionization chamber. By
sustaining a plasma in the supplemental chamber, reactive gases
such as F and F.sub.2 are produced, and these reactive gases,
introduced to the main ion source, chemically attack the deposited
materials. By-products released in the gas phase are drawn through
the extraction aperture of the ionization chamber, past the
extraction electrode, and are pumped away by the vacuum system of
the installation, cleaning the chamber and the ion extraction
electrode.
Deposition Model
[0099] It is a generally observed principle of physics that when
two objects interact, there can be more than one outcome.
Furthermore, one can assign probabilities or likelihoods to each
outcome such that, when all possible outcomes are considered, the
sum of their individual probabilities is 100%. In atomic and
molecular physics such possible outcomes are sometimes called
"channels" and the probability associated with each interaction
channel is called a "cross section". More precisely, the likelihood
of two particles (say, an electron and a gas molecule) interacting
with each other at all is the "total cross section", while the
likelihoods of certain types of interactions (such as the
interaction represented by the electron attaching itself to the gas
molecule thus forming a negative ion, or by removing an electron
from the gas molecule thus forming a positive ion, or by
dissociating the molecule into fragments, or by elastically
scattering from the molecule with no chemical change of the
molecule) are the "partial cross sections".
[0100] This state of affairs can be represented by a mathematical
relation which expresses the total cross section AT as the sum of
its i partial cross sections:
.sigma..sub.T=.sigma..sub.1+.sigma..sub.2+.sigma..sub.3+. . .
.sigma..sub.i, or (1) .sigma..sub.T=.SIGMA..sigma..sub.i. (2) The
ion sources used in ion implanters typically display modest
ionization fractions. That is, only a small fraction (from a few
per cent to a few tens of per cent) of the gas or vapor fed into
the ion source is ionized. The rest of the gas or vapor typically
leaves the source in the gas phase, either in its original state or
in some other neutral state. That is, the ionization cross section
is much smaller than the total cross section. Of course, some of
the gas components can stay in the ion source as deposited
materials, although this tends to be a small percentage of the
total for the commonly used implantation feed materials. While feed
materials vaporized by heating such as elemental As or P more
readily produce deposits than do normally gaseous feed materials,
the heated vapor tends to stay in the gas phase if the walls of the
ion source are at a higher temperature than the vaporizer, and do
not pose a severe deposition risk. However, significant detrimental
deposits may still be produced when producing boron beams from
gaseous BF3 feed gas, for example, as well as beam from In and
Sb.
[0101] Also, in general, over time, deposits of condensable
materials do occur on the extraction electrode and on certain other
components of ion producing systems, affecting their operational
life before disassembly and cleaning.
[0102] Furthermore, in the case of the borohydrides, the total
cross section representing all interactions with the ionizing
medium (i.e., electrons in the ion source) appears large, the
ionization cross section is small, and by far the largest cross
section represents the channel for dissociation of the borohydride
molecules into non-volatile fragments, which then remain on
surfaces in the ion source. The problem of deposition of these
fragments is adversely influenced by cooling of the ionization
chamber walls in an effort to reduce thermal decomposition of the
feed material. In sum, it appears that deposition from borohydrides
of boron-containing fragments in the source is a fundamental
phenomenon which would be observed in any type of ion source acting
on this material, and solution to the problem is of broad, critical
interest to the semiconductor manufacturing industry. It is also
found that contamination of the ion extraction electrode with
insulative deposits is a problem with borohydrides, as described
more fully below.
Electron Impact Ion Source Suitable For Borohydrides
[0103] An ion source particularly suitable for borohydrides is an
electron-impact ion source which is fully temperature-controlled
(see U.S. Pat. Nos. 6,452,338 and 6,686,595; also International
Application no. PCT/IUS03/20197, each herein incorporated by
reference); also see FIG. 7. Instead of striking an arc-discharge
plasma to create ions, the ion source uses a "soft" electron-impact
ionization of the process gas by energetic electrons injected in
the form of one or more focused electron beams. This "soft"
ionization process preserves these large molecules so that ionized
clusters are formed. As seen in FIG. 7, solid borohydride material
is heated in a vaporizer and the vapor flows through a vapor
conduit to a metal chamber, i.e., the ionization chamber. An
electron gun located external to the ionization chamber delivers a
high-current stream of energetic electrons into the ionization
chamber; this electron stream is directed roughly parallel and
adjacent to an extended slot in the front of the chamber. Ions are
extracted from this slot by an ion extraction electrode, forming an
energetic ion beam. During transport of the sublimated borohydride
vapor to the ionization chamber all surfaces are held at a higher
temperature than that of the vaporizer (but well below the
temperature of dissociation), to prevent condensation of the vapor.
Many hours of testing have confirmed that the surfaces of the vapor
feed and valves indeed remain clean when such temperature control
is implemented.
Extension of Ion Source Lifetime with Decaborane Between
Cleanings
[0104] The impact of vapor flow rate on source lifetime
(maintenance interval) was studied in a quantitative manner. The
electron impact ion source was run continuously with decaborane
feed material under controlled conditions at a given vapor flow,
until it was determined that the buildup of material was causing a
significant decrease in decaborane beam current. Five different
flow rates were tested, ranging from about 0.40 sccm to 1.2 sccm.
This resulted in mass-analyzed decaborane beam currents
(B.sub.10H.sub.x.sup.+) ranging from about 150 .mu.A to 700 .mu.A.
It is noted that typical feed gas flows in ion sources used in ion
implantation range from 1 to about 3 sccm, so this test range is
considered a "low" flow regime.
[0105] The results of these lifetime tests are summarized in FIG.
5. It suggests a simple model, a hyperbolic function. This is not
unexpected; if one assumes zero vapor flow, then the source
lifetime would, in principle, diverge; and if one assumes very high
vapor flow, then source lifetime would decrease asymptotically to
zero. Thus, the model can be expressed by: (flow rate).times.(flow
duration)=constant. (3)
[0106] Equation (3) simply states that lifetime (i.e., flow
duration) is inversely proportional to flow rate; the constant is
the amount of deposited material. If equation (3) is accurate, then
the fraction of deposited material is independent of the rate of
flow of material, which is consistent with our model describing a
fixed cross section for dissociation and subsequent deposition.
These data show that, using the electron impact ion source with
about 0.5 sccm decaborane vapor flow, dedicated decaborane
operation can be sustained for more than 100 hours. While this is
acceptable in many cases, in commercial semiconductor fabrication
facilities, source lifetimes of well over 200 hours are desired.
When the ion source is used in conjunction with the novel in-situ
cleaning procedure of the present invention, greatly extended
source lifetimes are achieved. The in situ cleaning includes
cleaning the ion extraction electrode assembly as has been fully
described below.
Advantages of Certain Features of in-situ Ion Source and Ion
Extraction Electrode Chemical Cleaning
[0107] There are several very important advantages to using a
supplemental ion source to produce the reactive gas for in situ
cleaning of the ion source and the ion extraction electrode. Such
plasma sources have been developed for effluent removal
applications from process exhaust systems (such as the Litmus 1501
offered by Advanced Energy, Inc.), and for cleaning large CVD
process chambers (such as the MKS Astron reactive gas generator),
but to the inventors' knowledge it has not been previously
recognized that a remote reactive gas generator could be usefully
applied to in situ cleaning the ionization chamber of an ion source
and the extraction electrode used to generate an ion beam. Remote
reactive gas generators such as the MKS Astron have been used to
clean process chambers (i.e., relatively large vacuum chambers
wherein semiconductor wafers are processed), an application which
uses high flows of feed gas (several Standard Liters per Minute
(SLM)), and high RF power applied to the plasma source (about 6
kW). The system of the present invention can employ a much more
modest feed gas rate, e.g. less than about 0.5 SLM of NF.sub.3, and
much less RF power (less than about 2.5 kW), for the very small
volume of the ionization chamber of the ion source being cleaned
(the volume of the ionization chamber for an implanter of
semiconductor wafers is typically less than about 100 ml, e.g. only
about 75 ml, with a surface area of less than about 200 cm.sup.2,
e.g. about 100 cm.sup.2). The reactive gas flow into the ionization
chamber is less than about 2 Standard Liters Per Minute.
[0108] One might think it strange to use an external ion source to
generate plasma by-products to introduce into the main ion source
of the system; why not just introduce the (e.g., NF.sub.3) gas
directly into the main ion source to create the plasma by-products
within that source directly? The reasons seem not obvious. In order
to achieve etch rates which far exceed deposition rates from the
feed gas during a small fraction of the uptime (productive period)
of the ion implantation system, it is found that the reactive gas
must be produced and introduced at relatively very high flows into
the small ionization chamber, e.g., flows on the order of 10.sup.2
to 10.sup.3 sccm, compared to typical feed flow rates for the main
ion source for ion implantation in the range of 1-3 sccm; such high
flows would raise the pressure within the ionization chamber of the
main ion source far beyond that for which it is designed to operate
for ion implantation. Furthermore, sustaining a high-density
NF.sub.3 plasma within the main ion source would etch away
sensitive components, such as hot tungsten filaments. This is
because halogen gases etch refractory metals at a high rate which
increases exponentially with temperature. (For example, Rosner et
al. propose a model for F etching of a tungsten substrate: Rate
(microns/min)=2.92.times.10.sup.-14T.sup.1/2N.sub.Fe.sup.-3900/T,
(4) Where N.sub.F is the concentration of fluorine in atoms per
cm.sup.3, and T is the substrate temperature in degrees
Kelvin.)
[0109] Since virtually all ion sources for ion implantation
incorporate hot filaments, and since in many cases the ion source
chambers are also made of refractory metals such as Mo and W, or
graphite (which is aggressively attacked by F), these ion sources
would quickly fail under high temperature operating conditions,
making the etch cleaning process unusable.
[0110] In the presently preferred embodiment, atomic fluorine is
caused to enter the cold ionization chamber of the de-energized
main ion source at a flow rate of 100 sccm or more, and the total
gas flow into the ionization chamber is 500 sccm or more. Under
these conditions, the gas pressure within the ionization chamber is
about 0.5 Torr, while the pressure within the vacuum source housing
of the implanter is a few tens of milliTorr or more. In a preferred
mode of operation, preceding the cleaning phase, an isolation valve
is closed between the vacuum housing of the ion source and the
implanter vacuum system, and the turbo-molecular pump of the ion
source is isolated. The housing of the ion source, including the
space containing the ion extraction electrode, is then pumped with
high-capacity roughing pumps of the vacuum system (i.e., the pumps
which normally back the turbomolecular pumps and evacuate the
vacuum system down to a "rough" vacuum).
[0111] A different embodiment of a related etch clean process,
shown in FIG. 5, utilizes a "dry etch" gas such as ClF.sub.3. As
has previously been observed, the ClF.sub.3 molecule breaks up on
contact with deposited surfaces to be cleaned; thus, atomic
fluorine and chlorine are released without requiring the generation
of a plasma. While handling of ClF.sub.3 gas requires special
equipment due to its highly reactive nature, it in principle can
simplify the chemical cleaning process to the extent of not
requiring an ancillary reactive gas plasma source. Since toxic
gases are routinely fed into an ion source for an ion implanter,
much of the equipment is already constructed to be
"toxic-gas-friendly", and a separate gas distribution system
incorporating ClF.sub.3 can be added in a straightforward
manner.
[0112] Advantages of the in-situ chemical cleaning of the ion
source and ion extraction electrode for an ion implanter include:
a) extending source life to hundreds, or possibly thousands, of
hours before service is required; b) reducing or eliminating
cross-contamination brought about by a species change, for example,
when switching from octadecaborane ion implantation to arsenic or
phosphorus ion implantation, and from arsenic or phosphorus ion
implantation to octadecaborane ion implantation; and c) sustaining
peak ion source performance during the service life of the ion
source. For example, performing a 10 minute chemical cleaning
protocol every eight hours (i.e., once every shift change of
operating personnel) and between each species change would have a
minimal impact on the uptime of the implanter, and would be
acceptable to a modern semiconductor fabrication facility.
Endpoint Detection
[0113] It is realized to be beneficial to provide endpoint
detection during the cleaning process, so that quantitative
information on the efficacy and required duration of the cleaning
process may be generated, and the reproducibility of the chemical
cleaning process may be assured. FIG. 3 shows a
differentially-pumped quadrupole mass analyzer (Residual Gas
Analyzer, RGA) sampling the cleaning process. By monitoring the
concentrations of cleaning gas products such as F, Cl, BF.sub.3,
PF.sub.3, AsF.sub.3, AlF.sub.3, WF.sub.6, for example, the cleaning
process may be tuned and verified. Alternatively, optical means of
monitoring the process may be utilized. An FTIR optical
spectrometer can monitor the gases resident in the vacuum housing
of the ion source of the implanter, through a viewport. This
non-invasive (ex-situ) means to identify chemical species may be
preferable to in-situ monitoring devices in certain cases.
Alternatively, see FIG. 4, an extractive FTIR spectrometer may be
coupled to the source vacuum housing for endpoint monitoring. A
novel means to accomplish endpoint detection consists of monitoring
the temperature of the ionization chamber during cleaning. Since
the chemical reaction is exothermic, energy is released during the
reaction, elevating the chamber temperature. This effect can in
principle be used to establish when the reaction rate is
diminished.
Novel Ion Extraction Electrode
[0114] Borohydrides such as decaborane and octadecaborane are
thermally sensitive materials. They vaporize and condense at
temperatures between 20 C. and 100 C. It is therefore important to
maintain all surfaces with which these materials come into contact
at a temperature higher than the vaporizer temperature (but below
the temperature at which they dissociate), to prevent condensation.
We have found that contamination of the extraction electrode is a
problem when using such a borohydride. Both direct ion beam strike
and condensed feed vapor or products of its molecular
disassociation can degrade operation of the ion beam formation
optics, since these boron-containing layers appear to be
electrically insulating. Once electrically insulating layers are
deposited, they acquire an electrical charge ("charge up") and
create vacuum discharges, or "glitches", upon electrical breakdown.
Such a discharge creates instabilities in the ion beam current and
can contribute to the creation of particles that may reach a
process chamber to which the ion beam is directed. An ion implanter
which has an ion beam-producing system that experiences many
glitches per hour is not considered production-worthy in modern
semiconductor fabrication facilities. Furthermore, even in absence
of such discharges, as insulating coatings become thicker, the
electric charge on electrode surfaces create unwanted stray
electric fields which can result in beam steering effects, creating
beam loss and may adversely affect ion beam quality.
[0115] Discovery of new information has led to a robust solution to
this problem. Most implanter ion extraction electrodes are made of
graphite. Graphite has been seen to have many advantages in this
application, including low materials cost, ease of machining, high
electrical conductivity, low coefficient of thermal expansion, and
good mechanical stability at high temperatures. However, using a
graphite extraction electrode, instabilities were observed after
producing an ion beam of borohydrides. It was suspected that the
surfaces of the electrode had become insulating. Samples of the
electrode deposits were collected and a chemical analysis performed
by x-ray fluorescence spectroscopy. The study revealed a chemical
stoichiometry consistent with a boron-carbon compound of the form
B.sub.2C,which was found to be insulating. In addition, it appeared
that metal surfaces in the vicinity of the ion source, including
the front plate (i.e., the ion extraction aperture plate) of the
ion source also had deposited insulating coatings after long use.
It was conceived to fabricate the electrode of aluminum, and
provide radiant heaters to keep the electrode plates, i.e., the
suppression and ground electrodes, at a well-controlled, elevated
temperature (see FIGS. 9, 11) sufficiently high to prevent
condensation of decaborane and octadecaborane. In addition, the
suppression electrode, which faces the ion source, was fabricated
of a single machined piece of aluminum, with a smooth, featureless
aspect and all fasteners were located at the backside of the
plates. This configuration dramatically reduced the number and
severity of discharge points in the event that insulating coatings
were formed, employing the principle that the electric field stress
at a "point", or sharp feature, is many times greater than at a
smooth surface.
[0116] The extraction electrode, thus produced, demonstrated
excellent performance, and operated reliably for more than 100
hours (at least ten times as long as the graphite electrode) with
very low glitch frequency. This great improvement is attributed to:
i) Al construction (i.e., metal versus graphite), ii) Active
heating and temperature control of the electrode plates, and iii)
smooth electrode surfaces. It was found that operating the
electrode plates at 200 C. gave good results when running
decaborane, significantly reducing the amount of deposited
material. In general, the temperature of the extraction electrode
should be kept below the dissociation temperature of the feed
material. In the case of decaborane the temperature should be kept
below 350 C., preferably in the range 150 C. to 250 C. For
octadecaborane operation, the temperature should not exceed 160 C.,
since chemical changes occur in octadecaborane above this
temperature; when running octadecaborane, an extraction electrode
temperature between 120 C. and 150 C. yields good results.
[0117] The radiative design shown in FIG. 11 demonstrated very good
temperature uniformity. Incorporating resistive heaters,
particularly using an aluminum electrode as illustrated in FIG. 12,
can also yield good uniformity and results in a more compact design
requiring less maintenance. A further design that incorporates
desirable features from both of the designs of FIGS. 11 and 12 is
described with reference to FIGS. 1A-1L.
[0118] For constructing a heated extraction electrode, other metals
would also work, for example molybdenum. Molybdenum has the
advantage of being refractory, so it can withstand very high
temperatures. It also has good thermal conductivity. Aluminum, on
the other hand, is a column III element like In and B of the
periodic table, and therefore offers the advantage of being only a
mild contaminant in silicon (it is a P-type dopant in silicon),
while transition metals such as molybdenum are very detrimental to
carrier lifetimes in integrated circuits. Aluminum is also not
readily attacked by halogens, whereas transition metals such as
molybdenum are susceptible to attack, particularly at elevated
temperatures. The primary disadvantage of aluminum, however, is
that it is not a high temperature material, and should be used
below about 400 C.
[0119] For these reasons, depending upon the particular use, the
heated electrode is constructed of a selected heat-resistant
material, aluminum or an aluminum containing alloy often being
preferred when used in association with in situ etch cleaning.
[0120] By providing the alternative of active electrode cooling as
well as active heating, a temperature-controlled ion extraction
electrode comprised of aluminum, suitable for halogen cleaning, may
be used with different types of interchangeable ion sources, or
with a multi-mode ion source. The aluminum electrode can be used
with cool ion sources (during which the extraction electrode is
heated to deter contamination, and avoid unstable operation), and
with hot ion sources (during which the extraction electrode is
cooled to keep its temperature below about 400 C., to maintain its
dimensional stability.)
BRIEF DESCRIPTION OF THE DRAWINGS
[0121] FIG. 1: Ion beam generation system incorporating reactive
gas cleaning.
[0122] FIG. 1A:Ion-extraction electrode assembly useful in the
system of FIG. 1.
[0123] FIGS. 1B and 1C: Cross-sections taken on lines 1B and 1C,
respectively, on FIG. 1A;
[0124] FIG. 1D: exploded view of the assembly;
[0125] FIGS. 1E and 1F: perspective views of the assembly mounted
on a manipulator.
[0126] FIGS. 1G, 1H and 1I: Orthogonal views of heater of the
assembly of FIG. 1A;
[0127] FIGS. 1J and 1K: cut away and side views of the end portion
of the heater;
[0128] FIG. 1L: Heater control circuit.
[0129] FIG. 2: Second embodiment of ion beam generation system
incorporating reactive gas cleaning.
[0130] FIG. 3: Ion beam generation system similar to FIG. 1 but
incorporating a vaporizer and certain gas distribution
elements.
[0131] FIG. 4: Ion beam generation system similar to FIG. 2 but
incorporating a vaporizer and certain gas distribution
elements.
[0132] FIG. 5: Ion generation system incorporating reactive gas
cleaning by the introduction of ClF.sub.3.
[0133] FIG. 6: Gas box for an ion implanter which includes a
reactive gas plasma source, feed vapor source, ion source
electronics, and facilities for the plasma source.
[0134] FIG. 6A: View similar to FIG. 6, showing a vapor flow
control system.
[0135] FIG. 6B: Valve schematic for an ion beam generating
system.
[0136] FIG. 7: Electron-impact ion source.
[0137] FIG. 7A: Magnified view of a portion of FIG. 7, showing
shielding of elements.
[0138] FIG. 7B: Control diagram for an embodiment.
[0139] FIG. 8: Ion extraction electrode.
[0140] FIG. 9: Ion extraction electrode optics.
[0141] FIG. 9A: B.sub.18H.sub.x.sup.+ beam profiles.
[0142] FIG. 9B: Extraction electrode having heating and cooling
features and two electrode elements.
[0143] FIG. 9C: Extraction electrode assembly having heating and
cooling features and three electrode elements.
[0144] FIG. 10: Extraction electrode and manipulator.
[0145] FIG. 11: Electrode head-exploded view.
[0146] FIG. 12: Second embodiment of electrode head.
[0147] FIG. 13: B.sub.10H.sub.x.sup.+beam current versus decaborane
flow rate.
[0148] FIG. 14: Lifetime versus decaborane vapor flow rate.
[0149] FIG. 15: Etch rate of Si coupon.
[0150] FIG. 16: Ion implanter.
DETAILED DESCRIPTION
Novel Ion Beam-Generating System
[0151] FIG. 1 shows an ion beam-generating system. As shown in this
example, it is adapted to produce an ion beam for transport to an
ion implantation chamber for implant into semiconductor wafers or
flat-panel displays. Shown are ion source 400, extraction electrode
405, vacuum housing 410, voltage isolation bushing 415 of
electrically insulative material, vacuum pumping system 420, vacuum
housing isolation valve 425, reactive gas inlet 430, feed gas and
vapor inlet 441, vapor source 445, feed gas source 450, reactive
gas source 455, ion source high voltage power supply 460, and
resultant ion beam 475. An ion beam transport housing is indicated
at 411. The ion source 400 is constructed to provide cluster ions
and molecular ions, for example the borohydride ions
B.sub.10H.sub.x.sup.+, B.sub.10H.sub.x.sup.-,
B.sub.18H.sub.x.sup.+, and B.sub.18H.sup.- or, or in addition, more
conventional ion beams such as P.sup.+, As.sup.+, B.sup.+,
In.sup.+, Sb.sup.+, Si.sup.+, and Ge.sup.+. Ion source 400 may be a
Bernas-style arc-discharge ion source, which is most commonly used
for ion implantation, or a "bucket"-type water-cooled ion source
which uses an immersed RF (radio frequency) antenna forming an RF
field to create ions, a microwave ion source, or an electron-impact
ionization source, for example. The gas and vapor inlet 441 for
gaseous state feed material to be ionized is connected to a
suitable vapor source 445, which may be in close proximity to gas
and vapor inlet 441 or may be located in a more remote location,
such as in a gas distribution box located elsewhere within a
terminal enclosure. A terminal enclosure is a metal box, not shown,
which encloses the ion beam generating system. It contains required
facilities for the ion source such as pumping systems, power
distribution, gas distribution, and controls. When mass analysis is
employed for selection of an ion species in the beam, the mass
analyzing system may also be located in the terminal enclosure.
[0152] In order to extract ions of a well-defined energy, the ion
source 400 is held at a high positive voltage (in the more common
case where a positively-charged ion beam is generated), with
respect to the extraction electrode 405 and vacuum housing 410, by
high voltage power supply 460. The extraction electrode 405 is
disposed close to and aligned with the extraction aperture 504 of
the ionization chamber. It consists of at least two
aperture-containing electrode plates, a so-called suppression
electrode 406 closest to ionization chamber 500, and a "ground"
electrode 407. The suppression electrode 406 is biased negative
with respect to ground electrode 407 to reject or suppress unwanted
electrons which otherwise would be attracted to the
positively-biased ion source 400 when generating positively-charged
ion beams. The ground electrode 407, vacuum housing 410, and
terminal enclosure (not shown) are all at the so-called terminal
potential, which is at earth ground unless it is desirable to float
the entire terminal above ground, as is the case for certain
implantation systems, for example for medium-current ion
implanters. The extraction electrode 405 may be of the novel
temperature-controlled metallic design, described below.
[0153] (If a negatively charged ion beam is generated the ion
source is held at an elevated negative voltage with other suitable
changes, the terminal enclosure typically remaining at ground.)
Novel Actively Heated Extraction Electrode
[0154] The ion accelerating and ion beam forming effects ("ion
optic effects") of extraction electrodes are well understood by
those skilled in the design of ion implantation systems.
[0155] Actively temperature-controlled extraction electrode designs
are shown in FIGS. 9, 9B, 9C, 11 and 12, described later herein. An
actively heated extraction electrode arrangement of "sandwich"
form, suitable for ion beams of decaborane and octadecaborane, is
shown in FIGS. 1A to 1L, which will be described now.
[0156] Referring to FIGS. 1A to 1L, extraction electrode 805 is
comprised of suppression electrode element 810 and ground electrode
element 820 mounted in close succession along the ion beam path.
The ions are drawn by electric field effects from the positively
biased ion source 400, FIG. 1, to the extraction electrode 805. The
ions propagate through electrode 805 along beam axis 530 as an
energetic, focused, ribbon-form ion beam 475. The ground electrode
is maintained at the potential of the surrounding vacuum housing
410 and establishes the potential of the ion beam as it proceeds
beyond the extraction electrode. Suppression electrode 810, biased
to a few thousand volts negative relative to the ground electrode,
serves to suppress secondary electrons which are generated
downstream from the suppression electrode due to beam strike. This
prevents such energetic electrons from back-streaming into the
positively-biased ion source 400 The suppression electrode element
810 and ground electrode element 820 are fabricated of aluminum and
have smooth, carefully polished surfaces to minimize local electric
fields. The extraction optic component 805 comprised of these
elements is mounted on a manipulator 610A, shown in FIGS. 1E and
1F. This manipulator is used e.g. to align electrode 805 with the
ion source and downstream components and to vary the focal length
of the ion optical system. As indicated, the manipulator enables
linear adjustment in the X dimension, transverse to the short
dimension of the slot-form aperture, and in the Z dimension, along
the axis of the ion beam. It also enables rotation about the X
axis.
[0157] Each electrode element, 810 and 820, is comprised of two
portions, inner aperture-defining portion, 810A and 820A, and
disc-form outer portion, 810B and 820B, respectively. Heater 830 is
disposed ("sandwiched") between these electrode elements, but is
spaced out of contact with them so that heat transfer from heater
to electrode elements is by radiation. Inner portions 810A and 820A
form elongated, slot-form apertures A in the electrode elements for
passage of the ions from the ions source 400 and serve to establish
the electric fields to which the ions are exposed. Outer portions
810B and 820B of the electrode elements serve multiple functions:
they support the inner electrode portions, they serve as
axially-directed, wide area heat receptors for absorbing heat that
radiates generally axially from the radiant heater 830 which is
disposed between them, and they define low-resistance thermal
conductive paths by which heat can flow by conduction radially from
the outer portions to the inner portions. In the preferred form
shown, each electrode element is of one piece, machined of
aluminum, and as such provides excellent heat conducting paths from
its outer to its inner electrode portion. In other designs, the
inner portions of the electrodes may be discretely formed as
replaceable units and may be thermally connected to permanently
mounted outer portions by heat conductive metal gaskets compressed
between the two portions. Also, instead of the outer portions of
the electrode elements being planar discs they may be of other heat
receptive forms, such as of conical or of curved cross-section.
[0158] The radiant heater 830, mounted between the two outer
electrode portions 810A and 820A, is configured to surround the
inner electrode portions 810B and 820B and the ion beam path. In
this implementation the heater is a circular tube heater, FIGS. 1A
and 1F. Heater 830, of overall diameter greater than the long
dimension L of the slot-form ion beam apertures A, surrounds the
apertures. It is centered on beam axis 530. Heater 830 is comprised
of a hollow outer, chemically-resistant radiating tube 831, e.g. of
stainless steel such as Incaloy.TM.. Inside of tube 831 is centered
an electric resistance heating element 832, e.g. nichrome wire. The
resistance wire is held in its central position by insulating
material 833, for instance magnesium oxide, see FIG. 1J. Such
heaters are available for instance as Watrod.TM. heaters, from
Watlow. For protecting the heater wire from chemical vapor during
reactive gas cleaning, glass hermetic end seals 835 are employed at
the ends of the heater tube. Nickel plated steel end conductors 834
extend centrally through the seals, from the exterior to electrical
connection with the resistance element 832 within the tube. With
suitably chosen insulative standoffs, e.g. of alumina
(Al.sub.2O.sub.3), and by employing stainless steel connectors, the
entire extraction electrode unit is fluorine-resistant, and
suitable for in situ cleaning by reactive gas.
[0159] An example of a suitable power control circuit for heater
830 is shown in FIG. 1L. Thermocouple 850 is connected with good
thermal contact to a thermally representative portion of the
extraction electrode assembly 805. Thermocouple 850 feeds back to a
closed-loop PID controller, 860, e.g. Omron E5CK. Controller 860 is
connected to solid state relay 865 of power circuit 870. In
operation, the set point for heating the extraction electrode
assembly is determined at the overall control unit 880 for the ion
beam producing system. The set point is fed to closed loop PID
controller 860. Controller 860 interprets the set point signal,
reads the temperature output of thermocouple 850 and controls the
on and off stages of relay 865 in manner to apply appropriate
electric power from power source 875 to heater 830 to achieve the
desired temperature at thermocouple 850.
[0160] As with the other embodiments described below, this heating
arrangement is capable of maintaining the extraction electrode at a
well-controlled, elevated temperature sufficiently high to prevent
condensation of decaborane or octadecaborane vapor emanating from
the relatively cool-operating ion source of FIGS. 7 and 7A, to be
described. The extraction electrode, made of fluorine-resistant
materials, enables periodic in situ cleaning of the electrode to
remove any deposits, employing fluorine vapors drawn through the
extraction aperture of the associated ion source 400. Such cleaning
systems will now be described.
[0161] Reactive Gas Cleaning
[0162] FIG. 1 shows the reactive gas source 455 at terminal
potential, with reactive gas inlet 430 incorporating a high voltage
break 431, which can be fabricated of an insulating ceramic such as
Al.sub.2O.sub.3, for example. Since ion sources for ion
implantation can in general be biased up to a maximum voltage of
about 90 kV, this high voltage break 431 must stand off 90 kV for
that application. As will be described below, the cleaning system
is used only with the ionizing source and high voltages off
(de-energized), so that there is only high voltage across break 431
when the vacuum housing 410 is under high vacuum, which makes high
voltage standoff clearance requirements easier to meet. A dedicated
endpoint detector 470, in communication with the vacuum housing
410, is used to monitor the reactive gas products during chemical
cleaning.
[0163] For ion sources suitable for use with ion implantation
systems, e.g. for doping semiconductor wafers, the ionization
chamber is small, having a volume less than about 100 ml, has an
internal surface area of less than about 200 cm.sup.2, and is
constructed to receive a flow of the reactive gas, e.g. atomic
fluorine or a reactive fluorine-containing compound at a flow rate
of less than about 200 Standard Liters Per Minute.
[0164] It is seen that the system of FIG. 1 enables in situ
cleaning, i.e. without the ion source and extraction electrode
being removed from operating position in the vacuum housing, and
with little interruption of service.
[0165] FIG. 2 illustrates another embodiment. The principal
difference in FIG. 2 over FIG. 1 is that the reactive gas source
455 and reactive gas inlet 430 are at ion source potential. The
benefits of this approach are twofold: it is a more compact
arrangement, and it allows the reactive gas source 455 and its
associated gas supplies to be contained in the gas box which, at
ion source potential, supplies gas and power to the ion source 400,
as is typical in commercial ion implantation systems.
Chemical Cleaning System
[0166] The embodiment of FIG. 3, having many features similar to
FIG. 1, is constructed to generate, selectively, both cluster ions
and monomer ions. It has a dedicated gas inlet 435 for feed
material in normally gaseous state and is in communication, through
valve 443, with a vapor source 445 for producing borohydride and
other vaporized feed materials. For conducting in-situ chemical
cleaning of the ion source and electrode, a remote plasma source
455 disassociates gas supplied by a cleaning gas supply 465, for
example NF.sub.3, into decomposition products such as F, F.sub.2,
and N-containing compounds. When cleaning is desired, after
de-energizing the ion source, the decomposition products are fed
into the ionization chamber from the outlet 456 of the remote
plasma source 455 by dedicated reactive gas inlet 430. The remote
plasma source 455 is mounted on the terminal potential side of
voltage isolation bushing 415. Since the ion source 400 runs at
high voltage, a high voltage break 431 in vacuum provides voltage
isolation.
[0167] To initiate a cleaning cycle, the ion source is shut down
and vacuum housing isolation valve 425 is closed; the high vacuum
pump 421 of the vacuum pumping system 420 is isolated and the
vacuum housing 410 is put into a rough vacuum state of <1 Torr
by the introduction of dry N.sub.2 gas while the housing is
actively pumped by backing pump 422. Once under rough vacuum, argon
gas (from Ar gas source 466) is introduced into the plasma source
455 and the plasma source is energized by on-board circuitry which
couples radio-frequency (RF) power into the plasma source 455. Once
a plasma discharge is initiated, Ar flow is reduced and the
F-containing cleaning gas feed 465, e.g. NF.sub.3, is introduced
into plasma source 455. Reactive F gas, in neutral form, and other
by-products of disassociated cleaning gas feed 465, are introduced
through reactive gas inlet 430 into the de-energized ionization
chamber 500 of ion source 400. The flow rates of Ar and NF.sub.3
(for example) are high, between 0.1 SLM (Standard Liters per
Minute) and a few SLM. Thus, up to about 1 SLM of reactive F as a
dissociation product can be introduced into the ion source 400 in
this way. Because of the small volume and surface area of
ionization chamber 500, this results in very high etch rates for
deposited materials. The ionization chamber 500 has a front plate
facing the extraction electrode, containing the extraction aperture
504 of cross sectional area between about 0.2 cm.sup.2 and 2
cm.sup.2, through which, during energized operation, ions are
extracted by extraction electrode 405. During cleaning, the
reactive gas load is drawn from ionization chamber 500 through the
aperture 504 by vacuum of housing 410; from housing 410 the gas
load is pumped by roughing pump 422. Since the extraction electrode
405 (constructed, for instance, as electrode 805 of FIG. 1A) is
near and faces aperture 504 of ionization chamber 500, the
electrode surfaces intercept a considerable volume of the reactive
gas flow. This results in an electrode cleaning action, removing
deposits from the electrode surfaces, especially from the front
surface of suppression electrode 406 (e.g. suppression electrode
810, FIG. 1A), which is in position to have received the largest
deposits. Thus, it is beneficial to fabricate extraction electrode
and its mounting of F-resistant materials, such as Al (either
aluminum or aluminum alloy) and the insulator elements of
Al.sub.2O.sub.3.
[0168] The embodiment of FIG. 3 also has an endpoint detector
consisting of a differentially-pumped, Residual Gas Analyzer (RGA),
constructed for corrosive service. Analyzer RGA is in communication
with vacuum housing 410. It is to be used as a detector for the end
point of the cleaning action by monitoring partial pressures of
F-containing reaction products (for example, BF.sub.3 gas resulting
from B combining with F). Other types of endpoint detectors can be
used, the RGA being shown to illustrate one particular embodiment.
When the boron-containing partial pressures decline at RGA, the
cleaning process is largely completed. Once the cleaning process is
ended, the plasma source 455 is turned off and is briefly purged
with Ar gas (which also purges the ionization chamber 500, the
housing 410 and elements contained therein). The roughing pump 422
is then isolated from direct communication with vacuum housing 410,
the high vacuum pump 421 isolation valve is opened, and vacuum
housing 410 is restored to high vacuum (about 1.times.10.sup.-5
Torr or below). Then, vacuum housing isolation valve 425 is opened.
The system is now ready to resume ion beam generation. The ion
source voltage supply 460 can be energized and ion source 400
operated normally.
[0169] An advantage of the embodiment of FIG. 3 is that the service
facilities needed to support the remote plasma source 455, such as
cooling water circulation and electrical power, can be at the
terminal potential of an ion implanter (see 208 in FIG. 16). This
enables sharing facilities denoted at S such as cooling water and
electrical power, with the mass-analyzer magnet 230 of the
implanter. During cleaning mode, when plasma source 455 is
energized, the analyzer 230 is de-energized and therefore does not
need water or power, and vise versa, during ion beam production
mode. This "sharing" can be accomplished by suitable control
arrangements represented diagrammatically at S', which direct
service facilities such as cooling water circulation and power
supply connection alternatively to the analyzer magnet 230, dashed
arrow S, or to the remote plasma source 455, solid arrow S,
depending upon the mode of operation being employed.
[0170] FIG. 4 shows an implementation similar to FIG. 2 for
conducting in-situ chemical cleaning of an source 400 and
extraction electrode 405. Three inlet passages are integrated into
ion source 400, respectively for reactive gas 430 from plasma
source 455, feed gas 435 from one of a number of storage volumes
450 selected, and feed vapor 440 from vaporizer 445. Unlike FIG. 3,
the embodiment of FIG. 4 has the plasma-based reactive gas source
455 at the high voltage of ion source 400. This enables the remote
plasma source 455 to share control points of the ion source 400,
and also enables the cleaning feed gas 465 and argon purge gas from
storage 466 to be supplied from the ion source gas distribution
box, which is at source potential, see also FIGS. 6 and 6A. Also
shown is a different type of endpoint detector, namely a Fourier
Transform Infrared (FTIR) optical spectrometer. This detector can
function ex-situ (outside of the vacuum housing), through a quartz
window. Instead, as shown in FIG. 4, an extractive type of FTIR
spectrometer may be used, which directly samples the gas in the
vacuum housing 410 during cleaning. Also a temperature sensor TD
may sense the temperature of the de-energized ionization chamber by
sensing a thermally isolated, representative region of the surface
of the chamber. The sensor TD can monitor heat produced by the
exothermic reaction of F with the contaminating deposit, to serve
as an end-point detection.
[0171] FIG. 5 shows an ion beam-generating system similar to that
of FIG. 4, but incorporating a fundamentally different type of
reactive gas source 455. In this case, reactive ClF.sub.3 gas
contained in a gas cylinder is fed directly into ion source 400
without use of a remote plasma source. This potentially reduces
equipment cost and footprint since power and controls for a remote
plasma source are not required. However, since ClF.sub.3 is
pyrophoric, it is dangerous and requires special gas handling,
whereas NF.sub.3 (for example) is primarily an asphyxiant, and is
less toxic than many semiconductor gases, such as BF.sub.3,
PH.sub.3, or AsH.sub.3, and therefore safer.
[0172] FIG. 6 shows plasma source 455, vapor source 445, source
electronics, and service facilities S for the plasma source
contained within a gas box B meant for retrofit into an existing
ion implanter installation.
[0173] The embodiment of FIG. 6a differs from the embodiment of
FIG. 6 above, by incorporating a preferred vaporizer and flow
control system described below. FIG. 6B is a valve schematic
diagram for the ion source and self-cleaning system of FIG. 4.
Preferred Ion Source and Vaporizer
[0174] FIG. 7 is a diagram of a preferred ion source 10 and its
various components, and see FIG. 7A. The details of its
construction, as well as its preferred modes of operation, are
similar to that disclosed by Horsky et al., International
Application No. PCT/US03/20197, filed Jun. 26, 2003: "An ion
implantation device and a method of semiconductor manufacturing by
the implantation of boron hydride cluster ions", and by Horsky,
U.S. patent application Ser. No. 10/183,768, "Electron impact ion
source", filed Jun. 26, 2002, both herein incorporated by
reference. The ion source 10 is one embodiment of a novel electron
impact ionization system. FIG. 7 is a cross-sectional schematic
diagram of the source construction which serves to clarify the
functionality of the components which make up the ion source 10.
The ion source 10 is made to interface to an evacuated vacuum
chamber of an ion implanter by way of a mounting flange 36. Thus,
the portion of the ion source 10 to the right of flange 36, shown
in FIG. 7, is at high vacuum (pressure<1.times.10.sup.-4 Torr).
Gaseous material is introduced into ionization chamber 44 in which
the gas molecules are ionized by electron impact from electron beam
70, which enters the ionization chamber 44 through electron
entrance aperture 71 such that electron beam 70 is aligned with
(i.e. extends adjacent, parallel to) ion extraction aperture 81.
Thus, ions are created adjacent to the ion extraction aperture 81,
which appears as a slot in the ion extraction aperture plate 80.
The ions are then extracted and formed into an energetic ion beam
475 by an extraction electrode 220 (FIGS. 8 and 9) located in front
of the ion extraction aperture plate 80. Referring to FIG. 7, gases
such as argon, phosphine, or arsine, for example, may be fed into
the ionization chamber 44 via a gas conduit 33. Solid feed
materials 29 such as decaborane or octadecaborane can be vaporized
in vaporizer 28, and the vapor fed into the ionization chamber 44
through vapor conduit 32 within the source block 35. Typically,
ionization chamber 44, ion extraction aperture plate 80, source
block 35 (including vapor conduit 32), and vaporizer housing 30 are
all fabricated of aluminum. Solid feed material 29 is held at a
uniform temperature by closed-loop temperature control of the
vaporizer housing 30. Sublimated vapor 50 which accumulates in a
ballast volume 31 feeds through conduit 39 and through throttling
valve 100 and shutoff valve 110. The nominal pressure of vapor 50
between throttling valve 100 and shutoff valve 110 is monitored by
heated pressure gauge 60, preferably a capacitance manometer. Since
the vapor 50 feeds into the ionization chamber 44 through the vapor
conduit 32, located in the source block 35, and gases feed in
through gas conduit 33, both gaseous and vaporized materials may be
ionized by this ion source, which is capable of creating ion beam
475 consisting of either molecular ions (such as
B.sub.18H.sub.x.sup.+)or monomer ions (such as As.sup.+), as
needed. The ion source may instead be a multi-mode ion source such
as described in U.S. Pat. No. 7,022,999, issued Apr. 4, 2006,
Entitled "Ion Implantation Ion Source, System and Method", or as
described in U.S. patent application Ser. No. 11/268,005, filed
Nov. 7, 2005, entitled "Dual Mode ion Source for Ion Implantation
and Forming N-Type Regions with Phosphorus and Arsenic Ions", the
contents of each of which, to the extent describing multi-mode ion
sources, being hereby incorporated by reference in their
entireties.
Vapor Flow Control Into the Ion Generating System
[0175] The flow of vapor to ionization chamber of FIG. 7, and see
FIG. 7B, is determined by the vapor pressure in the region just
before vapor feed passage 32, i.e., within shutoff valve 110 in
FIG. 7. This is measured by pressure gauge 60, e.g. a capacitance
monometer, located between throttling valve 100 and shut-off valve
110. In general, the flow rate is proportional to the vapor
pressure. This allows the pressure signal to represent flow, and to
be used as a set point to select flow. To generate a desired vapor
flow into the ion source, vaporizer housing 30 is brought to a
temperature such that when throttling valve 100 is in its fully
open position, the desired flow rate is exceeded. Then the
throttling valve 100 is adjusted to reach the desired pressure
output.
[0176] To establish a stable flow over time, separate closed loop
control of the vaporizer temperature and vapor pressure is
implemented using dual PID controllers, such as the Omron E5CK
control loop digital controller. The control (feedback) variables
are thermocouple output for temperature, and gauge output for
pressure. The diagram of FIG. 7B shows a digital vapor feed
controller 220 for performing these closed loop control
functions.
[0177] In FIG. 7B gauge output 250 from pressure gauge 60 is
applied to throttle valve position control 245 which applies
throttle valve position control signal 247 to throttle valve 100.
Thermocouple output 225 from vaporizer 28 is applied to vaporizer
heater control 215 which controls heater power 248 applied to the
vaporizer 28.
[0178] A second, slow level of control is implemented by digital
feed controller 220, accommodating the rate at which solid feed
material vaporizes being a function of its open surface area,
particularly the available surface area at the solid-vacuum
interface. As feed material within the vaporizer is consumed over
time, this available surface area steadily decreases until the
evolution rate of vapors cannot support the desired vapor flow
rate, resulting in a decrease in the vapor pressure upstream of the
throttle valve 100. This is known as "evolution rate limited"
operation. So, with a fresh charge of feed material in the
vaporizer, a vaporizer temperature of, say, 25 C. might support the
required vapor flow at a nominal throttle valve position at the low
end of its dynamic range (i.e., the throttling valve only partially
open). Over time (for example, after 20% of the feed material is
consumed), the valve position would open further and further to
maintain the desired flow. When the throttle valve is near the high
conductance limit of its dynamic range (i.e., mostly open), this
valve position is sensed by the controller 220, which sends a new,
higher heater set point temperature to the vaporizer heater control
215. The increment is selected to restore, once the vaporizer
temperature settles to its new value, the nominal throttle valve
operating point near the low end of its dynamic range. Thus, the
ability of the digital controller 220 to accommodate both
short-timescale changes in set point vapor pressure and
long-timescale changes in vaporizer temperature makes the control
of vapor flow over the lifetime of the feed material charge very
robust. Such control prevents over-feeding of vapor to the
ionization chamber. This has the effect of limiting the amount of
unwanted deposits on surfaces of the ion generating system, thus
extending the ion source life between cleanings.
[0179] FIG. 8 shows a top view (looking down) of an ion extraction
electrode 220 facing the novel ion source 10. The ion source 10 is
held at a positive potential V.sub.A with respect to the ion
extraction electrode 220, which is at local ground potential, i.e.,
at the potential of the vacuum housing. The ion extraction
electrode 220 is a simple diode; electrode plate 302 is the
"ground" electrode and plate 300 the "suppression" electrode,
typically held a few thousand volts below ground potential by
suppression power supply V.sub.S. The ionization chamber 44 and ion
extraction aperture plate 80 of ion source 10 are shown facing
extraction electrode 220. The three plates 80, 300, 302 contain
rectangular slots or apertures through which ions 90 are extracted;
FIG. 8 illustrates the slot profiles in the "short", or dispersive,
direction.
Further Embodiments of Novel Heated Electrode
[0180] During the decaborane lifetime tests shown in FIG. 14, a
novel heated aluminum electrode was used. FIG. 9 shows a top view
of the basic optical design of the extraction system, in the
dispersive plane of the one-dimensional "slot" aperture lenses. In
the implanter used, the ionization chamber 490 of the ion source
was held at the desired ion beam energy by positive high voltage
power supply V.sub.A, FIG. 8. For example, if a 20 keV ion beam is
desired, then V.sub.A=20 kV. Ion extraction aperture plate 500 is
electrically isolated from ionization chamber 490 such that it can
be biased by bipolar power supply V.sub.B from -750V-750V. The
isolation is accomplished by a thermally conductive, electrically
insulating polymeric gasket which is sandwiched between the ion
extraction aperture plate 500 and ionization chamber 490. The parts
of the ion source body that are exposed to vapor (source block 35,
ionization chamber 44, and extraction aperture plate 80 in FIG. 7)
are maintained in good thermal contact with each other to maintain
controlled temperature surfaces during source operation. Ions
produced in ionization chamber 490 are extracted through the
aperture in ion extraction aperture plate 500 by extraction
electrode 540 consisting of suppression electrode 510 and ground
electrode 520. The ions propagate as a focused ion beam along the
beam axis 530. Suppression electrode 510, biased to a few thousand
volts negative by power supply V.sub.S, serves to suppress
secondary electrons which are generated downstream from the
suppression electrode due to beam strike, preventing these
energetic electrons from back streaming into the positively-biased
ion source. The ionization chamber 490, ion extraction aperture
plate 500, suppression electrode 510, and ground electrode 520 are
all fabricated of aluminum, and have smooth, carefully polished
surfaces to minimize local electric fields.
[0181] An important effect of biasing ion extraction aperture plate
500 is to change the focal length of the ion optical system of FIG.
9. A negative bias increases the focal length, while a positive
bias decreases the focal length. For large biases, the effect can
be substantial. For diagnostic purposes, a scanning-wire
profilometer was installed, located at the entrance to the analyzer
magnet, just downstream of the source housing isolation valve (210
in FIG. 16). This scanner recorded the beam current distribution in
the dispersive plane, useful to determine how well the ion beam is
being focused in the dispersive plane. 20 keV octadecaborane beam
profiles are shown in FIG. 9a for three different biasing
conditions: -483V, 0, and +300V. The zero volt condition is
substantially over focused, the positive voltage condition more
over focused, and the negative voltage condition properly focused.
The electrode position was held constant during the three
measurements. As expected, the proper focusing condition yielded
the highest ion beam currents.
[0182] The ability to change the optical focal length, and thus
tune the optical system to obtain the highest ion beam current,
enables introduction of the least amount of feed material to the
vaporizer. Again, this has the beneficial effect of limiting the
amount of unwanted deposits on surfaces of the ion generating
system, extending the ion source life between cleanings.
[0183] Besides the biasing of the extraction aperture plate for
focusing the system just described, the invention provides means
for moving the extraction electrode optic element relative to other
components of the system. FIG. 10 shows the novel electrode 600
mounted on a three-axis manipulator 610 which allows for motion
(with respect to the ion source) in X, Z and .THETA., as defined by
coordinate system 620. Actuator 613 controls X-motion, actuator 612
controls Z-motion, and actuator 611 controls .THETA.-motion. The
manipulator 610 mounts to the side of the implanter vacuum housing
via mounting flange 615.
[0184] FIG. 11 shows a partial exploded view of the
radiatively-heated version of the novel electrode head. Shown are
suppression electrode 700, ground electrode 710, heater plate 720,
and radiant heater wire 730. The suppression and ground electrodes
are fabricated of aluminum, the heater plate of stainless steel,
and the heater wire 730 of nichrome. When the electrode was
operated at 200 C, power consumption was about 60W to maintain the
temperature. The heater power is controlled with a closed-loop PID
controller, the Omron E5CK, based on read back of a
thermocouple.
[0185] FIG. 12 shows a partial exploded view of a
resistively-heated version of the novel electrode head. Shown are
suppression electrode 800, ground electrode 810, and resistive
heaters 820. The four resistive heaters 820 fit into sleeves 830,
two into each electrode plate. The sleeves 830 are a split design
such that the heater press-fits into the sleeve, achieving intimate
contact. Intimate contact between heater and electrode is important
to insure proper heating of the electrode, and to prevent premature
burnout of the heaters. Again, the Omron E5CK or equivalent can
control the electrode temperature based on read back of a
thermocouple.
[0186] As described above, use of these heating arrangements for
the extraction electrode maintain a well-controlled, elevated
temperature sufficiently high to prevent condensation of decaborane
and octadecaborane such as produced by the relatively
cool-operating ion source of FIGS. 7 and 7A. The extraction
electrode made of fluorine-resistant materials, e.g. aluminum,
enables periodic in situ cleaning of the electrode to remove any
deposits by fluorine vapors drawn through the extraction
aperture.
[0187] A different situation is encountered with plasma ion sources
that inherently run so hot that the heat may harm the extraction
electrode assembly if made of low temperature material. Referring
to FIG. 9, shown in dotted lines are circular cooling coils, 512
and 522 secured in heat transfer relationship to the backs of
aluminum electrode members 510 and 522, respectively. Circulation
of cooling fluid through these cooling coils can cool the aluminum
electrodes to prevent deformation by heat from hot ion sources.
This enables use of fluorine-resistant materials for the extraction
electrode, for instance aluminum or a complex containing aluminum,
which provide resistance to attack by any fluorine present from
feed materials or from reactive cleaning gas.
[0188] Referring to FIG. 9B, a temperature-controlled extraction
electrode 540A is provided for use with a multimode ion source 490A
capable of operating, alternatively, at cool and high temperature
modes, or with replaceable units that operate at respective low and
high temperature modes. The extraction electrode assembly 540A,
formed for instance of aluminum, is equipped for active heating and
cooling.
[0189] In cool ion source mode, useful with vapors of decaborane
and octadecaborane, the extraction electrode is actively heated to
deter formation of deposits on the electrode surfaces. The ion
source may for instance operate by "soft" electron impact. In this
case it employs a focused electron beam 70 as described above in
relation to FIGS. 7 and 7A.
[0190] At the high temperature mode, the extraction electrode is
actively cooled to enable it to be formed of relatively low
temperature material such as aluminum. For a hot mode ion source,
for instance, a hot plasma may be maintained by an arc-discharge
within the ionization chamber, produced by an electron-emitting
cathode and negatively biased electron repeller disposed within a
confining magnetic field. Principles of design and construction of
arc-discharge plasma ion sources, per se, are well known, see for
instance Freeman, U.S. Pat. No. 4,017,403 and Robinson, U.S. Pat.
No. 4,258,266, incorporated herein by reference in this respect.
The arc discharge creates a relatively hot plasma which ionizes gas
introduced to the ionization chamber. In both hot and
cool-operating ion source modes the beam produced may be of ribbon
shape, its elongated cross-section produced by the similarly
elongated shape of the extraction aperture of the ion source and
the aperture through the extraction electrode.
[0191] As shown in FIG. 9B, circular, radiant tube heater 550, such
as the tubular heater described with reference to FIGS. 1A-1K, is
arranged coaxially with the ribbon-shaped path 530 of ions from the
ion source. Heater 550 is of diameter larger than the long
dimension of the slot-form aperture through the electrode elements.
The tubular heater is mounted downstream of the suppression
electrode element in position to directly radiate heat to outer
structure of both the extraction electrode element 520 at ground
potential and the suppression electrode element 510 at relatively
negative potential. The suppression electrode element is of the
form described above, with an outer disc-shaped portion defining a
broad, axially-directed face area adapted to receive and absorb
heat radiating from the heater 550, and to conduct the heat
radially to the inner portion of the electrode element. The ground
electrode element is of smaller radial extent, and is constructed
to be heated principally by heat radiating radially-inwardly from
the surrounding heater.
[0192] By suitable temperature sensing, as by the thermocouple of
FIG. 1L, a representative temperature of the extraction electrode
assembly is detected. By use of a suitable controller, electric
heating current flows through the internal resistive element of the
tubular heater to maintain the temperature of the thermocouple at a
set point. As with the controller shown in FIG. 1L, the set point
is selected to prevent condensation of feed vapors that reach the
extraction electrode. For instance, when ionizing decaborane and
octadecaborane, the temperature of the extraction electrode may be
maintained at about 150 C. by active heating by the heater.
[0193] The extraction electrode assembly of FIG. 9B is also
equipped with cooling coils, 512 and 522 that surrounds the beam
path. The coils are secured in conductive heat transfer
relationship to the backs of the aluminum electrode elements 510
and 522. As described regarding FIG. 9, circulation of cooling
fluid through the cooling coils, cooling the aluminum electrode
elements, can prevent electrode deformation by heat from a hot ion
source, that otherwise might disturb the electric fields produced
by the electrode elements.
[0194] When it is desired to employ the multimode ion source
apparatus in a hot mode to produce ion beams of suitable species,
the walls of the ionization chamber are permitted to operate at a
substantially elevated temperature, e.g. above 400 C. In this mode
of operation, heating of the extraction electrode assembly is
disabled and a flow of cooling liquid is maintained in coils 512
and 522 to cool the aluminum electrode elements below a temperature
at which detrimental distortion of the electrodes might occur, i.e.
below about 400 C.
[0195] This electrode assembly combined with the multimode ion
source is suitable for use in systems having in-situ reactive gas
cleaning, described in relation to FIGS. 1 to 6B and 7B. The
aluminum composition of the electrode elements and the
corrosive-resistant sheath of the tubular heater are resistant to
attack from fluorine.
[0196] Referring to FIG. 9C, the extraction assembly may instead
comprise three electrode elements: suppression electrode 510, as in
FIG. 9B, a central electrode 515, and the main extraction electrode
520'. Electrode 520' is maintained at ground (housing) potential,
the potential of central electrode 515 is variable and can be
maintained at a selected potential between ground and a
considerable value, for instance -30 KeV. The potential of
suppression electrode 510 may float at voltage V.sub.S, for
instance, -10 KeV, relative to the central electrode. In this
arrangement the ion-accelerating and focal-length adjusting effects
of the electrode system may be varied for obtaining desired effects
upon the ion beam.
[0197] The electrode elements are nested, as shown in FIG. 9C, so
that the beam passing through the electrode elements is exposed
only to the respective potentials of the three elements. In the
arrangement shown, the two outer electrode elements are supported
by circular radiant heat-receiving, heat-conductive disc portions,
while the central electrode is of less transverse extent, supported
between the outer electrodes by suitable support structure, not
shown. A tubular heater 560 of suitable dimension is disposed
between the two outer electrode structures 510, 520', and surrounds
inner electrode structure 515. The heater is constructed and
arranged to heat the three electrode structures by direct radiation
to each. Radiation proceeds generally axially toward the outer disc
portions of the outer two electrode elements, and radially inwardly
toward the center electrode. By suitable temperature sensing, as by
the thermocouple previously mentioned, a representative temperature
of the electrode assembly is detected. Electrical heating current
through the tubular heater maintains the temperature at a set point
to prevent condensation on the electrodes of feed vapors that reach
the assembly. For instance, when ionizing decaborane and
octadecaborane by "soft" electron impact, the temperature of the
electrode elements may be maintained at about 150 C. Similar to the
embodiment of FIG. 9B, each of the electrode structures is also
provided with a cooling coil 512 and 522, and the assembly of FIG.
9C may instead be cooled as described when an ion source operates
in hot mode.
[0198] While the heated arrangement of a three-electrode system has
been shown in FIG. 9C with respect to a multimode ion source, it is
of course useful with replaceable hot and cool mode ion sources.
For use with only cool-operating ion sources, such as shown in
FIGS. 7 and 7A, the cooling feature may be omitted from the
electrode assembly, or the cooling feature may be retained,
available for use with an arc-discharge hot plasma source that may,
from time to time, be substituted for the soft electron impact
source.
Source Lifetime Measurements When Running Decaborane
[0199] FIG. 14 shows the results of source lifetime testing over a
broad range of decaborane flows. The fit to these data is from
Equation (3). No failures of the ion source were recorded during
these tests; rather, the individual tests were ended when the
decaborane ion current dropped to roughly half of its initial
level. Upon inspecting the ion source, it was found that a
substantial amount of boron-containing material was deposited
within the ionization chamber, mostly adhering to the interior
walls of the chamber. In some cases, the ion extraction aperture
was also partially occluded. The model of Equation (3) seems to fit
the data well, and suggests that "lean" operation is the key to
prolonged ion source lifetime, between in situ chemical cleaning
procedures or disassembly.
Measurements of Etch Rates Within Ionization Chamber During F
Cleaning
[0200] The system with the ion source 10 of FIG. 7 was used to test
the F cleaning process on 1-mm-thick silicon coupons staged inside
of the ionization chamber 44, with the following modification:
rather than incorporating a dedicated reactive feed conduit, the
vapor feed conduit 32 was employed to introduce the reactive gas.
Si was used because etching of Si by F is well understood, and pure
Si material is available in the form of Si wafers. This test
required removing the vaporizer between cleaning cycles. Two coupon
locations were tested: one having line-of-sight relationship with
the reactive gas inlet (i.e., the vapor feed 32), and one not
having line-of-sight. The etch rates are shown in FIG. 15 as a
function of NF.sub.3 flow rate. During this process, a flow of 700
sccm of argon was maintained into the remote plasma source while
the NF.sub.3 flow rate was varied from 50 sccm to 500 sccm. A
line-of-sight geometry shows a factor of about five increase in
etch rate, and is therefore a preferred geometry if it can be done
uniformly. To this end, the geometry portrayed in FIG. 3 should
provide better etch uniformity of the ion source ionization chamber
44 than the geometry shown in FIG. 4. The test also indicated that
location of etch-sensitive components shielded from the gas flow is
effective to provide a degree of protection to those
components.
[0201] To extend the life of components of the self-cleaning ion
generating system construction materials are selected that are
resistive to the reactive gas, and provision can be made for
shielding of sensitive components.
[0202] For the interior of the ionization chamber, as indicated
above, aluminum is employed where the temperature of the ionizing
action permits because aluminum components can withstand the
reactive gas fluorine. Where higher temperature ionizing operation
is desired, an aluminum-silicon carbide (AlSiC) alloy is a good
choice for the surfaces of the ionization chamber or for the
extraction electrode. Other materials for surfaces in the
ionization chamber are titanium boride (TiB.sub.2), Boron Carbide
(B.sub.4C) and silicon carbide (SiC).
[0203] For components exposed to the fluorine but not exposed to
the ionizing action, for instance electron source components such
as electrodes, the components may be fabricated of Hastelloy,
fluorine-resistant stainless steels and nickel plated metals, for
instance nickel-plated molybdenum.
[0204] Both inert gas shields and movable physical barriers can
protect components of the system from the reactive gas during
cleaning. For example, referring to FIG. 7A, a conduit 113 for
inert gas, for instance argon, extends from a gas source, not
shown. Its outlet is at a strategic location in the ion source,
such that flow of the inert gas, when initiated for the cleaning
cycle, floods the component to be protected. In FIG. 7A the outlet
113a of inert gas conduit 113 aims a flooding stream of argon over
the active components of electron gun 112, including, the
electron-emitting cathode. In FIG. 7A a movable shield member 73 is
also shown, which is movable into position for the cleaning cycle.
In the example shown, it is movable over the aperture 71A leading
to beam dump 72, or to another electron gun when provided on that
side of the ionization chamber 44.
[0205] The cleaning process described above was conducted to
observe its effect on boron deposits within the ionization chamber
and on the interior of the ion extraction aperture of the novel ion
source 10 of FIG. 7. The observed etch rates had characteristics
similar to the plot of FIG. 15, but were a factor of 3 lower. Thus,
for a NF.sub.3 flow rate of 500 sccm, the etch rate for decaborane
deposits were 7 .mu.m/min (no line-of-sight), and 36 .mu.m/min
(line-of-sight). The interior of the ion extraction aperture after
running 4 hrs of decaborane at 0.8 sccm vapor flow had about 133
.mu.m thick boron-containing deposit prior to cleaning.
Observations were made after a 5 min F clean, and after a 15 min F
clean using these flow rates. One side of the aperture plate was in
line-of-sight with the vapor feed. It was observed from the
cleaning pattern that the vapor feed aperture is centered in the
vertical direction! After 15 minutes of cleaning, the plate was
almost completely free of deposits. Also, the novel heated aluminum
ion extraction electrode of FIG. 10 was removed and inspected after
long operation. It was very clean with no observable decaborane
deposits. This was undoubtedly due to exposure of the electrode to
reactive F (F can flow through the ion source ion extraction
aperture located in front of the vapor conduit, to the extraction
electrode directly in front of it). Also, elevated temperature of
the Al electrode assembly increased the effective etch rate of its
deposits.
[0206] With respect to the ionization chamber, again, a 15 min etch
clean left the chamber nearly free of deposits. A test was
conducted in which the system was repeatedly cycled in the
following manner: two hours of decaborane operation (>500 .mu.A
of analyzed beam current), the source was turned off and the
filament allowed to cool, followed by a 15 min chemical clean at
500 sccm of NF.sub.3 feed gas and 700 sccm of Ar, to see if
conducting repeated chemical cleaning steps was injurious to the
ion source or extraction electrode in any way. After 21 cycles
there was no measurable change in the operating characteristics of
the ion source or the electrode. This result demonstrates that this
F cleaning process enables very long lifetime in ion source
operation of condensable species.
The Ion Generating System Incorporated Into an Exemplary Ion
Implanter
[0207] FIG. 16 shows the basic elements of a commercial ion
implanter, with an embodiment of the novel ion beam generation
system incorporating the ion source of FIG. 7 installed. The ion
source 10 is inserted into the source vacuum housing 209 of the ion
implanter. It is electrically insulated from housing 209 by
insulator 211. The ion extraction electrode 220 extracts and
accelerates ions from the ion source 10 to form an ion beam 200.
Ion beam 200 propagates entirely in vacuum; from the electrode 220
it enters analyzer housing 290, 300 where it is bent and dispersed
by dipole analyzer magnet 230 into separate beamlets which differ
by their charge-to-mass ratio. The ion beamlet of interest passes
through mass resolving aperture 270 and into a final acceleration
(or deceleration) stage 310. The thus-produced,-selected
and-accelerated ion beam 240 leaves the ion beam forming system 208
and is introduced to the process chamber 330 where it intercepts
one or more device wafers 312 on rotating disk 314. The ion source
vacuum housing 209 can be isolated from the remainder of the
implanter's vacuum system by closing isolation valve 210. For
example, isolation valve 210 is closed prior to in situ cleaning of
the ion source and the ion extraction electrode 220. The extraction
electrode 220, may be temperature controlled in any of the ways
described above.
[0208] Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. Thus, it is
to be understood that, within the scope of the appended claims, the
invention may be practiced otherwise than as specifically described
above.
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