U.S. patent application number 10/801981 was filed with the patent office on 2005-12-22 for charged particle beam system.
This patent application is currently assigned to FEI Company. Invention is credited to Kimball, Brian T., Knowles, W. Ralph, Stewart, Diane K..
Application Number | 20050279934 10/801981 |
Document ID | / |
Family ID | 34838899 |
Filed Date | 2005-12-22 |
United States Patent
Application |
20050279934 |
Kind Code |
A1 |
Stewart, Diane K. ; et
al. |
December 22, 2005 |
CHARGED PARTICLE BEAM SYSTEM
Abstract
A charged particle beam system uses an ion generator for charge
neutralization. In some embodiments, the ion generator is
configured to maintain an adequate gas pressure at the ion
generator to generate ions, but a reduced pressure in the remainder
of the vacuum chamber, so that another column can operate in the
chamber either simultaneously or after an evacuation process that
is much shorter than a process that would be required to evacuate
the chamber from the full pressure required at the ion generator.
The invention is particularly useful for repair of photolithography
masks in a dual beam system.
Inventors: |
Stewart, Diane K.; (Ipswich,
MA) ; Knowles, W. Ralph; (Newbury, MA) ;
Kimball, Brian T.; (Georgetown, MA) |
Correspondence
Address: |
MICHAEL O. SCHEINBERG
P.O. BOX 164140
AUSTIN
TX
78716-4140
US
|
Assignee: |
FEI Company
Hillsboro
OR
|
Family ID: |
34838899 |
Appl. No.: |
10/801981 |
Filed: |
March 16, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10801981 |
Mar 16, 2004 |
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10665398 |
Sep 18, 2003 |
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10801981 |
Mar 16, 2004 |
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10330691 |
Dec 27, 2002 |
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60411699 |
Sep 18, 2002 |
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Current U.S.
Class: |
250/310 ;
250/309 |
Current CPC
Class: |
H01J 2237/0045 20130101;
H01J 2237/2817 20130101; H01J 2237/31742 20130101; H01J 37/244
20130101; H01J 2237/31737 20130101; G01N 23/225 20130101; H01J
2237/31798 20130101 |
Class at
Publication: |
250/310 ;
250/309 |
International
Class: |
G01N 023/00 |
Claims
1. A charged particle beam apparatus comprising: a work piece
vacuum chamber for containing a work piece and having a background
chamber pressure; a charged particle beam source; a charged
particle beam optical column for directing a particle beam along an
optical axis toward the work piece; a charged particle detector
comprising a volume including a detector gas ionizable by the
charged particles, electrodes to produce an electric field to cause
the ionization to take place, and a detector plate to detect
signals induced in the ionized gas, the charged particle detector
including a passage for delivery of the detector gas to maintain
the pressure of the detector gas around the detector sufficient to
operate the detector, while maintaining the pressure in the work
piece vacuum chamber at a significantly lower pressure.
2. The apparatus of claim 1 in which the charged particle beam
column comprises a scanning electron microscope column.
3. The apparatus of claim 1 in which the charged particle detector
comprises two plates, each plate having an aperture co-axial with
the optical axis.
4. The apparatus of claim 3 in which the gas passes in between the
two plates.
5. The apparatus of claim 1 in which the passage for delivery of
gas comprises a nozzle directing gas toward a region between the
detector plate and a work piece position.
6. A charged particle beam apparatus comprising: a work piece
vacuum chamber for containing a work piece and having a background
chamber pressure a charged particle beam source; a charged particle
beam optical column for directing a particle beam along an optical
axis toward the work piece; an ion generator in which secondary
particles generated by the impact of charged particle beam on a
work piece or particles from the primary beam backscattered by the
work piece ionize an ion producing gas, the ion generator
positioned such that at least some of the ions travel to work piece
to neutralize charge on the work piece, the ion generator including
a chamber containing a gas, the chamber connected to the work piece
vacuum chamber though an aperture that allows secondary or
backscattered particles from the work piece to enter the chamber
and allows ions to exit the chamber to neutralize charge on the
work piece.
7. The charged particle beam apparatus of claim 6 in which the
charged particle beam optical column includes an objective lens and
an optical axis and in which ion generator is positioned such that
a line drawn from the center of the aperture to the intersection of
the optical axis with the work piece is not parallel to the optical
axis.
8. The charged particle beam apparatus of claim 6 in which the
charged particle beam optical column comprises a scanning electron
microscope column.
9. A charged particle beam apparatus comprising: a work piece
vacuum chamber for containing a work piece and having a background
chamber pressure a charged particle beam source; a charged particle
beam optical column for directing a particle beam toward the work
piece; an ion generator in which secondary particles generated by
the impact of charged particle beam on a work piece or particles
from the primary beam backscattered by the work piece ionize an ion
producing gas, the ion generator positioned such that at least some
of the ions travel to work piece to neutralize charge on the work
piece, the ion generator configured such that the ion producing gas
is maintained at a sufficiently high pressure at the ion generator
to produce sufficient ions from the secondary or backscattered
particles to neutralize charge accumulation on the work piece,
while the background chamber pressure remains at a significantly
lower pressure.
10. The charged particle beam apparatus of claim 9 in which the ion
producing gas is maintained at a pressure greater than about 0.1
Torr and in which the background chamber pressure is maintained at
a pressure of less than about 0.01 Torr.
11. The charged particle beam apparatus of claim 9 in which the ion
producing gas is maintained at a pressure greater than about 0.3
Torr and in which the background chamber pressure is maintained at
a pressure of less than about 10.sup.-3 Torr.
12. The charged particle beam apparatus of claim 9 in which the ion
producing gas is maintained at a pressure greater than about 0.4
Torr and in which the background chamber pressure is maintained at
a pressure of less than about 10.sup.-3 Torr.
13. The charged particle beam apparatus of claim 9 in which the ion
generator comprises an a particle detector using gas ionization
amplification.
14. The charged particle beam apparatus of claim 13 in which the
particle detector comprises a plate having an aperture co-axial
with the charged particle beam.
15. The charged particle beam apparatus of claim 14 in which the
charged particle beam column includes a magnetic immersion
objective lens and in which the detector plate is positioned above
a work piece position and below a pole of the magnetic immersion
objective lens.
16. The charged particle beam apparatus of claim 13 in which the
particle detector includes a passage for transporting the ion
producing gas.
17. The charged particle beam apparatus of claim 9 in which the
charged particle beam includes an objective lens and an optical
axis and in which ion generator is positioned such that a line
drawn from the center of the aperture to the intersection of the
optical axis with the work piece is not parallel to the optical
axis.
18. The charged particle beam apparatus of claim 9 in which the ion
generator comprises a chamber containing a gas the chamber
communicating to the work piece vacuum chamber though an aperture
that allows secondary particles from the work piece to enter the
chamber and allows ions to exit the chamber to neutralize charge on
the work piece.
19. The charged particle beam apparatus of claim 9 in which the ion
producing gas increases the etch rate of charged particle beam or
decomposes in the presence of the charged particle beam to deposit
a material on the work piece.
20. The charged particle beam apparatus of claim 9, further
comprising: a second charged particle beam source; and a second
charged particle beam column for directing a second beam of charged
particles toward a work piece positioned in the work piece vacuum
chamber, a detector for detecting charged particles emitted from
the work piece upon impact of particles in the second charged
particle beam, the pressure in the work piece vacuum chamber being
sufficiently low to operate the second charged particle beam
column.
21. An ion generator for controlling charge on a sample that
produces secondary electrons as it is being worked on in a sample
chamber, comprising: a body having rear and forward ends and a gas
inlet opening to be controllably supplied with a gas, the forward
end having an aperture opening to receive the secondary electrons
and to emit positively charged ions; a detector electrode mounted
within said body; and a channel electrode mounted within the body
between the detector electrode and the aperture opening to channel
the secondary electrons toward the detector electrode, the channel
and detector electrodes defining an inner volume, wherein the body
is configured to maintain the supplied gas at least within the
inner volume at a working pressure sufficiently higher than that of
the sample chamber to promote gas ionization cascades thereby
generating positively charged ions to be emitted from the inlet
opening and providing an amplified secondary electron signal to the
detector electrode.
22. The ion generator of claim 21, wherein the channel electrode is
formed as part of the body.
23. The ion generator of claim 21, wherein the channel electrode is
conical in shape.
24. The ion generator of claim 21, wherein the channel and detector
electrodes are electrically isolated from one another.
25. The ion generator of claim 21, wherein the channel electrode
further comprises a plurality of discretely biased electrode
components.
26. The ion generator of claim 21, further comprising a
controllable magnetic field generation structure proximal to the
aperture opening for guiding secondary electrons into the aperture
opening.
Description
[0001] This application is a continuation-in-part of U.S. patent
application Ser. No. 10/665,398, filed on Sep. 18, 2003, which is
hereby incorporated by reference, which claims priority from U.S.
Prov. Pat. App. No. 60/411,699, filed Sep. 18, 2002, and which is a
continuation-in-part of U.S. patent application Ser. No.
10/330,691, filed on Dec. 21, 2002.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to the field of charged
particle beam tools for forming, altering and viewing microscopic
structures.
BACKGROUND OF THE INVENTION
[0003] Photolithography is a process that is used to create small
structures, such as integrated circuits and micromachines. The
photolithography process entails exposing a radiation-sensitive
substance, called photoresist, to a pattern of light or other
radiation. The pattern is typically created by passing the
radiation through a mask, that is, a substrate having a pattern on
the surface. The pattern blocks some of the radiation or changes
its phase to create exposed and unexposed areas on the
radiation-sensitive material. The size of the structure that can be
produced is limited by the wavelength of radiation used; shorter
wavelengths can produce smaller structures.
[0004] As photolithography processes are called upon to produce
ever-smaller structures, lithography systems are being developed
that use smaller wavelengths of radiation, including infrared and
even x-ray radiation. (The terms "light" and "photolithography" are
used in a general sense to also include radiation other than
visible light.) Photolithography Systems are now being developed
that can produce structures having dimensions of 70 nm and smaller.
Such structures can be fabricated using light having a wavelength
of 193 nm or 157 nm. Some photolithography masks for such short
wavelengths use a reflective, rather than a transmissive, pattern
on the mask because the mask substrate is not sufficiently
transparent to such radiation of short wavelengths. In such masks,
radiation is reflected from the mask onto the photoresist.
[0005] A photolithography mask must be free of imperfections if the
mask is to accurately produce the desired exposure pattern. Most
newly fabricated masks have defects such as missing or excess
pattern material. Before such masks can be used, the defects are
repaired, often by using a charged particle beam system. Dual beam
systems that include an ion beam and an electron beam can be used
in mask repair. The ion beam is used to etch away excess material
on the mask to deposit material onto the mask, or to form images of
the mask. The electron beam is also used to form images of the mask
and sometimes to deposit or etch material. When a charged particle
beam is applied to a mask, which is typically fabricated on an
insulating substrate, electrical charge tends to accumulate on the
substrate. The electric charge adversely affects the operation of
the charged particle beam by affecting the shape and positioning of
the beam spot.
[0006] One method of neutralizing or reducing accumulated charge
entails using an electron flood gun to direct electrons at a
positively charged substrate. Such a system is described, for
example in U.S. Pat. No. 4,639,301 to Doherty et al. Another
method, described in C. K. Crawford in "Charge Neutralization Using
Very Low Energy Ions," Scanning Electron Microscopy/1979/II, is to
use a beam of very low energy positive ions to neutralize a
build-up of negative charges. The ions are generated by a high
voltage that ionizes a gas within an ion generator, so the number
of ions produced in Crawford's system is determined by factors
unrelated to the charge accumulation on the work piece. Such
systems were not easy to use because they needed to be balanced for
any change in operational conditions, or sample properties. Use of
such systems declined with the introduction of low vacuum SEMs and
ESEMS and with the increased use of field emission gun SEMs, which
allowed satisfactory imaging at lower voltages, thereby reducing
work piece charging.
[0007] It is a common technique to use a charged particle beam to
form an image of the work piece by collecting secondary or back
scattered particles emitted as the primary beam scans the work
piece surface. The brightness of each point on the image
corresponds to the number of secondary particles collected as the
beam impacts each point on the substrate. (The term "secondary
particle" is used herein to include any particle coming off of the
work piece, including back-scattered particles.) Electrical
charging of the insulating substrate affects imaging by affecting
the paths of secondary particles.
[0008] One technique for detecting secondary particles emitted by
the impact of a primary electron beam is described in U.S. Pat. No.
4,785,182 to Mancuso, et al., which describes a secondary electron
detector for use in an environmental scanning electron microscope
("ESEM"). The detector device consists of an electrode, to which an
electrical potential is applied to produce an electric field.
Secondary particles emitted at the substrate are accelerated toward
the detector and collide with gas molecules, producing additional
charged particles, which in turn collide with other gas molecules
to produce even more charged particles. Such a process is called a
"cascade" effect. The ultimate number of charged particles produced
in this manner is proportional to the number of secondary particles
emitted at the substrate, thereby producing an amplified signal
corresponding to the number of secondary particles. The electron
source and much of the path of the primary beam is maintained at a
high vacuum by an aperture that passes the primary beam but
prevents most gas molecules from entering the column.
[0009] In an ESEM detector, the path length of the secondary
particles through the detector gas must be sufficiently long to
allow enough collisions with gas molecules to provide adequate
amplification. To increase the probability of collisions, detectors
are typically positioned away from the work piece to provide a
relatively long path length as the particles move from the work
piece to the detector. Increasing the gas pressure also increases
the probability of a collision while traversing a particular path.
Gas pressure in an ESEM is typically maintained at around 0.5 to 5
Torr between the work piece and the detector to provide sufficient
gas molecules to produce the cascade effect.
[0010] Another way of increasing amplification is to provide a
magnetic field that causes the particles to move in a curved path
or loops within the gas. U.S. Pat. No. 6,184,525 to Van Der Mast
describes the use of an electrostatic multipole structure to
increase the path length of the secondary electrons to increase the
probability of collisions with the gas molecules. Similarly, U.S.
Pat. No. 6,365,896 to Van der Mast describes adding an additional
magnetic fields between the detector and the specimen holder to
lengthen the path of the secondary electrons even further to
produce a higher degrees of ionization. Both van der Mast patents
are assigned to the assignee of the present application. Japanese
Pat. Publication No. 5-174768 also describes an ESEM with a
detector positioned in the magnetic field of an objective lens of
an electron microscope to increase amplification. Japanese Pat.
Publication No. 5-174768 also describes that ions generated by the
detector of an environmental scanning electron microscope can
neutralize charge build-up on a work piece.
[0011] The relatively high gas pressure required for an ESEM
detector makes such system unsuitable for use in the same vacuum
chamber as another charged particle beam system, such as an ion
beam system or a non-ESEM SEM, because most charged particle beam
systems cannot operate at the relatively high gas pressures
required by an ESEM. The gas molecules interfere with the ions or
electrons in the beam, reducing resolution or degrading signal to
noise ratio.
SUMMARY OF THE INVENTION
[0012] An object of the invention is to provide a system for
imaging or processing microscopic structures, and is particularly
useful for, though not limited to, imaging or processing
microscopic structures on a insulating substrate using a multiple
beam system in which at least one column includes an ion
generator.
[0013] The invention provides a method and apparatus for
controlling the electrical surface potential on a substrate, such
as a quartz-based lithographic mask, in a charged particle beam
system. A preferred embodiment includes an ion generator that uses
a gas that is ionized preferably by collisions with secondary
charged particles generated by the impact of the primary charged
particle beam with the substrate. In some embodiments, the ion
generator can be used to provide amplification for a secondary
particle signal, such as in an environmental scanning electron
microscope. In some embodiments, the ion generator can also be used
to provide a gas for chemical assisted charged particle beam
etching or deposition. While different embodiments of the invention
may be capable of performing charge neutralization, secondary
particle signal amplification, supplying gas for gas-assisted
charged particle beam operations, and other functions, not all
embodiments will provide all functions.
[0014] In some embodiments, an ion generator generates ions that
neutralize charge on a work piece while maintaining a relatively
low gas pressure in the vacuum chamber away from the ion generator.
The pressure is sufficiently high at the ion generator to generate
ions by collisions of secondary particles with the gas molecules,
while being sufficiently low in the remainder of the vacuum chamber
so that the time required to evacuate the system to a pressure
suitable for operating a non-ESEM charged particle beam system is
greatly reduced compared to the time required to evacuate the
chamber from the operating pressure of the ESEM.
[0015] The foregoing has outlined rather broadly some of the
features and technical advantages of various aspects of a preferred
system of the present invention in order that the detailed
description of the invention that follows may be better understood.
Additional features and advantages of the invention will be
described hereinafter. It should be appreciated by those skilled in
the art that the conception and specific embodiment disclosed
herein may be readily utilized as a basis for modifying or
designing other structures for carrying out the same purposes of
the present invention. It should also be realized by those skilled
in the art that such equivalent constructions do not depart from
the spirit and scope of the invention as set forth in the appended
claims. It should also be realized that while a preferred system
for repairing photomasks may implement many of the inventive
aspects described below, many of the inventive aspects could be
applied independently, or in any combination, depending upon the
goals of a specific implementation.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For a more thorough understanding of the present invention,
and the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0017] FIG. 1A is a schematic showing a dual beam system embodiment
of the invention in which two charged particle beam optical columns
are oriented approximately parallel to each other.
[0018] FIG. 1B is a schematic showing a dual beam system embodiment
of the invention in which two charged particle beam optical columns
are not oriented parallel to each other.
[0019] FIG. 2 is a schematic showing an embodiment of the invention
that includes a dual beam system when used with an electron column
with an immersion magnetic objective lens with an ESEM detector
that generates ions.
[0020] FIG. 3 shows an embodiment of the invention that uses an ion
generator for charge neutralization when used with an electron
column with a non-immersion magnetic objective lens.
[0021] FIGS. 4A and 4B show electron beam images of a binary mask,
FIG. 4A showing an image taken when the mask electrically charged
and FIG. 4B showing an image taken the charge neutralized in
accordance with an ESEM-type neutralizer.
[0022] FIGS. 5 and 6 show the magnetic and electric field structure
of an ion generator or detector suitable for use with a magnetic
immersion lens, and the trajectory of an electron in this field
structure
[0023] FIG. 7 shows an embodiment of the invention using an ESEM
detector with an immersion lens.
[0024] FIG. 8 shows an embodiment of the invention in which a gas
passage is incorporated into an ESEM detector with the work piece
close to the detector.
[0025] FIG. 9 shows an embodiment of the invention in which a gas
passage is incorporated through the anode of an ESEM detector.
[0026] FIG. 10A is a perspective view of one embodiment of an ion
generator with a detector.
[0027] FIG. 10B is a side sectional view of the ion generator of
FIG. 10B.
[0028] FIG. 10C is an end sectional view of the ion generator of
FIGS. 10A and 10B taken along line 10C-10C in FIG. 10B.
[0029] FIG. 10D is an end sectional view of the ion generator of
FIGS. 10A and 10B taken along line 10D-10D in FIG. 10B.
[0030] FIG. 11 is a side sectional view of an on-axis embodiment of
an ion generator with a detector.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] A preferred embodiment of the invention uses an ion
generator to neutralize negative charge in insulating samples in a
charged particle beam system. Because photolithography masks are
typically fabricated on an insulating substrate, such as quartz,
the invention is useful for charged particle beam operations on
photolithography masks.
[0032] The ion generator preferably uses secondary or backscattered
electrons emanating from the work piece to ionize a gas, in a
manner similar to the way a detector works in an environmental
scanning electron microscope (ESEM). The secondary particles
collide with and ionize the gas molecules as the particles pass
through the gas, producing free electrons which then collide with
and ionize other gas molecules in a cascading reaction. This ion
generation process can generate large quantities of ions for use in
stabilizing the charge on insulating samples or for imaging.
[0033] By using secondary particles from the work piece to generate
the ions, the number of ions generated will be related to the
amount of charge impinging on the workpiece from the charged
particle beam. Any change in the workpiece potential due the
charged particle beam works to automatically regulate the number of
ions reaching the work piece. The system can thus provide a
self-stabilizing ion generator for neutralizing charge on a work
piece in charged particle beam system. Once the gas pressure and
ion generator are set-up for controlling the charge, then other
microscope parameters, such as beam energy, scan speed, and beam
current, can be altered without upsetting the control--the system
can be self-regulating.
[0034] Some embodiments of the invention use an environmental
scanning electron microscope (ESEM) detector, which generates ions
to amplify the secondary electron signal, to also provide ions for
neutralizing charge on the work piece. Other embodiments of the
invention use an ion generator that is a separate device, not
connected with an ESEM detector.
[0035] Although an ESEM detector generates ions, and those ions can
be used for charge neutralization, ESEMs have been unsuitable for
use in a multiple beam system, such as a dual beam system including
an ESEM and a focused ion beam (FIB) column. ESEMs typically
operate at pressures of about 0.5 to 5 Torr, whereas a focused ion
beam typically operates at pressures of about 10.sup.-5 Torr. Thus,
in a dual beam system containing an ESEM and a focused ion beam
column, the user would have to reduce the pressure in the sample
vacuum chamber from 0.5 Torr to 10.sup.-5 Torr when switching from
using the ESEM to using the FIB. The time required to pump the
vacuum chamber down from 0.5 Torr to 10.sup.-5 Torr is a serious
disadvantage to using an ESEM together with a FIB in most
commercial applications. Even if an ESEM detector uses a gas jet
positioned in the detector area, rather than flooding the chamber
with the detector gas, the chamber still fills with the detector
gas, which must be evacuated before using the ion beam.
[0036] Some embodiments of the invention make practical the use of
an ESEM and a FIB or SEM in the same vacuum chamber. In some
embodiments, the system design tends to localize the gas in a
comparatively small volume near the ion generator. In such
embodiments, the system maintains a different pressure at different
locations in the system so that the gas pressure is higher at the
location where ions are to be generated, and lower at other parts
of the system so as to reduce interference with the charged
particle beams. For example, in one embodiment, when the pressure
in a small volume near the ESEM detector is about 0.5 Torr, the
background pressure in the chamber is maintained at about 10.sup.-4
Torr. When switching to the FIB, the chamber then only needs to be
pumped from 10.sup.-4 Torr to 10.sup.-5 Torr, instead of from 0.5
Torr to 10.sup.-5 Torr. Thus, switching from ESEM operation to FIB
operation is much quicker, and a dual beam ESEM-FIB is practical
for mask repair and other applications. In some embodiments, it may
be possible to operate the ESEM detector or other ion generator and
FIB simultaneously.
[0037] By returning some of the ions generated by the ESEM detector
or other ion generator to the work piece to neutralize charge, the
ESEM or other imaging system can produce images that show features
on chrome absorbers with a resolution of less than 2 nm. The charge
neutralization provided by the ion generator can be controlled in
part by controlling the pressure and identity of the gas or gas
mixture. The gases or gas mixtures used for ion generation can also
be used for charged particle beam assisted etching or deposition to
repair defects. The use of gases for etching or deposition in the
presence of a charged particle beam is referred to as "beam
chemistry." In some embodiments, gases coming from the ion
generator for charge neutralization can alternate with different
gases for etching or deposition, and in other embodiments gas
mixtures may be used.
[0038] Different electron final lens configurations will typically
require different designs to produce ions for neutralization for
secondary particle detection and/or charge neutralization. The
several embodiments described below provide example of designs that
can be used with different types of electron lenses and, by using
these examples and the principles disclosed, skilled persons can
design detection/neutralization configurations to work with other
types of lenses.
[0039] FIG. 1A shows schematically a dual beam system 100 that can
be used, for example, for advanced mask repair and metrology and
that can incorporate the present invention. The invention is not
limited to use in a dual beam system, but can be used in a single
beam system or a multiple beam system. System 100 comprises a first
charged particle beam column 102 and a second charged particle beam
column 104, the axes of columns 102 and 104 being oriented
approximately parallel to each other and approximately normal to
the surface of a work piece 106. A work piece holder or stage 108
can move the work piece 106 to accurately position it under either
column. Columns 102 and 104 and work piece 106 are contained within
a vacuum chamber 110.
[0040] The invention is not limited to any particular types of
columns. For example, an embodiment of the invention could comprise
any combination of ion beam columns, such focused beams columns or
shaped beam columns, and electron beam columns, such as ESEM and
non-ESEM columns. The term ESEM as used herein applies broadly to
any electron column configuration that uses the ionization of gas
by secondary electrons or backscattered electrons to generate ions
for charge neutralization, and/or as part of a detector, while
maintaining the electron column at a high vacuum using differential
pumping.
[0041] FIG. 1A shows, for example, an ESEM detector 120 used with
immersion lens column 104 and a separate ion generator 122 used
with non-immersion lens column 102 for charge neutralization.
Column 102 also includes a non-ESEM secondary particle detector
124, such as a scintillator detector or a channel plate detector.
The two electron columns offer different views of the sample. The
immersion lens column gives a high resolution image with a
symmetrical detector, whereas the non-immersion column allows for
detectors that offer a directional component to the image. The ion
generator 122 could be replaced with an ESEM detector for both
charge neutralization and signal detection. Different
implementations could use different combinations of ESEM detectors,
non-ESEM detectors, and ion generators.
[0042] FIG. 1B shows a system 150 in which a first charged particle
beam column 152 and a second charged particle beam column 154 are
arranged such that their beams are coincident or nearly so. That
is, one column is tilted with respect to the other column, so that
both beams impact the work piece at the same, or nearly the same,
point. If the impact points are offset, work piece holder 108 can
move the work piece 106 to accurately position it under the impact
point of either column. Columns 152 and 154 and work piece 106 are
contained within a vacuum chamber 160. Column 152 and 154 can use
any combination of ESEM ion generators, non-ESEM ion generators,
and conventional detectors and charge neutralizers, although the
tilt of one column may physically make some combinations difficult
to implement. FIG. 1B shows a charge generator 156 that provides
charge neutralization. Charge generator 156 could be an electron
flood gun or an ESEM type ion generator.
[0043] At least one of the two columns in a dual beam system used
for mask repair is preferably tilted or tiltable with respect to
the work piece surface. Using a tilted beam can provide
three-dimensional information about the work piece.
Three-dimensional information is useful, for example, in the repair
of quartz bump defects on a phase shift mask. Such defects, being
made of the same material as the substrate, do not exhibit much
contrast with the substrate in an image, and so can be difficult to
repair without damaging the substrate. U.S. patent application Ser.
No. 10/636,309 "Repairing Defects On Photomasks Using A Charged
Particle Beam And Topographical Data From A Scanning Probe
Microscope," describes a method of using three-dimensional
topographical information to repair defects in phase shift masks. A
tilted charged particle beam can be used to provide a
three-dimensional image instead of the scanning probe microscope
described in U.S. patent application Ser. No. 10/636,309. If a
charged particle beam system provides the three-dimensional data,
it becomes unnecessary to remove the work piece from the vacuum
chamber to obtain the information, thereby improving
productivity.
[0044] FIG. 2 shows a system 200 that includes an ion beam column
202 and an environmental scanning electron microscope column 204
positioned within a vacuum chamber 206. Environmental scanning
electron microscope column 204 includes a magnetic immersion
objective lens 207, which has a high magnetic field at the work
piece. Focused ion beam column 202 includes an ion source 203,
preferably a gallium liquid metal ion source (LMIS). The invention
is not limited to any particular type of ion source, and other ion
sources could be used, such as a silicon/gold eutectic LMIS or a
plasma ion source. Ion beam column 202 can use a focused beam or a
shaped beam. Ion beam column 202 can be used to remove material
from the surface of a work piece 208, either by sputtering or by
chemical-assisted etching, or to deposit material on the surface of
a work piece 208, using ion beam assisted deposition in which a
precursor gas is decomposed in the presence of the ion beam to
leave a deposit on the surface.
[0045] The combination of the large flat detector 210 and the large
flat work piece 208 provides some containment for the detector gas,
making it possible for a vacuum pump 214 to maintain a pressure
between detector 210 and work piece 208 that is several orders of
magnitude higher than the general pressure in other parts of
chamber 206.
[0046] ESEM column 204 includes an ESEM-type detector 210 connected
to an amplifier 216 and a gas injector 218 that provides the gas
used to amplify the secondary particles emitted from work piece
208. The gas can also be used for chemical assisted electron beam
etching or deposition. Stage 220 positions the work piece 208 under
either column 202 or column 204, as desired.
[0047] FIG. 3 shows an embodiment of the invention using a
self-stabilizing ion generator for neutralizing charge accumulated
on the work piece by operation of an electron beam column with a
non-immersion lens (i.e., a lens with low magnetic field at the
work piece). FIG. 3 shows an electron beam column 304 including a
non-immersion objective lens 305 that focuses an electron beam 306
onto the work piece 307, which is enclosed in a system vacuum
chamber 308 that may also contain a second charged particle beam
column, such as a focused ion beam system.
[0048] The ion generator 302 comprises an enclosed tube 310 having
an orifice 312 to let ions 314 leave tube 310 to reach impact point
316 where electron beam 306 impacts work piece 307. A high voltage
electrode 317 accelerates the secondary particles within tube 310.
A pipe 318 brings gas as shown by arrow 319 into tube 310.
[0049] The ion generator tube 310 is preferably maintained at a
small positive bias voltage, preferably between about 10 V and
about 500 V, and typically at about 200 V, to attract some of
secondary electrons 320 into the tube. High voltage electrode 317,
which is preferably maintained at a potential of between about 300
V and about 2000 V, and typically at about 500 V, further
accelerates secondary electrons 320 within tube 310 to trigger gas
ionization cascades in tube 310. The ions 314 flow away from the
electrode 317 and out of the orifice 312. The positive bias on the
tube (and any sample charging) accelerates the ions towards the
sample. If there were an excess of ions, the sample would become
positively charged, which would reduce the ion flow to the sample
and cause the excess ions to flow to the objective lens or back to
the tube. Thus, the neutralization is self-regulating. The self
regulation may be enhanced by the addition of an electrode 330
which may be grounded or biased to cause the excess ions to flow to
the electrode rather than the lens
[0050] The collection and detection of the majority of secondary
particles for imaging and analysis can be performed using any
conventional high vacuum detector 322, such as a scintillator
detector or a channel plate detector. Secondary particles can be
collected off-axis near the work piece as shown in FIG. 3 or
through the lens. Thus, by using ion generator 302, one can achieve
a system that can use non-ESEM detectors, while providing charge
neutralization by an ion generator. The vacuum chamber is not
flooded with a gas that would preclude operating a non-ESEM-type
charged particle beam, either simultaneously or after a relatively
brief pump down.
[0051] The switchover from using electron column 304 to using an
ion beam column (not shown) in chamber 308 can be very quick. The
orifice 312 is small enough so that the gas leakage into the system
vacuum chamber 308 is small, and gas pressure in chamber 308 can be
maintained at a lower level than the pressure in the more open ion
generator design of an ESEM detector. To switch from using column
304 to using the ion beam, an operator will typically turn off the
gas to the ion generator, and then vent the small ion generator
volume to the main chamber. Alternatively, a valve could be placed
in orifice 312. In some embodiments, it may be possible to use the
ion generator as a gas injector, that is, to direct toward the work
piece an etch enhancing gas or a gas that decomposes in the
presence of a charged particle beam to deposit a material. Such
gases are well known in the art.
[0052] A disadvantage of ESEMs is their relatively slow imaging
speed, typically less than about 1 microsecond per pixel. By using
an ion generator such as ion generator 302, one achieves the
benefits of ion generator charge neutralization while being able to
use a non-ESEM detector for faster imaging speed. This advantage
can also be achieved in a single beam system or a multiple beam
system, when at least one column that uses a non-ESEM detector.
[0053] Because of the amplification effect of the ion generator,
only a relatively small number of secondary particles need to be
collected by the ion generator to generate sufficient ions for
charge neutralization, and most of the secondary particles are
therefore available for collection by the imaging system. When the
ion generator is used for charge neutralization and not also as an
amplifier for the secondary particle signal, significantly lower
amplification can be used. Because the amplification depends on the
gas pressure in tube 310, a lower pressure can then be used, which
will reduce the pressure in chamber 308.
[0054] The appropriate pressure in tube 310 will depend upon the
voltage on electrodes inside the ion generator and the number and
energy of the captured secondary particles, which factors may vary
with the application. To determine an appropriate pressure for a
particular embodiment, one can measure the electron signal at the
electrode in order to monitor the ion generation and adjust the gas
pressure or voltages to achieve sufficient ions to neutralize the
substrate. The gas pressure can be reduced in embodiments in which
the ions are not used for ESEM-type signal amplification.
[0055] Gas pressure in the ion generator 302 is preferably greater
than 0.1 Torr and more preferably greater than about 0.3 Torr. A
preferred pressure in the tube of about 0.5 Torr would allow a
large ion multiplication factor. The pressure is preferably less
than 1.0 Torr and more preferably less than 0.7 Torr. An orifice of
about 0.2 mm would restrict the gas flow to keep the chamber in the
10.sup.-5 Torr range. The size of the orifice will vary with the
system parameters. The orifice should be sufficiently large to
allow a significant number of secondary electrons to pass into tube
310 and to allow most of the ions to pass out of the tube 310
orifice. Skilled persons can determine an appropriate orifice size
based on the guidance provided above. The pressure in the chamber
during operation of the ion generator is preferably less than
10.sup.-2 Torr, more preferably less than 10.sup.-3 Torr, more
preferably less than about 10.sup.-4 Torr, and most preferably less
than or equal to about 10.sup.-5 Torr.
[0056] Gases that are known to be suitable for use in ESEM
detectors are typically also suitable gases for use with ion
generator 302. Desirable properties of suitable gases include low
ionization energy, oxidizing, and non-corrosive. For example, water
vapor is a suitable detector gas for amplifying the secondary
particle signal. Other suitable gases include nitrogen, argon and
carbon monoxide. The detector gas can also be mixed with another
gas used in charged particle beam assisted deposition or etching.
For example, xenon difluoride enhances etching of several materials
including silicon. Gas molecules travel to the work piece surface,
and, when activated by the charged particle beam, etch the surface.
As another example, tungsten hexafluoride and tungsten hexacarbonyl
decompose in the presence of an electron beam to deposit
tungsten.
[0057] In some embodiments, the ion generator in FIG. 3 can be used
with an immersion lens. The magnetic field from the lens will
normally inhibit the secondary electrons from reaching the ion
generator tube. In this case, it is possible place the ion
generator in a position such that high energy backscattered
electrons (that can escape from the magnetic field) enter the tube
and trigger the ion generation process. The magnetic field will not
significantly affect the ion flow out. This configuration of ion
generator would allow the use of so called "through the lens"
detectors that operate in a high vacuum, and are integral with, or
above the immersion lens.
[0058] FIGS. 4A and 4B show an electron beam image of a binary
mask, with FIG. 4A showing the image obtained when the mask is
electrically charged due to the electron beam (without any charge
neutralization) and FIG. 4B showing the image obtained when the
charge on the mask is neutralized in accordance with an embodiment
of the invention
[0059] The embodiment of FIG. 3 uses a localized high pressure
region in tube 310 for ion generation, so that the vacuum chamber
can be maintained at a much lower pressure. The embodiments
described below combine the concept of localized high pressure with
the use of ESEM detection with immersion lenses. The embodiments
described below allow effective electron detection and electron
beam charge control while maintaining low chamber pressure, which
is preferred for electron beam chemistry and dual beam
operation.
[0060] Whereas the embodiment of FIG. 3 uses an ion generator that
is separate from a detector, the embodiments below use an ion
generator that is also used for secondary particle detection. These
embodiments allow ESEM-like secondary particle detection, as well
as charge control and chemically enhanced charged particle beam
operations, particularly when used with a magnetic immersion
objective lens. Some embodiments shown provide improved signal
amplification and charge control by using a specific combination of
a magnetic field and electrostatic field, in conjunction with a
gaseous environment. Several such configurations are described in
more detail is a US Patent Application entitled "Particle-optical
device and detection means," by Scholtz et al., filed concurrently
herewith by the assignee of the present application, and is hereby
incorporated by reference.
[0061] Japanese Pat. Publication No. 5-174768 shows a column
configuration in which a magnetic field from an objective lens is
parallel to the electric field, and the application claims that the
secondary electrons are trapped around the magnetic flux lines,
thereby increasing the path length and amplification.
"Particle-optical device and detection means" described above shows
that a longer path length can be provided by suitable choice of the
electric field shape to implement the so called "magnetic Penning
mechanism." This longer path length is in the form of a damped
oscillation. One can also configure an electrode such that the
electric field includes a component that is orthogonal to the
magnetic field. Such a configuration is similar to a structure
known as a "magnetron," in which electrons travel in a circular
orbit in the presence of a radial electric field perpendicular to a
magnetic field. This configuration can greatly extend the electron
path length and provide large gaseous amplification in the presence
of gas.
[0062] To illustrate the principle, FIG. 5 shows a magnetron
structure 500 that consists of two coaxial, cylindrical electrodes,
an inner cylinder 502 and an outer cylinder 504. Inner cylinder 502
is grounded, and the outer cylinder 504 is at a positive potential.
This potential difference produces a radial electric field between
the two electrodes, as illustrated by arrows labeled "E". In
addition, there is a magnetic field labeled B, perpendicular to and
directed into the plane of the page as indicated by the circled X
and preferably uniform throughout the space between the cylinders.
An electron that starts at a place between the two electrodes will
follow a path similar to that illustrated in FIG. 6.
[0063] In a high vacuum, with the right values of electric field
(E) and magnetic field (B), the electron can move around the
structure indefinitely. However, if there is gas present then the
electron will collide with a gas molecule. If the energy of the
electron is high enough then the gas molecule may be ionized, and
two electrons are then present. These two electrons will start to
move further out towards electrode 504 and will then move around
electrode 502 in a similar path to FIG. 6, but with a larger
radius. These two electrons may further collide with gas molecules
to repeat the multiplication process. The increased path length can
generate great amplification. It may be shown, however, that the
maximum amplification will only occur with the appropriate
combinations of the magnetic field, B, and electrostatic field,
E.
[0064] The combination of the two effects, that is, the magnetic
Penning effect from the component of electric field parallel to the
magnetic field and the "magnetron" effect from the component of the
electric field orthogonal to the magnetic field, creates a greatly
enhanced amplification of the signal. To achieve amplification
simultaneously using both methods in a single embodiment requires
specific combinations of three primary parameters: gas pressure,
magnetic field strength, and electric field strength.
[0065] FIG. 7 shows an embodiment comprising a charged particle
beam system 700 having an immersion lens 702 that focuses an
electron beam 704 onto a work piece 706 resting on a stage 708 that
rests on a second pole 710 of the immersion lens. System 700 uses
as a detector an electrode plate 714 connected to amplifier 716. A
pressure limiting aperture 717 maintains a pressure difference
between a detection space 718 and the immersion lens 702 by
reducing gas flow into the immersion lens 702. Electrode plate 714
is preferably a simple, thin conductive plate having a
substantially round central aperture 719 that is substantially
coaxial with a magnetic field 722. A positive voltage, in the range
of 100 V to 2000 V may be applied to the electrode plate 714 during
operation. The bias on electrode plate 714 produces an electric
field that, because of aperture 719, is in part parallel, and in
part orthogonal to the magnetic field 722 produced by immersion
lens 702, and system 700 can therefore achieve secondary particle
signal amplification by way of the magnetic Penning and magnetron
effects described above.
[0066] A large diameter aperture 719 will produce an electric field
close to the electrode 714 that is orthogonal to the magnetic field
722, thereby providing a region that can achieve amplification by
the magnetron effect. If the hole is too small, however, then the
magnetron effect does not occur. If the aperture is too big,
however, the enhancement due to the magnetic Penning effect does
not occur. Certain hole diameters, together with the certain
corresponding values of magnetic field, electric field, and gas
pressure, can achieve both amplification mechanisms simultaneously.
When these conditions are satisfied, the amplified signal from
magnetic Penning mechanism is then compoundly amplified by the
magnetron mechanism to achieve a correspondingly large overall
amplification.
[0067] The amplification of the secondary electron signal also
produces positive ions which are needed to avoid the charging of
the sample. However, the very large amplification produced by the
two mechanism described above may create too many ions. An
additional plate 730, which may be grounded, or may be biased, can
be provided to collect the excess ions. This plate may also be
connected to an amplifier to provide a detected signal.
[0068] There are many combinations of hole diameters, magnetic
field strengths, electric field strengths, and gas pressures that
will achieve the compound mechanism. However, for any specific hole
diameter, only certain combinations of bias voltage and magnetic
field will produce the compound effect.
[0069] Amplification due to the magnetic Penning mechanism will
occur only when the peak electric potential along the axis of the
electron beam exceeds the ionization potential of the gas.
Amplification due to the magnetron effect will only occur if the
radial electric field (E) and the magnetic field (B) are such that
2*m*(E/B).sup.2/q is greater than the ionization energy of the gas,
where m is the mass of an electron and q is the charge of an
electron. Skilled persons can use this guidance to determine
appropriate diameters for a particular application. As an example,
a high amplification of more than 5000 can be provided with an
anode hole diameter of 3 mm, anode voltage of 400V, magnetic field
of 0.1 Tesla and a pressure of 0.3 Torr of water vapor.
[0070] By achieving a large overall amplification, the distance
from the work piece to the detector can be kept short, which
decreases the working distance of the lens and increases its
resolution. Also, the gas pressure at the detector can be reduced,
which reduces the overall gas pressure in the chamber, thereby
decreasing or eliminating the time required to switch from ESEM
operation to FIB or other non-ESEM beam operation. The pressure in
detector space 718 can be reduced from about 0.5 Torr to about 0.3
Torr or lower, or even, for some embodiments, to 0.1 Torr or lower.
Reducing the gas pressure at the collector further reduces the gas
pressure in the vacuum chamber away from the detector.
[0071] When the work piece has a suitable shape, for example, a
large, flat object, such as a photolithography mask or a
semiconductor wafer, and the detector is placed close to the work
piece, the geometry provides some confinement to the gas in the
area in which amplification occurs. The gas pressure therefore
tends to remain greater in the space between the detector and the
work piece, and lower in the chamber away from the detector. Some
embodiments provide an operating pressure of one or more tenths or
a Torr in the amplification zone, while maintaining a reduced
pressure in the vacuum chamber in general. Gas pressures in the
amplification zone and in the remainder of the vacuum chamber can
be similar to those described with respect to the system of FIG. 3,
although the chamber pressure will typically be somewhat higher
using an ESEM detector, because the gas is less confined.
[0072] Thus, the ESEM detector in some embodiments of the invention
can operate in a vacuum chamber containing another charged particle
beam system, with the detector gas interfering minimally or not at
all with the other charged particle beam column, or with the gas
pressure in the chamber being raised to an extent at which the
chamber can be evacuated relatively quickly to operate the other
column.
[0073] The embodiment of FIG. 8 shows a lower potion of an electron
beam system 800 that includes a magnetic immersion lens 802 and
that uses the secondary particle signal amplification principles
described above. System 800 provides enhanced gas isolation to
localize the gas and to reduce interference with the operation of
other charged particle beam columns in the same vacuum chamber.
System 800 includes an insulator 804 that supports a pressure
limiting aperture 806 and an annular detector electrode 808. An ion
trap 810 surrounds the assembly including the detector electrode
808, the insulator 804, the tip of immersion lens 802, and the
pressure-limiting aperture 806. The work piece 814 is placed close
to underside of the ion trap. A pipe 812 is used to supply gas into
the detection region. A small amount of gas escapes into the
electron column through pressure limiting aperture 806, but the
flow is low enough that the electron column may be maintained at
the required high vacuum level. A small amount of gas also passes
between the ion trap and the sample, but the flow is again low
enough that the vacuum pump on the sample chamber can maintain a
very low gas pressure in the chamber.
[0074] A hole 820 in the detector 808 for the gas to exit
preferably has a diameter on the order of magnitude of millimeters,
depending upon the system parameters. The user alters the magnetic
field of the immersion lens when the image is focused. The
potential on the detector can then be adjusted, either
automatically or manually, to create the required electric field,
and optimize the detector gain. Lens 802 provides the required
magnetic field in the throughout the detection region.
[0075] Typical gaseous detectors use with the magnetic immersion
lens perform optimally at a gas pressure of about 0.5 Torr in the
chamber and in the path of the primary beam. This embodiment
produces the improved signal amplification and charging control
described above, but also provides for operation at much lower
chamber pressures by concentrating the gas in the region where the
amplification occurs. This embodiment is particularly appropriate
to the imaging or modification of photomasks or other similar work
pieces that are flat, and large in diameter.
[0076] The embodiment of FIG. 9 shows a lower potion of an electron
beam system 900 that is very similar to that shown in FIG. 8, and
similar elements have the same designation. System 900 is more
suitable for imaging or modifying of small or irregularly shaped
samples. System 900 localizes the gas by a different method. The
detector electrode comprises two plates 908, with the gas flowing
in between the two plates from an external source through pipe 912.
Applicants have found that the main signal amplification and ion
generation happens in the region of intense electric field close to
the detector electrode. In this embodiment, the electrode is hollow
so that gas can be passed through the electrode into the detection
region. This creates a high gas pressure close to the electrode,
giving high amplification and high ion generation. The gas will
then expand into the rest of the region, so that the gas pressure
at the beam axis is much lower. This is a major advantage because
the gas in the beam path creates a "skirt" of off-axis electrons
(particularly at low beam voltages) which have several undesirable
results: the off axis electrons create a background signal in the
electron image and reduce image contrast; the off-axis electrons
will cause etching/deposition outside of the beam impact area in
e-beam chemistry. By reducing the gas in the beam path, these
undesirable results are reduced or eliminated. Another major
advantage of this design for dual beam applications on irregular
shaped samples is that the chamber pressure is much lower so that
the changeover time between e-beam operation and ion beam operation
will be reduced.
[0077] In some embodiments, one can shape the detector electrode or
the hole in the ion trap or add a bias to the ion trap such as to
preferentially control the flow of ions through the hole in the ion
trap onto the sample. Embodiments can be used not only to
neutralize charge, but also to control an electrical bias on the
mask or other work piece. Controlling the bias can provide for
optimum imaging and can improve the use of beam chemistry, that is,
the use of gases to deposit material or to enhance etching.
[0078] The embodiments above described maintain the gas at a
sufficient pressure in the ionization region to support adequate
ionization by secondary particles, yet maintain the pressure in the
rest of the chamber that is either low enough to allow, or be
evacuated rapidly to allow, the use of gases directed toward the
work piece for charged particle beam deposition or
chemically-enhanced charged particle beam etching. The pressure is
also low enough in other parts of the vacuum chamber so as to not
interfere with the operation of the primary beam columns.
Other Embodiments
[0079] FIGS. 10A through 10D show another embodiment of an ion
generator for use with non-immersion lens electron columns, such as
with the system shown in FIG. 3. The depicted ion generator is
capable of both generating and streaming ions onto the sample and
detecting secondary electrons for imaging the work-piece. It works
well in a dual beam environment where it is desirable to quickly
change and/or maintain different pressures both at the work piece,
as well as with the gas used for ion generation, electron
amplification, or chemical processing. It can also be used in other
systems where the maintenance of separate pressures or gas
environments is beneficial. Such systems could include but are not
limited to SEM, ESEM, FIB and other imaging and charged particle
systems.
[0080] In the depicted embodiment, the ion generator includes a
body 1000 formed from a rear portion 1002 and a forward portion
1004 attached to, but electrically isolated from, each other. These
body portions are formed to contain and properly support
electrodes, along with the other components described herein. Both
rear and forward body portions are typically conductive with the
rear portion 1002 set to ground and the forward portion 1004
charged to a reasonably high positive value suitable for attracting
secondary work piece electrons to the ion generator. The rear
portion 1002 has gas inlet orifice 1003, while the forward portion
1004 has a gas outlet aperture 1005. Orifice 1003 is an
unrestricted input for receiving a gas suitable for both ion
generation and imaging. Process gasses can also be included in the
input gas stream. Aperture 1005 defines an aperture or other
restricted opening that limits the flow of imaging gas out of the
ion generator 1000 and into the work piece chamber.
[0081] The depicted ion generator also includes ion generator cell
1006 formed by detector electrode 1007 and channel electrode 1008.
In the depicted embodiment, the detector electrode 1007 is a disk
adjacent with, but electrically isolated from, a conically shaped
channel electrode 1008. The disk may be perforated to allow gas,
from orifice 1003, to more efficiently diffuse within the interior
1010 of the ion generator cell 1006. In the depicted embodiment,
the channel electrode 1008 is separate from the forward body
portion 1004, but in other embodiments, it could be formed
integrally with the forward body portion 1004.
[0082] The channel electrode 1008 has a relatively large opening
adjacent to detector electrode 1007. It may or may not be sealably
connected at this opening with the detector electrode 1007, but it
should be electrically isolated from it. At its other end, the
channel electrode 1008 has a smaller aperture opening 1009 (next to
aperture 1005) for passing charged particles, e.g., electrons
(indicated by the "-" character) and positively charged ions
(indicated by the "+" character), into and out from the ion
generator cell 1006. The interior 1010 of ion generator cell 1006
constitutes a volume of high pressure gas that is used for
generating the positively charged ions and amplifying the
negatively charged electrons. Depending on desired operational
parameters, gas pressure within the interior 1010 may be greater
than or equal to the gas pressure outside of the ion generator cell
1006 but still within the ion generator body 1000.
[0083] The detector electrode 1007 attracts the electrons toward it
inducing gas cascading, which generates the positively charged ions
for charge neutralization and free electrons for the amplified
image signal. The detector electrode 1007 also collects the
electrons for generating the image signal, which is further
amplified by amplifier 1017 and sent to an imaging system 1019. It
should be recognized, however, that other image detection schemes
could be used. For example, other gas cascade techniques such as
(1) detecting ions generated in the gas (e.g., as taught in U.S.
Pat. No. 4,785,182, incorporated herein by reference), or (2)
detecting the light generated in the gas during the cascade process
(e.g., as taught in U.S. Pat. No. 4,992,662, incorporated herein by
reference) could also be implemented. The channel electrode 1008 is
an electrostatic structure that facilitates efficient movement of
ions out of the ion generator cell in the direction of the imaged
area of the work piece.
[0084] The detector electrode 1007 is typically fixed at a fairly
high voltage level (e.g., 400 to 1000 volts), while the channel
electrode 1008 is typically biased at a lower positive value.
Accordingly, an electric field defined by the voltage levels and
geometry of the detector and channel electrodes, acts on the
charged particles within the ion generator chamber interior 1010.
It is normally desirable that proximal to the detector electrode
1007, charged particles are influenced more by the electric field
created by the detector electrode 1007 than by the electric field
coming from the channel electrode 1008. This can be achieved in a
variety of ways. For example, the voltage bias on the channel
electrode could vary over the surface of the channel electrode with
voltage values being greater on parts of it that are farther away
from the detector electrode. Such a non-uniform voltage across the
channel electrode 1008 can be obtained by making the channel
electrode 1008 from a number of separately biased, electrically
isolated electrode pieces. Alternatively, as with the depicted,
conically shaped chamber electrode, such a field could also be
achieved with a suitably shaped channel electrode 1008 (e.g.,
having non-uniform radii) to obtain a desired electric field
distribution with the channel electrode 1008 biased at a single
value that will generally be less than that of the detector
electrode 1007).
[0085] The depicted ion generator also has an annular magnetic
(and/or electro-static) field generating structure 1012 (similar to
a lens) substantially coaxially mounted in relation to the ion
generator cell 1006 and proximal to the ion generator aperture
opening 1005. The magnetic (and/or electro-static) field generating
structure 1012 may be controllable for adjusting the generated
field in order to funnel electrons through the aperture opening
1005 and into the ion generator cell 1006. Also included in the
depicted embodiment is an electrode 1014 (which may be annular) to
control the number and/or concentration of ions that ultimately
impinge on the imaged area of the work piece. These structures may
be either integrated into, or independent of, the ion generator
body. This will depend on the particular application and on the
geometries and biasing of the ion generator body 1000 and ion
generator cell 1006. Along with these structures, adjusting the
imaging gas pressure in the ion generator cell 1006 can also be
used to control the number and concentration of ions supplied to
the imaged area of the work piece.
[0086] FIG. 11 shows an on-axis version of the off-axis ion
generator of FIGS. 110A through 10D with corresponding components
numbered similarly. The ion generator of FIG. 11 operates similarly
to the off-axis embodiment but with the following exceptions. Ion
generator 1100 includes an additional upper disk electrode 1116
with an aperture opening 1117 for passing the charged particle
beam. This electrode, like detector electrode 1107 and channel
electrode 1108 is positively biased to attract electrons from the
work piece up through ion generator opening 1105 into the low
pressure bean space 1111 and force positively charged ions out of
opening 1105 and onto the work piece. The cylindrical electrode
1114 has an annular slit opening 1115 that passes both electrons
into ion generator cell 1106 through annular slit opening 1109 and
ions out into the low pressure, beam space 1111. When electrons in
the beam space 1111 get close to annular slit opening 1115, they
become more influenced by detector electrode 1107 and channel
electrode 1108, which causes them to enter the opening and pass
into the ion generator cell 1106 resulting in ion generation and
electron image signal amplification for electrons impinging upon
detector electrode 1107. Conversely, electro-static forces
dominated by detector electrode 1107 and channel electrode 1108
force ions out of the ion generator cell 1106 through openings 1109
and 1115 and into the beam space 1111. Once there, forces exerted
from disk electrode 1116 dominate, causing the ions to pass out of
opening 1105 and onto the work piece. In this embodiment,
cylindrical electrode 1114 serves as a gas pressure barrier between
the low pressure beam space 1111 and inner, high pressure ion
generator space 1110, but may also be biased to assist with the
control of the flow of ions and electrons.
[0087] Although the invention is not limited to any particular
application, some embodiments are particularly useful for repairing
lithography masks, especially masks used for the 70 nm lithography
node and beyond, including optical, x-ray, extreme ultra violet
(EUV), different absorbers, and alternating phase shift masks
(APSM) technologies.
[0088] Also, while the present invention and its advantages have
been described in detail, it should be understood that various
changes, substitutions and alterations can be made herein without
departing from the spirit and scope of the invention as defined by
the appended claims. Moreover, the scope of the present application
is not intended to be limited to the particular embodiments of the
process, machine, manufacture, composition of matter, means,
methods and steps described in the specification. As one of
ordinary skill in the art will readily appreciate from the
disclosure of the present invention, processes, machines,
manufacture, compositions of matter, means, methods, or steps,
presently existing or later to be developed that perform
substantially the same function or achieve substantially the same
result as the corresponding embodiments described herein may be
utilized according to the present invention. Accordingly, the
appended claims are intended to include within their scope such
processes, machines, manufacture, compositions of matter, means,
methods, or steps.
* * * * *