U.S. patent application number 11/806196 was filed with the patent office on 2007-10-11 for small electron gun.
This patent application is currently assigned to Hitachi High-Technologies Corporation. Invention is credited to Soichi Katagiri, Takashi Ohshima.
Application Number | 20070236143 11/806196 |
Document ID | / |
Family ID | 34138017 |
Filed Date | 2007-10-11 |
United States Patent
Application |
20070236143 |
Kind Code |
A1 |
Katagiri; Soichi ; et
al. |
October 11, 2007 |
Small electron gun
Abstract
To provide a small electron gun capable of keeping a high vacuum
pressure used for an electron microscope and an electron-beam
drawing apparatus. An electron gun constituted by a nonevaporative
getter pump, a heater, a filament, and an electron-source
positioning mechanism is provided with an opening for rough
exhausting and its automatically opening/closing valve, and means
for ionizing and decomposing an inert gas or a compound gas for the
nonevaporative getter pump. It is possible to keep a high vacuum
pressure of 10.sup.-10 Torr without requiring an ion pump by using
a small electron gun having a height and a width of approximately
15 cm while emitting electrons from the electron gun.
Inventors: |
Katagiri; Soichi; (Kodaira,
JP) ; Ohshima; Takashi; (Higashimurayama,
JP) |
Correspondence
Address: |
Stanley P. Fisher;Reed Smith LLP
Suite 1400
3110 Fairview Park Drive
Falls Church
VA
22042-4503
US
|
Assignee: |
Hitachi High-Technologies
Corporation
|
Family ID: |
34138017 |
Appl. No.: |
11/806196 |
Filed: |
May 30, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10873358 |
Jun 23, 2004 |
7238939 |
|
|
11806196 |
May 30, 2007 |
|
|
|
Current U.S.
Class: |
313/549 |
Current CPC
Class: |
H01J 2237/022 20130101;
H01J 7/18 20130101; H01J 37/18 20130101; H01J 2237/1825 20130101;
H01J 2237/188 20130101; H01J 37/07 20130101; H01J 7/16 20130101;
H01J 2237/062 20130101; H01J 2237/06316 20130101 |
Class at
Publication: |
313/549 |
International
Class: |
H01J 17/24 20060101
H01J017/24 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 10, 2003 |
JP |
2003-317703 |
Apr 6, 2004 |
JP |
2004-111682 |
Claims
1. An electron gun comprising: an electron source; a vacuum vessel
holding the electron source; a getter pump attached inside the
vacuum vessel; an opening configured to roughly exhaust the vacuum
vessel; a heating device configured to heat the getter pump; and a
decomposing device configured to decompose the gas produced while
the electron source operates.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a Continuation of U.S. application Ser.
No. 10/873,358 filed Jun. 23, 2004. This application claims
priority to U.S. application Ser. No. 10/873,358 filed Jun. 23,
2004, which claims priority to Japanese Patent Application Nos.
2003-317703 and 2004-111682 filed on Sep. 10, 2003, and Apr. 6,
2004, respectively, all of which is incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an electron gun of an
electron microscope or electronic drawing apparatus, particularly
to downsizing of an electron gun.
[0004] 2. Discussion of Background
[0005] A conventional scanning electron microscope (SEM) or
electron-beam drawing apparatus (EB) accelerates an electron beam
emitted from an electron gun constituted by a field-emission or
thermal-field-emission electron source, forms the electron beam
into a thin electron beam by an electron lens, forms the thin
electron beam into a primary electron beam, thereby scans the
surface of a sample with the primary electron beam by an electronic
deflector to obtain an image by detecting obtained secondary
electrons or reflected electrons when the SEM is used or to draw a
previously entered pattern on a resist film applied onto the sample
when the EB is used. The material of the electron source uses
tungsten when a general-purpose SEM is used. Moreover, an electron
source for semiconductor may use a material obtained by adding
zirconium to tungsten. Furthermore, LaB.sub.6 may be used for the
EB.
[0006] To emit a preferable electron beam from the above electron
source for a long time, it is necessary to keep the circumference
of an electron source at a high vacuum pressure (10.sup.-9 to
10.sup.-10 Torr). Therefore, a method has been used so far in which
the circumference of an electron gun 16 is forcibly exhausted by an
ion pump 13, as shown in FIG. 2. The ion pump 13 has an advantage
in that it is possible to keep a pressure of 10.sup.-10 Torr or
lower only by current-carrying because the pump has no movable
part. However, because the ion pump 13 has a size of tens of
centimeters square or more and generates a magnetic field, the pump
13 requires a considerable volume because a magnetic shield 15 is
necessary for the electron gun side.
[0007] Paragraph 0033 of JP-A No. 149850/2000 discloses a charged
particle beam apparatus having a built-in getter ion pump in a lens
tube as means for downsizing an electronic optical system.
Moreover, a charged particle beam apparatus having a built-in
nonevaporative getter pump in an electron gun chamber is disclosed
in FIG. 3 of U.S. Pat. No. 4,833,362; Paragraph 0033 of JP-A No.
149850/2000; and FIG. 2 of JP-A No. 111745/1994. The getter pump
mentioned above means a vacuum pump for activating and evaporizing
a getter by heating it and adsorbing impurities into the getter.
Moreover, the nonevaporative getter pump denotes a vacuum pump
constituted by using an alloy for adsorbing gas by only heating a
getter without evaporizing it. From the viewpoint of downsizing, it
is more preferable to use the nonevaporative getter pump.
[0008] Furthermore, Paragraph 0014 of JP-A No. 294182/2000
discloses an electron gun in which an axis adjustment screw for
adjusting the axis of an electron source is installed on the
circumference of a flange. Further, JP-A No. 188294/1994 discloses
a charged particle apparatus having a differential exhausting
structure for keeping the circumference of an electron source at an
ultrahigh vacuum pressure in its FIG. 9. Further, JP-A No.
325912/2001 discloses a technique for improving the exhausting
efficiency of a vacuum chamber by making a hydrocarbon-based gas
remaining in a sample chamber react with active oxygen introduced
into the sample chamber and thereby decomposing the gas and active
oxygen into water and carbon dioxide, which are easily
exhausted.
SUMMARY OF THE INVENTION
[0009] It has been recognized that what is needed is a high vacuum
pressure of between about 10.sup.-9 to 10.sup.-10 Torr when using a
field-emission electron gun. Accordingly, a dedicated ion pump 12
is provided to exhaust an electron-gun column 10, as shown in FIG.
1.
[0010] However, with conventional methods, it is difficult to
downsize an ion pump because the pump has a large size and a
magnetic field leaks and it is necessary to set the ion pump by
keeping a certain distance from an electron gun as shown in FIG.
2.
[0011] Moreover, a method may be used in which the housing of an
ion pump is formed like a doughnut so as to be coaxial with an
electron-gun column. However, because the diameter of the ion pump
housing is at least approximately tens of centimeters, there is a
limit in downsizing the ion pump.
[0012] By using a nonevaporative getter pump, it may be possible to
theoretically downsize an electronic optical system. However, when
using the nonevaporative getter pump, it is difficult to exhaust a
rare gas such as helium or argon and a chemically stable gas such
as methane, it is impossible to substantially keep a high vacuum
pressure, and thus the pump is not practically used yet. It is
necessary that gas has a micro potential for absorption. However,
when using a chemically stable gas such as a rare gas or
fluorocarbon gas, it is difficult to exhaust the gas because it is
completely equilibrium.
[0013] Moreover, when operating an electron source, some of
discharged electrons hit components of an electron gun and thereby,
miscellaneous gases are discharged. Thus, a vacuum pressure is
deteriorated and resultantly, a problem occurs that the service
life of the electron gun is shortened. Particularly, when the
volume of an electron gun is decreased by downsizing the gun, a
problem occurs that the total pressure of the above rare gas rises
and the trend that a vacuum pressure is deteriorated becomes
remarkable even if the partial pressure of the gas is low.
[0014] To address the problems of conventional methods, it is an
object of the present invention to provide an electron gun which is
able to keep a high vacuum pressure even while emitting an electron
beam and smaller than a conventional one. It is another object of
the present invention to provide an electron microscope or an
electron-beam drawing apparatus on which the small electron gun is
mounted.
[0015] The present invention attains the above objects using an
electron source, a vacuum vessel for holding the electron source, a
getter pump set in the vacuum vessel, an opening for exhausting the
vacuum vessel, and decomposing device for decomposing a gas
produced while the electron source operates.
[0016] By using the present invention, it is possible to obtain an
electron microscope or an electron-beam drawing apparatus capable
of keeping a high vacuum pressure of approximately 10.sup.-10 Torr
without using an ion pump.
[0017] The invention encompasses other embodiments of a system, an
apparatus, and a method, which are configured as set forth above
and with other features and alternatives.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] The present invention will be readily understood by the
following detailed description in conjunction with the accompanying
drawings. To facilitate this description, like reference numerals
designate like structural elements.
[0019] FIG. 1 explains an electron gun, in accordance with an
embodiment of the present invention;
[0020] FIG. 2 explains a configuration of a prior art electron
gun;
[0021] FIG. 3 is a schematic diagram of a scanning electron
microscope, in accordance with an embodiment of the present
invention;
[0022] FIG. 4 is a schematic diagram of an electron-beam drawing
apparatus, in accordance with an embodiment of the present
invention;
[0023] FIG. 5 is another configuration of an electron gun, in
accordance with an embodiment of the present invention;
[0024] FIG. 6A is a top view of an opening plate using an
automatically opening/closing valve, in accordance with an
embodiment of the present invention;
[0025] FIG. 6B is a cross-sectional view for explaining a structure
of an automatically opening/closing valve, in accordance with an
embodiment of the present invention;
[0026] FIG. 7A is a theoretical diagram for explaining an
electron-source positioning mechanism, in accordance with an
embodiment of the present invention;
[0027] FIG. 7B is a structure in which an electron-source
positioning mechanism is applied to an electron gun, in accordance
with an embodiment of the present invention;
[0028] FIG. 8A shows another configuration of an automatically
opening/closing valve, in accordance with an embodiment of the
present invention;
[0029] FIG. 8B shows still another configuration of an
automatically opening/closing valve, in accordance with an
embodiment of the present invention;
[0030] FIG. 8C shows still another configuration of an
automatically opening/closing valve, in accordance with an
embodiment of the present invention; and
[0031] FIG. 8D shows still another configuration of an
automatically opening/closing valve, in accordance with an
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] An invention for ***** is disclosed. Numerous specific
details are set forth in order to provide a thorough understanding
of the present invention. It will be understood, however, to one
skilled in the art, that the present invention may be practiced
with other specific details.
First Embodiment
[0033] FIG. 1 shows a configuration of an electron gun of this
embodiment. An electron source uses a thermal-field-emission
electron gun (TFE) 1. The electron source 1 is set to the flange of
an ICF 70 [WHAT IS "ICF 70"->It's a standard of flange used in a
vacuum industry] and connected with an introduction terminal 12 to
an electrode (suppressor, drawer, or chip) (not shown). The
electron source 1 is inserted into and fixed to an electron-gun
column 10.
[0034] The inside diameter of this column is approximately 37 mm.
The column 10 has a sheeted nonevaporative getter pump 2 along the
inside diameter. The nonevaporative getter pump 2 is activated when
overheated to take in air. Therefore, a first heater 4 is set to
the outside of the electron-gun column 10. This embodiment uses the
heater 4 by winding a sheath heater on the electron-gun column 10.
Note that a nonevaporative getter-pump heating device may be set in
the vacuum vessel, in other words, the electron-gun column 10. A
thermocouple 8 is set to a side face of the electron-gun column 10
to monitor a heating temperature of the nonevaporative getter pump
2. This embodiment uses a nonevaporative getter pump to be
activated at 400.degree. C. for 10 min.
[0035] A pump section 101 provided with an ionizing function is
connected to a part of the housing of the electron-gun column 10. A
filament 6 for decomposing miscellaneous gases produced while an
electron gun emits an electron beam is set in the pump section 101
provided with the ionizing function. The pump section 101 provided
with the ionizing function may be set to the housing of the
electron-gun column 10 as a special component or as a part of the
housing of the electron-gun column 10. An internal configuration of
the pump section 101 provided with the ionizing function will be
described later.
[0036] Operations of components of an electron gun are described
below. When the electron source 1 emits electrons, some emitted
electrons hit components to discharge gas containing hydrocarbon.
When the volume of an electron-gun column is small and forcible
exhausting is not performed by an ion pump, like the case of this
embodiment, hydrocarbon gas is not exhausted by the nonevaporative
getter pump 2. Therefore, there is a problem that the vacuum
pressure in an electron gun is deteriorated to affect the electron
source 1.
[0037] Accordingly, the ionizing-function-provided pump section 101
provided with the filament 6 made of tungsten is set to a side face
of the electron-gun column 10. The filament 6 is used to thermally
ionize and decompose the hydrocarbon (mainly, methane) in the
electron-gun column 10 into carbon and hydrogen. That is, by adding
a device for thermally ionizing and decomposing the hydrocarbon
which cannot be exhausted by a nonevaporative getter pump to an
electron gun, it is possible to exhaust the hydrocarbon. The
ionizing-function-provided pump section 101 is set to the housing
of the electron-gun column 10 by forming an opening on the housing.
A second nonevaporative getter pump is set to the inner wall
surface of the ionizing-function-provided pump section 101 to
adsorb ionized and decomposed hydrocarbon gas. Thus, by setting the
second nonevaporative getter pump nearby the filament, the
exhausting efficiency is further improved. A second heater 5 is set
to the outside of the housing of the ionizing-function-provided
pump section 101 in order to overheat the second nonevaporative
getter pump. To prevent an electron gun from overheating, it is
preferable to turn off the first heater for heating the housing of
an electron-gun column while an electron gun emits an electron
beam. Therefore, by setting the second heater set to the
ionizing-function-provided pump section 101 separately from the
first heater, it is possible to heat the second nonevaporative
getter pump even while an electron gun emits an electron beam. That
is, it is possible to exhaust only the circumference of a
hydrocarbon-gas producing source.
[0038] Because an opening 7 is formed below the electron gun 1,
emitted electrons pass through the opening 7 and are led to an
electronic optical system set in a column 9 for the electronic
optical system. Because the vacuum pressure in an electronic
optical system column is generally lower than the vacuum pressure
in an electron-gun column, the electronic optical system column and
electron-gun column constitute a differential exhausting structure
at both sides of the opening 7. Consequently, the above gas
containing hydrocarbon may also enter the electronic optical system
column through the opening 7. Therefore, an important function of
the present invention for thermally ionizing hydrocarbon gas is to
use a nonevaporative getter pump.
[0039] The orbit of an electron emitted from the electron source 1
may be influenced depending on an operation of the filament 6.
Therefore, at a position below the electron emission position of
the electron source 6 in the housing of the electron-gun column 10,
an electron emitted from the electron source may be influenced by
an operation of the filament 6. Thus, preferably a position to
which the filament 6 or ionizing-function-provided pump section 101
is set or the position of an opening where the electron-gun column
10 is connected with the ionizing-function-provided pump section
101 is present at an upper portion of the electron-beam generating
position of the electron source 1 (e.g., electron-beam takeout
electrode).
[0040] A procedure for operating the electron gun of this
embodiment is now described. First, to exhaust a sample chamber,
rough exhausting is started from the atmospheric pressure by
driving a vacuum pump (not shown). Then, baking is performed by
heating the first heater 4 and second heater 5. At the initial
stage of baking, a temperature is kept at approximately 200.degree.
C. to mainly bake water and hydrocarbon gas in a housing. By
performing baking for 6 to 12 hr, the gas released from the
inner-wall surface decreases up to a pressure which does not
matter. Then, the nonevaporative getter pumps 2 and 3 are activated
by increasing the power to be applied to the first and second
heaters to bring a target temperature to 400 to 500.degree. C. By
keeping the temperature for approximately 10 to 20 minutes after
temperature rise, the pumps 2 and 3 are sufficiently activated.
[0041] Exhausting and baking are performed by building up the
electron gun to achieve a vacuum pressure of 10.sup.-10 Torr.
Moreover, by applying 2 kV to the electron gun to emit electrons, a
vacuum pressure of 10.sup.-10 Torr may be kept. Because a high
vacuum pressure can be kept, a cold-cathode electron source (CFE)
or a Schottky electron source may be used instead of the
thermal-field-emission electron source used in this embodiment. It
is also possible to greatly decrease rough dimensions of the whole
electron gun. In the embodiment shown in FIG. 1, dimensions of the
whole electron gun is reduced in size as small as a width of
approximately 15 cm and a height of approximately 15 cm, compared
to a conventional configuration.
[0042] To exhaust the electron-gun column 10 from the atmospheric
pressure to a high vacuum pressure, a rough exhausting port may be
set to the electron-gun column 10 when exhausting from the opening
y is insufficient.
Second Embodiment
[0043] This embodiment describes the electron gun, described in the
first embodiment, as applied to a scanning electron microscope.
[0044] FIG. 3 shows a schematic configuration of the scanning
electron microscope of this embodiment. From the viewpoint of being
advantageous for downsizing, every electronic optical system used
in this embodiment uses a small electronic optical system
constituted by an electrostatic lens. In FIG. 3, an electron beam
18 field-emitted from a field-emission electron gun 17 is thinly
converged by electric fields formed between electrodes of an
electrostatic lens set below the electron gun 17 and applied onto a
sample 25. The electrostatic lens comprises a third lens electrode
19, a second lens electrode 20, and a first lens electrode 21.
[0045] At the same time, the electron beam 18 is deflected in the
internal space of the second lens electrode 20 by a deflector 24 to
two-dimensionally scan the surface of the sample 25. Moreover, to
align the optical axis of the electron beam 18 with that of the
electrostatic lens, the optical axis of the electron beam 18 can be
shifted by an alignment coil 23.
[0046] Furthermore, to perform astigmatism correction of the
electron beam 18, a stigma coil 22 is set. A secondary electron 33
generated from the sample 25 reaches a secondary-electron detector
26 and is detected. By supplying a detection signal of the electron
33 to image forming device 27, a two-dimensional secondary electron
image on the surface of the sample 25 can be obtained.
[0047] This embodiment aims at the observation at a low
acceleration voltage capable of decreasing the electrification or
damage of the surface of a sample due to irradiation with an
electron beam, so as to be suitable for surface observation of a
semiconductor.
[0048] Therefore, the acceleration voltage Va of the electron beam
18 is set to 3 kV or lower (mainly, approximately 1 kV).
[0049] For the embodiment shown in FIG. 3, an electronic optical
system is constituted by only an electrostatic lens. Therefore, an
electro optic lens tube has a very small size such as an outside
diameter of 34 mm and a height of 150 mm. Moreover, this embodiment
realizes a high resolution (6 nm at an acceleration voltage of 1
kV). Furthermore, the electronic optical system is inserted into a
vacuum vessel different from the vacuum vessel of the electron gun
17, both of which are located at both sides of an opening plate 7.
The former vacuum vessel is kept in a vacuum state by a
turbo-molecular pump. The opening plate 7 has an opening for
leading an electron beam to the outside of the electron gun.
[0050] Advantageously, it is possible to realize a small
high-resolution scanning electron microscope previously
unheard.
Third Embodiment
[0051] This embodiment describes the electron gun, described in the
first embodiment, as applied to an electron-beam drawing apparatus.
To provide a pattern drawing function for the small scanning
electron microscope described in the second embodiment, the
microscope may be used as an electron-beam drawing apparatus.
[0052] FIG. 4 is a schematic diagram of an electron-beam drawing
apparatus of the third embodiment. It is possible to draw a pattern
having a resolution of 6 nm on a resist film applied onto a sample
31 by sequentially reading data from a pattern record control
device 30, previously storing the data for the layout of a pattern
and the like, and deflecting an electron beam 18 by a deflector 24
so as to form the pattern. Moreover, it is possible to detect the
position of a pattern to be drawn by detecting a secondary electron
beam 33 generated from a region nearby a positioning mark, not
shown, by a secondary electron detector 26.
[0053] Because this embodiment uses an acceleration voltage at a
low acceleration of approximately 1 kv, it is impossible to draw a
pattern on a thick-film resist (1 .mu.m or more). Therefore, this
embodiment is suitable for a process for forming a pattern on the
surface of a thin film resist (0.3 .mu.m or less). As features,
because the influence of a proximity effect can be lowered, it is
possible to decrease the time for correction, greatly decrease an
apparatus in size, and comparatively easily obtain high-resolution
drawing.
Fourth Embodiment
[0054] This embodiment describes (1) a configuration of an electron
gun having a positioning mechanism for optical axis alignment of an
electron source and (2) an automatic change mechanism for rough
exhausting and main exhausting when exhausting an electron-gun
column.
[0055] When using an electron-beam-applied or
charged-particle-beam-applied apparatus, it is obviously necessary
to accurately perform optical axis alignment of an electron beam.
However, when downsizing an electron gun or an apparatus mounting
the electron gun, it is difficult to mount complex positioning
device or optical-axis alignment device on an apparatus because
downsizing is restricted.
[0056] Moreover, for exhausting of an apparatus, it is preferable
to minimize the number of exhausting apparatuses used for the whole
charged-particle-beam apparatus in order to decrease an apparatus
in size. Accordingly, it is preferable to communize an exhausting
device for an electronic optical system set below an electron gun
and measuring optical system on which various detectors are
arranged. Thus, it is an object of this embodiment to provide an
electron gun having a positioning mechanism for a small electron
source having a simple configuration and capable of accurately
performing optical axis alignment of the electron gun. It is
another object of this embodiment to provide and an automatic
rough-exhausting and main-exhausting change mechanism capable of
communizing rough exhausting device between an electron-gun column
and an electronic-optical system column.
[0057] FIG. 5 shows a configuration of the electron gun 17 of this
embodiment. Descriptions of portions common to those of the
electron gun having the configuration shown in FIG. 1 such as the
electron source 1 and ionizing-function-provided pump section 101
are omitted. The electron gun of this embodiment is different from
the electron gun of the first embodiment in that the following are
used: a positioning mechanism including a conflat flange 39 for
fixing an electron source 1 to an electron-gun column 10, bellows
40, or a knob 39 having an adjusting screw at its front end and a
differential exhausting section 200 having an automatically
opening/closing valve 102 for rough exhausting. To actually
constitute a charged-particle-beam apparatus, the electron gun 17
is combined with a column 9 and electronic optical system. The
electronic-optical-system column 9 has an electronic-optical-system
lens tube 36 storing a deflector and an objective and a sample
table 35. The column 9 for an electronic optical system connects
with a vacuum pump 34. Though not separately shown in FIG. 5, the
vacuum pump 34 includes a rough-exhausting vacuum pump and a
main-exhausting vacuum pump.
[0058] First, the configuration of a positioning mechanism of and
operations of an electron gun are described below. To make an
electron beam emitted from the electron source 1 efficiently pass
through an opening, it is necessary to adjust the position of the
electron source 1. Because the opening formed at the center of an
opening plate 7 has a diameter of approximately 0.5 mm, it is
necessary to realize a movement stroke of approximately 1 mm in a
plane vertical to the optical axis. The electron source 1 is fixed
to the conflat flange 39 having a diameter of 70 mm and various
types of electric wires are connected through an electrode 12 in
which the chip (not shown) and electrode (not shown) of the
electron source 1 are formed in feed-through. In this case, the
feed-through denotes an introducing section formed on a vacuum
vessel to lead various types of electric wires into the vacuum
vessel. These structures are connected to the electron-gun column
10 through the bellows 40. The electron source 1 is constituted so
as to be able to move to the electron-gun column 10 by a distance
equivalent to the deformation value of the bellows 40. A relative
position between both is adjusted so that an electron beam
transmitted to the electronic-optical-system column 9 is maximized
while turning an electron-source positioning knob 38. FIG. 5 shows
only one electron-source positioning knob. Actually, however, two
knobs are paired facing each other. The knobs are provided in
directions orthogonal to each other at the total of four places one
pair by one pair. When the position of the electron source 1 is
decided, it is possible to prevent a displacement by completely
fastening and locking faced knobs.
[0059] The configuration and operations of the differential
exhausting section 200 are described below. To exhaust the
electron-gun column 11 from the atmospheric pressure up to a high
vacuum pressure, exhausting from the opening 7 is insufficient.
Therefore, because a port for rough exhausting has been set to the
electron-gun column 10 so far, the external size has been
increased. For the electron gun of the first embodiment, however,
the opening formed on the opening plate 7 serves as a
rough-exhausting port. Therefore, rough-exhausting device can be
shared between the electron gun 17 and the
electronic-optical-system column 9. Accordingly, it is possible to
downsize an apparatus. However, the electron gun of the first
embodiment has a problem that the diameter of a hole formed on the
opening plate 7 is too small and the conductance for rough
exhausting is too small. However, it is impossible to greatly
increase the size of an opening formed on the opening plate 7.
Because the vacuum pressure in the electronic-optical-system column
9 is lower than that in the electron-gun column 10, when extremely
increasing the hole diameter on the opening plate 7, the gas
remaining in the electronic-optical-system column 9 reversely flows
into the electron-gun column 10. Consequently, it may be difficult
to keep a high vacuum pressure.
[0060] Accordingly, for the electron gun of this embodiment, a
rough-exhausting opening is formed on the opening plate 7
separately from an electron-beam-passing opening. Moreover, the
automatically opening/closing valve 102 is set to the
rough-exhausting opening.
[0061] The configuration and operations of the automatically
opening/closing valve 102 for rough exhausting are described below
in detail by referring to FIGS. 6A and 6B.
[0062] FIG. 6A shows a top view of the opening plate 7 and FIG. 6B
shows a sectional view of the opening plate 7 when cutting the
position A-A' in FIG. 6A along the alternate long and short dash
line shown in FIG. 6A. For the differential exhausting section 200
in FIG. 5, only one automatically opening/closing valve 102 is
shown for one opening plate 7. Actually, however, two valves are
added to one opening plate 7. The hatching at the left side in FIG.
6B denotes the inner wall surface of the differential exhausting
section 200 (or electron-gun column 10). Symbol 46 denotes a fixing
plate for fixing the opening plate 7 and a movable arm 45 to the
inner wall surface. A first opening 42 through which an electron
beam passes is formed at the center of the opening plate 7. A
second opening 43 is formed separately from the first opening 42.
Setting the hole diameter to a value larger than that of the
opening 43 is effective because the conductance for rough
exhausting increases. A lid 44 corresponds to the opening 43. The
lid 44 is connected with the inner wall surface by an autonomously
movable arm 45, which vertically moves on the basis of the inner
wall surface. This embodiment uses a bimetal to be thermally
deformed as the material of the arm 45. In this embodiment, the
autonomously movable arm 45 is constituted by a bimetal to be
thermally deformed.
[0063] However, it is also possible to obtain the same advantage by
using a shape memory alloy. A bimetal generally uses a magnetic
material such as a FeNi--NiFeCr alloy. When using a magnetic
material for a movable arm, the orbit of an electron beam passing
through the first opening 42 is bent. Therefore, it is preferable
to use a bimetal made of a non-magnetic material for a movable arm.
When considering the operation temperature of a nonevaporative
getter, it is confirmed through experiments that preferably a
bimetal has a high-temperature resistance. Particularly, it is
preferable to use a bimetal obtained by combining a stainless alloy
with a small thermal expansion metal such as tungsten.
[0064] To keep the air tightness of the electron-gun column 10, it
is necessary that the lid 44 closely adheres to the opening plate 7
when main exhausting is performed. When considering the
adhesiveness of the lid 44, preferably the lid 44 plastically
deforms when it adheres to the opening plate. Therefore, the
elastic modulus of a material constituting the lid 44 is preferably
smaller than that of a material constituting the opening plate 7.
Moreover, dirt may attach to the opening plate 7 because an
electron beam passes through the central opening 42. Furthermore,
the movable arm 45 and lid 44 may be deteriorated with time after
repeating adhesion and opening operation with the opening plate 7.
Accordingly, the opening plate 7, movable arm 45, and lid 44 must
easily be replaced.
[0065] In this embodiment, the opening plate 7 and movable arm 45
are respectively fixed to the inner wall surface by a fixing screw
202. Accordingly, separately using a fixing device to fix the
opening plate 7, lid 44, and movable arm 45 to the inner wall
surface provides an electron gun in which the opening plate 7,
movable arm 45, and lid 44 can easily be replaced. In FIG. 6B,
because the movable arm 45 and opening plate 7 use the same fixing
device, it is impossible to independently replace the movable arm
and opening plate. However, using two fixing screws and thereby
separately fixing the movable arm and opening plate to the inner
wall surface realizes an electron gun in which the opening plate 7
and movable arm 45 can independently be replaced. Thus, it is
possible to provide an electron gun that can be more easily
maintained.
[0066] Moreover, to keep the air tightness between upstream and
downstream sides of the opening plate 7, the opening plate 7 may be
welded to the inner wall surface. Note that another component must
be used for the opening 42 in order for the opening plate 7 to be
replaceable.
[0067] A procedure for exhausting the electron gun of this
embodiment and a charged-particle-beam applied apparatus is
described below. Components for constituting the apparatus are
assembled to execute exhausting. In this case, to activate
nonevaporative getter pumps 3 and 41, power is distributed to the
heaters 4 and 5 to heat the housing. It is possible to greatly
improve the efficiency for rough exhausting by using the heat in
the above case, opening the automatically opening/closing valve,
and increasing a conductance. When activation and exhausting of the
nonevaporative getter pumps are completed, power distribution to
the heaters is stopped to cool the housing up to approximately room
temperature. Because the lid 44 closes the opening 43 at
approximately room temperature, the conductance between the lid 44
and the electron-gun column 10 is decided by the opening 42, and
preferable differential exhausting is automatically realized. This
embodiment can obtain a differential exhausting characteristic of a
vacuum pressure of 10.sup.-8 Pa in the electron-gun column 10 which
is four digits higher than the vacuum pressure of 10.sup.-4 Pa in
the electronic-optical-system column 9.
[0068] The configuration described in this embodiment can greatly
decrease the schematic dimensions of the whole electron gun to 15
cm in width and 15 cm in height when compared to a conventional
configuration. Building up the electron gun and performing
exhausting and baking achieves a vacuum pressure of 10.sup.-10
Torr. Moreover, applying 2 kV to the electron source and
discharging electrons can keep a vacuum pressure of 10.sup.-10
Torr. Because a high vacuum pressure can be kept, a cold-cathode
electron source (CFE) or a Schottky electron source may be used
instead of the thermal field emission electron source used in this
embodiment.
[0069] Furthermore, as a result of applying the electron gun of
this embodiment to a scanning electron microscope having the
structure shown in FIG. 3, it is possible to realize a scanning
electron microscope having a size smaller than and a maintenance
period longer than the scanning electron microscope described for
the second embodiment, because the vacuum air tightness around the
electron source is preferable.
Fifth Embodiment
[0070] This embodiment describes an electron gun with a positioning
mechanism that realizes positioning more easily than that of the
electron gun described in the fourth embodiment as follows.
[0071] FIG. 7A shows a theoretical drawing of the positioning
mechanism of this embodiment. FIG. 7B shows a block diagram when
applying the positioning mechanism of this embodiment to the
electron gun shown in FIG. 5 together with an essential portion of
the electron gun.
[0072] First, the theory of this embodiment is described by
referring to FIG. 7A. The positioning mechanism of this embodiment
has a feature that an x-axis-directional reference plane is formed
separately from a y-axis-directional reference plane. As described
for the fourth embodiment, an electron source 1 is adjusted so that
an electron beam transmitted to the column 9 is maximized for
positioning. However, because a small charged-particle-beam applied
apparatus cannot have a high-accuracy positioning mechanism, a user
of the apparatus must manually perform adjustments. For the
electron gun of the fourth embodiment, all electron-beam-source
positioning knobs 38 are set at the same height. That is,
positioning along the x and y axes must be performed at the same
position. However, the electron gun in this embodiment
independently has x-axis alignment device 48 and y-axis alignment
device 49. Therefore, the user of the apparatus can initially
adjust the x-directional or y-directional position and then perform
appropriate positioning along the y or x axis. Therefore, the load
of the user of the apparatus is decreased when adjusting the axis
of the electron beam source.
[0073] FIG. 7B shows an important portion of the electron gun
having the positioning mechanism of this embodiment. An electron
source 1 is suspended in the electron-gun column 10 by a parallel
plate spring 47 for holding the electron source 1 at a
predetermined position in a vacuum vessel. Various electric wires
50 are held in the parallel plate spring. The upside of the
parallel plate spring 47 is fixed to a flange having a feed-through
and the electron source 1 is set to the lower end of the parallel
plate spring 47. The electric wires 50 are insulated by insulators
and led to the upper feed-through. The nonevaporative getter pump 2
surrounds the outer periphery of the parallel plate spring 47 so as
to cover it. A stress from x direction and a stress from y
direction are applied to the parallel plate spring 47 due to the
operation of a positioning mechanism 48 or 49. That is, a torsion
stress is applied to the parallel plate spring 47. Accordingly, it
is necessary that the positioning mechanism 48 and 49 respectively
have a rigidity to a torsion. Thus, this embodiment constitutes the
positioning mechanisms 48 and 49 by using two pairs of
lock-provided linear introduction terminals faced each other.
[0074] The drawing of the positioning mechanism shown in FIG. 7A
shows a bellows used to introduce a linear motion from the outside
of the electron gun housing. Moreover, the parallel plate spring 47
deforms by being distorted in x or y direction. However, the spring
47 has a large-enough rigidity and elastic modulus in the direction
orthogonal to x or y direction and is designed so as not to cause
buckling or plastic deformation. Note that fixing plates 51 and 52
for supporting the parallel plate spring 47 respectively have a
high rigidity and elastic modulus against a force applied for
positioning. A structure material other than a parallel plate
spring may comprise the electron-source holding device as long as
the material is a member not causing buckling or plastic
deformation.
[0075] The parallel plate spring 47 has flat-spring fixing plates
51 and 52 forming axis-alignment reference planes in x and y
directions. A pair of faced positioning mechanisms correspond to
each plate and are combined with operation knobs 38 present at the
outside of the housing of the electron-gun column 10 through the
linear motion feed through. Thereby, a structure which can be
operated from the atmospheric-air side is realized. To perform
x-directional positioning, it is only necessary to operate an
x-directional driving mechanism. The mechanism is preferable
because a temporal drift can be reduced by fastening and locking an
opposite driving mechanism when an x-directional position of the
former mechanism is decided. Y-directional positioning may be
performed basically the same as the case of the x-directional
positioning.
[0076] By using the above structure, the electron source 1 can
perform raster scanning not interfering in x or y direction. The
electron source 1 can comprehensively efficiently move in a movable
region of 1 mm.times.1 mm. The electron source 1 has a positioning
feature such that an electron beam can be efficiently emitted
through the opening 42. Another feature of the electron source 1 is
that it may be easily downsized. Therefore, preferably both
features can be coexistent.
[0077] Advantageously, a positioning mechanism can be simplified by
applying the positioning mechanism of this embodiment to an
electron gun having a structure other than that of the first
embodiment, as long as the electron gun makes it possible to
manually position an electron source.
Sixth Embodiment
[0078] This embodiment describes another configuration of the
automatically opening/closing valve shown by symbol 102 in FIG.
5.
[0079] FIG. 8A is a top view of an opening plate 7 when viewed from
its top, which is the same drawing as FIG. 6A. FIGS. 8B to 8d are
cross-sectional views obtained by cutting the opening plate 7 and
mounting various automatically opening/closing valves along the
alternate long and short dash line shown at the position A'-A in
FIG. 8A.
[0080] Characteristic points of the automatically opening/closing
valves shown in FIGS. 8B to 8D are described below. The
automatically opening/closing valve in FIG. 8B has a configuration
in which a conical piece is used for a lid 44. The valve has an
effect of preventing displacements of a hole and the valve due to
opening/closing of the lid 44 (automatic aligning). The
displacement of the above valve is a phenomenon due to a thermal
deformation of the valve caused as a result of repeatedly
opening/closing the valve accompanying a setting error of each
component or temperature rise or fall. When the above valve
displacement occurs, a problem occurs that the adhesiveness between
an opening and the valve is deteriorated, gas enters through the
gap between them, and a vacuum pressure is deteriorated. Therefore,
sufficient consideration is necessary.
[0081] The automatically opening/closing valve shown in FIG. 8C has
a configuration in which a contact face of the lid 44 with an
opening 43 is formed into a sphere. The valve has an automatic
aligning effect the same as the case of the valve shown in FIG. 8B.
Moreover, because it is possible to use a general-purpose ball for
the material of the lid 44, advantageously, the valve manufacturing
cost may be reduced.
[0082] FIG. 8D shows a valve having a configuration in which a
circular protrusion (circular edge) 203 is formed on the contact
face between the lid 44 and the opening plate 7. By forming the
circular edge on the lid 44, acceptable widths for displacements of
a hole and the valve are increased. The same effect can be obtained
even by forming the circular edge not at the lid-44 side but at the
opening plate-7 side.
[0083] Automatically opening/closing valves having the
configurations described by referring to FIGS. 8A to 8D improves
the adhesiveness between the opening plate 7 and the lid 44 and
therefore the vacuum air tightness in the electron-gun column
10.
[0084] In the foregoing specification, the invention has been
described with reference to specific embodiments thereof. It will,
however, be evident that various modifications and changes may be
made thereto without departing from the broader spirit and scope of
the invention. The specification and drawings are, accordingly, to
be regarded in an illustrative rather than a restrictive sense.
* * * * *