U.S. patent application number 12/866498 was filed with the patent office on 2011-02-17 for manufacturing method of electron source.
Invention is credited to Fumihiro Nakahara, Ryozo Nonogaki, Yoshinori Terui.
Application Number | 20110036810 12/866498 |
Document ID | / |
Family ID | 40951876 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110036810 |
Kind Code |
A1 |
Nakahara; Fumihiro ; et
al. |
February 17, 2011 |
MANUFACTURING METHOD OF ELECTRON SOURCE
Abstract
An electron gun with a truncated-cone-shaped cathode with
uniform emission current density is efficiently manufactured. A
manufacturing method of a cathode electron gun equipped with a
supply source for diffusing oxide of a metal element on a single
crystal needle of tungsten or molybdenum includes steps of forming
a truncated-cone-shape having a flat plane at a single crystal edge
serving as the cathode by machining beforehand, thereafter thinning
and removing a front layer of the flat plane by a focused gallium
ion beam, and re-flattening it.
Inventors: |
Nakahara; Fumihiro; (Gunma,
JP) ; Nonogaki; Ryozo; (Gunma, JP) ; Terui;
Yoshinori; (Gunma, JP) |
Correspondence
Address: |
STEIN MCEWEN, LLP
1400 EYE STREET, NW, SUITE 300
WASHINGTON
DC
20005
US
|
Family ID: |
40951876 |
Appl. No.: |
12/866498 |
Filed: |
March 25, 2008 |
PCT Filed: |
March 25, 2008 |
PCT NO: |
PCT/JP2008/055558 |
371 Date: |
October 28, 2010 |
Current U.S.
Class: |
216/66 ;
216/58 |
Current CPC
Class: |
H01J 9/025 20130101;
H01J 37/073 20130101; H01J 2237/31749 20130101; H01J 2237/06316
20130101; H01J 2201/30415 20130101; H01J 2237/06341 20130101; H01J
1/3044 20130101; H01J 9/042 20130101; H01J 1/16 20130101; H01J
2237/3174 20130101; H01J 37/3056 20130101 |
Class at
Publication: |
216/66 ;
216/58 |
International
Class: |
C25F 3/16 20060101
C25F003/16; C23F 1/00 20060101 C23F001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2008 |
JP |
2008-027796 |
Claims
1. A method of manufacturing an electron source having an electron
emission portion at one tip portion of a rod, the method of
manufacturing an electron source comprising a step of forming the
tip portion in the shape of a truncated cone having a flattened
surface by a machining process, and a step of removing a surface
layer of the flattened surface by focused ion beam processing or
vapor phase etching.
2. The method of manufacturing an electron source according to
claim 1, further comprising, between the step of forming in the
shape of a truncated cone and the step of removing a surface layer
of the flattened surface by focused ion beam processing, a step of
removing a processing scar layer on the surface of said tip portion
by means of vapor phase etching or electrolytic polishing.
3. The method of manufacturing an electron source according to
claim 1, wherein said step of removing a surface layer of the
flattened surface is a step of removing the surface layer of the
flattened surface by irradiating with a focused gallium ion beam in
an atmosphere of xenon difluoride gas.
4. The method of manufacturing an electron source according to
claim 1, wherein said step of removing a surface layer of the
flattened surface is a step of removing the processing scar layer
of the surface layer of said tip portion including the surface
layer of said flattened surface by performing vapor phase etching
under conditions of an oxygen pressure of at least
0.3.times.10.sup.-4 Pa and at most 8.times.10.sup.-4 Pa, and a
temperature of at least 1700 K and at most 1950 K.
5. The method of manufacturing an electron source according to
claim 1, wherein the diameter of said flattened surface is at least
10 .mu.m and at most 100 .mu.m.
6. The method of manufacturing an electron source according to
claim 1, wherein said electron source is an electron source
obtained by providing a reservoir for diffusion of oxides of metal
element to a monocrystalline rod of tungsten or molybdenum in the
<100>orientation.
7. The method of manufacturing an electron source according to
claim 6, wherein said metal element is chosen from among Ca, Sr,
Ba, Sc, Y, La, Ti, Zr, Hf and the lanthanoids.
Description
TECHNICAL FIELD
[0001] The present invention relates to a manufacturing method for
an electron source.
BACKGROUND ART
[0002] In recent years, electron guns using cathodes with
monocrystalline tungsten needle electrodes (electron sources
(hereinafter referred to as ZrO/W electron sources)) having coating
layers of zirconium and oxygen (hereinafter referred to as ZrO/W
electron guns) have been used to obtain electron beams that are
brighter and have a longer operating life than hot cathodes (see
Non-Patent Document 1).
[0003] ZrO/W electron guns are obtained by providing a diffusion
layer consisting of zirconium and oxygen (hereinafter referred to
as a ZrO coating layer) on a needle cathode composed of tungsten
monocrystals having an axial orientation in the <100>
orientation. This ZrO coating layer reduces the work function of
the (100) plane of the tungsten monocrystals from 4.5 eV to about
2.8 eV, so that only the small crystalline facet corresponding to
the (100) plane formed at the tip of this cathode forms an electron
emission region, as a result of which an electron beam that is
brighter than that of conventional hot cathodes can be obtained,
and the operating life is also prolonged. Additionally, they have
the advantages of being more stable than cold field-emission
electron guns, operating in a lower vacuum, and being easy to use
(see Non-Patent Document 2).
[0004] As shown in FIG. 1, a ZrO/W electron gun comprises a needle
cathode 1 of tungsten in the <100> orientation for emitting
an electron beam attached by welding or the like to a portion of a
tungsten filament 3 provided on a conductive terminal 4 anchored to
an insulator 5. A reservoir 2 of zirconium and oxygen is formed in
a portion of the cathode 1. While not shown in the drawings, the
surface of the cathode 1 is covered with a ZrO coating layer.
[0005] Since the cathode 1 is electrically heated to about 1800 K
by means of the filament 3, the ZrO coating layer on the surface of
the cathode 1 will evaporate. However, zirconium and oxygen are
continuously supplied to the surface of the cathode 1 by diffusion
from the reservoir 2, so the ZrO coating layer is maintained.
[0006] The tip portion of the cathode 1 of the ZrO/W electron gun
is positioned between the suppressor electrode 8 and the extractor
electrode 9 for use (see FIG. 3). A high negative voltage is
applied between the extractor electrode 9 and the cathode 1, while
a negative voltage of a few hundred volts is applied between the
cathode 1 and the suppressor electrode 8, to suppress thermal
electrons issuing from the filament 3.
[0007] In critical dimension SEM and wafer inspection equipment
where they are used at low accelerating voltages, ZrO/W electron
guns are operated at an angular current density of 0.1-0.2 mA/sr,
where the probe current is stable and broadening of the energy
width can be suppressed.
[0008] On the other hand, in electron beam lithography and Auger
spectroscopy devices, they are operated at a high angular current
density of about 0.4 mA/sr due to the emphasis on throughput. In
such applications emphasizing throughput, operation at even higher
angular current densities is desired, and angular current densities
as high as 1.0 mA/sr are sometimes called for.
[0009] However, in ZrO/W electron guns, there is still room for
improvement in that (1) angular current densities of about 1.0
mA/sr are the upper limit for operating at high angular current
densities, and (2) at such times, the extraction voltage applied
between the cathode and the extractor electrode can be greater than
4 kV, resulting in an extremely high field intensity of 0.4 to
1.0.times.10.sup.9 V/m at the tip, which increases the frequency of
damage due to arc discharge (see Non-Patent Document 3).
[0010] In order to overcome these drawbacks, an electron gun
wherein the cathode tip portion of the ZrO/W electron gun has a
truncated conical shape, with the diameter of the top surface of
the truncated cone forming the electron emission portion being at
least 5 .mu.m and at most 200 .mu.m, so as to enlarge the electron
emission area and enable operation at a high angular current
density with a low extraction voltage has been proposed (see Patent
Document 1). According to an embodiment of this Patent Document 1,
the cathode tip portion is formed into a cone, then the cathode tip
portion is shaped to a truncated cone by mechanical polishing.
Additionally, Patent Document 1 also proposes a method in which the
cathode tip portion is formed into a cone, then carved flat by a
focused gallium ion beam to form a truncated cone.
[0011] However, in electron guns having cathode tip portions that
are formed into truncated cones by mechanical polishing, the
emission current density can become uneven. This unevenness can
cause deviations in the axial current, making uniform electron beam
emissions difficult. This unevenness is attributed to processing
surface scars caused by machining of the monocrystalline rod. While
the depth of the processing scar layer depends on the roughness of
the abrasive grains used for machining and mechanical polishing,
they are known to extend as deep as tens of microns (see Non-Patent
Document 4).
[0012] As a solution for these problems, a method of improving the
non-uniformity of the current emission distribution by processing
the cathode tip portions into truncated cones by mechanical
polishing, then removing the processing scar layer by electrolytic
polishing has been proposed (see Patent Document 2). An example
from Patent Document 2 demonstrates the effects of improving
non-uniformity of the current emission distribution when providing
a flattened portion with a diameter of 20 .mu.m.
[0013] Patent Document 1: WO 2004/073010
[0014] Patent Document 2: WO 2006/075715
[0015] Non-Patent Document 1: D. Tuggle, J. Vac. Sci. Technol.,
1979, 16, p. 1699.
[0016] Non-Patent Document 2: M. J. Fransen, "On the
Electron-Optical Properties of the ZrO/W Schottky Electron
Emitter", Advances in Imaging and Electron Physics, Academic Press,
1999, Vol. III, pp. 91-166.
[0017] Non-Patent Document 3: D. W. Tuggle, J. Vac. Sci. Technol.,
1985, B3(1), p. 220.
[0018] Non-Patent Document 4: U. Linke and W. U. Kopp, "Surface
Analysis by X-ray Topography and Etching During the Preparation of
Single Crystal Surfaces", Microstructural Sciences, 1981, Vol. 9,
pp. 299-308.
DISCLOSURE OF THE INVENTION
Problems To Be Solved By the Invention
[0019] However, the conventional art described in the above
literature has the potential for improvement on the following
points.
[0020] First, while Patent Document 1 also proposes a method of
shaping the cathode tip portion into a cone, then carving flat with
a focused gallium ion beam to form a truncated cone, the
embodiments only describe a method of shaping the cathode tip
portion into a cone, then mechanically polishing to provide a
flattened portion with a diameter of 100 .mu.m at the cathode tip
portion, thereby resulting in a truncated cone.
[0021] By shaping the cathode tip portion into a cone, then carving
flat with a focused gallium ion beam to form a truncated cone, it
is indeed likely to be possible to obtain an electron emission
surface with a reduced scar layer, thereby resulting in a uniform
emission current density as described in Patent Document 1.
However, since the processing speed of a focused gallium ion beam
is extremely slow, several hours may be required to form a
flattened surface with a diameter of about 5 .mu.m from a conical
cathode tip portion, and when considering that in practice, it must
be applied to a manufacturing process on a production line, a
flattened surface with a diameter of 10 .mu.m or more would be very
difficult to form by means of a focused gallium ion beam.
[0022] Secondly, Patent Document 2 discloses a method of forming
the cathode tip portion into a truncated cone by means of
mechanical polishing, then removing the work scar layer by means of
electrolytic polishing, and the examples demonstrate effects of
improving non-uniformity of the current emission distribution when
providing a flattened portion with a diameter of 20 .mu.m.
[0023] However, with this method, the electrolytic polishing
results in sagging at the outer periphery of the electron emission
surface, so when the diameter of the top surface of the truncated
cone forming the electron emission surface is small, the electron
emission surface can become curved, thereby reducing the emission
current density.
[0024] In other words, as described above, the present inventors
have developed techniques for solving the respective technical
demands for (1) efficiently forming an electron emission surface
with a reduced scar layer in a short time in an electron source,
(2) improving the non-uniformity of current emission distribution,
and (3) increasing the emission current density. However, the
present inventors found, based on a number of discoveries as
described above, that these techniques are not capable of achieving
a good balance among these three technical demands which have a
conflicting relationship.
[0025] The fact that these three technical demands conflict with
each other in this way was discovered by the present inventors when
they observed, during the process of further improving the
technology of Patent Document 1, that the processing speed of the
focused gallium ion beam was extremely slow, with several hours
being necessary to form a flattened surface with a diameter of
about 5 .mu.m from a conical cathode tip portion, and observed,
during the process of further improving the technology of Patent
Document 2, that electrolytic polishing causes sagging at the outer
periphery of the electron emission surface, so that if the diameter
of the top surface of the truncated cone forming the electron
emission surface is small, the electron emission surface becomes
curved, thus reducing the emission current density.
[0026] The present invention is based on the above-described
technical problem which was recognized for the first time in the
relevant technical field, and has the object of offering a
technique that achieves a good balance between three technical
demands which have a conflicting relationship, the demands being
those of (1) efficiently forming an electron emission surface with
a reduced scar layer in a short time in an electron source, (2)
improving the non-uniformity of current emission distribution, and
(3) increasing the emission current density.
Means For Solving the Problems
[0027] The present invention offers a method of manufacturing an
electron source having an electron emission portion at one tip
portion of a rod, the method of manufacturing an electron source
comprising a step of forming the tip portion in the shape of a
truncated cone having a flattened surface by a machining process,
and a step of removing a surface layer of the flattened surface by
focused ion beam processing or vapor phase etching.
[0028] According to this method, the tip portion can be formed into
the shape of a truncated cone having a flattened surface by means
of machining which enables the electron emission surface to be
efficiently formed in a short period of time, then the surface
layer of the flattened surface can be removed by focused ion beam
processing or vapor phase etching to form an electron emission
surface with a reduced scar layer. Additionally, according to this
method, electrolytic polishing and liquid phase etching are not
performed (or electrolytic polishing or liquid phase etching is
followed by focused ion beam processing), so sagging will not occur
at the outer periphery of the electron emission surface.
[0029] Thus, this method is capable of efficiently forming an
electron emission surface with a reduced scar layer and little
sagging at the outer periphery in a short period of time, thus
achieving a good balance between three technical demands which have
a conflicting relationship, the three demands being those of (1)
efficiently forming an electron emission surface with a reduced
scar layer in a short time in an electron source, (2) improving the
non-uniformity of current emission distribution, and (3) increasing
the emission current density.
Effects of the Invention
[0030] The present invention achieves a good balance between three
technical demands which have a conflicting relationship, the three
demands being those of (1) efficiently forming an electron emission
surface with a reduced scar layer in a short time in an electron
source, (2) improving the non-uniformity of current emission
distribution, and (3) increasing the emission current density.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] [FIG. 1] A view showing the structure of a ZrO/W electron
gun.
[0032] [FIG. 2] An enlarged view of a cathode.
[0033] [FIG. 3] A view showing the constitution of a device for
evaluating electron emission properties.
[0034] [FIG. 4] FIG. 4(a) is a view for explaining a comparative
example of a rod 1, and FIG. 4(b) is a view for explaining an
embodiment of the rod 1.
[0035] [FIG. 5] A graph showing the results of measurements of the
current emission distribution (Example 1 and Comparative Example
1).
[0036] [FIG. 6] A graph showing the current emission distribution
in Example 2.
[0037] [FIG. 7] A graph showing the current emission distribution
in Comparative Example 2.
DESCRIPTION OF THE REFERENCE NUMBERS
[0038] 1 cathode
[0039] 2 reservoir
[0040] 3 filament
[0041] 4 conductive terminal
[0042] 5 insulator
[0043] 6 conical portion (of truncated conical portion)
[0044] 7 flattened portion (top surface portion of truncated
conical portion)
[0045] 8 suppressor electrode
[0046] 9 extractor electrode
[0047] 10 phosphor screen
[0048] 11 aperture
[0049] 12 cup-shaped electrode
[0050] 13 probe current measuring microcurrent mete
[0051] 14 filament-heating power source
[0052] 15 bias power source
[0053] 16 high voltage power source
[0054] 17 emitted electron beam
BEST MODES FOR CARRYING OUT THE INVENTION
Explanation of Terminology
[0055] The terminology used in the present specification and claims
are defined as below.
[0056] (Lower limit value) to (upper limit value): Means at least
the lower limit value and at most the upper limit value.
[0057] Hot cathode field emission type electron gun: Refers to an
electron gun of the type in which an emitter is heated in an
electric field to induce emission of electrons. The most common
types have conventionally been those in which a tungsten chip is
heated to about 2600 K to cause the electrons to jump across the
potential barrier (about 4.5 eV) to be emitted, but recently, those
in which an emitter formed by coating zirconium oxide to lower the
potential barrier (about 2.7 eV) is heated to about 1800 K and
using the Schottky effect to emit electrons have become more
common.
[0058] Etching: Refers to a method used in the fabrication of
samples of semiconductors and inorganic compounds, wherein surface
atoms are selectively removed from the surface of a solid by means
of chemical or physical reactions. They can be classified as either
liquid phase etching or vapor phase etching.
[0059] Chemical polishing: Refers to a method used for fabricating
samples of semiconductors and inorganic compounds. The sample is
immersed in a polishing solution based on a strong acid or strong
alkali, to form a thin film while keeping the sample surface
smooth. The advantage is that a sample can be fabricated without
applying any mechanical strain. In the present specification and
claims, it refers to methods using a polishing solution or the like
such that selective dissolution (etching) does not occur.
[0060] Mechanical polishing: Refers to physical polishing of a
sample. Examples include manual polishing using waterproof paper,
polishing with diamond powder or corundum using a rotating
polisher, polishing with corundum powder using a dimple grinder,
and polishing with diamond powder using a tripod polisher.
[0061] Electrolytic polishing: Refers to a method used for working
materials such as metals and alloys, in which they are immersed in
an appropriate electrolytic solution, and a DC current is applied
with the sample as the anode and platinum or stainless steel as the
cathode to cause elution of the material surface, so as to polish
the sample surface while keeping it smooth. The advantage is that
the material can be worked without applying any mechanical
strain.
[0062] Focused ion beam processing: Refers to a methods of
processing a sample by accelerating ions such as gallium ions to
about a few kV to 40 kV and focusing them on the sample. In some
cases, the sample can be processed while observing local areas by
secondary ion images (in some devices, a SEM image can be
seen).
Embodiment 1
[0063] Herebelow, a specific embodiment of the present invention
shall be described with reference to FIG. 1 to FIG. 4.
[0064] The present embodiment will describe a method of
manufacturing an electron source used in an electron gun for
electron beam appliances such as scanning electron microscopes,
Auger electron spectroscopes, electron beam lithography devices and
wafer inspection devices, the electron gun being especially
appropriate for use in electron beam lithography.
[0065] The present embodiment is a method of manufacturing an
electron source having an electron emission portion at one tip
portion of a rod-shaped cathode 1, the method of manufacturing an
electron source comprising a step of forming the tip portion into a
conical portion 6 by a machining process, then forming into the
shape of a truncated cone having a flattened portion 7, and a step
of removing a surface layer of the flattened portion 7 by focused
ion beam processing.
[0066] In the method of the present embodiment, a tip portion of a
rod-shaped cathode 1 of a tungsten or molybdenum monocrystal in the
<100> orientation is provided with a conical portion 6 by
mechanical polishing or electrolytic polishing, then a flattened
portion 7 is formed on the vertex by mechanical polishing using a
polishing film or the like covered with a diamond polishing
agent.
[0067] In order to obtain an electron source with a low total
emission current even during operation at high angular current
densities which is appropriate for use in an electron beam
lithography device or an Auger electron spectroscopy device, the
total angle of the conical portion 6 should be at least 25.degree.
and at most 95.degree., and the diameter of the flattened portion 7
should be 5 to 200 .mu.m. Furthermore, for similar reasons, the
line normal to the flattened electron emission surface formed on
the tip portion of the cathode should be within 2.degree. of the
<100> direction, and should more preferably be contained
within an angular difference of 0.5.degree..
[0068] The cathode 1 is affixed by welding via tungsten filaments 3
to a conductive terminal 4 brazed to an insulator 5, and can be
electrically heated.
[0069] While the processing scar layer of the flattened portion 7
of the cathode 1 will ultimately be removed by a focused gallium
ion beam, if the cut surface is initially too rough, a focused
gallium ion beam may not be sufficient to remove the processing
scar layer. Therefore, after forming the flattened portion 7, the
processing scar layer should preferably be removed by vapor phase
etching or liquid phase etching (chemical etching). In other words,
between the above-described step of forming the tip portion into a
conical portion 6 by a machining process, then forming into the
shape of a truncated cone having a flattened portion 7 (flattened
surface), and the step of removing a surface layer of the flattened
portion 7 by focused ion beam processing, it is preferable to
further include a step of removing the processing scar layer on the
surface of the tip portion by liquid phase etching (chemical
etching) or electrolytic polishing.
[0070] The vapor phase etching method may, for example, be
performed by evacuating a vacuum device, feeding oxygen gas into
the device to a pressure of 3.times.10.sup.-6 Torr
(4.times.10.sup.-4 Pa), and heating the cathode 1 to a temperature
of 1800 K to 2000 K. Additionally, the chemical etching method may,
for example, be performed by immersing the conical portion 8 of the
cathode 1 in 1 mol/L aqueous solution of sodium hydroxide, and
applying a voltage of 6 V between the conductive terminal 4 and an
electrode provided in the solution.
[0071] While these methods of removing the processing scar layer
can result in irregularities being formed or curvature of the
flattened portion 7 (due to sagging at the outer periphery), in the
present embodiment, they will have practically no effect on the
electron emission properties because they will be ultimately
re-flattened by a focused gallium ion beam.
[0072] The cathode 1 is provided with a reservoir consisting of
oxygen and a metal having the effect of reducing the work function
of the electron emission surface. In particular, electron sources
having a reservoir consisting of metal oxides comprising elements
chosen from the group consisting of zirconium (Zr), titanium (Ti),
hafnium (Hf), scandium (Sc), yttrium (Y), the lanthanoids, barium
(Ba), strontium (Sr) and calcium (Ca) can be appropriately applied
as an electron source for hot cathode field emission type electron
guns.
[0073] Using Zr--O as an example, a reservoir of zirconium and
oxygen can be formed by crushing zirconium hydride and mixing with
an organic solvent to form a paste, applying the paste to a portion
of the cathode, heating the cathode in an oxygen atmosphere of
about 3.times.10.sup.-6 Torr (4.times.10.sup.-4 Pa) to thermally
decompose the ZrH2,and further oxidizing.
[0074] Even with such heat treatments in an oxygen atmosphere, the
cathode 1 can be etched by the oxygen so as to form irregularities
on the flattened portion 7, but in this case as well, it will be
re-flattened by means of a focused gallium ion beam, and therefore
will not affect the electron emission properties.
[0075] When re-flattening the flattened portion 7 by means of the
focused gallium ion beam, the flattened portion 7 is positioned so
as to be parallel to the direction of the ion beam, and the ion
beam is uniformly emitted in a manner limited to the surface layer
of the flattened portion 7, thereby enabling a surface with a high
degree of flatness with no processing scars to be formed in a short
time. At this time, the surface layer of the flattened portion 7
should preferably be removed by shining a focused gallium ion beam
in a xenon difluoride atmosphere in view of the efficiency of
removal of the surface layer of the flattened portion 7.
[0076] This cathode is positioned between the extractor electrode 9
and the suppressor electrode 8, and a high negative voltage of
several kilovolts is applied between the extractor electrode 9 and
the cathode 1. Additionally, a negative voltage of a few hundred
volts is applied between the suppressor electrode 8 and the cathode
1, and the cathode 1 is heated to 1500 K to 1900 K so as to induce
electron emissions.
[0077] According to the method of the present embodiment, the
electron emission surface is processed by a focused gallium ion
beam to remove a processing scar layer caused by mechanical
polishing, thereby resulting in a uniform emission current density.
As a consequence, as shall be described in an example below, a hot
cathode field emission type electron gun comprising the electron
source of the present embodiment achieves a uniform emission
current density at high angular current densities of 1 mA/sr or
more, with an extremely low excess current, making it highly
reliable. Furthermore, this electron gun operates in a vacuum of
1.times.10.sup.-8 Torr (1.times.10.sup.-6 Pa) or less and is
tungsten-based, as a result of which the cathode undergoes very
little erosion and will have few changes in properties even when
operated over a long period of time, and the cathode surface can be
quickly repaired to a smooth surface even after being battered by
ion collisions.
Functions And Effects of Embodiment 1
[0078] Herebelow, the functions and effects of the method of
manufacturing an electron source according to the present
embodiment shall be explained.
[0079] The present embodiment offers a method of manufacturing an
electron source having an electron emission portion at one tip
portion of the cathode 1 (rod), the method of manufacturing an
electron source comprising a step of forming the tip portion into
the shape of a truncated cone having a flattened portion 7
(flattened surface), and a step of removing a surface layer of the
flattened portion 7 by focused ion beam processing.
[0080] According to this method, the tip portion is formed into the
shape of a truncated cone having a flattened portion 7 by means of
a machining process capable of efficiently forming the electron
emission surface in a short period of time, then the surface layer
of the flattened portion 7 is removed by focused ion beam
processing so as to form an electron emission surface with a
reduced scar layer. Additionally, according to this method,
electrolytic polishing and liquid phase etching are not performed
(or electrolytic polishing or liquid phase etching is followed by
focused ion beam processing), so sagging at the outer periphery of
the electron emission surface can be suppressed.
[0081] As a result, the method of the present embodiment enables a
flattened portion 7 (electron emission surface) with a reduced scar
layer and little sagging at the outer periphery to be efficiently
formed in a short period of time, thus achieving a good balance
between three technical demands which have a conflicting
relationship, the three demands being those of (1) efficiently
forming an electron emission surface with a reduced scar layer in a
short time in an electron source, (2) improving the non-uniformity
of current emission distribution, and (3) increasing the emission
current density
[0082] Furthermore, the method of the present embodiment should
preferably further include a step of removing the processing scar
layer on the surface of the tip portion by liquid phase etching or
electrolytic polishing, between the above-described step of forming
into the shape of a truncated cone and the above-described step of
removing the surface layer of the flattened portion 7 (flattened
surface). By doing so, even if the surface achieved by the initial
mechanical polish is too rough, the processing scar layer on the
surface of the tip portion can be removed to some degree by liquid
phase etching or electrolytic polishing, so the processing scar
layer of the flattened portion 7 of the cathode 1 can be finally
removed by the focused gallium ion beam.
[0083] Additionally, in the method of the present embodiment, the
above-mentioned step of removing the surface layer of the flattened
portion 7 (flattened surface) should preferably be a step of
removing the surface layer of the flattened surface by irradiating
with a focused gallium ion beam in a xenon difluoride gas
atmosphere. By doing so, the efficiency of removing the surface
layer of the flattened portion 7 by means of the focused gallium
ion beam can be improved, and a flattened portion 7 with a high
degree of flatness and few processing scars can be formed in an
even shorter period of time.
[0084] Furthermore, in the method of the present embodiment, the
diameter of the above-mentioned flattened portion 7 (flattened
surface) should preferably be at least 10 .mu.m and at most 100
.mu.m. In this manner, an electron source with a low total emission
current even when operating at high angular current densities which
is appropriate for use in electron beam lithography devices and
Auger spectroscopy devices can be obtained.
[0085] According to the present embodiment, sagging at the outer
periphery of the electron emission surface can be suppressed even
if the diameter of the flattened portion 7 is as small as 10 .mu.m,
thereby preventing the electron emission surface from becoming
curved and reducing the emission current density. On the other
hand, according to the method of the present embodiment, an
electron emission surface with a reduced scar layer can be formed
in a short time by using a focused gallium ion beam, even if the
diameter of the flattened portion 7 is as large as 100 .mu.m.
[0086] In the method of the present embodiment, the above-mentioned
electron source is preferably an electron source provided with a
reservoir for diffusion of oxides of metal elements to a
monocrystalline rod (cathode 1) of tungsten or molybdenum.
Additionally, the metal element should preferably be a metal
element chosen from among Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf and the
lanthanoids. Furthermore, the orientation of the above-mentioned
rod (cathode 1) should preferably be <100>. Additionally, the
above-mentioned electron source should preferably be an electron
source for a hot cathode field emission type electron gun.
[0087] In this manner, the work function of the tungsten
monocrystal <100> can be largely reduced from 4.5 eV by the
coating layer of these metal oxides, and the electron emission
region will be constituted by only the small crystal facet
corresponding to the flattened portion 7 formed on the tip portion
of the cathode, so when used as an electron source for a hot
cathode field emission type electron gun, it has the advantages of
providing an electron beam that is brighter than those of
conventional hot cathodes, and also prolonging the lifespan.
Additionally, the electron gun has the advantages of being more
stable than cold field emission type electron guns, being capable
of operating in a low vacuum, and being easy to use.
Embodiment 2
[0088] Herebelow, another embodiment of the present invention shall
be explained with reference to FIG. 1 to FIG. 4, with an emphasis
on the features differing from Embodiment 1. In all of the
drawings, the same reference numbers are used to designate elements
that are the same as those in Embodiment 1, and their explanations
will be omitted.
[0089] The present embodiment, as in the case of Embodiment 1, is a
method of manufacturing an electron source having an electron
emission portion at one tip portion of a cathode 1 (rod), the
method of manufacturing an electron source comprising a step of
forming the tip portion into the shape of a truncated cone having a
flattened portion 7 by a machining process, and a step of removing
a surface layer of the flattened portion 7 by vapor phase etching.
In the present embodiment, formation of the electron emission
portion by machining is followed by vapor phase etching of the
electron emission portion, thereby enabling the current emission
distribution to be made more uniform, and offering a large-current
electron source.
[0090] The gas used for vapor phase etching in the present
embodiment should preferably be oxygen due to its etching
efficiency and improved precision, the oxygen pressure should
preferably be 0.3.times.10.sup.-4 to 8.times.10.sup.-4 Pa, and the
temperature should preferably be at least 1700 K and at most 1950
K. For the same reasons as Embodiment 1, the rod in the present
embodiment should preferably have a reservoir of oxides of metal
elements chosen from the group consisting of Ca, Sr, Ba, Sc, Y, La,
Ti, Zr, Hf and the lanthanoids.
[0091] Regarding the method of manufacturing this reservoir, a
description shall be given for the case of zirconium oxide. In
order to manufacture a reservoir formed of zirconium oxide,
zirconium hydride is first crushed and mixed with isoamyl acetate
to form a paste which is then applied to a portion of the
monocrystalline rod constituting the cathode 1. Next, after the
isoamyl acetate has evaporated, it is placed in an ultrahigh vacuum
device. Next, the device is evacuated to an ultrahigh vacuum of
3.times.10.sup.-10 Torr (4.times.10.sup.-8 Pa) and electricity is
passed through the filament 3 to heat the monocrystalline rod 1 to
1800 K, so as to thermally decompose the zirconium hydride and form
metallic zirconium. Next, oxygen gas is fed into the device to a
pressure of 3 .times.10.sup.-6 Torr (4.times.10.sup.-4Pa) so as to
oxidize the metallic zirconium, resulting in a reservoir consisting
of zirconium oxide.
[0092] Here, the orientation of the rod constituting the cathode 1
should preferably be in the <100> direction for the same
reasons as Embodiment 1.
[0093] Additionally, the rod constituting the cathode 1 of the
present embodiment has a conical portion 6 on a tip portion, with
an electron emission portion at the vertex thereof, this electron
emission portion being shaped flat, and for the same reasons as
Embodiment 1, the diameter of the flattened portion 7 should be at
least 10 .mu.m and at most 100 .mu.m, and more preferably at least
10 .mu.m and at most 70 .mu.m. Furthermore, in the electron source
of the present embodiment, the rod constituting the cathode 1 has a
conical portion 6 at a tip portion, and for the same reason as in
Embodiment 1, a flattened electron emission portion should be
formed on the vertex of the conical portion 6 after removing the
processing scar layer by chemical etching.
[0094] On the other hand, it is not always necessary to perform
chemical etching, and in a method of manufacturing an electron
source having an electron emission portion on a tip portion of a
rod consisting of a monocrystal of tungsten or molybdenum
constituting the cathode 1, the electron emission portion can be
subjected to vapor phase etching after the electron emission
portion has been formed by machining.
[0095] To repeat, the gas used in vapor phase etching of the
present embodiment should preferably be oxygen for its etching
efficiency and improved precision, preferably at an oxygen pressure
of 0.3.times.10.sup.-4 to 8.times.10.sup.-4 Pa and a temperature of
at least 1700 K and at most 1950 K. The rod constituting the
cathode 1 in the present embodiment should preferably have a
reservoir of an oxide of a metal element chosen from among Ca, Sr,
Ba, Sc, Y, La, Ti, Zr, Hf and the lanthanoids for the same reason
as Embodiment 1. The orientation of the rod constituting the
cathode 1 in the present embodiment should preferably be in the
<100> direction for the same reason as in Embodiment 1.
[0096] Additionally, the rod constituting the cathode 1 of the
present embodiment has a conical portion 6 on a tip portion, with
an electron emission portion at the vertex thereof, this electron
emission portion being shaped flat, and for the same reasons as
Embodiment 1, the diameter of the flattened portion 7 should be at
least 10 .mu.m and at most 100 .mu.m, and more preferably at least
10 .mu.m and at most 70 .mu.m. Furthermore, in the electron source
of the present embodiment, the rod constituting the cathode 1 has a
conical portion 6 at a tip portion, and for the same reason as in
Embodiment 1, a flattened electron emission portion should be
formed on the vertex of the conical portion 6 after removing the
processing scar layer by chemical etching.
[0097] In this case, the etching of the surface of the rod
constituting the cathode 1 can be performed by welding the rod
constituting the cathode 1 to a filament, placing it in an
ultrahigh vacuum (3.times.10.sup.-10 Torr (4.times.10.sup.-8 Pa)),
then feeding oxygen gas into the device to a pressure of
3.times.10.sup.-6 Torr (4.times.10.sup.-4 Pa) and heating to a
temperature of 1800 K to 2000 K. By etching the monocrystalline
surface in this way, it is possible to remove machining scars
formed when shaping the rod constituting the cathode 1 into a
cone.
[0098] The rod functions as a cathode, and its surface may be
coated with oxygen and an oxide of a metal element chosen from
among Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf and the lanthanoids. Using
Zr--O as an example, zirconium hydride is crushed and mixed with an
organic solvent to form a paste which is applied to a portion of
the cathode, and the cathode is heated in an oxygen atmosphere of
about 1.times.10.sup.-6 Torr to thermally decompose the ZrH2, and
further oxidized to form a reservoir of zirconium and oxygen and
coat the surface of the cathode with zirconium and oxygen. This
cathode is placed between the extractor electrode and the
suppressor electrode, a high negative voltage of several kV
(kilovolts) is applied between the extractor electrode and the
cathode, and a negative voltage of a few hundred kV is applied
between the suppressor electrode 6 and the cathode 1 while heating
the cathode 1 to 1500 to 1900 K to induce electron emissions.
Functions And Effects of Embodiment 2
[0099] Herebelow, functions and effects of the method of
manufacturing an electron source of the present embodiment shall be
described.
[0100] The present embodiment offers a method of manufacturing an
electron source having an electron emission portion at one tip
portion of the cathode 1 (rod), the method of manufacturing an
electron source comprising a step of forming the tip portion into
the shape of a truncated cone having a flattened portion 7
(flattened surface), and a step of removing a surface layer of the
flattened portion 7 by vapor phase etching.
[0101] According to this method, the tip portion is formed into the
shape of a truncated cone having a flattened portion 7 by means of
a machining process capable of efficiently forming the electron
emission surface in a short period of time, then the surface layer
of the flattened portion 7 is removed by vapor phase etching so as
to form an electron emission surface with a reduced scar layer.
Additionally, according to this method, electrolytic polishing and
liquid phase etching are not performed (or electrolytic polishing
or liquid phase etching is performed before forming the flattened
portion by machining), so sagging at the outer periphery of the
electron emission surface can be suppressed.
[0102] As a result, the method of the present embodiment enables a
flattened portion 7 (electron emission surface) with a reduced scar
layer and little sagging at the outer periphery to be efficiently
formed in a short period of time, thus achieving a good balance
between three technical demands which have a conflicting
relationship, the three demands being those of (1) efficiently
forming an electron emission surface with a reduced scar layer in a
short time in an electron source, (2) improving the non-uniformity
of current emission distribution, and (3) increasing the emission
current density.
[0103] Furthermore, the method of the present embodiment should
preferably further include a step of removing the processing scar
layer on the surface of the tip portion by liquid phase etching or
electrolytic polishing, between the above-described step of forming
into the shape of a truncated cone and the above-described step of
removing the surface layer of the flattened portion 7 (flattened
surface). By doing so, even if the surface achieved by the initial
mechanical polish is too rough, the processing scar layer on the
surface of the tip portion can be removed to some degree by liquid
phase etching or electrolytic polishing. Thereafter, the flattened
portion can be formed and the processing scar layer of the
flattened portion 7 of the cathode removed by vapor phase
etching.
[0104] Additionally, in the method of the present embodiment, the
above-described step of removing the surface layer of the flattened
portion 7 (flattened surface) should preferably be a step of
removing the processing scar layer of the surface of the tip
portion including the surface layer of the flattened portion 7 by
performing vapor phase etching with an oxygen pressure of at least
0.3.times.10.sup.-4 Pa and at most 8.times.10.sup.-4 Pa and a
temperature of at least 1700 K and at most 1950 K. By doing so, the
efficiency of removal of the surface layer of the flattened portion
7 by vapor phase etching can be improved, and a flattened portion 7
with a high degree of flatness and few processing scars can be
formed in a much shorter period of time.
[0105] Furthermore, in the method of the present embodiment, the
diameter of the flattened portion 7 (flattened surface) should
preferably be at least 10 .mu.m and at most 100 .mu.m. By doing so,
an electron source with a low total emission current even during
operation at a high angular current density which is appropriate
for electron beam lithography devices or Auger spectroscopy devices
can be obtained.
[0106] According to the present embodiment, sagging at the outer
periphery of the electron emission surface can be suppressed even
if the diameter of the flattened portion 7 is as small as 10 .mu.m,
thereby preventing the electron emission surface from becoming
curved and reducing the emission current density. On the other
hand, according to the method of the present embodiment, an
electron emission surface with a reduced scar layer can be formed
in a short time by vapor phase etching, even if the diameter of the
flattened portion 7 is as large as 100 .mu.m.
[0107] In the method of the present embodiment, the above-mentioned
electron source is preferably an electron source provided with a
reservoir for diffusion of oxides of metal elements to a
monocrystalline rod (cathode 1) of tungsten or molybdenum.
Additionally, the metal element should preferably be a metal
element chosen from among Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf and the
lanthanoids. Furthermore, the orientation of the above-mentioned
rod (cathode 1) should preferably be <100>. Additionally, the
above-mentioned electron source should preferably be an electron
source for a hot cathode field emission type electron gun.
[0108] In this mariner, the work function of the tungsten
monocrystal <100>can be largely reduced from 4.5 eV by the
coating layer of these metal oxides, and the electron emission
region will be constituted by only the small crystal facet
corresponding to the flattened portion 7 formed on the tip portion
of the cathode, so when used as an electron source for a hot
cathode field emission type electron gun, it has the advantages of
providing an electron beam that is brighter than those of
conventional hot cathodes, and also prolonging the lifespan.
Additionally, the electron gun has the advantages of being more
stable than cold field emission type electron guns, being capable
of operating in a low vacuum, and being easy to use.
[0109] While embodiments of the present invention have been
described above with reference to the drawings, these are merely
examples of the present invention, and various other structures may
be employed.
[0110] For example, while focused ion beam processing was performed
after machining in Embodiment 1 and vapor phase etching was
performed after machining in Embodiment 2, the invention is not
limited to these two faults. For example, the method may involve
machining followed by vapor phase etching and further followed by
focused ion beam processing. By doing so, the scar layer of the
flattened portion 7 of the electron emission surface can be reduced
even further.
[0111] When performing both vapor phase etching and focused ion
beam processing as above, it is not necessary to perform
electrolytic polishing or liquid phase etching, but it is also
possible to perform electrolytic polishing or liquid phase etching
and further perform both vapor phase etching and focused ion beam
processing. In this case as well, the scar layer on the flattened
portion 7 of the electron emission surface can be further
reduced.
EXAMPLES
[0112] Herebelow, the present invention shall be further explained
by means of examples, but the present invention is not to be
construed as limited thereby.
Example 1
[0113] Herebelow, Example 1 shall be explained with reference to
FIG. 1 to FIG. 4. A tungsten filament 3 was spot-welded to a
conductive terminal 4 brazed to an insulator 5. A conical portion 6
with a total angle of 90.degree. was formed at the end portion of
the monocrystalline tungsten tip in the <100>direction, using
diamond paste and a polishing platen, and worked to the shape of
FIG. 4(a). Furthermore, the vertex portion of the conical portion 6
was polished with a polishing film coated with a diamond polishing
agent, to form a flattened portion 7 with a diameter of 35 .mu.m as
shown in FIG. 4(b). This rod was attached to the filament 3 by spot
welding. This rod functioned as a cathode 1.
[0114] Thereafter, the conical portion 6 of the cathode 1 was
immersed in a 1 mol/L aqueous solution of sodium hydroxide, and a
voltage of 6 V was applied for 10 seconds between the conductive
terminal 4 and an electrode set up in the solution, so as to
perform chemical etching.
[0115] Next, a paste formed by crushing zirconium hydride and
mixing with isoamyl acetate was applied to a portion of the cathode
1. Then, after evaporation of the isoamyl acetate, it was placed in
an ultrahigh vacuum device. Next, the device was set to an
ultrahigh vacuum of 3.times.10.sup.-10 Torr (4.times.10.sup.-8 Pa),
and electricity was passed through the filament 3 to heat the
monocrystalline rod 1 to 1800 K, so as to thermally break down the
zirconium hydride to form metallic zirconium. Subsequently, oxygen
gas was fed until the device was pressurized to 3.times.10.sup.-6
Torr (4.times.10.sup.-4 Pa), thereby oxidizing the metallic
zirconium and forming a diffusion supply 2 of zirconium and oxygen
consisting of zirconium oxide.
[0116] Next, the rod was placed in a focused gallium ion beam
device and oriented so as to make the flattened portion 7 parallel
to the direction of the ion beam, then it was uniformly irradiated
with an ion beam having an acceleration voltage of 30 kV and a beam
current of 5 nA, limited to the surface layer of the flattened
portion 7, thereby further removing about 2 .mu.m of the flattened
portion 7 for reflattening.
[0117] The tip of the resulting cathode 1 was positioned between
the suppressor electrode 8 and the extractor electrode 9. The
distance between the tip of the cathode 1 and the suppressor
electrode 8 was 0.1 mm, the distance between the suppressor
electrode 8 and the extractor electrode 9 was 0.8 mm, the hole
diameter of the extractor electrode 9 was 0.8 mm and the hole
diameter of the suppressor electrode was 0.6 mm.
[0118] Furthermore, the filament 3 was connected to a filament
heating power source 14 and further connected to a high voltage
power source 16, to apply a high negative voltage, i.e. an
extractor voltage V.sub.ex, with respect to the extractor electrode
9. Additionally, a suppressor electrode 8 was connected to a bias
power supply 15, and a negative bias voltage V.sub.b was applied to
the cathode 1 and the filament 3. This blocked the thermal
electrons emitted from the filament 3. The electron beam 17 emitted
from the tip of the cathode 1 passed through the hole in the
extractor electrode 9 to reach a phosphor screen 10. There was an
aperture 11 (hole) in the center of the phosphor screen 10, and the
probe current I.sub.p passing through to the cup-shaped electrode
12 was measured by a microcurrent meter 13. Defining the solid
angle computed from the distance between the aperture 11 and the
tip of the cathode 1, and the inner diameter of the aperture 11 as
.omega., the angular current density is I.sub.p/.omega..
Additionally, the aperture 11 and the cup-shaped electrode 12 can
be moved from outside the vacuum system, to measure the current
emission distribution.
[0119] Further, the measuring device was set to an ultrahigh vacuum
of 3.times.10.sup.-10 Torr (4 .times.10.sup.-8 Pa), and a
suppressor voltage V.sub.b=-300 V with respect to the
monocrystalline rod 1 was applied to the suppressor electrode 8
while keeping the monocrystalline rod 1 at 1800 K. Next, a high
emitter voltage of V.sub.ex=-4000 V was applied to the
monocrystalline rod 1 and held for several hours, and upon
stabilization of the emission current, the change in the probe
current I.sub.p when moving the aperture 11 was measured, to
determine the current emission distribution. The measurement
results of this current emission distribution are shown in FIG.
5.
Comparative Example 1
[0120] The current emission distribution of an electron gun
produced by the same manufacturing method as Example 1, with the
exception that flattening by chemical etching and focused gallium
ion beam were not performed, was determined by the same method as
the example. The measurement results of this current emission
distribution are shown in FIG. 5.
Analysis of Results of Example 1 and Comparative Example 1
[0121] While FIG. 5 shows the current emission distributions of
Example 1 and Comparative Example 1 converted to angular current
density, the comparative example can be observed to have an uneven
distribution with non-uniformities, whereas the comparative example
has a smooth and even distribution, as well as a high angular
current density of at least 2 mA/sr.
[0122] In Example 1, a flattened portion 7 was provided at the
vertex of the cathode 1 by mechanical polishing after forming a
conical portion 6 by means of mechanical polishing of the tip of
the cathode 1. Since mechanical polishing was used to form the tip
of the cathode 1 into a truncated cone, the processing surface was
scarred, but the processing scar layer was eliminated in a short
time by a focused gallium ion beam and therefore easily
removed.
[0123] That is, by first providing a flattened portion 7 by
mechanical polishing and then processing with a focused gallium ion
beam, an electron emission surface with a reduced scar layer was
able to be efficiently formed in a short time. Furthermore, since
it was processed by a focused gallium ion beam, sagging at the
outer periphery of the electron emission surface which tends to
occur when removing processing surface scars by liquid phase
etching or the like was also prevented. As a result, Example 1 was
able to achieve a uniform emission current density.
[0124] In other words, it was able to achieve a good balance
between three technical demands which have a conflicting
relationship, the demands being those of (1) efficiently forming an
electron emission surface with a reduced scar layer in a short time
in an electron source, (2) improving the non-uniformity of current
emission distribution, and (3) increasing the emission current
density.
Example 2
[0125] Herebelow, Example 2 will be explained with reference to
FIGS. 1-4 and 6-7.
(i) Electron Source
[0126] The electron source of the present example, as shown in FIG.
1, is formed by attaching a needle-shaped monocrystalline rod of
tungsten in the <100>orientation emitting an electron beam by
welding or the like at a predetermined position of a tungsten
filament 3 provided on a conductive terminal 4 affixed to an
insulator 5. This rod is a cathode 1 having a tip that is pointed
by electrolytic polishing, and electrons are mainly emitted from
this sharp tip. A reservoir 2 of zirconium and oxygen is provided
in a portion of the rod constituting the cathode 1, and while not
shown in the drawings, the surface of the rod constituting the
cathode 1 is covered with a ZrO coating layer.
[0127] The tip portion of the rod constituting the cathode 1 is
positioned between the suppressor electrode 8 and the extractor
electrode 9 for use as shown in FIG. 3. A high negative voltage is
applied between the rod, i.e. the cathode 1, and the extractor
electrode 9, and a negative suppressor voltage of about a few
hundred volts is applied between the suppressor electrode 8 and the
monocrystalline rod, so as to suppress thermal electron emissions
from the filament 3.
[0128] The rod tip portion constituting the cathode 1, as shown in
FIG. 2, has a conical portion 6 formed by mechanical polishing the
tip portion of a monocrystalline rod-shaped cathode 1 of tungsten
or molybdenum monocrystals in the <100>orientation, and a
flattened portion 7 formed by mechanical polishing of the vertex of
the conical portion 6. The flattened portion 7 is also an electron
emitting portion.
(ii) Method of Producing Electron Source
[0129] The method of producing the electron source shall be
explained with reference to FIG. 1.
[0130] As shown in FIG. 1, a tungsten filament 3 is attached by
spot welding to a conductive terminal 4 brazed to an insulator
5.
[0131] After forming a rod constituting a cathode 1 of tungsten
monocrystals in the <100>orientation, a rectangular
parallelepiped of 2 mm.times.0.4 mm.times.0.4 mm is cut out by
discharge processing, then one end is formed into a conical portion
6 with a total vertex angle of 90.degree. using diamond paste and a
polishing platen to achieve the shape of FIG. 4(a). The flattened
portion 7 which is an electron emitting portion is formed by
polishing with a polishing film coated with a chrome oxide
polishing agent until the diameter of the circle at the vertex of
the conical portion 6 is 40 .mu.m as shown in FIG. 4(b).
[0132] This rod constituting a cathode 1 is attached to a filament
3 by laser welding (see FIG. 1). The rod functions as a cathode 1.
The filament 3 is attached by welding to a conductive terminal 4
brazed to an insulator 5.
[0133] Subsequently, electricity is applied to the filament 3 in an
oxygen atmosphere of 3.times.10.sup.-6 Torr (4.times.10.sup.-8 Pa),
thereby etching the surface of the rod constituting the cathode
1.
[0134] The reservoir 2 shown in FIG. 1 was formed in a portion of
the rod constituting the cathode 1. The reservoir 2 was formed by
crushing a zirconium hydride powder and mixing with an organic
solvent to form a slurry that was then applied to a portion of the
rod constituting the cathode 1. After the isoamyl acetate
evaporated, the rod was heated in an oxygen atmosphere of about
1.times.10.sup.-6 Torr to thermally decompose the hydrides, and
further oxidized to result in zirconium oxide. Simultaneous to the
formation of this reservoir 2, the surface of the rod constituting
the cathode 1 was coated with zirconium and oxygen.
(iii) Measurement
[0135] Measurements were made using an electron emission property
measuring device of the structure shown in FIG. 3. As shown in FIG.
3, the tip of the rod constituting the cathode 1 was positioned
between the suppressor electrode 8 and the extractor electrode 9.
The distance between the tip of the rod constituting the cathode 1
and the suppressor electrode 8 was 0.10 mm, the distance between
the suppressor electrode 8 and the extractor electrode 9 was 0.8
mm, the hole diameter of the extractor electrode 9 was 0.8 mm and
the hole diameter of the suppressor electrode 8 was 0.6 mm. A
fluorescent material was applied to a screen electrode to form a
phosphor screen 10, enabling the electron emission distribution
pattern to be observed by eye.
[0136] As shown in FIG. 3, the filament 3 was connected to a
cathode high voltage power source 16, the cathode high voltage
power source 16 as connected to a filament heating power source 14,
and a high negative voltage (emitter voltage V.sub.ex) was applied
to the extractor electrode 9. The suppressor electrode 8 was
connected to a bias power source 15 functioning as a suppressor
power source, and a negative suppressor voltage V.sub.b was further
applied to the rod 1. As a result, the emission of thermal
electrons from the filament 3 was blocked.
[0137] By applying electricity to the filament 3, the rod
constituting the cathode 1 was heated to 1500-1900 K, as a result
of which an emission electron beam 17 was emitted from the electron
emitting portion at the tip portion of the rod forming the cathode
1. The emitted electron beam 17 was passed through a hole in the
extractor electrode 9 to reach a phosphor screen 10 consisting of a
screen electrode having an aperture 11 (hole) in the center.
[0138] When making the measurements, the inside of the measuring
device was set to an ultrahigh vacuum of 3.times.10.sup.-10 Torr
(4.times.10.sup.-5 Pa), and while keeping the rod 1 at 1800 K, a
voltage was applied to the suppressor electrode 8 to obtain a
suppressor voltage Vb=-300 V with respect to the rod constituting
the cathode 1, then a high voltage was applied to the rod
constituting the cathode 1 to achieve an emitter voltage
V.sub.ex=-4000 V which was maintained for several hours to
stabilize the emission current.
(iv) Measurement of Total Emission Current It of Electron
Source
[0139] The total emission current It from the electron source was
measured by a current meter (not shown) placed between the cathode
high voltage source 16 and ground.
(v) Measurement of Emitter Voltage V.sub.ex Dependence of Probe
Current I.sub.p (Angular Current Density)
[0140] The probe current I.sub.p passing through the aperture 11
and reaching the cup-shaped electrode 12 was measured by a
microcurrent meter 13 for measuring the probe current.
(vi) Measurement of Emission Current Distribution of Electron
Source
[0141] Defining the solid angle computed from the distance between
the aperture 11 and the tip of the rod constituting the cathode 1,
and the inner diameter of the aperture 11 as .omega., the angular
current density is I.sub.p/.omega.. Additionally, the aperture 11
and the cup-shaped electrode 12 can be moved from outside the
vacuum system, to measure the current emission distribution.
[0142] FIG. 6 shows the current emission distribution taking the
emission angle on the X axis and the angular current density
I.sub.p' when Vex=-4000 V on the Y axis. FIG. 6 is the emission
current distribution converted to angular current density on the
rod constituting the cathode 1 consisting of the shape shown in
FIG. 4(b) provided in the electron source described in Example
2.
Comparative Example 2
[0143] As a comparative example, an electron source with the
processing scar layer removed by electrolytic polishing was
produced. A similar rectangular parallelepiped of 2 mm.times.0.4
mm.times.0.4 mm aligned with the <100>orientation was cut out
and worked to the shape shown in FIG. 4(a) by means of mechanical
polishing, and an electron emitting portion 7 was provided on the
vertex (FIG. 4(b)). The total angle of the vertex of the conical
portion 6 was 90.degree., and the diameter of the electron emission
portion 9 was 40 .mu.m.
[0144] After mechanical polishing, the processing scar layer was
removed by electrolytic polishing. As the conditions for the
electrolytic polish, the conical portion 6 of the cathode 1 was
immersed in a 1 mol/L aqueous solution of sodium hydroxide, and a
voltage of 6 V was applied for 10 seconds between the conductive
terminal 4 and an electrode provided in the solution for chemical
etching.
[0145] FIG. 7 shows the current distribution with the emission
angle on the X axis and the angular current density I.sub.p' when
V.sub.ex=-4000 V on the Y axis. FIG. 7 is the emission current
distribution converted to angular current density on the rod
constituting the cathode 1 consisting of the shape shown in FIG.
4(b) provided in the electron source described in Comparative
Example 2.
Analysis of Results of Example 2 and Comparative Example 2
[0146] The electron source of Example 2 and the electron source of
Comparative Example 2, as shown in FIGS. 6 and 7, both have a
uniform emission current distribution. However, in FIG. 7 showing
Comparative Example 2, the angular current density was low. The
reason is that electrolytic polishing causes sagging at the outer
periphery of the electron emission surface, so that when the
diameter of the top surface of the truncated cone which is the
electron emission surface is small, the electron emission surface
becomes curved, thus reducing the emission current density.
[0147] That is, in the method for producing an electron source
according to Example 2, vapor phase etching was performed after
mechanical polishing, enabling a flattened portion 7 (electron
emission surface) with a reduced scar layer and little sagging at
the outer periphery to be formed in a short period of time, as a
result of which a smooth and even distribution was observed, while
obtaining a high angular current density of at least 2 mA/sr. In
other words, it was able to achieve a good balance between three
technical demands which have a conflicting relationship, the
demands being those of (1) efficiently forming an electron emission
surface with a reduced scar layer in a short time in an electron
source, (2) improving the non-uniformity of current emission
distribution, and (3) increasing the emission current density.
[0148] The present invention has been explained by means of
examples above. These examples are merely exemplary, and those
skilled in the art will understand that various modifications are
possible, and such modifications lie within the scope of the
present invention.
[0149] For example, while Example 1 above involved a method of
performing focused ion beam processing after machining and Example
2 involved a method of performing vapor phase etching after
machining, the invention is not necessarily restricted to these
two. For example, the machining may be followed by vapor phase
etching which is further followed by focused ion beam processing.
By doing so, the scar layer of the flattened portion 7 of the
electron emission surface can be further reduced.
INDUSTRIAL APPLICABILITY
[0150] An electron gun provided with an electron source produced by
the method of producing an electron source according to the present
invention is a highly reliable electron gun having a uniform
current emission distribution and operating with a large current,
so it is suitable for use as an electron gun requiring large
currents such as electron beam lithography apparatus, wafer
inspection apparatus and electron beam LSI testers, and is
extremely useful to industry.
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