GB2483182A

GB2483182A - Electron emitter for generating an electron beam

Info

Publication number: GB2483182A
Application number: GB1119712.6A
Authority: GB
Inventors: Shin Fujita; Mohamed El-Gomati; Torquil Wells
Original assignee: Shimadzu Corp
Current assignee: Shimadzu Corp
Priority date: 2006-06-30
Filing date: 2006-06-30
Publication date: 2012-02-29
Anticipated expiration: 2026-06-30
Also published as: GB2483182B; GB201119712D0

Abstract

An electron emitter for generating an electron beam is characterized in that the end of the emitter on the emission side of the electron beam is conical and has a radius of curvature of 1 um or more and is applied with an electric field to thereby emit electrons using a Schottky effect, and wherein the emitter has a form not to reveal a {100} crystal plane in a portion of the side surface.

Description

ELECTRON BEAM GENERATING APPARATUS AND ELECTRON EMITTERS

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The invention relates to electron beam generating apparatus and electron emitters.

(2) Description of the Related Art

Electron guns in electron beam based instruments use two types of cathodes (emitters); a thermionic emitter and a field emitter. A thermionic emitter uses a tungsten filament, a pointed emitter of a single crystal or a sintered compound of lanthanium hexaboride (LaB6) or cerium hexaboride (CeB6). The emitter is heated and caused to emit thermal excited electrons to thereby generate an electron beam. A field emitter uses a sharpened conical end of an electrode on the emission side of an electron beam and emits electrons by using a tunneling effect or a Schottky effect caused by a strong electric field applied to the conical end to thereby generate an electron beam.

Note that in a case where an analysis or observation is carried out in a small region, an electron beam with a high brightness is required in order to reduce a diameter thereof (here, the "brightness" is defined as the current density per unit solid angle of the electron beam). Therefore, in recent years, a field emitter has been adopted, instead of a thermionic emitter that has been conventionally employed, in a scanning electron microscope (hereinafter also referred to as "SEM" for short) and an electron probe microanalyzer (hereinafter also referred to as "EPMA" for short) as well as other electron beam based instruments; transmission microscopy, electron beam lithography, inspection tools, etc. in analysis or observation in a small region to thereby improve a spatial resolution.

There are two types of field emitters, a cold field emitter and a thermal field emitter. In the case of a cold field emitter, the conical end of an emitter is normally made from a single crystal fine tungsten wire and is subjected to a strong electric field at room temperature whereby electrons, in the single crystal, are emitted using a tunneling effect, so that an electron beam is generated. In the case of a thermal field emitter, the conical end of an emitter made from a single crystal fine tungsten wire coated with zirconium oxide (ZrO) is heated while being subjected to a strong electric field which causes electrons to be emitted using a Schottky effect, so that an electron beam is generated. Since a thermionic emitter uses a Schottky effect as described above, it is also called a Schottky emitter.

In a Schottky emitter, a zirconium oxide layer coating the conical end of the emitter has an effect of reducing a work function of a crystal face, formed in the conical end, and which is a (100) crystal plane. Therefore, a uniform, strong electron beam is emitted and extracted from the conical end. Note that a Schottky emitter technology is disclosed in US Patent Nos. 145042 and 145043.

In the case of a field emitter, as described above, the current density is, however, higher than that of a thermionic emitter. In the case of a field emitter, the electron source diameter, where an electron beam is emitted from in an electron gun configuration, is very small, as shown in Fig. 9B, in comparison with a thermionic emitter of Fig. 9A (Fig. 9B shows a Schottky emitter). An electron source diameter is several tens of pm in a case of a thermionic emitter, while in a field emitter represented by a Schottky emitter, an electron source diameter is several tens of nm. If an electron source area of a thermionic emitter is indicated by dSTE, and an electron source area of a field emitter is indicated by dSFE, the areas are different from each other by up to six orders of magnitude.

On the other hand, if a solid angle of an electron beam is indicated by dQ and a beam current value (current value is indicated by Ib), a solid angle dQ of the electron beam varies according to a beam current value b to be required. If an axial brightness of the electron beam is indicated by B, a beam current value b is given by the following equation (1) with an electron source area dS and a solid angle dQ.

Ib-BX(dSXdQ) In a case where a larger beam current is necessary, it is understood from equation (1) that an effective solid angle dQ increases for fixed brightness and source area.

A Schottky emitter is much higher in brightness than a thermionic emitter (by about three orders of magnitude). However, since an electron source area dSFE is smaller than dSTE by up to six orders of magnitude, a solid angle dQ of an electron beam in a case where the same beam current is secured is larger in a Schottky emitter than that in a thermionic emitter. That is, if a solid angle of an electron beam in a thermionic emitter is indicated by dikE and a solid angle of an electron beam in a field emitter represented by a Schottky emitter is indicated by dQFE, a relation expressed by the following equation (2) is established.

dQFE> dQTE... (2) That is, an angular current density which is the current per unit solid angle for a Schottky emitter is smaller than that of a thermionic emitter although the Schottky emitter has a higher axial brightness than the thermionic emitter.

Since, with a larger solid angle, an electron beam is diverged, collimation is required. As a result, in a field emitter, an aberration in an accelerating and condenser lens section downstream from the emission side exerts a large influence, so that a characteristic of the emitter, which would be by nature high in brightness, is degraded by an influence of the aberration, and the "apparent brightness" decreases as a beam current increases. Fig. 10 is a graph showing relationships between a beam current value and brightness in the case where a Schottky emitter is employed as an example of a field emitter and the case where a tungsten filament emitter is employed as an example of a thermionic emitter. The abscissa is assigned to a beam current and the ordinate is used for plotting brightness. A dotted line is a curve concerning a tungsten filament emitter and a solid line shows a curve concerning a Schottky emitter. Note that in the Schottky emitter, the curve was obtained under the conditions where the emission current density s is 1.0 x 1 A/cm2, an emitter temperature T is 1800 K and an angular current density Jc»=SE is 0.429 niA/str, while for the thermionic emitter, the curve was obtained under the conditions where the emission current density s is 3 A/cm2, an emitter temperature T is 2800 K and an angular current density Jçw = 140 niAlstr. The term "W filament" indicates the tungsten filament operated in the thermionic mode and the term "SE" indicates the Schottky emitter.

In a case of a thermionic emitter represented by a tungsten filament, the angular current density is high; therefore a decrease in brightness is not problematical in a practical aspect giving a reduced brightness when the current is in the neighborhood of a value in the range of 10 pAto 20 pA. On the other hand, in a case of a field emitter represented by a Schottky emitter, an angular current density is lower and an electron source diameter is smaller; therefore, the brightness begins to decrease when the beam current is in the neighborhood of 1 nA and the brightness decreases by 6 orders of magnitude at a beam current 1 pA.

Since a beam current employed in a case of a scanning electron microscope (SEM) is at a level of nA or less, no reduction in brightness is observed with a Schottky emitter in a case where the emitter is used in SEM. Thus, a Schottky emitter can be used in SEM. However, in a case of a device where a beam current at a level of sub pA or pA is required as in a electron probe micro analyzer (EPMA), reduction in the brightness is observed at a level of sub pA or pA for a Schottky emitter; therefore, even if a Schottky emitter is employed in instruments such as EPMA, only an electron beam with a low brightness can be used. Hence, it is impossible to employ a field emitter in instruments such as EPMA in a practical sense.

SUMMARY OF THE INVENTION

The invention has been made in light of such circumstances and it is an object of the invention to provide an electron beam generating apparatus and an electron emitter.

The following findings and knowledge have been obtained in order to achieve such an objective.

As shown in Fig. 1 1A, a Schottky emitter 201 has a construction in which a conical end 201 a of an emitter 201 on an emission side B of an electron beam is, as described above, sharpened in the shape of a cone. Note that Fig. 11B is a schematic diagram in which the conical end 201 a thereof is enlarged and if a radius of curvature of the conical end 201a is indicated by R, R is in the range of R < 0.5 pm to 0.6 pm.

Note that if trajectories of electron beams emitted from an electron gun (emitter) are called as "cathode trajectories", a primary characteristic of the cathode trajectories is characterized by electron gun focal length.(S.Fujita and H.Shimoyama, J. Electron Microscopy, 54(4), 331-343 (2005)) Fig. 12 is a diagram schematically showing the emitter (cathode) of an electron gun. If an angle is formed between an electron trajectory emitted normal to (at a given angle relative to) the cathode surface at position and an optical axis on a reference place (a drift region) is indicated by f3 as shown in Fig. 12, a focal length f is defined by the following differential equation (3).

1/f=-(dsinf3/S=oo *.. (3) It is seen in the equation (3) that the reciprocal of the electron gun focal length is the limiting ratio of the sine of the emerging ray angle to the off-axis distance of the starting position for the electrons emitted perpendicularly to the cathode surface. A crossover diameter, the minimum beam diameter of an electron beam formed along the optical axis (an electron source diameter) and an angular current density are obtained from the focal length f defined by the equation (3).

If an electron source diameter is indicated by d0, a Boltzmann's constant k, an absolute temperature T, the electronic charge e, a potential (an extraction potential) at an extractor electrode Vext and a current density at a cathode (a cathode current density) s, then an electron source diameter d0 and an angular current density Jç are given by the following equations (4) and (5), respectively.

=2 x fx {(k x T)/(e x Vext)}"2. . .(4) JQ=f2xj... (5) If a focal length f is longer, an electron source diameter d0 is larger as is understood from the equation (4) and an angular current density Jç is also raised as is understood from the equation (5). Hence, in order to set an angular current density Jc»= in a Schottky emitter with reasonable freedom, it is only required to adjust a focal length f.

Note that the increase (or decrease) of the angular current density Jc»= by the change in the electron gun focal length necessarily accompanies the increase (or decrease) of the electron source diameter d0. Consequently the brightness B itself is independent of the focal length as is shown below, B = Jç/ (d0/2)2 = (1/ )(e js/kT)Vext, (6) Hence, in order to control both of the electron optically important parameters, i.e. the brightness B and the angular current density Jc»=, it is necessary to have simultaneously under control the electron gun focal length f and the cathode current density s.

Let's start with the electron gun focal length. Then, attention will be paid to the equation (3), which defines a focal length f. It was found in this invention that the electron gun focal length can be adjusted by altering the shape of the Schottky emitter.

By scaling up or down the emitter tip radius R it is possible to increase or decrease the off-axis distance which corresponds to a fixed emerging ray angle 13. A focal length f defined by the equation (3) is obtained by fitting v ( sinl3) in Fig. 12 using the following equation (6).

v=(-1/Ox+Ex3... (7) Fig. 13 is a graph showing relationships between and v in cases where two Schottky emitters with different radii of curvature R were employed. The abscissa is assigned to and, also, the ordinate is used for plotting v. To be concrete, employed here is a Schottky emitter formed with a radius of curvature R of a conventional dimension of 0.6 pm and a Schottky emitter formed with a radius of curvature of 2.0 .im, which is larger than conventional size cathodes. As shown in Fig. 13, a curve of a conventionally standard Schottky emitter (R = 0.6 pm) is marked with the term "standard SE", while a curve of a Schottky emitter (R = 2.0 pm) having a radius of curvature R larger than the conventional size is marked with the term "Giant SE".

In Fig. 13, inclinations of both curves in the vicinity of = 0 take a value (-1/f).

By comparison in inclination, the Giant SE with a smaller inclination has a focal length f longer than the standard SE with a larger inclination.

As described above, findings and knowledge have been obtained that by adjusting the radius of curvature of the conical end of an emitter one can control a focal length and consequently a angular current density can be freely set. In particular, findings and knowledge have been obtained that if a radius of curvature of the conical end of an emitter is adjusted to be larger than those employed for conventional Schottky emitters, the focal length becomes longer and consequently the angular current density can be increased.

Next, we shall investigate the cathode current density. With Schottky emitters and cold field emitters the current densityj is a function of the electric field strength at the cathode. Since in the case of the point cathode tip the electric field is enhanced by its small curvature radius, the change in the radius usually influences the field strength.

Larger emitters would have a lower field if the other electrode configurations and the applied voltage were unchanged. Because the applied voltages cannot be augmented indefinitely without risk of the discharge, some compensation in the electrodes configuration is necessary in order to recover a high enough electric field with larger radius emitters. One effective way of its realization is to make the protrusion length of the emitter from the suppressor longer in the Schottky emitter module configuration which consists of an emitter, a suppressor and an extractor. By setting an appropriate protrusion length in accordance with the emitter tip radius it is possible to secure high enough tip field under reasonable applied voltage condition.

Therefore, the invention reported here based on the findings and knowledge obtained by the inventors has the following configuration.

According to one aspect of the present invention, there is provided an emitter generating an electron beam in which the conical end of an emitter on the emission side of an electron beam is sharpened in the shape of a cone and is applied with an electric field to thereby emit electrons using a Schottky effect, the emitter having a form not to reveal a { 1 OO} crystal plane in the emitter side surface portion.

In another aspect, the invention provides electron beam generating apparatus comprising such an emitter, wherein two electrodes apply an electric field to the conical end of the emitter so that an electron beam is generated using a Schottky effect, wherein said emitter side surface portion on the emission side outward from a suppressor electrode, located on the side opposite the emission side of among the two electrodes establishing an electric field, and carry a negative voltage.

Such an apparatus may further comprise processing means conducting a predetermined processing based on an electron beam generated by the electron beam generating means.

Preferably, a suppressor electrode and the emitter are disposed so that a protrusion length, that is a length to the topmost point of the conical end from the suppressor electrode, is in the range of 200 im or more and 1500 pm or less when the conical end is protruded on the emission side outward from the suppressor electrode, and a radius of curvature of the conical end is in the range of 1 pm or more and 4 pm or less.

By selecting a radius of curvature in the range of 1 pm or more, a focal length is controlled so as to be longer than in a conventional case where a radius of curvature is in the range from 0.5 pm to no more than 0.6 pm and, besides, a angular current density can be controlled to a higher value than in the conventional case.

Where the radius of curvature is selected in the range of 1 pm or more and 4 or less, and a protrusion length is selected in the range of 200 pm or more and 1500 pm or less (see Fig.4), this may enable a controlled increase of the angular current density to higher than conventional values to be realized while maintaining the high beam brightness of the Schottky emitter by ensuring the high cathode current densityj

with the appropriate electric field at the tip.

Where the emitter is formed not to reveal a (100) crystal plane in the side surface portion of an emitter on the emission side outward from the suppressor electrode, an unnecessary (100) plane is hidden in the rear part on the side opposite the emission side of the suppressor electrode, which enables an unnecessary extraction current to be suppressed. The term "emitter side surface" means a surface parallel to an emission direction of the electron beam.

An example of a device using electron beam generating apparatus related to the invention is an electron probe microanalyzer conducting an analysis or observation of a specimen, in which processing means conducts an analysis or observation of a specimen by irradiating the specimen with an electron beam to obtain an X-ray image based on X-rays generated from the specimen, or by irradiating a specimen with an electron beam to obtain an electron beam image based on secondary electrons or reflected electrons generated from the specimen.

An electron probe microanalyzer is optimal for analyzing or observing a small region of a specimen.

Another example of a device using electron beam generating apparatus related to the invention is an X-ray tube, in which processing means is a target generating X-rays by collision with an electron beam.

Since the X-ray tube is equipped with an emitter emitting an electron beam with a high brightness, an angle of an electron beam when a target is irradiated therewith can be suppressed to be small, thereby enabling an X-ray generating area on the target to be small. Therefore, a spatial resolution of an X-ray image can be improved.

Still another example of a device using electron beam generating apparatus related to the invention is an electron beam lithography system, in which processing means conducts lithography using an electron beam.

Since an electron beam lithography system is equipped with an emitter emitting an electron beam with a high brightness, an angle of an electron beam converging at one point on a pattern used in lithography can be suppressed to be small, thereby, enabling a spatial resolution of a pattern for lithography formed on a target of lithography to be better.

An example of electron beam generating apparatus or a device using electron beam generating apparatus of the invention is as follows: a suppressor electrode and the emitter are disposed so that a protrusion length, that is a length to the topmost point of the conical end from the suppressor electrode, is in the range of 200 pm or more and 1500 pm or less, and a radius of curvature of the conical end is in the range of 1 pm or more and 4 pm or less. Since a radius of curvature is in the range of 1 pm or more and 4 pm or less and a protrusion length is in the range of 200 pm or more and 1500 pm or less, an angular current density can be higher than in a conventional practice and, at the same time, an electric field at the conical end can be controlled.

The emitter has a form not to reveal a (100) crystal plane in the emitter side surface portion on the emission side outward from a suppressor electrode that is located on the side opposite the emission side among the two electrodes applying an electric

field, and applied with a negative voltage.

In this case, since unnecessary (100) planes are hidden in the rear part on the side opposite the emission side of the suppressor electrode, an unnecessary extraction current is suppressed.

BRIEF DESCIUPTION OF THE DRAWNGS

For the purpose of illustrating the invention, there are shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangement and instrumentation shown.

Fig. 1A is a schematic diagram showing a Schottky emitter related to one example of the invention; Fig. lB is an enlarged schematic diagram of the conical end of the emitter; Fig. 1C is an enlarged diagram for comparison with the conical end of a conventional emitter; Fig. 2 is a schematic diagram of an electron beam generating apparatus equipped with the Schottky emitter; Fig. 3 is a schematic block diagram of an electron probe microanalyzer (EPMA) equipped with the electron beam generating apparatus; Fig. 4 is a graph roughly showing a combined range of a protrusion length and a radius of curvature; Fig. 5A is a schematic diagram showing a configuration of a suppressor electrode and a Schottky emitter processed by means of a DC etching method or a similar means to produce similar shapes; Fig. 5B is a schematic diagram showing a configuration of a suppressor electrode and a Schottky emitter processed by means of an AC etching method or other means to produce similar shapes; Fig. 6 is a graph showing relationships between a beam current value and a reduced brightness in an example of the invention, a standard Schottky emitter of a conventional technology, and a tungsten filament emitter; Fig. 7 is a schematic block diagram of a microfocus X-ray tube equipped with a Schottky emitter; Fig. 8 is a schematic block diagram of an electron beam exposure system equipped with a Schottky emitter; Fig. 9A is a diagram schematically showing an electron source characteristics when an electron beam is emitted from an electron gun of a thermionic emitter; Fig. 9B is a diagram schematically showing an electron source characteristics when an electron beam is emitted from an electron gun of a field emitter; Fig. 10 is a graph showing relationships between a beam current value and brightness in a conventional Schottky emitter and a tungsten filament emitter; Fig. 1 1A is a schematic diagram showing a conventional Schottky emitter; Fig. 11B is an enlarged schematic diagram of the conical end of the emitter; Fig. 12 is a diagram schematically showing an emitter of an electron gun and the definition of the electron gun focal length; and Fig. 13 is a graph describing the findings and knowledge leading to the invention.

DESCRJPTION OF THE PREFERRED EMBODIMENTS

Detailed description will be given of a preferred embodiment of the invention below based on the accompanying drawings.

Fig. 1A is a schematic diagram showing a Schottky emitter related to one example of the invention. Fig. 1 B is an enlarged schematic diagram of the conical end of the emitter. Fig. 1 C is an enlarged diagram for comparison with the conical end of a conventional emitter. Fig. 2 is a schematic diagram of an electron beam generating apparatus equipped with the Schottky emitter. Fig. 3 is a schematic block diagram of an electron probe microanalyzer (EPMA) equipped with the electron beam generating apparatus.

A Schottky emitter 1 related to the example, as shown in Fig. 1A, has the conical end la sharpened in the shape of a cone on the emission side of an electron beam B (here, the symbol "B" in the figure denotes the electron beam and should not be confused with the quantity representing the brightness). The Schottky emitter 1 has a construction in which a zirconium oxide layer is coated on a single crystal wire of tungsten. As shown in Fig. 1B, a radius of curvature R of the conical end la is 2.0 pm and preferably in the range of 1 pm or more, which is larger as compared with a conventional conical end 201a in the range of 0.5 pm and not more than 0.6 pm (see Fig. 1 lB and Fig. 1C). Note that in Fig. 1C, the conical end la of a Schottky emitter 1 related to the example is shown with a two-dot chain line.

An electron beam generating apparatus 10 equipped with the Schottky emitter 1, as shown in Fig. 2, includes two electrodes 2 and 3 applying an electric field to the conical end 1 a of the Schottky emitter 1; an anode 4 extracting an electron beam B; and a condenser lens 5 converging the electron beam B. A portion consisting of the Schottky emitter 1 and the electrodes 2 and 3 (a suppressor electrode 2 and an extractor electrode 3 described later) is called an electron source. The electron source is easy to be understood with a potential of the Schottky emitter 1 as a reference (in Fig. 2, the potential is 0 V. In an actual case, a potential of the emitter 1 is usually at a negatively high potential). The conical end la is heated under a strong electric field applied thereto by the electrodes 2 and 3 to thereby emit electrons using a Schottky effect and the electron beam B is thus generated by the electron beam generating apparatus 10.

The electron beam generating apparatus 10 corresponds to an electron beam generating apparatus of the invention.

Of the two electrodes 2 and 3, the electrode 2 located on the side opposite the emission side and applied with a negative voltage (in Fig. 2, -300 V) is a suppressor electrode 2 and the electrode 3 located on the emission side and applied with a positive voltage (in Fig. 2, 6423 V) is an extractor electrode 3.

The anode 4 is disposed opposite the Schottky emitter 1 serving as a cathode and applied with a positive voltage with respect to the emitter 1. The anode 4 attracts the electron beam B emitted from the Schottky emitter 1. The electron beam B is accelerated by attraction by the anode 4.

The condenser lens 5 is constructed in the shape of a ring. A current is supplied into the condenser lens 5 from a lens power supply not shown to thereby generate a magnetic field to converge the electron beam B in a similar way to light in an optical condenser lens.

Description will be given of a configuration of the Schottky emitter 1, the electrodes 2 and 3, the anode 4 and the condenser lens 5 in the electron beam generating apparatus 10, again, with reference to Fig. 2. The suppressor electrode 2 and the extractor electrode 3 are disposed with a spacing of 700 pm therebetween. Even though the spacing is similar to the conventional Schottky emitter configurations, the disposition of each electrode is unique. The length from the suppressor electrode 2 to the topmost point of the conical end 1 a is indicated by On the other hand, a length to the extractor electrode 3 from the topmost point of the conical end is indicated by LTE.

Hence, a relation that + LTE = 700 pm is established. If the emitter is operated with the same protrusion length LST of 250 pm as conventional, an electric field strength F at the conical end la cannot be secured to be a necessary value (in this case, F = 1 x i09 Vim). Therefore, a protrusion length LST is set to be longer than conventional case in order to raise an electric field strength F to a necessary value (1 x i09 Vim). In a case of a Schottky emitter where a radius of curvature R of the conical end la is 2.0.im, the Schottky emitter 1 and the suppressor electrode 2 are disposed so that a protrusion length LST is 400 pm. Therefore, LTE is 300 pm (= 700 pm -LST).

In order to secure a necessary electric field strength F at the conical end 1 a, a protrusion length LST is adjusted so as to be adapted for a radius of curvature R. That is, a characteristic of an electric field versus a protrusion length LST also varies depending on a radius of curvature R. Hence, as shown in Fig. 4, a combined range of a protrusion length LST and a radius of curvature R are set in advance based on a value of an electric field necessary for field emission in the Schottky mode. In this case, a protrusion length LST and a radius of curvature R are individually altered to estimate the combined range suitable for the necessary electric field strength F (1 x i09 Vim) (see a crosshatched portion in Fig. 4). That is, a combined range of a radius of curvature R and a protrusion length LST necessary for an operation of the Schottky emitter 1 is defined by the hatched area in Fig. 4. Note that in Fig. 4, there is shown a combination of a radius of curvature R and a protrusion length LST (where R = 0.5 pm and LST = 250 pm) of a conventional standard Schottky emitter (with R = 0.5 pm) with a mark A distance between the anode 4 and the condenser lens 5 is indicated by L. In a case of a thermionic emitter, the anode 4 and the condenser lens 5 are spaced with a distance of a value of the order of L = 100 mm. Though L is longer and a lens aberration coefficient is larger, the problem of larger aberrations leading to larger beam diameter is not incurred since a thermionic emitter has a large angular current density.

Contrast thereto, in a case of a Schottky emitter, since an angular current density is smaller, the intrinsically high brightness of the Schottky emitter is degraded by an increase in lens aberration coefficients. Hence, in a case of a Schottky emitter, it is preferable that in order to suppress a lens aberration coefficient, L is set to be as close to o mm as possible to thereby locate the condenser lens 5 so as to be closer to the side of the Schottky emitter 1.

If a Schottky emitter 1 is processed using a direct current (DC) etching method or other suitable means, unnecessary (100) crystal planes are revealed, as shown in Fig. 5A, forward from the suppressor electrode 2, that is on the emission side (see hatching with oblique lines inclined to the right in the figure). A work frmnction of a (100) crystal plane decreases by the action of a zirconium oxide layer and an unnecessary extraction current is extracted with a result of increasing a load on a power supply. As a result, larger outgas rate, which is a gas load from the surrounding electrode surfaces, is generated to degrade the degree of vacuum in the vicinity of the emitter. The term, a DC etching method, is an etching conducted without altering polarities of electrodes used in the etching.

Contrast thereto, in a case where an alternate current (AC), or similar means are used in forming a Schottky emitter 1, the etching can be conducted, as shown in Fig. SB, so that the conical end la in the shape of a cone is longer with oblique lines in profile. The AC etching method enables not only an etched macro face with oblique lines in profile to be obtained, but also a crystal face different from a (100) to be microscopically produced. Hence, by processing the Schottky emitter 1 with an AC etching method, a (100) is not revealed on a crystal surface in the emitter side surface portion on the emission side outward from the suppressor electrode 2. The term "emitter side surface" is a surface parallel to the emission direction of the electron beam B. Therefore, hatching with oblique lines inclined to the right in the figure indicates (100) crystal planes in the emitter side surface portion. With such a construction adopted, unnecessary (100) crystal planes are hidden in the rear part on the side opposite the emission side of the suppressor electrode 2, which enables an unnecessary extraction current to be suppressed. The term, an AC etching method, is to conduct etching while polarities of electrodes used for the etching are alternated.

Then, description will be given of a method for controlling an electron beam.

To begin with, a radius of curvature R of the conical end la is adjusted. A radius of curvature R is adjusted to a value larger than conventional in order to control the electron gun focal length f to be longer and to control the angular current density to be higher. Since a radius of curvature of a conventional conical end 1 a is in the range of 0.5 pm to no more than 0.6 m, it is preferable to select a radius of curvature R in the range of 1 pm or more. In one embodiment given here by way of example, a radius of curvature R is selected to be 2 pm.

As described above, a combined range of a protrusion length LST and a radius of curvature R is set in advance based on an electric field value. The suppressor electrode 2 and the Schottky emitter 1 are disposed by determining a protrusion length LST, which is a length from the suppressor electrode 2 to the topmost point of the conical end of the Schottky emitter 1. In adjustment of a protrusion length LST, a protrusion length LST is selected from the combined range shown in Fig. 4 at an adjusted radius of curvature R. In Fig. 4, a desirable combined range of a radius of curvature R and the protrusion length LST is defined in the range selected in the range 1 pm < R < 4 pm and pm < LST< 1500 rim. In one embodiment shown here by way of example, a combination of a radius of curvature R and a protrusion length LST (R = 2.0 pm and LST 400 pm) is selected. By selecting a combination point in the combined range in Fig.4, an electric field strength F at the conical end la can be controlled (in the example, F= lx 10 V/rn).

A focal length for the electron beam B is controlled by a radius of curvature R thus adjusted. An angular current density of the electron beam B is controlled by a controlled focal length f, while the beam brightness is maintained at its intrinsically high value by guaranteeing a large enough tip electric field through the emitter protrusion length adjustment.

In a case where a radius of curvature of the conical end 1 a is set in the range of 1 m or more and 4 m or less and a protrusion length LST of the Schottky emitter 1 is adjusted in the range of 200 m or more and 1500 m or less, an angular current density can be higher than conventional and at the same time, an electric field F at the conical end la can be controlled to maintain the high beam brightness.

Since an angular current density is higher than conventional geometry, a brightness is scarcely reduced even at relatively high beam currents and in the EPMA of the example, the elementary analysis processing section 20 and the surface observation processing section 30 can conduct predetermined processing such as elementary analysis processing and surface observation processing, respectively with a high brightness electron beam. Therefore, the invention can be applied to various apparatuses represented by such an EPMA 50.

Note that in a case where an electron beam generating apparatus 10 related to the example is used in EPMA 50, the following effect is exerted. That is, EPMA 50 requires a beam current at a level of sub RA or RA, and it is also confirmed in Fig. 6 that in EPMA 50, no reduction in the brightness in the Schottky emitter 1 is observed even with a level of sub RA or RA.

Fig. 6 is a graph showing relationships between a beam current value and a brightness in a Schottky emitter 1 (R = 2.0 Rm) related to the example, a standard Schottky emitter (R = 0.6 Rm) of a conventional technology and a tungsten filament emitter as a thermionic emitter. That is, the graph of Fig. 6 is obtained by adding the graph showing a relationship between a beam current value and a brightness in the Schottky emitter 1 related to the example to Fig. 10. Fig. 6 was obtained in the same condition as in Fig. 10. In the Schottky emitter 1 related to the example, however, the relationships were obtained in the conditions that a current density s is 1.0 x 1 A/cm2, a temperature T is 1800 K and a angular current density Jc»=GSE is 2.22 mA/str. A curve drawn with a dotted line is a curve of the tungsten filament emitter and two curves drawn with a solid line in Fig. 6 is a curve of the Schottky emitter, wherein "Giant SE" in the graph indicates the curve of a Schottky emitter 1 (R 2.0 pm) related to the example having a radius of curvature R of the conical end 1 a larger than conventional and "Standard SE" indicates the curve of a conventional standard Schottky emitter (R = 0.6 pm). The mark of "W filament" in the graph indicates a tungsten filament.

It is found from Fig. 6 that in the case of a conventional standard Schottky emitter, an angular current density is low and an electron source diameter is small; therefore, a brightness begins to decrease at a beam current in the vicinity of 1 nA or greater and reduces by as much as 6 orders of magnitude at a level of 1 pA. Contrast thereto, in a case of a Schottky emitter 1 related to the example, an angular current density is high; therefore, it has been confirmed that a brightness is hard to decrease as compared to a standard Schottky emitter and a brightness decrease starts at about 1 pA if a position of the condenser lens 5 is properly selected. Hence, a Schottky emitter can be applied to a device requiring a beam current at a level of sub pA or A such as in an EPMA 50.

The invention can be modified in the following way without limiting to the embodiment.

(1) In the example, description was given of an electron probe microanalyzer (EPMA) as an example of a device using an electron beam generating apparatus, no specific limitation is imposed on a device as far as an electron beam generating apparatus is used therein. For example, the device may be a scanning electron microscope (SEM), a transmission electron microscope (hereinafter also referred to as "TEM" for short), a microfocus X-ray tube, an Auger electron spectrometer, an electron beam lithography system and an electron beam writer. A transmission electron microscope (TEM) can observe a projected image by causing an electron beam to be transmitted through a thin film specimen with a thickness of the order in the range of several tens to hundreds of nanometres. A microfocus X-ray tube generates an X-ray beam with a small diameter of the order in the range of from sub pm to several pm by causing an electron beam to collide with a target. An Auger electron spectrometer examines energy ofAuger electrons to conduct an elementary analysis on a specimen.

An electron beam lithography system conducts lithography with an electron beam instead of light in a conventional technology. An electron beam writer produces "masters" for high density optical disks.

Description will be given not only of the microfocus X-ray tube but also of the electron beam exposure system as an example of electron beam lithography system.

Fig. 7 is a schematic block diagram of a microfocus X-ray tube and Fig. 8 is a schematic block diagram of an electron beam exposure system.

A microfocus X-ray tube 70 equipped with an electron beam generating apparatus 10, as shown in Fig. 7, includes a target 60 generating X-rays by collision of an electron beam therewith. The electron beam generating apparatus 10 is equipped with not only the suppressor electrode 2, the extractor electrode 3, the anode 4 and the condenser lens 5, but also an iris lens 6 and an objective lens 7. The iris lens 6 has an aperture 6a having a diameter reducing hole that defines converging angle of an electron beam B. The condenser lens 5, the iris lens 6, the objective lens 7 and the target 60 are sequentially disposed in order from the upstream side (the emitter 1 side) to the downstream side in an irradiation direction of the electron beam B. The target 60 is formed from a material generating X-rays represented by tungsten. The target 60 corresponds to processing means of the invention.

Since the microfocus X-ray tube 70 has a Schottky emitter 1 in which an electron beam brightness does not deteriorate at a high beam current condition, an angle of the electron beam B when the target 70 is irradiated with the electron beam B can be suppressed to be small, thereby enabling the electron beam size focused on the target to be small. Consequently, an X-ray generating region on the target 60 can be smaller, and a spatial resolution of an X-ray image is improved.

The electron beam exposure system 90 equipped with the electron beam generating apparatus 10 includes an exposure processing section 80 conducting exposure on a substrate W as shown in Fig. 8. The electron beam generating apparatus is equipped with: the suppressor electrode 2; the extractor electrode 3; the anode 4; and the condenser lens 5. The exposure processing section 80 is equipped with: irradiation lens 81, image forming lens 82; a shaping aperture 83; a blanker 84; a reticle 85; and a contrast aperture 86. The reticle 85 is an original drawing of an exposure pattern. The exposure processing section 80 corresponds to processing means of the invention.

In the electron beam exposure system 80 shown in Fig. 8, each pair of lenses 5, 81 and 82 is disposed one on the other. Not only is the shaping aperture 83 disposed between the condenser lens 5 on the downstream side and the irradiation lens 81 in the upstream side in the irradiation direction of the electron beam B, but the blanker 84 is disposed between the irradiation lens 81 on the upstream side and the irradiation lens 81 on the downstream side in the irradiation direction of the electron beam B. Not only is the reticle 85 disposed between the irradiation lens 81 on the downstream side 81 and the image forming lens 82 on the upstream side, but the contrast aperture 86 is disposed between the image forming lens 82 on the upstream side and the image forming lens 82 on the downstream side.

Since the electron beam exposure system 90 is equipped with a Schottky emitter 1 emitting an electron beam B high in brightness, an angle of the electron beam B converging to one point on the reticle 85 can be suppressed to be small, thereby enabling a spatial resolution of an exposure pattern focused on the substrate W to be improved.

(2) In the example, an AC etching method is adopted to form the Schottky emitter 1 so as not to reveal a (100) crystal plane in the emitter side surface portion on the emission side outward from the suppressor electrode 2, while no limitation is placed on an AC etching method as far as a (100) crystal plane is not revealed in the emitter side surface portion.

(3) In the example, a Schottky emitter 1 has a shape where no (100) crystal plane is revealed in the emitter side surface portion on the emission side outward from the suppressor electrode 2, while the Schottky emitter 1 is not necessarily required to have a shape shown in Fig. SB unless an unnecessary extraction current is suppressed.

For example, as shown in Fig. SA, unnecessary (100) crystal planes may be revealed forward from the suppressor electrode 2.

The invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.

In this specification, the verb "comprise" has its normal dictionary meaning to denote non-exclusive inclusion. That is, use of the word "comprise" (or any of its derivatives) to include one fealure or more, does not exclude the possibility of also including finther features.

Claims

CLAIMS: 1. An emitter generating an electron beam in which the conical end of an emitter on the emission side of an electron beam is sharpened in the shape of a cone and is applied with an electric field to thereby emit electrons using a Schottky effect, the emitter having a form not to reveal a { 1 OO} crystal plane in the emitter side surface portion.
2. An electron beam generating apparatus comprising an emitter according to claim 1, wherein two electrodes apply an electric field to the conical end of the emitter so that an electron beam is generated using a Schottky effect, wherein said emitter side surface portion on the emission side outward from a suppressor electrode, located on the side opposite the emission side of among the two electrodes establishing an electricfield, and carry a negative voltage.
3. An apparatus according to claim 2, further comprising processing means conducting a predetermined processing based on an electron beam generated by the electron beam generating means.
4. An apparatus according to claim 3, wherein a suppressor electrode and the emitter are disposed so that a protrusion length, that is a length to the topmost point of the conical end from the suppressor electrode, is in the range of 200 pm or more and 1500 pm or less when the conical end is protruded on the emission side outward from the suppressor electrode, and a radius of curvature of the conical end is in the range of 1 pm or more and 4 pm or less.
5. An emitter substantially as hereinbefore described with reference to the accompanying drawings.Amendments to the claims have been filed as follows.CLAIMS: 1. An emitter generating an electron beam in which the end of an emitter on the emission side of an electron beam is sharpened in the shape of a cone and is applied with an electric field to thereby emit electrons using a Schottky effect, the emitter having a form not to reveal a {100} crystal plane in an emitter side surface portion and the radius of curvature of the conical end being 1 pm or more.2. An electron beam generating apparatus comprising an emitter according to claim 1, wherein two electrodes apply an electric field to the conical end of the emitter so that an electron beam is generated using a Schottky effect, wherein said emitter side (\J surface portion on the emission side is outward from a suppressor electrode, located on the side opposite the emission side of among the two electrodes establishing an electricQ field, and carrying a negative voltage. C)(\J 3. An apparatus according to claim 2, further comprising processing means conducting a predetermined processing based on an electron beam generated by the electron beam generating means.4. An apparatus according to claim 3, wherein a suppressor electrode and the emitter are disposed so that a protrusion length, that is a length to the topmost point of the conical end from the suppressor electrode, is in the range of 200 pm or more and 1500 pm or less when the conical end is protruded on the emission side outward from the suppressor electrode, and a radius of curvature of the conical end is in the range of 1 pm or more and 4 pm or less.5. An emitter substantially as hereinbefore described with reference to the accompanying drawings.