WO2011122687A1

WO2011122687A1 - Liquid metal ion source, method for manufacturing liquid metal ion source, and ion beam emission instrument

Info

Publication number: WO2011122687A1
Application number: PCT/JP2011/058375
Authority: WO
Inventors: Andreas Dirk Wieck; Daisuke Sakai; Toyosei Kawasaki
Original assignee: Ulvac-Phi, Incorporated
Priority date: 2010-03-29
Filing date: 2011-03-25
Publication date: 2011-10-06
Also published as: JP5479974B2; JP2011210489A

Abstract

A liquid metal ion source 10 consists of a needle electrode 11 which is made from two metal wires 11a and 11b arranged in parallel with their outer surfaces being in contact with each other. The needle electrode 11 has a guide passage 11c and an emission end T, both of which are also formed by two metal wires 11a and 11b. The guide passage 11c continuously extends to the emission end T. Towards the bottom of the guide passage 11c, the width of the guide passage 11c decreases to zero. When liquid metal is supplied to the guide passage 11c, capillary action is created in the region close to the bottom of the guide passage 11c, and "volume flow" instead of the usual "surface flow" of the liquid metal occurs. The principle and phenomenon of "volume flow" corresponds with that of a fountain pen. The guide passage 11c continuously extends to the emission end T. Thus, the flow of the liquid metal towards the emission end T is stably continued, and the emission current of the ion beam is stabilized even in case of liquid metal with low wettability. Furthermore, the emission end T is formed to be the hypothetical single emission end. Thus, the controllability of the point and direction of the ion beam is maintained.

Description

DESCRIPTION

LIQUID METAL ION SOURCE, METHOD FOR MANUFACTURING LIQUID METAL ION SOURCE, AND ION BEAM EMISSION INSTRUMENT

TECHNICAL FIELD

The present invention relates to a liquid metal ion source using liquid metal to emit ion beam, a method for manufacturing the liquid metal ion source, and an ion beam emission instrument such as a focused ion beam instrument or a secondary ion mass spectrometry instrument that employs the liquid metal ion source.

BACKGROUND ART As discussed in, for example, document 1 (L. W. S anson,

Appl. Surf. Sci., 76/77, 80(1994)) and document 2 (J. Van de Walle, P. Joyes, Phys . Rev. B, 35, 5509 (1987)), liquid bismuth, which is one of the liquid metals used for a liquid metal ion source, is a liquid metal having a large atomic weight. Furthermore, it has been known for about twenty years that liquid bismuth ion source provides cluster ions such as Bi_n ^p+. Since liquid bismuth ion source provides cluster ions, bismuth ion beam striking an object enhances sputtering yield per current unit. Thus, a focused ion beam instrument or secondary ion mass spectrometry instrument that uses liquid bismuth is allowed to obtain a benefit of the increased secondary ions released from the surface of a sample and, as a result, to shorten the analyzing or

processing time.

Fig. 8 is a front view showing the structure of a conventional liquid metal ion source. As shown in Fig. 8, the liquid metal ion source consists of a needle electrode 101,. which is formed by a straight wire having a conical tip end, and a reservoir 102, which is a spirally wound metal wire for a container of liquid metal. The liquid metal ion source also employs a filament heater 103 to heat the needle electrode 101 and the reservoir 102.

The needle electrode 101 penetrates the reservoir 102 with the tip end of the needle electrode 101 protruding from the lower end of the reservoir 102. The needle electrode 101 and the reservoir 102 are welded to each other at joint A, as shown in Fig. 8, so that the upper side of the reservoir 102 is fixed to the basal portion of the needle electrode 101. The portion of the needle electrode 101 protruding from the lower end of the reservoir 102 is welded to the filament heater 103 at joint B, as shown in Fig. 8. The two ends of the filament heater 103 are respectively welded to

cylindrical terminals 104a and 104b, through which current is supplied to the filament heater 103. The terminals 104a and 104b are brazed with an insulative support plate 105, which is made from, for example, a ceramic such as alumina. Finally, the needle electrode 101, the reservoir 102, the filament heater 103 and the support plate are integrated into an assembly. The liquid metal ion source is installed into an ion beam instrument by fixing the support plate 105 to a proper electrode in the instrument. When sufficient voltages are applied to an extraction electrode and the tip end of the needle electrode 101 in the ion beam instrument, liquid metal flows out of the reservoir 102 on the outer surface of the needle electrode. Hereby, "surface flow" is defined in the present invention as the flow on the outer surface of the needle electrode. The liquid metal reaching the very tip end of the needle electrode 101 is ionized and emitted as ion beam from the tip end of the needle electrode 101.

Here, the liquid metal must flow smoothly from the reservoir 102 to the very tip end of the needle electrode 101 to stabilize the emission current of the ion beam. In case that the liquid metal is liquid gallium or the like, which has high wettability with respect to electrode

materials such as tungsten, the surface flow of the liquid metal occurs and the liquid metal easily flows on the surface of the needle electrode 101. This allows for continuous supply of the liquid metal to the tip end of the needle electrode 101. In contrast, in case that the liquid metal is liquid bismuth, which has low wettability with respect to various needle electrode materials, the surface flow of the liquid bismuth hardly occurs and the liquid bismuth does not flow as easily as gallium or the like on the surface of the needle electrode 101. Thus, the flow of the liquid bismuth on the surface of the needle electrode 101 breaks more easily on the farther portion of the needle electrode 101 from the reservoir 102. Furthermore, the surface temperature of the needle electrode 101 that affects the flow of liquid metal becomes lower on the farther portion of the needle electrode 101 from the filament heater 103. This also breaks the flow of liquid bismuth. The residual gas molecules in the vacuum surrounding the liquid metal ion source oxidize the liquid bismuth on the surface of the needle electrode 101, and bismuth oxides are formed on the surface the needle electrode 101. This phenomenon also breaks the surface flow of the liquid metal. Thus, in case of liquid bismuth for the liquid metal ion source, the flow rate of the liquid metal supplied to the tip end of the needle electrode 101 gradually decreases, and the emission current of the ion beam emitted from the needle electrode 101 becomes less stabilized over time.

The problem that the emission current of the ion beam is not stabilized is not limited to the case that a

conventional liquid metal ion source described above is used for liquid bismuth. This problem may also be common to cases that liquid metal with similar physical property to that of liquid bismuth does not flow smoothly to the tip end of the needle electrode.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a liquid metal ion source that improves the stability of the ion beam emission current, a method for manufacturing the liquid metal ion source, and an ion beam emission instrument employing the liquid metal ion source.. A first embodiment according to the invention is a liquid metal ion source including a needle electrode having a tip end from which liquid metal ions are emitted. The needle electrode is composed of two metal wires arranged in parallel and having outer surfaces that are in contact with each other. The two metal wires form the tip end of the needle electrode with portions of the outer surfaces that are in contact with each other, and the contact portions of the outer surfaces extend continuously to the tip end of the needle electrode so that the two metal wires have a cross- sectional shape including a groove with a bottom formed by the contact portions of the outer surfaces and side walls formed by the outer surfaces. The groove also extends to the tip end of the needle electrode. In the first embodiment, the portions of contact between the outer surfaces of the two metal wires extend to the tip end of the needle electrode so as to form a groove extending to the tip end of the needle electrode. Thus, when the groove, formed by the outer surfaces of the two metal wires, is supplied with liquid metal, capillary action occurs through the groove, in which liquid metal flows toward the tip end of the needle electrode. As a result, the liquid metal supplied to the needle electrode easily flows to the tip end of the needle electrode. This stabilizes the supply of the liquid metal to the tip end of the needle electrode. Furthermore, the amount of ions generated at the very tip end of the needle electrode is stabilized. Thus, the liquid metal ion source that includes such a needle electrode improves the stability of the emission current of the ion beam.

Hereby, "volume flow" is defined in the present

invention as the flow of liquid metal by capillary action in the groove. The phenomenon and principle of "volume flow" corresponds with that of fountain pen.

Furthermore, the tip end of the needle electrode is formed by the contact portions of the outer surfaces of the two metal wires. In addition, the groove, which guides liquid metal to the tip end, is also formed by the contact portions of the outer surfaces of the two metal wires. Thus, the tip end of the needle electrode and the groove, which guides liquid metal to the tip end of the needle electrode, are continuously formed by the outer surfaces of the two metal wires. As a result, the tip end of the needle

electrode and the groove, which guides liquid metal to the tip end, are spontaneously arranged in alignment. In comparison with a needle electrode from a single metal wire with an additional groove, alignment of the tip end of the needle electrode and the groove through which liquid metal flows is ensured. This structure is just similar to that of a fountain pen.

When voltage is applied to the needle electrode for emission of liquid metal ion beam, a conical protrusion, so- called Taylor cone, is formed in the liquid metal on the tip end of the needle electrode. Then, liquid metal ions are emitted from the very tip of the Taylor cone, and the ion beam is directed to the orientation of the Taylor cone.

Thus, as described in the first embodiment mentioned above, as long as alignment of the tip end of the needle electrode and the groove is ensured, the tip of the Taylor cone, which is formed in liquid metal at the tip end of the needle electrode by applying voltage, is easily formed at a

predetermined position. As a result, the position at which the ion beam is emitted and the direction in which the ion beam is emitted is stabilized in predetermined position and direction, and the controllability of the ion beam is improved . A second embodiment according to the invention is that in the liquid metal ion of the first embodiment mentioned above, the two metal wires are intertwined with each other so that the outer surfaces are kept in contact with each other.

To form the groove, through which liquid metal flows, with the contact portions of the outer surfaces of the two metal wires, the outer surfaces of the metal wires must be kept in contact with each other. The state that the outer surfaces of the metal wires are kept in contact with each other can be maintained, for example, by welding at least one point of the contact portions of the two metal wires or by adhering the two metal wires to each other with an adhesive. However, in case, for example, that the metal wires are welded to each other, a metal mass might be formed in the groove when the molten metal created during the welding solidifies. If liquid metal flows into the groove, such a metal mass might block the flow of the liquid metal.

In this respect, as in the second embodiment mentioned above, the two metal wires are intertwined so that the two metal wires are kept in contact with each other. This easily avoids the formation of an obstacle that might block the flow of liquid metal in the groove.

A third embodiment according to the invention is that in the liquid metal ion source of the first or second

embodiment mentioned above, each of the outer surfaces is a circumferential surface.

In the third embodiment, the groove, which is formed by the outer surfaces of the two metal wires, is shaped so that the width of the groove gradually increases as receding from the tangential line formed by the two circumferential surfaces. In other words, the width of the groove decreases to zero towards the bottom of the groove. Generally, the contribution of capillary force acting on liquid increases as the surface area of the liquid

contacting with the capillary becomes large compared with the volume of liquid in the capillary. When the outer surfaces of the metal wires are

circumferential surfaces, as described above, the groove formed by the metal wires has a warped V-shaped cross- section on a perpendicular surface. to the tangential line. If the liquid volume is equal to each other, the contact area in the groove with sharp V-shaped cross-section is larger than that in a groove with rectangular cross-section. Thus, as long as the outer surfaces of the metal wires are circumferential surfaces, capillary action increases the wettability of the liquid metal with respect to the groove, and the volume flow of the liquid metal occurs. This allows for the stable supply of the liquid metal to the tip end of the needle electrode.

A fourth embodiment according to the invention is that in the liquid metal ion source of any one of the first to third embodiments mentioned above, each of the outer

surfaces has a cross-sectional shape of a circle, and the circular cross-sections of the other surfaces have the same diameter .

Electrolytic polishing and mechanical polishing are normally conducted to form a tip end of a needle electrode. Electrolytic polishing is a process that one end of a metal wire is electrochemically dissolved to be a tip by anodic dissolution in an acid or alkaline solution. Mechanical polishing is a process that one end of a metal wire is physically grinded to be a tip with a grind stone. In case of forming a tip end from a single metal wire in a

conventional needle electrode, either process of the two can be performed to form the tip end of the needle electrode. However, in case that performing electrolytic polishing on a needle electrode that consists of two wires, each tip end of each metal wire is conically polished. This increases the distance between the tip ends of the metal wires. As a result, it is technically impossible to form the contact portions of the two metal wires at the tip end of the needle electrode. Thus, mechanical polishing must be selected when forming the hypothetical single tip end of the needle electrode from two metal wires. In the fourth embodiment mentioned above, the outer surfaces of the two metal wires have circular cross-sections with the same diameter. Thus, when performing mechanical polishing, the contact portions of the two metal wires, namely, the portions of the hypothetical single tip end of the needle electrode, can easily be determined, and easily be set to the proper processing position. This facilitates the processing of the tip end through mechanical polishing.

A fifth embodiment according to the invention is that in the liquid metal ion source of any one of the first to fourth embodiments mentioned above, the metal wires are made from an element that is niobium.

Among metal wires, a niobium wire is a wire having superior ductilibility . Thus, the niobium wire is easy to shape, and the shape is easy to maintain. In contrast, a wire formed from tungsten, which is widely used as the material for forming a needle electrode, has high hardness and high elasticity. Thus, when mechanically processing a tungsten metal wire, the load required for processing is greater than when mechanically processing a niobium metal wire. The tungsten wire is difficult to shape, and the shape is difficult to maintain after the processing. In this respect, in the fifth embodiment mentioned above, even when mechanical processing is required to form the needle electrode, the use of niobium wires facilitates the mechanical processing and the shape subsequent to the formation of the needle electrode is easy to maintain.

A sixth embodiment according to the invention is that in the liquid metal ion source of any one of the first to fifth embodiments mentioned above, the tip end of the needle electrode has a semiangle of 5° or greater and 60° or less, and the tip end of the needle electrode has a radius of curvature of 0.1 μπι or greater and 50 μπ\ or less.

As described above, when sufficiently high voltage .is applied to the needle electrode for emission of liquid metal ion beam, a Taylor cone is formed in the liquid metal on the tip end of the needle electrode. Then, ions are emitted from the very tip of the Taylor cone, and the ion beam is

directed to the orientation of the Taylor cone. As the radius of the tip end of the needle electrode or the

semiangle of the tip end of the needle electrode is small, the radius of the basal end of the Taylor cone is small.

Furthermore, as the radius of the basal end of the Taylor cone is small, the position at which the ion beam is emitted and the direction in which the ion beam is emitted is stabilized.

In the sixth embodiment mentioned above, the tip end of the needle electrode has a semiangle of 5° or greater and 60° or less, and the tip end of the needle electrode has a radius of curvature of 0.1 μπι or greater and 50 μπι or less. This radius and semiangle allows for the stabilization of the position and the oriented direction of the Taylor cone. Therefore, the emission current of the ion beam is

stabilized . A seventh embodiment according to the invention is that in the liquid metal ion source of any one of the first to sixth embodiments mentioned above, the liquid metal is bismuth . Bismuth, which is an elementary metal, has low

wettability with respect to tungsten of the like that is used as the material of the needle electrode. In the seventh embodiment mentioned above, liquid bismuth, which has low wettability is used as the liquid metal. Thus, improvement in the stability of the emission current of the ion beam is noticeable .

An eighth embodiment according to the invention is a method for manufacturing a liquid metal ion source including a needle electrode having a tip end from which liquid metal ion beam is emitted. The method includes arranging two metal wires in parallel so that outer surfaces of the metal wires are in contact with each other and so that the metal wires have a groove with a cross-sectional shape of a bottom formed by a contact point of the outer surfaces and side walls formed by the outer surfaces. The groove extends to a tip end of the needle electrode. The method also includes mechanically polishing the outer surfaces of the two metal wires so that the contact point forms the tip end of the needle electrode.

In the eighth embodiment, the portions of contact between the outer surfaces of the two metal wires extends to the tip end of the needle electrode so as to form a groove extending to the tip end of the needle electrode. Thus, when the groove, formed by the outer surfaces of the two metal wires, is supplied with liquid metal, a capillary action occurs in the groove. Then, the volume flow of the liquid metal toward the tip end of the needle electrode occurs. As a result, the liquid metal supplied to the needle electrode easily flows to the tip end of the needle electrode. This stabilizes the supply of the liquid metal at the tip end of the needle electrode. Furthermore, the amount of ions generated at the very tip end of the needle electrode is stabilized. Thus, the liquid metal ion source that includes such a needle electrode improves the stability of the emission current of the ion beam.

electrode and the groove, which guides liquid metal to the tip end, are spontaneously arranged in alignment. This structure is just similar to that of fountain pen. In comparison with a needle electrode from a single metal wire with an additional groove, alignment of the tip end of the needle electrode and the groove, through which liquid metal flows, is ensured.

Electrolytic polishing and mechanical polishing are normally conducted to form a tip end of a needle electrode. Performing electrolytic polishing on a needle electrode that consists of two wires, each tip end of each metal wire is conically polished. This increases the distance between the tip ends of the metal wires. As a result, it is technically impossible to form the contact portions of the two metal wires at the tip end of the needle electrode. In this respect, in the eighth embodiment mentioned above,

mechanical polishing on the outer surfaces of the two wires can form the tip end in the contact portions of the two metal wires.

A ninth embodiment according to the invention is an ion beam emission instrument employing the liquid metal ion source according to any one of the first to seventh

embodiments mentioned above, and an extraction electrode facing toward the tip end of the needle electrode.

In the above-described configuration, the liquid metal supplied to the liquid metal ion source easily flows to the tip end of the needle electrode. This stabilizes the supply of liquid metal to the tip end of the needle electrode.

Thus, in an ion beam emission instrument employing such a liquid metal ion source, the amount of ions generated at the very tip end of the needle electrode is stabilized. Thus, this improves stability of the ion beam emission current.

Other embodiments and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing an ion beam emission instrument according to one embodiment of the present invention together with the front structure of a liquid metal ion source;

Fig. 2(a) is a front view showing a needle electrode of the liquid metal ion source, Fig. 2(b) is a cross-sectional view of the needle electrode taken along line 2A-2A in Fig. 2(a), Fig. 2(c) is a cross-sectional view of the needle electrode taken along line 2B-2B in Fig. 2(a), Fig. 2(d) is a bottom view showing the needle electrode from the tip end side of the needle electrode, and Fig. 2(e) is a cross- sectional view equivalent to Fig. 2(b), showing wetting menisci of liquid metal after wetting liquid metal ion source with the liquid metal;

Fig. 3 is an enlarged view showing the tip end of the needle electrode;

Figs. 4(a) and 4(b) are schematic diagrams showing processes for forming the tip end of the needle electrode;

Fig. 5(a) is a graph showing the relationship of the ion beam emission time in the ion beam emission instrument and the sample current in ah example, and Fig. 5(b) is a graph showing the ion beam emission time in the ion beam emission instrument and the sample current as a comparative example;

Fig. 6(a) is a front view showing a needle electrode of a liquid metal ion source according to a further embodiment of the present invention, Fig. 6(b) is a bottom view showing the needle electrode from the tip end side of the needle electrode ;

Fig. 7(a) is a front view showing a needle electrode of a liquid metal ion source according to another embodiment of the present invention, Fig. 7(b) is a bottom view showing the needle electrode from the tip end side of the needle electrode; and

Fig. 8 is a front view showing a conventional liquid metal ion source.

DESCRIPTION OF EMBODIMENTS

A liquid metal ion source, a method for manufacturing a liquid metal ion source, and an ion beam emission instrument according to one embodiment of the present invention will now be discussed with reference to Figs. 1 to 5.

Fig. 1 shows an ion beam emission instrument according to the present embodiment together with the front structure of a liquid metal ion source. As shown in Fig. 1, the ion beam emission instrument employs a liquid metal ion source 10, where liquid metal ions are produced, such as liquid bismuth or liquid gallium, and an extraction electrode 21, which extracts ions from the liquid metal ion source 10. The ion beam emission instrument employs a suppressor electrode 22 located between the liquid metal ion source 10 and the extraction electrode 21. The suppressor electrode 22

suppresses generation of thermal electron from filament heater 13 and stabilizes the emission current of the ions extracted from the liquid metal ion source 10.

The liquid metal ion source 10 includes a needle

electrode 11, which penetrates a reservoir 12, a spirally wound metal wire for a container of liquid metal, and the tapered tip-end portion D (bottom portion as viewed in Fig. ^" 1) protrudes from the lower end of the reservoir 12 toward the extraction electrode 21. The reservoir 12 has a top portion bent toward a basal portion (top portion as viewed in Fig. 1) of the needle electrode 11 and welded to the basal portion of the needle electrode 11 at joint A. The portion of the needle electrode 11 protruding from the reservoir 12 toward the extraction electrode 21 is welded at joint B to a filament heater 13, which is heated to heat the needle electrode 11 and the liquid metal on the needle electrode 11 and in the reservoir 12. The needle electrode 11, the reservoir 12, and the filament heater 13 are

mechanically and electrically connected to one another at joints A and B.

The two ends of the filament heater 13 are respectively welded to terminals 14a and 14b, and the filament heater 13 is connected via terminals 14a and 14b to a filament power supply 31. Terminals 14a and 14b are brazed to an insulative support plate 15, which is made from, for example, a ceramic such as alumina. When the filament power supply 31 applies current to the filament heater 13, the filament heater 13 is heated, the needle electrode 11 and the reservoir 12 are also heated, and the liquid metal contained on the needle electrode 11 and in the reservoir 12 is finally heated.

An extraction electrode 21 with a through hole 21a, which is coaxial to the tip end of the needle electrode 11, is located beneath the needle electrode 11. The extraction electrode 21 is negatively biased to the needle electrode 11 by an extraction power supply 33. A suppressor electrode 22 with a through hole 22a, which the needle electrode 11 penetrates, is located beneath the filament heater 13. The suppressor electrode 22 is negatively biased to the needle electrode 11 by a suppressor power supply 32. A beam

acceleration power supply 34 is connected between the needle electrode 11 and ground potential. The beam acceleration power supply 34 applies positive acceleration voltage to the needle electrode 11.

In the ion beam emission instrument, the filament power supply 31 first applies current to the filament heater 13. The filament heater 13 is heated, and the liquid metal is kept to be heated to the melting point temperature or over. Then, the extraction power supply 33 applies voltage to the extraction electrode 21, and an electric field generates in the region between the extraction electrode 21 and the needle electrode 11. A series of these processes generates the liquid metal ions on the very tip end, emission end T, of the needle electrode 11. Then, ion beam IB is emitted from the emission end T of the needle electrode 11 toward the extraction electrode 21. The suppressor electrode 22, biased by the suppressor power supply 22, suppresses generation of thermal electrons at the filament heater 13, and functions adjustment of emission current between the tip end of the needle electrode 11 and the extraction electrode 21. The beam acceleration power supply 34 applies

acceleration voltage to the needle electrode 11. This provides the ion beam just emitted from the emission end T with energy potential and allows for adjustment of the ion beam energy when the ions reach a sample. Figs. 2(a) to 2(e) are enlarged views showing two metal wires 11a and lib, which form the needle electrode 11. Fig. 2(a) is a front view showing the metal wires 11a and lib. Fig. 2(b) is a cross-sectional view of the metal wires 11a and lib taken along line 2A-2A in Fig. 2(a) . Fig. 2(c) is a cross-sectional view of the metal wires 11a and lib taken along line 2B-2B in Fig. 2(a). Fig. 2(d) is a bottom view showing the needle electrode 11 from the emission end T. and Fig. 2(e) is a cross-sectional view equivalent to Fig. 2(b), showing wetting menisci of liquid metal after wetting liquid metal ion source with the liquid metal

As shown in Fig. 2(a), the needle electrode 11 is formed by intertwining the two cylindrical metal wires 11a and lib, which are arranged in parallel, with their outer surfaces SI and S2 in contact with each other. The lower end surfaces Sa and Sb of the two metals wires 11a and lib respectively are outwardly bulging curved surfaces and form part of a

hypothetical conical surface VC, the apex of which is the emission end T of the needle electrode 11. The tip end portion D of the needle electrode 11 is part of a

hypothetical cone, the apex of which is also the emission end T of the needle electrode 11. The outer shape of the tip end portion D is formed by the two end surfaces Sa and Sb and part of the outer surfaces SI and S2 of the two wires 11a and lib respectively.

As shown in Fig. 2(b), the outer surfaces SI and S2 of the two metal wires 11a and lib are circumferential surfaces having the same diameter. In a cross-sectional surface taken along line 2A-2A in Fig. 2(a), the two outer surfaces SI and S2 are in contact with each other at a single contact point CP. As shown in Fig. 2(c), the outer surfaces SI and S2 of the two metal wires 11a and lib are in contact with each other along a tangential line TL, which extends from the basal end of the needle electrode 11 to the emission end T, and are intertwined so that the contact point CP lies in the tangential line TL. As shown in Fig. 2(d), the emission end T of the needle electrode 11 is the portion at which the outer surfaces SI and S2 of the two metal wires 11a and lib are in contact with each other and at which the two end surface Sa and Sb come into contact with each other. In this manner, the two metal wires 11a and lib are intertwined with each other so that the portion at which the outer surfaces SI and S2 is in contact with each other, namely, the

tangential line TL, includes the emission end T.

The two intertwined metal wires 11a and lib form two grooves (guide passages 11c) on opposite sides to each other with respect to the tangential line TL so that the bottom of each groove lies in the tangential line TL. Each guide passage 11c extends within the entire range in which the tangential line TL is formed, that is, from the emission end T of the needle electrode 11 to the basal end of the needle electrode 11. The side walls in the groove of each guide passage 11c are formed by portions of the outer surfaces SI and^'S2 facing with each other on opposite sides of the single tangential line TL. The cross-section of each guide passage 11c taken in a perpendicular direction to the tangential line TL has warped V-shape so that the width of the groove gradually increases as receding from the

tangential line TL . The two metal wires 11a and lib extend spirally along the tangential line TL. Thus, each guide passage 11c also extends spirally along the tangential line TL. In this manner in the present embodiment, the guide passages 11c extends from the basal end to the emission end T of the needle electrode 11. As shown in Fig. 2(e), when liquid metal is supplied to each guide passage 11c, part of the liquid metal comes into contact with the outer surfaces SI and S2 that form the side walls of the guide passage 11c, while most of the liquid metal flows through the guide passage 11c without contacting with the outer surfaces SI and S2. Furthermore, capillary action occurs in the guide passage 11c, and the volume flow of the liquid metal occurs. As a result, the liquid metal flow toward the emission end T of the needle electrode 11 does not break on the way, and the supply of the liquid metal to the emission end T of the needle electrode 11 is stabilized. Thus, the amount of ions generated at the emission end T of the needle electrode 11 is stabilized, and the stability of the emission current of the ion beam IB is improved in the liquid metal ion source 10 that includes the needle electrode 11. These advantages become further noticeable in liquid metals like liquid bismuth having a lower wettability with respect to metal wire .

As described above, the emission end T of the needle electrode 11 is formed by contact portions of the outer surfaces SI and S2 of the metal wires 11a and lib, and the guide passages 11c, which guide liquid metal, is also formed by contact portions of the outer surfaces SI and S2 of the metal wires 11a and lib. In other words, the emission end T of the needle electrode 11 and the guide passage 11c, which guide liquid metal to the emission end T, are continuously formed by the outer surfaces SI and S2 of the two metal wires 11a and lib. Therefore, the emission end T of the needle electrode 11 and the guide passage 11c, which guide liquid metal to the tip end, are spontaneously arranged in alignment. In comparison with a needle electrode from a single metal wire with an additional groove, the guide passages 11c leading to the emission end T of the needle electrode 11 are formed with higher positional accuracy.

To form the guide passages 11c with the contact portions of the outer surfaces SI and S2 of the two metal wires 11a and lib, the outer surfaces SI and S2 must be kept in contact with each other. The state that the outer surfaces SI and S2 of the metal wires 11a and lib are kept in contact with each other can be maintained, for example, by welding at least one point of the contact portions of the metal wires 11a and lib. However, when the metal wires 11a and lib are welded to each other, a metal mass might be formed in the guide passage 11c when the molten metal created during welding solidifies. When liquid metal flows into the guide passage 11c, such a metal mass might block the flow of the liquid metal.

In this respect, the needle electrode 11, which is formed by intertwining the two metal wires 11a and lib so that the two metal wires 11a and lib are kept in contact with each other, can easily avoid the formation of an obstacle that might . block the flow of liquid metal in the guide passage 11c.

The outer surfaces SI and S2 of the metal wires 11a and lib are circumferential surfaces having the same diameter. Thus, the guide passages 11c, which are formed by the metal wires 11a and lib, has warped V-shape cross-section in a perpendicular plane to the tangential line TL. If the liquid volume is equal to each other, the contact area of liquid metal in the guide passages 11c with sharp V-shaped cross- section is larger than that in a groove with rectangular cross-section. Thus, as in the present embodiment, as long as the outer surfaces SI and S2 of the metal wires 11a and lib are circumferential surfaces having the same diameter, capillary action increases the wettability of the liquid metal with respect to the guide passages 11c of the needle electrode 11, then the volume flow occurs. This allows for the stable supply of the liquid metal to the emission end T of the needle electrode 11.

When sufficiently high voltage is applied to the needle electrode 11 for emission of the liquid metal ion beam, a Taylor cone is formed in the liquid metal on the tip end of the needle electrode. Then, ions are emitted from the very tip of the Taylor cone, and the ion beam is directed to the orientation of the Taylor cone. As the radius of curvature of the emission end T of the needle electrode 11 or the tip end semiangle Θ of the needle electrode 11 is small, the radius of the basal end of the Taylor cone is small.

Referring to Fig. 3, for the tip end portion D of the needle electrode 11, the tip end semiangle Θ, which is formed by the axis CA extending through the emission end T of the needle electrode 11 and the end surfaces Sa and Sb of the two metal wires 11a and lib, is preferably 5° or greater and 60° or less. The tip end semiangle Θ from 15° to 45° is highly desirable. The radius of curvature r for the emission end T of the needle electrode 11 is preferably 0.1 μπι or greater and 50 μιτι or less. The radius of curvature r from 0.1 μιη to 10 μιη is highly desirable. When the semiangle Θ and the radius of curvature r are in the above range, the position and the oriented direction of the Taylor cone is stabilized. Therefore, the emission current of the ion beam is stabilized.

The needle electrode 11, the reservoir 12, and the filament heater 13 can be made from metal wires such as tungsten (W) , tantalum (Ta) , niobium (Nb) , and nickel (Ni). Among the wires from these elements, a niobium wire is a material having superior ductilibility . Thus, the niobium metal wire is easy to shape, and the shape is easy to maintain. In contrast, tungsten, which is widely used as the material for forming the needle electrode 11, has high hardness and high elasticity. Thus, when mechanically processing a tungsten metal wire, the load required for processing is greater than when mechanically processing a niobium metal wire. The tungsten wire is difficult to shape, and the shape is difficult to maintain after processing.

Thus, by selecting niobium as the material for making the needle electrode 11, the mechanical processing is

facilitated and the shape after the processing is easy to maintain.

A method for manufacturing a liquid metal ion source, particularly, a method for processing the tip end of the needle electrode 11 of the liquid metal ion source 10 will now be discussed with reference to Fig. 4.

Electrolytic polishing and mechanical polishing are normally conducted to form a tip end of a needle electrode from a single metal wire. Electrolytic polishing is a process that one end of a metal wire is electrochemically dissolved to be a tip by anodic dissolution in an acid or alkaline solution. Mechanical polishing is a process that one end of a metal wire is physically grinded to be a tip with a grind stone. In case of forming a tip end from a single metal wire in a conventional needle electrode, either process of the two can be performed to form the tip end of the needle electrode. However, in case that performing electrolytic polishing on a needle electrode that consists of two wires, each tip end of each metal wire is conically polished. This forms a total of two conical ends.

Therefore, it is technically impossible to form a

hypothetical single emission end T at the contact portions of the outer surfaces SI and S2 of the two metal wires 11a and lib. Thus, to form the emission end T of the needle electrode 11 from the contact portions of the two metal wires 11a and lib, mechanical polishing must be selected. Figs. 4(a) and 4(b) schematically show processes performed during mechanical polishing so that the contact portions of the two metal wires 11a and lib form the

emission end T of the needle electrode 11. First, prior to processing the tip end of the needle electrode 11 by mechanical polishing, the two metal wires 11a, intertwined together, are inserted into the reservoir 12, and the reservoir 12 is welded to the metal wires 11a and lib at joint A. The metal wires 11a and lib are welded to the filament heater 13 at joint B. The two ends of the filament heater 13 are welded to the terminals 14a and 14b, respectively. Finally, mechanical polishing is performed to process the liquid metal ion source 10 without the emission end T.

As shown in Fig. 4(a), the support plate 15 of the liquid metal ion source 10 is rotated around the axis CI, which includes the tangential line TL of the two metal wires 11a and lib. A grinder 41, which is for polishing the two metal wires 11a and lib, is arranged so that the angle between the axis CI and a normal (axis C2) extending through the rotational center of a grinding surface is in

correspondence with 90°-θ as the semi-angle of the tip end is Θ. Fig. 4(b) shows the front of the polishing surface of the grinder 41. As shown in Fig. 4(b), the polished portions of the metal wires 11a and lib in the liquid metal ion source 10 are arranged to be in contact with an off-center position on the polishing surface of the grinder 41. Then, the liquid metal ion source 10 is rotated slowly and the grinder 41 is rotated fast when polishing the metal wires 11a and lib. Thus, this grinding of the two metal wires 11a and lib described above forms a hypothetical conical surface VC with the apex semi-angle Θ, and the ground end surfaces Sa and Sb of the two metal wires 11a and lib also form part of a conical surface VC . The emission end T of the needle electrode 11 is located at the apex of the conical surface VC, and is simultaneously located at an end of the guide passage 11c just on the tangential line TL. The arrangement of each equipment shown in Fig. 4(b) and the rotation speed setting that the grinder 41 rotates much faster than the liquid metal ion source 10 does, results in small polishing scratches, formed on the ground end surfaces Sa and Sb during polishing, being directed toward the tip end of the needle electrode 11. This minimizes the possibility that the polishing scratches block the flow of the liquid metal ion source. In the present embodiment, the outer surfaces SI and S2 of the two metal wires 11a and lib are circumferential surfaces having the same diameter. This allows for easily determining the axis CI including the contact portions of the two metal wires 11a and lib and easily setting the axis CI to the proper processing position. Thus, the processing of the emission end T through mechanical polishing is facilitated . [Examples ]

As an example, the liquid metal ion source 10 with the needle electrode 11, formed by intertwining the two metal wires 11a and lib to supply liquid metal to the tip end by volume flow, was produced. As a comparative example, a conventional liquid metal ion source with a needle electrode from a single metal wire to supply liquid metal to the tip end by surface flow, was produced. Niobium was used as the material of the metal wires, and niobium wires were also used for the reservoir 12 and the filament heater 13. Each liquid metal ion source was installed in an ion beam

emission instrument. Liquid bismuth was the liquid metal that serves as ion source. When the ion beam was emitted by applying predetermined voltage to the extraction electrode and the stabilized emission of the ion beam was obtained, the emission current, derived from primary ions generated at the tip end of the needle electrode, and the sample current, derived from the primary ions reaching the sample, were measured.

[Example 1]

Emission Current 2 μΑ

Extraction Voltage 6.5 kV

Heater Current 2.5 A

When an extraction voltage of 6.5 kV was applied to the extraction electrode, emission current of 2μΑ was stably obtained. As shown in Fig. 5(a), the sample current under the emission condition was maintained at approximately 13 nA for 4000 seconds from the measurement start. This is attributed to the stable supply of liquid bismuth to the emission end T of the needle electrode 11 through the guide passage 11c formed in the needle electrode. As a result, the amount of ionized bismuth per time unit was stabilized, and the emission current, which is determined by the ionized amount of bismuth, was stabilized.

[Comparative Example 1]

Emission Current 2 to 3 μΑ

Extraction Voltage 5.2 kV

Heater Current 2.4 A

When an extraction voltage of 5.2 kV was applied to the extraction electrode, the ion beam emission was obtained. Even if the several hours was spent for warming up to stabilize the ion beam emission, the ion beam emission was not stabilized and decrease of the emission current was still observed. Emission current of 3 μΑ was obtained when starting the measurement. However, the emission current decreased to 2 μΑ after 3600 seconds from the measurement started. In this measurement, the sample current,

simultaneously measured, decreased from 25 nA at the

measurement start to about 12 nA after 4000 seconds, as shown in Fig. 5(b) .

The decrease of the ion beam emission is attributed to the decreased supply of the liquid bismuth to the tip end of the needle electrode, because bismuth oxides, which are formed by the residual gases in the vacuum surrounding the liquid metal ion source and have a higher melting point than bismuth metal, blocks the surface flow of the liquid metal on the outer surfaces of the needle electrode. Furthermore, the jitter of the sample current during the measurement was large. This is also attributed to the instability of the liquid bismuth supply to the tip end of the needle electrode per unit time. In this manner, bismuth metal ion beam even with a conventional liquid metal ion source can be obtained.

However, the emission current decreases and destabilizes over time, because the supply of liquid bismuth to the emission end of the needle electrode is suppressed. Finally, the amount of liquid bismuth supplied to the emission end of the needle electrode falls below the minimum amount for maintaining the ion beam emission, and the ion beam emission stops . In contrast, as described above, in the liquid metal ion source of the example, capillary action occurs in the guide passage 11c formed by the two metal wires 11a and lib. The liquid bismuth that does not come into contact with the walls of the guide passage 11c or the outer vacuum

surrounding the liquid metal ion source 10 is little

oxidized, and the fluidity of the liquid bismuth is hardly affected. These advantages of the volume flow stabilize the amount of liquid bismuth supplied to the emission end T of the needle electrode 11. Thus, the emission current in the ion beam emission instrument and, consequently, the sample current of the primary ions are stabilized.

The liquid metal ion source, the method for

manufacturing a liquid ion source, and the ion beam emission instrument according to the present embodiment has the advantages described below.

(1) The contact portion between the outer surfaces SI and S2 of the two metal wires 11a and lib extends continuously to the emission end T of the needle electrode 11 so as to form the guide passages 11c, which extend to the emission end T of the needle electrode 11. Thus, when the guide passages 11c are supplied with liquid metal, some of the liquid metal comes into contact with the outer surfaces SI and S2 that form the side walls of the guide passages 11c, while most of the liquid metal flows through the guide passages 11c without contacting the outer surfaces SI and S2. Furthermore, capillary action occurs in the guide passages 11c, and the volume flow of the liquid metal toward the emission end T of the needle electrode 11 occurs. Then, the liquid metal supplied to the needle electrode 11 easily flows to the emission end T of the needle electrode 11, and as a result, the supply of the liquid metal to the emission end T of the needle electrode 11 is stabilized. Thus, the amount of ions generated at the emission end T of the needle electrode 11 is stabilized, and the stability of the

emission current of the ion beam IB is improved in the liquid. metal ion source 10 that includes the needle

electrode 11.

(2) The tip end of the needle electrode 11 is formed by the contact portions of the outer surfaces SI and S2 of the two metal wires 11a and lib. Furthermore, the guide passages 11c, which guide liquid metal to the tip end, is also formed by the contact portions of the outer surfaces SI and S2 of the two metal wires 11a and lib. Thus, the emission end T of the needle electrode 11 and the guide passages 11c, which guide liquid metal to the emission end T, are continuously formed by the outer surfaces SI and S2 of the two metal wires 11a and lib. Therefore, the emission end T of the needle electrode 11 and the guide passages 11c, which guide liquid metal to the emission end T, are spontaneously arranged in alignment. This structure is quite similar to that of a fountain pen. In comparison with a needle

electrode from a single metal wire with an additional groove, the guide passages 11c leading to the emission end T of the needle electrode 11 are formed with higher accuracy.

(3) The needle electrode 11 is formed by intertwining the two metal wires 11a and lib so that the two metal wires 11a and lib are kept in contact with each other. This structure easily avoids the formation of an obstacle that might block the flow of liquid metal in the guide passage 11c, such as a metal mass formed during welding. (4) The guide passages 11c, which are formed by the outer surfaces SI and S2 of the two metal wires 11a and lib, is shaped so that the width of the guide passage 11c

gradually increases as receding from the tangential line TL. In other words, the width of the groove decreases to zero towards the contact portions of the outer surfaces SI and S2 of the two metal wires 11a and lib. Thus, the ratio of the area, keeping in contact with the side walls of the guide passage 11c, to the volume of the liquid metal is large in the region close to the bottom of the guide passage 11c, and capillary action, occurring in the region of the bottom of the guide passage 11c, increase the wettability of the liquid metal with respect to the guide passages 11c of the needle electrode 11. Therefore, the volume flow of the liquid metal occurs. This allows for the stable supply of the liquid metal to the emission end T of the needle

electrode 11.

The outer surfaces SI and S2 of the two metal wires 11a and lib are circumferential surfaces having the same diameter. This allows for easily determining the axis CI including the emission end T of the needle electrode 11 and easily setting the axis CI to the proper processing

position. Thus, the processing of the tip end by mechanical polishing is facilitated.

(6) Niobium is used for the metal wires 11a and lib. Thus, when forming the needle electrode 11, niobium

facilitates the mechanical processing and easily maintains the shape after processing.

(7) The tip end semiangle Θ of the emission end T of the needle electrode 11 is preferably set to 5° or greater and 60° or less, and the tip end semiangle Θ from 15° to 45° is highly desirable. The radius of curvature for the tip end of the needle electrode 11 is 0.1 μιτι or greater and 50 μκι or less, and the radius of curvature r from 0.1 μιιι to 10 μπι is highly desirable. When the semiangle Θ and the radius of curvature r are in the above range, the position and the oriented direction of the Taylor cone is stabilized.

Therefore, the emission current of the ion beam is

stabilized.

(8) Liquid bismuth, which has low wettability with respect to various metals, can be used for liquid metal ion source. The emission current of the ion beam IB from the bismuth liquid metal ion source can be noticeably

stabilized.

It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the scope of the invention. Particularly, it should be understood that the present invention may be embodied in the following forms.

The suppressor electrode 22 and the suppressor power supply 32, which applies voltage to the suppressor electrode 22, can be eliminated from the ion beam emission instrument.

The needle electrode 11 does not have to be formed by the two metal wires 11a and lib. As shown in Fig. 6, a needle electrode 51 can be formed by three of more metal wires, for example, three metal wires 51a, 51b, and 51c. In this case, as shown in Fig. 6(a), the metal wire 51c is wound around the two metal wires 51a and 51b, which are shaped identically to the metal wires 11a and lib shown in Fig. 2. Furthermore, as shown in the bottom view of Fig.

6(b), the emission end T of the needle electrode 51 is the contact point CP of the metal wire 51a and 51b. In

comparison to forming a needle electrode with two metal wires, the increase in the quantity of the metal wires 51a, 51b, and 51c forms more grooves (guide passages) with the contacting metal wires in such a structure of needle

electrode .

The number of metal wires wound around the tangential line TL is not limited to two and can be one or three or more .

The reservoir 12 is discrete from the needle electrode 11, and the needle electrode 11 is inserted into the

reservoir 12. However, the present invention is not limited in such a manner, and a reservoir can be formed integrally with a needle electrode. In this case, one end of a single metal wire can be used to form the spiral reservoir, and the other end of the metal wire may be used to form the needle electrode that protrudes from the reservoir.

The reservoir 12 does not have to be discrete from the filament heater 13. A reservoir can be formed integrally with a filament heater. In this case, two ends of a single metal wire can be used as a filament heater connected to a filament power supply, and the remaining part of the metal wire can be used, for example, as a spiral reservoir.

The radius of curvature r for the emission end T of the needle electrode 11 does not have to be 0.1 m to 10 μπι. The emission end T may have any radius of curvature as long as it is 0.1 μπι or greater and 50 μιη or less.

The tip end semiangle Θ of the emission end T of the needle electrode 11 does not have to be 15° to 45° as long as it is 5° or greater and 60° or less. Welding is performed to couple the needle electrode 11 to the reservoir 12 and the needle electrode 11 to the filament heater 13. However, the present is not limited in such a manner and, for example, an adhesive agent can be used to fix the reservoir 12 to the needle electrode 11 and the needle electrode 11 to the filament heater 13.

The needle electrode 11, the reservoir 12, and the filament heater 13 do not have to be made from the same material. For example, the needle electrode 11, the

reservoir 12, and the filament heater 13 can be made from different conductive material from one another.

The outer surfaces SI and S2 of the two metal wires 11a and lib do not have to have the same diameter and can have different diameters instead as shown in Fig. 7(a). More specifically, a needle electrode 61 can be formed winding a metal wire 61b, which has a relatively small diameter, around a metal wire 61a, which has a relatively large diameter. In this case, as shown in the bottom view of Fig. 7 (b) , in comparison with a needle electrode formed by two metal wires having the same diameter, the difference in the diameters between the metal wires 61a and 61b results in the emission end T of the needle electrode 61 being off-center toward the metal wire 61b. The numbers of the turn of the metal wire around the metal wire 61a can be arbitrary.

The outer surfaces SI and S2 of the metal wires 11a and lib do not have to be circumferential surfaces and can be of any shape. For example, the outer surfaces can be formed so as to have polygonal with three or more sides in a cross- sectional plane perpendicular to the direction the metal wire extends toward.

The two metal wires 11a and lib are intertwined so that they are kept in contact with each other. However, the present invention is not limited to this manner. The metal wires 11a and lib can be welded to each other so that they are kept in contact with each other.

The tangential line TL extends throughout the entire needle electrode 11. However, the tangential line TL can extend only through part of the needle electrode 11 that includes the emission end T of the needle electrode 11.

The aforementioned examples and embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims .

Claims

1. A liquid metal ion source (10) including a needle electrode (11) having a tip end (T) from which liquid metal ions are emitted, the liquid metal ion source being

characterized in that:

the needle electrode (11) includes two metal wires (11a, lib) arranged in parallel and having outer surfaces (SI, S2) that are in contact with each other; and

the two metal wires (11a, lib) form the tip end (T) of the needle electrode (11) with portions (CP) of the outer surfaces (SI, S2) that are in contact with each other, and the contact portions (CP) of the outer surfaces of the two metal wires extends continuously to the tip end (T) of the needle electrode so that the two metal wires (11a, lib) have a cross-sectional shape including a groove (11c) with a bottom formed by the contact portions (CP) of the outer surfaces (SI, S2) and side walls formed by the outer

surfaces (SI, S2), and the groove (11c) extends to the tip end (T) of the needle electrode (11) .

2. The liquid metal ion source (10) according to claim 1, characterized in that the two metal wires (11a, lib) are intertwined so that the outer surfaces (SI, S2) are in contact with each other.

3. The liquid metal ion source (10) according to claim 1 or 2, characterized in that each of the outer surfaces (SI, S2) is a circumferential surface.

4. The liquid metal ion source (10) according to any one of claims 1 to 3, characterized in that each of the outer surfaces (SI, S2) has a cross-sectional shape of a circle, and the circular cross-sections of the other

surfaces (SI, S2) have the same diameter.

5. The liquid metal ion source (10) according to any one of claims 1 to 4, characterized in that the metal wires (11a, lib) are made from an element that is niobium.

6. The liquid metal ion source (10) according to any one of claims 1 to 5, characterized in that the tip end (T) of the needle electrode (11) has a semiangle (Θ) of 5° or greater and 60° or less; and the tip end (T) of the needle electrode (11) has a radius (r) of 0.1 pm or greater and 50 μιη or less.

7. The liquid metal ion source (10) according to any one of claims 1 to 6, characterized in that the liquid metal is bismuth.

8. A method for manufacturing a liquid metal ion source (10) including a needle electrode (11) having a tip end (T) from which liquid metal ions are emitted, the method being characterized by:

arranging two metal wires (11a, lib) in parallel so that outer surfaces (SI, S2) of the metal wires are in contact with each other and so that the metal wires (11a, lib) have a cross-sectional shape including a groove (11c) with a bottom formed by the contact portions (CP) of the outer surfaces (SI, S2) and side walls formed by the outer

surfaces (SI, S2), in which the groove (11c) extends

continuously to the tip end (T) of the needle electrode (11) ; and

mechanically polishing the outer surfaces (SI, S2) of the two metal wires (11a, lib) so that a contact point (CP) between the outer surfaces (SI, S2) forms the tip end (T) of the needle electrode (11).

9. An ion beam emission instrument being characterized by:

the liquid metal ion source (10) according to any one of claims 1 to 7; and

an extraction electrode (21) facing toward the tip end (T) of the needle electrode (11).