JP3250724B2 - Manufacturing method of needle electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam emitting cathode used for an electron microscope, a length measuring machine, an electron beam exposure machine, an electron beam tester, etc., a mask repair, an ion implanter, a semiconductor device cross-sectional analyzer, The present invention relates to a method of manufacturing a needle-shaped electrode applied to an ion source used for a focused ion beam (FIB) source such as a thin film forming apparatus for a transmission electron microscope.

[0002]

2. Description of the Related Art In order to increase the accuracy and efficiency of electron beam-based devices such as electron microscopes, higher-luminance electron beams are required. Various electron emission cathodes such as a Schottky emission cathode are being studied.

[0003] For example, a cold field cathode having a sharp electron emission surface obtained by cutting a thin line of a single crystal of tungsten by electropolishing as a high-brightness electron source is widely used as an electron source for a high-resolution electron microscope. In recent years, a ZrO / W thermal field emission cathode in which the work function of the (100) plane is reduced from about 4.5 eV to about 2.8 eV by coating a surface of a tungsten single crystal chip with a ZrO layer has been attracting attention.

The emission current density of any of the electron emission cathodes is determined by Fowler-Nordheim equation or Scho
As can be seen from the equation of ttky, it is determined by the work function and the electric field intensity distribution at the tip of the cathode. The electric field intensity distribution strongly depends on the voltage applied between the cathode and the anode, and particularly on the shape of the tip of the cathode, and is one of the important factors governing the characteristics of the electron emission cathode.

Regarding the tip shape of the electron emission cathode, for example, in the above-mentioned ZrO / W thermal field emission cathode, when the radius of curvature is increased to 1 μm or more, the energy width of the emission current is reduced or the stability is reduced. Is known to be improved (J. Vac. Sci. Techno).
l. , 16 (6) 1979, 1704-1708;
Applied Physics. , 46, 5 (197
5) 2029-2050).

Japanese Patent Application Laid-Open No. 7-105834 discloses that
In order to obtain a thermal field emission cathode having a low energy width and stable emission, a method of discharging a conventionally known thermal field emission cathode is disclosed. Further, although a conventional method of heat-treating the field emission cathode to about 2800 K has been tried, it also discloses that the half angle of the cone becomes as large as 40 degrees or more, which is not practical. Furthermore, it also suggests that a thermal field emission cathode having a large radius of curvature and a small tip shape having a half cone angle cannot be dealt with only by the conventional electropolishing method.

In fact, in the conventional electropolishing method, as the radius of curvature increases, the half angle of the cone also increases. Even when the radius of curvature is 0.6 μm or less, the half angle of the cone is controlled to be 5 degrees lower. Was 0.6 to 2.0 μm, it was difficult to control the half cone angle to 10 degrees or less.

The present inventors have disclosed in Japanese Patent Application Laid-Open No. 8-36981 that a large radius of curvature of 1.2 to 10 μm and a thermal field emission cathode having a total cone angle of 25 ° or less have an energy width of 0.5 eV or less. , Angular current density is 0.02 mA
Discloses that an electron beam of at least / sr can be stably obtained with a variation rate of 5% or less, and that the thermal field emission cathode can be obtained by combining a dry etching method and the like with a conventional electrolytic polishing method. are doing.

However, the method disclosed in the above-mentioned known document employs a process with low productivity such as electric discharge or dry etching, and requires an expensive dedicated apparatus. In particular, there is a problem that an electron emission cathode having a tip shape having a tip curvature radius of 0.6 μm or more and a conical half angle of 10 degrees or less cannot be obtained.

On the other hand, FIB is also used in various semiconductor inspection devices and semiconductor processing devices, and has been receiving attention in recent years. In particular, a liquid metal ion source using gallium as an ion species has been widely used as an ion source for FIB. I have.

The liquid metal ion source wets a needle electrode of a high melting point metal with a liquid metal, and applies a strong electric field to a sharp end of the needle electrode to ionize the liquid metal. At the sharp end of the needle-like electrode, a conical protrusion of liquid metal called a Taylor cone is formed by a strong electric field, and ions are emitted from this tip. It is said that the cone angle of the Taylor cone is about 97 degrees in full angle, and it is known that it is important to form the cone angle of the sharp end of the needle electrode in accordance with the cone angle of the Taylor cone. I have.

For this reason, the sharp end is generally formed by an electrolytic polishing method as in the case of the above-mentioned needle electrode of the electron emission cathode. Is also used. However, the machining method using mechanical polishing requires a special jig due to the processing of minute parts, and as a result, there is a problem that the cost is high. In the electrolytic polishing method, the cone angle of the tailor cone is easily increased. There is a problem that a needle electrode having a cone angle close to the above cannot be stably obtained.

That is, as a method of processing a needle-shaped electrode by electric field polishing, for example, “Solid State Physics” Vol. 2 No. 2 (196)
6) Although disclosed on pages 33 to 38, the conventional method has a strong correlation between the half cone angle and the tip radius and cannot be controlled independently. Particularly, when the tip radius is large, the half cone angle is reduced. There is a problem that is difficult.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been experimentally studied by the present inventors to form a narrow portion in advance in a fine wire of a high melting point metal by an electropolishing method. When the method of cutting from the constricted portion is adopted, a needle-like electrode having a desired tip shape can be obtained stably and therefore at a low cost, and the needle-like electrode is used as an electron emission cathode or an ion source. The present inventors have found that they are suitable for use, and have arrived at the present invention.

An object of the present invention is to provide a needle-shaped electrode having a desired tip shape stably at a low cost, and as a result, an electron microscope, a length measuring machine, an electron beam exposure machine, an electron beam tester, etc. Electron beam emitting cathodes used for masks, mask repairs, ion implanters, semiconductor device cross-sectional analyzers,
An object of the present invention is to provide an inexpensive and stable ion source used for a focused ion beam (FIB) source such as a thin film forming apparatus for a transmission electron microscope.

In addition, an object of the present invention is to provide a tip having a large radius of curvature, preferably 1.5 μm or more, and a small half angle of a cone, preferably 10 degrees or less, so that the energy width is small and the high angle An object of the present invention is to provide a thermal field emission cathode having a current density and stable electron beam characteristics for a long period of time.

[0017]

SUMMARY OF THE INVENTION The present invention relates to a method of manufacturing a needle-like electrode in which a constricted portion is formed in a thin wire of a high melting point metal and cut from the constricted portion, wherein the constricted portion is formed of a thin line of a high melting point metal. Is formed by electropolishing while reciprocating in the axial direction of the electrode.

According to the present invention, the constricted portion is formed at 0.01 μm /
forming the needle electrode by electropolishing at an initial speed of not less than sec and not more than 0.1 μm / sec.
Is the way. Further, the present invention provides a method for forming a narrow wire of a high melting point metal.
A method of manufacturing a needle-shaped electrode, comprising forming a constricted portion and heating and fusing the constricted portion.

The present invention is characterized in that, at the time of the heating and fusing, a narrow wire is heated and blown by Joule heat while energizing a thin wire;
Preferably, one end of the fine wire is immersed in a liquid metal and an electric current is passed through the liquid metal.
a. The method for producing a needle-like electrode described above, wherein

The present invention is characterized in that a direct current electrolytic polishing is carried out by using an aqueous solution of sodium hydroxide and / or potassium hydroxide of 0.1N or more and 0.8N or less as an electrolytic solution. The manufacturing method, preferably, measuring the reduction rate of the electrolytic current during the DC electrolytic polishing,
The method of manufacturing a needle-shaped electrode according to the above, wherein the electropolishing is stopped when the reduction rate reaches a predetermined value, and more preferably, the predetermined value is 10% or more.

The present invention is the above-mentioned method for producing a needle-like electrode, wherein the fine wire of the refractory metal is at least one selected from the group consisting of tungsten, molybdenum, tantalum, and rhenium. Preferably, the fine wire of the high melting point metal is made of a single crystal of tungsten or molybdenum, and more preferably, the fine wire of the high melting point metal is <
The method for producing a needle-shaped electrode according to the above, wherein the single-crystal has a 100> orientation.

Further, the present invention provides a method of manufacturing a needle-shaped electrode obtained by the above-described needle-shaped electrode, wherein the needle-shaped electrode is further heat-treated in a vacuum to adjust a tip radius of curvature of the needle-shaped electrode.
A method for producing a needle electrode for a field emission cathode, comprising adjusting the radius of curvature of the needle electrode by vapor phase etching by heating while introducing oxygen and / or water under reduced pressure. .

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method for producing a needle-shaped electrode according to the present invention will be described with reference to an electron emitting cathode as an example. First, a conventionally known electron emission cathode is shown below (Step 1).
To (Step 5).

(Step 1) A V-shaped tungsten filament is attached to the tips of two metal posts brazed to the insulator by spot welding. Further, the top of the V-shaped tungsten filament has a length of 3.0 mm and a diameter of about 0.3 mm.
A 13 mm <100> oriented tungsten single crystal fine wire is attached by spot welding.

(Step 2) An aqueous solution of NaOH is applied to the ring-shaped electrode, and the tip of the tungsten single crystal fine wire is immersed.
A tungsten single crystal fine wire is polished by electrolytic polishing to cut off a part of the fine wire to form a needle-like electrode having a sharp tip.

(Step 3) Zirconium hydride is pulverized in an organic solvent to form a slurry at a position of about 0.2 mm from the apex of the tungsten filament of the tungsten single crystal thin wire.

(Step 4) 3 × 10 ⁻⁹ T in an ultra-high vacuum apparatus
After evacuation to orr, the tungsten filament is heated to about 1800 K by energizing a tungsten filament through a metal support, and then oxygen is supplied to 3 × 10 ⁻⁶ Torr.
and maintained for 48 hours. By this operation, the hydrogenated zirconium is thermally decomposed and oxidized to form a reservoir made of zirconium oxide.

(Step 5) A suppressor electrode is put on the insulator, a needle-like electrode is protruded from a hole of the suppressor electrode, mounted on an electron gun paired with an extraction electrode, and installed in an ultra-high vacuum device. Vacuum down to 5 × 10 ⁻¹⁰ Torr, apply electricity to the tungsten filament and set the needle electrode to 1
Heat to 800K. Subsequently, a negative high voltage (extraction voltage) is applied to the needle electrode with respect to the extraction electrode, and electron emission is started. The on-axis current passing through the extraction electrode is monitored, and the electron emission is maintained until the current is stabilized. still,
The high voltage application and measurement system is as shown in FIG. The on-axis angular current density I′p = Ip / ω is calculated from the measured on-axis current Ip and the solid angle ω.

On the other hand, in the present invention, in order to control the shape of the tip of the needle electrode, instead of (step 2),
(Step A) and (Step B) shown below , or (Step A)
(Step C) or (Step 2) is omitted, and (Step A) and (Step C) are interposed between (Step 4) and (Step 5). Further, the present invention is characterized in that an additional step is performed via (step D).

(Step A) A part of a tungsten single crystal thin wire is polished by applying an NaOH aqueous solution to the ring-shaped electrode, immersing the tungsten single crystal thin wire, and electropolishing while moving the tungsten thin wire up and down. A constriction is formed (see FIG. 1). Here, the constricted portion referred to in the present invention means a portion formed on a part of a fine wire having a diameter of D1 and having a diameter of D2 which satisfies the relationship of D2 <D1. Satisfying the relationship is preferable because it is easy to obtain a small needle-like electrode having a half cone angle.

After the above (Step A), electrolytic polishing (Step B) or (Step C) under the following specific conditions found by the present inventors provides a tungsten single crystal thin wire. Is cut off to form a needle electrode.

(Step B) As in step A, an aqueous NaOH solution is applied to the ring-shaped electrode, a tungsten single crystal thin wire is immersed, and the tungsten single crystal thin wire is vertically polished while being electropolished, and finally cut. In this case, the initial rate of the electrolytic polishing is set to 0.01 μm / sec or more and 0.1 μm / s.
ec or less. 0.01 μm / sec
If it is less than 0.1, crystal defects may be exposed on the surface of the needle-shaped electrode obtained by polishing, resulting in poor electron emission characteristics.
If it exceeds μm / sec, the controllability of polishing becomes poor. The rate of electrolytic polishing (also referred to as a polishing rate) is determined by polishing a fine wire for a predetermined time and dividing the change in diameter of the fine wire before and after polishing by the time required for polishing. The diameter of the thin wire is, for example, 5
Measure on a 0x magnification projector.

In the above-mentioned (Step A) and (Step B), it is preferable to perform electropolishing with a direct current using a 0.1 to 0.8 N aqueous solution of sodium hydroxide or potassium hydroxide as an electrolytic solution. . If it is less than 0.1 normal, crystal defects may be exposed on the surface of the needle-shaped electrode obtained by polishing, resulting in poor electron emission characteristics. If it exceeds 0.8 normal, the controllability of polishing is deteriorated. Further, focusing on the rate of change of the electrolytic current during electrolytic polishing, since the electrolytic current gradually decreases, it is necessary to stop the electrolytic polishing when the rate of change (that is, the reduction rate) exceeds a predetermined value (desired). This is preferable because it is possible to obtain a needle-like electrode having the above-mentioned tip shape with good reproducibility), particularly when stopping the electropolishing when the reduction rate becomes 10% or more. If the electropolishing is not forcibly stopped, the electropolishing proceeds and a needle electrode having a desired tip curvature and a half cone angle cannot be obtained. In addition, (Step A) and (Step B) can be performed as a single step by performing a series of operations as well as being performed as separate steps. The rate of change of the electrolytic current is determined by, for example, using a digital ammeter for electrolytic current measurement, integrating time of 2.5 ms, and sampling interval of 20 m.
The moving average of the current for approximately 2 seconds can be measured from the average value obtained by sampling s and 100 samples, which is the same in the present invention.

In the present invention, the (step B)
(Step C) described below can be adopted instead of the above.

(Step C) In this step, the constricted portion formed in (Step A) is heated and melted to obtain (Step A)
To form a needle-shaped electrode having a tip radius of curvature substantially equal to the radius of the constricted portion formed by the above. For example, the tip of a tungsten single crystal thin wire is immersed in a liquid metal in a metal boat in a vacuum device. Subsequently, the pressure is reduced to a vacuum of 5 × 10 ⁻⁷ Torr, the tungsten filament is energized through a metal support, and the temperature is raised and maintained until the vicinity of the constricted portion of the tungsten single crystal thin wire reaches 1800 K. Further, a thin single-crystal tungsten wire can be energized via a liquid metal by a thin-wire power supply, and only the narrow portion can be locally heated and melted. The liquid metal is also liquid at low temperatures and preferably has a low vapor pressure under vacuum conditions. For example, Ga, Hg, solder, or the like can be used. In particular, Ga is liquid at room temperature, and It is preferable because it has low reactivity with various materials and is easy to handle.

In the step (C), the local heating of the constricted portion by applying a current to the single-crystal thin wire having the constricted portion and the melting and cutting of the constricted portion are exemplified above. Local heating using electron beams, infrared rays, etc., can also be melted and cut, or the single crystal thin wire having the constricted portion is fixed to a high melting point filament, and the filament is heated by energization to form the single crystal thin wire. It is also possible to heat all or a part including the constricted portion and melt-cut at the constricted portion. Furthermore, when the (step C) of the present invention is employed, it is preferable that (step 4) be before (step A) and (step C) according to the study of the present inventors. If (Step 4) is performed after (Step C), a gas phase etching phenomenon may be exhibited in (Step 4), and the tip may become a pyramid.

According to the present invention, a constricted portion having a desired size is formed in (Step A), and further cut in the constricted portion by the operation of (Step B) or (Step C) to form the constricted portion in (Step A). It is characterized in that a needle-shaped electrode having a tip radius of curvature substantially equal to the radius of the constricted portion can be easily formed with good reproducibility. Further, the method of the present invention has a feature that the tip shape of the needle-shaped electrode obtained by (Step A) can be almost determined, but (Step A) and (Step B) are formed by the electrolytic polishing method. In addition, since (Step C) is a method of melting by heating, it has a feature that the thin wire can be applied regardless of whether it is a single crystal or a polycrystal.

As the fine wire of the refractory metal applied to the present invention, tungsten, molybdenum, tantalum, rhenium and other refractory metals which are heat-resistant under a high vacuum are preferable due to the limitation in use. , Molybdenum, tantalum, rhenium and at least one selected from the group consisting of fine wires having a diameter of about 0.1 to 0.5 mm are preferred.

[0039] In the present invention, (step A) and (step B), or (step A) and is the basic methods via (step C), it is also possible to through further (Step D) In particular, it is effective when trying to control the tip shape of the obtained needle-shaped electrode to a minute degree.

(Step D) This step is a step of controlling the tip shape of the needle-shaped electrode obtained by the above method in more detail. According to the study by the present inventors, the needle-shaped electrode obtained by the above-described operation can be heat-treated in a vacuum to increase the radius of curvature of the tip of the needle-shaped electrode. In addition, by heating while introducing oxygen and / or water under reduced pressure,
The radius of curvature of the tip of the needle electrode can be reduced. By appropriately selecting the above two methods, the tip shape of the needle electrode obtained by the above method can be controlled in more detail.

Hereinafter, the present invention will be described in more detail with reference to examples.

[0042]

【Example】

[Examples 1 to 3, Comparative Examples 1 to 3] Examples 1 to 3 correspond to (Step 1), (Step 3), (Step 4), (Step A), (Step C), and (Step 5) described above. ), The electropolishing conditions in (Step A) were adjusted to produce a thermal field emission cathode having needle electrodes having various radii of curvature. on the other hand,
Comparative Examples 1 to 3 are conventionally known (Step 1), (Step 2),
(Step 3), (Step 4), and (Step 5), which were sequentially formed. The electropolishing conditions in Step 2 were adjusted to prepare a thermal field emission cathode having needle electrodes having various radii of curvature. . Table 1 summarizes the tip shape and electron emission characteristics of these thermal field emission cathodes.

[0043]

[Table 1]

It is clear from Table 1 that according to the present invention, the radius of curvature can be easily controlled in the range of 2.0 to 100 μm, particularly 2.0 to 20 μm, while keeping the conical half angle at 10 degrees or less.

[Examples 4 to 7, Comparative Example 4] Examples 4 to 7
As a result, a thermal field emission cathode was prepared through (Step 1), (Step A), (Step B), (Step 3), (Step 4) and (Step 5). In each case, (Step A) and (Step B)
And were a series of operations. For Examples 4 to 7, the electrolyte of (Step A), and thus (Step B),
Using 0.5, 0.7, 1.0 normal sodium hydroxide aqueous solution, direct current electrolytic polishing was performed using a tungsten single crystal thin wire as a positive electrode. The tungsten single crystal fine wire is about 1
It was moved up and down with a stroke of 50 μm, and the applied voltage was 6 V. Further, the electrolytic current was measured, and the rate of change of the electrolytic current was 10%.
%, The polishing was completed.

With respect to the thermal field emission cathode, the tip shape and electron emission characteristics were examined, and further, the rate of change of the tip radius of curvature after 5000 hours of electron emission was examined. As a comparative example, a thermal field emission cathode obtained by the same operation as Comparative Examples 1 to 3 was examined. Table 2 shows the results.

[0047]

[Table 2]

As shown in Table 2, even when the electron emitting cathode obtained by the method of the present invention is operated for as long as 5000 hours, the change in the radius of curvature is smaller than that of the electron emitting cathode obtained by the conventional manufacturing method. Good.

Further, the relationship between the electrolyte concentration and the initial polishing rate was examined from the above Examples 4 to 7 and the experimental results according to the present invention not described above. Obtained. Similarly, FIG. 4 shows the relationship between the radius of curvature of the needle electrode obtained by the method of the present invention and the half angle of a cone. According to the method of the present invention, it is possible to obtain a needle-like electrode having a large radius of curvature while keeping the half cone angle small.
When the concentration is specified, the initial polishing rate is from 0.01 to 0.1.
It is clear that a needle-shaped electrode controlled at 1 μm / sec and having a radius of curvature of 2.0 μm or less can be obtained while maintaining the half cone angle at 10 degrees or less.

[0050]

According to the present invention, it is possible to obtain a needle-shaped electrode having a tip curvature radius of 0.6 μm or more and a conical half angle of 10 ° or less without using a special device, which was difficult to obtain conventionally. It is possible to provide inexpensively a thermal field emission cathode with low energy width and stable electron emission characteristics for a long time,
Very useful in industry.

In the method of the present invention, the shape of the tip of the needle electrode is controlled based on operations such as electropolishing and fusing, which have a small crystal anisotropy. The present invention can also be applied to a shape electrode.

[Brief description of the drawings]

FIG. 1 is an explanatory view of an electropolishing method for forming a constricted portion according to the present invention.

FIG. 2 is a circuit diagram for measuring electron emission characteristics of the thermal field emission cathodes according to the example of the present invention and the comparative example.

FIG. 3 is a diagram showing a relationship between an electrolyte concentration and an initial polishing rate in the present invention.

FIG. 4 is a diagram showing a relationship between a radius of curvature of a needle-shaped electrode according to the present invention and a half cone angle.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Tungsten single crystal thin wire 12 Ring electrode 13 Electrolyte 21 Thermoelectric field emission cathode 22 Suppressor electrode 23 Leader electrode 24 Filament heating power supply 25 High voltage generation power supply 26 Aperture 27 Probe current measurement electrode 28 Microcurrent meter for probe current measurement 29 Bias power supply

Claims (14)

(57) [Claims] 1. A method of manufacturing a needle-shaped electrode in which a constricted portion is formed in a fine wire of a high melting point metal and cut from the constricted portion, wherein the constricted portion is reciprocated by moving a thin line of a high melting point metal in an axial direction thereof.
A method for producing a needle-like electrode, characterized in that the electrode is formed by electropolishing while performing polishing. 2. The constricted portion has a thickness of 0.01 μm / sec or more.
2. The method according to claim 1, wherein the electrode is formed by electropolishing at an initial speed of 1 [mu] m / sec or less. 3. A method for producing a needle-shaped electrode, comprising forming a constricted portion in a fine wire of a high melting point metal and fusing the constricted portion by heating. 4. The method for producing a needle-shaped electrode according to claim 3, wherein a current is applied to the thin wire to heat and melt the constricted portion by Joule heat. 5. The method according to claim 4, wherein one end of the thin wire is immersed in a liquid metal, and a current is supplied through the liquid metal. 6. The method according to claim 5, wherein the liquid metal is Ga. 7. A method according to claim 1, wherein an aqueous solution of sodium hydroxide and / or potassium hydroxide having a concentration of 0.1 N or more and 0.8 N or less is used as an electrolytic solution, and the constriction is cut off by DC electropolishing. The method for producing a needle-shaped electrode according to claim 1 or 2. 8. A method for manufacturing a needle-shaped electrode according to claim 7, wherein a reduction rate of the electrolytic current during DC electrolytic polishing is measured, and when the reduction rate reaches a predetermined value, the electrolytic polishing is stopped. . 9. The method according to claim 8, wherein the predetermined value is 10% or more. 10. The method for manufacturing a needle-shaped electrode according to claim 1, wherein the fine wire of the refractory metal is at least one selected from the group consisting of tungsten, molybdenum, tantalum and rhenium. . 11. The fine wire of a high melting point metal is made of a single crystal of tungsten or molybdenum.
A method for producing the needle-like electrode according to the above. 12. The method for producing a needle-shaped electrode according to claim 11, wherein the fine wire of the refractory metal is a single crystal having a <100> orientation. 13. The method according to claim 10, 11 or 12.
Needle-like electrode obtained through the method for manufacturing a needle-like electrode described,
Further, a method for producing a needle electrode for a field emission cathode, comprising adjusting the radius of curvature of the needle electrode by heat treatment in a vacuum. 14. A method according to claim 10, wherein the method comprises the steps of:
Needle-like electrode obtained through the method for manufacturing a needle-like electrode described,
Further, a method of manufacturing a needle electrode for a field emission cathode, comprising adjusting the radius of curvature of the needle electrode by vapor phase etching by heating while introducing oxygen and / or water under reduced pressure. .