JPH0145695B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a field emission cathode, in which a point formed in the shape of a needle is held so as to face an anode, and electrons are emitted from the point by an electric field. be.

In a field emission cathode, electrons are emitted from the needle point by applying a negative potential to the needle point and a positive potential to the opposing anode. At this time, the anode is connected to a phosphor screen. By doing so, a field emission electron image can be obtained. This field emission electron image usually exhibits a geometric pattern that reflects the crystallographic regularity of the metal that makes up the point, and if the range in which the image appears is defined by the radiation angle, the radiation angle is from the point to the point. It emanates over an area of approximately 1 rad.

However, when such a field emission cathode is actually used, only a small part of the wide emission angle as described above is used. The following description will be made with reference to the schematic diagram shown in FIG. FIG. 1 schematically shows an example of an electron optical system for focusing electron beams using a field emission cathode. A negative voltage is applied to the first anode 3 by a power source 5 to the needle-shaped field emission cathode needle 1 welded to the center of the hairpin-shaped filament 2, and electrons are emitted by field emission. . The spread of the emitted electrons at this time is about 1 rad. in terms of radiation angle, as described above. Aperture hole 1 of the first anode
The electron beam that has passed through the anode 2 is converged by an electrostatic lens effect caused by the potential difference caused by the power supply 6 applied between the first anode and the second anode 4, and is focused on a suitable convergence surface 13 to form fine electrons. Provide beam. Alternatively, a finer electron beam can be obtained by repeating convergence by combining a magnetic lens or the like. At this time, the emitted electrons used as the focused electron beam are restricted by the aperture hole 12 of the first anode 3, and the reason for this is as follows. Regardless of whether it is an electrostatic lens or a magnetic lens, electronic lenses have aberrations that cannot be corrected, and among these aberrations, the quantitatively largest aberration is spherical aberration. Since this amount of aberration is large, the electron beam used must be limited to the vicinity of the optical axis. When the spherical aberration coefficient is Cs and the aperture angle of the electron beam is α, Csα ³
The amount becomes the amount of blur based on aberration. Therefore, in order to reduce the amount of aberration and obtain a fine electron beam, α must be limited to a small region. In a practical device, it is about α10 ^-3 rad. Assuming that the current density distribution at the first anode 3 is uniform,
The ratio of the solid angle (~1sr) of the total electron radiation to the solid angle (πα ² ) of the electron beam passing through the aperture hole 12 is the ratio of the total current of field radiation to the current used as a fine electron beam. In reality, as mentioned above, the current density distribution at the first anode 3 is not uniform due to the crystallinity, and the current density at the center of the field emission electron image varies depending on the crystal axis direction of the pointed needle 1. In the above case, approximately 1000:1 is the ratio between the total current and the utilized fine electron beam current.

On the other hand, in a practical device, the electron beam flux is as fine as possible, and the current value (hereinafter referred to as probe current) is
is required to be brought out to a large extent. For example, the probe current incident on the aperture hole 12 with an opening angle (2α) of 20 mrad is
In the case of a tungsten needle of 310>, a probe current of approximately 1 μA is obtained when 1 mA is drawn as a total emitted current.

On the other hand, the field emission current under a constant degree of vacuum is
The smaller the current value is, the more stable it is, but the larger the current value is, the larger the current fluctuation becomes, making it more likely to become unstable.
Also, the better the degree of vacuum when drawing a constant current, the better
The current is stable. Therefore, due to the above-mentioned requirement, even if an attempt is made to obtain a large total radiation current, current fluctuations will become large, making the device unusable. In fact, the degree of vacuum in a normal vacuum container is 5×10 ^-10 Torr.
It is extremely difficult to stably draw out a field emission current of 100 μA to 1 mA over a long period of time, even if the probe current is small, and therefore it has been impossible to obtain a larger probe current.

In order to solve these conventional problems, by adsorbing a single layer of each of oxygen gas molecules and metal atoms on the surface of the needle of a needle-shaped field emission cathode, field emission electrons can be absorbed by the crystal of the needle. The present invention provides a monoatomic layer adsorption type field emission cathode that generates radiation only in a narrow region of a specific crystal plane and can suppress the total radiation field value to a low radiation angle of about 1/4 rad. They have already proposed it. The materials used for the acicular cathode include tungsten (W) and molybdenum (M _p ), which can maintain the shape of the acicular point and clean the surface of the acicular point by heating at high temperatures.
A high-temperature resistant metal material, such as, which can be processed into a needle-like point by electric field polishing, is used.
A metal having a work function lower than that of the material of the point and whose oxide forms a high temperature resistant oxide is deposited on the clean surface of the point, and by oxygen aeration and heat treatment, W and M _p In this case, electrons are emitted only from the (100) plane of the needle, and the emission angle is reduced to about 1/4 rad. In terms of reducing the radiation angle, the above technique satisfactorily achieves the objective, but new measures are required to stably obtain an even larger probe current from the field emission cathode.

In the monoatomic layer adsorption type field emission cathode mentioned above,
750℃~ to reduce the drift amount of radiation current
Stability will be improved if the needle is heated in the range of 1000℃. The optimal heating temperature varies depending on the material used and the degree of vacuum. Furthermore, since the adsorption is on the order of a monoatomic layer, if the heating temperature is too high, the lifetime will be shortened. However, for vapor-deposited metals with relatively high melting points, current stability becomes much better when heated to 1000°C or higher. In order to achieve this state over a long period of time, it is necessary to stabilize the amount consumed by evaporation so that even when the needle is heated to high temperatures, adsorption of about a monoatomic layer on the needle surface remains stable. must be replenished.

It has been proposed (US Pat. No. 3,814,975) to utilize zirconium (Zr) as an adsorbed metal for such a replenishment means. In this, a single crystal is bonded to the center of a W pin part as a heating filament, and the tip is electrolytically polished to form a pointed needle. By attaching zirconium hydride (ZrH) powder to the joint between this hairpin-shaped filament and the single-crystal needle and heat-treating it in a vacuum atmosphere with oxygen gas partial pressure, ZrH can be used even at high temperatures of 1500℃ or higher. This enables the replenishment of The disadvantage of this method is, firstly,
Since the ZrH powder adhesion is a mere adhesion with no bonding force, it is easy to peel off in vacuum before heat treatment.
Second, when heated to a high temperature and radiated in an electric field, a large amount of material is deposited at the joint between the hairpin-shaped filament and the pointed needle.
The presence of Zr or oxidized Zr causes a large amount of unnecessary thermionic emission current to be mixed into the field emission current, and thirdly, impurities are likely to be mixed in.

Therefore, an object of the present invention is to provide a method for manufacturing a field emission cathode that overcomes these conventional difficulties and has even higher performance characteristics.

In order to achieve the above object, in the present invention, by adsorbing single layers of oxygen gas molecules and metal atoms on the surface of the needle of a needle-shaped field emission cathode, field emission electrons are absorbed into the crystal of the needle. The monoatomic layer adsorption type field emission cathode is operated at high temperatures, and the emission is limited to a narrow region of a specific crystal plane, and the emission angle is about 1/4 fad, which keeps the total emission current low. In order to constantly and stably replenish the amount consumed by evaporation even when the temperature is increased, a high-temperature-resistant thin metal wire is formed into a hairpin shape to form a filament, and the work of the point needle to be attached to the apex of this filament. A metal member having a work function lower than that of the filament and whose oxide has high temperature resistance is then entangled with the filament, and then heated under a high vacuum to make the point of the filament. It is characterized in that the field emission cathode is manufactured in such a way that at least one joint is formed between the filament and the metal member at a location remote from the apex position to be attached. The method for producing a field emission cathode, which is characteristic of the present invention, makes it possible to reliably fix the metal member serving as the replenishment source to the filament at a location away from the point where the point is attached to the filament, and as a result, High stability and long life of the field emission cathode can be achieved. In particular, according to the present invention, since the joint of the metal member serving as a supply source to the filament is formed at a distance from the point where the point is attached to the filament, an unnecessarily large amount of the metal member is formed near the point where the point is attached to the filament. Since there is no low work function member, there is no worry that a large amount of unnecessary thermionic radiation current will be mixed in and emitted during operation.

Hereinafter, the method for manufacturing a field emission cathode according to the present invention will be explained in detail with reference to the embodiments shown in FIGS. 2 to 4. The field emission cathode is made of tungsten or molybdenum, with a diameter of 0.15 mm.
A polycrystalline wire of mm is formed into a hairpin shape to form a filament 2, and the center of this filament 2 has an axial direction <
100> single crystal wires are joined and the tips are electrolytically polished in an aqueous _NaOH solution to produce a pointed needle 1. Figure 2 shows such a state, where 1 is a tungsten or molybdenum point, 2 is a hairpin-shaped filament made of tungsten or molybdenum polycrystalline wire, and 7 is Kovar etc. bonded to filament 2. and 8 is a glass base. Next, as shown in Fig. 3, one end of a hafnium (H _f ) wire 9' with a diameter of about 80 μm is spot welded to the stem 7, entwined with the hairpin-shaped filament 2, and separated again along the shape of the filament 2 like a twisted wire. One end of the stem 7 is spot welded to the stem 7 of the stem 7. Electricity is applied to both ends of the stem 7 in a high vacuum using a power source 14, and the temperature is increased until the Hf wire 9' is melted. When it melts, you can see with the naked eye that the temperature instantly increases further, so turn off the electricity. In this way, a field emission cathode having a shape as shown in FIG. 4 is produced. That is, the Hf wire 9' is melted at approximately the center of both sides of the hairpin portion of the filament 2 to form a spherical supply source 9, and the Hf wire 9' on the stem 7 side is not melted and is joined to the stem 7 while remaining linear. has been done. The moment the Hf wire 9' melts can be confirmed with the naked eye as described above, but when heating is performed using a constant voltage source as the power source 14, the moment can be easily determined from changes in the applied current.

In this case, the Hf ray 9' melts into a spherical supply source 9.
It is not necessarily necessary to form a bonded portion, and it is sufficient that at least one joint portion is formed between the filament 2 and the Hf wire 9'. That is,
This joint will perform the same function as the supply source 9.

Furthermore, it is not necessary to spot-weld both ends of the Hf wire 9' to the stem, and both ends may be left free. In this case, by heating the filament 2 with electricity, the filament 2 and Hf
A joint portion with line 9' is formed. Also, Hf line 9'
may be electrically heated by a separate power source. Furthermore, the heating means does not depend on energization; other heating means may be, for example, a gas burner.

Note that the Hf wire 9' may not be wrapped around the entire filament 2, but only on one side, and in this case, there will be only one joint or supply source.

The field emission cathode thus produced as shown in FIG. 4 was placed in a high vacuum container as shown in FIG. 5, and the basic characteristics of field emission were measured. When the field emission cathode shown in Fig. 4 is heated in vacuum, the Hf supply source 9 becomes W or
The surface of the Mo hairpin filament 2 is diffused to cover the surface of the single crystal needle 1. In FIG. 5, when the filament 2 is energized to heat the needle 1 to about 1400° C. and a high voltage is applied by the power source 5, a field emission electron image is obtained on the anode plate 10 whose surface is coated with a phosphor. From the point needle 1 near the center of the anode plate 10
A small hole 15 is provided so that an opening angle of 20 mrad.
For the reasons mentioned above, oxygen gas was introduced to reduce the radiation angle, and the oxygen gas partial pressure was reduced to 1.5×10 ^-7 Torr.
When this is the case, the work function is the lowest and a field emission electron image with a reduced radiation angle as shown in FIG. 6 is obtained. When only a single crystal of W is used, the radiation angle has an opening angle of approximately 1 rad., as shown by the dotted line in Figure 5.
In Figure 6, the opening angle is approximately 1/4 rad. Also,
At 1400°C, the current density is extremely small in the area outside the opening angle of 1 rad, but the electron image based on thermionic radiation (horizontal dotted line in Figure 6) and the four {100} planes on the side of the point 1 are clearly visible. Field emission electron beams are also observed. In the radiation angle reduced electron image at the center, the current density around the ring shape is 2 to 4 times higher than near the center, but the current stability is several times better near the center. In the case of this embodiment, when the total radiation current measured by the anode plate 10 is 200 μA (including 50 μA of thermionic electrons outside the electron image), the probe current measured by the Faraday cage 11 is 1 μA, and the above-mentioned W<310> This is equivalent to drawing out a total radiated current of about 1 mA with a pointed needle.
Furthermore, the current stability in this state is not particularly shown, but it shows stability comparable to thermionic emission, and ΔI _P /I _P (probe current
The amount of variation ΔI _P ) with respect to I _P is. Furthermore, the major difference and revolutionary point from conventional field emission cathodes is
It can be operated by constantly introducing oxygen gas on the order of 10 ^-7 Torr. This has become a necessary condition for reducing the radiation angle and achieving high stability. However, as the temperature increases, the ratio of thermionic radiation current included in the total radiation current increases. Considering that high current stability is the ultimate goal, this is no problem.

As shown in Figure 6, there is a correlation between the point temperature and the oxygen partial pressure at which the radiation angle is reduced and the work function is minimized.In the case of Hf, when the point temperature is 1200℃,
10 ^-5 Torr, 1300℃ 6×10 ^-8 Torr, 1500℃
At 1400℃, it was 2.8×10 ^-7 Torr, and as mentioned above, it was 1.5×10 ^-7 Torr. However, in actual use, it is not necessary to use the tip temperature and oxygen partial pressure strictly given by this correlation, and as mentioned above, the rate of radiation angle reduction can greatly contribute to the stability of the probe current. Therefore, it can be used in a fairly wide range of needle temperatures and oxygen partial pressures. However, if no oxygen gas is introduced at all (or if there is no partial pressure of oxygen gas as a residual gas), the W
Only the adsorption pattern on the (100) surface can be obtained. In the same figure, C shows almost the minimum work function of the (100) plane, B shows a smaller oxygen gas partial pressure than C at a certain point temperature, and D shows radiation angle reduced electrons when the oxygen gas partial pressure is high. Show the image. The current density is
As shown in FIG. 6, the (100) plane at the center is 1/2 to 1/4 smaller than the surrounding ring-shaped portion, but is extremely stable. In addition, in FIGS. 7B to 7D, thermionic emission images as shown in FIG. 6 appear when the needle temperature is 1400° C. or higher, but are omitted. Note that the current density in the figure is an approximate representation.

Even when oxygen gas species coexists, the current stability described above does not deteriorate. For example, if air (including oxygen, nitrogen, water, etc.) is a residual gas,
When ×100 ^-7 Torr is present, stable operation can be achieved by introducing oxygen gas at 2 ×10 ^-7 Torr at a needle temperature of 1500°C. Also, carbon monoxide (CO) and carbon dioxide (CO ₂ ), which are adversely affected by ordinary field emission cathodes,
No deterioration in characteristics was observed even with gases such as.

Other embodiments include zirconium (Zr) instead of Hf.
This is the case when using The method for producing the pointed needle of the present invention is exactly the same as shown in FIGS. 2 to 4. As the melting point of Zr is lower than that of Hf,
The heating temperature for making the diagram is slightly lower. Fifth
When field emission similar to that shown in the figure is performed, field emission electron images completely equivalent to those shown in FIGS. 6 and 7 are obtained. In addition, the correlation between the tip temperature and oxygen partial pressure that gives the minimum work function is almost the same as that for Hf in the initial state of use, but after 1 to 2 hours, it becomes 1/2 for a constant tip temperature. An oxygen partial pressure of 3 to 1/4 is sufficient, and becomes about 1/10 after 3 to 4 hours, and in a steady state, an oxygen partial pressure of 1/10 to 1/100 of that in the case of Hf is sufficient. In the case of Zr, this is a source of diffusion,
This is considered to be due to oxidation progressing to some extent.

Another example uses titanium (Ti), but the method is equivalent to Zr and Hf. However, Ti
A major feature of this device is that it can operate stably at temperatures around 1100°C without the need for supplemental oxygen.

In addition, the case where binary or ternary alloy wires made of Hf, Zr, and Ti are used, and the case where two or three metal wires made of Hf, Zr, and Ti are used are almost the same as in the above embodiment. The effect of this can be obtained.

It should be noted that when the point according to the present invention is used as a field emission cathode, it is basically sufficient to maintain the shape shown in FIG. 4, and the order of manufacturing is not dependent on the shape.
That is, before joining the single crystal wire 1 to the W or Mo hairpin filament 2, the operations shown in FIGS.
After joining the single crystal wire 1 to the hairpin filament 2, the steps in FIGS. 3 to 4 may be performed and finally electrolytic polishing may be performed.

In addition, in the present invention, in addition to fabricating wires of Hf, Zr, Ti, etc. by intertwining them with the hairpin filament 2, it is also possible to prepare wires 9' of Hf, Zr, Ti, etc. on the hairpin filament 2 in advance as shown in FIG. 8A. Alternatively, a spherical supply source 9 as shown in FIG. Of course, it is not necessary to make it spherical as described in the above-mentioned embodiments, and only bonding is sufficient. Further, the single crystal wire 1 is electrolytically polished to form the pointed needle 1 either before or after the production of the spherical supply source 9.

In the present invention, the needle temperature that maintains good current stability is
In the case of Hf, Zr, and Ti, the temperature is in the range of 860℃ to 1500℃, and the oxygen gas partial pressure required at that time depends on the needle temperature and type of needle as described above, but the maximum 5×10 ^-7 Torr, usually 5×
An appropriate range may be selected depending on the needle temperature and the type of needle at a pressure of 10 ^-7 Torr or less.

Regarding the life of the point needle manufactured according to the present invention, Ti, Zr, and Hf can be used for a longer time if used at the same temperature, but in the case of Zr, which is in the middle, it is used at 1500°C in the example shown in Fig. 4. It has a lifespan of more than 10,000 hours, and it does not affect the shape of the needle, so it does not pose a practical problem. Ti can have the lowest operating temperature, and Zr,
They are in the order of Hf.

In addition, the point needle manufactured according to the present invention has a probe current spot diameter that is similar to that used in electron beam lithography equipment.
It is ideal when a larger value is desired for the probe current at approximately 0.1 μm, and the stability of the probe current is less than 1% for both hourly current level fluctuation (drift) and short-term fluctuation (noise). , which showed better values than any conventional field emission cathode.

As detailed above, according to the present invention, the field emission cathode, which is essentially a high-brightness electron beam source, can draw out a large probe current while ensuring excellent long-term stability and long life. Moreover, the quality of the vacuum used (residual gas, etc.) is hardly limited so that it can be put to practical use.

[Brief explanation of drawings]

Fig. 1 is a block diagram of an electron optical system for converging electron beams using a field emission cathode, and Figs. 2 to 4 are block diagrams of a field emission cathode according to the present invention and explanatory diagrams of the manufacturing process thereof.
FIG. 5 is a configuration diagram of a characteristic measurement system of a field emission cathode according to the present invention, and FIGS. 6 and 7 are schematic diagrams of field emission electron images of a field emission cathode according to the present invention. 8A and 8B are other configuration diagrams of the field emission cathode according to the present invention and explanatory diagrams of the manufacturing process thereof. 1... point needle, 2... hairpin type filament,
3...First anode, 4...Second anode, 5, 6, 14
...Power supply, 7...Stem, 8...Glass base,
9... Deposit, 9'... Line, 10... Fluorescent plate, 1
1... Faraday gauge, 12, 15... throttle hole, 13... convergence surface.

Claims (1)

[Claims] 1. A filament made by forming a thin wire of a high-temperature-resistant metal into a hairpin shape, which has a work function lower than that of the point to be attached to the apex of the filament, and whose oxide has a high-temperature-resistant property. After entangling a metallic member with the filament, the filament and the metallic member are heated at a high vacuum to form a bond between the filament and the metallic member at a portion of the filament away from the apex position where the point is to be attached. A method for manufacturing a field emission cathode, characterized in that at least one junction is formed between the cathodes. 2. The method for manufacturing a field emission cathode according to item 1, characterized in that the point needle is attached after the joint portion is formed. 3. The method for producing a field emission cathode according to item 1 or 2, wherein the filament is made of a fine wire of W or Mo. 4. The method for producing a field emission cathode according to any one of items 1 to 3, wherein the metal member is made of at least one metal selected from the group consisting of Zr, Hf, and Ti. 5. A first device characterized in that the point is made of a single crystal of W or Mo and has an axial orientation of <100>.
A method for manufacturing a field emission cathode according to any one of paragraphs 1 to 4.