JP2008159499A - Manufacturing method of schottky emitter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To shorten a time required for manufacturing a Schottky emitter by easing a condition with respect to a processing temperature and electric field intensity in a facet forming process. <P>SOLUTION: A method of manufacturing the Schottky emitter comprises a process of sharpening a tip by electrolytic etching against an emitter base material 2 formed of a single crystal tungsten wire having a <100> orientation, a process of desorbing and removing adhered materials and smoothening a tip surface by heating the emitter base material 2 in a vacuum, a process of thereafter making a zirconium oxide diffused and permeated into the emitter base material 2, and a faceting treatment process of thereafter heating the emitter base material 2 in the vacuum and forming a flat (100) surface at the tip by applying an electric field between the emitter base material 2 and an extraction electrode 10 arranged opposite to the tip of the emitter base material 2. In the faceting treatment process, the electric field is applied so that the extraction electrode 10 side becomes to have negative potential with respect to the emitter base material 2. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a point cathode called a Schottky emitter used as an electron source such as an electron microscope, SEM (scanning electron microscope), EPMA (electron beam probe microanalyzer), and microfocus X-ray tube.

As a high-brightness electron source, a point cathode called a Schottky emitter in which a single crystal tungsten (W) chip having a (100) axis orientation is coated with a single atomic layer of zirconium oxide (ZrO) is widely used. This is because the work function can be significantly lowered by forming a ZrO layer on tungsten as a base material. That is, the work function φ _{W of} tungsten is 4.5 eV, whereas the work function φ _{ZrO / W} when a zirconium oxide layer is formed on tungsten is 2.7 eV. As a result, a Schottky emitter in which a ZrO layer is formed on a tungsten emitter can achieve a high cathode current density, and when used as an electron source emitter, high luminance can be obtained (see Patent Document 1).

  Also, among such Schottky emitters, what is called a dispenser-type cathode is a device in which a ZrO composite is diffused and penetrated into a tungsten substrate in advance, so that even if the ZrO layer on the surface evaporates, Replenishment enables long-time stable operation (see Patent Document 2).

  Therefore, at present, Schottky emitters have come to be used exclusively as electron sources in high performance electron microscopes and the like.

  A Schottky emitter is manufactured through several steps. A manufacturing process is schematically shown in FIGS.

(A) As shown in FIG. 7A, an emitter substrate 2 made of tungsten wire is fixed in advance to a heating filament 4 made of tungsten by spot welding. The emitter substrate 2 is a single crystal tungsten wire having a <100> orientation.
First, as shown in FIG. 6, the tip of the emitter base material 2 is sharply sharpened by an electrolytic etching technique. In FIG. 6, 1, 2, and 3 are sequentially shown in the progress of the electrolytic etching, and the enlarged view below it is the tip portion of the emitter substrate 2 after the completion of the electrolytic etching.

  (B) Next, the emitter base material 2 is heated at a high temperature in a vacuum to desorb and remove deposits, and the tip surface 6 is made smooth by the effect of surface tension as shown in FIG. 7B.

(C) Thereafter, as shown in FIG. 8, zirconium hydride (ZrH ₂ ) is attached to the base of the emitter substrate 2 and heated at a high temperature in a low-pressure oxygen partial pressure in a vacuum chamber. At this time, ZrH ₂ attached to the base of the emitter base material 2 is oxidized and diffuses and penetrates into the emitter base material 2.

  (D) When the emitter base material 2 having been subjected to the oxidation treatment is mounted on a module having the extraction electrode 10 and the suppressor 12 and the emitter base material 2 is heated at a high temperature in an ultra-high vacuum, diffusion penetration into the emitter base material 2 The ZrO that has appeared again appears on the emitter surface, forms a monoatomic layer, and lowers the work function of the cathode. The faceting process is to form a flat (100) surface at the tip of the emitter substrate 2 in order to obtain a uniform emission distribution. At this time, when an electric field having a certain level of intensity is applied to the emitter base 2 in an ultra-high vacuum, electron emission occurs as indicated by reference numeral 14 in the upper diagram of FIG. These atoms are rearranged and a flat (100) plane grows at the tip.

The upper diagram of FIG. 10 schematically shows changes in the tip shape of the emitter base material 2 in time series as (a) to (d). The lower four images (a) to (d) of FIG. 10 show the results of receiving the emitted electrons 14 in the facet formation process on the fluorescent plate and observing them as an emission pattern. By performing the faceting process, it can be seen that the emission distribution, which initially showed a ring-shaped intensity distribution, becomes uniform. This faceting process is indispensable for using a Schottky emitter as a stable electron source.
s
U.S. Pat. No. 3,814,975 U.S. Pat. No. 4325000

Since the faceting process is performed by the movement of atoms on the surface of the tungsten emitter base, the emitter base is kept at a relatively high temperature of about 1800 ° K. and about 5 × 10 ⁸ V / m or more in order to induce atomic movement. It is necessary to apply the electric field strength of a certain level or more. For this reason, a considerable amount of electron emission is inevitably involved in the faceting process. Since the magnitude of the electron emission depends on the temperature of the emitter substrate and the intensity of the applied electric field, it is difficult to arbitrarily increase these in order to shorten the processing time. This is because when the electron emission limit is exceeded, the emitter base material is damaged by discharge.

  For this reason, it is necessary to gradually increase the processing temperature and electric field strength while paying attention to the deterioration of the degree of vacuum due to electron emission and accompanying degassing, so the faceting process usually takes about 10 hours. Yes. Further, it takes a longer time to process the emitter substrate having a large radius of curvature at the tip.

  An object of the present invention is to reduce the time required for manufacturing a Schottky emitter by relaxing the conditions for the processing temperature and electric field strength in the facet forming process.

  In the present invention, the polarity of the electric field applied in the facet forming process is reversed from that used in the usual manner so that electron emission does not occur, so that the processing temperature and electric field strength in the facet forming process can be arbitrarily increased. did.

  That is, the present invention provides a step of sharpening the tip of an emitter substrate made of a single crystal tungsten wire having a <100> orientation by electrolytic etching, and heating the emitter substrate in vacuum to remove the deposits. Removing the surface and smoothing the tip surface, then diffusing and infiltrating zirconium oxide into the emitter substrate, and then heating the emitter substrate in vacuum and facing the tip of the emitter substrate A Schottky emitter manufacturing method including a faceting treatment step in which an electric field is applied between an arranged extraction electrode and an emitter substrate to form a flat (100) surface at a tip portion. The electric field is applied so that the extraction electrode side becomes a negative potential with respect to the emitter base material.

  According to the inventors' research, the formation of facets effectively reduces the surface tension at the tip of the emitter substrate by the electrostatic field energy stored around the tip of the emitter substrate, resulting in anisotropy of the surface tension. Promoted by becoming prominent. This is because the shape of the tip of the emitter substrate is determined by the “Equilibrium Crystal Shape (ECS)” obtained from the γ-plot indicating the anisotropy of the surface tension using the Wulff method.

  Since the magnitude of the electrostatic field energy does not depend on the polarity of the electric field, facets can be formed in the same manner as usual even when an electric field with the polarity reversed is used.

  The electric field applied in the facet formation process increases the electric field strength at a speed at which the tip of the emitter base material can change the thermal equilibrium crystal shape while maintaining the thermal equilibrium state of the crystal at the processing temperature at that time. It is preferable. If the electric field strength is increased at this speed, the thermal equilibrium shape is always maintained by the movement of tungsten atoms on the surface of the tip of the emitter substrate, and the facet formation proceeds smoothly. If a high voltage is applied at a higher speed than this, the tip of the emitter base material will be put out of the thermal equilibrium state, the transition to the thermal equilibrium crystal shape will not be performed smoothly, and the shape may be fixed in the middle. is there. On the other hand, if the electric field strength is increased at a speed slower than this speed, there is no problem in causing the transition to the thermal equilibrium crystal shape, but the processing time becomes longer, so the electric field strength at a slower speed. It is not preferable to increase the value.

  In the faceting process in the conventional method, since an electric field is applied so that the extraction electrode side is positive with respect to the emitter base, electron emission from the emitter base occurs. In order to suppress the electron emission, it is necessary to provide a suppressor electrode that is set at a negative potential with respect to the emitter substrate on the side of the emitter substrate in the faceting process. On the other hand, in the present invention, in the faceting process, since an electric field is applied so that the extraction electrode side becomes a negative potential with respect to the emitter base, no electron emission occurs from the emitter base. Therefore, in the faceting process, a suppressor electrode that should be set to a negative potential with respect to the emitter base material may be provided on the side of the emitter base material. Then, if the tip of the emitter base material is disposed opposite to the extraction electrode without providing the suppressor electrode on the side of the emitter base material, a larger electric field can be applied to the tip of the emitter base material. This is an effective means for ensuring a sufficient electric field strength, for example, in the facet forming process of a Schottky emitter having a larger radius of curvature.

  In the faceting treatment step in the conventional method, electron emission from the emitter base material occurs, and the electron emission is more likely to occur as the treatment temperature becomes higher. Therefore, the treatment temperature cannot be arbitrarily increased. On the other hand, in the present invention, no electron emission occurs, so that the problem due to the electron emission does not occur even if the processing temperature is increased in order to shorten the processing time.

  In the present invention, since the electric field is applied so that the extraction electrode side becomes a negative potential with respect to the emitter base material in the faceting process, electron emission from the emitter base material does not occur at all. Therefore, the conditions for the processing temperature and electric field strength are relaxed, and the time required for manufacturing the Schottky emitter can be shortened by appropriately setting the processing temperature and electric field strength. .

  The Schottky emitter manufacturing method according to the present invention is the same as the manufacturing process described with reference to FIGS. 6 to 8 except for the polarity of the electric field applied to the emitter substrate in the faceting process. An embodiment will be described with reference to FIGS. 6 to 8 again and further using the apparatus of FIG.

  (A) As shown in FIG. 7A, the emitter substrate 2 is made of a single crystal tungsten wire having a <100> orientation, and is fixed to a heating filament 4 made of tungsten in advance by spot welding.

  First, as shown in FIG. 6, the tip of the emitter base material 2 is sharply sharpened by an electrolytic etching technique. The electrolytic etching is performed until the curvature radius R of the tip reaches about 0.1 to 0.5 μm.

  (B) Next, the heating filament 4 is energized and heated in a vacuum, and the temperature of the emitter base material 2 is increased to about 2000 ° K to desorb and remove the deposit, as shown in FIG. As shown, the tip surface 6 is smoothed by the effect of surface tension.

(C) Thereafter, as shown in FIG. 8, ZrH ₂ is attached to the base of the emitter base material 2, and again put into the vacuum chamber, and the pressure is 1800 in an oxygen partial pressure of about 1 × 10 ⁻⁶ mbar. Heat at a temperature of about ° K for about 20 hours. As a result, ZrH ₂ attached to the base of the emitter base 2 is oxidized and diffused and penetrates into the emitter base 2.

(D) Faceting process:
The faceting process is for forming a flat (100) surface at the tip of the emitter substrate 2 in order to obtain a uniform emission distribution.

  As shown in FIG. 1, the emitter substrate 2 that has been subjected to the oxidation treatment is placed in an ultra-high vacuum chamber in a state of being mounted on a module having an extraction electrode 10 and a suppressor electrode 12. The emitter substrate 2 is spot welded to the tip of the heating filament 4 attached to the emitter insulator 16. The emitter insulator 16 is inserted into the suppressor electrode 12, and is fixed so that only the tip of the emitter base 2 protrudes from a hole formed in the upper portion of the suppressor electrode 12. The state is the same as in FIG. Appropriate height of the tip of the emitter base material 2 protruding from the hole above the suppressor electrode 12 is 0.2 to 0.3 mm. The extraction electrode 10 is placed on the counter electrode of the suppressor electrode 12 with the insulator 18 interposed therebetween.

The emitter base material 2 is heated to about 1800 ° K by energizing the heating filament 4 to generate heat in an ultra-high vacuum (pressure of about 10 ^-9 mbar or higher). As a result, ZrO diffused and permeated into the emitter substrate 2 appears again on the emitter surface, forming a monoatomic layer and lowering the work function of the cathode. What is important at this time is that since the ZrO layer can selectively lower the work function of the (100) plane of tungsten, electron emission from other planes can be suppressed. In other words, the electron emission distribution will be non-uniform unless the tungsten surface is uneven and a flat (100) surface is formed.

Then, a high voltage Vext is applied to the extraction electrode 10 while causing the current If to flow through the heating filament 4. The high voltage Vext is set so that an electric field strength of 5 × 10 ⁸ V / m or more is applied between the emitter base material 2 and the extraction electrode 10 so that the extraction electrode 10 is on the negative side with respect to the emitter base material 2. To do.

  The suppressor electrode 12 is applied with a negative voltage of about several hundred volts, for example, −300 V, to the heating filament 4 to suppress thermionic emission from the heating filament 4.

  On the other hand, FIG. 11 shows a method of applying a voltage to the emitter substrate 2 in the faceting process in the conventional method. The difference from the embodiment of FIG. 1 is that a voltage is applied so that the extraction electrode 10 is on the positive side with respect to the emitter substrate 2.

  The facet formation was performed using the voltage application sequence shown in FIG. 2 in order to compare the facet formation by the conventional “normal method” and the two methods “inversion method” according to the embodiment, and the results were compared. The temperature of the emitter base material 2 was both 1750 ° K.

  In the faceting process, it is necessary to carry out the process while monitoring the degree of vacuum. Therefore, in the conventional method, the current discharge is small and the risk of deterioration in the degree of vacuum is eliminated. Therefore, Vext = 2.0 kV is relatively overnight. The next day, the operator set the voltage low and gradually increased the voltage manually to encourage facet formation. This is based on a conventional process method which has been conventionally performed.

  On the other hand, in the inversion method according to the present invention, since there is no risk of discharge due to electron emission, the program was set so that the voltage Vext increased from 0 V to −6.0 kV overnight.

  FIG. 3 shows changes in the emission pattern observed to monitor the facet formation. The upper column is the reversal method according to the present invention, and the lower column is the normal method.

  According to the normal method, it can be seen that the emission pattern at the stage after one night is in a ring shape, and the growth of the (100) plane has not ended. This is because the electric field strength is not sufficient. It can be seen from the pattern change that the facet grows when the operator gradually raises the applied voltage Vext to 3.2 kV and waits for about 3 hours.

  In the inversion method according to the present invention, the facet formation was already completed at the stage of dawning overnight. This is because an electric field having a sufficiently high applied voltage Vext of −6.0 kV is already applied to the tip of the emitter substrate.

  As described above, in the inversion method according to the present invention, the time required for manufacturing the Schottky emitter can be shortened by reducing the condition for the electric field strength in the facet forming process.

  FIG. 4 compares SEM images of the tip of a Schottky emitter faceted by the normal method (A) and the inversion method (B) according to the present invention. It can be seen that the tips can be faceted in almost the same shape in both methods.

  In the embodiment, the voltage is gradually increased from 0 V when facet formation is performed using the inversion method. It is meaningful not to apply the final applied voltage from the beginning. This is because the thermal equilibrium crystal shape at the tip of the emitter substrate can be slowly changed by gradually increasing the electric field. The movement of tungsten atoms on the emitter substrate tip surface always maintains a thermal equilibrium shape and the facet formation proceeds smoothly. If suddenly a high voltage is applied, the tip of the emitter substrate will be placed in a state greatly deviated from the thermal equilibrium shape, and the transition from here to the thermal equilibrium crystal shape will not be performed smoothly, and the shape will be fixed in the middle. There is. An example is shown in FIG. Here, the voltage Vext was set to -6.0 kV from the beginning. Although it can be seen from the emission pattern that the tip (100) surface grows, the shape is finally fixed in a state where the tip surface is divided into two stages.

  In the embodiment, a suppressor electrode 12 for applying a negative voltage to the heating filament 4 is provided in order to suppress thermionic emission from the heating filament 4 in the faceting process. However, in the present invention, in the faceting process, an electric field is applied to the emitter base material 2 and the heating filament 4 so that the extraction electrode 10 side has a negative potential. No electron emission occurs. Therefore, as another embodiment, in the faceting process, a suppressor electrode to be set to a negative potential with respect to the emitter base material 2 and the heating filament 4 on the side of the emitter base material 2 and the heating filament 4 is used. 12 is not provided. In this case, a large electric field can be applied to the tip of the emitter base material 2 by the extraction electrode 10, which is convenient for the facet forming process of the Schottky emitter having a large curvature radius.

  Further, in the embodiment, the processing temperature of the faceting process is set to about 1800 ° K. As still another embodiment, the processing temperature of the faceting process is increased to 1900 ° K. Even in that case, electron emission from the emitter substrate did not occur. Thus, the processing time can be shortened by increasing the processing temperature of the faceting process.

It is the schematic of the apparatus which shows the voltage application state at the time of the faceting process in one Example. It is a time chart which shows the voltage application sequence at the time of the faceting process about the conventional "normal method" and the "inversion method" by an Example. It is an image of an emission pattern showing how facets are formed. The upper column is for the present invention, and the lower column is for the conventional method. It is a SEM image which shows the front-end | tip part of the Schottky emitter by which facet formation was carried out, (A) is a normal method, (B) is based on the inversion method by this invention. It is a figure which shows the image of the emission pattern at the time of applying a high voltage from the beginning at the time of a faceting process. It is a figure which shows an electrolytic etching process. It is a figure which shows the process of smoothing the front-end | tip of an emitter base material. It is a figure which shows the diffusion process of ZrO to an emitter base material. It is a schematic sectional drawing which shows the conventional faceting process. It is a figure which shows the conventional faceting process process in time series, the upper figure is sectional drawing which shows the shape change of the emitter base-material front-end | tip part, and the lower figure is a figure which shows the image of the emission pattern in each step. It is the schematic of the apparatus which shows the voltage application state at the time of the faceting process in the conventional method

Explanation of symbols

2 Emitter base material 4 Heating filament 10 Extraction electrode 12 Suppressor electrode

Claims (3)

A step of sharpening the tip of an emitter base material made of a single crystal tungsten wire having a <100> orientation by electrolytic etching, heating the emitter base material in a vacuum, and removing and removing deposits. Smoothing the surface, then diffusing and infiltrating zirconium oxide into the emitter substrate, and then heating the emitter substrate in a vacuum and providing an extraction electrode disposed opposite the tip of the emitter substrate; In a method for manufacturing a Schottky emitter, including a faceting treatment step in which an electric field is applied to an emitter substrate to form a flat (100) surface at a tip portion.
In the faceting process, the electric field is applied so that the extraction electrode side has a negative potential with respect to the emitter base material.   2. The electric field strength of the electric field is increased at a speed at which the tip of the emitter base material can change the thermal equilibrium crystal shape while maintaining the thermal equilibrium state of the crystal at the processing temperature at that time. Manufacturing method of Schottky emitter.   3. The method of manufacturing a Schottky emitter according to claim 1, wherein, in the faceting process, a suppressor electrode to be set to a negative potential with respect to the emitter substrate is not provided on a side of the emitter substrate.