JPS6223418B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to 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, and a method for manufacturing the same. The field emission cathode emits electrons by applying a negative potential to the needle-like point and a positive potential to the opposing anode, but at this time,
A field emission electron image can be obtained by using a fluorescent screen as the anode. This field emission electron image is
Usually, it exhibits a geometric pattern that reflects the crystallographic regularity of the metals that make up the inferior needle, and if the range in which the image appears is defined in terms of radiation angle, it covers an area of approximately 1 rad. from the inferior needle. However, when the field emission cathode is put to practical use, only a small portion 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 beam focusing optical system in an electron gun using a field emission cathode. A needle-shaped field-emitting cathode needle 1 welded to the center of a hairpin-shaped filament 2
A negative voltage is applied to the first anode 3 by a power source 5, and electrons are emitted from its tip by field emission. The spread of the emitted electrons at this time is about 1 rad. in terms of radiation angle, as described above.
The electron beam that has passed through the aperture of the first anode 3 is focused by an electrostatic lens effect caused by the potential difference caused by the power source 6 applied between the first anode 3 and the second anode 4, and Provides a fine electron beam spot on the converging surface. Alternatively, by repeating convergence by combining a magnetic lens or the like, an even finer electron beam spot can be obtained. At this time, the emitted electrons used as the focused electron beam are restricted by the aperture 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. Because of this large amount of aberration, the electron beam used is limited to the vicinity of the optical axis, and the spherical aberration coefficient
Cs, and when α is the aperture angle of the electron beam, the amount ^Csα3 is 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 area. In a practical device, α is about 10 ^-3 rad. Assuming that the current density distribution at the first anode 3 is uniform, the ratio of the solid angle of the total electron radiation (1sr) to the solid angle of the electron beam passing through the aperture (πα ² ) is This is the ratio of the total radiation current to the current used as a fine electron beam. In reality, as mentioned above, the current density at the first anode 3 is not uniform due to the crystallinity, and the current density in the field emission electron image is higher in the center of the axial direction of the pointed needle 1. Therefore, in the above case, the ratio of the total current to the current to be used is approximately 1000:1. On the other hand, in a practical device, it is required to converge the electron beam as finely as possible and draw out a large current (referred to as probe current). For example, in order to obtain a probe current on the order of 0.1 μA, a total radiation current on the order of 1 mA is required. 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, even if an attempt is made to obtain a large total radiation current due to the above-mentioned requirements, current fluctuations will become large, making the device unusable.
In fact, in a normal vacuum container, the degree of vacuum is 5×
Even if the current is about 10 ^-10 Torr, it is extremely difficult to draw out a field emission current of 100 μA stably over a long period of time, and therefore it is impossible to obtain a larger probe current. Ta. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a novel field emission cathode from which a stable and large probe current can be extracted, and a method for manufacturing the same. In this invention, by adsorbing a single layer of each of oxygen gas molecules and metal atoms on the inferior needle surface of a needle-shaped field emission cathode, electron emitted electrons can This was done by focusing on the fact that the electric field radiation tends to occur only in a narrow area, and the total radiation current can be suppressed only in the area where the radiation angle of the electric field radiation is about 1/4 rad. or less. It is. The field emission cathode according to the present invention will be explained in detail below. The material of the needle-like cathode used in this invention is a high-temperature-resistant metal material such as tungsten or molybdenum, which can maintain the shape of the needle-like point and clean the surface of the point by heating at high temperature. A material that can be processed into a needle-like point by electrolytic polishing is used. The clean surface of the point is made of a metal whose oxide has a work function lower than the work function of the point material and whose oxide forms a high temperature resistant oxide, that is, the point is made of tungsten (W) or molybdenum (Mo). If available, deposit a monoatomic layer or more of aluminum (Al), chromium (Cr), cerium (Ce), magnesium (Mg), titanium (Ti), and silicon (Si). Next, oxygen gas is introduced into the vacuum where the needle is located, and aerated to an extent that a monomolecular layer of oxygen gas molecules is adsorbed on the surface of the needle. In the case of oxygen gas, this aeration is equivalent to about 1L (Langmiure), so
Exposure is for 1 second at 1×10 ^-6 Torr and 100 seconds at 1×10 ^-8 Torr. After that, the oxygen gas is evacuated, and in a vacuum atmosphere where electric field radiation can be performed, the temperature is 1300 to 1500℃, depending on the vapor deposition material, for 10 to 10 minutes.
The needle points of the present invention can be formed by heat-treating the needle points for 60 seconds. The field emission electron image of the needle-shaped cathode produced in this way shows that for the needles made of W, Mo, etc., electrons are mainly emitted only from the (100) plane, and the field emission is reduced to about 1/4 rad. An electronic image is obtained. Regarding the principle of the present invention, since the radius of curvature of the tip of the needle is a minute region of about 1000 Å, and the adsorption of atoms or molecules is about a single layer, the problem is dependent on the crystal plane of the needle, so the details are still unknown. Although some parts are not clear, the basic idea is as follows. For the sake of simplicity, when considering W as a point material, the change in work function on the W surface when a substance with a lower work function than W is deposited on the W surface is expressed as the work function of W (φ _W ), the work function reaches its lowest value when the thickness of the deposited metal is about 0.7 monoatomic layer, then increases again, and becomes almost saturated above a monoatomic layer, and that value is equal to the work function of the deposited metal (φ
_M ) is known to be asymptotic. This is W
This is because the surface potential of the surface changes due to the adsorption of vapor-deposited metal, and is generally understood as follows. The phenomenon that occurs on the W surface is a many-body problem consisting of bonds between a large number of surface atoms and adatoms, but it can be approximated as a bond between one surface atom and one adatom. The electric dipole moment μ generated between a surface atom and an adatom can be expressed as follows by Malone ⁽¹⁾ using the concept of electronegativity. μ=χ _ad −χ _p (1) Here, χ _ad is the electronegativity of the deposited metal, and χ _p is the electronegativity of W. On the other hand, there is a Gordy-Thomas relational expression ⁽²⁾ between electronegativity χ _p and work function φ, which can be expressed as follows. χ _p =0.44φ−0.15 (2) Since the work function change Δφ caused by adsorption is proportional to the magnitude of the dipole moment μ and the number n of adatoms per unit area, Δφ=2πμn=2πn(χ _ad −χ _p ) (3) ⁽³⁾ Therefore, the change in work function as shown in Figure 2 is caused by depositing a metal with a smaller work function than W, that is, a metal with smaller electronegativity, up to the level of a monoatomic layer where the effect of adsorption becomes apparent. I understand that. Adsorption of a monoatomic layer means that adsorption is performed commensurate with the atomic density of the W surface, and n in equation (3) does not increase with adsorption of a monoatomic layer or more. Moreover, the fact that the minimum value is between 0.7 and 1 atomic layer suggests that the electric dipole (moment .mu.) increases because the degree of freedom of adatoms that can diffuse on the W surface is large to some extent. Next, consider the case where oxygen gas molecules are also adsorbed in addition to metals. As shown in Figure 3a, even if metal M is first vapor-deposited on the W surface and oxygen gas O ₂ molecules are adsorbed afterwards, the reaction activity of the vapor-deposited metal M with respect to oxygen gas O ₂ will differ. However, when left as is or by slight heating, a chemical adsorption reaction similar to normal oxidation causes tertiary oxidation.
As shown in FIG. b, it can be assumed that oxygen atoms O (molecules) are arranged between W and the deposited metal M. This reaction is
The more active the metal is against oxidation, the more likely it is to occur at temperatures close to room temperature. However, FIG. 3b is only a model, and the details of the connection are unknown. Unlike the case where only metal is adsorbed, if the arrangement is as shown in the model shown in FIG. 3b, the work function change experienced at that time can be predicted to be even larger due to the magnitude of the dipole moment. When the ionic radius of the adsorbed metal atom is r _M and the ionic radius or covalent bond radius of the oxygen atom is r _G , the following equation is assumed as the approximate Δφ. Δφ=2πn(χ _ad −χ _p )r _G +r _M /r _G (4) If r _G and r _M have the same value, Δφ is almost twice the value in the case of adsorption of metal atoms. is expected. Even if a polycrystalline material is used, the needle-like point has a pseudospherical surface with a radius of curvature of only about 1000 Å.
Contained into one large grain due to grain growth due to heating. In other words, the needle surface can be regarded as a single crystal surface in any case. As is well known, the work function of W differs depending on each crystal plane, and therefore the above φ or χ _p
Strictly speaking, they must also be labeled as φ _hkl and χ _hkl depending on the crystal plane. Table 1 shows values for some crystal planes as examples. When adsorption of W-O-M occurs as shown in Figure 3b, if the work function change occurs uniformly on each crystal plane, there will be no change in the distribution of the field emission electron image even after adsorption. The extraction electric field simply decreases, but when the work function changes as shown in equation (4), the distribution of the field emission electron image clearly changes. Also, as the value of n in equation (4), when a single layer of oxygen and metal is adsorbed, W
If the adsorption probability on each crystal plane of is 1, then n
is the atomic density at the surface of each crystal plane, and has the values shown in Table 1. Based on these values, calculation examples for Ce and Ti as adsorbed metals are shown in Table 1. Note that r _G is the ionic radius of oxygen.

[Table] This calculation result shows that the (110) plane has the lowest work function, followed by the (100) plane. This is contrary to the fact that in the case of W, the emitted electron image is mainly limited to the (100) plane, but the reason is as follows. In the above calculation, the adsorption probability on each crystal plane is assumed to be 1, and the atomic density on the surface is set to n, but in reality, it is thought that the adsorption probability differs depending on the crystal plane. That is, the (110) plane is the most thermally stable plane in a W crystal, and if it is made into a needle-like point, it has the flattest and widest surface. It is known that the probability of adsorption is small on such a surface, and therefore, it is expected that the work function of the (110) surface is actually not as low as the values shown in Table 1. There is no significant difference in surface area on other surfaces. For the above reasons, on the W point needle,
It is semi-quantitatively understood that when a single layer consisting of oxygen and a metal with a lower work function than W is adsorbed, the emitted electron image is mainly limited to the (100) plane. The work functions on the (100) plane after adsorption in Table 1 are in good agreement with the values experimentally determined by the Fowler-Nordheim plot of field emission. These phenomena are exactly the same for Mo, which has a slightly lower work function than W, and even for other metals, the crystal plane on which the radiated electron image appears may be different, but the radiated electron image is They are similar in that they are restricted to specific crystal planes. Furthermore, the single layer of the adsorbed metal and oxygen of the present invention is clearly chemically bonded as determined by analysis results using a surface analyzer such as X-ray excitation photoelectron spectroscopy, which indicates chemisorption. In other words, it can be regarded as a type of oxidation of a monoatomic layer metal, but when a monoatomic layer or more of an adsorbed metal is evaporated and oxidized, the work function increases, and unless a very high electric field is applied, electrons cannot be oxidized. It becomes impossible to emit radiation. Therefore, the single layer of adsorbed metal and oxygen according to the present invention is significantly different from a case where a thin film of an ordinary oxide is formed on the surface of the needle. Hereinafter, this will be explained in detail using examples. FIG. 4 shows one embodiment of a field emission cathode according to the invention. A hairpin-shaped tungsten wire 2 with a diameter of 0.15 mm is welded to the center of the stem 14 made of Kovar and fixed to the glass base 7.
A pointed needle 1 made by electropolishing a 0.15 mm single crystal with an axial orientation of <100> using an aqueous NaOH solution is instantaneously heated to a high temperature in an ultra-high vacuum container by applying electricity to a hairpin-shaped tungsten wire 2. Clean the surface. At this time, if the fluorescent screen 11 is used as an anode, a field emission electron image of the W-cleaned surface centered on the (100) plane as shown in FIG. 5a can be obtained. In the figure, the dark areas have low current densities, and the horizontal lines, diagonal lines, and white areas indicate higher densities in this order. The area where this electron image is visible corresponds to the radiation angle of about 1 rad. (1 sr in solid angle) shown in Figure 1. Approximately 5 mm to the needle 1, diameter 10 to 15 mm on the anode side
A power source 1 is connected to a Ti wire 8 with a diameter of 0.3 mm formed into a circular shape.
2 and heated to 1400 to 1500°C to evaporate Ti to the point needle 1. The amount of vapor deposition can be controlled by the following operation. When a high voltage is applied to the point needle 1 by the power source 5, the current received by the fluorescent screen 11 of the anode is
The current value was set to about 0.1 μA, and as the Ti was deposited, the current value increased due to the decreasing work function as explained in Figure 2, and reached the maximum value as shown in Figure 6, with a tendency completely opposite to the work function in Figure 2. After having a value, it approaches a constant value. The point with the maximum value can be regarded as the 0.7 monoatomic layer, or the monoatomic layer can be determined directly from the inflection point in Figure 6. The amount of Ti deposited is a minimum monoatomic layer. Excessive deposition can be reduced later by evaporation. After that, as mentioned above, introduce oxygen gas for at least 1 hour.
Perform aeration to the level of langumiure. After exhausting the oxygen gas and restoring the original degree of vacuum, the W filament 2 is energized using the power source 13 to heat the needle 1.
The heating temperature and time vary depending on the amount of Ti evaporated and the amount of oxygen gas aeration, but heating may be performed for 10 to 60 seconds in the range of 1300 to 1500°C. Even if the temperature is less than 1300°C, heating for a longer time will result in the same treatment, but it will be inefficient.
Furthermore, since heat treatment has little effect at temperatures below 800°C, it must be carried out at temperatures above 800°C. Also,
If heated for more than 60 seconds at 1500°C, the adsorption layer of the present invention will be destroyed. In the above manufacturing process, oxidation can be further promoted by aerating with oxygen gas and then heating the point 1 to an appropriate temperature by applying electricity to the filament 2. FIG. 5b shows a field emission electron image of the field emission cathode prepared in this manner. In the electron image of the clean surface in Figure 5a, the current density on the central (100) plane is very small, whereas in Figure 5b, the electron image is limited to a spot centered on the (100) plane. . The radiation angle corresponds to the distribution of the electron image, so if Fig. 5 is 1 rad., the radiation angle in Fig. 5b is about 1/5 rad. In FIG. 4, another embodiment uses a heater 9 made of a W wire or the like with a diameter of 0.1 to 0.3 mm instead of the Ti wire 8.
This is a case where the vapor-deposited metals 10 of Al, Mg, Ce, Si, and Cr are melted in advance and used. The amount of evaporation can be controlled using Ti.
Although it cannot be done in the same way as with a wire, it is possible to accurately measure the temperature of the deposited metal and determine the vapor pressure to approximately evaporate a monoatomic layer. Other processing is the same as in the previous embodiment. The radiation electron image obtained by the above embodiment gives a radiation angle of about 1/4 to 1/5 rad. or less, and the efficiency of the total radiation current with respect to the probe current is very good, and the total radiation A large probe current, which has been difficult to obtain in the past, can be obtained without increasing the current. In the embodiment for the electron beam device, the first
When the opening angle α in the figure is approximately 1×10 ^-3 rad. and the probe current is 0.1 μA, the total radiation current is 30 μA.
On the other hand, when using a point needle with the axial direction <310> where the ratio of the total radiation current and probe current of normal W or Mo is the largest, it takes about 1 mA to draw out the same probe current. Requires total radiated current. Stably drawing out a total emission current of 1 mA with a field emission cathode is particularly important for electron microscopes,
In general-purpose equipment such as electron beam lithography equipment, there is a limit to creating an ultra-high vacuum in the electron gun chamber, so it is extremely difficult to do so. On the other hand, with the cathode of the present invention, as in the above comparison, a large probe current, which has been difficult in the past, can be easily obtained. The point used in the present invention is mainly made of W or Mo, but if it has suitability as a field emission cathode,
Other materials can also be used in combination with the adsorbed metals described below. The conditions under which it can be used as an adsorbed metal are: (i) It must have a lower work function than the pointed material, and if the bonding with oxygen is considered to be equivalent to oxidation, considering high temperature resistance, (ii) The oxide of the adsorbed metal be high temperature resistant, and
(iii) With regard to adsorption processing, adsorption can be performed using the simplest possible method as shown in the examples. In addition, since the point needle of the present invention has a lower work function than the point needle which is not subjected to any treatment such as W or Mo, the amount of attenuation of the radiation current increases when used at room temperature. In order to reduce the amount of current drift including this effect, stable current characteristics can be obtained over a long period of time by heating the point needle of the present invention to 750 to 1000°C. The above temperature range is determined by (i) the lower limit of the heating temperature with respect to the amount of attenuation of the current caused by the adsorption of residual gas molecules in vacuum to the needle, that is, the attenuation is saturated at a constant value; on the other hand,
(ii) The upper limit of the heating temperature is determined so that the single layer consisting of the adsorbed metal and oxygen of the present invention does not thermally decompose. When using the present invention for W and Mo, as described in the examples, if a single crystal with an axial orientation of <100> is used, the center of the radiation angle and the optical axis coincide, which is very convenient in practice. Therefore, it can be used even when the center of the radiation angle with an axial orientation of <310> is slightly shifted from the optical axis. References (1) JGMalone: J.Chem.Phys. 29 (1958),
1154. (2) W. Gordy and WJOTomas: J. Chem.
Phys. 24 (1956) 439. (3) LWSwanson and AEBell: Advances in
Electronics and Electron Physics 32 ,
(1973) 285.

[Brief explanation of the drawing]

Figure 1 is a configuration diagram of an electron beam focusing optical system in an electron gun using a field emission cathode, Figure 2 is a graph showing the relationship between adsorption amount and work function, and Figures 3a and b explain the adsorption state. FIG. 4 is an explanatory diagram of the manufacturing process of the field emission cathode according to the present invention, FIGS. 5 a and b are schematic diagrams for explaining the field emission electron image of the field emission cathode according to the present invention, and FIG. The figure is a graph showing the relationship between the adsorption amount of the electron emitting cathode and the field emission current according to the present invention. 1... point needle, 2... filament, 3... first
Anode, 4... Second anode, 5, 6, 12, 13...
DC power supply, 8... Ti wire loop, 9... heater, 10... evaporated metal, 11... fluorescent plate.

Claims (1)

[Claims] 1. Cr, Al, Ce, Mg,
A field emission cathode characterized in that at least one metal selected from the group consisting of Ti and Si is adsorbed via oxygen to a thickness not exceeding a monoatomic layer. 2. The field emission cathode according to claim 1, wherein the crystal axis orientation of the single crystal is <100> or <310>. 3. In a vacuum atmosphere where field emission can occur, Cr, Al, Ce, Mg,
A vapor deposition step of depositing at least one metal selected from the group consisting of Ti and Si to a thickness of approximately a monoatomic layer, and then introducing an appropriate amount of oxygen gas into the vacuum atmosphere to deposit the metal monoatomic layer on the metal monoatomic layer. An adsorption step in which oxygen gas is adsorbed to a thickness of approximately a monomolecular layer, followed by an evacuation step in which the oxygen gas is evacuated and returned to a vacuum atmosphere in which the electric field emission can be performed; A method for producing a field emission cathode in which the above metal is adsorbed via oxygen to a thickness not exceeding a monoatomic layer, the method comprising a heat treatment step of heating in a temperature range of 10 to 60 seconds at a temperature of .degree. 4. The method of manufacturing a field emission cathode according to claim 3, wherein after the adsorption step, the evacuation step is performed after passing through a heating step of heating the needle cathode to an appropriate temperature.