JP2008098087A - Electron source - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electron source which has a small energy width and is operated in high angle current density. <P>SOLUTION: The electron source is composed of a (100) direction single crystal rod 1 of tungsten or molybdenum and has an electron emitting face with (100) crystal face exposed at one end part, and furthermore, has a diffusion source 2 containing one kind or more of a metal element oxide selected from a group of Ho, Gd, Nd, Sm or Pr. It is preferable that its operation temperature is 1,200 K or more and 1,500 K or less. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a surface analysis apparatus such as a scanning electron microscope, a transmission electron microscope, an Auger electron spectrometer, an electron beam apparatus for a semiconductor process, or an electron source for an electron beam exposure machine.

In recent years, a Schottky electron emission source using a tungsten single crystal needle electrode has been used to obtain an electron beam having a longer lifetime and higher brightness than a hot cathode. In this electron source, a tungsten single crystal rod having an axial orientation of <100> is provided with a coating layer made of zirconium and oxygen (hereinafter referred to as “ZrO coating layer”), and the ZrO coating layer is used to form a tungsten single crystal { The work function of the 100} plane is lowered from 4.5 eV to about 2.8 eV, and only the minute crystal plane corresponding to the {100} plane formed at the sharp end of the rod becomes the electron emission region. The electron beam with higher brightness than that of the conventional hot cathode can be obtained and has a long life. Further, it is more stable than a cold field emission cathode, operates at a low vacuum, and is easy to use. (Hereinafter referred to as “ZrO / W electron source”.)

As shown in FIG. 1, the ZrO / W electron source has a <100> orientation of tungsten that emits an electron beam to a predetermined position of a tungsten filament 3 provided on a conductive terminal 4 fixed to an insulator 5. A needle-like single crystal rod 1 is fixed by welding or the like. This single crystal rod has a sharp end by electropolishing, and this sharp end becomes an electron emission portion. A part of the single crystal rod 1 is provided with a zirconium and oxygen diffusion source 2 made of zirconium oxide. Although not shown, the surface of the single crystal rod 1 is covered with a ZrO coating layer.

Since the single crystal rod 1 is energized and heated by the filament 3 and is normally used at a temperature of about 1800 K, the ZrO coating layer on the surface of the single crystal rod 1 is consumed by evaporation. However, since zirconium and oxygen are diffused from the diffusion source 2 and continuously supplied to the surface of the single crystal rod 1, the ZrO coating layer is maintained as a result.

In electron beam equipment for semiconductor processes such as a scanning electron microscope, a length measuring SEM, or a review SEM, etc., the ZrO / W electron source is widely used because it has a high brightness and a long life. These apparatuses generally use a low acceleration electron beam of 1 kV or less in order to observe and measure the subject as it is.

In the case of using a low acceleration electron beam, the diameter of the electron beam focused by electrostatic and magnetic lenses is governed by chromatic aberration (see Non-Patent Document 1).
J. Pawley, 'Journal of Microscopy', 136, Pt1, 45 (1984)

In order to reduce this chromatic aberration, it is necessary to reduce the energy width of the electrons emitted from the electron source. Energy width of the Schottky electron emission source will not be less than 2.45K _B at a minimum. Here, k _B is the Boltzmann constant, and T is the absolute temperature of the electron emission region (see Non-Patent Document 2).
R. D. Young, 'Phys. Rev. '113 (1959) p110

Therefore, it is effective to lower the temperature of the electron source in order to reduce chromatic aberration. On the other hand, in the case of Schottky electron emission or thermionic emission, the radiation current decreases drastically when the operating temperature is lowered. For this reason, an electron source having a lower work function must be used to lower the temperature of the electron source. From the above viewpoints, search for adsorption species and diffusion sources on a tungsten single crystal having a low work function in place of the ZrO adsorption layer has been actively conducted in recent years (Non-Patent Documents 3, 4, 5, 6). reference).
Hidetoshi Nishiyama, Takashi Oshima, Hiroyuki Shinada, Applied Physics, Vol. 71, No. 4 (2002), p. 438 H. Nishiyama, T .; Ohshima, H .; Shinada, 'Applied Surface Science' 146 (1999), p382. Yasushi Saito, Keiji Yada, Hiroshi Adachi “Science Technical Report” ED 2001-175 (2001-12), p. 15 Y. Saito, K .; Yada, K .; Minami, H .; Nakane, H .; Adachi J. et al. Vac. Sci. Technol. B22 (6), 2004, p2743

As an adsorption species on a tungsten single crystal having a low work function in place of the ZrO adsorption layer, an example in which BaO is coated as an adsorption layer and operated at a temperature of about 1000 K has already been reported. Barium or (Ba, Sr, Ca) oxide is used (see Non-Patent Document 7 and Patent Document 2).
Hidetoshi Nishiyama, Takashi Oshima, Hiroyuki Shinada, "Applied Physics" Vol. 71, No. 4 (2002), p. 438 JP-A-10-154477

According to these research examples, when (Ba, Sr, Ca) oxide is used and a BaO adsorbing layer is provided on a <100> oriented needle of a tungsten single crystal, it operates at 1000 K after heat treatment at 1500 K or more. Thus, it has been reported that it exhibits desirable characteristics as an electron source.

However, on the other hand, the stable operation time is as short as several hours, and it is stated that it is necessary to repeatedly perform heat treatment of 1500 K or more, and it is considered that it cannot withstand industrial practical use. A case where barium oxide is used as a diffusion source has also been reported. In this case, the electron emission distribution is four-fold symmetric, and the electron dose at the center of the emission axis is small, and the reproducibility is poor. ing.

As a result of intensive studies in view of the above circumstances, the present inventor has solved the above-described problems, operates at a low temperature, has a small energy width, has a low work function, and operates at a high angular current density. The present inventors have found that a source can be provided and have arrived at the present invention.

The present invention consists of a <100> -oriented single crystal rod of tungsten or molybdenum, has an electron emission surface with an exposed {100} crystal plane at one end, and is composed of Ho, Gd, Nd, Sm, and Pr. An electron source having a diffusion source containing at least one metal element oxide selected from the group consisting of:

Furthermore, the present invention is the above-described electron source, wherein the operating temperature is 1200 K or more and 1500 K or less.

Furthermore, the present invention is a scanning electron microscope, a transmission electron microscope, a surface analysis apparatus, an electron beam apparatus for semiconductor processes, or an electron beam exposure apparatus characterized by comprising the above-described electron source.

Since the electron source of the present invention has a work function lower than that of the ZrO / W electron source, it is possible to operate at an operating temperature of 1200 K to 1500 K lower than that of the conventionally known ZrO / W electron source. Since an electron beam having a small energy width is emitted, it contributes to improving the resolution of a scanning electron microscope or an electron beam apparatus for semiconductor processing that operates at a low acceleration voltage and to drawing a finer pattern in an electron beam exposure apparatus.

Since the scanning electron microscope, transmission electron microscope, surface analyzer, semiconductor process electron beam apparatus or electron beam exposure apparatus of the present invention has the electron source having the characteristics described above, As a preferable result, for example, in a transmission electron microscope, when analyzing a light element by energy loss spectroscopy, a low background spectrum is obtained, which contributes to an improvement in sensitivity.

Specific embodiments for carrying out the present invention will be described below.

A <100> single crystal rod of tungsten formed by machining is welded to a tungsten filament for a heater. Both ends of the filament are fixed to two conductive terminals brazed to an insulator, and energization Joule heating can be performed through the conductive terminals. Subsequently, a sharp end having a radius of curvature of submicron to several microns is formed at one end of the single crystal rod by electrolytic polishing using an aqueous solution of sodium hydroxide.

Next, in one part of the single crystal rod, one or more metal element oxides selected from the group consisting of Ho, Gd, Nd, Sm, and Pr are slurried in a mortar using an organic solvent such as isoamyl acetate as a dispersion medium. The mixture is crushed and mixed until it becomes, and dried to form a diffusion source.

This electron source is introduced into a vacuum apparatus, the degree of vacuum is pulled to the 10 ⁻¹⁰ Torr level, and the temperature of the single crystal rod is heated to about 1500 K by energizing through a conductive terminal.

Next, a high electric field is applied to the sharp end of the single crystal rod by applying a negative high voltage to the single crystal rod. By maintaining this state, the {100} crystal plane is exposed at the sharp end, and the work function of only that portion is lower than that of tungsten, and electrons are emitted at a high current density.

The operating temperature of this electron source is 1200 K to 1500 K, which is lower than the operating temperature of 1800 K, which is the operating temperature of the ZrO / W electron source, and emits an electron beam with a low energy width.

As the diffusion source, one or more metal element oxides selected from the group consisting of Ho, Gd, Nd, Sm, and Pr are preferable in the case of a diffusion source made of Pr oxide according to the present inventors' investigation. Electron emission characteristics are shown.

The electron source of the present invention has a diffusion source made of a specific metal element oxide, and can emit electrons at a considerably low temperature as compared with a conventionally known ZrO / W electron source. As a result, the energy width is low. It has the characteristics that an electron beam can provide. For this reason, in the electron beam utilization apparatus using the electron source of the present invention, advantages that have not been obtained in the past can be obtained in each apparatus. For example, the resolution can be improved in a scanning electron microscope, a transmission electron microscope, a surface analyzer, and an electron beam apparatus for semiconductor processing, and a finer pattern can be drawn in an electron beam exposure apparatus.

(Examples 1-5, comparative example)
After fixing the tungsten filament 3 to the conductive terminal 4 brazed to the insulator 5 by spot welding, a <100> oriented single crystal tungsten rod prepared by machining is attached to the filament by spot welding, and Then, one end of the rod was electropolished to obtain a sharp tip having a radius of curvature of about 1 μm (see FIG. 1).

Next, powders of oxides of Ho, Gd, Nd, Sm, and Pr (Ho ₂ O ₃ , Gd ₂ O ₃ , Nd ₂ O ₃ , Sm ₂ O ₃ , Pr ₂ O ₃ ) are dispersed in isoamyl acetate. As a medium, the mixture was pulverized on a mortar to obtain a slurry. The slurry was applied to a part of the rod to form the diffusion source 2.

After the isoamyl acetate in the slurry was evaporated, it was introduced into the apparatus shown in FIG. The tip of the rod 1 is disposed between the suppressor electrode 6 and the extraction electrode 7. The distance between the tip of the needle 1 and the suppressor electrode 6 is 0.25 mm, the distance between the suppressor electrode 6 and the extraction electrode 7 is also 0.6 mm, the hole diameter of the extraction electrode 7 is 0.6 mm, and the hole diameter of the suppressor electrode 6 is 0.4 mm. It is.

Filament 3 is connected to a filament heating power source 14 is further connected to a high voltage power supply 13, a negative high voltage, that is, the emitter voltage V _E is applied to the rod 1 and the filament 3. Further, the suppressor electrode 6 is connected to a bias power source 12, and a negative voltage and a bias voltage V _b are further applied to the rod 1 and the filament 3. Thereby, the radiant thermoelectrons from the filament 3 are blocked. Total emission current I _t from the electron source is measured by the ammeter 15 is placed between the high voltage source 13 and ground. The electron beam 16 emitted from the tip of the rod 1 passes through the hole of the extraction electrode 7 and reaches the fluorescent plate 8. A current I _S (hereinafter referred to as a screen current) reaching the fluorescent plate is measured by an ammeter 17. There is an aperture 9 (small hole) in the center of the total fluorescent plate 8, and the probe current _Ip that has passed through and reached the cup-shaped electrode 10 is measured by the microammeter 11. If the solid angle calculated from the distance between the aperture 9 and the tip of the rod 1 and the inner diameter of the aperture 9 is ω, the angular current density is I _p / ω.

The created electron source is introduced into a vacuum apparatus, the inside of the apparatus is set to an ultra-high vacuum of 3 × 10 ⁻¹⁰ Torr (4 × 10 ⁻⁸ Pa), the filament 3 is energized, the needle 1 is heated to 1500 K, and a diffusion source Is fired. Further, the needle is heated to 1800K for about 1 minute to clean the tip surface, readjusted to 1500K, a bias voltage V _b = −300V is applied to the suppressor, and then the emitter voltage V _E = −6.5 kV. A high voltage was applied and maintained for several hours to several tens of hours.

During this time, the total emission current gradually increased, and when the emitter voltage was lowered to −4.5 kV and electron emission was continued for several hours, only the center on the fluorescent plate 8 was brightened, and the electron emission angle distribution was narrowed around the axis. It was confirmed that it was confined.

Further, the temperature range in which the operation temperature was changed stably was investigated, and the screen current I _S , the angular current density I _p / ω vs. the emitter voltage, that is, the current vs. voltage characteristics were measured. The bias voltage V _b at the time of measuring the current-voltage characteristic was 1/10 of the emitter voltage V _E.

Thereafter, the electron source was taken out from the apparatus, and the tip of the tungsten rod was observed with a scanning electron microscope. As a result, it was confirmed that a {100} crystal plane was formed on the rod tip.

Next, as a comparative example, a commercially available ZrO / W electron source having a single crystal rod with a sharp radius of curvature of 1 μm was introduced into the same apparatus, and the electron emission characteristics were measured in the same manner.

Table 1 shows stable operation regions of the electron source according to the present invention and the electron source of the comparative example. It should be noted that the stable operation region was determined by observing the electron emission distribution pattern only at the center of the fluorescent plate and the probe current _Ip drift being 5% or less per 10 hours. In the ZrO / W electron source which is a comparative example, stable operation was observed between 1650K and 1850K, whereas in the electron source according to the present invention, stable operation was observed between 1200K and 1500K which is lower than the comparative example. It was.

The measurement results of the effective work function regarding the electron source according to Example 1 and the electron source of the comparative example are shown below. The effective work function was determined by the following method.

First, the radius of the {100} crystal plane formed at the tip of the rod is measured by observation with a scanning electron microscope. The crystal plane calculates the current density J by dividing the corresponding screen-current I _s a [pi (radius) ² electron emitting surfaces. By creating a figure of (logarithm of current density J) versus (absolute value of emitter voltage) ^1/2 for each operating temperature and fitting with a straight line, and extrapolating (absolute value of emitter voltage) ^1/2 to zero The current density (saturation current density) corresponding to the zero electric field is calculated. The relationship between the saturation current density J, the operating temperature T, and the work function φ is as follows from the Richardson equation (see Non-Patent Document 8).
J = AT ² exp (−φ / k _B T)
Here, J is the saturation current density, kB is the Boltzmann constant, φ is the work function, and T is the absolute temperature. When the work function when the Richardson constant A is the theoretical value 120 A / cm ² / K ² is the effective work function φ _E ,
φ _E = −k _B Tln (J / 120T ² )
Thus, the effective work function φ _E can be determined from the saturation current density J and the operating temperature T.
S. G. Christov, phys. Stat. Sol. 17, 11, 1966, p22

FIG. 3 shows the temperature dependence of the effective work function of the electron source according to Example 1 of the present invention and the electron source according to the comparative example. It was confirmed that the electron source of Example 1 has a much lower effective work function than that of the comparative example.

Further, FIG. 4 shows the relationship between the angular current density Ip / ω versus the emitter voltage of the electron source according to Example 1 of the invention and the electron source according to the comparative example. The electron source of the present invention showed a higher angular current density than that of the comparative example, and it was confirmed that the electron source had higher performance as an electron source.

Further, when the electron source according to Example 1 of the present invention was mounted on a scanning electron microscope and operated at 1300 K and observed at an acceleration voltage of 1 kV, the resolution was about 30% that of the comparative example. Improved.

The electron source of the present invention operates at 1200 K to 1500 K, which is lower than 1650 K, which is the stable operating temperature of the conventional ZrO / W electron source, and as a result, emits an electron beam with a low energy width. Can do. As a result, it can contribute to improving the resolution of a scanning electron microscope, surface analysis equipment, and electronic equipment for semiconductor processes and miniaturizing the drawing pattern of an electron beam exposure apparatus, which is extremely useful industrially.

The block diagram of an electron source. The block diagram of the evaluation apparatus of electron emission characteristics. The figure which shows the temperature dependence of the effective work function of the electron source which concerns on the Example and comparative example of this invention. The figure which shows the relationship of the angular current density Ip / (omega) vs. emitter voltage of the electron source which concerns on the Example and comparative example of this invention.

Explanation of symbols

1: Needle 2: Diffusion source 3: Filament 4: Conductive terminal 5: Insulator 6: Suppressor electrode 7: Extraction electrode 8: Fluorescent plate 9: Aperture 10: Cup-shaped electrode 11: Microammeter 12 for probe current measurement: Bias power supply 13: High-voltage power supply 14: Filament heating power supply 15: Ammeter for total emission current measurement 16: Emission electron beam 17: Microammeter for screen current measurement

Claims (3)

1 consisting of a <100> oriented single crystal rod of tungsten or molybdenum, having an electron emission surface with an exposed {100} crystal plane at one end, and being selected from the group consisting of Ho, Gd, Nd, Sm, and Pr An electron source comprising a diffusion source containing a metal element oxide of more than one species. 2. The electron source according to claim 1, wherein the operating temperature is 1200 K or more and 1500 K or less. A scanning electron microscope, a transmission electron microscope, a surface analyzer, an electron beam apparatus for semiconductor processes, or an electron beam exposure apparatus, comprising the electron source according to claim 1.

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