JP4032057B2 - Manufacturing method of electron source - Google Patents

Description

The present invention provides an electron source for a surface analysis apparatus such as a scanning electron microscope, a transmission electron microscope, an Auger electron spectrometer, a semiconductor wafer inspection apparatus, or an electron beam exposure machine, particularly an electron beam acceleration voltage of 1 kV. the following scanning electron microscope used in the low acceleration, regarding the CD (critical dimension) SEM, DR (defect review) producing how the electron source used in the semiconductor wafer inspection apparatus such as a SEM.

  In recent years, a Schottky electron emission source (hereinafter referred to as an electron source) using a tungsten single crystal needle-like electrode has been used in order to obtain a brighter electron beam. In this electron source, a tungsten single crystal needle having an axial orientation of <100> is provided with a coating layer composed of zirconium and oxygen (hereinafter referred to as a ZrO coating layer), and the tungsten single crystal (100) is formed by the ZrO coating layer. The work function of the surface is lowered from 4.5 eV to about 2.8 eV. Since only the minute crystal plane corresponding to the (100) plane formed at the tip of the needle of the electron source becomes the electron emission region, an electron beam with higher brightness than the conventional hot cathode can be obtained. It is known that this (100) plane can be developed by applying a negative high voltage to the tungsten single crystal needle, that is, by balancing the electrostatic stress due to a high electric field with the surface tension of the needle tip. . This electron source has a long life, is more stable than a cold field emission cathode, operates at a low vacuum, and is easy to use.

  As shown in FIG. 1, the electron source has a tungsten <100> orientation needle 1 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. It is fixed by welding or the like. A part of the needle 1 is provided with a supply source 2 of zirconium and oxygen. Although not shown, the surface of the needle 1 is covered with a ZrO coating layer.

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

  In a semiconductor inspection apparatus such as a scanning electron microscope or a CD SEM or DR SEM, an electron source in which a zirconium and oxygen coating layer is provided on a single crystal needle 1 of tungsten <100> orientation, a so-called ZrO / W Schottky electron source, Is widely used because of its high brightness and 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.

When a low acceleration electron beam is used, the diameter of the electron beam focused by the lens is governed by chromatic aberration. (For example, refer nonpatent literature 1) In order to reduce this chromatic aberration, it is necessary to make small the energy width of the electron radiated | emitted from an electron source. The energy width of the Schottky electron radiation source is not less than 2.45 kBT. Here, kB is the Boltzmann constant, and T is the absolute temperature of the electron emission region (Non-Patent Document 2). Therefore, it is effective to lower the operating temperature of the electron source to reduce chromatic aberration. On the other hand, in the case of Schottky electron radiation or thermionic radiation, the radiation current decreases drastically when the operating temperature is lowered. For this reason, an electron source with a low work function must be used to lower the operating temperature of the electron source. From the above viewpoint, the search for the adsorbed species and the supply source on the tungsten single crystal having a low work function in place of the ZrO adsorbing layer has been actively carried out in recent years (for example, Non-patent Documents 3, 4, 5 reference).
J. et al. Pawley, Journal of Microscopy, 136, Pt1, 45 (1984). R. D. Young, Phys. Rev. 113 (1959) p110. Hideyama Nishiyama, Taku Oshima, Hiroyuki Shinada Applied Physics Vol. 71, No. 4 (2002) p438. 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) p15.

On the other hand, as an electron source for a cathode ray tube, barium oxide, barium carbonate or barium oxide and calcium oxide, aluminum oxide, etc. are added to tungsten and sintered, and a Ba or BaO adsorption layer is provided on the surface of the tungsten sintered body. Dispenser cathodes, L-type cathodes or impregnated cathodes that lower the work function have been known for a long time (for example, Non-Patent Document 6). It is summarized in.
A. H. W. Beck, The Institution of Electric Engineers Paper No. 2750R Nov. 1958, p372, p378-381.

  Since the impregnated cathode operates at a temperature of about 1000-1300K, the work function is lowered by providing a BaO adsorption layer on the tungsten single crystal using a similar method. Therefore, it can be easily predicted that the electron emission with a low energy width can be performed by operating at a temperature of about 1000-1300K.

Apart from the energy width at the time of low acceleration voltage operation, the operating angular current density is noticed in applications in which throughput is important. In applications such as an electron beam exposure apparatus and DR SEM, high throughput is required. In such applications, the ZrO / W Schottky electron source is operated at a high angular current density of about 0.4 mA / sr, and the higher angle An HfO / W Schottky electron source has been proposed to cope with current density operation (Patent Document 1).
JP 2001-319559 A.

A barium supply source made of barium oxide or (Ba, Sr, Ca) oxide is provided in a part of the tungsten single crystal, and barium is diffused on the surface of the tungsten single crystal to lower the work function to about 1.2 eV. Electron sources that operate at such a low temperature are known. Although this electron source has not been directly verified, it is considered to have a BaO adsorption layer on a tungsten single crystal (see Non-Patent Document 7 or Patent Document 2). According to this research example, when (Ba, Sr, Ca) oxide is used and a BaO adsorption layer is provided on a <100> orientation needle of tungsten single crystal, it operates at 1000 K after heat treatment at 1500 K or more. It has been reported that the electron radiation is confined at a narrow angle along the radiation axis, and exhibits favorable radiation 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. In addition, the case where barium oxide is used as the supply source has also been reported. In this case, however, it is pointed out that the electron emission becomes four-fold symmetric and cannot be confined uniformly along the emission axis, and the reproducibility is poor. ing.
Nishiyama Hidetoshi, Oshima Takashi, Shinada Hiroyuki “Applied Physics” Vol. 71, No. 4 (2002) p438. Japanese Patent Laid-Open No. 10-154477.

Patent Document 3 describes an HfO / W Schottky electron source having an operating angular current density of 1.0 mA / sr and a total radiation current of 350 μA or less. Recently, however, an angular current density operation (3 to 5 mA) is further increased. / Sr) is required. During such extremely high angular current density operation, the total radiation current also increases, so that electrons are emitted to the extraction electrode and the metal diaphragm on the electron beam radiation axis, resulting in significant gas release, and the collision of the electron beam with the gas. Often, the generated ions bombard and damage the electron source or cause an arc discharge to destroy the electron source.
JP 2001-319559 A.

  As a result of various studies in view of the above circumstances, the present inventor has found a supply source suitable for providing a BaO-containing adsorption layer on a single crystal of tungsten or molybdenum, and solved the above problems to arrive at the present invention. Is.

The present invention has the following features.
(1) A positive potential is applied to a needle provided with a barium supply source made of a double oxide of barium oxide and a metal oxide other than barium on a part of a single crystal needle of tungsten or molybdenum, and 1000K to 1700K The manufacturing method of an electron source characterized by heating below.
(2) The method for manufacturing an electron source according to (1), wherein the heating temperature is 1350K or higher and 1650K or lower.

  The present invention relates to a method for manufacturing and using a BaO / W Schottky electron source having a BaO adsorbing layer and capable of stable operation for a long time. Thus, it is possible to operate at 900 to 1450 K, preferably 1000 to 1300 K, capable of low energy width operation without requiring high temperature heat treatment of 1500 K or more over several times once in a few hours. In addition, since the total radiation current is suppressed to 350 μA or less under the operating condition of the angular current density of 4.0 mA / sr, it is possible to remove from the extraction electrode and the metal diaphragm during the high angular current density operation, which has been a problem in the past. Degradation of electron emission characteristics due to outgassing or deterioration of reliability such as damaging the needle tip with an arc can be suppressed, and the throughput of electron beam application equipment such as an electron beam exposure machine and DR-SEM can be improved. It has the characteristics.

  According to the manufacturing method and the usage method of the present invention, it is possible to reduce the probability of needle breakage due to arc discharge during manufacturing and to obtain an effect of obtaining an electron source that emits electrons with less noise.

  The electron source obtained by the present invention is, for example, an electron source (BaO / W emitter) in which a barium and oxygen coating layer is provided on a needle of <100> orientation or <210> orientation of a tungsten or molybdenum single crystal. Further, there is a feature that a double oxide of barium oxide and an oxide of a metal other than barium is provided in a part of the single crystal needle as a barium supply source.

Examples of metal elements other than barium in double oxides of metal oxides other than barium include group IIIA (boron, aluminum, gallium, indium, thallium) and group IVB (titanium, zirconium) of the periodic table (short-period type). , Hafnium), and group IIIB (scandium, yttrium, lanthanoid elements, actinoid elements), one or more metal elements selected from the group are preferred. As the above-mentioned double oxide, in particular, one or more selected from the group consisting of BaAl ₂ O ₄ , BaAl ₁₂ O ₁₉ , Ba ₃ Sc ₄ O ₉ , BaSc ₂ O ₄ , BaTiO ₃ , BaZrO ₃ , BaHfO ₃ Double oxides are preferred.

  Note that a part of barium in the barium oxide can be replaced with a group IIA element other than barium (magnesium, calcium, strontium, etc.). Further, the supply source may contain free barium oxide, carbonate and other metal oxides in addition to the double oxide.

  The electron source of the present invention can be used in a temperature range of 900 to 1450K, preferably 1000 to 1300K.

  Also, the electron source of the present invention operates at a temperature extremely lower than 1800 K, which is the operating temperature of the ZrO / W Schottky electron source, and therefore has a small energy width and a scanning electron microscope or semiconductor wafer inspection apparatus used with a low acceleration electron beam. Can contribute to improving the resolution.

  Furthermore, since the electron source of the present invention has a low total radiation current of 350 μA or less even at an angular current density operation of 4.0 mA / sr or higher, the possibility of destruction of the electron source due to ion damage or arc is low, and high reliability is achieved. Can be achieved.

  The BaO adsorption layer has a large effect of lowering the work function, and the oxide containing barium radiates a large amount of thermionic electrons at a high temperature as is known as an oxide cathode. For this reason, when a cathode having barium oxide as a supply source is heated in vacuum and a negative high voltage is applied to the cathode, not only the surface of the tungsten single crystal needle but also a large amount of thermoelectrons from the supply source made of barium oxide. Radiate. This thermoelectron strikes the anode, induces gas emission, and is further ionized by an electron beam to generate an arc discharge, and the probability of destroying the single crystal needle becomes extremely high. For this reason, it is often possible that a sufficient electric field cannot be applied to the single crystal needle, and the crystal plane cannot be sufficiently developed at the tip of the single crystal needle.

However, according to the manufacturing method of the present invention, since a positive potential is applied to the single crystal needle, no electron emission occurs, and therefore no ionization of the residual gas occurs and no arc discharge occurs. Therefore, a high voltage necessary to develop a sufficient crystal plane at the tip of the single crystal needle can be applied, and the development of the sufficient crystal plane contributes to noise reduction of the radiation current. The applied voltage is 4 × 10 ⁶ V / cm to 2 × 10 ⁷ V / cm at the maximum when converted into electric field strength. Furthermore, since it is accompanied by mass transfer at the tip of the single crystal needle in order to develop the crystal plane, the temperature is preferably high. However, since the supply source rapidly evaporates and disappears, 1700K or less is preferable. On the other hand, if the temperature is low, mass transfer is suppressed and the crystal plane does not develop. Especially, 1350-1650K is especially preferable.

  In this way, a positive potential is applied to a single crystal needle provided with a barium supply source made of barium oxide and a metal oxide other than barium on a part of the single crystal needle of tungsten or molybdenum, and 1000K or more and 1700K. Hereinafter, the heating is preferably performed at 1350 to 1650 K, whereby the frequency of arc discharge is greatly suppressed, and a stable electron source with a high yield can be efficiently manufactured.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention should not be construed as being limited to such examples.

  After fixing a tungsten filament to a conductive terminal brazed to an insulator by spot welding, a needle with a <100> oriented single crystal tungsten fine wire cut is attached to the filament by spot welding, and the tip of the needle is further attached. Electrolytic polishing was performed so that the radius of curvature was about 1 μm to obtain an electron source intermediate.

In addition, as a material constituting the supply source, commercially available barium aluminate (BaAl ₂ O ₄ ) powder,
・ Barium oxide powder,
A mixture of barium oxide, calcium oxide and aluminum oxide powder (molar ratio 5: 3: 2),
Comparison was made using each of the barium oxide, strontium oxide and calcium oxide powder mixtures (molar ratio 5: 4: 1). Each powder was pulverized on a mortar using isoamyl acetate as a dispersion medium to obtain a slurry. The slurry was applied to the needle of the electron source intermediate (position at the center between the position fixed to the filament and the tip of the needle) to preform the supply source.

After the isoamyl acetate in the slurry was evaporated, it was introduced into the apparatus shown in FIG.
The tip of the needle 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 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.

  The filament 3 is connected to a filament heating power source 14 and further connected to a high voltage power source 13 whose polarity can be changed as necessary, and a negative high voltage, that is, an extraction voltage Vex is applied to the extraction electrode 7. Further, the suppressor electrode 6 is connected to a bias power source 12, and a negative voltage and a bias voltage Vb are further applied to the needle 1 and the filament 3. Thereby, the radiant thermoelectrons from the filament 3 are blocked. The total radiation current It from the electron source is measured by an ammeter 15 placed between the high-voltage power supply 13 and the ground. The electron beam 16 emitted from the tip of the needle 1 passes through the hole of the extraction electrode 7 and reaches the fluorescent plate 8. There is an aperture 9 (small hole) in the center of the fluorescent plate 8, and the probe current Ip that has passed through and reached the cup-shaped electrode 10 is measured by a microammeter 11. If the solid angle calculated from the distance between the aperture 9 and the tip of the needle 1 and the inner diameter of the aperture 9 is ω, the angular current density is Ip / ω.

Examples 1 to 4 An electron source using barium aluminate as a supply source was introduced into a vacuum apparatus, and the inside of the apparatus was placed in an ultrahigh vacuum of 3 × 10 ⁻¹⁰ Torr (4 × 10 ⁻⁸ Pa). 3 is energized to heat the needle 1 to 1500K, and the supply source is fired.
Thereafter, the two types of the procedure 1 and the procedure 2 (Examples 1 and 2) and the procedure 1 ′ and the procedure 2 (Examples 3 and 4) were compared.
(Procedure 1)
While maintaining the needle at 1000 to 1600 K, the bias voltage Vb of the suppressor was set to 0 V, and a positive high voltage was applied to the needle for several tens of hours.
(Procedure 1 ')
While maintaining the needle at 1000 to 1600 K, several hundred volts of the bias voltage of the suppressor was applied, and a negative high voltage was applied to the needle and maintained for several tens of hours.
(Procedure 2)
The needle was set to an operating temperature of 1000-1300 K, a bias voltage was applied to the suppressor, and then a negative high voltage was applied to needle 1.

  In Procedure 1, a maximum positive voltage of 6 kV was applied to the needle 1. In the case of processing in the order of procedure 1 and procedure 2, when the bias voltage was 50 V and the extraction voltage of Vex = 3 kV was applied to needle 1 in procedure 2, the radiation current gradually increased in all electron sources.

  In Example 3, the electron radiation beam observed on the fluorescent plate 8 was symmetric four times on the electron emission axis and the probe current on the axis was weak as shown in FIG. The probe current on the axis gradually increased with the increase. (FIG. 3 (b)) After that, when the operating temperature is further lowered to 1100K and the extraction voltage is further lowered to 1kV and the electron emission is continued for 12 hours, the pattern of the electron radiation beam on the fluorescent plate 8 is as shown in FIG. Axisymmetrically and irradiated on the axis. Further, a probe current corresponding to an angular current density of 1 mA / sr was observed, and thereafter stable electron emission could be confirmed over 1500 hours. Further, during this time, the heat treatment at 1500 K or more was performed only once, and it was not necessary to repeat the heat treatment. In addition, the total radiation current at the time of 4 mA / sr operation was about 120 μA from the relationship between the extraction voltage-angular current density and the total radiation current, and it was confirmed that the operation angular current density was 10 mA / sr. (Fig. 4)

  However, in the case where the procedure 1 ′ and the procedure 2 were performed (Examples 3 and 4), the radiation current suddenly increased in the initial stage of high voltage application, and arc discharge frequently occurred and the needle 1 was broken. Only a low negative high voltage could be applied. In rare cases, the electron radiation beam pattern shown in FIG. 3C was obtained, but the crystal plane was not sufficiently developed, and the noise of the probe current was large.

  Thereafter, the electron source was taken out of the apparatus, and the tip of the tungsten needle was observed with a scanning electron microscope. As a result, it was confirmed that a flat (100) crystal plane was formed on the tip of the needle. In particular, the samples processed in the order of the procedure 1 and the procedure 2 (Examples 1 and 2) are those in which the diameter of the crystal plane formed at the tip of the needle 1 is the procedure 1 ′ and the procedure 2 (Examples 3 and 4). It was bigger.

  (Comparative Examples 1 to 12) Next, Table 1 summarizes the results of the electron sources in which the supply source was composed of barium aluminate and other materials. Other materials are easily depleted due to their high vapor pressure, and are not suitable for long-term treatment at high temperatures. On the other hand, barium aluminate is easy to obtain an electron source with favorable characteristics without being depleted by high temperature treatment for a long time.

The results of Examples 1 to 4 and Comparative Examples 1 to 12 are summarized in Table 1. As is apparent from Table 1, the electron source of the present invention has a feature that it operates stably for a long time. In particular, according to the present invention, an electron source with extremely small noise in the probe current can be obtained.

  The electron source obtained by the present invention is a BaO / W Schottky electron source having a BaO adsorbing layer and capable of stable operation for a long period of time, and more than 1500K over several times once in several hours as in the prior art. An operation at 900 to 1450K, preferably 1000 to 1300K is possible without the need for high-temperature heat treatment. Under the operating condition of angular current density of 4.0 mA / sr, the total radiation current is suppressed to 350 μA or less, so that gas is emitted from the extraction electrode and metal diaphragm during high angular current density operation, which has been a problem in the past. It is possible to suppress the deterioration of the reliability due to the deterioration of the electron emission characteristics caused by the damage or the damage to the needle tip by the arc, and the throughput of the electron beam application equipment such as the electron beam exposure machine and DR-SEM can be improved.

  Further, according to the method of use according to the present invention, it is possible to reduce the probability of needle breakage due to arc discharge during production and to perform electron emission with less noise.

Schematic structure diagram of an electron source. The schematic block diagram of the evaluation apparatus of an electron emission characteristic. Angular distribution pattern of electron radiation beam observed on a fluorescent screen Measurement example of extraction voltage-angular current density and extraction voltage-total radiation current

Explanation of symbols

1: Needle 2: Supply 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 12: Bias power supply 13: High-voltage power supply 14: Filament heating power supply 15: Ammeter for total radiation current measurement 16: Radiation electron beam

Claims (2)

  A positive potential is applied to a needle provided with a barium supply source made of a double oxide of barium oxide and a metal oxide other than barium on a part of a single crystal needle of tungsten or molybdenum, and heated at 1000K to 1700K. A method for manufacturing an electron source. 2. The method of manufacturing an electron source according to claim 1, wherein the heating temperature is 1350 K or more and 1650 K or less.

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