Patent Grant
8766542
The present invention relates to a field-emission electron gun and a method for controlling it, and in particular to an electron gun mounting a W<310> field-emission electron gun and a method for controlling it.
If a strong electric field is applied to a metal surface, a potential barrier has an inclination at a boundary to vacuum. If the electric field becomes at least 107 V/cm, the barrier becomes superfine-pointed and electrons are emitted into the vacuum. This is called field emission. Dispersion of energy of emitted electrons becomes small and it is approximately 0.3 eV. An apex of a field-emission electron source is finished to have a radius of curvature of approximately 100 nm in order to generate a strong electric field.
Since the size of the spot diameter of the field-emission electron gun is as small as 5 to 10 nm, the field-emission electron gun has a feature that the brightness is extremely high and it is used often as an electron gun for high resolution SEMs and TEMs. Furthermore, since the energy width of emitted electrons is small, it is easy to obtain a high resolution even at a low acceleration voltage.
On the other hand, since it operates at the room temperature, the emission current is apt to become unstable due to gas adsorption. Ultra-high vacuum is needed. There is a possibility that the cathode surface will become rough due to shocks of residual gas molecules which are ionized by emitted electrons and the cathode will be destroyed finally. As described in Patent Literature 1, therefore, instantaneous heating of the cathode called flashing is conducted sometimes to remove adsorbed gas.
As the field-emission electron source, a needle (tip) of tungsten (W) is usually used. If flashing is conducted on the W electron source at a temperature of at least 1,500 K, the adsorbed layer is evaporated and a clean surface is obtained.
If the W surface is clean, electrons are emitted mainly from a (111) plane and a (310) plane which are relatively low in work function among all planes as indicated by a field emission pattern shown in
The present inventor has found features in the way of decrease of the emission current from the W(310) plane. An object of the present invention is to disclose a method for using a field-emission electron source more stably by using this knowledge.
The present invention provides a charged particle beam apparatus including a field-emission electron source having <310> single crystal of tungsten, a vacuum chamber having the electron source disposed therein, an exhausting system for exhausting the vacuum chamber, a filament connected to the electron source and let flow a current to heat the electron source, a power supply for letting a current flow through the filament, an ammeter for measuring a total current emitted from the electron source, and a controlling unit for exercising control to cause the power supply to let a current flow through the filament when the total current measured periodically has become a predetermined ratio or less as compared with a total current from the electron source obtained immediately after first electron beam emission, or a total current from the electron source obtained immediately after a current is let flow through the filament.
Furthermore, the exhausting system keeps a pressure in the vacuum chamber at 10−9 Pa level or less.
According to the present invention, it is possible to make the most of the original high brightness of the W<310> field-emission electron source effectively.
Prior to description of embodiments, a principle of the present invention will now be described.
If residual gas (for example, hydrogen) is adsorbed to a clean surface of a field-emission electron source, the work function increases and the emission current decreases. The decrease time τ is inversely proportional to pressure P, and τ·P is constant (Non Patent Literature 1). Therefore, the pressure change around the electron source can be known by measuring the change of the decrease time τ.
In an electron microscope, only a very small part of a probe current (Ip) which gets out from a plane of a tip apex is used out of a total emission current (Ie) of an electron beam emitted from an electron source as shown in
Hereafter, how the emission current decreases in an experiment in the case of the W<310> field-emission electron source will be described.
If flashing cleaning is conducted in the case of the W<310> field-emission electron source, then decrease of the emission current obtained immediately after the flashing becomes as shown in
The current Ip decreases very slowly until approximately 90% of the initial current is reached. Thereafter, the decrease becomes fast and the current falls abruptly. This area of falling is designated as decrease area III.
On the other hand, as for Ie, the decrease speed changes in the order of rapid (area I), moderate (area II) and rapid (area III) as shown in
Different ways of decrease of Ie and Ip are considered to be caused by the fact that the change of the work function of the tungsten surface due to hydrogen adsorption differs from plane to plane. Since the work function of the (111) plane increases from the initial value due to hydrogen adsorption, the emission current from the (111) plane continues to decrease as far as several % of the initial value. On the other hand, the work function of the (310) plane does not increase in the initial hydrogen adsorption. Denoting time when the emission current decreases to 90% of the initial value by τ90, rapid decrease of the emission current from the (310) plane begins after the time τ90 in average (Ip curve in
Therefore, it is appreciated that the fast initial decrease (area I) of Ie reflects the fast initial decrease of I(111). On the other hand, the reason why the decrease of Ie becomes fast in the last stage of decrease (area III) is that Ie and Ip decrease rapidly at the same time, i.e., the decrease speed of I(310) increases abruptly. This is indicated by the following ground. If only the (310) plane which is the apex plane of the W<310> field-emission electron source is caused to adsorb carbon to prevent Ip from decreasing, Ie does not decrease in the area III which reflects the decrease of the emission current from the (310) plane as shown in
It is appreciated from the above-described experimental results that the W<310> field-emission electron source has a feature that the probe current Ip does not decrease in the areas I and II.
Relations between the degree of vacuum of the electron gun and the emission current will now be described.
In the conventional electron gun having a pressure of the level of 10−8 Pa, the emission current decreases completely as shown in
Even if Ie is increased to its initial value, however, the probe current Ip increases only to a current which is a half or less of the initial value before the decrease. This means that the brightness of the electron beam from the tip apex (310) plane used for image observation falls to a half or less after the decrease. Emission angle current density (probe current per unit solid angle) I′ data measured while changing the total emission current Ie also shows that I′ becomes half or less after the decrease as compared with that before decrease. If high brightness is needed when observing an image by using an electron microscope mounting the conventional field-emission electron source of the 10−8 Pa level, observation is conducted for approximately 15 minutes before Ip from the (310) plane decreases (τ90).
On the other hand, it has been found that τ90 increases to a little less than ten times and the stable image observation time before the decrease lengthens to several hours as shown in
If flashing is conducted in accordance with the time when rapid decrease begins by using these kinds of knowledge, it becomes possible to observe for a long time with a very large emission current (high brightness). In addition, if the pressure is increased to the 10−9 Pa level, the flashing interval becomes long and an electron microscope which is excellent in the practical use as well is obtained.
As for the timing of the flashing, an ammeter for measuring the actual Ip is provided, timing (τ90) when Ip has reached 90% of the initial value which is time when Ip decreases rapidly is monitored, and a controlling unit controls flashing at that time point.
On the other hand, in the case where gas molecules other than hydrogen remain on the surface of the electron source, or in the case where a peripheral electrode such as the anode is contaminated, it is more desirable to use a different scheme in some cases.
In the case where the W<310> field-emission electron source is used as the electron source of an electron microscope, a high brightness image observation can always be conducted by conducting the flashing in accordance with τf, where τf is time when rapid decrease of the probe current Ip begins (in
a) shows an decrease curve in the case where the surface of the electron source is clean, and rapid decrease begins in the vicinity of τ90. On the other hand, in the case where gas molecules other than hydrogen remain on the surface of the electron source, or in the case where a peripheral electrode such as the anode is contaminated, initial decrease of the emission current becomes relatively earlier as shown in
Since the contamination of the electron source surface and peripheral parts is possible, a method different from a method of monitoring Ip and looking for τ90 also becomes necessary. Hereafter, the method will be described.
Since the total emission current Ie is emitted from the whole surface of the electron source, noise is small as compared with the probe current Ip which gets out from a part of the apex of the electron source and deviations in the initial surface clean condition and the decrease curve caused by gas emission of peripheral parts are also small. By comparing decrease curves of Ie and Ip with each other, it has been found that time τf when rapid decrease of Ip begins and 50% decrease time τe50 of Ie nearly coincide with each other regardless of the pressure as shown in
Pressure around the electron source will now be described.
For prolonging the high brightness observation time by at least several hours, degasing by heating components in a vacuum furnace having a pressure of the 10−3 Pa level or less and 400° C. or above and electrolytic polishing are executed, a W<310> single crystal field-emission electron source is mounted on an electron gun with a gas emission amount reduced in this way, and the pressure around the electron source is kept at the 10−9 Pa level or less by using an exhausting system capable of lowering the pressure to the 10−9 Pa level or less.
Hereafter, a concrete form for implementing the present invention described above will be described in detail with reference to the drawings.
In
The flashing is conducted by letting a current flow through the filament 6 for a determinate time and raising the temperature of the W<310> field-emission electron source 1 to, for example, 1500 K or above as a result of heating. The time period for which the current is let flow is approximately several seconds at its maximum, and the current is let flow a plurality of times depending upon the surface state. If flashing is conducted in accordance with the time τf when rapid decrease of Ip begins as shown in
The timing of the flashing is determined according to the procedure described hereafter. The flashing power supply 7 is controlled to conduct flashing in accordance with the parameters such as the current, voltage, power, and time which are input by using the operating unit 5. If the surface of the W<310> field-emission electron source 1 becomes clean owing to the flashing, then the electron source controlling unit 4 controls an extraction power supply 9, applies a positive extraction voltage to an extraction electrode 3, and causes an electron beam having a total emission current value Ie0 determined by the operating unit 5 to be emitted from the W<310> field-emission electron source 1. Immediately thereafter, the electron source controlling unit 4 stores the current value Ie0 measured by the ammeter 8 in a memory. While the electron beam is being emitted from the W<310> field-emission electron source 1, the ammeter 8 measures the total emission current measured values Ie at intervals of several seconds and sends the measured values to the electron source controlling unit 4. The electron source controlling unit 4 compares the total emission current measured values Ie which are sent with the initial value Ie0 stored in the memory. If the total emission current measured value Ie becomes a determined ratio in the vicinity of 50% (for example, a determined value in the range of 30% to 70%) of the initial value R×Ie0 or less, flashing is conducted to prevent the probe current from decreasing.
As for the timing of the flashing, the flashing is conducted automatically at time when the electron source controlling unit 4 has detected that the relation Ie≦R×Ie0 is satisfied. However, the electron source controlling unit 4 monitors the operation state of the electron microscope, and exercises control to prevent the flashing from being conducted while acquiring an electron microscope image such as a secondary electron image, a reflected electron image, an EDX mapping image, or an EELS mapping image, or spectrum of EDX, WDX, or EELS and conduct flashing a predetermined time after the image or spectrum is acquired.
In another method, the electron source control unit 4 starts monitoring of the operation state of the electron microscope when it has detected that the relation Ie≦R×Ie0 is satisfied. When an operator of the electron microscope has interrupted the electron microscope image observation, flashing is conducted automatically. For example, timing of sample interchange is detected, and flashing is conducted automatically during the sample interchange.
In another method, timing of a valve operation for intercepting an electron beam path between the W<310> field-emission electron source 1 and the sample is detected. Flashing is conducted automatically while the valve intercepts the electron beam path between the W<310> field-emission electron source 1 and the sample.
In another method, the operating situation, such as the focus adjustment, visual field movement, or magnification adjustment, of the electron microscope operator is monitored. If the electron microscope operating is not conducted for some determinate time, flashing is conducted automatically.
In the case where the electron source controlling unit 4 starts monitoring of the operation state of the electron microscope when it has detected that the relation Ie≦R×Ie0 is satisfied and the electron microscope operator does not interrupt the electron microscope image observation, however, the electron source controlling unit 4 may exercise control to cause flashing to be conducted automatically. Or, for example, R′ satisfying the relation R′<R may be predetermined and the electron source controlling unit 4 may exercise control to cause flashing to be conducted automatically at time when a relation Ie≦R′×Ie0 is satisfied.
The electron source controlling unit 4 may execute an instruction for displaying a message to urge to conduct flashing on an observation monitor of the operator at time when the relation Ie≦R×Ie0 is satisfied. The operator executes flashing manually by pressing a flashing start button disposed on the operating unit.
A measurement method of the total emission current value Ie using a micro-ammeter will now be described. In the case where an acceleration voltage V0 is low and a leak current from a high voltage side including the W<310> field-emission electron source 1, the filament 6, the extraction electrode 3, and an acceleration power supply 10 is slight, the micro-ammeter may be disposed in series with either of an interconnection on the W<310> field-emission electron source 1 side and an interconnection on the extraction electrode 3 side as shown in
On the other hand, in the case where a high negative acceleration voltage V0 is applied, a leak current occurs on the high voltage side in some cases as shown in
As shown in
If a superfine-pointed extraction electrode 15 fabricated by using a thin wire or the like to be hardly struck by the electron beam is adopted and a Faraday cup 23 is provided on a ground side struck by the electron beam as shown in
For using the electron gun mounting the W<310> field-emission electron source 1 during a time period between immediately after the flashing and before the decrease (the areas I and II in
All parts and a vacuum vessel in the vacuum chamber 2 of ultra-high vacuum having the W<310> field-emission electron source 1 disposed therein are fabricated by using materials which can be heated to at least 300° C. and degassed in a vacuum furnace which maintains a pressure of the 10−3 Pa level or less, for at least one hour. If metal parts are subject to electrolytic polishing before degassing, the gas emission amount is further decreased. However, it is not indispensable.
As an exhausting system of the vacuum chamber 2 of ultra-high vacuum having the W<310> field-emission electron source 1 disposed therein, a non-evaporable getter pump 16 and an ion pump 17 having an exhausting speed of at least 1 l/s are used jointly as shown in
In another method, a titanium sublimation pump 18 and an ion pump 17 are used jointly as an exhausting system of the vacuum chamber 2 of ultra-high vacuum having the W<310> field-emission electron source 1 disposed therein as shown in
In another method, a cryo pump 19 is used as an exhausting system of the vacuum chamber 2 of ultra-high vacuum having the W<310> field-emission electron source 1 disposed therein as shown in
In still another method, as an exhausting system of the vacuum chamber 2 of ultra-high vacuum having the W<310> field-emission electron source 1 disposed therein, the vacuum chamber 2 of ultra-high vacuum is exhausted by a first stage turbomolecular pump 20 having an exhausting speed of at least 100 l/s as shown in
If the pressure around the W<310> field-emission electron source 1 is brought to the 10−9 Pa level or less by using a technique as described above, it is possible to prolong the time in the areas I and II and the original high brightness of the W<310> field-emission electron source can be utilized effectively.
1: W<310> field-emission electron source, 2: vacuum chamber, 3: extraction electrode, 4: electron source controlling unit, 5: operating unit, 6: filament, 7: flashing power supply, 8: ammeter, 9: extraction power supply, 10: acceleration power supply, 11: exhausting system, 12: ammeter A1, 13: ammeter A2, 14: ammeter A3, 15: superfine-pointed extraction electrode, 16: non-evaporable getter pump (NEG), 17: ion pump, 18: titanium sublimation pump, 19: cryo pump, 20: first stage turbomolecular pump, 21: second stage turbomolecular pump, 22: auxiliary pump, 23: Faraday cup.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-033048 | Feb 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/000233 | 1/19/2011 | WO | 00 | 9/26/2012 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2011/102077 | 8/25/2011 | WO | A |
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