Patent Application
20230230794
The disclosure in the present application relates to an electron gun, an electron beam applicator, and an emission method of an electron beam.
Electron guns equipped with a photocathode and electron beam applicators such as an electron microscope, a free electron laser accelerator, an inspection device, or the like including such an electron gun (hereafter, a device from which an electron gun is removed from electron beam applicators therefrom may be referred to as a “counterpart device”) are known. For example, Patent Literature 1 discloses an electron microscope device with a photocathode that emits an electron beam in response to being irradiated with excitation light from a light source.
In electron beam applicators such as an electron microscope device, it is required to stably maintain emission of an electron beam. In a photocathode, however, continued light irradiation causes deterioration of electron emission characteristics and a decrease in the number of emitted electrons. Thus, in an electron beam source with such a photocathode, the intensity of an electron beam decreases over operation time. Thus, Patent Literature 1 discloses increasing the intensity of excitation light and applying deposition of Cs to recover the intensity of the electron beam.
Further, an electron gun that measures the intensity of an electron beam emitted from a photocathode by using a measuring electron beam blocked by an electron beam shield member and, in accordance with a measurement result, adjusts the intensity of the electron beam emitted from the photocathode is also known (see Patent Literature 2).
Patent Literature 2: Japanese patent No. 6578529
As disclosed in Patent Literatures 1 and 2, it is known to recover the intensity of an electron beam by increasing the intensity of excitation light or applying redeposition of Cs when the function of a photocathode deteriorates due to continued use of the photocathode. To improve the operation rate of an electron beam applicator to which an electron gun with a photocathode is mounted, however, a lower frequency of redeposition of Cs, in other words, a longer lifetime of the photocathode is preferable. However, there is no known method for extending the lifetime of a photocathode for an electron gun with a photocathode.
The present inventors have made an intensive study and newly found that it is possible to extend the lifetime of a photocathode by causing emission of an electron beam while heating the photocathode.
Accordingly, an object of the disclosure of the present application is to provide an electron gun that can extend the lifetime of a photocathode, an electron beam applicator to which the electron gun is mounted, and an emission method of an electron beam. Other optional, additional advantageous effects of the disclosure in the present application will be apparent in embodiments of the invention.
(1) An electron gun comprising:
It is possible to extend the lifetime of a photocathode by causing emission of an electron beam while heating the photocathode.
An electron gun, an electron beam applicator, and an emission method of an electron beam will be described below in detail with reference to the drawings. Note that, in the present specification, members having the same type of functions are labeled with the same or similar references. Further, duplicated description for the members labeled with the same or similar references may be omitted.
Further, the position, size, range, or the like of respective components illustrated in the drawings may be depicted differently from the actual position, size, range, or the like for easier understanding. Thus, the disclosure in the present application is not necessarily limited to the position, size, range, or the like disclosed in the drawings.
An electron gun 1A according to the first embodiment will be described with reference to
The electron gun 1A according to the first embodiment includes at least a photocathode 2, a light source 3, an anode 4, the heater device 5, and an output adjustment device 6.
In the example illustrated in
The photocathode film 22 is not particularly limited as long as it can emit an electron beam by being irradiated with excitation light, and the material thereof may be a material requiring EA surface treatment, a material not requiring EA surface treatment, or the like.
Further,
The light source 3 is not particularly limited as long as it can irradiate the photocathode 2 with the excitation light L to cause emission of an electron beam B. The light source 3 may be, for example, a high power (watt class), high frequency (several hundred MHz), ultrashort pulse laser light source, a relatively inexpensive laser diode, an LED, or the like. The excitation light L for irradiation can be either pulse light or continuous light and can be adjusted as appropriate in accordance with the purpose. Note that, although the light source 3 is arranged outside the vacuum chamber CB and the excitation light L is emitted from the second face 21b side of the photocathode 2 in the example illustrated in
The anode 4 is not particularly limited as long as it can generate an electric field together with the cathode 3, and an anode generally used in the field of electron guns can be used.
In the example illustrated in
There is no particular restriction on the arrangement of the power supply as long as the electron beam B can be emitted from the cathode 2 to the anode 4. In the example illustrated in
The heater device 5 is used for heating the photocathode film 22 and/or the substrate 21. As illustrated in Examples described later, in irradiation of the photocathode film 22 with the excitation light L for emission of the electron beam B, the irradiation of the excitation light L with the photocathode 2 (the photocathode film 22) being heated extends the lifetime of the photocathode 2, in other words, slows the deterioration rate of the photocathode film 22.
The heater device 5 is not particularly limited as long as it can heat the photocathode film 22 during emission of the electron beam B. Further, the heater device 5 may directly heat the photocathode film 22 or may indirectly heat the photocathode film 22 by directly heating the substrate 21. Furthermore, the heater device 5 may heat the photocathode film 22 in a more indirect manner by indirectly heating the substrate 21.
In the example illustrated in
Note that, in the example illustrated in
When the photocathode film 22 has deteriorated, the photocathode 2 is heated to restore the photocathode film 22. Note that, in the present specification, restoration of the photocathode film 22 means (1) to clean the EA surface 22b by heating and retreat the same (reform an EA surface) with a surface treatment material described later when using the photocathode film base material 22a requiring EA surface treatment and (2) to remove contaminants deposited to the surface of the photocathode film 22c by heating when using the photocathode film 22c not requiring EA surface treatment. In the electron gun 1A according to the first embodiment, the heater device 5 can be used both for restoration of the photocathode film 22 and for extension of the lifetime of the photocathode 2.
The temperature in restoring the photocathode film 22 will be significantly elevated, though differing in accordance with the composition (crystal structure) of the photocathode film 22. However, an excessively high heating temperature of the photocathode 2 (the photocathode film 22) when emitting the electron beam B may destroy the photocathode film 22. Therefore, the temperature for heating the photocathode film 22 and/or the substrate 21 during emission of the electron beam B will be lower than the temperature in restoring the photocathode film 22.
Thus, the electron gun 1A according to the first embodiment includes the output adjustment device 6 for adjusting the heating temperature applied by the heater device 5 so that the photocathode 2 can be heated at different temperatures. For example, when the heater device 5 is an electrical resistance heater, the output adjustment device 6 can control the value of current output to the electrical resistance heater. In the case of a high-frequency heater device, the output adjustment device 6 can control alternating current output to the coil of the high-frequency heater device.
The lower limit of the heating temperature of the photocathode film 22 during emission of the electron beam B may be any temperature that is higher than room temperature, such as, for example, 50° C. or higher, 75° C. or higher, 100° C. or higher, 120° C. or higher, 140° C. or higher, 160° C. or higher, 180° C. or higher, or the like, and may be adjusted as appropriate in accordance with the material forming the photocathode film 22. Further, the upper limit of the heating temperature of the photocathode film 22 during emission of the electron beam B may be any temperature at which the photocathode film 22 is not destroyed due to heating, in other words, may be any temperature that is lower than the temperature in restoration of the photocathode film 22. Although differing due to the composition and the crystal structure of the photocathode film 22, this temperature may be, for example, 600° C. or less, 550° C. or less, 500° C. or less, 450° C. or less, 300° C. or less, 250° C. or less, 200° C. or less, or the like.
The output adjustment device 6 can adjust the output of the heater device 5 that directly or indirectly heats the photocathode film 22 so that the photocathode film 22 (the photocathode 2) is within the temperature range described above. As described above, the heater device 5 can switch the output between at least two temperatures, namely, the temperature for restoring the photocathode film 22 and the temperature suitable for emission of the electron beam B. Therefore, it can be said that the output adjustment device 6 is a device that can supply at least two types of output having different power to the heater device 5. Although illustration is omitted in
Further, as for the electron gun 1A according to the first embodiment illustrated in
There is no particular restriction on the material of the housing container 9, and the housing container 9 can be formed of a heat resistant material that can withstand heat of 300° C. or higher, more preferably 400° C., such as glass, molybdenum, ceramic, sapphire, titanium, tungsten, tantalum, or the like, for example.
The surface treatment material 91 arranged inside the housing container 9 is not particularly limited as long as it is a material that enables EA surface treatment. The element forming the surface treatment material 91 may be, for example, Li, Na, K, Rb, Cs, Te, Sb, or the like. Note that Li, Na, K, Rb, and Cs out of the above elements will spontaneously ignite by itself and are unable to be stored and utilized. Thus, Li, Na, K, Rb, and Cs are required to be used in a form of a composite element of the above element or a compound containing the above element. On the other hand, when the element is used in a form of a compound, it is required to prevent an impurity gas from emitting during deposition of the element. Therefore, when the element selected from Li, Na, K, Rb, and Cs is used as the surface treatment material 91, it is preferable to combine and use a compound such as Cs2CrO4, Rb2CrO4, Na2CrO4, or K2CrO4 and a reducing agent that suppresses emission of an impurity gas. The surface treatment material 91 is vaporized inside the housing container 9 by using a heating unit and deposited on the photocathode 2.
In the example illustrated in
In the example illustrated in
In the example illustrated in
Further, in the examples illustrated in
An electron gun 1B according to the second embodiment will be described with reference to
The electron gun 1B according to the second embodiment differs from the electron gun 1A according to the first embodiment in that a heater device 52 arranged outside the vacuum chamber CB and configured to heat the photocathode film 22 and/or the substrate 21 from outside of the vacuum chamber CB is provided and in that the heater device 5 and the output adjustment device 6 of the electron gun 1A according to the first embodiment are optional, additional features, and other features are the same as those in the first embodiment. Therefore, in the second embodiment, features different from those in the first embodiment will be mainly described, and duplicated description for the features that have already been described in the first embodiment will be omitted. Thus, it is apparent that, even when not explicitly described in the second embodiment, any feature that has already been described in the first embodiment can be employed in the second embodiment.
The heater device 52 provided to the electron gun 1B according to the second embodiment heats the photocathode film 22 and/or the substrate 21 from outside of the vacuum chamber CB. Thus, the heater device 52 is not particularly limited as long as it is a heater device that can heat a distant heating target, and may be a lamp heater, a laser heater, a high-frequency heater device, an electrical resistance heater, or the like.
In the electron gun 1B according to the second embodiment, the heater device 52 is not used for the purpose of restoring the photocathode 2 but used for the purpose of extending the lifetime of the photocathode 2. Therefore, since the heater device 52 may be driven by one type of output that has been set in advance, the output adjustment device 6 is not essential in the electron gun 1B according to the second embodiment. However, this does not mean exclusion of heating the photocathode film 22 and/or the substrate 21 at two or more different temperatures for the purpose of extending the lifetime of the photocathode 2. Therefore, the electron gun 1B according to the second embodiment may optionally, additionally include the output adjustment device 6 that adjusts the output of the heater device 52. When the heater device 52 is a lamp heater, a laser heater, or the like, the output adjustment device 6 can adjust the value of current for adjusting the power of the lamp heater, the laser heater, or the like. When the heater device 52 is an electrical resistance heater, the output adjustment device 6 can control the value of current output to the electrical resistance heater. When the heater device 52 is a high-frequency heater device, the output adjustment device 6 can control alternating current output to the coil of the high-frequency heater device. Further, in the case of the electron gun 1B according to the second embodiment, by using a lamp heater, a laser heater, an electrical resistance heater, a high-frequency heater device, or the like, the photocathode film 22 may be directly heated from outside of the vacuum chamber CB, or the substrate 21 may be heated and thereby the photocathode film 22 may be heated via the substrate 21.
Note that, in the electron gun 1B according to the second embodiment, the heater device 5 for restoration of the photocathode 2 is not essential as a solution to achieve the object of the present application. However, the electron gun 1B according to the second embodiment may optionally, additionally include the heater device 5.
The case of the electron gun 1A according to the first embodiment achieves an advantageous effect that the heater device 5 can be shared for restoration of the photocathode 2 and for extension of the lifetime of the photocathode 2. On the other hand, in the case of the electron gun 1B according to the second embodiment, the heater device 52 that is more frequently used than the heater device 5 for restoration of the photocathode 2 can be arranged outside the vacuum chamber CB. Therefore, the electron gun 1B according to the second embodiment achieves an advantageous effect that the heater device 52 can be replaced without the vacuum chamber CB being opened even when the heater device 52 fails.
An electron gun 1C according to the third embodiment will be described with reference to
The electron gun 1C according to the third embodiment differs from the electron gun 1A according to the first embodiment and the electron gun 1B according to the second embodiment in that a measuring unit 11 that measures a change in the intensity of the electron beam B emitted from the photocathode 2 due to deterioration of the photocathode and a control unit 12 that controls the output adjustment device 6 in accordance with a measurement result from the measuring unit 11 are provided, and other features are the same as those in the above embodiments. Therefore, in the third embodiment, features different from those in the first embodiment and the second embodiment will be mainly described, and duplicated description for the features that have already been described in the first embodiment and the second embodiment will be omitted. Thus, it is apparent that, even when not explicitly described in the third embodiment, any feature that has already been described in the first embodiment and the second embodiment can be employed in the third embodiment.
In the example illustrated in
The electron beam shield member 11a includes a hole 11a1 through which a part of the electron beam B emitted from the photocathode 2 passes. The width of the hole 11a1 is smaller than the width of the electron beam B. As illustrated in
The material of the electron beam shield member 11a is not particularly limited as long as it is a conductor or a semiconductor. For example, in a case of a conductor, the material may be a metal such as stainless steel (SUS), copper, or the like.
The measuring instrument 11b measures the intensity of the electron beam B by using the measuring electron beam that is a part of the electron beam B blocked by the electron beam shield member 11a. The measuring instrument 11b is not particularly limited as long as it can measure the intensity of the electron beam B. For example, when the electron beam shield member 11a is a conductor, a measuring electron beam causes current to occur between the electron beam shield member 11a and the measuring instrument 11b. Thus, the intensity of the electron beam B can be measured as a value of current at the measuring instrument 11b. Note that the value of current can be measured by using a known ammeter. Further, since the measured value of current depends on the intensity of the electron beam B, monitoring of a change in the value of current enables monitoring of a change in the intensity of the electron beam B. Further, it is also possible to use a semiconductor as the electron beam shield member 11a and measure the value of current occurring in response to the measuring electron beam hitting the semiconductor.
Further, the measuring instrument 11b may measure the intensity of the electron beam B by a fluorescent intensity instead of a value of current. More specifically, a conductor to which a fluorescent material has been applied in advance may be used as the electron beam shield member 11a, and the fluorescent intensity of light emitted in response to a measuring electron beam hitting the fluorescent material may be measured by the measuring instrument 11b. Note that a fluorescent intensity can be measured by using a known fluorophotometer.
As disclosed in Patent Literature 2, it is known to use the electron beam shield member 11a to measure a change in the intensity of the electron beam B emitted from the photocathode 2 due to deterioration of the photocathode 2, adjust the intensity of the light source 3 or the like in accordance with a measurement result from the measuring unit 11, and thereby adjust intensity of the electron beam B emitted from the photocathode 2. In contrast, the control performed by the electron gun 1C according to the third embodiment differs from the control disclosed in Patent Literature 2 in that the control unit 12 controls the output adjustment device 6 in accordance with a measurement result from the measuring unit 11 and thereby adjusts the temperature of the photocathode 2.
Note that, although
Further, although illustration is omitted, the measuring unit 11 may be formed of a member such as an aperture that is provided to the electron gun 1 or the counterpart device E and blocks the electron beam B and the ammeter (measuring instrument) 11b in combination. Alternatively, since there is a correlation between the light amount of the light source 3 and the intensity of the electron beam B, the measuring unit 11 may be formed of a measuring instrument that measures leak current from the electron gun 1C (or the counterpart device E) and a calculation unit that calculates a change in the intensity of the electron beam B in accordance with the ratio of leak current relative to a light amount. Further alternatively, the measuring unit 11 may be formed of an ammeter that measures current supplied to the photocathode from an acceleration power supply accelerating the electron beam B.
The electron gun 1C according to the third embodiment includes the control unit 12, and this enables the following control presented as an example, for example.
(1) Heating of the photocathode 2 by the heater device 5 may be performed by starting heating the photocathode 2 in accordance with the measurement result from the measuring unit 11 without heating the photocathode 2 when starting emission of the electron beam B. In such a case, the load and running cost of the heater device 5 can be reduced.
(2) The heating temperature of the photocathode 2 is changed in accordance with the measurement result of the measuring unit 11.
Further, the control unit 12 of the electron gun 1C according to the third embodiment may adjust the intensity of the electron beam B emitted from the photocathode 2 as needed in addition to the control of the output adjustment device 6. In such a case, for example, the following control presented as an example is possible.
(1) When the intensity of the electron beam B decreases, the light source 3 or the like may be adjusted first to adjust the intensity of the electron beam B, and after further adjustment of the light source 3 or the like becomes difficult, heating of the photocathode 2 by the heater device 5 may be performed. In such a case, the load and running cost of the heater device 5 can be reduced.
(2) The control of the light source 3 or the like and the heating control by the heater device 5 may be performed in combination.
Note that adjustment of the intensity of an electron beam is disclosed in Patent Literature 2. The content disclosed in Patent Literature 2 (Japanese Patent No. 6578529) is incorporated in the present specification.
The electron beam applicator E to which the electron gun 1 is mounted may be a known device to which the electron gun 1 is mounted. For example, the electron beam applicator E may be a free electron laser accelerator, an electron microscope, an electron holography device, an electron beam drawing device, an electron diffractometer, an electron beam inspection device, an electron beam metal additive manufacturing device, an electron beam lithography device, an electron beam processing device, an electron beam curing device, an electron beam sterilization device, an electron beam disinfection device, a plasma generation device, an atomic element generation device, a spin-polarized electron beam generation device, a cathodoluminescence device, an inverse photoemission spectroscopy device, or the like.
An emission method of an electron beam includes, with use of the electron gun 1 or the electron beam applicator according to any of the first to third embodiments, an electron beam emission step of irradiating the photocathode 2 with the excitation light L from the light source 3 while heating the photocathode film 22 and/or the substrate 21 and emitting the electron beam B from the photocathode 2 in response to receiving the excitation light L.
The emission method of an electron beam may perform a photocathode temperature adjustment step of adjusting the heating temperature of the photocathode film 22 and/or the substrate 21 as needed. The photocathode temperature adjustment step can switch the output of the heater device 5 by using the output adjustment device 6. The output to the heater device 5 from the output adjustment device 6 may be switched by an operator at any timing. Alternatively, the output to the heater device 5 from the output adjustment device 6 may be switched in accordance with a measurement result from the measuring unit 11 that measures a change in the intensity of the electron beam B emitted from the photocathode 2 due to the deterioration of the photocathode 2.
While Examples are presented below to specifically describe the embodiment disclosed in the present application, these Examples are only for the purpose of illustration of the embodiment. The Examples neither limit the scope of the invention disclosed in the present application nor express restriction.
Example 1
A laser light source (iBeamSmart by Toptica) was used for the light source 3. An InGaN photocathode was fabricated as the photocathode 2 by a known method disclosed in Daiki SATO et al. 2016 Jpn. J. Appl. Phys. 55 05FH05. EA treatment on the photocathode surface was performed by a known method. The electron beam shield member 11a was fabricated by using stainless steel. Further, an electrical resistance heater made of tantalum was used for the heater device 5 and arranged so that the heater device 5 is in contact with the substrate 21. Further, the temperature of the photocathode 2 when a predetermined amount of current is conducted to the heater device 5 was measured in advance to obtain the correlation between the temperature and a value of current required when the photocathode 2 is adjusted at a desired temperature.
The photocathode 2 was caused to emit the electron beam B by irradiating the photocathode 2 with excitation light from the light source 3 and applying an acceleration voltage of 30 kV between the photocathode 2 and the anode 4. The value of current of a measuring electron beam acquired by the electron beam shield member 11a was then acquired by measuring and converting the voltage between both ends of a shunt resistor of 100 kΩ by using a data logger (MW100 by Yokogawa Electric Corporation). The measurement was performed every 1 second. The value of current measured by the measuring unit 11 was set to be approximately 2×1031 5 A, and the result of the value of current measured by the measuring unit 11 was fed back to the light source 3 every 5 seconds. Further, current was conducted to the heater device 5 for heating so that the temperature of the photocathode 2 becomes about 100° C. at the same time as irradiation of the excitation light L. The measurement result is illustrated in
An experiment was made by the same procedure as in Example 1 except that the heater device 5 was not heated and the photocathode 2 was irradiated with the excitation light L at room temperature (about 27° C.). The measurement result is illustrated in
As illustrated in
An experiment was made by the same procedure as in Example 1 except that GaAs was used instead of the InGaN photocathode. GaAs was fabricated by molecular beam epitaxy (MBE). The measurement result is illustrated in
An experiment was made by the same procedure as in Comparative example 1 except that GaAs was used instead of the InGaN photocathode. The measurement result is illustrated in
As illustrated in
According to the above results, even when the type of the semiconductor forming the photocathode film 22 was changed, the lifetime of the photocathode 2 was extended when the electron beam B was emitted with the photocathode 2 being heated. The heat resistance temperature of the photocathode film 22 varies in accordance with the type of the semiconductor. Therefore, from the results of Examples 1 and 2, it was confirmed that the upper limit of the heating of the photocathode 2 can be set at a suitable temperature in accordance with the type of the semiconductor.
The use of the electron gun, the electron beam applicator, and the emission method of an electron beam disclosed in the present application can extend the lifetime of a photocathode. Therefore, the electron gun, the electron beam applicator, and the emission method of an electron beam disclosed in the present application are useful for business entities that handle an electron gun.
1, 1A electron gun
2 photocathode
21 substrate
21a first face of the substrate
21b second face of the substrate
22 photocathode film
22a photocathode film base material requiring EA surface treatment
22b EA surface
22c photocathode film not requiring EA surface treatment
23 rod
3 light source
4 anode
5 heater device
51 clamping member
52 heater device
6 output adjustment device
7 holder
8 drive device
81 in-vacuum chamber drive device
9 housing container
91 surface treatment material
11 measuring unit
11a electron beam shield member
11a1 hole
11b measuring instrument
12 control unit
B electron beam
CB vacuum chamber
D1 width of an electron beam
D2 width of the hole
E counterpart device
L excitation light
| Number | Date | Country | Kind |
|---|---|---|---|
| 2020-073311 | Apr 2020 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2021/013455 | 3/30/2021 | WO |
