Patent Application
20080169743
1. Field of the Invention
The present invention relates to a field emission electron gun and a method of operating the same.
2. Description of the Prior Art
Japanese Patent Application Laid-open Publication No. 2005-243389 (hereinafter referred to as “Patent Document 1”) has disclosed a field emission electron source for emitting electrons from an extremity portion of a carbon nanotube (CNT) connected to a cathode and an anode by applying an electric field to the cathode and the anode. Patent Document 1 has disclosed that an admolecular layer (contamination) is removed by heating at a flashing temperature of 100° C. to 1300° C. for 0.1 to 1.0 hour. This removal of an admolecular layer by heating is termed as a “flashing” process.
In addition, Patent Document 1 describes a thermal field emission electron source which performs field emission while being heated at a temperature of 0° C. to 1000° C., and which requires no flashing process to be carried out.
A chief cause of fluctuation in a field emission current is a localized change in the work function which takes place due to a repeated series of a residual gas's adsorption (contamination) to, and desorption from, a field emission site in the extremity of a carbon nanotube. Specifically, when a residual gas which has a lower work function than the carbon nanotube adsorbs to the field emission site, this adsorption facilitates the field emission from the field emission site to which the gas has adsorbed, and thereby the field emission current increases locally. When the adsorbed gas desorbs from the field emission site later, the field emission current returns to the original level.
Even when the adsorbed gas is removed by flashing of the field emission electron source, the gas adsorbs to the field emission site during the field emission. This brings about a problem that the field emission current fluctuates again.
Furthermore, suppose a case where a carbon nanotube electron source is caused to perform field emission by introducing the carbon nanotube electron source in the vacuum after exposed to the atmosphere. In this case, it is impossible to completely remove the adsorbed gas only by heating the carbon nanotube electron source at a temperature of approximately 600K, and it is also impossible to obtain a stable field emission current without heating the carbon nanotube electron source at a temperature of not lower than 1000K once. Moreover, when the heating temperature is raised unnecessarily, the energy distribution of the field emission electrons spreads, although the field emission current is stabilized. This widely-spread energy distribution presents a cause of deteriorating the resolution of an electron microscope in a case where the carbon nanotube electron source is installed in the electron microscope.
An object of the present invention is to provide a field emission electron gun and a method of operating the same which make it possible to constantly obtain a stable field emission current for a long period of time, and to minimize the energy distribution of field emission electrons, which would be widely spread due to heating.
An aspect of the present invention for the purpose of achieving the foregoing object is a field emission electron gun including: an electron source having a conductive base material to which at least a carbon nanotube is fixed; a drawer device for causing the electron source to emit electrons by field emission; and an accelerator for accelerating the electrons emitted from the electron source. The field emission electron gun also includes a heater for heating the carbon nanotube, and the heater includes a switching device for switching between a flashing temperature and a field emission temperature. Moreover, the field emission electron gun is characterized in that the switching device includes a field-emission-temperature controller for determining the field emission temperature according to a value representing a field emission current. The electron source is heated and kept at the heating temperature before field emission, and the heating temperature is thereafter lowered. Thereby, the electron source is caused to perform the field emission while kept at a certain temperature. Furthermore, the field emission electron gun is provided with a means for detecting and monitoring the field emission current, and the temperature for heating the electron sources is thus controlled when needed in order that the field emission current can fluctuate within a prescribed range. In sum, the present invention is to heat and keep the carbon nanotube at the heating temperature, and to thereafter control the heating temperature while monitoring the current fluctuation.
It should be noted that Saito Yahachi (2002) Surface Science (hyomen kagaku), vol. 23, no. 38 has disclosed an example of a method of stabilizing a field emission current from a field emission electron source constituted of a carbon nanotube (hereinafter referred to as a “CNT electron source”). According to the above document, a field emission current is stabilized when a CNT electron source is caused to perform field emission at room temperature after being heated at 1300 K for one minute. In addition, Niels de Jonge (2005). Phys. Rev. Lett. vo. 94, 186807 has disclosed that a field emission current is stabilized when a CNT electron source is caused to perform field emission while kept at a temperature of not lower than 600K.
The present invention makes it possible to provide an electron gun capable of obtaining a stable field emission current constantly for a long period of time, and to control an energy distribution of the field emission electrons so as not to spread widely due to heating.
In addition, by applying an electron gun according to the present invention to electron-beam-applied apparatuses, for example, a scanning electron microscope (SEM) and an electron beam lithography, it is possible to provide the apparatuses which have a low noise level and a high resolution even while operated continuously for a long period of time.
The present invention is carried out as a field emission electron gun including: an electron source configured of a single fibrous carbon substance and a conductive base material supporting the fibrous carbon substance; a drawer device for causing electrons to be emitted by field emission; an accelerator for accelerating the electrons; and a means for heating the electron source. The field emission electron gun is characterized in that the electron source is heated and kept at the heating temperature before the field emission, thereafter the heating temperature is lowered. The field emission is then performed while the electron source is kept at a certain temperature.
The fibrous carbon substance is a fibrous substance essentially containing carbon, for example, a carbon fiber produced by vapor phase growth. A carbon nanotube can be cited as such a fibrous substance. A carbon nanotube with boron, nitrogen, a metal and the like mixed therein may be used.
The indirect heating of a carbon nanotube by electrifying a V-shaped filament to which a base material for the carbon nanotube is connected can be cited as a method of heating the electron source. A heater heats the carbon nanotube up to a flashing temperature once, and thereafter lowers the temperature of the carbon nanotube to the field emission temperature at which the carbon nanotube is kept. In this event, while the temperature of the carbon nanotube is being lowered, the field emission current is monitored. When the current fluctuates over a range of a predetermined value, the temperature is stopped from further lowering. Subsequently, the temperature of the carbon nanotube is raised up to a temperature at which a value representing the current fluctuates within the predetermined value, and then, the field emission starts to be performed.
It is desirable that an extremity of the carbon nanotube be not opened, and be closed with a cap including a five-membered ring.
Another characteristic of the present invention is that the electron gun of the invention is used for various electron-beam-applied apparatuses by use of the above-mentioned method of operating the electron gun.
Detailed descriptions will be provided for embodiments of the present invention by seeing the drawings.
With regard to the shape of the carbon nanotube, it is desirable to have 10 nm to 200 nm in diameter in view of a field emission characteristic, electrical resistance and durability. In addition, it is desirable that the carbon nanotube be not more than 20 μm in length in view of electric resistance control of the carbon nanotube and vibration control of the carbon nanotube during field emission. Any other substance may be applicable as the field emission electron source according to this embodiment as long as the substance is a fibrous substance which takes on the same shape as the carbon nanotube, and which essentially contains carbon.
No specific restriction is imposed on the material for the conductive base material. It is desirable, however, that the material be a noble metal (specifically, gold, silver and an equivalent), crystalline carbon and a refractory metal (specifically, tungsten, tantalum, niobium, molybdenum and the like) in view of a heat resisting property, oxidation resistance and mechanical strength.
In addition, by an FIM process or the like, a flat surface is formed in the extremity portion of the conductive base metal whose extremity is sharpened by chemical etching or the like, for the purpose of controlling an angle between the center axis of the conductive base material and the carbon nanotube. Judging from a radiation angle at which an electron beam is emitted from the carbon nanotube, it should be noted that it is difficult to control the optical axis of the electron beam without having the angle between the center axis of the conductive base material and the carbon nanotube confined within a range of ±5 degrees.
Subsequently, descriptions will be provided of a method of forming the conductive covering layer in the section where the carbon nanotube and the conductive base material are jointed to each other. In a chamber into which a gas containing a conductive element is introduced, a beam of electrons is emitted on at least a part of an area where the carbon nanotube and the conductive base material are in contact with each other. This makes it possible to form the conductive covering layer with a sufficient thickness in a short period of time. By using this method, it is possible to locally cover and reinforce the section where the carbon nanotube and the conductive base material are jointed to each other with the conductive covering layer without adhering the conductive covering element to the carbon nanotube jutting out from the conductive base material.
A gas which is dissolved only by a high-energy heavy-ion beam, such as a gallium ion beam, which is usually used for an FIB process and the like, can not be used as the gas containing the conductive element. This is because, when the high-energy heavy-ion beam is emitted on the carbon nanotube, the carbon nanotube itself is instantaneously damaged so that the carbon nanotube is fractured or an irradiation defect takes place. With this taken into consideration, it is desirable that an electron beam with an energy of not larger than 100 KeV which does not damage the carbon nanotube, be employed as a particle beam used for dissolving the gas. In addition, it is desirable that an organic metal gas essentially containing a metal, such as carbon, platinum, gold, tungsten or the like, and a fluoride gas be used as the gas containing the conductive element, the organic metal gas and the fluoride gas dissolved by use of an electron beam with an energy of not larger than 100 KeV and concurrently vaporized at a temperature of not higher than 100° C. The irradiation of an electron beam on these gases makes it possible to locally form the conductive covering layer only in the section where the carbon nanotube and the conductive base material are jointed to each other.
It should be noted that the extremity of the fibrous carbon substance constituting the electron source according to the first embodiment has a closed structure. In a case of a carbon nanotube having an opened structure, in which the extremity of a fibrous carbon substance is opened and shaped like a tube, the current was not stabilized even while the carbon nanotube was heated.
The current fluctuation which takes place during field emission is beforehand examined, and an appropriate range of the current fluctuation is set. Thereby, the field emission current monitoring device constantly monitors whether or not the field emission current falls within a predetermined range of the current fluctuation. In addition, the heater is controlled in accordance with a result of the monitoring.
Thereafter, in a case where the field emission current goes beyond the predetermined range of the current fluctuation, the heating temperature is raised to temperature (T4) which causes the field emission current to fall within the predetermined range of the current fluctuation. Afterward, the heating temperature is lowered to temperature (T3) which causes the field emission current to go beyond the predetermined range of the current fluctuation. In the process of lowering the heating temperature, the lowest temperature (T2) which causes the field emission current to fall within the predetermined range of the current fluctuation is determined. Subsequently, the temperature of the electron source is raised to temperature T2 again, and is kept at temperature T2.
This operation method is capable of being controlled manually. However, an automated operation using the monitoring device makes it possible to continuously obtain a stable field emission current for a long period of time, and to minimize the spread of ΔE which takes place due to heating. Moreover, the holding of the heating temperature to a minimum is advantageous in a view of the heat resistance of the section where the fibrous carbon substance and the conductive base material are jointed to each other in the electron source according to this embodiment.
In the scanning electron microscope, an alignment coil, a condenser lens, an astigmatic correction coil, a deflecting/scanning coil, an object lens and an object stop are arranged sequentially along an electron beam of emitted from the electron gun. A sample is placed on a sample stage, and the electron beam is emitted on the sample. A secondary electron detector is provided in a sidewall portion of a sample chamber. In addition, the sample chamber is designed to be held in high vacuum by a discharge system. With this configuration, the electron beam emitted from the electron gun is accelerated by an anode, and is condensed by the electron lens. Thereby, the resultant electron beam is emitted on a minute area on the sample. This irradiated area is scanned over two-dimensionally. Thereby, secondary electrons, reflection electrons and the like which are emitted from the sample are detected by a secondary electron detector. According to the difference in the amount of detected signals, an magnified image is formed.
The application of the electron gun and the operating method according to the present invention to a scanning electron microscope makes it possible for the scanning electron microscope to be operated continuously for a long period of time. In addition, data to be obtained has a low noise level, and ΔE is small. Hence, a scanning electron microscope with a high resolution can be made. For this reason, the electron gun and the operating method according to the present invention can also be applied to a measurement SEM used for observing micro-processed patterns and measuring the dimensions in a semiconductor process which requires the SEM to be operated continuously for a long period of time. A basic configuration of an electron-optical system of the measurement SEM is common with that of the regular SEM (as shown in
A conventional type of field emission electron gun is configured of a single crystal tungsten electron source, and needs heat flashing in intervals of approximately 10 hours. No emission can be carried out during the flashing process. In addition, the emission current is not stabilized in one hour after the flashing process. These obstruct a stable observation. The semiconductor process line needs to be in operation continuously for 24 hours a day, and is accordingly not allowed to be out-of-operation each time a flashing process is carried out. This makes it impossible to install the conventional type of field emission electron gun in the scanning electron microscope.
It should be noted that the configuration of the scanning electron microscope in which the field emission electron gun is installed includes, but is not limited to, the configuration shown in
The electron beam rendering is carried out by emitting the electron beam while the electron beam is being turned on and off by the blanking electrode, and while the electron beam is deflected and scans over the sample by the deflection/scanning coil. The electron beam lithography forms various circuit patterns by emitting the electron beam on a sample substrate to which a resist sensitive to the electron beam is applied. As the various circuit patterns are constructed in an increasingly fine scale, a demand for an electron gun whose probe is extra fine in diameter has become stronger. Electron sources of a thermal-electron-emission type which is made of a tungsten filament or LaB6 have been heretofore in use for electron guns. These electron guns are advantageous in that a larger amount of beam current can be gained from the electron guns. On the other hand, these electron guns have an astigmatism which takes place due to the larger diameters of their respective emitter extremities in an absolute term. This makes it impossible for these electron guns to render lines which are 20 nm or narrower in width. For this reason, field emission electron guns of a type which is configured of a single crystal tungsten electron source are increasingly in use recently. Nevertheless, field emission electron guns of this type are incapable of performing a secure rendering operation because the beam current is small in amount and is unstable.
Use of the electron source according to the present invention makes it possible for the operation to be performed continuously. Moreover, the smallness of ΔE makes it possible to obtain a fine rendering operation. The application of the electron gun and the operating method according to the present invention makes it possible to solve the foregoing problems.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-219139 | Aug 2006 | JP | national |
