Charged particle beam device

Abstract

In order to provide a charged particle beam device capable of returning to a usable state in a short time after completion of flushing processing for continuously and stably using an electron beam having a high luminance and a low energy dispersion and capable of shortening the downtime of the device, the charged particle beam device is configured as follows.　The charged particle beam device comprises: an electron source that emits electrons; an extraction electrode that faces the electron source and generates an electric field for causing the electron source to emit electrons; a first voltage power supply that applies a first voltage to the electron source in order to emit electrons from the electron source; a second voltage power supply that applies a second voltage to the extraction electrode; and a cooling device that cools the electron source. The charged particle beam device further comprises: a third voltage power supply that, in order to remove gas molecules adsorbed onto the electron source in the process of a specimen inspection, applies a third voltage having a polarity opposite to that of the first voltage to the electron source and thereby can set an electric field generated between the electron source and the extraction electrode such that the electron source is positive and the extraction electrode is negative; and a control unit for performing control so as to stop applying the first voltage from the first voltage power supply when applying the third voltage from the third voltage power supply to the electron source.

Description

charged particle beam equipment

[0001] The present invention relates to a charged particle beam device.

[0002] The electron beam is focused through an electromagnetic lens, and the sample is scanned while being irradiated with the focused electron beam. The charged particles (secondary electrons) emitted from the sample are detected, allowing the structure of the sample surface to be observed. This is called a scanning electron microscope (SEM).

[0003] SEMs are specimen observation instruments with nanometer-order resolution, exceeding the limits of optical microscopes, and are making a wide range of contributions to scientific research and industry. A representative example is the critical dimension scanning electron microscope (CD-SEM), which is used to inspect the dimensions of semiconductor devices, and this technology is now indispensable for device production management.

[0004] Schottky emitters (SEs) have long been used as the electron source for SEMs dedicated to semiconductor device inspection and measurement, such as CD-SEMs. Because these electron sources operate at high temperatures via filament heating, they are less susceptible to electron beam current fluctuations caused by impurity gases near the electron source, making them ideal for SEMs used in semiconductor device production management, where throughput is particularly important. However, with the advances in miniaturization and three-dimensionalization mentioned above, there are an increasing number of inspection and measurement applications that require electron sources with higher brightness and smaller energy spread.

[0005] Cold field emitters (CFEs) are known as electron sources capable of emitting high-brightness electron beams with small energy dispersion. Unlike SE electron sources, CFEs operate their emitters at room temperature, resulting in larger electron beam current fluctuations due to residual gases than SE electron sources. This characteristic makes them unsuitable for semiconductor device production control applications.

[0006] Residual gases increase the work function of the emitter surface by adsorbing onto it or diffusing through the surface. This phenomenon affects the performance of the electron source, such as increasing noise in the electron beam current or attenuating the current amount. The above phenomenon occurs because the adsorption of residual gases changes the work function of the surface.

[0007] One technique for controlling the effects of residual gas is to heat the emitter to a high temperature before operation, a process called flashing. Heating removes impurity gases from the emitter surface, reducing the work function and making it easier to emit electron beams.

[0008] Patent Document 1 discloses a technology that determines whether the time elapsed since a flushing operation was performed on a cold cathode field emission electron gun has exceeded a set time, and if it is determined that the set time has been exceeded and that a predetermined operation has been started, performs a flushing operation on the cold cathode field emission electron gun, thereby providing a good electron emission state without the user being aware of the flushing operation.

[0009] Japanese Patent Application Laid-Open No. 2020-27779

[0010] In order to inspect and measure advanced semiconductors in the future, a resolution of approximately 1 nanometer will be required using an SEM. To achieve this performance, it is desirable to adopt a CFE electron source as the SEM's electron source. However, as mentioned above, the CFE electron source suffers from large fluctuations in electron beam current due to residual gas. If the aforementioned flushing is performed to reduce the effects of this residual gas, the emitter heating during flushing is carried out at a temperature that changes the emitter shape, resulting in changes in the operating conditions of the electron source before and after the flushing. Changing the operating conditions of the electron source necessitates changes to the SEM's optical conditions, which places a burden on the user.

[0011] It is known that cooling a CFE electron source improves the vacuum level of the electron source due to the cryogenic effect. This improved vacuum level reduces residual gas around the electron source and slows the diffusion rate of residual gas adsorbed to the emitter, potentially suppressing electron beam current fluctuations. However, when flashing is performed with the electron source cooled, the temperature of the surrounding components, particularly the electron source, which is directly heated, rises, and it takes time for the temperature to return to a steady state. The optical conditions of the SEM are unstable during this time, making it unsuitable for the inspection and measurement of advanced semiconductors.

[0012] An object of the present invention is to provide a charged particle beam device that can be restored to a usable state in a short time after completion of a flushing process to enable continuous and stable use of a high-brightness, low-energy dispersion electron beam, thereby reducing downtime of the device.

[0013] To achieve the above object, the present invention is configured as follows: A charged particle beam device includes an electron source that emits electrons, an extraction electrode that faces the electron source and generates an electric field that causes the electron source to emit electrons, a first voltage power supply that applies a first voltage to the electron source to emit electrons from it, a second voltage power supply that applies a second voltage to the extraction electrode, and a cooling device that cools the electron source, and further includes a third voltage power supply that applies a third voltage having an opposite polarity to the first voltage to the electron source in order to remove gas molecules adsorbed to the electron source during sample observation, thereby making the electric field generated between the electron source and the extraction electrode positive for the electron source and negative for the extraction electrode, and a control unit that controls the first voltage power supply to stop applying the first voltage when the third voltage is applied to the electron source from the third voltage power supply.

[0014] Alternatively, a charged particle beam device includes an electron source that emits electrons, an extraction electrode that faces the electron source and generates an electric field that causes the electron source to emit electrons, a first voltage power supply that applies a first voltage to the electron source to emit electrons from the electron source, a second voltage power supply that applies a second voltage to the extraction electrode, and a cooling device that cools the electron source, and further includes a control unit that controls either or both of a first voltage applied from the first voltage power supply to the electron source or a second voltage applied from the second voltage power supply to the extraction electrode so that the electric field generated between the electron source and the extraction electrode becomes positive for the electron source, in order to remove gas molecules adsorbed to the electron source during the process of observing a sample.

[0015] According to the present invention, it is possible to provide a charged particle beam device that can be restored to a usable state in a short time after completion of a flushing process to ensure continuous and stable use of a high-brightness, low-energy dispersion electron beam, thereby reducing the downtime of the device.

[0016] 1 is a configuration diagram of a charged particle beam device according to an embodiment of the present invention. It shows the tip of an emitter electrode and an emitted electron beam. It shows the tip of an emitter electrode and a diagram showing that the emitted electron beam is obstructed by residual gas molecules. It shows the movement of residual gas molecules at the tip of the emitter electrode. It shows the removal of residual gas molecules from the tip of the emitter electrode. It shows that an emitter electrode sharpened by an electropolishing process is blunted by a field evaporation forming process to form a field end form. It shows that an emitter electrode sharpened by a gas etching process is blunted by a field evaporation forming process to form a field end form. It shows an example of stabilizing an electron beam by intermittent electric field flashing. It is an enlarged view of a portion of FIG. 8. It is a diagram explaining an example in which the field evaporation process voltage is increased stepwise. It is a diagram explaining an example in which the extraction voltage for emitting an electron beam after field evaporation process is increased linearly with time. It is a configuration diagram of a conventional charged particle beam device.

[0017] To facilitate understanding of the present invention, the following will first explain the problems with current charged particle beam devices, then provide an overview of the basic principles of the present invention, and then explain the specific configuration of a charged particle beam device according to an embodiment of the present invention with reference to the drawings.

[0018] First, the configuration of a conventional charged particle beam device 1000 will be described with reference to FIG.

[0019] The electron gun 1 includes an emitter electrode (emitter tip) 11 having a needle-like tip, and an extraction electrode 13. The extraction electrode 13 has an opening facing the emitter electrode 11. The emitter electrode 11 is cooled by cooling means 14, such as a mechanical refrigerator, typified by a Gifford-McMahon (GM) refrigerator, or a cryogen such as liquid nitrogen or liquid helium. A high-voltage power supply 21 applies a voltage to the emitter electrode 11, and a high-voltage power supply 22 applies a voltage to the extraction electrode 13, and the potential difference between them forms an electric field that emits electrons near the tip of the emitter electrode 11.

[0020] The high voltage power supplies 21 and 22 can be controlled independently of each other, which allows the acceleration voltage of the electron beam and the extraction voltage for forming the field emission field to be controlled independently. The high voltage applied to the emitter electrode by the high voltage power supply 21 is generally a negative potential of 0.1 kV to 30 kV.

[0021] In order to emit an electron beam 15 from the emitter electrode 11 of the electron gun 1, a high voltage is first applied between the emitter electrode 11 and the extraction electrode 13. This is called the extraction voltage. The voltage applied to the extraction electrode 13 is generally maintained at a positive potential of 2 kV to 7 kV relative to the potential applied to the emitter electrode 11. For example, when the potential of the emitter electrode 11 is, say, negative 20 kV, applying a voltage of negative 14 kV to the extraction electrode can set the potential of the extraction electrode 13 to a positive potential of 6 kV.

[0022] Application of this extraction voltage concentrates an electric field at the tip of the emitter electrode 11. It is preferable that the tip of the emitter electrode 11 be sufficiently sharpened so that an electric field is generated that emits electrons. To increase the brightness of the emitted electron beam, the material constituting the emitter electrode, or the crystal orientation of that material, should be selected so as to lower the work function of the electron-emitting region at the tip of the emitter electrode. For example, if a single crystal of tungsten is used and the electron-emitting region at the tip is selected to have a (310) plane, a bright electron beam can be extracted.

[0023] The vacuum chamber 101 containing the electron gun 1 is －６ Ultra-high vacuum below 10 Pa or －９ It is preferable that the vacuum chamber 101 be maintained at an ultra-high vacuum of not more than Pa. In order to make the vacuum chamber 101 reach an ultra-high vacuum, the start-up operation of the electron gun 1 may include so-called baking, in which the entire vacuum chamber is heated to 100 degrees or more.

[0024] The electron beam 15 is focused by an electromagnetic lens 16. The electromagnetic lens 16 is composed of a lens using an electromagnet with a coil as a component, and there are two types: one that focuses the electron beam by magnetic interaction, and one that uses a lens with electrodes made of multiple metal plates including an electrode to which a high voltage is applied, and focuses the electron beam by electrostatic interaction.

[0025] The focused electron beam 15 is irradiated onto the sample 18, and various signals emitted from the irradiated position are detected by a detector (not shown) and analyzed by an analysis unit (not shown), thereby making it possible to obtain information on the surface of the sample 18. The sample 18 is held by a stage 17. In some cases, the stage 17 may be configured to move horizontally or vertically relative to the electron beam 15, so that the electron beam 15 can be irradiated to any position on the sample 18.

[0026] 2 is an enlarged view of the tip of the emitter electrode 11. When an extraction voltage is applied, a strong electric field is generated at the tip 110 of the emitter electrode 11, causing an electron beam 15 to be emitted from the electron emission region 111.

[0027] 3 is a diagram of the tip 110 after a certain time has elapsed since the start of emission of the electron beam 15. As shown in the figure, after a certain time has elapsed, residual gas molecules 112 may be adsorbed onto the surface of the tip 110. When these residual gas molecules 112 cover the surface of the electron emission region 111 and increase the work function, the amount of current of the electron beam 15 decreases, which means that the brightness decreases.

[0028] In order to remove the adsorbed residual gas molecules 112, a method commonly known as flashing, which heats the emitter electrode 11, is generally used in the prior art. By heating the entire emitter electrode 11 including the electron emission region 111 with a filament or the like (not shown) supporting the emitter electrode 11, the residual gas molecules 112 are desorbed from the surface. By desorbing the residual gas molecules 112 from the surface, the work function of the electron emission region 111 decreases, and the current amount of the electron beam 15 is restored.

[0029] When flashing by heating is performed, the shape of the emitter electrode 11 may be deformed by heat. For example, the radius of curvature of the tip 110 may increase. This change in shape reduces the magnitude of the electric field generated at the tip 110 even if the same extraction voltage as before heating is applied. In this case, a larger extraction voltage must be applied to generate the same electric field magnitude as before heating. When the extraction voltage changes, the optical conditions required for electron beam convergence, such as the conditions of the electromagnetic lens 16, change. When flashing by heating is performed in this way, it becomes necessary to readjust the overall conditions of the charged particle beam device 1000. The fact that this condition adjustment requires a certain amount of time is a problem with the conventional technology.

[0030] In this embodiment, cooling means 14 is used to increase the stability of the electron beam. The cooling means 14 cools the inside of the electron gun 1, mainly the emitter electrode 11. It also cools the members around the emitter electrode 11, such as the extraction electrode 13. The cooling means 14 may be a mechanical refrigerator such as a Gifford-McMahon type (GM type) or pulse tube type, or may use a refrigerant such as liquid helium, liquid nitrogen, or solid nitrogen.

[0031] Mechanical refrigerators and cooling means using liquid helium or liquid nitrogen usually generate vibrations. For this reason, it is possible to indirectly cool the emitter electrode 11 by transporting gases cooled by these cooling means to the vicinity of the emitter electrode 11. By not directly connecting the refrigerator or the like mechanically, it is possible to suppress the transmission of generated vibrations.

[0032] The residual gas inside the electron source adheres to the area cooled by the cooling means 14 due to the cryogenic effect, thereby improving the degree of vacuum inside the electron source. As shown in Figure 4, residual gas molecules 114 adhering to an area at a certain distance from the electron emission region 111, for example, the shank 115 of the emitter electrode 11, can adhere to the electron emission region 111 due to a surface diffusion effect 116, thereby increasing the work function. When the emitter electrode 11 is cooled by the cooling means 14, the speed of this surface diffusion effect 116 decreases.

[0033] In other words, generally, cooling the emitter electrode 11 using the cooling means 14 has the effect of extending the time required for the current amount of the electron beam 15 to attenuate, i.e., the time required for the state shown in FIG. 2 to change to the state shown in FIG. 3.

[0034] However, cooling the emitter electrode 11 presents new challenges. Although the decay time is extended, the presence of residual gas means that, in principle, the current of the electron beam 15 will eventually decay. To restore the decayed current, the residual gas molecules 112 must be desorbed from the surface. However, if conventional thermal flashing is performed while the emitter electrode 11 is cooled, the time required for the emitter electrode 11 to return to its original temperature after heating is longer than when the emitter electrode 11 is not cooled. Even if the optical conditions are adjusted in the transient state until the temperature returns to its original state, the conditions may no longer be optimal after the temperature is restored, requiring time for readjustment. Therefore, after thermal flashing, it is generally necessary to wait until the steady state is restored. In other words, the issue of additional waiting time after thermal flashing arises.

[0035] To solve this problem, in this embodiment, a voltage of the opposite polarity to the polarity of the voltage applied to the emitter electrode 11 when observing the sample (a voltage in the opposite direction to the direction in which electrons are emitted from the emitter electrode 11) is applied to the emitter electrode 11, thereby removing the residual gas molecules 112 adsorbed to the tip 110 and restoring the current.

[0036] When a high voltage is applied in the direction opposite to the direction in which electrons are emitted from the emitter electrode 11, an electric field is generated at the tip 110, which has the effect of removing charge from surrounding molecules and adsorbed molecules, causing them to become positively ionized. As shown in Figure 5, the positively ionized molecules 117 move away from the surface due to the electrostatic force from the electric field. This phenomenon is called field ionization or field desorption ionization. Depending on the strength of the electric field, the atoms that make up the emitter electrode 11 may also be positively ionized and desorb from the tip 110. This phenomenon is called field evaporation.

[0037] A charged particle beam device according to this embodiment is shown in FIG. A high-voltage power supply 23 having a polarity opposite to that of the high-voltage power supply 21 is connected to the emitter electrode 11 to generate an electric field at the tip 110 that generates field desorption ionization. The high-voltage power supplies 21 and 23 are further connected to the emitter electrode 11 via a switch circuit 20 controlled by a control unit 100. The control unit 100 switches the switch circuit 20 so that the high-voltage power supply 21 and the emitter electrode 11 are connected when emitting an electron beam 15 from the emitter electrode 11, and so that the high-voltage power supply 23 and the emitter electrode 11 are connected and an ionization voltage is applied when removing residual gas molecules 112 by field desorption ionization. The configuration consisting of these two high-voltage power supplies 21 and 23 and the switch circuit 20 can also be replaced with a single voltage power supply capable of outputting voltages of both polarities, in which case the switch circuit 20 can be omitted.

[0038] The technique of field desorption / ionization of residual gas molecules 112 by connecting the high-voltage power supply 23 to the emitter electrode 11 and restoring the current of the electron beam 15 is called electric field flashing. When the emitter electrode 11 is cooled by a cooling means 14, electric field flashing has the effect of significantly shortening the waiting time after processing compared to thermal flashing. Depending on the connection method between the emitter electrode 11 and the high-voltage power supply, particularly the capacitance of the wiring, it is easy to reduce the waiting time to less than one second. On the other hand, in the case of thermal flashing, the waiting time until the temperature returns to its original pre-heating temperature is longer than one second in the simulation. This simulation assumes the minimum components, such as the emitter electrode and the heating means (the filament supporting the emitter electrode 11, the electrode supporting the filament, and the insulator components supporting the electrode). However, adding more realistic components would result in longer time required for the temperatures of those components to return to their original steady-state temperatures.

[0039] When performing electric field flashing, the voltage applied must generate an electric field at the tip 110 sufficient to positively ionize the residual gas molecules 112. On the other hand, if the applied voltage is further increased, the atoms constituting the emitter electrode 11 are desorbed by field evaporation. As a result, the shape of the tip 110 of the emitter electrode 11 changes, and the electric field generated at the tip 110 changes with respect to the applied voltage. Therefore, when the shape of the emitter electrode 11 changes due to electric field flashing, the optical conditions of the charged particle beam device 1000 change before and after the process, and time is required to optimize those conditions. Therefore, when performing electric field flashing, it is necessary to generate an electric field at the tip 110 that is weaker than the electric field at which the atoms constituting the emitter electrode 11 are desorbed by field evaporation.

[0040] It is generally difficult to know the strength of the electric field generated at the tip 110 of the emitter electrode 11 relative to the applied voltage because it depends on the shape of the emitter electrode 11. In other words, it is difficult to set the electric field strength during electric field flashing to a value that will cause field desorption but not field evaporation.

[0041] For this reason, in this embodiment, as a preparation prior to generating the electron beam 15, a high-voltage power supply 23 is connected between the emitter electrode 11 and the extraction electrode 13 to apply an ionization voltage. By setting this voltage sufficiently high, field evaporation is intentionally caused to occur at the tip 110 of the emitter electrode 11. This is called the field evaporation formation process.

[0042] If there were no upper limit to the voltage applied by the high-voltage power supply 23, the electric field generated at the tip would eventually exceed the electric field strength at which field evaporation occurs. In reality, the maximum voltage applied by the power supply is regulated by the product used, and there is also an upper limit to the withstand voltage between the emitter electrode 11 and the extraction electrode 13, so there is an upper limit to the applied voltage.

[0043] Therefore, before performing the field evaporation formation process, a process for sharpening the tip 110 of the emitter electrode 11 in advance can be performed. This may be a process for creating the emitter electrode 11 before introducing it into the charged particle beam device 1000, such as an electropolishing process. In the electropolishing process, a metal wire that will become the emitter electrode is introduced into an electrolyte, and a direct or alternating current is passed through it to sharpen the metal wire. By keeping the conditions of this process constant, it is possible to set the applied voltage during the field evaporation formation process (field evaporation process voltage) to a practically achievable value.

[0044] As shown in FIG. 6, the tip 120 of the emitter electrode, which has been sharpened by the electrolytic polishing process, becomes blunted to tip 122 as atoms 121 are desorbed from the tip by the field evaporation forming process.

[0045] Alternatively, the sharpening process may be a process performed after the emitter electrode 11 is introduced into the charged particle beam device 1000, such as a sharpening process of the emitter electrode 11 using gas etching, as shown in FIG. 7 . When the emitter electrode is made of tungsten, it is known that introducing nitrogen gas or oxygen gas around the emitter electrode and applying an appropriate ionization voltage to the emitter electrode causes atoms constituting the sidewall of the emitter electrode to be chemically desorbed, resulting in a sharpened emitter electrode 11. As shown in FIG. 7 , the tip 123 of the emitter electrode before gas etching changes to tip 124 after gas etching. The ionization voltage applied during sharpening by gas etching is called the gas etching voltage. When the emitter electrode tip 110 is sharpened using this process, the field evaporation process voltage can be set using the gas etching voltage as an indicator, making this a suitable pre-processing method. This is because, in this process, atoms are removed from the shank portion 115 by a chemical reaction, sharpening the tip 110, but at the same time, field evaporation occurs from the tip 110 due to the action of the electric field generated by the gas etching voltage. In other words, at the tip 124 of the emitter electrode after gas etching, the field evaporation initiation voltage from the tip is equal to the gas etching voltage. In other words, by applying an ionization voltage equal to or higher than the gas etching voltage, it is possible to reliably execute the field evaporation formation process.

[0046] Therefore, when performing a field evaporation process after a gas etching process, if the ionization voltage is set to be higher than the gas etching voltage, it is guaranteed that field evaporation will occur at the tip 110 .

[0047] When performing the field evaporation process after these sharpening processes, a constant ionization voltage, i.e., the field evaporation process voltage, is applied for a sufficiently long time. Specifically, this time may be 10 or 100 times longer than the time the voltage is applied during electric field flashing. As field evaporation progresses and the tip becomes blunt, the electric field at the tip weakens, and a steady state is reached where no field evaporation occurs. The shape 122 of the tip of the emitter electrode 11 in this state is called the field end form (Figures 6 and 7). The field end form is a shape determined by conditions such as the distance between the emitter electrode 11 and the extraction electrode 13 and the field evaporation process voltage.

[0048] In the process of forming the field end forms 122, the processing time can be reduced by heating the emitter electrode 11. This is because the probability of field evaporation of atoms varies depending on the temperature, and the higher the temperature, the higher the probability of field evaporation occurring.

[0049] In this state where field end forms have been formed, even if an ionization voltage lower than the field evaporation voltage is applied, the shape of the emitter electrode 11 does not change. Therefore, if the ionization voltage for the electric field flashing process is set to a voltage lower than the field evaporation voltage, the shape of the emitter electrode 11 does not change even if electric field flashing is performed multiple times constantly, and only the residual gas molecules 112 are removed from the surface, making it possible to restore the current of the electron beam 15.

[0050] Figure 8 shows the results of stable electron beam 15 emission using this embodiment. The vertical axis of this graph represents the current of the electron beam 15, and the probe current 31 measured is limited to an angle of approximately 90 mrad. "Limited to an angle of approximately 90 mrad" means that only electron beams emitted from the emitter electrode with a half-angle of approximately 90 mrad or less are used as probes. The base surrounding the emitter electrode 11 is cooled to 16 K, and the emitter electrode 11 is presumably cooled to a similar temperature. An extraction voltage of -2.2 kV is applied between the emitter electrode 11 and the extraction electrode 13, which extracts the electron beam 15 from the emitter electrode 11. Normally, the probe current 31 decays over time. To prevent this, a voltage of +18 kV is applied between the emitter electrode 11 and the extraction electrode 13 every 30 minutes. This has enabled us to successfully extract a stable probe current of approximately 50 nA for 140 hours.

[0051] Figure 9 is a graph enlarging a region 30 in the transition of probe current 31. At time points 33 and 34 in this graph, a voltage of +18 kV was applied for approximately 5 seconds each as electric field flushing. At the time points when these electric field flushing operations were performed, a slight recovery of probe current 31 was observed. This is thought to be due to residual gas molecules being desorbed from the tip of the emitter electrode.

[0052] Before the current measurement and field-type flashing, a field evaporation voltage of +20 kV was applied between the emitter electrode 11 and the extraction electrode 13 for a sufficiently long time. This process resulted in the formation of a field end foam at the tip of the emitter electrode 11. Because of the formation of the field end foam, the shape of the emitter electrode 11 was not changed by the subsequent field-type flashing process of +18 kV. Therefore, even after multiple field-type flashing processes, the probe current of approximately 50 nA was maintained by the extraction voltage of −2.2 kV applied at the start.

[0053] The field evaporation process voltage may be determined by alternately repeating the field evaporation process and measuring the current of the electron beam 15 by electron emission. In this case, the field evaporation process voltage may be increased stepwise. Figure 10 shows the time-series changes in the voltage between the emitter electrode 11 and the extraction electrode 13 during this process, as well as the transition in the current of the electron beam 15. Vertical axis 40 represents the change in voltage, and vertical axis 49 represents the change in current.

[0054] In this example, an untreated emitter electrode 11 is assumed as the initial state. Therefore, no current is detected from the tip even when the extraction voltage Ve (45) is applied (region 50). After checking the current amount, the field evaporation process voltage V1 (44) is applied for a certain period of time. Even when the extraction voltage Ve (45) is applied again, no current is detected in region 46. A field evaporation process voltage V2 (43) greater than the field evaporation process voltage V1 (44) is then applied. When the extraction voltage Ve (45) is then applied, a finite current is detected in region 47. A field evaporation process voltage V3 (42) greater than the field evaporation process voltage V2 (43) is then applied. When the extraction voltage Ve (45) is then applied, a current greater than the current detected in region 47 is detected in region 48. In this way, by gradually increasing the field evaporation process voltage, the minimum necessary voltage value can be determined. In this case, it is advisable to determine a threshold current amount 51 for the extraction voltage Ve (45) in advance. In this example, since the threshold current amount 51 is exceeded in the region 48, the process can be ended and the field evaporation process can be terminated.

[0055] 11, the extraction voltage for emitting the electron beam after each stage of the field evaporation process may be configured to increase linearly with time. At this time, in region 61 or region 62 where the electron beam is detected, the amount of current increases nonlinearly in conjunction with the increase in the extraction electrode. Instead of determining the end of the field evaporation process based on the threshold current amount 51, the dependency of the current amount on the extraction voltage may be used to determine the end of the field evaporation process. For example, the ratio of the increase in the current amount to the increase in the extraction voltage may be used as a criterion.

[0056] In the above explanation, based on the configuration of the conventional charged particle beam device 1000, a configuration has been described in which the polarity of the voltage applied to the emitter electrode 11 is reversed using the high-voltage power supply 23. For example, by applying a positive 16 kV voltage to the emitter electrode 11 using the high-voltage power supply 23 and a negative 2 kV voltage to the extractor electrode 13, the electric field formed between the emitter electrode 11 and the extractor electrode 13 can be such that the potential of the emitter electrode 11 is 18 kV higher than the potential of the extractor electrode 13, thereby achieving the effect of ionizing and desorbing adsorbed gas molecules. In other words, in order to desorb adsorbed gas molecules from the emitter electrode 11, it is sufficient to make the potential of the extractor electrode 13 lower than the potential of the emitter electrode 11.

[0057] For example, even if the voltage applied to the emitter electrode 11 is the same as that applied by the high-voltage power supply 21 (which applies a voltage of negative 20 kV) as in the configuration of the conventional charged particle beam device 1000, by changing the negative potential applied to the extraction electrode 13 from the high-voltage power supply 22 to negative 38 kV, which is the potential applied to the extraction electrode 13 during sample observation, the electric field formed between the emitter electrode 11 and the extraction electrode 13 can be such that the potential of the emitter electrode 11 is 18 kV higher than the potential of the extraction electrode 13, making it possible to desorb adsorbed gas molecules from the emitter electrode 11. It is expected that the effect of desorbing adsorbed gas molecules from the emitter electrode 11 can be enhanced by increasing the potential difference in the electric field formed between the emitter electrode 11 and the extraction electrode 13. Therefore, when performing the process of desorbing adsorbed gas molecules from the emitter electrode 11, if the potential of the emitter electrode 11 is changed from the normal negative 20 kV to negative 2 kV, an electric field can be formed in which the potential of the emitter electrode 11 is 12 kV higher than the potential of the extraction electrode 13, even if the potential applied to the extraction electrode 13 remains at the normal negative 14 kV.

[0058] 1: electron gun, 11: emitter electrode (emitter tip), 13: extraction electrode, 14: cooling means, 15: electron beam, 16: electromagnetic lens, 17: stage, 18: sample, 110: tip, 111: electron emission region, 112: residual gas molecules, 114: residual gas molecules, 115: shank portion, 116: surface diffusion effect, 117: positively ionized molecules, 120: emitter electrode sharpened by electropolishing process, 121: desorbed atoms, 122: field end form, 123: emitter tip electrode before gas etching, 124: emitter tip electrode after gas etching, 30: region, 31: probe current amount, 33: time point, 34: time point, 1000: charged particle beam device.

Claims

1. A charged particle beam device comprising: an electron source that emits electrons; an extraction electrode that faces the electron source and generates an electric field that causes the electron source to emit electrons; a first voltage power supply that applies a first voltage to the electron source to emit electrons; a second voltage power supply that applies a second voltage to the extraction electrode; and a cooling device that cools the electron source, wherein the charged particle beam device further comprises: a third voltage power supply that applies a third voltage having an opposite polarity to the first voltage to the electron source in order to remove gas molecules adsorbed to the electron source during a sample observation process, thereby making the electric field generated between the electron source and the extraction electrode positive for the electron source and negative for the extraction electrode; and a control unit that controls the first voltage power supply to stop applying the first voltage when the third voltage is applied to the electron source from the third voltage power supply.

2. A charged particle beam device according to claim 1, wherein the control unit controls the application of the third voltage to remove gas molecules adsorbed on the electron source in the process of gas-etching the tip of the electron source by applying a voltage greater than the third voltage from the third voltage power supply to the electron source, and then using the gas-etched electron source to observe a sample.

3. A charged particle beam device according to claim 1, wherein the control unit controls the application of the third voltage to the electron source in order to remove gas molecules adsorbed on the electron source during the process of observing a sample using the sharpened electron source by applying a voltage greater than the third voltage from the third voltage power supply to the electron source, thereby sharpening the tip of the electron source by field evaporation.

4. A charged particle beam device according to claim 2 or 3, characterized in that it comprises a heating unit for heating the electron source when gas etching or field evaporation is performed on the tip of the electron source.

5. A charged particle beam device according to any one of claims 1 to 3, characterized in that the control unit controls the application of the third voltage for removing gas molecules adsorbed on the electron source so as to be performed intermittently.

6. A charged particle beam device according to claim 2 or 3, wherein the control unit, when gas etching or field evaporation of the tip of the electron source, alternates between application of the first voltage from the first voltage power supply while gradually increasing the voltage applied by the third voltage power supply, and controls to determine the processing voltage to be applied to the electron source by the third voltage power supply when gas etching or field evaporation of the tip of the electron source is performed based on the value of the current that flows when the first voltage is applied by the first voltage power supply.

7. A charged particle beam device comprising: an electron source that emits electrons; an extraction electrode that faces the electron source and generates an electric field that causes the electron source to emit electrons; a first voltage power supply that applies a first voltage to the electron source to emit electrons; a second voltage power supply that applies a second voltage to the extraction electrode; and a cooling device that cools the electron source, wherein the charged particle beam device further comprises a control unit that controls either the first voltage applied to the electron source from the first voltage power supply or the second voltage applied to the extraction electrode from the second voltage power supply to change or both of the first voltage applied to the electron source from the first voltage power supply or the second voltage applied to the extraction electrode from the second voltage power supply so that the electric field generated between the electron source and the extraction electrode becomes positive in order to remove gas molecules adsorbed to the electron source during a sample observation process.

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