DE112016007631B3 - electron microscope - Google Patents

Abstract

The present invention seeks to provide an electron microscope which can be activated at an appropriate temperature by placing an NEG on an extraction electrode around an electron source. The present invention is an electron microscope provided with an electron gun, the electron gun comprising an electron source, an extraction electrode and an accelerating tube, the accelerating tube being connected to the extraction electrode at a connection portion, the extraction electrode comprising a first heater and a first NEG and the first heater and the first NEG are spaced apart from each other in an axial direction of an electron beam emitted from the electron source.

Description

technical field

The present invention relates to an electron microscope provided with an electron gun.

State of the art

An electron microscope is an observation device that obtains an enlarged image of a sample and information about constituents using an electron beam. In the electron microscope, a precise magnified image of the sample and a result of elemental composition analysis of the sample are obtained by an electron beam. For this reason, an electron gun that generates an electron beam plays a large role.

It is important for the electron gun to keep its internal residual gas pressure low. In order to pump residual gas molecules into the electron gun and keep the molecules at low pressure, the electron gun is provided with a vacuum pump such as an ion pump (sputter ion pump). An ion pump is a device that ionizes residual gas molecules in a vacuum vessel by collision with an electron beam and pumps residual gas through the confinement of the gas in an inner wall.

Also, in recent years, a non-evaporable getter (NEG) pump for pumping residual hydrogen molecules has been used in combination with an ion pump. PTL 1 discloses an electron gun provided with an ion pump and a NEG. Furthermore, PTL 2 discloses an electron gun provided with an NEG around an electron source.

citation list

patent documents

• PTL 1: JP 2015 - 015 200 A
• PTL 2: WO 2005/124 815 A1
• PTL 3: U.S. 2008 0 284 332 A1
• PTL 4: U.S. 2013 0 087 703 A1
• PTL 5: DE 11 2014 006 978 T5
• PTL 6: DE 112016 007 160 T5

Summary of the Invention

Technical problem

It is important to keep the residual gas in the electron gun low to reduce the frequency of cleaning and the frequency of changing the electron source. In the electron microscope described in PTL 1, it is possible to keep the residual gas low by providing both the ion pump and the NEG. The NEG is a porous metal rod and uses chemisorption on metal atoms on a surface as the pumping principle. Before using the NEG, atmospheric molecules are absorbed on a metal surface and are in an inactive state. By heating the NEG to a high temperature of about 450°C under negative pressure, the gas molecules on the pump surface are released into the vacuum or sorbed in the NEG, and the metal atoms on the surface become chemically active <activation>. The activated NEG sorbs and pumps residual gas molecules such as hydrogen, nitrogen, carbon monoxide.

In an electron microscope as described in PTL 1, it is not possible to activate a NEG unless a heater dedicated to the NEG is provided to activate the NEG. In addition, at the time of baking an electron gun or the like, a heater associated with the NEG needs to be activated and heated to a temperature of an activated state. In PTL 2, a coil-shaped heater is wrapped around a NEG and the NEG is electrically heated to activate it. Therefore, it is necessary to arrange a coil dedicated to the NEG around the electron source. In addition, when the NEG is activated, a large amount of gas molecules are emitted from the NEG and absorbed by surrounding components or the electron source, which may adversely affect the performance of the electron source.

The present invention has been conceived in view of the above problems, and its object is to provide a means for suitably activating a NEG and at the same time arranging a NEG around an electron source without using a heater to activate the NEG.

the solution of the problem

The features of the present invention for solving the above problems are as follows.

The present invention is therefore an electron microscope provided with an electron gun, the electron gun having an elec ron source, an extraction electrode, and an acceleration tube, wherein the acceleration tube is connected to the extraction electrode at a connection portion, the extraction electrode includes a first heater and a first NEG, and the first heater and the first NEG are spaced in an axial direction of an electron beam emitted from the electron source are.

Advantageous Effects of the Invention

According to the present invention, it is possible to obtain a stable emission current over a long period of time without exchanging the electron source and without using a separate heater for the NEG by arranging the appropriately activated NEG particularly at the extraction electrode around the electron source. Thereby, it is possible to obtain an electron microscope that does not require long-term maintenance and obtains an observation image that has been more stabilized for a longer period of time.

character list

• [ 1 ] 1 Fig. 12 is a diagram showing an electron microscope in an embodiment of the invention.
• [ 2 ] 2 Fig. 14 is a diagram showing an electron gun in an embodiment of the invention.
• [ 3 ] 3 Fig. 12 is a diagram showing a configuration of a vacuum pump of a cold field emission type electron gun.
• [ 4 ] 4 Fig. 12 is an enlarged view showing a structure around an electron source.

Description of Embodiments

An electron microscope mainly consists of an electron gun, an electron optical system, a specimen holder, a detector, an operation unit, and a power supply unit. The electron gun is a device for generating a beam of electrons. The electron optical system is a device that transports electrons generated by the electron gun and irradiates a sample with the electrons. The electron optical system also has a function of converging and deflecting an electron beam through an electromagnetic lens. The sample holder is a device that fixes a material (a sample) to be observed in an electron beam path in the electron optics and moves and tilts the material as needed.

In order to irradiate the sample with the electron beam emitted from the electron gun without collision with air molecules, the inside of the electron gun must be kept at negative pressure. Therefore, the electron microscope is provided with a vacuum pump. Electrons irradiated to the sample generate reflected electrons, secondary electrons, transmitted electrons, scattered electrons, X-rays and the like by interacting with atoms constituting the sample. In the detector, these electrons and X-rays are measured. In addition, the fluorescence generated by the electron beam transmitted from the sample is obtained as a sample image using a fluorescent plate.

The power supply unit supplies electric power required for the operation of the electron gun, the electron optical system, the detector and the like, and performs accurate control. The operation unit controls the power supply, analyzes the information obtained from the detector, processes the information as a magnified image of the sample and the elemental composition of the sample into a state easily recognizable for an operator, displays and records the information. Here, the electron gun is a device that generates electrons used for observation as free electrons under negative pressure. Also, in the electron gun, the generated free electrons are accelerated by the differential electric potential to form a group of electrons with kinetic energy, i.e., electrons. H. an electron beam. The amount of electrons generated by the electron source in the electron gun per unit time is called the emission current.

a cold cathode field emission electron gun (cold field emission electron gun), which is one type of electron gun, will be described below. In the case of the cold field emission electron gun, a tungsten monocrystal is used as the electron source, the tip of which is sharpened by electrolytic grinding. An extraction electrode is provided near the electron source. An extraction voltage of a few kilovolts is applied between the extraction electrode and the electron source. Then, due to the pointed shape of the electron source, an electric field concentration occurs at the tip of the electron source and a high electric field is generated. Due to this high electric field, electrons are emitted from the tip of the electron source due to field emission. In the cold field emission type electron gun, the total amount of electron beams thus emitted from the electron source by field emission is the emission current described above.

The magnitude of the emission current is not only determined by the extraction voltage (and ultimately the electric field strength at the tip of the electron source) but also by the surface condition of the electron source. A cold field emission electron source is arranged in the electron gun, the residual gas pressure of which is lowered by the vacuum pump. However, when some remaining gas molecules are absorbed by the electron source, the work function of the electron source generally decreases and the amount of emission current at the same electric field strength decreases. Therefore, when the remaining gas molecules collide with the electron source and are absorbed, the amount of emission electrons decreases. As a result, the emission current gradually decreases during the operation of the electron gun provided that a constant extraction voltage is applied.

Since, as described above, the emission current of the cold field emission electron gun is affected by the surface state of the electron source, it is necessary for stable use to clean the surface of the electron source by re-emitting gas molecules absorbed on the surface of the electron source in the negative pressure or the like. This is achieved by momentarily heating the single crystal. Therefore, many cold-field emission electron guns have a mechanism that welds an electron source to the tip of a filament, heats the electron source by briefly electrically heating the filament, and performs cleaning. This operation is called flashing. Flashing cleans the tip of the electron source. In the case of the cold field emission electron gun, this flashing is carried out regularly in order to obtain a constant emission current.

Here, by keeping the residual gas pressure in the electron gun low, a necessary frequency of cleaning decreases, and the electron source can be used for a long time (about several hours to a day) without performing cleaning. This is because the frequency of adsorption of the residual gas molecules on the electron source decreases and the change in the surface state with time becomes small. In order to perform better sample observation, e.g. For example, to obtain different sample images under the same conditions and obtain element analysis results, it is important to achieve stable emission current over a long period of time without operating cleaning and extraction voltage.

In addition, the advantage of reducing the cleaning frequency also has the effect of increasing the lifetime of the electron source. Flashing is a process of heating an electron source for a short time to remove absorbed gas molecules, but since flashing causes thermal displacement of the tungsten atoms at the tip of the electron source, the physical shape of the tip portion gradually changes with each repetition of flashing. This change is usually a change such that the physical shape of the sharply pointed tip portion is gradually rounded (this is called “rounded” at the tip portion of the electron source) due to the attractive force acting on the atoms in the crystal. Therefore, when the flashing is repeated, the radius of curvature of the tip portion of the electron source gradually increases, resulting in the electric field concentration at the tip portion of the electron source being weakened accordingly. Since the electric field concentration is weakened, in order to obtain the same emission current, it is necessary to increase the extraction voltage. If the extraction voltage is set to maintain a constant emission current, the extraction voltage will gradually increase over the service life (roughly months to years) of the electron gun. If this extraction voltage exceeds a voltage limit (depending on the performance of the power source and the insulating property within the wiring and the electron gun) of the electron gun, sufficient emission current cannot be obtained from this electron source and it becomes necessary to replace the electron source. In order to replace the electron source and restart the device, the vacuum pump must normally be stopped. In particular, in order to reduce the molecular weight of the residual gas in the electron gun to a level required for long-term stabilization of the emission current, it is necessary to heat (bake) the electron gun in addition to pumping by the vacuum pump. Since the gas molecules adsorbed on the inner wall of the electron gun are instantaneously discharged by the baking and the molecular weight of the residual gas is greatly reduced after being pumped out by the vacuum pump, the baking is essential. The electron gun is baked with various heaters and then cooled to room temperature for use, but this process requires about a week's work.

Therefore, it is possible to reduce the frequency of replacement of the electron source by reducing the frequency of cleaning, and it is possible to reduce the frequency of necessary maintenance and suppress the running cost of the apparatus.

1 Fig. 12 shows an electron microscope provided with a cold field emission electron gun as an example of an embodiment of the invention. The electron microscope has an electron gun 1 , an electron optical system 2 , a specimen holder 3 , a detector 4 , an operation unit 5 and a power supply unit 6 . In 1 For example, the electron gun 1 and the electron optical system 2 each have a vacuum pump 17 and 21, respectively, but depending on the size of the electron microscope, it may be the case that a single vacuum pump is provided, or it may be the case that a large number of divided ones Vacuum pumps is provided.

An electron gun 1 provided with a cold-field emission electron gun 101 generates an electron beam 10. The electron optical system 2 converges and deflects the electron beam 10, and irradiates a sample 31. The sample holder 3 holds the sample 31 and moves, tilts, and switches the sample 31 as needed. The detector 4 measures reflected electrons, secondary electrons, transmitted electrons, scattered electrons, X-rays and the like generated from the sample 31. FIG. The power supply unit 6 supplies power to the electron gun 1 and the electron optical system 2, adjusts the output power, and controls the electron beam to a state requested by the operator. In addition, information from the detector 4 is converted into a digital signal. The operation unit 5 controls the electron gun 1 and the electron optical system 2 via a power supply system 6, processes information from the detector 4 and displays or records the information in a form visible to the operator.

2 FIG. 12 shows details of the structure of the cold-field emission electron gun 1 with an acceleration voltage of several hundreds of kilovolts according to an embodiment of the present invention. The electron gun 1 is provided with an electron source 101 (cold field emission electron source), an extraction electrode 103 and an accelerating tube 105 . The electron source 101 and the extraction electrode 103 are connected to an electron gun power supply 62 which is a part of the power supply system 6 in 1 is. The acceleration tube 105 is a cylindrical tube provided with electrodes 104, 106, 107, 108, 109 and 110 and connecting the electrodes and an insulator to each other. Since the electron gun power supply 62 is isolated from the ground potential by the accelerating tube 105, it can apply a high potential to the electron source 101, the extraction electrode 103 and the like. The electrode 110 has ground potential.

An extraction voltage (V1) of several kilovolts with respect to the electron source 101 from the electron gun power supply 62 is applied to the extraction electrode 103 . Since the electron source 101 has a sharpened tip shape, a strong electric field is generated at the tip portion of the electron source 101. FIG. At this time, the electrons emitted from the electron source 101 are initially accelerated and irradiated onto the extraction electrode 103 by the potential difference between the electron source 101 and the extraction electrode 103 based on the principle of field emission. This is the emission current. A part of the emission current passes through the hole of an anode aperture 121 provided in the extracting electrode 103 and enters the accelerating tube 105 . The accelerating tube 105 is provided with a plurality of space electrodes 104, 106, 107, 108 and 109, and the electron beam passes through the space electrodes and is further accelerated. The electron source 101 and the extraction electrode 103 are under a high negative voltage (VO) of several hundred kilovolts from the electron gun power supply 62 . The anode 110 is at ground potential and zero potential. The electron beam is accelerated by this potential difference, and the electrons that have passed through the anode 110 are electron beams with an energy of VO.

In order to prevent collisions with the air molecules of the electron beam and to prevent collisions of residual gas molecules with the surface of the electron source 101 and keep the surface of the electron source 101 in a clean state, the electron gun 1 including the accelerating tube 105 forms a vacuum container. The inside of the electron gun is evacuated by the vacuum pump 17 .

Next, the configuration of the vacuum pump of the cold field emission electron gun will be explained with reference to FIG 3 described. The electron gun 1 is a vacuum container provided with the electron source 101, a filament 102, the extraction electrode 103, and the accelerating tube 105. FIG. The inside of the electron gun 1 is evacuated by the vacuum pump 17 . The vacuum pump 17 consists of an ion pump 171 and a NEG 172. The ion pump 171 is a sputtering ion pump that pumps out residual gas molecules in the electron gun according to the principle of ionization and absorption. The NEG 172 is a pump with non-evaporable getter, the residual gas molecules in the Electron gun chemically sorbed to pump out the residual gas molecules. In 3 the NEG 172 is installed between the ground potential electrode 110 of the accelerating tube and the ion pump 171. As described above, in order to obtain an electron gun that generates a stable emission current, it is necessary to reduce residual gas molecules around the electron source 101 as much as possible. The ion pump 171 and the NEG 172 can pump out the residual gas molecules. On the other hand, however, the electron gun 1 basically has a complicated structure such as the accelerating tube 105 as its structure. For this reason, since the ion pump 171 and the NEG 172 provided at a position remote from the electron source 101 must pump residual gas molecules around the electron source 101, the pumping efficiency is poor. It is difficult to sufficiently exhaust residual gas molecules around the electron source 101 to obtain a stable emission current for a long time (several hours to a day).

Here, in the embodiment of the present invention, the NEG 173 is provided in the vicinity of the electron source 101 . The NEG 173 efficiently sorbs residual gas molecules around the electron source 101 and enables the electron gun 1 to emit the emission current stably for a long time. A NEG 173 is near the electron source 101 in 4 provided, however many NEGs 173 may be provided instead of one. It is possible to have residual gas molecules absorbed more efficiently by providing one or more of them.

In addition to the ion pump 171 and the NEG 172, the NEG 173 is provided near the electron source 101, particularly at the extraction electrode, and therefore the number of residual gas molecules colliding with the electron source can be greatly reduced.

When the NEG 173 is provided in the vicinity of the electron source 101 in the embodiment of the present invention, the extraction electrode 103 is provided with the NEG 173 . An enlarged view showing the structure around the electron source 101 and the NEG 173 is in 4 shown.

4 12 is a schematic diagram showing the structure of the electron source 101 in a state where the electron gun 1 is being set. The inside of the electron gun 1 is baked by a heater 191 arranged around the electron gun while being pumped out by a vacuum pump (not shown). The heater 191 is placed in the air outside the vacuum tank.

In the embodiment of the present invention, the extraction electrode 103 is provided with an internal heater 192 . The internal heater 192 is a heater arranged in the vacuum tank of the electron gun 1 . An electric current is introduced into the vacuum through a bushing (not shown) and serves as a heat source for the internal heater 192. The internal heater 192 is always energized during the baking of the electron gun and serves to heat the extraction electrode 103 particularly intensely . Specifically, since the role of the extraction electrode 103 is to absorb a large part of the electron beam generated by the electron source 101 during use of the electron source, the internal heater 192 bakes the extraction electrode 103 at a high temperature and causes the gas molecules absorbed in the extraction electrode perimeter 194 are emitted. Thereby, collisions of residual gas molecules with the surface of the electron source 101 can be prevented, and the electron source surface 101 can be maintained in a clean state. The suitable temperature of the heater 191 is about 350°C, which is 300°C or higher, but the suitable temperature of the inner heater 192 is about 450°C, which is 400°C or higher.

Here, the NEG 173 is attached between the inner heater 192 of the extracting electrode 103 and a connecting portion 181 where the extracting electrode 103 is in contact with the outer wall 193 of the accelerating tube, and is heated by a thermal gradient. Here, the temperature around a front end portion 182 of the extraction electrode is heated to about 450°C by the internal heater 192 . While being heated via the air by the heater, the temperature of the connection portion 181 of the outer wall 193 of the accelerating tube is lower than that of the heater 191 and is about 300°C. By fixing the NEG 173 approximately in the middle of a heat transmission path between the connection portion 181 and the internal heater 192, the temperature of the NEG 173 can be heated to approximately 350°C, which is higher than the temperature of the connection portion 181 and lower than the temperature of the internal heater 192.

In this way, it is possible to easily activate the NEG 173 without heating the NEG 173 with another heater during the baking of the electron gun.

Further, the NEG 173 is activated by being at a relatively low tempera for a long time temperature of about 350°C instead of being heated to 450°C for a short time, which is an activation temperature of the NEG and has the ability to sorb residual gas around the electron source 101.

The advantages of heating the NEG 173 at about 350°C for a long time instead of heating the NEG 173 at about 450°C for a short time are the following three points.

(1) Since the NEG can be activated using the heater 192 to heat the extraction electrode without using a separate heater and heater power supply, the structure of the inside of the electron gun is simplified, there are advantages in terms of reducing the amount of residual gas molecules and at the same time it is possible to provide an inexpensive device.

(2) There is a case where a large amount of gas molecules emitted from the NEG at the time of activation to heat the NEG to about 450°C for a short time are absorbed by the surrounding components or the electron source and adversely affect the performance of the electron source. But it is possible to slow down the generation of gas molecules from the NEG to such an extent that the gas molecules are pumped out by another vacuum pump by heating the NEG at 350°C for a long time. This suppresses the adsorption of gas molecules on the electron source.

(3) When the NEG is heated at about 450°C for a long time, an active activation action takes a long time and a large amount of gas molecules are trapped in the NEG. This shortens the lifetime of the NEG and reduces the NEG pump speed when the NEG is reactivated by electron exchange or the like. Also, it may be necessary to replace the NEG to achieve the required pump speed.

According to experimental results using a large number of electron guns, it was found that by keeping the NEG at about 350 °C within the baking time of 24 hours to 96 hours, the NEG can be activated to such an extent that a required pumping speed is achieved and a Overactivation does not occur. In the embodiment of the present invention, the extraction electrode 103 is provided with the NEG 173 to keep the NEG at about 350°C using the internal heater 192 heated to about 450°C.

According to these embodiments, compared to the prior art cold-field emission electron gun, residual gas molecules around the electron source are reduced, and a more stable emission current can be obtained for a longer period of time without exchanging the electron source. Thereby, it is possible to provide an electron microscope that does not require maintenance for a long period of time and can obtain a higher-quality observation image.

Reference List

1

electron gun

10

electron beam

101

electron source

102

filament

103

extraction electrode

104

adjustment electrode

105

acceleration tube

106

interstitial electrode

107

interstitial electrode

108

interstitial electrode

109

interstitial electrode

110

anode

110

anode

121

anode screen

17

Electron gun vacuum pump (ion pump and the like)

171

ion pump

172

NEG (Accelerating Tube Base)

173

NEG (around the electron source)

181

Section where the extraction electrode is in contact with the outer wall of the electron gun <connection section>

182

Front end portion of the extraction electrode

191

Heater to bake the electron gun

192

Inner heating

193

outer wall of the acceleration tube

194

electron source scope

2

electron optics system

21

Electron optics system vacuum pump

3

sample holder

31

sample

4

detector

5

operating unit

6

power supply unit

62

Electron Gun Power Supply

Claims (3)

electron microscope, with an electron gun (1), wherein the electron gun (1) comprises an electron source (101), an extraction electrode (103) and an acceleration tube (105), wherein the acceleration tube (105) is connected to the extraction electrode (103) at a connection portion (181), the extraction electrode (103) comprises a first heater (192) and a first NEG (173), the first heater (192) and the first NEG (173) are spaced from each other in the axial direction of an electron beam (10) emitted from the electron source (101), wherein the electron gun (1) further comprises a second heater (191) disposed around the acceleration tube (105), wherein the first NEG (173) is provided between the first heater and the connection portion, the first heater (192) is heated to 400 °C or higher, the second heater (191) is heated to 300 °C or higher, the first heater (192) is disposed in a vacuum vessel within the electron gun (1), and the second heater (191) is arranged outside the vacuum container. electron microscope claim 1 wherein the first NEG (173) is mounted between the first heater (192) and the connection section (181) and the first NEG (173) by means of a thermal gradient from the first heater (192) to the connection section (181) at about 350° C is heated by applying the thermal gradient over several hours to a day. electron microscope claim 1 , further comprising an ion pump (171), wherein a second NEG is provided between an electrode having a ground potential of the accelerating tube (105) and the ion pump (171).

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