Ion beam system

Abstract

Ion beam system with a gas field ionization ion source (1), comprising: a vacuum vessel (15), an emitter tip holder (35) located in the vacuum vessel (15), an emitter tip (21) connected to the emitter tip holder (35), an extraction electrode (24) opposite the emitter tip (21), a gas supply section (26) for supplying a gas to the emitter tip (21), and a heat transfer element (33) located in the vacuum vessel (15) and transferring cooling energy to the emitter tip holder (35), wherein the surface of the heat transfer element (33) is covered with a heat-insulating material (34) to prevent condensation of the gas, and the heat transfer element (33) is a thin metal film (85) or a metal stranded wire (56) and has a heat-insulating layer (87) adhered to its surface, characterized by an adhesive element (81), which covers the surface of the cold transfer element (33),wherein the heat-insulating material (34) adheres to the surface of the cold transfer element (33) via the adhesive element (81) and covers the adhesive element (81).

Description

PRIORITY CLAIMThe present application claims priority from Japanese Patent Application No. 2016-133700 filed on Jul. 5, 2016, the contents of which are hereby incorporated by reference into this application.BACKGROUND1. Technical FieldThe present invention relates to an ion beam system provided with a gas field ionization ion source.2. Background ArtThe surface structure of a sample can be viewed by a device which scans a sample with an electron beam and detects secondary charged particles emitted from the sample. This device is referred to as a scanning electron microscope. On the other hand, the sample surface structure may also be viewed by a device that scans the sample with an ion beam and detects secondary electrons, secondary ions, reflected ions, and the like emitted from the sample. This device is referred to as a scanning ion microscope.The ion beam is more sensitive to information about the sample surface than the electron beam. This is because the excitation region of the secondary electrons is located closer to the sample surface than the electron beam. The electron beam, which has not negligible wave properties, suffers aberrations induced by diffraction effects. Because ions are heavier than electrons, the ion beam suffers much less diffraction-induced aberrations than the electron beam. In particular, in a case where a gas field ionization ion source having a high luminance is used as compared with the electron beam, the ion beam can be focused on a very fine object.It is noted at the edge that the gas field ionization ion source is a device in which gas molecules are field ionized by supplying a gas such as helium gas to a metal emitter tip and applying a high voltage of at least several kilovolt to the emitter tip having a tip radius of curvature on the order of 100 nm and extracting the resultant ions as an ion beam. The present ion source is characterized by the ability to form a very fine ion beam because of a small ion energy distribution width and a small ion generation source.The ion beam system irradiates the sample with the very fine ion beam to detect the secondary electrons (or ions), reflected electrons (or ions), transmitted electrons (or ions), and the like. Accordingly, the system enables a highly accurate observation of the surface structure of the sample, a highly accurate measurement of the surface structure of a miniaturized semiconductor sample, or an elemental analysis of the sample by determining the energy of the electrons (ions) emitted from the sample or the angle of the energy emission thereof. Further, information reflecting the internal structure of the sample can also be obtained by irradiating a thin film sample with a very fine ion beam and detecting the ions transmitted by the sample. Moreover, irradiation with an ion beam of neon, argon, krypton, xenon, nitrogen, oxygen, carbon monoxide or the like enables very fine processing of the sample due to a severe sputtering phenomenon.Patent Literature 1 discloses a charged particle irradiation system including a charged particle beam source for emitting a charged particle beam, a charged particle optical system for focusing the charged particle beam on a sample, and a cooling mechanism for cooling a tip. The charged particle beam source includes a heat insulating structure member disposed between the tip and a high voltage terminal, and includes a heat insulating structure for suppressing heat inflow from the atmosphere side. In the heat insulation structural member, a heat transfer path from an end on the high-voltage terminal side to an end on the tip side is longer than a direct distance from the end on the high-voltage terminal side to the end on the tip side of the heat insulation structural member.PRIOR ART DOCUMENTPatent LiteraturePatent Literature 1: JP 2016-27525 ASUMMARYIn order to observe a sample having a high signal-to-noise ratio or process the sample at a high speed, an ion beam having a high current density on the sample needs to be generated. In order to meet this requirement, the density of the ion emission angular current of the gas field ionization ion source needs to be increased. In order to increase the density of the ion emission angular current, the molecular density of an ionic material gas (ionizable gas) in the vicinity of the emitter tip can be increased. The molecular density of the gas supplied to the emitter tip increases as the gas temperature decreases. It is therefore necessary to cool the emitter tip to a very low temperature. In practice, the ionizable gas can be maintained near the emitter tip at a pressure in the range of 1×10 -2 to 10 Pa.In a gas field ionization ion source having a nanopyramidic structure at a tip end of the emitter tip, the following problem occurs. If an ionizable gas having a high mass such as neon, oxygen, nitrogen, argon, krypton or xenon is used instead of hydrogen or helium, the sputtering effect of the ion beam is enhanced to be suitable for processing the sample. Although an ion beam system using helium or neon gas has been practically used, ion beam systems using other gas species have not necessarily reached a level to be practically used even though ion emission has been experimentally confirmed. In particular, an ion beam processing system using a gas having a high boiling point such as argon (boiling point 87 K), krypton (boiling point 120 K) or xenon (boiling point 156 K) as the ion source material has not been put into practical use.As described above, the emitter tip in the gas field ionization ion source is cooled. Helium used in a conventional gas field ionization ion source had a low boiling point of 4 K, so that the current could be increased to the lowest possible temperature by cooling the emitter tip. Therefore, the gas field ionization ion source has been designed to achieve the lowest possible temperature by the cooling mechanism of the ion source. However, it has been difficult to put this ion source into practical operation by introducing krypton gas instead of helium gas. Because krypton gas had a boiling point of 120 K, a krypton ion stream emitted from the emitter tip cooled to a temperature of about 120 K was a fraction to an order of magnitude lower than a helium ion stream. Therefore, a very fine ion beam having a high current density could not be formed. When the temperature of the emitter tip was lowered further, the ion current tended to increase, but became too unstable to be practically applied. Further, when the temperature of the emitter tip was increased, the vacuum degree of the ion source was promptly deteriorated, sometimes resulting in a stop of a turbo molecular pump of an evacuation system.Patent Literature 1 teaches that the amount of heat radiation from elements at room temperatures to very low temperature regions such as a tip, a copper stranded wire, heat absorbing members, and the tip holder is reduced when the components are enclosed with a heat shield. Further, a second heat exchanger is configured to include a heat exchanger to also block heat radiation to the heat exchanger. This is effective to reduce the thermal stress, so that it is expected that an even lower peak cooling temperature can be obtained. The patent literature suggests that the total amount of the transferred heat and the radiant heat may be limited to about 0.5 [W]. However, even the structure of Patent Literature 1 is not capable of stably generating a high-current ion beam from the ionizable gas having a high boiling point such as argon, krypton, and xenon.DE 10 2008 060 270 A1 describes an ion beam system having the features in the preamble of present claim 1. further conventional ion beam systems are disclosed in DE 11 2014 007 059 T5, US 2008 / 217 555 A1, US 2009 / 173 888 A1 and US 2011 / 147 609 A1.The present invention is defined in the appended claims. It provides a gas field ionization ion source which can obtain a high current sufficient for processing and stabilize an ion beam current when forming an ion beam of gas species using gases having a high boiling point such as argon, krypton, xenon, nitrogen, oxygen and carbon monoxide. The present invention further provides an ion beam system provided with this gas field ionization ion source, which can perform highly accurate and high-speed processing of a very fine region of a sample.The present invention provides an ion beam system provided with a gas field ionization ion source, which can perform highly accurate and high-speed processing of a very fine region of the sample by irradiation with an ion beam of argon, krypton, xenon, nitrogen, oxygen, carbon monoxide or the like.Problems, arrangements, and effects other than those mentioned above will be understood from the following descriptions of embodiments.BRIEF DESCRIPTION OF THE DRAWINGThe following are shown: FIG. 1 is a schematic configuration diagram of an example of an ion beam system not falling within the scope of the claims, FIG. 2 is an enlarged view of a gas field ionization ion source, FIG. 3 is a schematic configuration diagram of an example of a control system of the ion beam system not falling within the scope of the claims, FIG. 4 is a schematic configuration diagram of an example of the gas field ionization ion source of the ion beam system according to the present invention, FIG. 5 is a schematic diagram of a gas field ionization ion source and a cooling mechanism thereof according to an embodiment not falling within the scope of the claims, FIG. 6 is a schematic diagram of a gas field ionization ion source and a cooling mechanism thereof according to an embodiment not falling within the scope of the claims, FIG. 7 is a schematic configuration diagram of an example of the ion beam system according to the present invention; and FIG. 8 is a detailed view of the vicinity of a heat exchanger of the gas field ionization ion source of the ion beam system according to the present invention.DETAILED DESCRIPTIONIn a gas field ionization ion source, an emitter tip is cooled as described above. The present inventors have experimentally made a gas field ionization ion source to which krypton gas has been supplied as an ionizable gas for ion emission. The emission current was measured while the pressure of krypton gas was set to 3×10 -2 Pa and the temperature of the emitter tip was changed. At a temperature of 105K, the ion beam current was 8 pA. At a temperature of 90K, the ion beam current was 10 pA.When the experiment was attempted to be continued while the temperature was lowered to 90 K or below, if the krypton gas pressure was lowered significantly, it was impossible to carry out measurement of the emission current. Therefore, it has been attempted to measure the emission current while increasing the gas flow rate by two orders of magnitude. Thereby, it was found that at a temperature of 60 K, the ion beam current was 50 pA, which is a drastic increase in the emission current. However, it was also found that it was difficult to maintain the gas pressure at a constant level or stabilize it for a long period of time. When the temperature was lowered while the krypton gas pressure was kept constant, it could be measured how the gas pressure rapidly decreased. The system was examined and it was found that when the emitter peak temperature was 90 K, a cooling machine was cooled to about 56 K.A high luminance operating condition of the gas field ionization ion source serves to reduce the temperature of the emitter tip to the lowest possible level as described above. In a conventional gas field ionization ion source configured such that the emitter tip is cooled with cooling energy from the cooling machine, the cooling machine is at the lowest temperature in a cold transfer path between the cooling machine and the emitter tip, while the emitter tip is at the highest temperature. For example, a transfer section of the refrigeration machine which is in vacuum is 25 K, a cold transfer member made of copper is 35 K, the emitter holder is 55 K, and the emitter tip is 60 K. That is, if the emitter tip temperature is to be lowered to the lowest possible level so as not to induce gas condensation, the ionizable gas is condensed halfway through the cold transfer path, thereby making it difficult to control the gas pressure. Further, when the ionizable gas is condensed, a gas liquefied during the temperature rise of the ion source is evaporated at once and discharged in the vacuum vessel, so that a considerable load is applied to the exhaust system of the system. Consequently, the luminance of the ion source decreases as the emitter peak temperature is increased. This makes it difficult to operate the conventional gas field ionization ion source in a high luminance state in which the ion source can be basically operated.Accordingly, the present invention is based on the finding that when the emitter tip is cooled to the lowest possible temperature, it is very important not to allow gas condensation on the cold transfer path which is at lower temperatures than the emitter tip. In the conventional gas field ionization ion source, the current density of the krypton ion beam and the like could not be increased because the ion source was not designed in consideration of gas condensation on the cold transfer path.Based on such a cause study, the present invention applies a cold transfer member to the cold transfer path to transfer cooling energy to the emitter tip. The cold transfer element has a temperature controlled structure with its inner part maintained at relatively low temperatures while its surface is maintained at relatively high temperatures. For example, the surface of the cold transfer member is covered with a heat insulating material to suppress the gas condensation. The structure of the cold transfer path according to the present invention need not be applied to the entire length of the cold transfer path, but is effective even when it is applied to a part of the path. It is particularly effective to apply the structure of the present invention to a portion of the refrigerant transfer path which is in the vicinity of the refrigerating machine and whose surface temperature is lower than the condensation temperature of the gas.According to the present invention, the ion source luminance is maximized or the density of the ion emission angular current is maximized because the ionizable gas does not condense in the vacuum vessel while the ions are discharged from the emitter tip whose temperature is maintained at the lowest level. Accordingly, the very fine high current ion beam can be generated by processing ion species such as argon, krypton, xenon or nitrogen. Therefore, very fine processing can be performed more quickly than before.Before describing the present invention in detail, individual components and effects thereof will be mentioned and described.(1) For example, an ion beam system includes a gas field ionization ion source including a vacuum vessel, an emitter tip holder located in the vacuum vessel, an emitter tip connected to the emitter tip holder, an extraction electrode opposite to the emitter tip, a gas supply portion for supplying a gas to the emitter tip, and a cold transfer member located in the vacuum vessel and transferring cooling energy to the emitter tip holder. The surface of the cold transfer member is covered with a heat insulating material to prevent the gas condensation.Such a configuration prevents the condensation of the ionizable gas on the surface of the cold transfer member even when the emitter tip is cooled to the lowest temperature within a temperature range that does not allow the ionizable gas to condense. First, the emitter tip is cooled to low temperatures, so that the ion emission current increases. In this way, the density of the angular emission current of ion species having a high boiling point is increased. In particular, a very fine high-current ion beam can be formed. Accordingly, the very fine processing can be performed more quickly than before. Further, this ion beam enables observation of a sample surface with a higher resolution and a higher signal-to-noise ratio. Because the ionizable gas does not condense on the surface of the cold transfer element, the amount of gas supplied to the emitter tip is stabilized. This results in a stable ion emission current. Therefore, the very fine processing can be performed with higher precisions than before. Further, the sample surface can be viewed at a low noise level.In contrast to the prior art, according to the present invention, there does not arise a problem that when the temperature rises after the condensation of the ionizable gas on the surface of the cold transfer member, the condensed gas is evaporated, thereby promptly degrading the vacuum degree of the vacuum chamber. In some cases, this results in the breakdown of the conventional suction system. The present invention is free from such problems and can provide an ion beam system having high reliability.Conventionally, it has been considered that it is inadequate to cool the emitter tip below the boiling point of the ionizable gas because the ion current becomes unstable when the emitter tip is cooled below the boiling point of the ionizable gas. The inventors have found that the problem is not cooling the emitter tip to below the boiling point of the ionizable gas, but rather gas condensation on the surface of the cold transfer element.The inventors have also found that although the flow rate of the ionizable gas is constant, the amount of gas supplied to a tip end of the emitter significantly decreases by the gas condensation on the surface of the cold transfer member. In the prior art, there was the wrong perception that too much cooling of the emitter tip leads to a reduction in the ion current. Therefore, in the prior art, sufficient ion current for providing a gas field ionization ion source and ion beam system suitable for very fine and fast processing and very high resolution observation has not been achieved.(2) For example, an ion beam system includes the gas field ionization ion source according to section (1), and further includes: an adhesive member disposed so as to cover the surface of the cold transfer member; and a heat insulating material adhered to the surface of the cold transfer member so as to cover the adhesive member via the adhesive member.Such a configuration provides the effects described in Section (1). Because the heat insulating material adheres to the surface of the cold transfer member through the adhering member, the heat insulating material is prevented from being separated or cracking when cooled or heated. Further, in this configuration, there is no fear that the condensation of the gas enters a space between the heat insulating material and the cold transfer member. That is, there are provided a gas field ionization ion source and an ion beam system capable of performing the very fine and fast processing and the observation with very high resolution with high reliability and stability.(3) For example, an ion beam system includes the gas field ionization ion source according to section (1), and further includes: the heat insulating material disposed to cover the surface of the cold transfer member; a metal material disposed to cover the surface of the heat insulating material; and a heating mechanism for heating the metal material.Such a configuration provides the effects described in Section (1). Further, this configuration can controllably uniform the temperature of the metal surface because of the inclusion of the metal material covering the surface of the heat insulating material and the heating mechanism for heating the metal material. In particular, the configuration reliably prevents the gas condensation because the temperature of the entire surface is controlled to be at a level not permitting the gas condensation. Accordingly, there are provided a gas field ionization ion source and an ion beam system capable of performing the very fine and fast processing and the observation with very high resolution with high reliability and stability.(4) In the configuration according to the above item (1), (2) or (3), the cold transfer member is a heat exchanger that transfers cooling energy of a cooling mechanism to a vacuum vessel.Such a configuration provides the effects described in Section (1), (2), or (3). Further, since the cold transfer member is the heat exchanger for transferring the cooling energy of the cooling mechanism to the vacuum vessel of the ion source, the heat exchanger is free from the gas condensation. The heat exchanger is a cold transfer element cooled to the lowest temperature. Therefore, a considerable effect can be obtained in cases where only the surface of the heat exchanger is treated in the manner described in the sections (1), (2) and (3).(5) In the configuration according to the above item (4), the cooling mechanism is such that it circulates a helium gas cooled by the cooling machine while cooling the heat exchanger with the helium gas.Such a configuration provides the effects described in Section (4). Further, this configuration reduces transmission of mechanical vibrations of the cooling machine to the emitter tip. That is, the configuration due to the vibration-free emitter tip enables processing with higher precisions. Further, the configuration enables higher resolution viewing.(6) In the configuration according to the above item (4), the cooling mechanism is a cooling mechanism for cooling helium gas in a helium gas vessel by the cooling machine, while the heat exchanger is the helium gas vessel.Such a configuration provides the effects described in Section (4). Further, this configuration reduces transmission of mechanical vibrations of the cooling machine to the emitter tip. That is, the configuration due to the vibration-free emitter tip enables processing with higher precisions. Further, the configuration enables higher resolution viewing.(7) In the configuration according to the above item (1), (2) or (3), the cold transfer member is a metal thin film having a heat insulating layer adhering to its surface.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration reduces transmission of mechanical vibrations of the cooling machine to the emitter tip. That is, the configuration due to the vibration-free emitter tip enables processing with higher precisions. Further, the configuration enables higher resolution viewing. Moreover, by the metal thin film without an intermediate adhesive layer, strong adhesion with the heat insulating layer is achieved, thereby eliminating the gas condensation on the surface of the metal thin film. Moreover, this configuration can achieve cost reduction.(8) In the configuration according to the above item (1), (2) or (3), the cold transfer member is made of a metal, while the heat insulating layer is made of a fluororesin.Such a configuration provides the effects described in Section (1), (2), or (3). Moreover, this configuration can achieve cost reduction.(9) In the configuration according to the above item (1), (2) or (3), the cold transfer member is made of a metal, while the heat insulating layer is made of ceramic.Such a configuration provides the effects described in Section (1), (2), or (3). Moreover, this configuration can achieve cost reduction.(10) In the configuration according to the above item (1), (2) or (3), the cold transfer member is made of a metal stranded wire whose surface is covered with the heat insulating layer.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration reduces transmission of mechanical vibrations of the cooling machine to the emitter tip. That is, the configuration due to the vibration-free emitter tip enables processing with higher precisions and observation with higher resolution. Moreover, the metal stranded wire achieves an increased adhesive strength with the heat insulating layer without an intermediate adhesive layer, thereby eliminating the gas condensation on the surface of the metal stranded wire. Moreover, this configuration can achieve cost reduction.(11) In the configuration according to the above item (1), (2) or (3), the gas supply section supplies a gas containing neon, argon, krypton or xenon.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration enables processing or observation with chemical reactions with the sample suppressed.(12) In the configuration according to the above item (1), (2) or (3), the gas supply section supplies a gas containing carbon monoxide, oxygen or nitrogen.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration is suitable for processing the sample by chemical reactions.(13) In the configuration according to the above item (1), (2) or (3), the gas supply portion supplies an argon-based gas to cool the cold transfer member to 30 K or below.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration is suitable for emitting a stable high current argon ion beam for very fine and very fast sample processing and sample observation with very high resolution.(14) In the configuration according to the above item (1), (2) or (3), the gas supply section supplies a krypton-based gas to cool the cold transfer member to 50K or below.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration is suitable for emitting a stable high current krypton ion beam for very fine and very fast sample processing and sample observation with very high resolution.(15) In the configuration according to the above item (1), (2) or (3), the gas supply portion supplies a neon-based gas to cool the cold transfer member to 25K or below.Such a configuration provides the effects described in Section (1), (2), or (3). Further, this configuration is suitable for emitting a stable high current neon ion beam for very fine and very fast sample processing and very high resolution observation.(16) In the configuration according to the above item (1), (2) or (3), the gas supply section is capable of switchably supplying at least two types of gases including a krypton gas, switching between two types of gases while maintaining the emitter tip at a substantially constant temperature of 60 K or below.(17) In the configuration according to the above item (1), (2) or (3), the gas supply portion is adapted to switchably supply argon gas and helium or oxygen gas, switching between these gases while maintaining the emitter tip at a substantially constant temperature of 45 K or below.[First Embodiment]FIG. 1 is a schematic sectional view of an example of an ion beam system according to a first embodiment. FIG. 2 is a schematic diagram of a gas field ionization ion source 1 in enlarged dimensions.The following description relates to an ion beam processing system as an example of an ion beam system. The ion beam processing system according to the embodiment includes a gas field ionization ion source 1, an ion beam irradiation system column 2, a sample chamber 3, a cooling mechanism 4, and a gas supply mechanism 26. An ionizable gas supplied from the gas supply mechanism 26 is argon gas.The gas field ionization ion source 1 includes a needle-like emitter tip 21, an extraction electrode 24 opposite to the emitter tip and having an opening 27 for passage of ions, a thin wire filament 22, a columnar filament holder 23, an electrically insulating column 36, an emitter tip holder 35, and a columnar emitter base holder 37. A valve 29 capable of vacuum interruption is provided between the vacuum vessel 15 and the ion source vacuum suction pump 12, and the extraction electrode 24 is provided with a heater 30.The gas field ionization ion source 1 has a tilt mechanism 63 for changing the tilt of the emitter tip 21, and is fixed to the emitter base holder 37. The tilt mechanism 63 is used for accurately aligning the emitter tip end with an ion beam irradiation axis 64. The distortion of the ion beam is reduced by this off-axis angle adjustment. Bellows 61, 62 are deformed when the emitter tip 21 is tilted. Bellows 62 also allows very little variation in argon pressure around the emitter when emitter tip 21 is tilted. Gas contaminants around the emitter tip can be reduced as the temperature of the extraction electrode 24 is increased to 150° C. by the heater 30.The ion beam irradiation system includes a focusing lens 5 for focusing ions emitted from the gas field ionization ion source 1, a first movable aperture 6 for limiting an ion beam 14 passing through the focusing lens 5, a first deflector 32 for scanning or aligning the ion beam passing through the first aperture 6, a second deflector 7 for deflecting the ion beam, a second aperture 38 for limiting the ion beam, and an objective lens 8 which is an electrostatic ion lens for focusing the ion beam on the sample. The objective lens 8 has three electrodes.In the sample chamber 3, a sample stage 10 on which a sample 9 is placed, a charged particle detector 11, and an electron gun 16 for neutralizing a charged sample during irradiation with the ion beam are provided. The sample chamber 3 is evacuated by a sample chamber vacuum suction pump 13. The sample chamber 3 is further provided with a gas gun (not shown) for supplying an etching gas or a deposition gas in the vicinity of the sample. A base plate 18 is attached to a system supporting structure 17 via a vibration isolation mechanism 19. The system support structure is arranged on a floor 20. The gas field ionization ion source 1, the ion beam irradiation system column 2, and the sample chamber 3 are supported by the base plate 18.The cooling mechanism 4 serves to cool the emitter tip 21 in the gas field ionization ion source 1. The cooling mechanism 4 includes a cooling machine 31 located within a vacuum vessel 60. The details of the cooling mechanism will be described below, and an outline thereof will be described here. The cooling energy generated by the cooling machine 31 is transmitted to a cold transfer rod 33 as a cold transfer member. The cold transfer rod 33 passes through a vacuum baffle 39 on its way. In Fig. 1, the left side of the vacuum partition 39 belongs to the vacuum vessel 60 in which the cooling machine 31 is accommodated, while the right side of the vacuum partition 39 leads to the ion source vacuum vessel 15. The surface of the cold transfer rod 33 is covered with a heat insulating material 34 to suppress the condensation of argon gas. The cold transfer rod 33 cools the emitter tip holder 35 connected thereto, and the surface of the emitter tip holder 35 is also covered with the heat insulating material 34 to suppress the condensation of argon gas. The emitter tip holder 35 is connected to the filament holder 23 via the electrically insulating column 36. The surface of the electrically insulating column 36 is also covered with the heat insulating material 34 to suppress the condensation of argon gas. The cooling energy is transferred to the emitter tip 21 through the filament 22 attached to the filament holder 23, thereby cooling the emitter tip 21.Preferable examples of the heat insulating material 34 include ceramics and resins having low thermal conductivities. Preferred ceramics include steatitute (magnesium and silicon oxide containing ceramics), zirconia (zirconium oxide), cordierite (magnesium, aluminum and silicon oxide containing ceramics), and the like. These ceramics have thermal conductivities of 5 W / m·K or below. Particularly preferred resins include fluororesins and silicone resins usable at low temperatures. For example, the cold transfer rod 33 is made of oxygen-free copper, the emitter tip holder 35 is made of a metal such as copper or stainless steel, the electrically insulating column 36 is made of aluminum nitride or ceramic such as aluminum oxide, the filament holder 23 is made of aluminum oxide, and the filament 22 is made of tungsten.For example, when a Gifford-McMahon (GM) cooler or a pulse tube cooler is used for the cooling mechanism 4, a compressor unit using helium gas as a working gas is installed on the floor 20. The vibrations of the compressor unit are transmitted to the system support structure 17 via the floor 20. The vibration isolation mechanism 19 is disposed between the system supporting structure 17 and the base plate 18, so that high-frequency vibrations of the bottom are less easily transmitted to the gas field ionization ion source 1, the ion beam irradiation system column 2, the vacuum sample chamber 3, or the like. Here, the refrigerating machine 31 and the compressor are mentioned as causes of the vibrations of the floor 20. However, the causes of the vibrations of the floor 20 are not limited to these. The vibration isolation mechanism 19 may include a vibration proof rubber, a spring, a damper, or a combination thereof.The characteristics of the emitter tip 21 of the gas field ionization ion source lie in a microscopic projection structure of atomic size. An ion can be generated in the vicinity of an atom at a tip end of the emitter tip by adjusting the strength of the electric field formed at the tip end of the emitter tip 21. Therefore, a region where the ion is emitted or the ion beam source is a very narrow region having a nanometer or less. Therefore, the ion beam source has an increased current value per unit area or per unit cube at a high luminance. When a beam from the ion beam source is focused on the sample at a single magnification or at a fractional magnification, a beam having a diameter of 0.1 nm to 1 nm can be formed. That is, a very high resolution view or very fine processing on the order of 0.1 nm to 1 nm is provided.The microscopic projection structure on the order of one nanometer based on the atom of the emitter tip 21 typically has an atom at its tip end and an underlying layer of three to six atoms. A thin tungsten or molybdenum wire is used as the material for the emitter tip 21. A method for forming the nanopyramidic structure at the tip end of the emitter tip includes depositing iridium, platinum, rhenium, osmium, palladium, rhodium, or the like at the tip end thereof, followed by heating the emitter tip to a high temperature by supplying energy to the filament. Other methods include field evaporation in vacuum, gas etching, irradiation with an ion beam, redesign, and the like. According to such a method, the nanopyramidic structure may be formed at the tip end of the tungsten wire or molybdenum wire. In the case of <111> tungsten wire, the tip end consists of one or three tungsten atoms or iridium atoms. Otherwise, a wire of tungsten, molybdenum, platinum, iridium, rhenium, osmium, palladium, rhodium or the like is sharpened by etching the wire tip with oxygen gas or nitrogen gas introduced under the influence of an applied voltage. Alternatively, a similar microscopic projection structure may also be formed by modelling, wherein a voltage is applied to the emitter tip in a heated state.In a case where the microscopic projection structure having an atom at the tip end is formed using platinum, rhenium, osmium, iridium, palladium, rhodium, or the like, the current emitted per unit area or per unit cube or the ion source luminance may be increased, so that the emitter tip is capable of decreasing the beam size on the sample or increasing the current. However, if the emitter tip is sufficiently cooled and enough gas is supplied thereto, not only one atom is necessarily present at the emitter tip, but an emitter tip having three, six, seven or ten atoms can provide an appropriate operation. In particular, in a case where the tip end is made of four or more and less than ten atoms, the ion source luminance can be increased and the emitter tip is capable of stable operation in addition to which atoms at the tip end are less easily evaporated. Also in this case, the ion beam emitted from the vicinity of an atom reaches the sample.FIG. 3 is a schematic diagram showing an example of a control unit of the ion beam system. This control unit comprises an ion source controller 91 for controlling the gas field ionization ion source 1, a cooling mechanism controller 92 for controlling the cooling mechanism 4, a lens controller 93 for controlling the focusing lens 5 and the objective lens 8, a first aperture controller 94 for controlling the first movable aperture 6, a first deflector controller 195 for controlling the first deflector 32, a second deflector controller 95 for controlling the second deflector 7, a sample stage controller 97 for controlling the sample stage 10, a vacuum suction pump controller 98 for controlling the sample chamber vacuum suction pump 13, respectively, a plurality of electric power sources for applying voltages to electrodes and the like of the sample stage 10 and the charged particle detector 11 and a controller 96 therefor, and a system controller 99 having a computational processing capability. The system controller 99 includes a computation processing section, a storage section, an image display section, and the like. The image display section displays images generated from detection signals from the charged particle detector 11 and information input by input means.The sample table 10 includes a mechanism for linearly moving the sample 9 in two mutually orthogonal directions in a sample placement surface, a mechanism for linearly moving the sample 9 in a direction perpendicular to the sample placement surface, and a mechanism for rotating the sample 9 in the sample placement surface. The sample table 10 further has a tilt function for changing the emission angle of the ion beam 14 to the sample 9 by rotating the sample 9 about the tilt axis. These mechanisms are controlled based on a command from the system controller 99 through the sample stage controller 97.Next, operations of the gas field ionization ion source according to the embodiment will be described. After an appropriate time has elapsed since evacuation, the cooling mechanism 4 is operated. As described above, the cooling energy generated by the cooling machine 31 is transferred to the cold transfer rod 33, the vacuum partition wall 39, the emitter tip holder 35, the electrically insulating column 36, the filament holder 23, the filament 22, and the emitter tip 21 in this order. In this way, the emitter tip 21 is cooled. First, a positive high voltage is applied as an ion accelerating voltage to the emitter tip 21. A high voltage is applied to the extraction electrode 24 so that the extraction electrode is at a negative potential with respect to the emitter tip 21. Then, a strong electric field is formed at the tip end of the emitter tip 21. When argon gas is supplied from an ionizable gas supply tube 25 connected to the gas supply mechanism 26, the argon gas is drawn toward the emitter tip surface by the strong electric field. Further, the argon gas reaches the vicinity of the tip end of the emitter tip 21 where the electric field is strongest. In this manner, the argon gas is ionized to form an ion beam. The ion beam is introduced into the ion beam irradiation system column 2 through the opening 27 of the extraction electrode 24.Next, the operations of the ion beam irradiation system will be described. The operations of the ion beam irradiation system are controlled based on a command from the system controller 99. The ion beam 14 generated from the gas field ionization ion source 1 passes through the focusing lens 5, the first aperture 6, and the second aperture 38 for limiting the beam, and is focused by the objective lens 8 on the sample 9 located on the sample stage 10. An ion optical condition is defined to focus the beam from the ion beam source at a magnification of at least 0.5 to an image on the sample to obtain a high current. The signal from the charged particle detector 11 is transmitted as a luminance modulation signal to the system controller 99. The system controller 99 again generates a scanning ion microscopic image and displays the resultant image on the image display section. In this way, the sample surface can be viewed.In the cooling machine 31 according to the embodiment, a cooling stage is cooled to about 10 K. Next, the cold transfer rod 33 made of copper having a high thermal conductivity is cooled to 15 K. In the conventional ion source, the surface of the cold transfer rod 33 is exposed, resulting in the phenomenon that argon gas condenses on the surface of the rod. The ion source according to the embodiment is configured such that the emitter tip is enclosed by the extraction electrode 24 and the bellows 62. Outside an ionization chamber including the argon gas, the argon gas pressure is low. Therefore, in the related art, the phenomenon that argon gas condenses on the surface of the cold transfer rod 33 has not been observed. However, the inventors have found that in the case of gas condensation, a small temperature change induces vaporization of the condensed gas, whereby the ion beam becomes unstable. When the cold transfer rod 33 is raised to high temperatures, a large volume of argon gas is generated, thereby causing damage to an evacuation system in some cases. According to this embodiment, the surface of the cold transfer rod 33 was covered with the heat insulating material 34 to prevent the gas condensation. The surface of this heat insulating material 34 is maintained at a temperature of about 40 K. At the gas pressure used by the ion source according to this embodiment, the argon gas is condensed at temperatures of about 38 K. On the other hand, the surface of the heat insulating material is maintained at about 40 K so that the argon gas does not condense on the surface of the heat insulating material. Although the boiling point of the argon gas was about 87 K, the argon gas did not condense at temperatures below the boiling point at the argon gas pressure at which this ion source operated. Note that a case where a small amount of argon gas condenses in a narrow gap without interfering with stable operation of the ion source is included in the scope of this embodiment.The cold transfer rod 33 is connected to the emitter tip holder 35. The surface of the emitter tip holder 35 is also covered with the heat insulating material 34 to prevent the condensation of argon gas. The surface of this heat insulating material is at a temperature of about 50 K. The emitter tip holder 35 is further connected to the filament holder 23 via the electrically insulating column 36. At this point, the emitter tip holder 35 is at a temperature of about 30 K, while the filament holder 23 is at about 40 K. The emitter tip 21 connected to a distal end of the filament holder was at a temperature of about 45 K which was below the boiling point of 87 K of the argon gas. However, the argon ion beam could be stably generated.The existence of gas contaminants other than the ionizable gas in the ion source vacuum vessel is a factor leading to destabilization of the ion beam. Therefore, the inside of the vacuum vessel 15 is kept as clean as possible to improve its degree of vacuum. For high vacuum purposes, the ion source may sometimes be heated to about 150°C. Therefore, it has become common practice to use copper to form the cold transfer rod and to coat the rod surface with gold. The conventional heat insulating material is used for shielding heat radiation from the chamber. When the conventional heat insulating material is disposed on the outside of the cold transfer rod, it has been common to provide a gap between the cold transfer rod and the heat insulating material. When the gap exists between the cold transfer rod and the heat insulating material, the argon gas easily enters the gap and is easily condensed on the surface of the cold transfer rod. The cold transfer member covered with the heat insulating material to prevent the condensation of argon gas as proposed by the embodiment is not used in the related art.According to the above-described embodiment, even when the emitter tip is cooled to a temperature below the boiling point of the argon gas, but in a temperature range in which condensation of argon gas is not permitted, the argon gas does not condense on the surface of the cold transfer member. In particular, the argon gas does not condense on the surfaces of the cold transfer rod and the emitter tip holder made of copper. Further, the emitter tip is cooled to 40 K to 45 K, so that the argon ion emission current increases. Accordingly, the density of the angular emission current of an argon ion beam is increased. In particular, a very fine ion beam having a high current can be formed. Therefore, the ion beam system according to this embodiment can perform very fine processing faster than the conventional systems. This system also allows the sample surface to be viewed with a higher resolution and a higher signal-to-noise ratio. Further, the amount of argon gas supplied to the emitter tip is stabilized because the argon gas does not condense on the surface of the cold transfer member. Therefore, the argon ion emission current is stabilized. Accordingly, the ion beam system according to this embodiment can perform the very fine processing with higher precisions than the conventional systems. This system also allows the sample surface to be viewed with less noise.Further, this embodiment eliminates the problem occurring in the prior art that, when the temperature rises after the ionizable gas is condensed on the surface of the cold transfer member, the condensed gas vaporizes, thereby promptly degrading the vacuum degree of the vacuum chamber. In some cases, this results in the breakdown of the conventional suction system. The embodiment does not have this difficulty and can provide an ion beam system with high reliability.It has conventionally been considered that it is not appropriate to cool the emitter tip below the boiling point of the argon gas because the ion current becomes unstable when the emitter tip is cooled below the boiling point. The inventors have found that there is a problem in the condensation of argon gas on the surface of the cold transfer member.The inventors have also found that although the flow rate of the argon gas is constant, the amount of gas supplied to the tip end of the emitter tip is significantly reduced by the gas condensation on the surface of the cold transfer member. There has been a false perception in the prior art that excessive cooling of the emitter tip results in the reduction of the ion current. Therefore, in the prior art, sufficient ion current for providing a gas field ionization ion source and ion beam system suitable for very fine and fast processing and very high resolution observation has not been achieved.If the cold transfer member is made of a metal while the heat insulating layer is made of a fluororesin or ceramic, a gas field ionization ion source and an ion beam system are provided which are capable of performing very fine and very rapid processing and observation of very high resolution at low cost.Although the embodiment uses argon gas, the ionizable gas may be a gas containing carbon monoxide, oxygen, or nitrogen. This approach can achieve that the sample can be processed by a chemical reaction. When carbon monoxide is used as the ionizable gas, the typical gas pressure is on the order of 1×10 -3 Pa and the condensation temperature is about 35 K. For example, the emitter tip at this time may be maintained at a temperature of about 38 K, the cold transfer member may be maintained at a temperature of about 20 K, and the surface of the heat insulating material may be maintained at a temperature of about 45 K. When oxygen is used as the ionizable gas, the typical gas pressure is on the order of 1×10 -3 Pa and the condensation temperature is about 38 K. For example, the emitter tip at this time may be maintained at a temperature of about 40 K, the surface of the heat insulating material may be maintained at a temperature of about 50 K, and the cold transfer member may be maintained at a temperature of about 25 K. When nitrogen is used as the ionizable gas, the typical gas pressure is on the order of 1×10 -3 Pa, and the condensation temperature is about 32 K. For example, the emitter tip at this time may be maintained at a temperature of about 35 K, the surface of the heat insulating material may be maintained at a temperature of about 40 K, and the cold transfer member may be maintained at a temperature of about 20 K.When an argon-based gas is introduced into the gas field ionization ion source according to the embodiment, the cold transfer member is cooled to a temperature below 30 K which is lower than the boiling point of the argon gas as described above. Accordingly, there are provided a gas field ionization ion source and an ion beam system particularly well suited to perform the very fine processing and the observation with a very high resolution upon irradiation in argon gas.Although argon gas is used according to the embodiment, gases containing other inactive gases such as neon, krypton and xenon are also usable. Such an approach allows processing and inspection to be performed on the sample with fewer chemical reactions.When a mixture of argon gas and hydrogen gas or a mixture of argon gas and helium gas is introduced into the gas field ionization ion source according to the embodiment, the emitter tip is cooled to a substantially constant temperature higher than the condensation temperatures of the individual gases and at 45 K or below, for example, at 40 K. Such an approach allows neither argon gas, helium gas, nor hydrogen gas to condense. This allows the emitter tip to emit two types of ion beams while hardly changing the emitter tip temperature. The two types of gases can be selected by switching an extraction voltage applied to the emitter tip. This voltage switching may be implemented by software for controlling the electric power source.In this way, the ion beams of two or more gas species containing at least argon gas are stably generated, and a variety of effects are also obtained. That is, the use of the argon ion beam enables rapid processing. On the other hand, when a hydrogen or helium ion beam is emitted to the sample, observation or measurement of the sample with little damage occurring is enabled. Further, more detailed information on the sample surface or the inside of the sample can be obtained by comparing a viewing image obtained by irradiation with the hydrogen or helium ion beam with a viewing image obtained by irradiation with the argon ion beam, or by performing calculations on these viewing images. It should be noted that the two gas species may also be introduced through separate feed tubes.[Second Embodiment]FIG. 4 is a schematic diagram showing another embodiment of the gas field ionization ion source and the cooling mechanism therefor in the ion beam system shown in FIG. 1. Here, the cooling mechanism will be described. This embodiment uses neon gas as the ionizable gas.According to this embodiment, a cooling mechanism in which a GM cooling machine 40 and a helium gas container 43 are combined is used as the cooling mechanism 4 of the gas field ionization ion source 1. The central axis of the GM cooling machine is oriented parallel to the optical axis of the ion beam irradiation system, which extends through the emitter tip 21 of the ion beam system. This structure can achieve both improvement of convergence of the ion beam and improvement of cooling function.The GM cooling machine 40 includes a heat sink 41, a first cooling stage 42A, and a second cooling stage 42B. The heat sink 41 is supported by a support stand 103. The first cooling stage 42A and the second cooling stage 42B are suspended from the heat sink 41. The outer diameter of the first cooling stage 42A is larger than the outer diameter of the second cooling stage 42B. The first cooling stage 42A has a cooling capacity of about 5 W, while the second cooling stage 42B has a cooling capacity of about 0.2 W.The upper end of the first cooling stage 42A is enclosed by a bellows 69. The lower ends of the first cooling stage 42A and the second cooling stage 42B are covered by the gas seal helium gas container 43. The helium gas container 43 includes a large diameter portion 43A configured to include the first cooling stage 42A and a small diameter portion 43B configured to include the second cooling stage 42B. The helium gas container is supported by a support stand 104. The support stand 104 is supported by the base plate 18 shown in FIG. 1. The bellows 69 and the helium gas container 43 have sealing structures in which a helium gas 46 is loaded as a heat transfer medium. Although the two cooling stages 42A, 42B are surrounded by the helium gas 46, they are not in contact with the helium gas container 43.In the GM cooling machine 40 according to the embodiment, the first cooling stage 42A is cooled to about 50 K. Therefore, the helium gas 46 is cooled to about 70 K around the first cooling stage 42A. The second cooling stage 42B is cooled to about 4 K. The helium gas 46 around the second cooling stage 42B is cooled to about 6K. Accordingly, the lower end of the helium gas container 43 is cooled to about 6 K.This helium gas container is located in the ion source vacuum vessel 15, which also serves as a cold transfer element for transferring the cooling energy of the GM cooling machine 40 to the emitter tip 21. The surface of this helium gas container is covered with the heat insulating material 34 to prevent the condensation of neon gas. This embodiment uses, as the heat insulating material 34, a fluororesin material having a thickness of at least 5 mm. The surface of this heat insulating material is at a temperature of about 20 K. The pressure of the neon gas was about 1×10 -3 Pa. While the boiling point of neon is about 27 K, its condensation temperature at neon gas pressure during the operation of this ion source was about 10 K. Therefore, the neon gas did not condense at temperatures below its boiling point. This is because the condensation temperature of neon under this condition is about 10 K, and the surface temperature of the heat insulating material 34 covering the helium vessel surface and coming into contact with the neon gas is higher than the condensation temperature of the neon gas. Because the lower end of the container is about 6K, the neon gas condenses as it comes into contact with the container surface. However, the neon gas is only slightly condensed because the helium gas container 43 is covered with the heat insulating material 34. Note that a case where a small amount of neon gas condenses in a narrow gap without interfering with stable operation of the ion source is included in the scope of this embodiment.On the other hand, the lower end of the helium gas container 43 is connected to the cold transfer rod 33 made of copper having high thermal conductivity. The cold transfer rod 33 also serves as a cold transfer member, and its surface is covered with the heat insulating material 34 to prevent the condensation of neon gas. The heat insulating material 34 is a silicone resin material having a thickness of about 10 mm. While the cold transfer rod 33 is at a temperature of about 15 K, the surface of the heat insulating material is at a temperature of about 30 K, so that the neon gas does not condense thereon. Further, the cold transfer rod 33 is connected to a copper stranded wire 56. The stranded wire is formed by braiding about 1000 thin copper lines having a size of about 0.05 mm. The surface of each of the thin lines of the stranded wire 56, thereby forming the cold transfer member, is covered with a heat insulating fluororesin to prevent the condensation of neon gas. Therefore, the neon gas does not condense on the stranded wire. Further, the stranded wire 56 is connected to the emitter tip holder 35. The emitter tip holder 35 is connected to the filament holder 23 via the electrically insulating column 36. This filament holder 23 has its own emitter tip tilt mechanism. At this point, the emitter tip holder 35 is at a temperature of about 20 K, while the filament holder 23 is at a temperature of about 25 K.The emitter tip holder 35 and the electrically insulating column 36 also constitute the cold transfer member, and their surfaces are each covered with an yttrium-containing zirconia ceramic material having a thickness of about 5 mm as the heat insulating material 34 to prevent the condensation of neon gas. The surface of the heat insulating material on the emitter tip holder 35 is at a temperature of about 45 K, while the surface of the heat insulating material 34 on the electrically insulating column 36 is at a temperature of about 45 K. The emitter tip 21 connected to the tip end of the emitter tip holder is at a temperature of about 25 K which is lower than the boiling point of neon gas of 27 K but higher than the condensation temperature of 10 K of neon gas. Therefore, the neon beam is stably generated.According to this embodiment, as described above, the neon gas does not condense on the surface of the cold transfer member even when the emitter tip is cooled to a temperature below the boiling point of the neon gas but in a temperature range in which condensation of neon gas is not permitted. In particular, the neon gas does not condense on the surface of the helium gas container and the surfaces of the cold transfer rod, the stranded wire, the emitter tip holder, the electrically insulating column, the filament holder, and the like, which are made of copper. Further, the emitter tip 21 is cooled to low temperatures, so that the neon ion emission current is increased. In this way, the density of the emission angular current of the neon beam is increased. In particular, a very fine ion beam having a high current can be formed. Therefore, the ion beam system according to this embodiment enables the very fine processing to be performed more quickly than the conventional systems. The system according to the embodiment further enables the sample surface to be viewed with a higher resolution and a higher signal-to-noise ratio. The amount of neon gas supplied to the emitter tip is stabilized because of the elimination of the condensation of neon gas on the surface of the cold transfer member. Therefore, the neon ion emission current is stabilized. Accordingly, the ion beam system according to this embodiment can perform the very fine processing with higher precisions than the conventional systems. Furthermore, this system also allows the sample surface to be viewed with less noise. The embodiment shown in FIG. 4 can achieve the same effects as the first embodiment.The embodiment uses the GM chiller 40, which may be replaced with a pulse tube cooler or a Stirling cooler. While the cooling machine according to the embodiment has two cooling stages, it may also have a single cooling stage. The number of cooling stages is not particularly limited.The cooling mechanism according to the embodiment cools the helium gas in the helium container by the GM cooling machine. The cooled helium gas cools the helium vessel. Also, since the cooling stages of the GM refrigerator are separated from the helium vessel, the embodiment is characterized in that it prevents the mechanical vibrations of the GM refrigerator from being transmitted to the helium vessel.When the neon-based gas is introduced into the gas field ionization ion source according to the embodiment, the cold transfer member is cooled to a temperature below 20 K lower than the boiling point of the neon gas as described above. Accordingly, there are provided a gas field ionization ion source and an ion beam system particularly suitable for performing very fine processing and observation with very high resolution in neon irradiation.Although the temperature of the emitter tip was about 25 K in the above example, its temperature may be further lowered to about 20 K, for example. At this time, the temperature of the cold transfer member is maintained at about 8 K, and the surface temperature of the heat insulating material is maintained at about 15 K.When a gas mixture of neon and hydrogen or a gas mixture of neon and helium is introduced into the gas field ionization ion source according to the embodiment, the emitter tip is cooled to a temperature of 30 K to 35 K, for example. This approach allows neither neon gas nor helium gas or hydrogen gas to condense. This allows the emitter tip to emit two types of ion beams while hardly changing the emitter tip temperature. In this way, the ion beams of two or more gas species containing at least neon gas are stably generated, and a variety of effects are also obtained. That is, the use of the neon ion beam enables rapid processing. On the other hand, observation or measurement of the sample with little damage is possible when the hydrogen or helium ion beam is irradiated on the sample. Alternatively, more detailed information on the sample surface or the inside of the sample can be obtained by comparing a viewing image obtained by irradiation with the hydrogen or helium ion beam with a viewing image obtained by irradiation with the neon ion beam, or by performing calculations on these viewing images. It should be noted that the two gas species may also be introduced into the ion source by separate feed tubes.Although neon is used as the ionizable gas according to the above embodiment, neon may be replaced with krypton or xenon. When krypton is used as the ionizable gas, the typical gas pressure is on the order of 1×10 -3 Pa and the condensation temperature is about 56 K. For example, the temperature of the emitter tip at this time may be maintained at about 60 K, the surface temperature of the heat insulating material may be maintained at about 65 K, and the temperature of the cold transfer member may be maintained at about 45 K.FIG. 5 is a schematic diagram of another embodiment of the gas field ionization ion source 1 and a cold transfer path of the cooling mechanism 4 thereof in the ion beam system shown in FIG. 1.The emitter tip holder 35, the electrically insulating column 36, the filament holder 23, the filament 22, the emitter tip 21, and the extraction electrode 24 of the gas field ionization ion source 1 are disposed in an ionization chamber 75. Krypton gas is introduced as, for example, an ionizable gas from the gas supply mechanism 26 into the ionization chamber 75 via the gas supply tube 25 to provide a krypton gas pressure of about 0.2 Pa. The cooling energy of the cooling machine 31 is transferred through the cold transfer copper rod 33 to the emitter tip holder 35 of the gas field ionization ion source. The surface of the cold transfer rod 33 is covered with the heat insulating material 34 with an intermediate adhesive layer 71 interposed therebetween. The intermediate adhesive layer 71 serves to cause the heat insulating material 34 to adhere closely to the cold transfer rod. A chromium-aluminum-yttrium alloy is used for the intermediate adhesion layer, for example. An yttrium-containing zirconium ceramic was used as the heat insulating material 34.The temperatures of the cold transfer path from the cooling machine 31 to the emitter tip 21 are as follows. When the temperature of the cooling machine 31 was 20 K, the cold transfer rod 33 was about 30 K, the emitter tip holder 35 was about 50 K, and the emitter tip 21 was about 60 K. The krypton gas used as the ionizable gas has a condensation temperature of about 56 K. Under this condition, it is necessary to cover the surfaces of the cold transfer rod 33 and the emitter tip holder 35 with the heat insulating material and to increase the surface temperature of the heat insulating material to about 56 K in order to prevent krypton condensation. The surface temperature of the heat insulating material 34 covering the surface of the cold transfer rod 33 was about 65 K, whereby krypton gas did not condense. The emitter tip holder 35 is provided with a temperature controller 76 having a resistance heater, a temperature sensor and the like. The temperature of the emitter tip 21 is controllable by the temperature control device 76.The heat insulating material need not necessarily be applied to the entire length of the cold transfer path, but need only be applied to at least a portion thereof whose temperature is lower than or equal to the condensation temperature of the ionizable gas. Accordingly, depending on the temperature condition of the cold transfer path, the species of the ionizable gas, and the like, a configuration shown in FIG. 5 can be formed in which the heat insulating material 34 covers only the surface of the cold transfer rod 33 via the intermediate adhesive layer 71, while the surfaces of the emitter tip holder 35 and the electrically insulating column 36 are not covered with the heat insulating material but are exposed. Although FIG. 5 shows that the heat insulating material 34 covers the entire length of the cold transfer rod 33 via the intermediate adhesive layer 71, the heat insulating material 34 does not necessarily need to cover the entire length of the cold transfer rod 33. Depending on conditions, the prevention of the condensation of the ionizable gas can be achieved by covering only the surface of a portion of the cold transfer rod 33 with the heat insulating material 34 via the intermediate adhesive layer 71, the portion being in the vicinity of the cooling machine 31 and cooled to a lower temperature.FIG. 6 is a schematic diagram of another embodiment of the gas field ionization ion source 1 and the cold transfer path of the cooling mechanism 4 thereof in the ion beam system shown in FIG. 1. Nitrogen gas was used as the ionizable gas.This embodiment is different from the embodiment shown in Fig. 5 in that an aluminum thin film having a thickness of 0.1 mm is formed as the metal layer 72 via the intermediate adhesive layer 71 on the heat insulating material 34 covering the cold transfer rod 33, and that a resistance heater 73 is disposed in contact with the metal layer 72. The other components are the same as those shown in FIG. 5, so detailed description thereof is omitted.According to the embodiment, the surface temperature of the cold transfer path for transferring the cooling energy through the cold transfer rod 33 can be freely controlled by controlling the output of the resistance heater 73. For example, when the temperature of the cooling machine 31 was 20 K, the cold transfer rod 33 was at a temperature of about 30 K, the emitter tip holder 35 was at a temperature of about 50 K, the emitter tip 21 was at a temperature of about 60 K, and the metal layer 72 was at a temperature of about 65 K. Therefore, the cold transfer rod 33 according to the embodiment can cool the emitter tip 21 to a desired temperature by transferring the cooling energy of the cooling machine 31 to the emitter tip holder 35 without allowing the condensation of the ionizable gas.[Third Embodiment]FIG. 7 is a schematic diagram of an example of the ion beam system. According to this embodiment, an example of the cooling mechanism of the ion beam system will be described in detail.A cooling mechanism 4 according to the embodiment uses a helium circulation system. The gas field ionization ion source 1 uses krypton gas as an ionizable gas. The embodiment is constructed such that a region around the emitter tip is enclosed by the cylindrical wall and the extraction electrode, so that the pressure of the supplied gas is increased in the region around the emitter tip. The gas supply tube is connected to this ionization chamber.The cooling mechanism 4 includes a compressor unit 400, a pulse tube cooler 401, a vacuum heat insulating vessel 416, a transfer tube 404, and the like. The pulse tube cooler 401 generates the cooling energy by circulating the helium gas. A compressor unit 116 is attached to the pulse tube cooler. Transfer tubes 111, 112 allow the helium gas to pass through to circulate. The vacuum heat insulating vessel 416 is evacuated by a vacuum pump having a degree of vacuum of 1×10 -4 Pa. In particular, the vacuum heat insulation vessel blocks heat transfer from the outside by keeping the vacuum inside the vessel. The vacuum heat insulation vessel 416 includes cooling stages 408, 411 of the pulse tube cooler 401, a heat exchanger, helium gas tubes, and the like. The helium gas is circulated from the compressor unit 400 through the tubes.The helium gas is cooled by the pulse tube cooler 401 and the heat exchangers 402, 409, 410, 412. The helium gas brought to about 0.9 MPa by the compressor unit 400 flows into the heat exchanger 402 at a temperature of 300 K through a pipe 419 so as to be cooled to a temperature of about 60 K by heat exchange with returning helium gas at a low temperature, as described below. The cooled helium gas is transported through a pipe 403 in the heat insulated transfer tube 404 and flows to a heat exchanger 405 disposed in the vicinity of the gas field ionization ion source 1. Here, the helium gas cools a heat conductor 54 thermally integrated with the heat exchanger 405 to a temperature of about 65 K, thereby cooling a radiation shield in the ion source. The radiation shield is not shown in the figure.The heated helium gas flows out of heat exchanger 405 and through a tube 407 to heat exchanger 409 which is thermally integrated with the first cooling stage 408 of pulse tube cooler 401. The helium gas is cooled to a temperature of about 15 K by heat exchange with the returning helium gas having a low temperature as described below. Then, the helium gas flows into the heat exchanger 412 thermally integrated with the second cooling stage 411 of the pulse tube cooler 401. The helium gas is cooled to a temperature of about 9K and transported through a tube 413 in the transfer tube 404. The helium gas flows into a heat exchanger 414 located near the gas field ionization ion source 1. In the heat exchanger 414, the helium gas cools a cold transfer rod 53 having a high thermal conductivity to a temperature of about 10 K. The transfer rod is thermally connected to the heat exchanger 414.Next, the helium gas heated by the heat exchanger 414 flows through a pipe 415 and sequentially into the heat exchangers 410, 402. The helium gas is substantially returned to a normal temperature of about 275K by heat exchange with the mentioned helium gas. Then, the resultant helium gas flows through the pipe 415 to be recovered by the compressor unit 400. The aforementioned low temperature portion is accommodated in the vacuum heat insulating vessel 416 and is adiabatically connected to the transfer tube 404, this connection not being shown in the figure. In the vacuum heat insulating vessel 416, entry of heat from a room temperature portion is blocked by heat radiation into the low temperature portion by a radiation shielding plate, a multilayer heat insulating material or the like, which is not shown in the figure.It is noted here that a gap is located between the radiation shielding plate and the multilayer heat insulating material and the low temperature portion. This is because a heat insulating effect is provided by vacuum. If krypton gas is introduced into the vacuum heat insulating vessel 416, the gas condenses on the surface of the low temperature portion. The conventional arrangement of the heat insulating material is intended to prevent the entry of heat into the low temperature portion, so that a gas having a high boiling point does not need to be considered. Therefore, the heat insulating material was not used for covering the low temperature portion to prevent the gas condensation. Further, such a vacuum heat insulating vessel does not need to achieve a very high vacuum, so that it is not necessary to heat the ion source to above 150° C. in order to improve its degree of vacuum. That is, no measures need be taken to prevent deterioration or cracking of the heat insulating material.The transfer tube 404 is securely fixed to and supported by the floor 20 or a support body 417 anchored to the floor 20. Although not shown in the figure, the tubes 403, 407, 413, 415 securely held in the transfer tube 404 by a heat insulating material having a low thermal conductivity made of glass fiber-filled resin are also securely supported by the floor 20.As described above, the cooling mechanism according to the embodiment is a cooling mechanism for cooling a cooling target by a refrigeration generating portion for generating cooling energy by expanding a first high-pressure gas generated by the compressor unit 116, the helium gas being cooled as a second moving refrigerant by the cooling energy of this refrigeration generating portion, and being circulated by the compressor unit 400.FIG. 8 is a schematic diagram illustrating the details of heat exchanger 414. The heat exchanger 414 is disposed in the same vacuum vessel 15 in which the gas field ionization ion source is accommodated, and its surface is covered with the heat insulating material 34 to prevent the condensation of krypton gas. The heat exchanger is made of oxygen-free copper having a high thermal conductivity. The tubes 413, 415 through which the cooled helium gas flows are formed in the heat exchanger 414. Specifically, the cooling energy of the helium cooled by the heat exchanger 414 is transferred to the oxygen-free copper. An adhesive member 81 is disposed between the heat exchanger 414 and the heat insulating material 34. The chromium-aluminum-yttrium alloy is used for the heat insulating material 34, for example. The adhering member 81 is used to cause the heat insulating material 34 to adhere closely to the cold transfer member, and uses, for example, the chromium-aluminum-yttrium alloy. The heat insulating material 34 covers this adhesive member 81, thereby preventing the gas from being absorbed to the surface of the adhesive member. This approach prevents the separation or cracking of the heat insulating material when the heat exchanger is cooled to achieve its primary function or when the heat exchanger is heated to above 150° C. by baking or the like to improve the degree of vacuum. Further, this approach also eliminates the problem that the gas enters and condenses in the gap between the heat insulating material and the cold transfer member.The krypton gas is not condensed on the heat exchanger. The heat exchanger is a cold transfer element cooled to the lowest temperature. Therefore, a considerable effect can be obtained by treating only the surface of the heat exchanger. Further, the surface of the heat insulating material is covered with a thin film 82 of oxygen-free copper, a part of which is in contact with a heating mechanism 83. The heating mechanism 83 is, for example, a resistance heater whose output is controllable. A thermometer 84 is attached to the surface of the oxygen-free copper thin film 82. The thin film can be controlled to a predetermined temperature by inputting a temperature measured by the thermometer 84 to the temperature controller. According to this embodiment, the surface of the oxygen-free copper thin film 82 is controlled to a constant temperature of about 65 K. On the other hand, the heat exchanger is maintained at a temperature of about 30 K. The heating mechanism 83 may also use lamp heating.Next, the heat exchanger 414 is joined to a cold transfer thin film 85. The cold transfer thin film 85 is characterized in that it transfers vibrations of the heat exchanger 414 very weakly. As shown in the circle of the figure in an enlarged sectional view, this thin film has an oxygen-free copper inner portion 86 whose periphery is covered with a magnesium-silicon oxide ceramic heat insulating thin film 87. The heat insulating thin film is covered with an aluminum thin film 88. The inner oxygen-free copper is maintained at about 45 K, while the surface of the aluminum thin film is maintained at about 65 K. As shown in Fig. 7, the cold transfer thin film 85 is connected to the emitter tip holder 35. The surface of the emitter tip holder 35 is also covered with zirconia as the heat insulating material 34 to prevent the condensation of krypton gas. The emitter tip holder 35 is connected to the filament holder 23 via the electrically insulating column 36. The emitter tip 21 is cooled to about 60 K.Under the gas pressure of 1×10 -3 Pa used by this gas field ionization ion source 1, the condensation temperature of krypton gas is about 56 K. When cooled to below 56 K, krypton condenses on a solid surface. However, krypton gas does not condense because the surfaces of the heat exchanger 414 and the cold transfer thin film 85 are maintained at temperatures above 65K. Since the emitter tip 21 is cooled to 60 K, a large amount of gas is supplied to the tip end of the emitter tip 21, thereby generating a high ion beam current.This embodiment can control the temperature of the metal surface to be equalized because the embodiment has the metal material covering the surface of the heat insulating material and the heating mechanism for heating the metal material. The embodiment can reliably eliminate the gas condensation because the entire surface can be brought to a temperature at which gas condensation is not possible. The embodiment can provide a gas field ionization ion source and an ion beam system capable of performing the very fine and very fast processing and observation at a very high resolution with high reliability and high stability. Further, krypton has a higher atomic weight than argon, thereby providing a higher processing speed per ion current. Moreover, krypton is less likely to generate bubbles in the sample because krypton remains in a flat area on the sample surface.Although the pulse tube cooler 401 causes vibration of the bottom, the gas field ionization ion source 1, the ion beam irradiation system column 2, the vacuum sample chamber 3, and the like are insulated from the pulse tube cooler 401. Further, the tubes 403, 407, 413, 415 connected to the heat exchangers 405, 414 installed in the vicinity of the gas field ionization ion source 1 do not vibrate because they are held and supported by the bottom 20 and the base plate 18 which hardly vibrate. Furthermore, the tubes are insulated from the vibrations of the floor. Therefore, mechanical vibrations of the system are greatly reduced.The embodiment uses the pulse tube cooler 401, which may be replaced with the GM chiller or the Stirling cooler. While the cooling machine according to the embodiment has two cooling stages, it may also have a single cooling stage. The number of cooling stages is not particularly limited. For example, a compact and inexpensive ion beam system can be produced if a compact Stirling cooler having a cooling stage in which the lowest cooling temperature is set to 50 K is used as the helium circulation cooler. In this case, neon gas or oxygen gas may be used instead of helium gas.The ion beam system according to the embodiment enables observation at high resolution because the emitter tip is less susceptible to the vibrations from the cooling mechanism and a fixing mechanism for the emitter base holder is provided. The inventors have further found that the noises of the compressor units 116, 400 induce the vibrations of the gas field ionization ion source 1 and deteriorate the resolutions thereof. According to this embodiment, therefore, a sound proof cover 418 is provided for spatially separating the compressor unit and the gas field ionization ion source. Accordingly, the embodiment can reduce the influence of the vibration induced by the noise of the compressor units and enables observation with high resolution. Gap elimination is particularly important to achieve sound suppression. The ion beam source has a high optical magnification of the sample, so that the vibrations of the emitter tip appear as vibrations of the beam on the sample. Therefore, the vibration control is essential for improving the operation of the ion beam system.When the krypton-based ionizable gas is introduced into this gas field ionization ion source, the cold transfer member is cooled to a temperature below 50 K lower than the boiling point thereof, as described above. Then, the gas field ionization ion source and the ion beam system capable of performing the very fine and very fast processing and the observation with a very high resolution upon irradiation of krypton are provided.When a gas mixture of krypton and hydrogen or a gas mixture of krypton and helium is introduced into the gas field ionization ion source according to the embodiment, the emitter tip is cooled to a temperature that is 60 K at the maximum and higher than the condensation temperature of krypton. Such an approach enables neither krypton gas nor helium gas nor hydrogen gas to condense because the condensation temperature of the krypton gas is about 56 K, while the condensation temperatures of the helium gas and the hydrogen gas are 20 K or below. This allows the emitter tip to emit two types of ion beams while hardly changing the emitter tip temperature. In this way, the ion beams of two or more gas species containing at least krypton gas are stably generated, and a variety of effects are also obtained. That is, the use of the krypton ion beam enables fast processing. On the other hand, observation or measurement of the sample with little damage is possible when the hydrogen or helium ion beam is irradiated on the sample. Further, more detailed information on the sample surface or the inside of the sample can be obtained by comparing a viewing image obtained by irradiation with the hydrogen or helium ion beam with a viewing image obtained by irradiation with the krypton ion beam, or by performing calculations on these viewing images. It should be noted that the two gas species may also be introduced into the ion source by separate feed tubes.[Fourth Embodiment]An ion beam system according to this embodiment has substantially the same system configuration as that illustrated in FIG. 1. However, it is to be understood that the embodiment uses a direct coupling pulse tube refrigerator as the cooling mechanism 4. The embodiment used nitrogen as the ionizable gas.The cooling mechanism according to the embodiment is configured such that a cooling head of the cooler is disposed in the same vacuum vessel in which the gas field ionization ion source is accommodated. The cooler's cooling head is connected to the emitter tip to cool the emitter. The embodiment uses a thin wire bundle of oxygen-free copper as the intermediate cold transfer member. The surface of each of the thin wires is covered with a heat insulating material. This bundle of thin wires is characterized in that it does not transmit the mechanical vibrations of the pulse tube cooler to the emitter. The surface of the cooling head of the radiator is covered with the heat insulating material, the surface of which is covered with a thin film of oxygen-free copper. A portion of the oxygen-free copper thin film is in contact with a resistive heating mechanism. The surface of the oxygen-free copper thin film was controlled to a constant temperature of about 40 K. On the other hand, the inside of the cold head is maintained at about 20 K. The thin wire bundle of oxygen free copper is connected to the emitter holder to cool the emitter tip to about 35K.Under the gas pressure of 1×10 -3 Pa used by this gas field ionization ion source 1, the condensation temperature of the nitrogen gas is about 32 K. When cooled to below 32 K, the nitrogen gas condenses on a solid surface. However, the nitrogen gas did not condense because the surface of the cold head was maintained at temperatures above 35 K. Since the emitter tip was cooled to 35 K, a large amount of gas was supplied to the tip end of the emitter, thereby generating a high ion beam current. This allowed the very fine processing to be achieved more quickly than the conventional system.It should be noted that the present invention is not limited to the above embodiments and includes a variety of modifications. The above embodiments explained are intended to explain the present invention by way of example. The present invention is not necessarily limited to having all the above-described components. A component of one embodiment may be replaced with a component of another embodiment. Further, a component of one embodiment may be added to arrange another embodiment. A part of the arrangement of each embodiment enables addition of a component of another embodiment, omission or replacement thereof.The present invention further discloses the following gas field ionization ion sources and ion beam systems.An ion beam system comprising a gas field ionization ion source, comprising: a vacuum vessel; an emitter tip holder located in the vacuum vessel; an emitter tip connected to the emitter tip holder; an extraction electrode opposite to the emitter tip; a gas supply portion for supplying a gas to the emitter tip; and a cold transfer member located in the vacuum vessel and transferring cooling energy to the emitter tip holder, the system further comprising at least two temperature control devices each comprising a temperature sensor and a heater and having a structure wherein: the gas supply portion supplies krypton gas and the emitter tip is maintained at a substantially constant temperature of 60 K at the most.An ion beam system comprising a gas field ionization ion source, comprising: a vacuum vessel; an emitter tip holder located in the vacuum vessel; an emitter tip connected to the emitter tip holder; an extraction electrode opposite to the emitter tip; a gas supply portion for supplying a gas to the emitter tip; and a cold transfer member located in the vacuum vessel and transferring cooling energy to the emitter tip holder, the system further comprising at least two temperature controllers each comprising a temperature sensor and a heater and having a structure, wherein:the gas supply portion supplies argon gas, andthe emitter tip is maintained at a substantially constant temperature of at most 45 K.DESCRIPTION OF SYMBOLS1 Gas field ionization ion source 2 Ion beam irradiation system column 3 Sample chamber 4 Cooling mechanism 5 Focusing lens 8 Objective lens 9 Sample 10 Sample table 14 Ion beam 15 Vacuum vessel 21 Emitter tip 22 Filament 23 Filament holder 24 Extraction electrode 26 Gas supply mechanism 31 Cooling machine 33 Cold transfer rod 34 Heat insulating material 35 Emitter tip holder 36 Electrically insulating column 37 Emitter base holder 40 GM cooling machine 63 Tilt mechanism 81 Adhesion member 82 Oxygen-free copper thin film 83 Heating mechanism 84 Thermometer 85 Cold transfer thin film 414 Heat exchanger

Claims

An ion beam system comprising a gas field ionization ion source (1) comprising: a vacuum vessel (15); an emitter tip holder (35) located in the vacuum vessel (15); an emitter tip (21) connected to the emitter tip holder (35); an extraction electrode (24) opposed to the emitter tip (21); a gas supply section (26) for supplying a gas to the emitter tip (21); and a cold transfer member (33) located in the vacuum vessel (15) and transferring cooling energy to the emitter tip holder (35), wherein the surface of the cold transfer member (33) is covered with a heat insulating material (34) to prevent condensation of the gas, The cold transfer member (33) is a metal thin film (85) or a metal stranded wire (56), and has a heat insulating layer (87) adhered to its surface, characterized bya sticking member (81) covering the surface of the cold transfer member (33), wherein the heat insulating material (34) adheres to the surface of the cold transfer member (33) via the sticking member (81) and covers the sticking member (81).The ion beam system according to claim 1, wherein the cold transfer member (33) comprises a metal, and the heat insulating material comprises a fluororesin or a ceramic.The ion beam system of claim 1, wherein the gas includes neon, argon, krypton, or xenon.The ion beam system of claim 1, wherein the gas includes carbon monoxide, oxygen, or nitrogen.The ion beam system according to claim 1, wherein the gas supply section (26) supplies a mixture of krypton gas and another gas or supplies the krypton gas and the other gas by switching between these gases, and the emitter tip (21) is maintained at a substantially constant temperature higher than the condensation temperatures of the gases and at most 60 K.The ion beam system according to claim 1, wherein the gas supply section (26) supplies a mixture of argon gas and helium gas or a mixture of argon gas and hydrogen gas, and the emitter tip (21) is maintained at a substantially constant temperature higher than the condensation temperatures of the gases and at most 45 K.The ion beam system according to claim 1, further comprising: a metal material (82) covering the surface of the heat insulating material (34); and a heating mechanism (83) for heating the metal material (82).

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