Abstract:
Ion sources, systems and methods are disclosed. In some embodiments, the ion sources, systems and methods can exhibit relatively little undesired vibration and/or can sufficiently dampen undesired vibration. This can enhance performance (e.g., increase reliability, stability and the like).

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates to ion sources, systems, and methods. 
       BACKGROUND 
       [0002]    Ion sources and systems can produce ion beams which are used to investigate and/or modify a sample. 
       SUMMARY 
       [0003]    In some embodiments, the ion sources, systems and methods can exhibit relatively little undesired vibration and/or can sufficiently dampen undesired vibration. This can enhance performance (e.g., increase reliability, stability and the like). 
         [0004]    In one aspect, the disclosure features a field emission charged particle source that includes: (a) an outer structure mounted to a carrying structure being at environmental temperature; (b) an intermediate structure mounted to the outer structure; (c) an inner structure mounted to the intermediate structure; (d) a charged particle emitter mounted to the inner structure; and (c) a cooling device thermally connected to the intermediate structure. The outer structure is made of a material having at cryostatic temperature a lower thermal conductivity than the material of the inner structure. 
         [0005]    In another aspect, the disclosure features a field emission charged particle source that includes: (a) an outer structure mounted to a carrying structure being at environmental temperature, the outer structure having a thermal conductivity; (b) an intermediate structure mounted to the outer structure; (c) an inner structure mounted to the intermediate structure; (d) a field emitter mounted to the inner structure; and (e) a cooling device thermally connected to the intermediate structure. The intermediate structure has a thermal capacity which is larger than 1.5 times the thermal conductivity of the outer structure. 
         [0006]    In a further aspect, the disclosure features a gas field beam system that includes: (a) a field emitting source generating a beam of charged particles with a main direction of propagation; (b) a cooler thermally connected to the field emitter or a structure to which the field emitter is mounted; (c) a scanning system by the aid of which a beam generated by the field emission source can be deflected in direction perpendicular to the main direction of propagation; and (d) a control system for operating the field emission system, the control system providing at least a first and a second mode of operation. In the first mode of operation the cooler is operated to cool the field emitter, and in the second mode of operation the cooler is switched off and the scanning system is operated so that beam is deflected to scan a sample. 
         [0007]    In another aspect, the disclosure features a gas field beam system that includes: (a) a field emitting source generating a beam of charged particles with a main direction of propagation; (b) a cooler thermally connected to the field emitter or a structure to which the field emitter is mounted; (c) a scanning-system by the aid of which a beam generated by the field emission source can be deflected in direction perpendicular to the main direction of propagation; and (d) a control system for operating the field emission system, the control system providing at least a first and a second mode of operation. In the first mode of operation the cooler is operated to cool the field emitter, and in the second mode of operation the cooler is switched off and the scanning system is operated so that beam is deflected to scan a sample. 
         [0008]    In a further aspect, the disclosure features a method that includes exposing a sample to a charged particle beam generated by a tip of a charged particle system, where during exposure of the sample, a constant phase is maintained between a vibrational displacement function of the tip and corresponding portions of an exposure pattern of the charged particle beam on the sample. 
         [0009]    In another aspect, the disclosure features a system that includes a tip configured to cause ionization of gas particles to form an ion beam, the tip being mounted on a support structure, and a vibrational damper connected to the support structure and to a cooling device, where the vibrational damper includes a first plurality of flexible members connected to the support structure, a second plurality of flexible members connected to the cooling device, and a solid member disposed between the first and second pluralities of flexible members. 
         [0010]    In a further aspect, the disclosure features an ion microscope system that includes a first member that includes a first curved surface, and a second member connected to a tip and including a second curved surface complementary to the first curved surface and configured to permit relative motion between the first and second members, where the second curved surface includes a plurality of annular protrusions, and where when the first and second members are drawn together, at least some of the annular protrusions contact the first curved surface to form annular contact regions between the first and second surfaces. 
         [0011]    Embodiments can include one or more of the following features. 
         [0012]    The outer structure can have a first wall thickness and the inner structure can have a second wall thickness, where the first wall thickness is smaller than the second wall thickness. 
         [0013]    The inner structure can be made of copper and the outer structure can be made of stainless steel. 
         [0014]    The outer structure can have a cylindrical shape with a first diameter and the inner structure can have a cylindrical shape with a second diameter, the second diameter being smaller than the first diameter. 
         [0015]    The cooling device can be thermally connected to the intermediate structure along a first bundle of thermally conducting, flexible wires. The cooling device can be connected to the intermediate structure along a second bundle of flexible wires and along one or more rigid rods, where the one or more rigid rods are arranged, in series, between the first and the second bundle of thermally conductive flexible wires. The first and/or the second bundle of flexible wires can be made of copper. The first and/or the second bundle of flexible wires can be made of a carbonated pitch material. The rigid rod can include copper. 
         [0016]    The charged particle source can include an extraction electrode electrically isolated from the field emitter and where, in operation, a high voltage is applied between the field emitter and the extraction electrode. 
         [0017]    The field emitter can be mounted to the inner structure via a material with a thermal conductivity which is higher than the thermal conductivity of the material of the outer structure. 
         [0018]    The field emission charged particle source can include a gas conducting tube for feeding a gas to a region within the inner structure, the gas conducting tube terminating in an intermediate region between the outer and the inner structure, and the inner structure comprising holes to provide a gas flow from the intermediate region to a region surrounded by the inner structure. 
         [0019]    The field emission charged particle source can include a control by which the cooling device can be switched off for a defined period of time. 
         [0020]    The cooling device can be thermally connected to the intermediate structure isothermically. 
         [0021]    The inner structure can be a cylinder, where the intermediate structure has the form of a ring and where the cooling device is thermally connected to the intermediate structure along a series of connection regions which are arranged along the intermediate structure. 
         [0022]    The outer structure can be made of a material having at cryostatic temperature a lower thermal conductivity than the material of the inner structure. 
         [0023]    The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description, drawings, and claims. 
     
    
     
       DESCRIPTION OF DRAWINGS 
         [0024]      FIG. 1  is a schematic diagram of an ion microscope system. 
           [0025]      FIG. 2  is a schematic diagram of a gas field ion source. 
           [0026]      FIG. 3  is a schematic diagram of a portion of an ion microscope system that includes a vibration damper. 
           [0027]      FIG. 4  is a schematic diagram of another embodiment of a vibration damper. 
           [0028]      FIGS. 5A and 5B  are schematic diagrams of images of a line of material on a sample surface. 
           [0029]      FIG. 6A  is a schematic diagram of the vibrational amplitude of a tip. 
           [0030]      FIG. 6B  is a schematic diagram of a sample image. 
           [0031]      FIG. 7  is a schematic diagram showing phase-locking of an image scan sequence to a vibrational displacement function of a tip. 
           [0032]      FIG. 8  is a schematic diagram of a tip manipulator. 
           [0033]      FIG. 9  is a schematic diagram of a portion of a tip manipulator. 
           [0034]      FIG. 10  is a schematic diagram of a portion of a tip manipulator. 
       
    
    
       [0035]    Like reference symbols in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
     Introduction 
       [0036]    When used to investigate properties of various samples, ion beams can provide qualitative and/or quantitative measurements that are precise and accurate to atomic resolution. Sample images measured with an ion beam (e.g., images that are derived from measurements of secondary electrons and/or scattered ions and/or scattered neutral atoms) can have very high resolution, revealing sample features that are difficult to observe using other imaging techniques. Optionally, ion beams can be used to provide qualitative and/or quantitative material constituent information about a sample. 
         [0037]    An example of a sample is a semiconductor article. Semiconductor fabrication typically involves the preparation of an article (a semiconductor article) that includes multiple layers of materials sequentially deposited and processed to form an integrated electronic circuit, an integrated circuit element, and/or a different microelectronic device. Such articles typically contain various features (e.g., circuit lines formed of electrically conductive material, wells filled with electrically non-conductive material, regions formed of electrically semiconductive material) that are precisely positioned with respect to each other (e.g., generally on the scale of within a few nanometers). The location, size (length, width, depth), composition (chemical composition) and related properties (conductivity, crystalline orientation, magnetic properties) of a given feature can have an important impact on the performance of the article. For example, in certain instances, if one or more of these parameters is outside an appropriate range, the article may be rejected because it cannot function as desired. As a result, it is generally desirable to have very good control over each step during semiconductor fabrication, and it would be advantageous to have a tool that could monitor the fabrication of a semiconductor article at various steps in the fabrication process to investigate the location, size, composition and related properties of one or more features at various stages of the semiconductor fabrication process. As used herein, the term semiconductor article refers to an integrated electronic circuit, an integrated circuit element, a microelectronic device or an article formed during the process of fabricating an integrated electronic circuit, an integrated circuit element, a microelectronic device. In some embodiments, a semiconductor article can be a portion of a flat panel display or a photovoltaic cell. Regions of a semiconductor article can be formed of different types of material (electrically conductive, electrically non-conductive, electrically semiconductive). Exemplary electrically conductive materials include metals, such as aluminum, chromium, nickel, tantalum, titanium, tungsten, and alloys including one or more of these metals (e.g., aluminum-copper alloys). Metal silicides (e.g., nickel silicides, tantalum silicides) can also be electrically conductive. Exemplary electrically non-conductive materials include borides, carbides, nitrides, oxides, phosphides, and sulfides of one or more of the metals (e.g., tantalum borides, tantalum gennaniums, tantalum nitrides, tantalum silicon nitrides, and titanium nitrides). Exemplary electrically semiconductive materials include silicon, germanium and gallium arsenide. Optionally, an electrically semiconductive material can be doped (p-doped, n-doped) to enhance the electrical conductivity of the material. Typical steps in the deposition/processing of a given layer of material include imaging the article (e.g., to determine where a desired feature to be formed should be located), depositing an appropriate material (e.g., an electrically conductive material, an electrically semiconductive material, an electrically non-conductive material) and etching to remove unwanted material from certain locations in the article. Often, a photoresist, such as a polymer photoresist, is deposited/exposed to appropriate radiation/selectively etched to assist in controlling the location and size of a given feature. Typically, the photoresist is removed in one or more subsequent process steps, and, in general, the final semiconductor article desirably does not contain an appreciable amount of photoresist. 
         [0038]      FIG. 1  shows a schematic diagram of a gas field ion microscope system  100  that includes a gas source  110 , a gas field ion source  120 , ion optics  130 , a sample manipulator  140 , a front-side detector  150 , a back-side detector  160 , and an electronic control system  170  (e.g., an electronic processor, such as a computer) electrically connected to various components of system  100  via communication lines  172   a - 172   f . A sample  180  is positioned in/on sample manipulator  140  between ion optics  130  and detectors  150 ,  160 . During use, an ion beam  192  is directed through ion optics  130  to a surface  181  of sample  180 , and particles  194  resulting from the interaction of ion beam  192  with sample  180  are measured by detectors  150  and/or  160 . 
         [0039]    As shown in  FIG. 2 , gas source  110  is configured to supply one or more gases  182  to gas field ion source  120 . Gas source  110  can be configured to supply the gas(es) at a variety of purities, flow rates, pressures, and temperatures. In general, at least one of the gases supplied by gas source  110  is a noble gas (helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe)), and ions of the noble gas are desirably the primary constituent in ion beam  192 . In general, as measured at surface  181  of sample  180 , the current of ions in ion beam  192  increases monotonically as the pressure of the noble gas in system  100  increases. In certain embodiments, this relationship can be described by a power law where, for a certain range of noble gas pressures, the current increases generally in proportion to gas pressure. 
         [0040]    Optionally, gas source  110  can supply one or more gases in addition to the noble gas(es); an example of such a gas is nitrogen. Typically, while the additional gas(es) can be present at levels above the level of impurities in the noble gas(es), the additional gas(es) still constitute minority components of the overall gas mixture introduced by gas source  110 . 
         [0041]    Gas field ion source  120  is configured to receive the one or more gases  182  from gas source  110  and to produce gas ions from gas(es)  182 . Gas field ion source  120  includes an electrically conductive tip  186  with a tip apex  187 , an extractor  190  and optionally a suppressor  188 . 
         [0042]    Electrically conductive tip  186  can be formed of various materials. In some embodiments, tip  186  is formed of a metal (e.g., tungsten (W), tantalum (Ta), iridium (Ir), rhenium (Rh), niobium (Nb), platinum (Pt), molybdenum (Mo)). In certain embodiments, electrically conductive tip  186  can be formed of an alloy. In some embodiments, electrically conductive tip  186  can be formed of a different material (e.g., carbon (C)). 
         [0043]    During use, tip  186  is biased positively (e.g., approximately 20 kV) with respect to extractor  190 , extractor  190  is negatively or positively biased (e.g., from −20 kV to +50 kV) with respect to an external ground, and optional suppressor  188  is biased positively or negatively (e.g., from −5 kV to +5 kV) with respect to tip  186 . Because tip  186  is formed of an electrically conductive material, the electric field of tip  186  at tip apex  187  points outward from the surface of tip apex  187 . Due to the shape of tip  186 , the electric field is strongest in the vicinity of tip apex  187 . The strength of the electric field of tip  186  can be adjusted, for example, by changing the positive voltage applied to tip  186 . With this configuration, un-ionized gas atoms  182  supplied by gas source  110  are ionized and become positively-charged ions in the vicinity of tip apex  187 . The positively-charged ions are simultaneously repelled by positively charged tip  186  and attracted by negatively charged extractor  190  such that the positively-charged ions are directed from tip  186  into ion optics  130  as ion beam  192 . Suppressor  188  assists in controlling the overall electric field between tip  186  and extractor  190  and, therefore, the trajectories of the positively-charged ions from tip  186  to ion optics  130 . In general, the overall electric field between tip  186  and extractor  190  can be adjusted to control the rate at which positively-charged ions are produced at tip apex  187 , and the efficiency with which the positively-charged ions are transported from tip  186  to ion optics  130 . 
         [0044]    In general, ion optics  130  are configured to direct ion beam  192  onto surface  181  of sample  180 . Ion optics  130  can, for example, focus, collimate, deflect, accelerate, and/or decelerate ions in beam  192 . Ion optics  130  can also allow only a portion of the ions in ion beam  192  to pass through ion optics  130 . Generally, ion optics  130  include a variety of electrostatic and other ion optical elements that are configured as desired. By manipulating the electric field strengths of one or more components (e.g., electrostatic deflectors) in ion optics  130 , He ion beam  192  can be scanned across surface  181  of sample  180 . For example, ion optics  130  can include two deflectors that deflect ion beam  192  in two orthogonal directions. The deflectors can have varying electric field strengths such that ion beam  192  is rastered across a region of surface  181 . 
         [0045]    When ion beam  192  impinges on sample  180 , a variety of different types of particles  194  can be produced. These particles include, for example, secondary electrons, Auger electrons, secondary ions, secondary neutral particles, primary neutral particles, scattered ions and photons (e.g., X-ray photons, IR photons, visible photons, UV photons). Detectors  150  and  160  are positioned and configured to each measure one or more different types of particles resulting from the interaction between He ion beam  192  and sample  180 . As shown in  FIG. 1 , detector  150  is positioned to detect particles  194  that originate primarily from surface  181  of sample  180 , and detector  160  is positioned to detect particles  194  that emerge primarily from surface  183  of sample  180  (e.g., transmitted particles). As described in more detail below, in general, any number and configuration of detectors can be used in the microscope systems disclosed herein. In some embodiments, multiple detectors are used, and some of the multiple detectors are configured to measure different types of particles. In certain embodiments, the detectors are configured to provide different information about the same type of particle (e.g., energy of a particle, angular distribution of a given particle, total abundance of at given particle). Optionally, combinations of such detector arrangements can be used. 
         [0046]    In general, the information measured by the detectors is used to determine information about sample  180 . Typically, this information is determined by obtaining one or more images of sample  180 . By rastering ion beam  192  across surface  181 , pixel-by-pixel information about sample  180  can be obtained in discrete steps. Detectors  150  and/or  160  can be configured to detect one or more different types of particles  194  at each pixel. 
         [0047]    The operation of microscope system  100  is typically controlled via electronic control system  170 . For example, electronic control system  170  can be configured to control the gas(es) supplied by gas source  110 , the temperature of tip  186 , the electrical potential of tip  186 , the electrical potential of extractor  190 , the electrical potential of suppressor  188 , the settings of the components of ion optics  130 , the position of sample manipulator  140 , and/or the location and settings of detectors  150  and  160 . Optionally, one or more of these parameters may be manually controlled (e.g., via a user interface integral with electronic control system  170 ). Additionally or alternatively, electronic control system  170  can be used (e.g., via an electronic processor, such as a computer) to analyze the information collected by detectors  150  and  160  and to provide information about sample  180  (e.g., topography information, material constituent information, crystalline information, voltage contrast information, optical property information, magnetic information), which can optionally be in the form of an image, a graph, a table, a spreadsheet, or the like. Typically, electronic control system  170  includes a user interface that features a display or other kind of output device, an input device, and a storage medium. 
         [0048]    In certain embodiments, electronic control system  170  can be configured to control various properties of ion beam  192 . For example, control system  170  can control a composition of ion beam  192  by regulating the flow of gases into gas field ion source  120 . By adjusting various potentials in ion source  120  and ion optics  130 , control system  170  can control other properties of ion beam  192  such as the position of the ion beam on sample  180 , and the average energy of the incident ions. 
         [0049]    In some embodiments, electronic control system  170  can be configured to control one or more additional particle beams. For example, in certain embodiments, one or more, types of ion beam source and/or electron beam sources can be present. Control system  170  can control each of the particle beam sources and their associated optical and electronic components. 
         [0050]    Detectors  150  and  160  are depicted schematically in  FIG. 1 , with detector  150  positioned to detect particles from surface  181  of sample  180  (the surface on which the ion beam impinges), and detector  160  positioned to detect particles from surface  183  of sample  180 . In general, a wide variety of different detectors can be employed in microscope system  200  to detect different particles, and a microscope system  200  can typically include any desired number of detectors. The configuration of the various detector(s) can be selected in accordance with particles to be measured and the measurement conditions. In some embodiments, a spectrally resolved detector may be used. Such detectors are capable of detecting particles of different energy and/or wavelength, and resolving the particles based on the energy and/or wavelength of each detected particle. 
         [0051]    Detection systems and methods are generally disclosed, for example, in U.S. Patent Application Publication No. US 2007/0158558, the entire contents of which are incorporated herein by reference. 
       Ion Beam Measurements 
       [0052]    In general, the accuracy of ion beam measurements depends, in part, on the stability of the ion beam during measurement. For example, fluctuations in the position of the ion beam on the surface of a sample during a measurement can lead to errors in spatially resolved measurements. If such errors are too severe, the suitability of ion beams for certain applications can be unduly limited. Vibrations in ion microscope systems—which can be generated within the system (e.g., via vacuum pumps) or coupled into the system from external sources (e.g., floor vibrations)—can cause the tip to vibrate, producing fluctuations in the position of the ion beam on the sample surface. Accordingly, the ion beam systems disclosed herein include certain features that, at least in part, help to reduce the effects of such vibrations. 
         [0053]      FIG. 3  shows a schematic expanded view of a portion of an ion microscope system. The system includes an outer structure  500  (e.g., a vacuum chamber) and an inner structure  510  that contacts outer structure  500 . Attached to inner structure  510  is a support  530  that supports a tip  186 . During operation, as discussed above, gas particles (e.g., helium gas particles or particles of another noble gas) are ionized in the vicinity of tip  186 , and the newly formed ions propagate in a directly approximately parallel to a central axis of outer structure  500 . To improve the precision with which newly formed ions are directed along the axis of outer structure  500 , extractor  550  is positioned adjacent to tip  186 . As discussed above, extractor  550  selects from among the ions produced in the vicinity of tip  186  a certain subset of ions, which form the microscope&#39;s ion beam. Further, the system includes radiation shields  540  to prevent stray ions from propagating at large angular deviations within outer structure  500 . 
         [0054]    The system also includes an intermediate structure  520  that contacts both inner structure  510  and outer structure  500 . To cool the system (and particularly tip  186 ), intermediate structure  520  contacts cooler  600  (e.g., isothermically) through a thermal contact device  560 . 
         [0055]    In general, intermediate structure  520  contacts both outer structure  500  and inner structure  510 , and is therefore capable of cooling both structures. However, it is generally more important that tip  186  be cooled in preference to outer structure  500 , because cooling tip  186  to very low temperature can be an important step in operating a field ion microscope. 
         [0056]    Tip  186  is mounted on support  530 , which is typically formed from a material such as a ceramic material. Support  530  (which can have a thermal conductivity greater than outer structure  500 ) is in thermal contact with inner structure  510 , and therefore, cooler  600  can cool tip  186  by withdrawing heat through support  530 , inner structure  510 , intermediate structure  520 , and thermal contact device  560 . 
         [0057]    In general, inner structure  510  and outer structure  500  are constructed so that heat flow occurs more readily between inner structure  510  and intermediate structure  520  (and, ultimately, to cooler  600 ) than between outer structure  500  and cooler  600 . In some embodiments, for example, inner structure  510  is formed from a material (e.g., copper, oxygen-free high conductivity copper) that has a higher thermal conductivity than a material such as stainless steel from which outer structure  500  is formed. The thermal conductivity of inner structure  510  can be larger than the thermal conductivity of outer structure  500  by a factor of 1.1 or more (e.g., 1.3 or more, 1.5 or more, 1.7 or more, 2.0 or more, 2.5 or more, 3.0 or more, 3.5 or more, 4.0 or more, 5.0 or more, 10.0 or more, 100 or more, 1000 or more). The thermal conductivity of inner structure  510  can be larger than the thermal conductivity of outer structure  500  at room temperature, for example, and/or at temperatures less than room temperature, including liquid nitrogen temperature and/or liquid helium temperature. 
         [0058]    Outer structure  500 , in the embodiment shown in  FIG. 3 , has a cylindrical shape with a central axis  620 . Inner structure  510  has a similar cylindrical shape and shares a common central axis. In general, the shapes of inner structure  510  and outer structure  500  can be selected as desired for particular applications and to accommodate differently-shaped tips. 
         [0059]    The thicknesses of each of outer structure  500  and inner structure  510  are measured along a direction perpendicular to axis  620  in  FIG. 3 . In general, the thicknesses of these structures can be selected to ensure that the thermal capacity of inner structure  510  is larger than the thermal capacity of outer structure  500 . Accordingly, in certain embodiments, the thickness of inner structure  510  can be larger than the thickness of outer structure  500  by a factor of 1.1 or more (e.g., 1.2 or more, 1.3 or more, 1.5 or more, 2.0 or more, 2.5 or more, 3.0 or more, 3.5 or more, 4.0 or more, 4.5 or more, 5.0 or more, 6.0 or more, 7.0 or more, 8.0 or more, 10.0 or more). 
         [0060]    Typically, during operation, ions generated by tip  186  are directed to be incident on a sample, and particles leaving the sample in response to the incident ions are measured to determine properties of the sample (e.g., to obtain one or more images of the sample). During this process, vibrations introduced by cooler  600  into the system can introduce errors into the measured results. To reduce the amplitude of vibrations introduced by cooler  600  into the system, cooler  600  can be turned off during exposure of the sample to the ion beam generated by tip  186 . For example, in some embodiments, cooler  600  can be operated to cool tip  186  to a particular operating temperature, and then cooler  600  can be turned off for a period of one minute or more (e.g., two minutes or more, three minutes or more, five minutes or more, seven minutes or more, nine minutes or more, 11 minutes or more, 13 minutes or more, 15 minutes or more, 20 minutes or more) while the sample is exposed to the ion beam and particles leaving the sample are measured. Intermediate structure  520  is typically formed from a material with a high thermal conductivity such as copper, and forms a type of “thermal battery” so that intermediate structure  520  acts as a cooling reservoir when cooler  600  is turned off. In this way, the temperature increase of tip  186  can be limited, in certain embodiments, to a few degrees K or less when cooler  600  is turned off. 
         [0061]    The system includes thermal contact device  560 , as shown in  FIG. 3 . Thermal contact device  560  includes a first plurality of flexible contact members  570  that are connected to intermediate support  520  on one end, and to central member  580  on the other end. A second plurality of flexible contact members  590  connects central member  580  to cooler  600 . One central member  580  is shown in  FIG. 3  but in general, any number (e.g., two or more, three or more, four or more, or even more) of central members  580  can be used. 
         [0062]    Thermal contact device  560  provides a conduit for heat transfer between cooler  600  and intermediate structure  520 . Thus, flexible contact members  570  and  590  and central member  580  are typically formed from one or more materials with relatively high thermal conductivity, such as copper. Other materials from which some or all of contact members  570  and  590  and central member  580  can be formed include carbon-based materials such as carbonaceous pitch, silver, and/or gold. In some embodiments, for example, flexible contact members  570  and/or  590  can be formed from large numbers of small diameter, flexible strands of one or more thermally conductive materials such as copper, that are woven and/or wrapped around one another to form rope-like braids corresponding to contact members  570  and/or  590 . 
         [0063]    Thermal contact device  560  functions as a vibration damper, reducing the amplitude of vibrations transmitted to structures  500 ,  510 , and  520  (and also support  530  and tip  186 ) from cooler  600 . By forming contact members  570  and  590  from flexible materials, vibration transfer through these materials is reduced relative to vibration transfer through more rigid materials. Further, central member  580  acts as a type of pendulum to counteract vibrations that are coupled into flexible contact members  590  from pump  600 . 
         [0064]    In general, the geometric properties of flexible contact members  570  and  590  and central member  580  are selected to counteract the vibrational properties of cooler  600 . In particular, by selecting and/or changing the geometric properties of contact members  570  and  590  and/or central member  580 , the damping ability of thermal contact device  560  can be specifically tuned to a resonance frequency of cooler  600  (or higher harmonics thereof). Typically, for example, cooler  600  has a harmonic resonance frequency of about 46 Hz, and thermal contact device  560  can be tuned to damp high frequency vibrations at multiple harmonics of this resonance frequency (e.g., second harmonic and higher, third harmonic and higher, fourth harmonic and higher, fifth harmonic and higher, sixth harmonic and higher, eighth harmonic and higher, tenth harmonic and higher). 
         [0065]    The damping ability of thermal contact device  560  can be tuned several ways. Generally, thermal contact device  560  has an effective band of damping frequencies that includes a central damping frequency which depends, in part, on the mass of central member  580  and an effective harmonic spring constant of contact members  570 . In certain embodiments, to tune the central damping frequency of thermal contact device  560 , the position of central member  580  can be selected and/or changed relative to contact members  570  and  590  (e.g., to change the lengths of contact members  570 ). By changing the lengths of contact members  570 , the effective spring constant of these members can be changed, altering the central damping frequency of device  560 . In general, shortening the length of contact members  570  makes the members stiffer, increasing central damping frequency of device  560 . Conversely, increasing the length of contact members  570  decreases the central damping frequency of device  560 . 
         [0066]    In some embodiments, central member  580  is implemented as a solid cylinder, and changing the position of central member  580  relative to contact members  570  and  590  effectively corresponds to shortening or lengthening contact members  570  during fabrication of device  560 . In certain embodiments, central member  580  is implemented as a hollow cylinder that slides over contact members  570  and  590  (e.g., contact members  570  and  590  are continuous members), and contact members  570  can be shortened or lengthened by sliding member  580  along the continuous members and then securing member  580  in position (e.g., with a fastener such as a screw). 
         [0067]    In some embodiments, the central damping frequency of device  560  can be changed by adjusting the mass of central member  580 . In general, increasing the mass of central member  580  leads to a decrease in the central damping frequency of device  560 , while decreasing the mass of central member  580  increases the central damping frequency of device  560 . The mass of central member  580  can be selected during fabrication of device  560  to compensate a known vibration frequency of cooler  600  (and/or harmonics thereof), for example, and/or the mass of central member  580  can be adjusted following fabrication (e.g., by adding or removing annular strips of material that are concentric with central member  580 , not shown in  FIG. 3 ) to tune the central frequency of device  560 . 
         [0068]    In some embodiments, either or both of contact members  570  and  590  can be shaped to reduce transmission of vibrations by these members. For example, either or both of contact members  570  and  590  can include U-shaped bends between central member  580  and intermediate structure  520  or cooler  600 , respectively. The U-shaped bends assist in preventing efficient vibrational amplitude transfer along the lengths of contact members  570  and  590 . Each of members  570  and/or  590  can include multiple U-shaped bends, as desired. 
         [0069]    In certain embodiments, the central damping frequency of device  560  can be changed by applying an axial rotation to members  570  and/or  590 . For example, before attaching members  570  to intermediate support  520 , a torsional force can be applied to members  570  to twist the members, so that residual torsional force remains in members  570  after device  560  is mounted between structure  520  and cooler  600 . The residual torsional force increases the effective spring constant of members  570 , increasing the central damping frequency of device  560 . 
         [0070]    As shown in  FIG. 3 , contact members  570  are attached to only one side of intermediate structure  520 . Generally, contact members  570  can be attached to structure  520  (and, in some embodiments, to central member  580 ) using a deformable, thermally conductive material such as indium foil, which fills in gaps in the mating surfaces of contact members  570  and structure  520  and/or central member  580 , improving the thermal contact between these surfaces. Similarly, in certain embodiments, contact members  590  can be attached to central member  580  and/or cooler  600  via a deformable material such as indium foil. 
         [0071]    To achieve a relatively uniform temperature distribution along the circumference of structure  520 , structure  520  is formed from a material that has relatively high thermal conductivity. As shown in  FIG. 3 , contact members  570  are attached to only one side of structure  520  and thus, if the thermal conductivity of structure  520  is not large enough, a temperature gradient will form along the circumference of structure  520 . In some embodiments, to reduce the likelihood of such a gradient forming, contact members  570  can be attached at various points along structure  520 . 
         [0072]      FIG. 4  shows an embodiment in which contact members  570  are spaced at intervals along intermediate structure  520  (the remaining portions of  FIG. 3  are not shown, for clarity). Contact members  570  are also joined to a ring member  630 , which effectively functions in a similar manner to central member  580  in  FIG. 3 . Ring member  630  is connected via contact members  590  to cooler  600 , for example. By spacing contact members  570  along intermediate structure  520 , the magnitude of any thermal gradients formed in intermediate structure  520  can be reduced. 
         [0073]    In some embodiments, central member  580  can be attached to a support structure. For example, central member  580  can be connected through a support structure (e.g., wires) to outer structure  500 , or to another external structure (an external structure that is positioned on a vibration-damping base, for example, so that it is vibrationally decoupled from cooler  600 ). 
         [0074]    When a sample is imaged by exposing the sample to the ion beam generated by tip  186  and detecting particles that leave the sample as a result of the incident ions, mechanical vibration of tip  186  can lead to imaging errors.  FIG. 5A  shows a sample  180  that includes a line of material  650  on the sample surface. Line  650  has straight, parallel sides. However, if sample  180  is imaged as tip  186  vibrates (e.g., due to vibrations coupled into tip  186  from cooler  600  and/or other sources), line  650  can appear as shown in  FIG. 5B , with wavy, irregular sides. 
         [0075]      FIG. 6A  shows the amplitude A of mechanical vibration  660  of tip  186  as a function of time t.  FIG. 6B  shows an image  663  of a sample that is exposed to the ion beam formed by tip  186 . Four image pixels  661   a - d  are shown in  FIG. 6B . With reference to  FIG. 6A , when pixel  661   a  is exposed to the ion beam, tip  186  is not vibrationally displaced from its equilibrium position—the vibrational amplitude is zero (position  662   a ). Accordingly, vibration of tip  186  does not contribute any error to the position measurement of pixel  661   a . When pixel  661   b  is exposed to the ion beam, tip  186  is vibrationally displaced at position  662   b  from its equilibrium position. Thus, pixel  661   b  is measured not at its true position, but at position  664   a  in the image (e.g., positively displaced). When pixel  661   c  is exposed to the ion beam, tip  186  is vibrationally displaced at position  662   c  from its equilibrium position, and so pixel  661   c  in the image appears not at its true position, but at position  664   c  (e.g., negatively displaced). When pixel  661   d  is exposed to the ion beam, tip  186  is once again positively displaced from its equilibrium position, and pixel  661   d  appears in position  664   d  in the image. By connecting pixels  664   a - d  in  FIG. 6B , it is evident how the waviness and irregularity in the sides of line  650  can be produced. 
         [0076]    The imaging irregularities discussed above arise, in part, from the random phase at which pixel data in the image is acquired, relative to the vibrational motion of tip  186 . To reduce these irregularities, the pixel scanning pattern of the ion beam on the sample can be phase-locked to the mechanical vibration of tip  186 . For example, as shown in  FIG. 7 , the mechanical vibration amplitude function  660  can be phase locked to the scanning voltage  670  that is applied to scan the ion beam across the surface of the sample. The effect of the phase-locking is to ensure that rather than exposing pixels  661   a - d  of the image at random vibrational displacements of tip  186  from its equilibrium position, each of pixels  661   a - d  is exposed with tip  186  at approximately the same vibrational displacement from equilibrium (e.g., at points  671   a - d  on amplitude function  660 ). As a result of the phase-locking, dynamic imaging errors that result from phase fluctuations between scans of certain pixels and the vibrational amplitude of tip  186  can be significantly reduced and/or eliminated. 
         [0077]    Images can still include static errors, because different pixels (different pixels along a common horizontal line, for example) are sampled at different vibrational displacements of tip  186 . In certain embodiments, the images can be corrected (e.g., following acquisition) by applying a pixel-dependent offset that is derived from knowledge and/or estimates of the vibrational displacement of tip  186  from its equilibrium position at each pixel position. Due to the phase-locking between the pixel scanning pattern and the vibrational displacement of the tip, the pixel position measurement errors arise largely from systematic, position-dependent errors rather than random, phase-related errors, and are significantly easier to correct as a result. 
         [0078]    In some embodiments, to eliminate both phase-related (e.g., dynamic) and static errors in images, each image pixel can be exposed with tip  186  at a common vibrational displacement. For example, referring to  FIG. 7 , each pixel in an image of a sample can be exposed when tip  186  is at a position corresponding to position  671   a . That is, each pixel can be exposed when tip  186  is maximally displaced in one direction from its equilibrium position. Because the relative phase between each of the image pixels (e.g., not just the first pixel in each row) and the vibrational displacement of tip  186  is the same, each image pixel corresponds to a common vibrational displacement of tip  186  from its equilibrium position. As a result, both dynamic and static errors in the image due to vibration of tip  186  during imaging can be significantly reduced. 
         [0079]    Although simple linear raster-scanning of the ion beam on the sample surface has been described above, in general, any scanning pattern can be used. For example, in some embodiments, checkerboard scanning patterns can be used, with the scanning pattern phase-locked to vibrational displacement of tip  186  from its equilibrium position. More sophisticated scanning patterns can also be phase-locked to the vibrational displacement of tip  186  from its equilibrium position as discussed herein. 
         [0080]    Another potential source of vibrational instability in the ion microscope system is the tip manipulator, which includes a dome-shaped surface of motion and a translator connected to tip  186 , with a mating surface shaped to permit movement along the surface of motion. The tip manipulator permits both translation of tip  186  in the x-y plane, and tilting of tip  186  with respect to axis  1132  of ion optics  130 .  FIG. 8  is a cross-sectional view of a portion of an ion microscope system including tip  186 , support assembly  1520  and an embodiment of a tip manipulator. The tip manipulator includes a shaft  1502 , a dome  1504 , a shoulder  1510  and a translator  1514 . Translator  1514  is connected to shaft  1502 , which is dimensioned to fit through an opening  1516  in shoulder  1510 . Shaft  1502  is further connected to base  1508 , which in turn is connected to assembly  1520 . Shoulder  1510  is in a fixed position relative to dome  1504  by static frictional forces between surfaces  1512  and  1513 , and translator  1514  is in a fixed position relative to shoulder  1510  by static frictional forces between surfaces  1518  and  1519 . 
         [0081]    The tip manipulator provides for translation of tip  186  in the x-y plane. To translate tip  186 , a high pressure gas is introduced into inlet  1503 . The high pressure gas introduced into inlet  1503  can be a gas such as room air, for example. Typically, the gas can be introduced at a pressure of 50 pounds per square inch (psi) or more (e.g., 75 psi or more, 100 psi or more, 125 psi or more). As a result of introducing the high pressure gas, a force is applied to translator  1514  in the −z direction, away from shoulder  1510 . The applied force lessens (but does not reduce to zero) the frictional force between surfaces  1518  and  1519 , and permits repositioning of translator  1514  with respect to shoulder  1510  by applying a lateral force in the x-y plane. Tip  186  is translated in the x-y plane when translator  1514  is repositioned. When tip  186  is in its new position, the supply of high pressure gas is turned off and strong static frictional forces between surfaces  1518  and  1519  are re-established by evacuating the interior of the tip manipulator using one or more vacuum pumps. Tip  186  is rigidly fixed in position as a result of the re-established strong frictional forces. 
         [0082]    The tip manipulator also provides for tilting of tip  186  with respect to axis  1132  of ion optics  130 . To tilt tip  186 , a high pressure gas is introduced into inlet  1505 . The high pressure gas introduced into inlet  1505  can be a gas such as room air, for example. Typically, the gas can be introduced at a pressure of 50 pounds per square inch (psi) or more (e.g., 75 psi or more, 100 psi or more, 125 psi or more). As a result of introducing the high pressure gas, a force is applied to shoulder  1510  in the −z direction, away from dome  1504 . The applied force lessens (but does not reduce to zero) the frictional force between surfaces  1512  and  1513 . Shoulder  1510  can then be re-positioned with respect to dome  1504  by applying a lateral force to translate shoulder  1510  in a direction indicated by arrows  1506 . Translation of shoulder  1510  corresponds to relative movement along the curved surface of dome  1504 . As a result of this movement, the angle between axes  1132  and  207  (which corresponds to the tilt angle of tip  186 ) changes. When adjustment of the tilt of tip  186  is complete, the supply of high pressure gas is turned off and strong static frictional forces between surfaces  1512  and  1513  are re-established by evacuating the interior of the tip manipulator. Tip  186  is rigidly fixed in position as a result of the re-established strong frictional forces. 
         [0083]    If the mating surfaces  1512  and  1513  are not both very smooth, however, small protrusions on either surface can lead to the formation of points of contact at the interface between surfaces  1512  and  1513 . In other words, as shown in  FIG. 9 , instead of an entire annular contact region at the interface between surfaces  1512  and  1513 , a small number of contact points  1512   a  and  1512   b  exist between the surfaces. As a result, the frictional force which holds tip  186  in place is greatly reduced, and external vibrations can cause undesired motion of tip  186 . 
         [0084]    To increase the area of contact between surfaces  1512  and  1513  in the presence of surface irregularities such as small protrusions, surface  1512  can include two or more annular protrusions instead of a continuous mating surface, as shown in  FIG. 10 . Annular protrusions  1515   a  and  1515   b  are formed in surface  1512 , with a recess  1517  between the surfaces. Each of the protrusions  1515   a  and  1515   b  has a thickness t p  measured in a direction normal to the surface of the protrusion. 
         [0085]    Typically, the thickness t p  is 1 mm or less (e.g., 800 microns or less, 600 microns or less, 500 microns or less, 400 microns or less, 300 microns or less, 200 microns or less, 100 microns or less, 50 microns or less, 25 microns or less, 10 microns or less). Due to the relatively small thickness of protrusions  1515   a  and  1515   b , when the interior of the tip manipulator is evacuated, the clamping force between shoulder  1510  and dome  1504  causes each of protrusions  1515   a  and  1515   b  to deform, establishing two regions of intimate contact with surface  1513 . These regions are annular, extending around the curved surface of dome  1504 . As a result of the annular contact regions between shoulder  1510  and dome  1504 , the frictional force that holds tip  186  in place is greater than in the situation shown in  FIG. 9 , in which only relatively small points of contact exist between surfaces  1512  and  1513 . Accordingly, the stability of tip  186  is improved and the amplitude of the vibrational motion of tip  186  can be reduced. 
         [0086]    In the embodiment shown above, surface  1512  includes two protrusions  1515   a  and  1515   b , with a recess  1517  between the protrusions. In general, surface  1512  can include any number of protrusions (e.g., three or more, four or more, five or more, six or more, eight or more, ten or more, or even more). Recesses can be positioned between the protrusions to allow for deformation of the protrusions when the interior of the tip manipulator is evacuated. 
         [0087]    In some embodiments, it can be desirable to improve gas utilization (e.g., utilization of a beam-forming gas such as helium) to increase the ion beam current, for example. Low signal-to-noise ratio in sample measurements that are performed with ion beams can limit the suitability of ion beams for certain measurement applications. For example, low signal-to-noise ratios can introduce errors in measurement precision, making such measurements less reliable. When the ion beam is used to obtain images of a sample, certain fine details of the sample surface can be obscured by noise in the acquired images. One method for improving the signal-to-noise ratio in measured images is to increase the ion beam current. 
         [0088]    The ion beam current can be increased by using a tip  186  with a slightly larger radius of curvature. Ionization of the gas occurs in the vicinity of the tip apex. By using a tip with a slightly larger radius of curvature, the region of space surrounding tip  186  in which ionization of gas particles can occur is larger. As a result, the ion current in the ion beam can be increased. 
         [0089]    Typically, to produce a tip with a larger radius of curvature, the tip is first formed in a fabrication process (suitable fabrication processes are discussed, for example, in U.S. Patent. Application Publication No. US 2007/0158558). The fabrication process can be performed in the absence of oxygen gas, to prevent some sharpening of the tip. As a result of the fabrication process, tip  186  typically has a full cone angle of between 30 degrees and 45 degrees. 
         [0090]    The radius of curvature of tip  186  is typically 100 nm or more (e.g., 120 nm or more, 140 nm or more, 160 nm or more, 180 nm or more, 200 nm or more). A gas (e.g., helium gas) is introduced through a tube into cooling channels  610  in inner structure  510 , where it is pre-cooled before entering the ion microscope system through support  530 . Ionization of the gas occurs in the vicinity of tip  186 , producing an ion beam which is then directed by extractor  550  (and, more generally, ion optics  130 ) to propagate along a main direction (e.g., along axis  620  in  FIG. 3 ) and to be incident on a sample. 
       OTHER EMBODIMENTS 
       [0091]    As an example, while embodiments have been described in which a gas field ion source is used, other types of ion sources may also be used. In some embodiments, a liquid metal ion source can be used. An example of a liquid metal ion source is a Ga ion source (e.g., a Ga focused ion beam column). 
         [0092]    As another example, while embodiments have been described in which an ion source is used, more generally any charged particle source can be used. In some embodiments, an electron source, such as an electron microscope (e.g., a scanning electron microscope) can be used. 
         [0093]    As a further example, while embodiments have been described in which samples are in the form of semiconductor articles, in some embodiments, other types of samples can be used. Examples include biological samples (e.g., tissue, nucleic acids, proteins, carbohydrates, lipids and cell membranes), pharmaceutical samples (e.g., a small molecule drug), frozen water (e.g., ice), read/write heads used in magnetic storage devices, and metal and alloy samples. Exemplary samples are disclosed in, for example, U.S. Patent Publication No. US 2007/0158558. 
         [0094]    As an additional example, while embodiments have been described in which a sample is inspected, alternatively or additionally, the systems and methods disclosed herein can be used to modify (e.g., repair) a sample (e.g., to repair a region of the article at or near the portion of the article exposed by the cross-section). Such modification can involve gas assisted chemistry, which can be used to add material to and/or remove material from a sample (e.g., a given layer of the sample). As an example, gas assisted chemistry can be used for semiconductor circuit editing in which damaged or incorrectly fabricated circuits and/or circuit elements formed in semiconductor articles are repaired. Typically, circuit editing involves adding material to a circuit (e.g., to close a circuit that is open) and/or removing material from a circuit (e.g., to open a circuit that is closed). Gas assisted chemistry can also be used in photolithographic mask repair. Mask defects generally include an excess of mask material in a region of the mask where there should be no material, and/or an absence of mask material where material should be present. Thus, gas assisted chemistry can be used in mask repair to add and/or remove material from a mask as desired. Typically, gas assisted chemistry involves the use of a charged particle beam (e.g., ion beam, electron beam, or both) that interacts with an appropriate gas (e.g., Cl 2 , O 2 , I 2 , XeF 2 , F 2 , CF 4 , H 2 O, XeF 2 , F 2 , CF 4 , WF 6 ). As another example, modification of a sample can involve sputtering. In some instances, when fabricating articles, it can be desirable during certain steps to remove materials (e.g., when removing undesired material from a circuit to edit the circuit, when repairing a mask). An ion beam can be used for this purpose where the ion beam sputters material from the sample. In particular, an ion beam generated via the interaction of gas atoms with a gas field ion source as described herein can be used for sputtering a sample. Although He gas ions may be used, it is typically preferable to use heavier ions (e.g., Ne gas ions, Ar gas ions, Kr gas ions, Xe gas ions) to remove material. During the removal of material, the ion beam is focused on the region of the sample where the material to be removed is located. Examples of such inspection are disclosed, for example, in U.S. Patent Publication No. US 2007/0158558. 
         [0095]    Combinations of features disclosed herein can be used in various embodiments. Other embodiments are covered by the claims.