Abstract:
The disclosure relates to ion beams systems, such as gas field ion microscopes, having multiple modes of operation, as well as related methods. In some embodiments, the disclosure provides a method of operating a gas field ion microscope system that includes a gas field ion source, where the gas field ion source includes a tip including a plurality of atoms.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of, and claims priority under 35 U.S.C. §120 to, international application PCT/US2010/033339, filed May 3, 2010, which claims priority under 35 U.S.C. §119(e)(1) to U.S. Provisional Application No. 61/177,439, filed May 12, 2009. The contents of both of these applications are hereby incorporated by reference in entirety. 
     
    
     FIELD 
       [0002]    The disclosure relates to ion beams systems, such as gas field ion microscopes, having multiple modes of operation, as well as related methods. 
       BACKGROUND 
       [0003]    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. A semiconductor article can be, for example, a portion of a flat panel display or a photovoltaic cell. 
         [0004]    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 germaniums, 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. 
         [0005]    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. 
         [0006]    Ion microscope systems can be used to produce ions that are used, for example, to image a semiconductor sample, obtain chemical information about a semiconductor sample, and/or perform chemistry on a semiconductor sample. Microscope systems that use a gas field ion source to generate ions that can be used in sample analysis (e.g., imaging) are referred to as gas field ion microscopes. A gas field ion source is a device generally includes a tip (typically having an apex with 10 or fewer atoms) that can be used to ionize neutral gas species to generate ions (e.g., in the form of an ion beam) by bringing the neutral gas species into the vicinity of the electrically conductive tip (e.g., within a distance of about four to five angstroms) while applying a high positive potential (e.g., one kV or more relative to the extractor) to the apex of the electrically conductive tip. 
       SUMMARY 
       [0007]    In general, the disclosure relates to gas field ion microscope systems and related methods. The microscope systems can provide enhanced flexibility for operation in different modes. In some embodiments, a microscope system can be switched between a first mode of operation involving a relatively low ion beam current and/or a relatively small spot size and a second mode of operation involving a relatively high ion beam current and/or a relatively large spot size without removing the sample from the microscope system (e.g., without moving the sample within the microscope system). This can, for example, result significant improvements in efficiency and cost in handling, preparing and/or analyzing various types of samples, such as semiconductor samples. The first mode of operation may be desirable, for example, when seeking to image a sample with relatively high precision, and/or when seeking to perform sample chemistry (e.g., etching and/or deposition) with relatively high precision. The second mode of operation may be desirable, for example, when obtaining information about a sample which would take a relatively long period of time with a lower ion beam current and/or smaller spot size. By using a relatively high ion beam current and/or a relatively large spot size, the potential loss in precision can be sufficiently compensated for with the increase in speed. As an example, in some cases, the type of particle detected (e.g., a scattered ion) to obtain information (e.g., chemical information) about the sample may be generated at relatively low abundance and/or be difficult to detect with relatively high efficiency. In such cases, increasing the ion beam current can reduce the time used to collect a sufficient number of particles to obtain the desired information. Alternatively or additionally, in certain instances, it may be desirable to perform chemistry on a sample (e.g., etch the sample and/or deposit material on the sample) in a relatively short period of time (e.g., in a relatively high throughput process). In such circumstances, increasing the ion beam current can reduce the time used to process a given sample or a given collection of samples. In some embodiments, a microscope system can be designed to operate in more than two different modes. In certain embodiments, a microscope system can be switched between various modes of operation multiple different times. 
         [0008]    In one aspect, the disclosure generally provides a method of operating a gas field ion microscope system including a gas field ion source. The gas field ion source includes a tip including a plurality of atoms. The method includes operating the gas field ion microscope system in a first mode including interacting a first ion beam with a sample. At least about 80% of the ions in the first ion beam are generated by a first number of atoms of the plurality of atoms of the tip. The method also includes operating the gas field ion microscope system in a second mode in a second mode including interacting a second ion beam with the sample. At least about 80% of the ions in the second ion beam are generated by a second number of atoms of the plurality of atoms of the tip. The first mode is different from the second mode, and the first number of atoms is different from the second number of atoms. 
         [0009]    In another aspect, the disclosure generally provides a method of operating a gas field ion microscope system including a gas field ion source. The method includes interacting a first ion beam generated by the ion source with a sample. The first ion beam having a current of at most one nanoamp at a surface of the sample. The method also includes interacting a second ion beam generated by the ion source with the sample. The second ion beam having a current of at least 10 picoAmps at the surface of the sample. 
         [0010]    In a further aspect, the disclosure generally provides a method of operating a gas field ion microscope system including a gas field ion source. The method includes interacting a first ion beam generated by the ion source with a sample. The first ion beam having a first ion current at a surface of the sample. The method also includes interacting a second ion beam generated by the ion source with the sample. The second ion beam having a second current at the surface of the sample, and the second ion current is at least two times the first ion current. 
         [0011]    In an additional aspect, the disclosure generally provides a method of operating a gas field ion microscope system including a gas field ion source. The method includes interacting a first ion beam generated by the ion source with a sample to obtain information about a sample, and interacting a second ion beam generated by the ion source with the sample to perform chemistry on the sample. The first ion beam has a first ion current at a surface of the sample, and the second ion beam has a second ion current at the surface of the sample that is different from the first ion current at the surface of the sample. 
         [0012]    In another aspect, the disclosure generally provides a method of operating a gas field ion microscope system including a gas field ion source. The method includes interacting a first ion beam generated by the ion source to etch a sample at a first rate, and interacting a second ion beam generated by the ion source with the sample to etch the sample at a second rate greater than the first rate. 
         [0013]    Other features and advantages of the disclosure will be apparent from the description, drawings, and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a schematic diagram of an ion microscope system. 
           [0015]      FIG. 2  is a schematic diagram of a gas field ion source. 
           [0016]      FIG. 3  is a schematic diagram of a gas field ion microscope system. 
           [0017]      FIG. 4  is a top view of an embodiment of a multi-opening aperture.  FIG. 5  is a top view of an embodiment of a multi-opening aperture. 
       
    
    
     DETAILED DESCRIPTION 
     General Discussion 
       [0018]    Information relating to gas field ion microscope systems and related methods is generally disclosed, for example, in U.S. Patent Application Publication No. US 2007/0158558, U.S. Ser. No. 61/092,919 and U.S. Ser. No. 61/074,361, each of which is hereby incorporated by reference in its entirety. 
         [0019]      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 . 
         [0020]    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 . 
         [0021]    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 . 
         [0022]    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 . 
         [0023]    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)). 
         [0024]    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 . 
         [0025]    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 , 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 . 
         [0026]    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 ion beam  192  and sample  180 . As shown in  FIG. 2 , 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). 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 a given particle). Optionally, combinations of such detector arrangements can be used. Exemplary detectors include Everhart-Thornley detectors (e.g., to detect secondary electrons), photon detectors, microchannel plate detectors (e.g., to detect condary electrons, neutral atoms and/ or ions), conversion plates (e.g., to detect ions), channeltron detectors (e.g., to detect secondary electrons, ions and/or neutral atoms), phosphor detectors (e.g., to detect photons), solid state detectors (e.g., to detect secondary electrons, ions, and/or neutral atoms), scintillator detectors (e.g., to detect photons), energy detectors for ions (e.g., electrostatic prism detectors, magnetic prism detectors, quadrupole detectors), energy detectors for secondary electrons (e.g., electrostatic prism detectors, magnetic prism detectors, detectors with a negatively biased particle selector disposed in the flight path of the electrons to be detected), time-of-flight detectors (e.g., to detect ions) 
         [0027]    In general, the information measured by the detectors is used to determine information about sample  180 . In some embodiments, such as when the detected particles are secondary electrons, the detected particles are used to form an image of sample  180 . In certain embodiments, such as when the detected particles are Auger electrons, secondary ions, secondary neutral particles, primary neutral particles, scattered ions and/or photons, the detected particles are used to determine chemical information about sample  180 . 
         [0028]    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. 
         [0029]    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, chemical information 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. 
         [0030]    In some embodiments, electronic control system  170  can be configured to control additional devices. For example, electronic control system  170  can be configured to regulate a supply (e.g., control flow rate and/or gas composition) of a reactive gas delivered to sample  180  in the vicinity of ion beam  192 . The reactive gas can be used, for example, in one or more beam-induced chemical etching operations to selectively remove substrate material. Additionally or alternatively, a reactive gas can be used to deposit conductive material. 
         [0031]    Detectors  150  and  160  are depicted schematically in  FIG. 2 , 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 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 can 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. 
         [0032]      FIG. 3  shows a schematic diagram of a He ion microscope system  200 . Microscope system  200  includes a first vacuum housing  202  enclosing a He ion source and ion optics  130 , and a second vacuum housing  204  enclosing sample  180  and detectors  150  and  160 . Gas source  110  delivers He gas to microscope system  200  through a delivery tube  228 . A flow regulator  230  controls the flow rate of He gas through delivery tube  228 , and a temperature controller  232  controls the temperature of He gas in gas source  110 . The He ion source includes a tip  186  affixed to a tip manipulator  208 . The He ion source also includes an extractor  190  and a suppressor  188  that are configured to direct He ions from tip  186  into ion optics  130 . Ion optics  130  include a first lens  216 , alignment deflectors  220  and  222 , an aperture  224 , an astigmatism corrector  218 , scanning deflectors  219  and  221 , and a second lens  226 . Aperture  224  is positioned in an aperture mount  234 . Sample  180  is mounted in/on a sample manipulator  140  within second vacuum housing  204 . Detectors  150  and  160 , also positioned within second vacuum housing  204 , are configured to detect particles  194  from sample  180 . Gas source  110 , tip manipulator  208 , extractor  190 , suppressor  188 , first lens  216 , alignment deflectors  220  and  222 , aperture mount  234 , astigmatism corrector  218 , scanning deflectors  219  and  221 , sample manipulator  140 , and/or detectors  150  and/or  160  are typically controlled by electronic control system  170 . Optionally, electronic control system  170  also controls vacuum pumps  236  and  237 , which are configured to provide reduced-pressure environments inside vacuum housings  202  and  204 , and within ion optics  130 . 
         [0033]    As noted above, in general, tip  186  can be formed of any appropriate electrically conductive material. In certain embodiments, tip  186  can be formed of a single crystal material, such as a single crystal metal. Typically, a particular single crystal orientation of the terminal shelf of atoms of tip apex  187  is aligned with a longitudinal axis of tip  186  to within 3° or less (e.g., within 2° or less, within 1° or less). In some embodiments, apex  187  of tip  186  can terminate in an atomic shelf having a certain number of atoms (e.g., 20 atoms or less, 15 atoms or less, 10 atoms or less, nine atoms or less, six atoms or less, three atoms or less). For example, apex  187  of tip  186  can be formed of W(111) and can have a terminal shelf with three atoms (a trimer).  FIGS. 4 and 5  show schematic representations of enlarged top and side views, respectively, of the two atomic shelves of a W tip  186  that are nearest to the apex of tip. The terminal shelf, which includes three W atoms  302  arranged in a trimer, corresponds to a (111) surface of W. Without wishing to be bound by theory, it is believed that this trimer surface is advantageous (in terms of its ease of formation, re-formation and stability) because the surface energy of the W(111) crystal face favorably supports a terminal shelf formed by three W atoms arranged in an equilateral triangle to form a trimer. The trimer atoms  302  are supported by a second shelf of W atoms  304 . 
         [0034]    In some embodiments, tip  186  can have a terminal shelf that includes fewer than three atoms or more than three atoms. For example, a W(111) tip can have a terminal shelf that includes two atoms, or a terminal shelf that includes only one atom. Alternatively, a W(111) tip can have a terminal shelf that includes four or more atoms (e.g., five or more atoms, six or more atoms, seven or more atoms, eight or more atoms, nine or more atoms, ten or more atoms, more than ten atoms). 
         [0035]    Alternatively, or in addition, tips that correspond to other W crystalline orientations (e.g., W(112), W(110) or W(100)) can be used, and such tips can have terminal shelves that include one or more atoms (e.g., two or more atoms, three or more atoms, four or more atoms, five or more atoms, six or more atoms, seven or more atoms, eight or more atoms, nine or more atoms, ten or more atoms, more than ten atoms). 
         [0036]    In some embodiments, tips formed from a material other than single crystal W can be used in the ion source (e.g., a single crystal of a metal, such as a single crystal of one of the metals noted above), and such tips can have terminal shelves that include one or more atoms (e.g., two or more atoms, three or more atoms, four or more atoms, five or more atoms, six or more atoms, seven or more atoms, eight or more atoms, nine or more atoms, ten or more atoms, more than ten atoms). 
       Multi-Mode Systems 
       [0037]    A gas field ion microscope system can designed to operate in at least two different modes. In a first mode, for example, the ion beam has a relatively low ion current and/or relatively small spot size at the surface of the sample. In a second mode, for example, the ion beam has a relatively high ion current and/or a relatively large spot size at the surface of the sample. 
         [0038]    In general, the ion beam current at the surface of the sample in the second mode is greater than the ion beam current at the sample in the first mode. In some embodiments, the ion beam current at the surface of the sample in the second mode is at least two times (e.g., at least three times, at least five times, at least 10 times, at least 25 times, at least 50 times, at least 100 times, at least 500 times, at least 1,000 times) the ion beam current at the surface of the sample in the first mode, and/or the ion beam current at the surface of the sample in the second mode is at most about 5,000 times the ion beam current at the surface of the sample in the first mode. As an example, if the first mode is a relatively high resolution mode, then the ion beam. In certain embodiments, in the first mode the ion current at the surface of the sample is 250 picoAmps or less (e.g., 100 picoAmps or less, 50 picoAmps or less, 25 picoAmps or less, 10 picoAmps or less, five picoAmps or less, one picoAmp or less), and/or in the second mode the ion current at the surface of the sample is 10 picoAmps or more (e.g., 25 picoAmps or more, 50 picoAmps or more, 100 picoAmps or more, 500 picoAmps or more). 
         [0039]    In some embodiments, in the first mode the maximum dimension of the ion beam at the surface of the sample is 25 nanometers or less (e.g., 15 nanometers or less, 10 nanometers or less, five nanometers or less), and/or at lest one nanometer. In certain embodiments, in the second mode the maximum dimension of the ion beam spot at the surface of the sample is 10 nanometers or more (e.g., 25 nanometers or more, 50 nanometers or more, 100 nanometers or more), and/or at most 500 naometers. 
         [0040]    In general, a gas field ion microscope system is switched between the first mode of operation and the second mode of operation by manipulating the system such that most of the ions in the ion beam that interact with the sample are generated by only one atom of the tip (first mode) or from a plurality of atoms of the tip (second mode). For example, in some embodiments, in the first mode at least 80% (e.g., at least 90%, at least 95%, at least 98%) of the ions in the ion beam that interact with the surface are generated by only one atom of the tip of the ion source (e.g., only one atom of a trimer that forms the terminal shelf of the tip), and/or in the second mode at least 80% (e.g., at least 90%, at least 95%, at least 98%) of the ions in the ion beam that interact with the surface are generated by more than one atom of the tip of the ion source (e.g., two atoms of a trimer that forms the terminal shelf of the tip, three atoms of a trimer that forms the terminal shelf of the tip). 
         [0041]    In some embodiments, the gas field ion microscope system can be switched between the first mode and the second mode by changing one or more apertures along the ion beam path between the tip of the ion source and the sample. Generally, in the first mode, the aperture(s) is(are) smaller than in the second mode. For example, referring again to  FIG. 3 , aperture  224  can be changed from having a relatively small opening in the first mode to having a relatively large opening in the second mode. In some embodiments, aperture  224  can include a plurality of openings having different widths w. For example,  FIG. 4  is a top view (along the z-direction of  FIG. 3 ) of a disk-shaped aperture  224   a  that includes multiple openings  225   a - 225   g.  Aperture  224   a  is configured to rotate about a pivot point  227  that coincides with the center of aperture  224   a.  The centers of each of openings  225   a - 225   g  are positioned at the same distance from pivot point  227 . An aperture opening of a particular size can therefore be selected by rotating aperture disk  224   a  such that a selected opening is positioned in the path of the ion beam, and then translating aperture disk  224   a,  if desired, to ensure correct alignment of the opening with the ion beam.  FIG. 5  is a top view (along the z-direction of  FIG. 3 ) of a rod-shaped aperture  224   b  that includes multiple openings  229   a - 229   e  extending through aperture  224   b.  The aperture size can be chosen by selecting an opening in aperture  224   b.  This selection is performed by translating aperture  224   b  in a direction parallel to arrow  221  to align one of the openings  229   a - 229   e  with the ion beam. Typically, openings  225   a - 225   g  and  229   a - 229   e  have diameters that can be chosen as desired. For example, in some embodiments, the diameter of any of the openings can be five μm or more (e.g., 10 μm or more, 25 μm or more, 50 μm or more) and/or 200 μm or less (e.g., 150 μm or less, 100 μm or less). In certain embodiments, the diameters of openings  225   a - 225   g  and/or  229   a - 229   e  can be from five μm to 200 μm (e.g., five μm to 150 μm, five μm to 100 μm). 
         [0042]    In certain embodiments, the gas field ion microscope system can be switched between the first mode and the second mode by manipulating one or more components in the ion optics (e.g., first lens  216  and/or alignment deflectors  220 ,  222 ). This can be achieved, for example, by appropriate selection of the electrical potentials applied to first lens  216  and/or deflectors  220 ,  222 , with or without changing the size of aperture  224  (see discussion above). Such selection can result in switching from the first mode to the second mode by, for example, moving the beam crossover toward the aperture plane so that its image size becomes smaller than the size of the aperture. 
         [0043]    Optionally, the gas field ion microscope system can be switched between the first and second modes without removing the sample from the gas field ion microscope system (e.g., without moving the sample). 
       Exemplary Applications of Multi-Mode Systems 
       [0044]    In some embodiments, the first mode can be used to image a sample. For example, secondary electrons can be detected to provide information that results in the image of the sample. A relatively low ion beam current (e.g., created by only one atom of the terminal shelf of the tip) can be used when collecting secondary electrons because the yield of secondary electrons is generally relatively high. In addition, the detector used with secondary electrons (e.g., an Everhardt-Thornley detector) can be custom shaped to conform to the ion optics, allowing the detector to be located relatively close to the area of the sample with which the ion beam interacts, which can enhance secondary electron detection. Further, the energy of the secondary electrons is relatively low and is in some instances of relatively little analytical value, thereby allowing the use of a biased electron grid to accelerate and steer the secondary electrons to the detector, which can augment the effective collection solid angle, without substantially interfering in obtaining the desired information. Further, using a relatively low ion beam current (e.g., created by only one atom of the terminal shelf of the tip) can also result in formation of a relatively small ion beam spot size at the surface of the sample, resulting in a relatively high precision image of the sample. 
         [0045]    In certain embodiments, the second mode can be used to detect particles that have a relatively low yield and/or for which it is difficult to position the detector to obtain a relatively high yield of the particles. Examples of such particles can include 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). In many instances, such particles can be used to determine chemical information about the sample (e.g., qualitative and/or quantitative information regarding the chemical constituents at the sample surface and/or in the subsurface region of the sample). In such embodiments, using a relatively high ion beam current and/or a relatively large ion beam spot size can increase the number of particles generated at the surface, which can decrease the amount of time used to determine the desired information about the sample. 
         [0046]    In embodiments such as discussed in the preceding two paragraphs, switching between the first and second modes of operation typically involves not only manipulating the gas field ion microscope system to change the ion current and/or ion beam spot size at the sample, but also involves switching the detector. Optionally, this can be achieved without removing the sample from the gas field ion microscope system (e.g., without moving the sample). 
         [0047]    In some embodiments, the second mode of operation (relatively high ion beam current and/or relatively large ion beam spot size) is used to perform chemistry on the sample at a relatively high rate, while the first mode of operation is used to image the sample (see discussion above) and/or to perform chemistry on the sample at a relatively low rate. Examples of types of chemistry that can be performed on a sample (e.g., a semiconductor sample) include, for example, etching the sample and/or depositing material on the sample. Such embodiments may be implemented, for example, when etching a cross-section of a sample for subsequent inspection of the resulting exposed area of interest of the sample. Often, etching the sample to form the cross-section can take substantially more time than inspecting the article, so using a relatively high ion beam current and/or a relatively large ion beam spot size can reduce the time involved in preparing/analyzing the sample. Alternatively or additionally, it may be desirable to first form a relatively rough cut of the sample, followed by a more precise etch of the sample, prior to inspecting the exposed area of interest of the sample. Thus, it can be advantageous to use a relatively high ion beam current and/or a relatively large ion beam spot size during the initial preparation of the cross-section because precision can be sacrificed for speed, and also because the cross-section can subsequently be precisely refined using a relatively low ion beam current and/or relatively small ion beam spot size. In some embodiments, a relatively high ion beam current and/or a relatively large spot size is used to form a cross-section in a sample, followed by taking an image of the sample without using a relatively precise intermediate process to refine the cross-section. In certain embodiments, a relatively ion beam current and/or a relatively large ion beam spot size is used to form a cross-section in a sample, followed by using a relatively low ion beam current and/or a relatively small ion beam spot size to modify the cross-section of the sample (e.g., to provide a more precisely formed cross-section), which can optionally be followed by taking an image of the sample. In these embodiments, switching between the first and second modes of operation may involve not only manipulating the gas field ion microscope system to change the ion current and/or ion beam spot size at the sample, but may also involve using a detector in one mode but not another mode. Optionally, the relatively high ion current and/or relatively large ion beam spot size is operated for at least two times (e.g., at least five times, at least 10 times) as long as the relatively low ion current and/or relatively small ion beam spot size. 
         [0048]    In some embodiments when etching a sample, it may be advantageous to use a relatively heavy gas to enhance the etch rate. For example, Ne, Ar, Kr and/or Xe may be used. This may be particularly beneficial, for example, when attempting to form a cross-section in a sample. 
         [0049]    When depositing material, in some embodiments it may be beneficial to use a relatively light gas (e.g., He,  3 He, molecular hydrogen) to reduce ion beam-induced sputtering that may compete and/or interfere with the desired deposition process. In some cases, deposition of a material on a sample involves the interaction of the ion beam 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 ). 
         [0050]    In certain embodiments when obtaining chemical information about a sample, it may be advantageous to use a relatively heavy gas (e.g., Ne, Ar, Kr and/or Xe) to enhance sensitivity for higher mass constituents of the sample (e.g., when detecting scattered ions). 
         [0051]    In some embodiments, such as when performing secondary ion mass spectrometry as the detection technique, it could be beneficial to use molecular oxygen as the gas because this could result in a higher ionization fraction of sputtered material. 
         [0052]    While the first mode has been described as involving a relatively low ion current, the brightness and/or etendue in the first mode can still be relatively high. In certain embodiments of the first mode, the ion beam has a brightness at the surface of the sample of 5×10 8  A/m 2 srV or more (e.g., 1×10 9  A/cm 2 srV or more, 1×10 10  A/cm 2 srV or more). As used herein, the reduced brightness is as defined in U.S. Patent Application Publication No. US 2007/0158558. In certain embodiments of the first mode, the ion beam has an etendue at the surface of the sample of 5×10 −21  cm 2 sr or less (e.g., 1×10 −22  cm 2 sr or less, 1×10 −23  cm 2 sr or less, 1×10 −23  cm 2 sr or less, 1×10 −24  cm 2 sr or less). As used herein, the etendue is as defined in U.S. Patent Application Publication No. US 2007/0158558. In some embodiments of the first mode, the ion beam has a reduce etendue at the surface of the sample of 1×10 −16  cm 2 sr or less (e.g., 1×10 −17  cm 2 sr or less, 1×10 −18  cm 2 sr or less, 1×10 −19  cm 2 sr or less). As used herein, the reduced etendue is as defined in U.S. Patent Application Publication No. US 2007/0158558. In some embodiments of the first mode, the ion beam has a reduced brightness at the surface of the sample of 1×10 9  A/cm 2 sr (e.g., 1×10 10  A/cm 2 sr or more, 1×10 11  A/cm 2 sr or more). As used herein, the brightness is as defined in U.S. Patent Application Publication No. US 2007/0158558. 
         [0053]    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. Published Patent Application 2007-0158558. 
         [0054]    While embodiments have been described in which first and second modes are used for etching, in some embodiments, one mode is used for etching and another mode is used for deposition. In certain embodiments, the first and second modes are used for deposition. In some embodiments, the rate of chemistry performed using a relatively large ion beam current and/or a relatively large ion beam spot size can be at least two times (e.g., at least five times, at least 10 times) the rate of chemistry performed on the sample using a relatively low ion beam current and/or a relatively small ion beam spot size. 
         [0055]    While embodiments have been described in which a gas field ion microscope system is operated in two different modes, in some embodiments, more than two modes (e.g., three modes, four modes, five modes, six modes, seven modes, eight modes, nine modes,  10  modes, etc.) can be used. The different modes may be defined by the ion beam current, the ion beam spot size, the detector used, the particles detected, and/or the chemistry performed on the sample. 
         [0056]    While embodiments have been described in which most of the ions in the ion beam that interact with the sample are generated by only one atom of the tip in the first mode and by a plurality of atoms of the tip in the second mode, more generally most of the ions in the ion beam that interact with the sample in the first mode are generated by a number of atoms of the tip that is less than (e.g., by one atom, by two atoms, by three atoms, by four atoms, etc.) the number of atoms in the tip that generate most of the ions in the ion beam that interact with the sample in the second mode. 
         [0057]    In general, any of the systems and/or methods described herein can be implemented and/or controlled in computer hardware or software, or a combination of both. The systems and/or methods can be implemented in computer programs using standard programming techniques following the methods and figures described herein. Program code is applied to input data to perform the functions described herein and generate output information. The output information is applied to one or more output devices such as a display monitor. Each program may be implemented in a high level procedural or object oriented programming language to communicate with a computer system. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language. Moreover, the program can run on dedicated integrated circuits preprogrammed for that purpose. Each such computer program can be stored on a storage medium or device (e.g., ROM or magnetic diskette) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein. The computer program can also reside in cache or main memory during program execution. The methods or portions thereof can also be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein. 
         [0058]    In general, various aspects of the foregoing embodiments can be combined as desired. 
         [0059]    Other embodiments are covered by the claims.