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). In certain embodiments, the ion sources, systems and methods can enhance the ability to make tips having desired physical attributes (e.g., the number of atoms on the apex of the tip). This can enhance performance (e.g., increase reliability, stability and the like).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of, and claims priority under 35 USC 120 to, U.S. application Ser. No. 12/997,371, filed Jun. 3, 2011, now U.S. Pat. No. 8,461,557, which is the National Stage of International Application No. PCT/US2008/066933, filed Jun. 13, 2008. U.S. application Ser. No. 12/997,371 and international application PCT/US2008/066933 are hereby incorporated by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to ion sources, systems and methods. 
     BACKGROUND 
     Ion sources and systems can produce ion beams which are used to investigate and/or modify a sample. 
     SUMMARY 
     The disclosure relates to ion sources, systems and methods. 
     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). 
     In certain embodiments, the ion sources, systems and methods can enhance the ability to make tips having desired physical attributes (e.g., the number of atoms on the apex of the tip). This can enhance performance (e.g., increase reliability, stability and the like). 
     In one aspect, the disclosure generally features a system that includes a charged particle column, a detector and an optical reflective element having a first position and a second position. When in the first position, the optical reflective element can reflect light passing through the charged particle column to the detector. When in the second position, the optical reflective element cannot reflect light passing through the charged particle column to the detector. In some embodiments, the system further includes a positioning device configured to move the optical reflective element between its first and second positions. In certain embodiments, the charged particle column is an ion column. In some embodiments, the system further includes a charged particle source. The charged particle source can be configured so that during use at least some of the charged particles generated by the charged particle source pass through the charged particle column. The charged particle source can be configured so that, when it emits light, the light goes into the column and can be reflected by the optical reflective element when it is in the first position. In certain embodiments, the detector is configured to detect light reflected by the optical reflective element. 
     In another aspect, the disclosure generally features a system that includes a charged particle column and an optical reflective element having an optical reflective portion and an aperture. The optical reflective element is in the charged particle column. The optical reflective portion of the optical reflective element can light passing through the charged particle column. Charged particles emitted by a charged particle source can pass through the aperture of the optical reflective element. In some embodiments, the optical reflective element is fixed with respect to the charged particle column. In certain embodiments, the charged particle column is an ion column. In some embodiments, the system further includes the charged particle source. The charged particle source can be configured so that during use at least some of the charged particles generated by the charged particle source pass through aperture in the optical reflective element. The charged particle source can be configured so that, when it emits light, the light goes into the column and can be reflected by the optical reflective portion of the optical reflective element. The charged particle column has an axis, and the optical reflective element can be positioned along the axis of the charged particle column. In some embodiments, the system further includes a detector configured to detect light reflected by the optical reflective element. 
     In a further aspect, the disclosure generally features a system that includes a charged particle column having an axis, and an optical reflective element positioned within the charged particle column and displaced off-axis with respect to the axis of the charged particle column. The optical reflective element is coupled to the charged particle column. In some embodiments, the optical reflective element is fixed with respect to the charged particle column. In certain embodiments, the system further includes a support to which the optical reflective element is mounted. The support can be fixed with respect to the charged particle column. In some embodiments, the charged particle column is an ion column. In certain embodiments, the system further includes a charged particle source. The charged particle source can be configured so that during use at least some of the charged particles generated by the charged particle source pass through the charged particle column without interacting with the optical reflective element. The charged particle source can be configured so that, when it emits light, the light goes into the column and can be reflected by the optical reflective element. In some embodiments, the system further includes a detector configured to detect light reflected by the optical reflective element. 
     In an additional aspect, the disclosure generally features a system that includes a charged particle column and a moveable optical reflective element having a first position in the charged particle column and a second position outside the charged particle column. 
     In one aspect, the disclosure generally features a charged particle system that includes any of the preceding systems. In some embodiments, the charged particle system can be a gas field ion microscope. 
     In another aspect, the disclosure generally features a method that includes emitting light from a charged particle source so that the light enters a charged particle column, and reflecting at least a portion of the light in the charged particle column to a detector. In some embodiments, the method also includes using the detected light to determine one or more parameters for preparing a tip of the charged particle source. Examples of parameters include the temperature of the tip of the charged particle source, the gas pressure of a chamber housing the charged particle source, and the intensity of light emitted by the charged particle source. In certain embodiments, the method further includes, based on the detected light, increasing at least one parameter selected from the group consisting of a charged particle source temperature and a gas pressure in a chamber housing the charged particle source. In some embodiments, the charged particle source is an ion source, such as a gas field ion source. 
     In an further aspect, the disclosure generally features a method that includes using any of the systems described above to make a tip of a charged particle source. 
     In an additional aspect, the disclosure generally features a system that includes a vacuum housing having a door and a stage assembly. The stage assembly includes a stage configured to support a sample, and a support member connected to the door. The stage is connected to the support via a friction mechanism. In some embodiments, the friction mechanism includes at least one friction bearing. In certain embodiments, the friction mechanism includes a tube that is friction fit within an aperture. In some embodiments, the stage is tiltable relative to the door. In certain embodiments, the friction mechanism can be used to tilt the stage relative to the door. In some embodiments, the system further includes a charged particle source, such as an ion source (e.g., a gas field ion source). In certain embodiments, the system is a gas field ion microscope. 
     In another aspect, the disclosure generally features a system that includes a sample holder having a first surface and a second surface opposite the first surface. The second surface has a plurality of holes. They system also includes a stage having a surface with support positions. The holes in the second surface of the sample holder are configured to engage with the support positions of the stage. The system further includes at least one magnet configured to secure the sample holder to the stage. In some embodiments, the at least one magnet is a plurality of magnets. In certain embodiments, an exposed surface of the at least one magnet coincides with the second surface of the sample holder. In some embodiments, the system further includes a charged particle source, such as an ion source (e.g., a gas field ion source). In certain embodiments, the system is a gas field ion microscope. 
     Other features and advantages will be apparent from the description, drawings, and claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of an ion microscope system. 
         FIG. 2  is a schematic diagram of a gas field ion source. 
         FIG. 3  is a schematic diagram of a sample holder assembly. 
         FIG. 4  is a schematic diagram of a magnetic sample holder. 
         FIG. 5  is a schematic diagram of a sample chamber that includes a retractable positioner. 
         FIG. 6  is a schematic diagram of a positioner. 
         FIG. 7  is a schematic diagram of a chamber that includes an off-axis mirror. 
         FIG. 8  is a schematic diagram of a chamber that includes an on-axis mirror with an aperture. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     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. 
     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 thrilled 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 clement, 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 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. 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. 
       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 . 
     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. 
     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 . 
     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 . 
     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)). 
     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 . 
     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 . 
     When ion beam  192  impinges on sample  180 , a variety of different types of particle  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 parades 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 a given particle). Optionally, combinations of such detector arrangements can be used. 
     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. 
     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. 
     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. 
     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. 
     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. 
     Detection systems and methods are generally disclosed, for example, in US 2007-0158558, the entire contents of which are incorporated herein by reference. 
     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. 
     One source of such fluctuations in the position of the ion beam can be mechanical vibrations which lead to displacement of the sample relative to the ion beam during the course of a measurement. Ion beam sources typically use a variety of components such as pumps and drive mechanisms that produce low frequency vibrations when activated. Such low frequency vibrations can couple through intermediate components, inducing motion of the sample relative to the ion beam. As an example, such low frequency vibrations can couple through components formed of relatively rigid materials (e.g., stainless steel) and into the sample holder. 
     A sample holder assembly  1510  that provides for improved stability and reduced vibrational coupling to a sample is shown in  FIG. 3 . Assembly  1510  is mounted to a body  1511  having an opening  1512  to insert a sample. In some embodiments, body  1511  does not include an opening, and is instead a solid member that corresponds to a door of a sample chamber. To insert a sample, body  1511  swings open on a side-mounted hinge, exposing the sample holder assembly for sample mounting. 
     Body  1511  is connected to arms  1518  of the sample holder assembly through adjustable connectors  1522 . Arms  1518  support a sample stage  1514  via friction bearing  1520 . Sample stage  1514  includes a mounting surface  1516  having an aperture  1524 . 
     Sample holder assembly  1510  can be connected to an ion microscope such that a tip from which the ion beam is generated is pointed towards aperture  1524  on sample stage  1514 . Body  1511  can be formed from suitable rigid materials such as hardened steel, stainless steel, phosphor bronze, and titanium. Typically, for example, body  1511  corresponds to a door of a sample chamber and is square or rectangular in shape, with a thickness of between 0.25 inches and 2 inches or more. By forming body  1511  from a relatively thick piece of metal, body  1511  is relatively highly resistant to deformation, and therefore does not transmit mechanical vibrations efficiently. 
     Sample stage  1514  is supported by arms  1518  connected to body  1511  along adjustable connectors  1522 . Adjustable connectors  1522  comprise rails with recesses that mate cooperatively with flanges  1521  of arms  1518 . Arms  1518  are movable in the vertical direction of  FIG. 3  with respect to body  1511  by sliding flanges  1521  within the recesses of adjustable connectors  1522 . In  FIG. 3 , the vertical direction is parallel to the optical axis  1513  of the ion beam system. In other words, anus  1518  are movable in a direction that is parallel to optical axis  1513  of the ion beam system. 
     Following movement in the vertical direction, arms  1518  (and stage  1514  connected thereto) can be locked in a specific position. Stage assembly  1510  includes pneumatic or vacuum clamps (not shown in  FIG. 3 ) positioned on the opposite side of body  1511  from arms  1518 , and connected to arms  1518  through apertures in body  1511 . To lock arms  1518  in position relative to body  1511 , the pneumatic or vacuum clamps are engaged, pulling arms  1518  tight against body  1511  and preventing further relative motion between body  1511  and arms  1518 . 
     During operation of assembly  1510 , body  1511 , which corresponds to a door of the sample chamber, swings open to expose stage  1514 . A sample is mounted on stage  1514 , and then body  1511  swings closed to seal the sample chamber. A suitable height for the mounted sample is selected by releasing the pneumatic (or vacuum) clamps that fix the position of arms  1518  relative to body  1511 , and then translating arms  1518  along the vertical direction in  FIG. 3 . Flanges  1521  of arms  1518  move relative to connectors  1522  during the vertical translation of arms  1518 . When the sample has been positioned at a desired vertical position, the pneumatic (or vacuum) clamps are re-engaged, rigidly locking arms  1518  in place against body  1511  and preventing further relative motion between arms  1518  and body  1511  in the vertical direction. The rigid locking of arms  1518  to body  1511  has the added benefit of increasing the resistance of body  1511  to flexural deformation when vibrations (e.g., from pumps and other sources) are coupled to body  1511 . 
     Sample stage  1514  is connected to arms  1518  via friction bearings  1520 . Friction bearings  1520  include a hollow cylindrical shaft that extends from arm  1518  and into a mating aperture on stage  1514 . Stage  1514  includes two such friction bearings, as shown in  FIG. 3 . The cylindrical shaft is sized to provide an interference fit with the mating aperture on stage  1514 . As a result, the two friction bearings  1520  allow stage  1514  to tilt relative to arms  1518 , out using moving parts such as oil coated ball bearings that can introduce contaminants into the sample chamber. As shown in  FIG. 3 , the tilt axis (e.g., the axis about which stage  1514  is rotatable) is perpendicular to the optical axis of the ion beam system (e.g., optical axis  1513 ). 
     In certain embodiments, friction bearings  1520  include a hollow cylindrical shaft that extends from stage  1514  and into a mating aperture on arm  1518 . Two such friction bearings  1520  can be provided, one on each side of stage  1514  as shown in  FIG. 3 . The cylindrical shaft is sized to provide an interference fit with the mating aperture on arm  1518 . As a result, the friction bearings  1520  allow stage  1514  to tilt relative to arms  1518 . The tilt axis (e.g., the axis about which stage  1514  is rotatable), as shown in  FIG. 3 , is perpendicular to the optical axis of the ion beam system (e.g., optical axis  1513 ). 
     The interference fit in each friction bearing  1520  is sufficiently restrictive so that stage  1514  can be tilted to an angle of 45 degrees or more without undergoing slip relative to arms  1518 . Generally, a motor is used to adjust the tilt angle of stage  1514 . Due to the friction hearings, tilt motion of the stage is typically not continuous, but occurs in a series of tiny jumps, each corresponding to an angular displacement of less than about 0.25 degrees (e.g., less than 0.20 degrees, less than 0.15 degrees, less than 0.10 degrees, less than 0.05 degrees). In other words, the diameter of the cylindrical shaft and of the mating hole in friction bearing  1520  are selected so that the tiny jumps in angular displacement of stage  1514  relative to arms  1518  during relative motion are about 0.25 degrees or less. 
     Sample stage  1514  further includes mounting surface  1516  which can have an opening  1524 . A sample can be placed on mounting surface  1516  and a sample position control system can be used to move the sample in the plane of surface  1516 . In certain embodiments, surface  1516  (or a portion thereof) can be rotated about its center to rotate the sample. As shown in  FIG. 3 , in some embodiments, the tilt angle of stage  1514  is zero. Accordingly, a rotation axis of surface  1516  (e.g., the axis about which surface  1516  is rotated) is oriented in the vertical direction of  FIG. 3 , parallel to the direction of optical axis  1513  of the ion beam system. Surface  1516  can be formed from various types of rigid materials, such as stainless steel, ceramic, glass and polymers. 
     Movement of surface  1516  in the horizontal place (e.g., perpendicular to the vertical direction in  FIG. 3 ) is typically controlled by piezoelectric devices. The relatively high stiffness of piezoelectric devices ensures that surface  1516  remains rigidly fixed in position in the horizontal plane of  FIG. 3  (e.g., the plane perpendicular to optical axis  1513 ), and external vibrations do not effectively couple into surface  1516  along the horizontal plane of  FIG. 3 . 
     A particular advantage of the stage assembly  1510 , as discussed above, is the absence of ball bearings in the assembly, which are typically coated with a hydrocarbon-based lubricants. Such lubricants act as impurities within a sample chamber, depositing on chamber surfaces and even on the surface of the sample during exposure to the ion beam. By eliminating the use of such bearings, a potential source of contaminants is also eliminated from the ion beam system. To ensure secure but removable mounting, samples are mounted to stage  1514  using magnetic sample holder. An embodiment of a magnetic sample holder  1600  is shown in  FIG. 4 .  FIG. 4  depicts the underside of sample holder  1600  which mates with mounting surface  1516  of assembly  1510 . Sample holder  1600  includes three support structures  1610  and three magnetic contacts  1620 . 
     Each of the three support structures  1610  includes two holes  1630  that are sized to accommodate two corresponding conical pins that extend upwards from surface  1516 . By positioning sample holder  1600  with each of the six pins that extend upwards from surface  1516 , sample holder  1600  can be reproducibly positioned relative to surface  1516  with a tolerance of a one micron or less. 
     To rigidly affix sample holder  1600  to surface  1516 , each of the three magnetic contacts  1620  is positioned adjacent to a corresponding piece of magnetic steel which is mounted in surface  1516 . The correct positioning of the magnetic contacts  1620  is achieved automatically by engaging holes  1630  with the conical pins of surface  1516 . Strong magnetic field interactions between contacts  1620  and the corresponding steel magnets in surface  1516  ensure that sample holder  1600  is affixed to surface  1515  with significant force. 
     Each of the magnetic contacts includes two strong permanent magnets  1635  encased in a 5-sided enclosure of mu-metal. Only the lower surface of the mu-metal enclosure is left open (e.g., the surface adjacent to the steel magnets in surface  1516 . The other surfaces of the mu-metal enclosure are closed to restrict the spatial extent of the magnetic field extending from magnets  1635 . 
     The two permanent magnets  1635  in each contact  1620  are oriented to that their poles are opposed. As a result, magnetic field lines extending from the two magnets are relatively restricted spatially. Because of this, and because of the mu-metal enclosure, the magnetic fields generated by contacts  1620  do not perturb the ion beam during sample exposure. 
     To introduce a sample onto mounting surface  1516 , the sample is first mounted to the underside of sample holder  1600  in  FIG. 4 . Then, sample holder  1600  is placed on a mounting arm, which engages with recessed lip  1640  of holder  1600 . The mounting arm (not shown in  FIG. 4 ) is extended toward surface  1516 , and rotated to ensure alignment of the conical pins with holes  1630 . As the mounting arm is lowered toward surface  1516 , the magnetic force between contacts  1620  and the corresponding magnets in surface  1516  fix holder  1600  in place atop surface  1516 , supported by the six conical pins extending from surface  1516 . In this fixed position, contacts  1620  are positioned within 500 microns of the magnets in surface  1516 . The mounting arm is then carefully withdrawn, and sample holder  1600  (and the sample mounted thereon) remain fixed to surface  1516 . 
     In some embodiments, the magnets positioned in surface  1516  are permanent magnets. In certain embodiments, the magnets positioned in surface  1516  can be switched on and off (e.g., by changing the position of the magnets in surface  1516  via rotation, and/or by applying a counteracting electromagnetic field via one or more magnetic coils that balances the magnetic field of the magnets in surface  1516 ). Switchable magnets can be particularly advantageous when positioning sample holder  1600  relative to surface  1516 . For example, with the magnets switched off, sample holder  1000  can be positioned atop the supporting conical pins that extend from surface  1516 . When sample holder  1600  is in the correct position, the magnets can be switched on to lock sample holder  1600  in place relative to surface  1516 . Typically, the magnetic force between contacts  1620  and the magnets in surface  1560  is sufficiently strong to prevent relative movement of sample holder  1600  at tilt angles of 45 degrees or more. 
     As noted above, tips for ion beam sources can be produced by first forming a tip from a material such as, for example, tungsten. In some embodiments, forming the tip involves sharpening a rod (e.g., a tungsten rod) to form a sharpened tip, and field evaporating the sharpened tip to produce a desired terminal shelf of the apex of the tip. In some embodiments, it is desirable for the terminal shelf of the apex of the tip that includes only a small number of atoms (e.g., from 1 to 20 atoms). During field evaporation, the tip is usually heated, and light emanating from the tip can be observed optically (e.g., using the eye, using a light detector). In some instances, the temperature of the tip can be estimated based on the observed tip color. During field evaporation of the tip, the geometry of the tip apex can be monitored by observing the field emission pattern from the tip under an appropriate applied potential (by using heat and electrical potential during field evaporation). 
     Observing the tip during fabrication can be difficult because the sharpening and field evaporation steps are typically performed under vacuum in a sample chamber. In addition, the tip is typically oriented such that at least some of the light goes through the ion optics (ion column) which can make it difficult to observe the light. To facilitate observation of the ion source (e.g., during field evaporation when making the tip), the sample chamber can include one or more source viewing optics fixed on a retractable positioner.  FIG. 5  shows a retractable positioner  1700  configured for use with a sample chamber in an ion beam system. As shown in  FIG. 5 , retractable positioner is mounted within a flange  1704  of a sample chamber  1702 . Positioner  1700  includes a first actuator  1706  for moving positioner  1700  in an axial direction, and a second actuator  1708  for moving positioner  1700  within a plane perpendicular to the axial direction of the positioner. Also mounted to a flange of chamber  1702  is a measurement device  1710  (e.g., a camera, or a bolometer, or a linear sensor, or another device). 
     Positioner  1700  can include a variety of optical elements. In some embodiments, positioner  1700  includes a mirror inclined at an angle to the central axis of chamber  1702  (e.g., perpendicular to the plane of  FIG. 5 ). The inclined mirror  1712  is configured to direct light  1714  emitted by the glowing tip to measurement device  1710 , as shown in  FIG. 6 . Light captured by measurement device  1710  can be used to monitor the tip during fabrication. For example, in certain embodiments, measurement device  1710  can be a camera, and photons produced by the glowing tip during sharpening can be detected by the camera to form an image of the glowing tip. By monitoring the color of the tip, for example, the temperature of the tip can be estimated. 
     In some embodiments, positioner  1700  can include an angled mirror  1716  as discussed above, and a scintillator material  1718 . The scintillator material can be positioned to convert ions produced during field evaporation of the tip to photons. The photons are then directed by angled mirror  1716  to be incident on a camera or other measurement device (e.g., measurement device  1710 ). 
     In certain embodiments, positioner  1700  can include a device to measure ion current such as a Faraday cup  1720 . The Faraday cup  1720  can be moved into the beam path to capture ions from the tip during field evaporation of the tip. The ion current due to the captured ions can then be measured, and the information used to assess the progress of tip building. 
     In some embodiments, positioner  1700  can include one or more apertures  1722 . Aperture  1722  provides spatial filtering for the ion beam generated from the tip, and can be used to produce an ion beam with particular properties. When multiple apertures are present on positioner  1700 , the desired ion beam properties can be selected by selecting a particular aperture. 
     In certain embodiments, chamber  1702  does not include a positioner  1700 . Instead, chamber  1702  includes an angled mirror  1724  mounted in an off-axis position within chamber  1702  to a fixed mount  1726 , as shown in  FIG. 7 . Mirror  1724  is positioned to direct oblique light rays  1728  emerging from the tip to measurement device  1710  for observation. The position of angled mirror  1724  is selected so that the mirror does not interfere with the ion beam when the ion beam system is in use. 
     In some embodiments, chamber  1702  includes an angled mirror  1730  mounted (via a fixed mount  1732 ) in the path of the ion beam, as shown in  FIG. 8 . Angled mirror  1730  includes a central aperture  1734  that permits the ion beam to pass through the mirror. However, the portions of the mirror surface surrounding aperture  1734  are positioned to direct optical radiation from the glowing tip to measurement device  1710  for observation. 
     Embodiments of positioner  1700  (and also fixed mounts  1726  and/or  1732 ) can also include a variety of other elements to perform various beam filtering and tip observation functions. For example, positioner  1700  (and mounts  1726  and/or  1732 ) can include optical filters, adjustable apertures, phosphor-based devices, materials for frequency conversion of optical radiation, various types of electronic measurement devices (e.g., cameras, line sensors, photodiodes, bolometers), and, in general, any type of device that can be mounted on positioner  1700  (and/or mounts  1726  and  1732 ) and which is suitable for use in the environment of chamber  1702 . 
     Other Embodiments 
     As an example, while examples 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). 
     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. 
     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, US 2007-0158558. 
     As an additional example, while embodiments have been disclosed in which a sample is inspected, alternatively or additionally, the systems and methods 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 to 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 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, 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 US 2007-0158558. 
     Combinations of features can be used in various embodiments. 
     Other embodiments are covered by the claims.