Patent Publication Number: US-11024481-B2

Title: Scanning electron microscope

Description:
The present application is a divisional application of U.S. application Ser. No. 16/085,727, filed Sep. 17, 2018, which was a National Phase of PCT Application No. NL2017/050128, filed Mar. 3, 2017, which claimed the priority benefit of Netherlands Patent Application Serial No. 2016367, filed Mar. 4, 2016. The disclosures of the foregoing applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the field of scanning electron microscopes. 
     BACKGROUND TO THE INVENTION 
     Scanning electron microscopes are known per se. The scanning electron microscope includes an electron optical imaging system. The electron optical imaging system can include an electron beam source and an electron detector. The electron beam source provides an electron beam which is focused onto a sample. The impinging electron beam causes secondary electrons to be emitted from the surface of the sample. Also some electrons from the electron beam are scattered in forward direction and backscattered. The electron detector detects the secondary, scattered and/or backscattered electrons and generates a signal representative of the amount of electrons detected. The electron beam is moved relative to the sample so as to scan the surface of the sample with the electron beam. The variation in the electron detector signal per location of the electron beam on the sample provides image information of the sample. 
     SUMMARY OF THE INVENTION 
     The invention relates to a scanning electron microscope. 
     It is an objective of the invention to provide an improved scanning electron microscope. It is also an objective of the invention to at least provide a useful alternative for known scanning electron microscopes. 
     According to a first aspect of the invention is provided a scanning electron microscope including an electron optical imaging system and a sample carrier, wherein the sample carrier is movable between a loading position for loading a sample and an imaging position for imaging the sample. The scanning electron microscope includes a sliding vacuum seal between the electron optical imaging system and the sample carrier. The sliding vacuum seal includes a first plate having a first aperture associated with the electron optical imaging system and resting against a second plate having a second aperture associated with the sample carrier. The first and second plates are slideably movable with respect to each other. The first and second apertures overlap in the imaging position. The first and second apertures do not overlap in the loading position. The first plate and/or the second plate includes a groove circumscribing the first and/or second aperture, wherein the groove is arranged for being in communication with a vacuum system. 
     It is noted that a scanning electron microscope having a sliding vacuum seal with a first plate having a first aperture associated with the electron optical imaging system and resting against a second plate having a second aperture associated with the sample carrier, the first and second plates being slideably movable with respect to each other, is known from US2011/133083A1. In the sliding vacuum seal of US2011/133083A1, however, the first plate and/or the second plate does not include a groove circumscribing the first and/or second aperture, wherein the groove is arranged for being in communication with a vacuum system. 
     The first plate and/or the second plate including a groove circumscribing the first and/or second aperture, wherein the groove is arranged for being in communication with a vacuum system, provides the advantage that a vacuum can be applied to the groove, hence, efficiently providing a suction force between the first and second plate. The suction force allows for a firm and/or mechanically stiff abutment of the first and second plates. Especially when the sample carrier is rigidly connected to the second plate, and the electron optical imaging system is rigidly connected to the first plate, the mechanically stiff abutment allows for rigid positioning of the sample with respect to the electron optical imaging system. Rigid positioning of the sample allows for high image quality. 
     Optionally, the vacuum system is arranged for having the electron optical imaging system and the sample carrier at a first vacuum level while imaging. The first vacuum level is preferably chosen such as to allow an electron beam of the electron optical imaging system to irradiate the sample, as is known in the art. 
     Optionally, the vacuum system is arranged for having the groove at a second vacuum level, different from the first vacuum level. Preferably the second vacuum level is between the first vacuum level and ambient pressure. Hence, the second vacuum level aids in maintaining a good vacuum seal between the first plate and the second plate. Moreover, having the groove at the second vacuum level between the first vacuum level and ambient pressure improves (e.g. allows to lower) the first vacuum level. 
     Optionally, the groove is arranged to circumscribe both the first and second apertures both in the loading position and in the imaging position. This provides the advantage that while moving the sample carrier from the loading position to the imaging position, the groove is not exposed to the first or second aperture. Hence, contamination of the groove can be avoided since it is not exposed. Further, it is possible to maintain the vacuum level inside the groove while moving the sample carrier from the loading position to the imaging position. The groove circumscribing the second aperture in the loading position, and/or at least a position not being the imaging position, also provides the advantage that the groove can be used as a pre-pump stage for the sample carrier. Providing a vacuum to the groove allows to pump the sample carrier from ambient pressure to (close to) the second vacuum level. It is noted that exposing the imaging system directly to a sample carrier at atmospheric pressure will damage the imaging system, while exposing the groove to a sample carrier at atmospheric pressure will not. After the sample carrier has been pre-pumped by the groove it will not damage the imaging system while moving to the imaging region. 
     According to a second aspect of the invention is provided a scanning electron microscope including an electron optical imaging system including an electron beam source and an electron detector, and a sample carrier between the electron beam source and the electron detector. The sample carrier is movable relative to the electron beam for moving a sample. The sample carrier can be movable e.g. in a plane orthogonal to the electron beam. The sample carrier can also be arranged such that a sample carried by the sample carrier is movable in a direction parallel to the electron beam. The electron detector is movable relative to the electron beam. The electron detector can be movable in a direction non-parallel to the electron beam, e.g. orthogonal to the electron beam. 
     The electron detector being movable relative to the electron beam provides the advantage that the scanning electron microscope is rendered more flexible in modes of detecting secondary and/or (back) scattered electrons. 
     Optionally, the electron detector is arranged for moving in synchronism with the sample, e.g. with the sample carrier. The electron detector can be arranged to move in the same direction as the sample carrier, e.g. simultaneously with the sample carrier. The electron detector can be arranged to move over the same distance as the sample carrier, e.g. simultaneously with the sample carrier. This provides the advantage that when the sample carrier is moved, for having the electron beam impinge on a different spot on the sample, the electron detector is also moved. The electron detector can be fixedly connected to the movable sample carrier. 
     Optionally, the scanning electron microscope includes a magnetic lens between the sample and the electron detector. Optionally, the scanning electron microscope includes a magnetic lens between the sample carrier and the electron detector. The magnetic lens can e.g. be electromagnetically driven and/or can include permanent magnets. The magnetic lens can be movable relative to the electron beam. The magnetic lens can be movable in a direction non-parallel to the electron beam, e.g. orthogonal to the electron beam. 
     Optionally, the magnetic lens is arranged for moving in synchronism with the sample, e.g. with the sample carrier. The magnetic lens can be arranged to move in the same direction as the sample carrier, e.g. simultaneously with the sample carrier. The magnetic lens can be arranged to move over the same distance as the sample carrier, e.g. simultaneously with the sample carrier. This provides the advantage that when the sample carrier is moved, for having the electron beam impinge on a different spot on the sample, the magnetic lens is also moved. The magnetic lens can be fixedly connected to the movable sample carrier. 
     Optionally, the electron detector is positioned in the focal plane of the magnetic lens. Optionally the electron detector is positioned in the focal point of the magnetic lens. Thus, when both the magnetic lens and the electron detector are moved in synchronism with the sample, the electron beam travelling through the sample is deflected by the magnetic lens towards the electron detector. It is noted that this allows for simple constructions, e.g. the magnetic lens and the electron detector being rigidly connected to the sample carrier, in which the position of the electron detector with respect to the sample is well defined. In case the sample is movable relative to an electron detector that is stationary relatively to the electron beam impinging on the sample, as in prior art scanning electron microscopes, it is harder to maintain mechanical stability and accuracy of the position of the inspected part of the sample relative to the electron detector. 
     Optionally, a bright-field area of the electron detector is positioned in the focal plane of the magnetic lens. Optionally, the bright-field area of the electron detector is positioned in the focal point of the magnetic lens. This provides the advantage that the primary electrons of the electron beam that are not disturbed by the sample are deflected to the bright-field area of the electron detector. Scattered electrons can then be deflected to the edges of the electron detector surrounding the bright-field area. 
     According to a third aspect of the invention is provided a scanning electron microscope including a vacuum chamber including internal structures, such as an electron optical system, and a sample carrier for carrying a sample, movable within the vacuum chamber. The scanning electron microscope includes a motion control unit including an input unit arranged for receiving user commands relating to movement of the sample carrier. The motion control unit includes a memory storing a three-dimensional model of the internal structures of the vacuum chamber. The three-dimensional model of the internal structures of the vacuum chamber can include a three-dimensional mode of the geometry of the internal structures of the vacuum chamber. The memory stores a three-dimensional model of the sample carrier. The three-dimensional model of the sample carrier can include a three-dimensional model of the geometry of the sample carrier. The motion control unit is arranged for moving the sample carrier on the basis of the received user commands and the three-dimensional models of the internal structures and the sample carrier while avoiding collision of the sample carrier and the internal structures. 
     Optionally, the memory further stores a three-dimensional model of the geometry of the sample carried on the sample carrier. The three-dimensional model of the sample can be a three-dimensional model of the geometry of the sample. The motion control unit can then be arranged for moving the sample carrier further on the basis of the three-dimensional model of the sample while avoiding collision of the sample, sample carrier and the internal structures. 
     Optionally, the motion control unit is arranged for determining a collision free path from the current location of the sample carrier to a target location of the sample carrier. Sample-based planning algorithms such as probabilistic roadmaps (PRMs) or rapidly-exploring random trees (RRTs), or rapidly-exploring dense trees, can be used to compute collision-free paths. However, other algorithms can be used. Optionally the collision-free path can be smoothed, e.g. to reduce jerk. 
     Optionally, the motion control unit is arranged for calculating a minimum distance between the three-dimensional models of on the one hand the sample carrier and optionally the sample, and on the other and the internal structures. The calculated minimum distance can be used for determining a collision free path. 
     Instead of avoiding collision of the sample carrier and the internal structures, or in addition thereto, the motion control unit can be arranged for moving the sample carrier on the basis of the received user commands and the three-dimensional models of the internal structures and the sample carrier and optionally the sample while maintaining distance between the sample carrier and internal structures, said distance being larger than or equal to a predetermined minimum distance. This allows for maintaining a predetermined safety distance. The predetermined minimum distance can be of special use when the internal structures are at a voltage different from the sample carrier, e.g. at a high (positive or negative) voltage. For example, the calculated minimum distance can be used for determining a path that maintains a distance of at least the predetermined minimum distance from internal structures at a voltage different from the sample carrier, e.g. at a high (positive or negative) voltage. Hence, electric discharge can be avoided. 
     Optionally, the motion control unit takes into account a first predetermined minimum distance and a different second predetermined minimum distance. The first minimum distance being maintained relative to internal structures of a first type and the second predetermined minimum distance being maintained relative to internal structures of a second type. The internal structures of the first type may e.g. be at a voltage different from the sample carrier (or differ by more than a threshold voltage), while the internal structures of the second type are at a substantially the same voltage as the sample carrier. The first predetermined minimum distance may then be larger than the second predetermined minimum distance. 
     Optionally, the motion control unit is arranged for simulating execution of a user command prior to executing the user command, wherein the motion control unit is arranged for ignoring the user command if the simulation indicates a collision of the sample carrier and/or the sample with the internal structures. Hence, any user command that would result in a collision can be ignored so as to avoid the collision. It will be appreciated that the motion control unit can be arranged for generating a message to the user that the user command will not be performed. The message can e.g. be displayed on a display device of the scanning electron microscope. Optionally, the motion control unit can determine an alternative route, alternative to the route according to the user command, resulting in positioning of the sample carrier and sample at the final position according to the user command, the alternative route being free of collision. The motion control unit can be arranged for suggesting the alternative route to the user, e.g. by displaying a message on the display device. The motion control unit can also be arranged for automatically substituting the alternative route for the route according to the user command. 
     Optionally, the input unit is arranged for receiving data representative of a geometry of the sample. It is for instance possible that the input unit is arranged for receiving measurements of the sample geometry. The motion control unit can be arranged for determining the three-dimensional model of the sample on the basis of the data representative of the geometry of the sample. The scanning electron microscope can include one or more templates for determining an approximate geometry of the sample. The templates can for example have a cavity of progressively increasing dimensions. The template with the smallest cavity that fits over the sample can be taken to be representative for the geometry of the sample. The cavity can e.g. be cylindrical or hemispherical, although other shapes can be envisaged. 
     Optionally, the scanning electron includes an optical camera and a geometry determination unit arranged for determining a geometry of the sample on the basis of at least one image of the sample provided by the optical camera. The optical camera can be arranged for providing a top plan image of the sample. The top plan image can be used for determining a perimeter of the sample. The geometry determination unit can be arranged for determining an outline of the sample by obtaining a first image and a second image wherein in the second image the sample is rotated over a first angle relative to its position in the first image, and determining pixels that are different in the first and second images. The differing pixels are representative for the perimeter of the sample. Alternatively, or additionally, the optical camera is arranged for providing a side view image of the sample. The geometry determination unit can be arranged for determining the three-dimensional model of the sample on the basis of two or more two-dimensional images of the sample. Optionally, the sample carrier includes one or more reference features, such as markers, appearing in the two-dimensional images for three-dimensional reconstruction of the model of the sample. 
     The invention also relates to a method for loading a sample into a vacuum chamber of a scanning electron microscope. The method includes providing a sample carrier that is movable between a loading position for loading the sample and an imaging position for imaging the sample. The scanning electron microscope includes a sliding vacuum seal between the electron optical imaging system and the sample carrier. The sliding vacuum seal includes a first plate having a first aperture associated with the electron optical imaging system and resting against a second plate having a second aperture associated with the sample carrier. The first and second plates are slideably movable with respect to each other. The first and second apertures overlap in the imaging position. The first and second apertures do not overlap in the loading position. The first plate and/or the second plate includes a groove circumscribing the first and/or second aperture. The groove is arranged for being in communication with a vacuum system. The method includes placing the sample in the sample carrier when the sample carrier is in the loading position. The method can also include applying a vacuum to the groove. 
     The invention also relates to a method for obtaining an electron microscope image of a sample including irradiating the sample with an electron beam from a first side, and using an electron detector detecting electrons transmitted through the sample at an opposite second side. The method includes moving the sample through the electron beam and moving the electron detector in synchronism with the sample. 
     Optionally, the method includes providing a magnetic lens between the sample and the electron detector and moving the magnetic lens in synchronism with the sample and the detector. 
     The invention also relates to a method for moving a sample within a vacuum chamber of a scanning electron microscope. The vacuum chamber includes internal structures, such as an electron optical imaging system. The method includes positioning the sample on a sample carrier that is movable within the vacuum chamber. The method includes inputting a user command relating to a desired movement of the sample into a motion control unit. The motion control unit includes a memory storing a three-dimensional model of the internal structures of the vacuum chamber and a three-dimensional model of the sample carrier. The method includes moving, by the motion control unit, the sample carrier on the basis of the received user commands and the three-dimensional models of the internal structures and the sample carrier while avoiding collision of the sample carrier and the internal structures. 
     Optionally, the memory further stores a three-dimensional model of the sample carried on the sample carrier. The method can include moving, by the motion control unit, the sample carrier further on the basis of the three-dimensional model of the sample while avoiding collision of the sample, sample carrier and the internal structures. 
     It will be appreciated that any of the aspects of the invention can be combined. It will also be clear that all features and options mentioned in view of the scanning electron microscope apply equally to the method. 
     It will be appreciated that any one or more of the above options can be combined. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which: 
         FIG. 1A  shows an example of a scanning electron microscope with a sample carrier in a first position; 
         FIG. 1B  shows an example of a scanning electron microscope with a sample carrier in a second position; 
         FIG. 1C  shows an example of a scanning electron microscope with a sample carrier in a third position; 
         FIG. 2A  shows an example of a scanning electron microscope with a sample carrier in a first position; 
         FIG. 2B  shows an example of a scanning electron microscope with a sample carrier in a second position; 
         FIG. 2C  shows an example of a scanning electron microscope with a sample carrier in a third position; 
         FIG. 3A  shows an example of a scanning electron microscope with a sample carrier in a first position; 
         FIG. 3B  shows an example of a scanning electron microscope with a sample carrier in a second position; 
         FIG. 4A  shows an example of a scanning electron microscope; 
         FIG. 4B  shows an example of a scanning electron microscope; and 
         FIG. 4C  shows an example of a scanning electron microscope. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A, 1B and 1C  shows an example of a scanning electron microscope  1 . The scanning electron microscope  1  includes an electron optical imaging system  2 . In this example, the electron optical imaging system  2  includes an electron beam source  4  and a magnetic lens  6 . The electron optical imaging system  2  includes an electron detector  8 . The scanning electron microscope  1  includes a sample carrier  10 . The sample carrier  10  is arranged for carrying a sample  12 . The electron beam source  4  is arranged for generating an electron beam  14 . The magnetic lens  6  focuses the electron beam  14  so as to impinge on the sample  12 . Part of the electron beam  14  may pass the sample  12  undisturbed. Another part of the electron beam  14  has electrons scattered by the sample  12 . The scattered electrons, and optionally the undisturbed electron beam  14 , are detected by the electron detector  8 . The sample carrier  10  is movable relative to the electron beam  14  for moving the sample  12  relative to the electron beam  14 . 
     In the example of  FIGS. 1A, 1B and 1C  the sample carrier  10  is movable between a loading position  16  for loading a sample and an imaging position  18  for imaging the sample. The scanning electron microscope  1  includes a sliding vacuum seal  20  between the electron optical imaging system  2  and the sample carrier  10 . In  FIGS. 1A, 1B and 1C  the sliding vacuum seal  20  includes a first plate  22  having a first aperture  24  associated with the electron optical imaging system  2 . In  FIG. 1  the sliding vacuum seal  20  includes a second plate  26  having a second aperture  28  associated with the sample carrier  10 . The first plate  22  rests against the second plate  26 . The second plate  26  is slideably movable with respect to the first plate  22 . The contacting surfaces of the first and second plates  22 ,  26  are sufficiently smooth to act as vacuum seal. 
     When the sample carrier  10  is positioned in the imaging position  18  the first and second apertures  24 ,  28  overlap as shown in  FIG. 1C . Hence the electron beam  14  can pass from the electron beam source  4  through the first and second apertures  24 ,  28  and impinge upon the sample  12 . When the sample carrier  10  is positioned in the loading position  16  the first and second apertures  24 ,  28  do not overlap as shown in  FIG. 1A . Hence the electron beam source  4  and a magnetic lens  6  are sealed from ambient air by the sliding vacuum seal  20 . Thus, the electron beam source  4  and a magnetic lens  6  can be maintained at vacuum conditions. Thereto the scanning electron microscope  1  includes a first connector  30  connecting an inner space  32  of the electron optical system  2  to a vacuum system  34 . The sample carrier  10  can in the loading position  16  be opened to ambient air for loading and/or unloading a sample  12  onto the sample carrier  10 . It will be appreciated that after loading a sample  12  the sample carrier  10  can be closed and pumped to vacuum conditions as well. Thereto the scanning electron microscope  1  includes a second connector  36  connecting an inner space  38  of the sample carrier  10  to the vacuum system  34  as will be described below. 
     In the example of  FIGS. 1A, 1B and 1C  the first plate  22  includes a circumferential groove  40  circumscribing the first aperture  24 . The groove  40  is in fluid communication with a third connector  42  connecting to the vacuum system  34 . As can be seen in  FIGS. 1A, 1B and 1C  the groove  40  circumscribes both the first and second apertures  24 ,  28  both in the loading position  16  and in the imaging position  18 . The groove  40  circumscribing both the first and second apertures  24 ,  28  both in the loading position and in the imaging position provides the advantage that the apertures  24 ,  28  do not cross the groove  40  during movement hence avoiding the risk of mechanical damage to the groove or the apertures. It will be appreciated that it is possible that the groove  40  circumscribes both the first and second apertures at least in the imaging position  18 . The groove may circumscribe the first aperture  24 , but not the second aperture  28 , in the loading position. 
     It will be appreciated that alternatively, or additionally, the second plate  26  may include a circumferential groove circumscribing the second aperture  28 . This groove too can be in fluid communication with a connector connecting to the vacuum system  34 . This groove too can circumscribe both the first and second apertures  24 ,  28  both in the loading position  16  and in the imaging position  18 . 
     The scanning electron microscope  1  described with respect to  FIGS. 1A, 1B and 1C  can be operated as follows. 
     The scanning electron microscope  1  is brought into a condition ready for operation. The sample carrier  10  is positioned in the loading position  16  for receiving a sample  12  as shown in  FIG. 1A . The inner space  32  of the electron optical system  2  is brought at a predetermined first vacuum level by the vacuum system  34 . The groove  40  is brought at a predetermined second vacuum level by the vacuum system  34 . In this example, the second vacuum level is chosen to be different from the first vacuum level. Here the second vacuum level is chosen to be between the first vacuum level and ambient pressure. 
     With the sample carrier  10  in the loading position  16 , a sample  12  is positioned in the sample carrier  10 . The sample carrier  10  is closed from ambient air. In this example, the sample carrier  10  is closed by sliding the second plate  26  to an intermediate position  44  as shown in  FIG. 1B . In the intermediate position  44  the sample carrier  10  is closed off by the first plate  22 . In this example, in the intermediate position the inner spaced  38  of the sample carrier  10  is evacuated via the second connector  36 . The inner space  38  of the sample carrier  10  in this example is brought at a vacuum level that is, at least approximately, equal to the first vacuum level in the inner space  32  of the electron optical system  2 . 
     From the intermediate position  44  the sample carrier is moved to the imaging position  18  as shown in  FIG. 1C . In the imaging position the first aperture  24  and the second aperture  28  overlap. As a result, the inner space  38  of the sample carrier  10  is in communication with the inner space  32  of the electron optical system  2 . The electron beam source  4  and magnetic lens  6  are activated to generate the electron beam  14 . The electron beam impinges on the sample  12 . Both undisturbed electrons of the electron beam  14  and forward scattered electrons are detected by the electron detector  8 . For providing an image of (a part of) the sample  12 , the electron beam  14  is scanned across the surface of the sample  12 . The scanned region of the sample  12  can be increased by tiling, i.e. scanning multiple portions of the sample  12  at different locations of the sample  12  relative to the electron beam  14 . In this example the sample  12  is scanned in a plane orthogonal to the electron beam  14  by moving the sample carrier, and the second plate  26 , along the first plate  22 . The sample  12  can be scanned in two orthogonal directions. 
     The groove  40  being maintained at a second vacuum level provides a suction force clamping the first and second plates  22 ,  26  together. Hence a rigid connection can be obtained of the sample carrier  10  with respect to the electron optical imaging system  2 . This provides a good spatial stability of the sample  12  with respect to the electron beam  14 . It is noted that the suction force may cause stick-slip effects when moving the second plate  26  relative to the first plate  22 . In an embodiment the sample carrier  10  is controlled to always approach a target location from the same direction, so as to avoid hysteresis effects in positioning. For example when moving in a plane orthogonal to the electron beam  14  in two orthogonal directions, X and Y, the sample carrier  10  may be controlled to always approach a target location in the positive X direction and in the positive Y direction. 
       FIGS. 2A, 2B and 2C  show an example of a scanning electron microscope  1  similar to the example shown in  FIGS. 1A, 1B and 1C . Like reference numerals refer to like features. In the example of  FIGS. 2A, 2B and 2C , the groove  40  extends around the first aperture  24 . In this example the groove  40  extends around the first aperture  24  and second aperture  28  in the imaging position  18 . In this example the groove  40  extends around the first aperture  24  but not around the second aperture  28  in the loading position  16 . 
     It is noted that the second connector  36  is not necessary in this example. Instead, in the intermediate position  44 , as shown in  FIG. 2B , the inner space  38  of the sample carrier  10  can be evacuated via the third connector  42 . 
       FIGS. 3A and 3B  show an example of a scanning electron microscope  1 . Similarly as described with respect to  FIGS. 1A, 1B, 1C, 2A, 2B and 2C  the scanning electron microscope  1  includes an electron optical imaging system  2 . In this example, the electron optical imaging system  2  includes an electron beam source  4  and a magnetic lens  6 . The electron optical imaging system  2  includes an electron detector  8 . The scanning electron microscope  1  includes a sample carrier  10 . The sample carrier  10  is arranged for carrying a sample  12 . The electron beam source  4  is arranged for generating an electron beam  14 . The magnetic lens  6  focuses the electron beam  14  so as to impinge on the sample  12 . Part of the electron beam  14  may pass the sample  12  undisturbed. Electrons of another part of the electron beam  14  are scattered by the sample  12 . The forward scattered electrons, and optionally the undisturbed electron beam  14 , are detected by the electron detector  8 . The sample carrier  10  is movable relative to the electron beam  14  for moving the sample  12  relative to the electron beam  14 . 
     In the example of  FIGS. 3A and 3B  the scanning electron microscope  1  includes a second magnetic lens  46 . The second magnetic lens  46  is positioned between the sample  12  and the electron detector  8 . The electron detector  8  is positioned in the focal plane of the second magnetic lens  46 . In this example the second magnetic lens  46  is rigidly connected to the sample carrier  10 . Hence, the position of the second magnetic lens  46  with respect to the sample  12  is well defined. In this example the electron detector  8  is rigidly connected to the sample carrier  10 . Hence, the position of the electron detector  8  with respect to the sample  12  is well defined. 
     The sample carrier  10  is movable relative to the electron beam  14 . In this example, the sample carrier is movable in a plane orthogonal to the electron beam. The sample carrier  10  can e.g. be the movable part of a sample stage. The sample carrier can also be movable as explained with respect to  FIGS. 1A, 1B and 1C  or  FIGS. 2A, 2B and 2C . Since the second magnetic lens  46  is rigidly connected to the sample carrier, the second magnetic lens  46  moves in synchronism with the sample carrier  10 . Since the electron detector  8  is rigidly connected to the sample carrier, the electron detector moves in synchronism with the sample carrier  10 . Since the electron detector  8  is positioned in the focal point of the second magnetic lens  46 , the electron beam  14 ′ travelling through the sample  12  will be deviated by the second magnetic lens  46  towards the focal point, i.e. towards the electron detector  8 . This can for instance be seen in  FIG. 2B . 
     In the example of  FIGS. 3A and 3B  the electron detector  8  includes a central bright-field area  50  and a dark-field area  52  surrounding the bright-field area. The bright-field area  50  of the electron detector  8  is positioned in the focal plane of the second magnetic lens  46 . Hence the undisturbed electron beam  14 ′ is deviated towards the bright-field area  50 . The forward scattered electrons are deviated towards the dark-field area  52 . 
       FIGS. 4A, 4B and 4C  show an example of a scanning electron microscope  1 . Similarly as described with respect to  FIGS. 1A, 1B, 1C, 2A, 2B, 2C, 3A and 3B  the scanning electron microscope  1  includes an electron optical imaging system  2 . In this example, the electron optical imaging system  2  includes an electron beam source  4  and a magnetic lens  6 . The electron optical imaging system  2  includes an electron detector  8 . The scanning electron microscope  1  includes a sample carrier  10 . The sample carrier  10  is arranged for carrying a sample  12 . The electron beam source  4  is arranged for generating an electron beam  14 . The magnetic lens  6  focusses the electron beam  14  so as to impinge on the sample  12 . Part of the electron beam  14  may pass the sample  12  undisturbed. Electrons of another part of the electron beam  14  is scattered by the sample  12 . The forward scattered electrons, and optionally the undisturbed electron beam  14 , are detected by the electron detector  8 . However, also other detectors may be present, e.g. a backscattered electron detector (BSD), a secondary electron detector (SE), an energy dispersive X-ray detector (EDX), a detector for sample current, a cathodoluminescence detector (CL), an Auger detector, an optical camera, etc. The sample carrier  10  is movable relative to the electron beam  14  for moving the sample  12  relative to the electron beam  14 . 
     The scanning electron microscope  1  includes a vacuum chamber  54 . The vacuum chamber  54  includes internal structures, such as the electron optical imaging system  2 . The sample carrier  10  is movable within the vacuum chamber  54 . A movement system  55  is provided for moving the sample carrier  10 . The movement system  55  may include guides and actuators as is known in the art. The movement system may be arranged for moving the sample carrier along one or more axes and/or for rotating the sample carrier about one or more axes. The movement system may be arranged for moving the sample carrier in six degrees of freedom. The movement system  55  may be arranged for moving the sample carrier in two directions in a plane orthogonal to the electron beam  14 . The movement system  55  may be arranged for moving the sample carrier  10  in a direction parallel to the electron beam. The movement system may be arranged for tilting the sample carrier in one or two directions relative to the electron beam. The movement system may be arranged for rotating the sample about an axis that is parallel to the electron beam. 
     The scanning electron microscope includes a motion control unit  56 . The motion control unit  56  is arranged for controlling the movement system  55  for moving the sample carrier  10 . The motion control unit  56  includes an input unit  58  arranged for receiving user commands relating to movement of the sample carrier  10 . The input unit  58  can include a keyboard, a pointing device, a touchscreen, a voice activated unit or the like. The motion control unit  56  includes a memory  60 . In this example, the memory  60  stores a three-dimensional model of the internal structures of the vacuum chamber  54 . The three-dimensional model of the internal structures can include a three dimensional model of a geometry of the internal structures of the vacuum chamber  54 . The three-dimensional model of the internal structures can also include a three dimensional model of inner walls of the vacuum chamber  54 . In this example the memory  60  further stores a three-dimensional model of the sample carrier  10 . The three-dimensional model of the sample carrier can include a three dimensional model of a geometry of the sample carrier  10 . 
     The scanning electron microscope  1  as described with respect to  FIGS. 4A, 4B and 4C  can be operated as follows. 
     The sample carrier  10  carrying the sample  12  is positioned inside the vacuum chamber  54  at a first position. The electron beam  14  can impinge on the sample  12 . The user can input a user command related to a desired movement of the sample carrier  10  into the input unit  58 . In this example, the user command relates to an absolute position, e.g. coordinates, of a desired second location of the sample carrier  10 . It is also possible that the user command relates to a relative position of a desired second location of the sample carrier  10  e.g. a desired displacement—e.g. 0.125 mm in a predetermined direction—of the sample carrier  10 . Based on the current location of the sample carrier  10  at the first position and the desired location of the sample carrier  10  at the second location the motion control unit  56  calculates a path to be followed by the sample carrier from the first position to the second position. In calculating the path the motion control unit  56  uses the three-dimensional model of the internal structures and the three-dimensional model of the sample carrier  10  for calculating a path that is free of collision of the sample carrier  10  and the internal structures. In this example, the motion control unit  56  uses a sample-based planning. The sample-based planning algorithm can e.g. be a probabilistic roadmap (PRM), or rapidly-exploring random tree (RRT), or rapidly-exploring dense tree. However, other algorithms can be used. Optionally the collision-free path can be smoothed, e.g. to reduce jerk of the sample carrier  10 . Once the collision free path has been determined, the motion control unit  56  controls the movement system  55  for moving the sample carrier  10  along the collision free path. 
     It is noted that in determining a possible path a collision can be determined if on at least one position along the path the three-dimensional model of the sample carrier touches or intersects the three-dimensional model of the internal structures. It will be appreciated that also a safety margin may be used, wherein collision is taken as a distance less than a predetermined safety distance. In that case the motion control unit determines the path to include a collision if on at least one position along the path the minimum distance between the three-dimensional model of the sample carrier  10  and the three-dimensional model of the internal structures is less than the predetermined safety distance. 
     It will be appreciated that the movement system  55  be embodied as a sample stage. Alternatively, the sample carrier  10  can be a movable part of a sample stage. Various parts of the sample stage can move at different rates or strokes when the sample carrier  10  is moved. It will be appreciated that the memory can also store a three-dimensional model of the sample stage. In calculating the path the motion control unit  56  can also use the three-dimensional model of the sample stage for calculating a path that is free of collision of the sample stage and the internal structures. More in general, in calculating the path the motion control unit  56  can also use a three-dimensional model of the movement system for calculating a path that is free of collision of the sample carrier, the movement system, and the internal structures. 
     In a mode the memory  60  further stores a three-dimensional model of the sample  12  carried on the sample carrier  10 , The three-dimensional model of the sample can include a three dimensional model of a geometry of the sample  12 . The motion control unit  56  can use the three-dimensional model of the sample  12  in calculating path to be followed by the sample carrier  10  from the first position to the second position so as to avoid collision of the sample  12 , sample carrier  10 , optionally the movement system  55 , and the internal structures. 
     The motion control unit  56  controls the sample carrier  10  to move from the first position to the second position along the calculated path that is free of collision. 
     The motion control unit  56  in this example is arranged for simulating execution of a user command prior to executing the user command. The simulation includes calculating a path from the current position of the sample carrier  10  to an updated position according to the user command. If the path calculated in the simulation indicates a collision between the sample carrier  10  (and optionally the sample  12  and/or the movement system  55 ) on the one hand and the internal structures on the other hand, the motion control unit refrains from performing the user command. Thus, the motion control  56  unit is arranged for ignoring the user command if the simulation indicates a collision. This prevents collisions or collision hazards in case the safety margin is observed. The motion control unit  56  can be arranged for generating a message to the user that the user command cannot, should not, or will not be performed. The message can e.g. be displayed on a display device  59  of the scanning electron microscope. The motion control unit  56  can be arranged for determining an alternative route, alternative to a route according to the user command, resulting in positioning of the sample carrier  10  at the final position according to the user command, the alternative route being free of collision. The motion control unit  56  can be arranged for suggesting the alternative route to the user, e.g. by displaying a message on the display device  59 . The motion control unit  56  can also be arranged for automatically substituting the alternative route for the route according to the user command. The motion control unit  56  may generate a message indicating use of an alternate route. The motion control unit  56  may be arranged for indicating the alternative route at the display device  59 , e.g. in overlay with a microscope image. 
     In the example of  FIGS. 4A, 4B and 4C  the input unit  58  is arranged for receiving data representative of a geometry of the sample  12 . It is possible that the geometry of the sample is known. Then the data can include a three-dimensional model of the geometry of the sample. Alternatively, the data representative of the geometry of the sample can be indicative of the geometry of the sample. 
     In the example of  FIG. 4A  the scanning electron microscope  1  includes a plurality of templates  62  for determining an approximate geometry of the sample  12 . In this example each template includes a cavity of predetermined dimensions. Here the cavities are cylinders of different diameter and/or height. However, alternate shapes can be used, such as hemispheres, lozenges, cubes, cones, or the like. The user can fit one or more of the templates  62  over the sample  12  on the sample carrier  10  to select the template  62  having the smallest cavity fitting over the sample  12 . An indication of the selected template  62 , e.g. a code or number, can be provided to the input unit  58 . From this, the scanning electron microscope  1  can determine an approximated geometry of the sample  12 . 
     In the example of  FIG. 4B  the scanning electron microscope  1  includes an optical camera  64  and a geometry determination unit  66 . The geometry determining unit  66  is arranged for determining a geometry of the sample  12  on the basis of at least one image of the sample  12  provided by the optical camera  64 . The optical camera  64  is positioned for providing a top plan image of the sample  12 . 
     Here the geometry determination unit  66  determines a perimeter of the sample  12 . The optical camera  64  obtains a first image of the sample  12  in top plan view. Next, the sample is rotated over a predetermined angle about an axis substantially parallel to the optical axis of the camera  64 . Then the optical camera  64  obtains a second image of the sample  12  in top plan view. The geometry determination unit  66  compares the first image with the second image. In this example the geometry determination unit compares an intensity of each pixel in the first image with an intensity of the corresponding pixel in the second image. Pixels for which the difference of the intensity in the first and second images exceeds a predetermined threshold level are determined to be indicative of the outline of the sample  12 . The geometry determination unit  66  determines an approximate geometry of the sample on the basis of the determined outline. It will be appreciated that a height of the sample may be determined as a predetermined height, or as a function of a dimension of the outline, e.g. a predetermined percentage of a maximum diameter of the top plan outline of the sample. 
     In the example of  FIG. 4C  an optical camera  68  is arranged for providing a side view image of the sample  12 . The optical camera  68  obtains a first image of the sample  12  in side view. Next, the sample is rotated over a predetermined angle about an axis substantially orthogonal to the optical axis of the camera  68 . Then the optical camera  68  obtains a second image of the sample  12  in side view. The geometry determination unit  66  determines the three-dimensional model of the sample  12  on the basis of the first image and the second image. Optionally, the optical camera  68  may obtain further images of the sample at different rotational positions for the geometry determination unit  66  to use. The geometry determination unit  66  can use techniques for reconstructing a three-dimensional model on the basis of two or more two-dimensional images known in the art. In the example of  FIG. 4C  the sample carrier  10  includes markers  70 . The markers  70  are positioned to be in the field of view of the optical camera  68 . Hence, the markers  70  appear in the two-dimensional images obtained by the camera  68 . The markers can be used in three-dimensional reconstruction of the model of the sample from the two-dimensional images. 
     It will be appreciated that the camera  68  of  FIG. 4C  can also be combined with the camera  64  of  FIG. 4B . Camera  64  can then provide information on the outline of the sample  12 , while the camera  68  can provide information on the height of the sample. 
     Herein, the invention is described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein, without departing from the essence of the invention. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, alternative embodiments having combinations of all or some of the features described in these separate embodiments are also envisaged. 
     It will be appreciated that the scanning electron microscope as described with respect to  FIGS. 3A, 3B, 4A, 4B and 4C  may also include a sliding vacuum seal, e.g. as described with respect to  FIGS. 1A, 1B, 1C, 2A, 2B and 2C . 
     It will be appreciated that the scanning electron microscope as described with respect to  FIGS. 1A, 1B, 1C, 2A, 2B, 2C, 4A, 4B and 4C  may also include a second magnetic lens and/or electron detector movable in synchronism with the sample carrier, e.g. as described with respect to  FIGS. 3A and 3B . 
     It will be appreciated that the scanning electron microscope as described with respect to  FIGS. 1A, 1B, 1C, 2A, 2B, 2C, 3A and 3B  may also include a motion control unit, e.g. as described with respect to  FIGS. 4A, 4B and 4C . 
     In the example of  FIGS. 4A, 4B and 4C , the motion control unit is arranged for avoiding collision. It will be appreciated that it is also possible that the motion control unit is arranged for maintaining a distance between the sample carrier and internal structures, said distance being larger than or equal to a predetermined minimum distance. This can provide for a predetermined safety margin. Fore example, the calculated minimum distance can be used for determining a path that maintains a distance of at least the predetermined minimum distance from internal structures at a voltage different from the sample carrier, e.g. at a high (positive or negative) voltage. Hence, electric discharge between the sample carrier and/or sample and the respective internal structure can be avoided. 
     It will be appreciated that the motion control unit and geometry determination unit can be embodied as dedicated electronic circuits, possibly including software code portions. The motion control unit and geometry determination unit can also be embodied as software code portions executed on, and e.g. stored in, a memory of, a programmable apparatus such as a computer, tablet or smartphone. 
     Although the embodiments of the invention described with reference to the drawings comprise computer apparatus and processes performed in computer apparatus, the invention also extends to computer programs, particularly computer programs on or in a carrier, adapted for putting the invention into practice. The program may be in the form of source or object code or in any other form suitable for use in the implementation of the processes according to the invention. The carrier may be any entity or device capable of carrying the program. 
     For example, the carrier may comprise a storage medium, such as a ROM, for example a CD ROM or a semiconductor ROM, or a magnetic recording medium, for example a floppy disc or hard disk. Further, the carrier may be a transmissible carrier such as an electrical or optical signal which may be conveyed via electrical or optical cable or by radio or other means, e.g. via the internet or cloud. 
     When a program is embodied in a signal which may be conveyed directly by a cable or other device or means, the carrier may be constituted by such cable or other device or means. Alternatively, the carrier may be an integrated circuit in which the program is embedded, the integrated circuit being adapted for performing, or for use in the performance of, the relevant processes. 
     However, other modifications, variations, and alternatives are also possible. The specifications, drawings and examples are, accordingly, to be regarded in an illustrative sense rather than in a restrictive sense. 
     For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. 
     In the claims, any reference sign placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other features or steps than those listed in a claim. Furthermore, the words ‘a’ and ‘an’ shall not be construed as limited to ‘only one’, but instead are used to mean ‘at least one’, and do not exclude a plurality. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to an advantage.