Scanning electron microscope

Abstract

A scanning electron microscope (1) including a sliding vacuum seal (20) between an electron optical imaging system (2) and a sample carrier (10) with a first plate (22) having a first aperture (24) associated with the electron optical imaging system and resting against a second plate (26) having a second aperture (28) associated with the sample carrier. The first plate and/or the second plate includes a groove (40) circumscribing the first and/or second aperture. The scanning electron microscope may include a detector (8) movable relative to the electron beam. The scanning electron microscope may include a motion control unit for moving a sample carrier along a collision free path.

Claims

1. 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, wherein the scanning electron microscope includes a sliding vacuum seal between the electron optical imaging system and the sample carrier, wherein 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 being slideably movable with respect to each other, the first and second apertures overlapping in the imaging position, and the first and second apertures not overlapping in the loading position, wherein 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.

2. Scanning electron microscope according to claim 1, wherein the groove is arranged to circumscribe both the first and second apertures both in the loading position and in the imaging position.

3. Scanning electron microscope according to claim 1, wherein the vacuum system is arranged for having the electron optical imaging system and the sample carrier at a first vacuum level while imaging.

4. Scanning electron microscope according to claim 3, wherein the vacuum system is arranged for having the groove at a second vacuum level, different from the first vacuum level.

5. Scanning electron microscope according to claim 4, wherein the second vacuum level is between the first vacuum level and ambient pressure.

6. Scanning electron microscope according to claim 1, wherein the electron optical imaging system includes an electron beam source and an electron detector, wherein the sample carrier is positionable between the electron beam source and the electron detector, the sample carrier being movable relative to the electron beam for moving the sample, wherein the electron detector is movable relative to the electron beam.

7. Scanning electron microscope according to claim 1, including: a vacuum chamber including internal structures, such as the electron optical imaging system, wherein the sample carrier is movable within the vacuum chamber, a motion control unit including an input unit arranged for receiving user commands relating to movement of the sample carrier, the motion control unit including a memory storing a three-dimensional model of the internal structures of the vacuum chamber and a three-dimensional model of the sample carrier, wherein 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.

8. Scanning electron microscope according to claim 7, wherein the memory further stores a three-dimensional model of the sample carried on the sample carrier, and the motion control unit is arranged for moving the sample carrier further on the basis of the three-dimensional model of the sample.

9. Scanning electron microscope according to claim 8, further comprising an optical camera for providing an image of the sample, and a geometry determination unit for determining the three-dimensional model of the sample.

10. Scanning electron microscope according to claim 1, further comprising an electron beam source, an electron detector, and a magnetic lens, wherein the sample carrier is positioned between the electron beam source and the electron detector, and the magnetic lens is positioned between the sample carrier and the electron detector.

11. Method for loading a sample into a vacuum chamber of a scanning electron microscope including: positioning a sample carrier in a loading position, wherein the scanning electron microscope includes a sliding vacuum seal between an 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 being slideably movable with respect to each other, the first and second apertures overlapping in an imaging position, and the first and second apertures not overlapping 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; placing the sample in the sample carrier when the sample carrier is in the loading position; and moving the sample carrier from the loading position to the imaging position.

12. The method of claim 11, wherein the sample carrier is moved from the loading position to the imaging position by sliding the second plate of the sliding vacuum seal with respect to the first plate of the sliding vacuum seal so that the first and second apertures are overlapped.

13. The method of claim 11, further comprising passing an electron beam from the electron optical imaging system through the overlapped first and second apertures to impinge the sample in the sample carrier when the sample carrier is in the imaging position.

14. The method of claim 11, wherein moving the sample carrier from the loading position to the imaging position includes: sliding the second plate to an intermediate position after placing the sample in the sample carrier and before moving the sample carrier to the imaging position; and evacuating the inner space of the sample carrier.

15. The method of claim 14, wherein the inner space of the sample carrier is evacuated via a connector connecting the inner space of the sample carrier and the vacuum system.

16. The method of claim 15, wherein the inner space of the sample carrier is evacuated via a connector in fluid communication with the groove.

17. The method of claim 11, further comprising providing the electron optical imaging system a first vacuum level and providing the groove a second vacuum level between the first vacuum level and ambient pressure.

18. The method of claim 11, further comprising determining a model of the sample, and the sample carrier is moved based on the model of the sample.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described in detail with reference to the accompanying drawings in which:

(2) FIG. 1A shows an example of a scanning electron microscope with a sample carrier in a first position;

(3) FIG. 1B shows an example of a scanning electron microscope with a sample carrier in a second position;

(4) FIG. 1C shows an example of a scanning electron microscope with a sample carrier in a third position;

(5) FIG. 2A shows an example of a scanning electron microscope with a sample carrier in a first position;

(6) FIG. 2B shows an example of a scanning electron microscope with a sample carrier in a second position;

(7) FIG. 2C shows an example of a scanning electron microscope with a sample carrier in a third position;

(8) FIG. 3A shows an example of a scanning electron microscope with a sample carrier in a first position;

(9) FIG. 3B shows an example of a scanning electron microscope with a sample carrier in a second position;

(10) FIG. 4A shows an example of a scanning electron microscope;

(11) FIG. 4B shows an example of a scanning electron microscope; and

(12) FIG. 4C shows an example of a scanning electron microscope.

DETAILED DESCRIPTION

(13) 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 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. 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.

(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.

(15) 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.

(16) 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.

(17) 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.

(18) The scanning electron microscope 1 described with respect to FIGS. 1A, 1B and 1C can be operated as follows.

(19) 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.

(20) 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.

(21) 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.

(22) 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.

(23) 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.

(24) 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.

(25) 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 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 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.

(26) 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.

(27) 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.

(28) 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.

(29) 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.

(30) 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.

(31) 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.

(32) The scanning electron microscope 1 as described with respect to FIGS. 4A, 4B and 4C can be operated as follows.

(33) 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 displacemente.g. 0.125 mm in a predetermined directionof 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.

(34) 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.

(35) 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.

(36) 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.

(37) 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.

(38) 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.

(39) 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.

(40) 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.

(41) 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.

(42) 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.

(43) 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.

(44) 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.

(45) 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.

(46) 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 slicing vacuum seal, e.g. as described with respect to FIGS. 1A, 1B, 1C, 2A, 2B and 2C.

(47) 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

(48) 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.

(49) 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.

(50) 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.

(51) 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.

(52) 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.

(53) 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.

(54) 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.

(55) 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.