Imaging sample with scanning probe microscope

Abstract

A method of imaging a sample with a scanning probe microscope, the scanning probe microscope comprising: a platform; a sample on the platform; a probe comprising a probe mount, a cantilever extending from the probe mount, and a probe tip extending from the cantilever; a coarse lateral alignment system; a fine lateral actuation system; a coarse Z-actuator comprising a static part and a moving part; and a fine Z-actuator. The method comprises: in a set-up phase, operating the coarse Z-actuator so that the moving part of the coarse Z-actuator moves from a first position to a second position, relative to the static part, and brings the probe towards the sample on the platform; after the set-up phase, obtaining a sequence of two or more images of the sample, each image being obtained in a respective imaging phase by: generating a lateral scanning motion between the probe and the platform with the fine lateral actuation system so that the probe tip interacts with the sample across a respective imaging region, expanding and contracting the fine Z-actuator so that the probe mount moves towards and away from the platform to react to topographic changes of the sample across the imaging region, and monitoring the probe to obtain an image of the imaging region; and in a transit phase between each pair of imaging phases which are adjacent in the sequence, generating a relative transit motion between the probe and the platform, the relative transit motion following a trajectory in which the probe is retracted from the sample by contraction of the fine Z-actuator, laterally aligned with a new imaging region by the coarse lateral alignment system, and returned to the sample at the new imaging region by expansion of the fine Z-actuator. The moving part of the coarse Z-actuator remains fixed in its second position relative to the static part of the coarse Z-actuator for a full duration of each imaging phase and for a full duration of each transit phase.

Description

[0001] IMAGING SAMPLE WITH SCANNING PROBE MICROSCOPE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to methods of imaging a sample with a scanning probe microscope; and a scanning probe microscope for performing such a method.

[0004] BACKGROUND OF THE INVENTION

[0005] In conventional probe microscopy systems, the maximum Z actuation distance for a fine Z-actuator (such as a Z-piezo) is not considered to be sufficiently large to enable the probe to be retracted by the Z-piezo and then translated laterally during a transit phase between imaging sites without potentially damaging the sample or probe due to unexpected height variations. In such conventional systems, in the transit phase the probe is retracted from the sample and then returned to the sample by operation of a coarse Z-actuator, such as a motor. This enables the probe to be retracted sufficiently far to avoid clashing with the sample during the transit phase, but the coarse Z-actuator tends to operate slowly.

[0006] US7562564 discloses a scanning probe microscope capable of measuring accurate 3- D shape information of a sample with high through-put without damaging a sample. In a method for acquiring an accurate 3-D shape of a sample without imparting damage to the sample by bringing a probe into contact at only a measurement point, once pulling up and retracting the probe when it moves towards a next measurement point, moving the probe towards the next measurement point and allowing it to approach, a deflection signal of the probe and its twist signal area are analyzed so that measurement can be made at a minimum necessary retraction distance.

[0007] SUMMARY OF THE INVENTION

[0008] A first aspect of the invention provides a method according to claim 1 . Optional features are set out in the dependent claims.

[0009] A further aspect of the invention provides a scanning probe microscope according to claim 21. Optional features are set out in the dependent claims. A further aspect of the invention provides a method according to claim 22. Optional features are set out in the dependent claims.

[0010] A further aspect of the invention provides a method according to claim 23. Optional features are set out in the dependent claims.

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0013] Figure 1 shows functional elements of a scanning probe microscope;

[0014] Figure 2 shows a probe head and support structure of the scanning probe microscope of Figure 1 ;

[0015] Figure 3 shows a coarse Z-actuation system;

[0016] Figure 4 shows details of the components carried by the probe head of the scanning probe microscope of Figure 2;

[0017] Figure 5 is a schematic plan view of an XY-scanner;

[0018] Figure 6 is a sectional side view showing the parts of the XY-scanner under the objective lens;

[0019] Figure 7 is a sectional side view showing the XY-scanner under the objective lens;

[0020] Figure 8 is a flow diagram showing steps in a method of obtaining sample data from a sample;

[0021] Figure 9 shows an exemplary topography of the surface of a sample;

[0022] Figure 10 shows the platform on adjustment screws;

[0023] Figure 11 shows the platform after adjustment;

[0024] Figure 12 shows a wafer being mounted on the platform;

[0025] Figure 13 shows a vacuum clamp applying a clamping force to the wafer;

[0026] Figure 14a shows an unmodified transit trajectory;

[0027] Figure 14b shows a modified transit trajectory for an upwardly tilted sample;

[0028] Figure 14c shows a further modified transit trajectory for the upwardly tilted sample;

[0029] Figure 14d shows a modified transit trajectory for a downwardly tilted sample;

[0030] Figures 15a and 15b shows a process of obtaining height information from a test item;

[0031] Figure 16a shows a modified transit trajectory for a domed sample;

[0032] Figure 16b shows a further modified transit trajectory for the domed sample;

[0033] Figure 16c shows a further modified transit trajectory for the domed sample; Figure 17a shows a modified transit trajectory for an undulating sample;

[0034] Figure 17b shows a further modified transit trajectory for the undulating sample;

[0035] Figure 18a shows a modified transit trajectory for a further undulating sample;

[0036] Figure 18b shows a further modified transit trajectory for the further undulating sample; Figures 19a-c are plan views of a semiconductor wafer indicating contour lines and three imaging sites.

[0037] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0038] A scanning probe microscope 1 shown in Figure 1 is configured to measure a topography of a sample 2 on a platform 3 (conventionally known as a chuck).

[0039] The microscope 1 has various control elements which together provide a control system configured to operate the scanning probe microscope 1 to image the sample 2 as described below. These control elements (and other various functional elements of the microscope 1) may be implemented in computer software running on one or more computer processors, or in dedicated hardware.

[0040] The scanning probe microscope 1 comprises a probe mount 4; and a probe comprising a cantilever 5 and probe tip 6. The cantilever 5 is carried by the probe mount 4, the cantilever 5 extending from a proximal end at the probe mount to a distal end remote from the probe mount. The probe tip 6 is at the distal end of the cantilever 5.

[0041] A detection system 30 is shown schematically in Figure 1. A laser light source (not shown) generates a detection beam 32 which is steered onto the distal end of the cantilever 5 by a steering mirror 33 and focused onto the distal end of the cantilever 5 by an objective lens 37. The distal end of the cantilever 5 reflects the detection beam 32 to generate a return beam 34.

[0042] An XY-align actuator 16 is used to move the probe relative to the objective lens 37 so that the detection beam 32 falls onto the cantilever 5. A detection system focusing stage 109 (shown in Figure 4) can be also operated to move the detection system 30 and objective lens 37 up and down (in the Z direction) so that the focal plane of the objective lens 37 coincides with the cantilever 5. The detection system 30 incorporates an interferometer which generates a height measurement signal 116. Details of a suitable interferometer are disclosed in US11733265, the contents of which are incorporated herein by reference.

[0043] A signal processor 50 is configured to monitor the height measurement signal 116 to obtain a series of topography measurements indicative of a topography of the sample 2. These topography measurements are output as a topography measurement signal 51 to an image collection module 52.

[0044] A photothermal actuation system 60 is configured to bend the cantilever 5 by illuminating the cantilever with an actuation beam 61 under control of a photothermal offset signal 62 and an oscillation signal 63. An oscillation signal generator 64 is configured to generate the oscillation signal 63. The oscillation signal 63 may be substantially sinusoidal, at a resonant frequency of the cantilever 5.

[0045] The cantilever 5 has a thermal bimorph structure, the materials of which undergo differential expansion when heated. Details of a suitable cantilever are disclosed in US11733265, the contents of which are incorporated herein by reference.

[0046] As explained above, the detection beam 32 is directed onto the cantilever by a steering mirror 33 and an objective lens 37. As the steering mirror 33 tilts, the reflected detection beam 32 deflects and retains its position on the distal end of the cantilever 5. The actuation beam 61 is also directed onto the cantilever by the steering mirror 33 and the objective lens 37. As the steering mirror 33 tilts, the actuation beam 61 deflects and retains its position on the cantilever 5 (in this case towards the proximal end of the cantilever). Thus, the steering mirror 33 retains the positions of both beams 32, 61 on the cantilever 5. The steering mirror 33 is driven by steering signals 65 from a scan controller 11 .

[0047] A feedback control system 80 is configured to adjust the photothermal offset signal 62 and a height control signal 21 on a basis of the height measurement signal 116, so that the probe tip follows the topography of the sample 2.

[0048] The signal processer 50 may be arranged to extract data from a position within each oscillation cycle of the height measurement signal 116 that satisfies a predetermined measurement criterion. This ensures that the measurement point is extracted at a position most likely to reflect the true height of the sample. For example, at a minimum (or maximum) path difference it can be inferred that the probe tip is in contact with the sample surface. This improves the accuracy of information extracted by the detection system 30, which may then be used by the image collection module 52 to generate an image of the sample surface. This image may reflect surface height, or any other aspect of the topography of the sample.

[0049] In this example, an image collection module 52 constructs an image on the basis of a topography measurement signal 51 , and an XY position signal 55 from the scan controller 11 which indicates the current XY position of the probe tip. Alternatively, the image may be constructed on the basis of the height control signal 21.

[0050] In other embodiments of the invention, rather than generating a two-dimensional image with the image collection module 52, a series of topography measurements may be measured with only a single line scan (for instance across a trench) to generate a onedimensional topographical image rather than scanning in two-dimensions to generate a two-dimensional topographical image.

[0051] The detection system 30, steering mirror 33, and objective lens 37 are housed in a probe head 101 shown schematically in Figure 2. The probe head 101 is held by a support structure comprising multiple individual granite blocks, secured together to form a base 105a, upright 105b and supporting arm 105c. The base 105a supports an XY-stage 8 which in turn supports the platform 3. The sample 2 is positioned on the platform 3, for example by using a robot and wafer handling system (not shown). The support structure 105a-c rests on a mechanical isolation system 106.

[0052] As shown schematically in Figure 1 , a lateral scanning system 10, 11 is configured to generate a relative lateral scanning motion between the probe mount 4 and the sample 2 in a horizontal (X,Y) plane. In this case the lateral scanning motion is achieved by motion of the probe mount 4, but in other embodiments the relative lateral scanning motion may achieved by motion of the platform 3 (which carries the sample 2). The scanning system in this case comprises a fine lateral actuation system 10 (referred to below as an XY-scanner 10), such as a pair of piezoelectric actuators which each move a fine Z-actuator 20 and the probe mount 4 in a respective horizontal direction (X or Y). The XY-scanner 10 is driven by scanning drive signals 12 from the scan controller 11. Large-scale relative motion between the probe mount 4 and the sample 2 is provided by a lateral alignment system in the form of an XY stage 8 which carries the platform 3 and is arranged to translate the platform 3 in a horizontal (XY) drive plane. The XY stage 8 may comprise an X-motor for driving the platform 3 in an X-direction and a Y- motor for driving the platform 3 in a Y-direction. The X and Y directions define the drive plane of the XY stage 8, which is ideally precisely horizontal.

[0053] A Z-actuation system 14, 20 is configured to adjust a height of the proximal end of the cantilever 5 by moving the probe mount 4. The Z-actuation system 14, 20 comprises two Z-actuators 14, 20.

[0054] The first Z-actuator is a coarse Z-actuator 14 arranged to translate the probe head 101 in a vertical (Z) direction over a long distance. The coarse Z-actuator 14 may comprise a motor shown in detail in Figure 3.

[0055] The coarse Z-actuator 14 comprises a static part 94 and a moving part 92. The moving part of the coarse Z-actuator 14 moves from a first position to a second position, relative to the static part, and moves the probe head 101 to bring the probe 5, 6 towards the sample 2 on the platform 3 and into close proximity with the sample, preferably within the working range of the fine Z-actuator 20 (described below).

[0056] In this case the coarse Z-actuator 14 comprises a motor 90 which rotates a screw 91 connected to an internally threaded moving part 92. The moving part 92 is connected to the probe mount 4 via the probe head 101 and various other components shown in Figures 4 and 7. The moving part 92 of the coarse Z-actuator 14 is arranged to move vertically along a bearing 93 supported by a frame 94. Therefore, the frame 94 forms the static part of the coarse Z-actuator 14. In other embodiments the coarse Z-actuator may instead move the sample 2 by moving the platform 3 up and down.

[0057] The second Z-actuator is a fine Z-actuator 20 arranged to translate the probe mount 4, cantilever 5, and probe tip 6 in a vertical (Z) direction quickly and over a short distance. The fine Z-actuator 20 is a piezoelectric actuator (referred to below as a Z-piezo 20) which is configured to translate the probe mount 4 up and down in an essentially vertical (Z) direction on a basis of a height control signal 21 . The Z-piezo 20 is arranged to translate the probe mount 4, cantilever 5, and probe tip 6 in the vertical direction by expansion and contraction of the Z-piezo 20. The Z-piezo 20 and XY-scanner 10 are shown in plan view and side view in Figures 5 and 6, respectively. Figure 7 is a more detailed cross-sectional side view including the probe head 101.

[0058] As shown in Figure 5, the XY-scanner 10 comprises a piezoelectric actuator (X-piezo) 47 which can be driven to move the probe mount 4 in a lateral X-direction; and a piezoelectric actuator (Y-piezo) 48 which can be driven to move the probe mount 4 in a lateral Y-direction. The X-piezo 47 and Y-piezo 48 may act between an inner frame

[0059] 44 and an outer frame 45 so that each actuator 47, 48 causes the inner frame 44 to move laterally relative to the outer frame 45. The probe mount 4 is connected to the inner frame 44 via the Z-piezo 20 so that lateral movement of the inner frame 44 will result in lateral movement of the probe mount 4. As shown in Figure 4, the outer frame

[0060] 45 of the XY-scanner 10 is attached to the X-Y align actuator 16, which is attached in turn to the probe head 101 . Hence the X-Y align actuator 16 and the outer frame 45 of the XY-scanner 10 are moved up and down together by the coarse Z-actuator 14.

[0061] The Z-piezo 20 is mounted at its upper end to the inner frame 44 by a support structure 49 (shown in Figure 6 but omitted in Figure 5 so the Z-piezo 20 is visible). The lower end of the Z-piezo 20 is connected to the probe mount 4, and thus, in turn, the probe 6. Hence as the X-piezo 47 and Y-piezo 48 are actuated to scan the inner frame 44 laterally, the Z-piezo 20, probe mount 4 and probe 5, 6 move with the inner frame 44.

[0062] The inner frame 44 is connected to the outer frame 45 by flexures 42 or any other suitable means for holding the inner frame 44 while allowing movement relative to the outer frame 45. Flexures 42 are shown in only one of the four corners of the XY-scanner 10 in Figure 5, but typically there will be similar flexures at each corner.

[0063] The X-piezo 47 and the Y-piezo 48 together provide a lateral scanning system which can be operated to generate a lateral scanning motion between the probe 5, 6 and the sample 2, in this case by motion of the probe mount 4. In other cases, the lateral scanning motion between the probe 5, 6 and the sample 2 may be caused by motion of the sample 2 (such motion being caused by motion of the platform 3 supporting the sample 2). The Z-piezo 20 can be driven to move the probe mount 4 up and down in a Z-direction perpendicular to the sample 2. The XY-scanner 10 and the Z-piezo 20 are arranged in a gap below the objective lens 37 as shown in Figure 6.

[0064] When the detection beam 32 is focused onto the cantilever as in Figure 6, the distance between the lowest point of the objective lens 37 and the cantilever is about 20 mm, which is the working distance of the objective lens 37. The objective lens 37 can be moved up and down by the focusing stage 109 so the objective lens 37 is positioned correctly at this working distance. The working distance affects the numerical aperture of the objective lens 37, which in turn affects the spot size of the focused detection beam 32. A small spot size is desirable, which means that the working distance is ideally kept low (for instance not above 20mm).

[0065] Typical piezoelectric actuators have a range of motion of about 1 pm per mm. Hence a 15pm range of motion, for example, requires a Z-piezo 20 which is 15mm long. As shown in Figure 6, if the working distance of the objective lens 37 is 20mm, then the Z- piezo 20 must be shorter than this working distance, making a 20pm range of motion the theoretical maximum.

[0066] Note that the detection beam 32 and the return beam 34 pass through a small aperture 44a in the inner frame 44a, and as shown in Figure 6 this aperture 44a is not sufficiently large to receive the objective lens 37. So the working distance of the objective lens 37 places a fundamental limit on the possible size (and hence range of motion) of the Z- piezo 20.

[0067] Figure 8 shows method steps 120 which are performed to image a sample 2, such as a semiconductor wafer.

[0068] A first step 121 is a set-up phase. In a first part of the set-up phase 121 , the XY stage 8 is operated to generate a lateral registration motion between the probe and the sample so that the probe becomes laterally aligned with a first imaging region of the sample. Typically the probe crosses a peripheral edge of the sample 2 during this lateral registration motion so the probe head 101 is held at some distance from the sample to avoid clash. In a second part of the set-up phase 121 the coarse Z-actuator 14 is operated so that the moving part 92 of the coarse Z-actuator 14 moves down from a first position to a second position, relative to the static part 94, and brings the probe down to an operating distance in close proximity with the sample 2 on the platform 3.

[0069] After the set-up phase 121 , a sequence of two or more images of the sample is obtained, each image being obtained in a respective imaging phase 122, 124. Each imaging phase 122, 124 may comprise generating a lateral scanning motion between the probe and the platform with the XY scanner 10 so that the probe tip interacts with the sample across a respective imaging region, expanding and contracting the Z-piezo 20 so that the probe mount moves towards and away from the platform to react to topographic changes of the sample across the imaging region, and monitoring the probe 5, 6 to obtain an image of the imaging region.

[0070] A variety of different types of imaging modes may be used in the imaging phases 122, 124, for example the imaging mode described above with reference to Figure 1 ; the adaptive imaging mode described in US2014 / 0026263A1 ; or the dynamic imaging mode described in LIS2011 / 0247106A1. In each case the Z-piezo 20 expands and contracts so that the probe mount 4 moves towards and away from the platform 3 to react to topographic changes of the sample across the imaging region. The image may be a two-dimensional image (based on a two-dimensional scan) or a one-dimensional image (based on a single line scan).

[0071] During imaging, the detection beam 32 is directed through a series of optics (not shown) such that the detection beam 32 propagates to the steering mirror 33 and is reflected towards the objective lens 37. As the steering mirror 33 tilts, the reflected detection beam 32 deflects, with the result that the angle and point of incidence of the detection beam 32 into the objective lens 37 changes. Synchronisation of the angle of the steering mirror 33 with the XY scanning pattern followed by the probe mount 4 as it is driven by the XY-scanner 10 means that the detection beam 32 retains its position on the distal end of the cantilever 5.

[0072] After the first imaging phase 122 is complete, the method comprises a transit phase 123 in which the probe transits between imaging regions or sites. Specifically, the transit phase occurs between each pair of imaging phases 122, 124 which are adjacent in the sequence. During the transit phase 123, a relative transit motion between the probe and the platform is generated so the probe becomes aligned with a new imaging region. During the transit phase 123, the relative transit motion between the probe and platform follows a trajectory in which the probe is retracted from the sample 2 by contraction of the Z-piezo 20, laterally aligned with a new imaging region by operation of the coarse lateral alignment system (XY stage 8), and returned to the sample 2 at the new imaging region by expansion of the Z-piezo 20. In this case the probe is laterally aligned by motion of the sample 2, but in other embodiments of the invention the probe may be laterally aligned by motion of the probe.

[0073] After the transit phase 123 is complete and the probe is at the next imaging site in the sequence, the further imaging phase 124 can be carried out.

[0074] In conventional probe microscopy systems, the maximum Z actuation distance for the Z-piezo is not considered to be sufficiently large to enable the probe to be retracted by the Z-piezo and then translated laterally during the transit phase without potentially damaging the sample or probe due to unexpected height variations. In such conventional systems, in the transit phase the probe is retracted from the sample and then returned to the sample by operation of a coarse Z-actuator, such as a motor. This enables the probe to be retracted sufficiently far to avoid clashing with the sample during the transit phase, but the coarse Z-actuator tends to operate slowly.

[0075] Surprisingly, it has been found that for some samples (for example semiconductor wafers) it is possible to retract by a small distance and still reliably avoid clashing with the sample. Hence in contrast to conventional probe microscopy systems, during the transit phase 123 the coarse Z-actuator 14 is inactive and the retraction and return of the probe in the Z-direction are driven solely by a fine Z-actuator (in this case, the Z- piezo 20). The moving part 92 of the coarse Z-actuator 14 remains fixed in its second position relative to the static part 94 of the coarse Z-actuator 14 for a full duration of each imaging phase 122, 124 and for a full duration of each transit phase 123. This enables the transit phase 123 to be performed more quickly than in conventional probe microscopy systems.

[0076] The method may repeat steps 123 and 124 so that more than two imaging sites are imaged whilst the moving part of the coarse Z-actuator 14 is in the second position. In other words, two or more sites on the sample may be imaged without moving the probe head 101 up and down. This can decrease the time taken to carry out the full scanning process when a large number of imaging sites on one sample are imaged.

[0077] Figure 9 gives an example of a small-scale topography of the surface of the sample 2. In this case, the surface of the sample 2 has a series of trenches with steep sidewalls. The X-Y scanner 10 produces a scanning motion between the probe and sample over a first imaging region with a lateral width which is typically in the range of 0.1 m to 100pm. During the imaging phase, the Z-piezo 20 expands and contracts (as indicated by dashed line 19) to follow the profile of the sample - in this case to follow the profile of the series of trenches. The maximum range of motion of the Z-piezo 20 in the imaging phase is typically of the order of 5pm, although it can be as low as 1nm in the case of a relatively planar sample.

[0078] In the transit phase, the probe is retracted away from the sample by a retraction distance of the order of 10pm, then the XY stage 8 is moved laterally so that the probe becomes aligned with the next imaging region. This lateral transit motion could span a distance as small as 100pm or as large as 300mm (in the case of a 300mm semiconductor wafer). During the imaging and transit phases, the coarse Z-actuator 14 is inactive and fixed in position.

[0079] As shown in Figure 9, for each imaging region the Z-piezo 20 expands and contracts over a respective imaging range (for instance a range of 1 pm or 5pm) to react to topographic changes of the sample across the imaging region, and during each transit phase 123 the Z-piezo 20 contracts outside the imaging range of the immediately preceding imaging phase (for instance retracting by 10pm). Optionally the Z-piezo 20 is contracted to a limit of its range as it retracts the probe away from the sample.

[0080] In the example of Figure 9, the upper surface of the sample is flat (on a large scale) and perfectly horizontal. Also, the coarse lateral alignment system (the XY stage 8) drives the sample in a precisely horizontal direction 119, parallel with the upper surface of the sample 2. During the transit phase of Figure 9 the probe is retracted away from the sample and then returned to the sample over approximately the same distance, and the relative transit motion 119 between the probe and the sample is precisely horizontal. Hence the probe can be retracted away from the sample by a small and optionally predetermined distance - in this case about 10pm - with no risk of clashing with the sample. This scale of motion is possible for the Z-piezo 20, but the range of motion of the Z-piezo 20 cannot be extended much further because it is limited by the working distance of the objective lens 37, for the reasons explained above.

[0081] During the relative transit motion between the probe and the platform 3 it is important that the probe does not clash with the sample, and a large retraction is desirable to avoid this. However there is a trade-off with speed of transit, and use of the Z-piezo 20 to drive the retraction sets a limit on how far the probe can be retracted. In the case of Figure 9, a small retraction distance (for instance about 10pm) may be sufficient to avoid clashing with the sample.

[0082] However in other cases such a small retraction distance may be insufficient - for instance if the upper surface of the sample is not flat (on a large scale) or not perfectly horizontal, or if the drive plane of the coarse lateral alignment system (the XY stage 8) is mis-aligned so it does not drive the platform 3 in a precisely horizontal direction.

[0083] Two classes of solution to this problem will now be described. A first class of solution (Figures 10-13) employs strategies to ensure that the upper surface of the sample 2 is more flat (on a large scale) and / or more horizontal. A second class of solution (Figures 14-18) obtains height information to measure large-scale deviation of the upper surface of the sample and / or mis-alignment of the drive plane of the XY stage 8, and uses this height information to modify the trajectory of the relative transit motion during the transit phase 123, resulting in a trajectory which is both fast and avoids clash.

[0084] Figure 10 illustrates a strategy to ensure that the upper surface of the sample is more horizontal (and hence more precisely aligned with the lateral drive plane of the XY stage 8) by adjusting the tip / tilt angle of the platform 3 relative to the X-Y stage 8. The platform 3 is carried by the XY-stage 8 on three or more adjustable screws 142a, 142b, only two of which are shown in Figure 10 (the other screw(s) being out of the plane of the cross-section of Figure 10). Each screw can be individually turned to adjust the gap between the platform 3 and the XY-stage 8, and hence adjust the tip / tilt angle of the platform 3 relative to the lateral drive plane of the XY stage 8.

[0085] Figure 10 shows the platform 3 and sample 2 mis-aligned with the lateral drive plane, and Figure 11 shows the platform 3 after the screw 142b has been adjusted to make the sample 2 and the platform 3 more parallel with the lateral drive plane. When the sample is in its adjusted state shown in Figure 11 , the probe is less likely to clash with the sample 2 during the transit phase 123.

[0086] Figure 12 illustrates a further adjustment which can be applied to make the sample 2 more parallel with the lateral drive plane. In this case the adjustment comprises a deformation of the sample 2, caused by clamping the sample 2 to the platform 3.

[0087] The platform 3 in Figure 12 is a vacuum chuck with a vacuum port 147, and an upper surface with an array of pips 148, and a ring 149 around the edge to form a vacuum seal with the sample 2. The sample 2 is placed on the vacuum chuck 3 in contact with the ring 149. Initially the sample 2 has a non-planar bowed shape shown in Figure 12. The vacuum port 147 is coupled to a vacuum manifold (not shown). A vacuum 150 is applied from the vacuum manifold via the vacuum port 147 which creates a vacuum between the pips 148 and pulls the sample 2 into contact with the pips 148 as shown in Figure 14, clamping the sample 2 to the vacuum chuck 3. The vacuum clamping force deforms the sample 2 so it adopts a more planar shape as shown in Figure 13. When the sample 2 is in its adjusted planar shape shown in Figure 13, the probe is less likely to clash with the sample 2 during the transit phase 123.

[0088] In this example a vacuum clamping force is used to deform the sample 2, but in other examples the clamping force may be applied to the sample 2 mechanically, or using electrostatic force (i.e. an electrostatic chuck).

[0089] The pips 148 may be arranged concentrically about a centre of the vacuum chuck 3 of Figure 12. The pips 148 provide a plurality of contact points between the vacuum chuck and sample 2 whilst allowing a negative pressure to form around the pips 148 to hold the sample 2 to the vacuum chuck. By providing a large number of contact points, the likelihood of the sample 2 adhering to the vacuum chuck can be reduced. Although a single outer ring 149 is shown, one or more additional rings with a smaller radius may be provided. The inner rings can provide a seal with the underside of the sample so that concentric vacuum regions can be formed. This is advantageous when the vacuum chuck supports a sample which is bowed or distorted as the sample can be pulled towards the vacuum chuck in stages to counteract the curved surface of the sample.

[0090] The platform 3 (i.e. the platform 3 of Figure 10 or the vacuum chuck 3 of Figure 12) may comprise a ceramic material, such as silicon carbide, aluminium nitride, silicon nitride, Yttrium oxide (Y2O3), or silicon dioxide (SiO2). The platform may be formed by this ceramic material in its entirety, or only an upper surface of the platform (i.e. the part of the platform which contacts the sample) may comprise ceramic material. In either case, the platform 3 comprises a ceramic material which is configured to contact the sample. In the case of the vacuum chuck of Figure 12, at least the pips 148 and the ring 14 comprise the ceramic material, and optionally the body of the vacuum chuck also comprises the same ceramic material.

[0091] It can be advantageous to contact the sample 2 with a ceramic material due to the chemical and physical properties of ceramic material. For example hardness which makes it more resistant to damage, such as chips, meaning that the platform surface can provide greater flatness. Additionally, ceramic material has a lower thermal expansion coefficient than most metals. The ceramic material that contacts the sample may have a composition of 50% silicon and 50% silicon carbide. This can provide a density of around 2.8 g / cm3, a hardness of around 20 GPa, and a coefficient of thermal expansion of around 3.1x10'6 / °C. The ceramic material that contacts the sample may have a composition of 25% silicon and 75% silicon carbide. Silicon carbide is particularly advantageous when the sample is a semiconductor wafer as the platform 3 and sample have a similar coefficient of thermal expansion. Silicon carbide is conductive, meaning that a silicon carbide chuck can be used to electrostatically hold the sample on the chuck.

[0092] Figures 14-18 illustrate methods which obtain height information to predict large-scale deviation of the upper surface of the sample 2 and / or mis-alignment of the drive plane of the XY stage 8, and use this height information to modify the trajectory of the relative transit motion during the transit phase 123.

[0093] In Figure 14a the sample 2 is flat and parallel with the drive plane so the relative transit motion follows a trajectory in which the probe is retracted from the sample by a short distance D1 by a Z-actuation system (for instance the Z-piezo 20 or the coarse Z- actuator 14), laterally aligned (by a horizontal relative motion 130) with a new imaging region by the coarse lateral actuation system (in this case the XY stage 8 which moves the sample 2) and then returned to the sample 2 at the new imaging region by the same short distance D1. In Figure 14b the surface of the sample 2 is tilted up so that the second imaging region is higher than the first imaging region. This upward tilt could be caused by a variety of systematic errors: for example a sample 2 with a varying thickness, a platform 3 with a varying thickness, or inaccurate adjustment of the adjustment screws 142a, 142b.

[0094] These systematic errors are first measured by periodically (for instance once a month) obtaining height information by measuring heights of an array of test sites of a test item on the platform 3.

[0095] As shown in Figure 4, the scanning probe microscope comprises a proximity sensor 107 which can be used to provide data representative of the height of the probe head 101 above the sample. The proximity sensor 107 may be a capacitance sensor for example. In this case the proximity sensor 107 is attached to the frame 45 of the XY scanner 10, but in other examples the proximity sensor 107 may be attached to the probe head 101 or any other suitable part.

[0096] In another example, the proximity sensor 107 may be an optical proximity sensor (such as an interferometer or a confocal optical sensor) in the probe head 101 which measures a distance to the sample 7 using a detection beam which optionally passes through the objective lens 37.

[0097] Figures 15a and 15b shows an example of obtaining the height information. A test item 140 (such as a semiconductor wafer) is placed on the platform 3; and a lateral mapping motion 141 is generated between the proximity sensor 107 and the platform 3 so that the proximity sensor 107 becomes sequentially aligned with a series of test sites. At each test site, the proximity sensor 107 is operated to obtain the height information by measuring a proximity of the test item 140. Figure 15a shows a first test site where a height H1 is measured by the proximity sensor 107, and Figure 15b shows a second test site in the series where a height H2 is measured by the proximity sensor 107. Note that the proximity sensor 107 may measure an average proximity of the test item 140 over a relatively large lateral distance, which averages out the effects of any small trenches or other features in the surface of the test item 140.

[0098] The lateral mapping motion 141 is preferably generated by the XY-scanner 8, which also drives the relative transit motion between imaging regions in the transit phase 123. Hence the height information H1 , H2 not only picks up the systematic errors mentioned above, but also picks up systematic errors caused by mis-alignment of the lateral drive plane of the XY-scanner 8.

[0099] The height information H1 , H2 etc. output by the proximity sensor 107 provides a topographic large scale map (e.g. a two-dimensional array) of height measurements indicating the height of the test item 140, which is stored in a memory. A series of samples 2 subsequently imaged can each be expected to have a similar large-scale variation of height to the test item 140, and hence the height information H1 , H2 can be used during imaging of each subsequent sample 2 as described below.

[0100] Firstly, the height information may be used to check that a range of the Z-piezo 20 is sufficient to avoid the probe clashing with the sample 2 as it is laterally aligned with a new imaging region during a transit phase 123. If so, then the Z-piezo 20 is used. If not, then the coarse Z-actuator 14 is used instead during the transit phase 123.

[0101] Secondly, if the height information indicates that the sample is relatively planar and horizontal, then a fast and un-modified relative transit motion is used as shown in Figure 14a.

[0102] Thirdly, if the height information indicates that the sample has significant large-scale height variations, as shown in Figures 14b, 14e, 16a for example, then the relative transit motion is adapted so that it follows a modified trajectory as explained below.

[0103] When the probe is aligned with the second imaging region by the relative transit motion shown in Figure 14b, part of the control system of the scanning probe microscope (such as a transit controller 18 shown in Figure 18) uses the stored height information to modify the trajectory of the relative transit motion, compared with Figure 14a. In this case, the transit controller 18 determines that there is an upward tilt between the first and second region, on a basis of the stored height information and the positions of the two imaging regions. As a result the transit controller 18 commands a relative transit motion which follows a modified trajectory shown in Figure 14b in which the probe is retracted from the sample by a relatively large distance D2 by the Z-piezo 20 and / or the Z-motor 14, laterally aligned (by a horizontal relative motion 130) with a new imaging region by operation of the coarse lateral actuation system (in this case the XY stage 8 which moves the sample 2), and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor at the new imaging region by the distance D1 (D1 <D2). Retracting by an increased distance D2 prevents clash, by ensuring that the distance during the lateral alignment is not less than D1 despite the upward tilt.

[0104] Figure 14c shows another way of modifying the trajectory of the relative transit motion to account for an upward tilt. In this case the relative transit motion follows a modified trajectory in which the probe is retracted from the sample by the Z-piezo 20 and / or the Z-motor 14 by a short distance D1 , laterally aligned so that the distance remains D1 (by a non-horizontal motion 131 which follows the upward tilt angle of the sample 2 and is driven by simultaneous operation of both the Z-piezo 20 and the XY-stage 8) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by the same short distance D1 . This prevents clash and reduces transit time by ensuring that the distance during the motion 131 remains D1 throughout.

[0105] Figure 14d shows a way of modifying the trajectory of the relative transit motion to account for a downward tilt. In this case the relative transit motion follows a modified trajectory in which the probe is retracted from the sample by the Z-piezo 20 and / or the Z-motor 14 by a short distance D1 , laterally aligned so that the distance remains D1 (by a non-horizontal motion 132 which follows the downward tilt angle of the sample and is driven by simultaneous operation of both the Z-piezo 20 and the XY-stage 8) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by the same short distance D1 . This prevents clash and reduces transit time by ensuring that the distance during the motion 132 remains D1 throughout.

[0106] Figure 16a shows a way of modifying the trajectory of the relative transit motion to account for a sample 2 with a domed profile. In this case, the transit controller 18 determines that there is a domed profile between the first and second region, on a basis of the height information. As a result the transit controller 18 commands a relative transit motion which follows a modified trajectory shown in Figure 16a in which the probe is retracted from the sample by a distance D2 by the Z-piezo 20 and / or the Z- motor 14, laterally aligned (by a horizontal relative motion 130) with a new imaging region by the coarse lateral actuation system (in this case the XY stage 8 which moves the sample 2) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by the same distance D2. D2 is chosen to be sufficiently large to avoid clashing with the sample at the high point 133 of the domed profile. Figure 16b shows another way of modifying the trajectory of the relative transit motion to account for a sample 2 with a domed profile. In this case the relative transit motion follows a modified trajectory in which the probe is retracted from the sample by the Z- piezo 20 and / or the Z-motor 14 by a short distance D1 (which is less than D2 in Figure 16a), laterally aligned so that the distance remains D1 (by a curved non-horizontal motion 134 which follows the domed profile of the sample and is driven by simultaneous operation of both the Z-piezo 20 and the XY-stage 8) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by the same short distance D1. This prevents clash and reduces transit time by ensuring that the distance during the curved motion 134 remains D1 throughout.

[0107] In the trajectories of Figures 14a-d, 16a and 16b the relative transit motion follows a trajectory in which the probe is retracted from the sample by the Z-actuation system (Z-piezo 20 and / or Z-motor 14) by a distance D1 or D2; then laterally aligned with a new imaging region by the lateral actuation system (XY stage 8); and then returned to the sample at the new imaging region by the Z-actuation system. During retraction and return the lateral actuation system is inactive.

[0108] In the trajectory of Figure 16c, the retraction and return the lateral actuation system (XY stage 8) is active so the relative transit motion follows a continuously curved trajectory 134a-c. The probe is retracted from the sample by the Z-actuation system (Z-piezo 20 or Z-motor 14) by a distance D1 during a first phase 134a; then laterally aligned with a new imaging region by the lateral actuation system (XY stage 8) in a second phase 134b and then returned to the sample at the new imaging region by the Z-actuation system in a third phase 134c. The XY stage 8 is active during all three phases 134a-c. The trajectories of Figures 14a-d may be modified to follow a similar continuously curved trajectory.

[0109] Figure 17a shows a way of modifying the trajectory of the relative transit motion to account for a sample 2 with a domed profile with a downward tilt. In this case, the transit controller 18 determines that there is a domed profile between the first and second region, and a downward tilt, on a basis of the height information. As a result the transit controller 18 commands a relative transit motion which follows a modified trajectory shown in Figure 17a in which the probe is retracted from the sample by a distance D3 by the Z-piezo 20 and / or the Z-motor 14, laterally aligned (by a horizontal relative motion 130) with a new imaging region by the coarse lateral actuation system (in this case the XY stage 8 which moves the sample 2) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by a distance D4 (D4>D3). D3 is chosen to be sufficiently large to avoid clashing with the sample at the high point of the domed profile.

[0110] Figure 17b shows another way of modifying the trajectory of the relative transit motion to account for a sample 2 with the profile of Figure 17b. In this case the relative transit motion follows a modified trajectory in which the probe is retracted from the sample by the Z-piezo 20 and / or the Z-motor 14 by a short distance D1 (which is less than D3 in Figure 17a), laterally aligned so that the distance remains D1 (by a curved and nonhorizontal motion 135 which follows the domed profile of the sample and is driven by simultaneous operation of both the Z-piezo 20 and the XY-stage 8) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by the same short distance D1. This prevents clash and reduces transit time by ensuring that the distance during the curved motion 135 remains D1 throughout.

[0111] Figure 18a shows a way of modifying the trajectory of the relative transit motion to account for a sample 2 with an undulating profile. In this case, the transit controller 18 determines that there is an undulating profile between the first and second region, on a basis of the height information. As a result the transit controller 18 commands a relative transit motion which follows a modified trajectory shown in Figure 18a in which the probe is retracted from the sample by a distance D5 by the Z-piezo 20 and / or the Z-motor 14, laterally aligned (by a horizontal relative motion 130) with a new imaging region by the coarse lateral actuation system (in this case the XY stage 8 which moves the sample 2) and then returned to the sample 2 by the Z-piezo 20 and / or the Z-motor 14 at the new imaging region by a distance D6 (D5>D6). D5 is chosen to be sufficiently large to avoid clashing with the sample at the high point of the undulating profile.

[0112] Figure 18b shows another way of modifying the trajectory of the relative transit motion to account for a sample 2 with the undulating profile of Figure 18a. In this case the relative transit motion follows a modified trajectory in which the probe is retracted from the sample by the Z-piezo 20 and / or the Z-motor 14 by a short distance D1 (which is less than D5 in Figure 18a), laterally aligned (by a curved non-horizontal motion 136 which clears the high point by distance D1 and is driven by simultaneous operation of both the Z-piezo 20 and the XY-stage 8) and then returned to the sample 2 by the Z- piezo 20 and / or the Z-motor 14 at the new imaging region by the same short distance D1.

[0113] Figures 19a-c show a further method of using the height information measured in Figures 15a and 15b to minimise the total transit time between imaging sites.

[0114] A sample 2 shown in Figure 19a has a planar surface with a systematic tilt illustrated by three contours lines 140a, 140b, 140c. The contour line 140a is the highest and the contour line 140c is the lowest.

[0115] The scanning probe microscope 1 is operated to obtain a sequence of three images of the sample 2 of Figure 19a on the platform 3, at three imaging regions or sites A, B, C. Each image is obtained in a respective imaging phase by: generating a lateral scanning motion between the probe and the platform with the fine lateral actuation system so that the probe tip interacts with the sample across a respective imaging region, operating the Z-actuation system (i.e. the Z-piezo 20 and / or the coarse Z actuator 14) so that the probe moves towards and away from the platform to react to topographic changes of the sample across the imaging region, and monitoring the probe to obtain an image of the imaging region.

[0116] In a transit phase between each pair of imaging phases which are adjacent in the sequence, a relative transit motion between the probe and the platform is generated, the relative transit motion following a trajectory in which the probe is retracted from the sample by the Z-actuation system (i.e. the Z-piezo 20 and / or the coarse Z actuator 14), laterally aligned with a new imaging region by the coarse lateral actuation system (XY scanner 8) and returned to the sample at the new imaging region by the Z-actuation system. As noted above, the coarse lateral actuation system (XY scanner 8) may be active throughout the transit phase, or it may be inactive during the retraction and return stages of the transit phase.

[0117] There are six potential orders in which the sequence of three images could be obtained: ABC, ACB, BAC, BCA, CAB or CBA. The systematic tilt illustrated by the three contour lines 140a, 140b, 140c means that the order will affect the total transit time. So for instance if the sequence of three images is obtained in the order ACB then the probe must travel down from A to C, then back up from C to B. The height information can therefore be used by the transit controller 18 to determine an optimal order in which the sequence of three or more images is obtained. In this case the order ABC or CBA provided the quickest total transit time.

[0118] A sample 2 shown in Figure 19b has a domed surface illustrated by three contours lines 141a, 141b, 141c. The contour line 141a is the highest and the contour line 141c is the lowest. There are six potential orders in which the sequence of three images could be obtained: ABC, ACB, BAC, BCA, CAB or CBA. The domed profile illustrated by the three contour lines 141a, 141b, 141c means that the order will affect the total transit time. So for instance if the sequence of three images is obtained in the order ACB then the probe must travel up from A to C, then back down from C to B. The height information can therefore be used by the transit controller 18 to determine an optimal order in which the sequence of three or more images is obtained. In this case the orders ABC, BAC, CAB and CBA provide the quickest total transit time, whereas ACB or BCA will take longer.

[0119] A sample 2 shown in Figure 19c has a saddle-shaped surface profile illustrated by two sets of contours lines 142a, 142b. The contour lines 142a are the highest and the contour linesl 41 b are the lowest. There are six potential orders in which the sequence of three images could be obtained: ABC, ACB, BAC, BCA, CAB or CBA. The saddle- shaped profile illustrated by the contour lines 142a, 142b means that the order will affect the total transit time. So for instance if the sequence of three images is obtained in the order ACB then the probe must travel up and down from A to C, then back up from C to B. The height information can therefore be used by the transit controller 18 to determine an optimal order in which the sequence of three or more images is obtained. In this case the orders ABC and CBA provide the quickest total transit time.

[0120] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A method of imaging a sample with a scanning probe microscope, the scanning probe microscope comprising: a platform; a sample on the platform; a probe comprising a probe mount, a cantilever extending from the probe mount, and a probe tip extending from the cantilever; a coarse lateral alignment system; a fine lateral actuation system; a coarse Z-actuator comprising a static part and a moving part; and a fine Z-actuator, the method comprising: in a set-up phase, operating the coarse Z-actuator so that the moving part of the coarse Z-actuator moves from a first position to a second position, relative to the static part, and brings the probe towards the sample on the platform; after the set-up phase, obtaining a sequence of two or more images of the sample, each image being obtained in a respective imaging phase by: generating a lateral scanning motion between the probe and the platform with the fine lateral actuation system so that the probe tip interacts with the sample across a respective imaging region, expanding and contracting the fine Z-actuator so that the probe mount moves towards and away from the platform to react to topographic changes of the sample across the imaging region, and monitoring the probe to obtain an image of the imaging region; and in a transit phase between each pair of imaging phases which are adjacent in the sequence, generating a relative transit motion between the probe and the platform, the relative transit motion following a trajectory in which the probe is retracted from the sample by contraction of the fine Z-actuator, laterally aligned with a new imaging region by the coarse lateral alignment system, and returned to the sample at the new imaging region by expansion of the fine Z-actuator; wherein the moving part of the coarse Z-actuator remains fixed in its second position relative to the static part of the coarse Z-actuator for a full duration of each imaging phase and for a full duration of each transit phase.

2. A method according to claim 1 , wherein the coarse lateral alignment system generates a relative lateral motion between the probe and the platform in a lateral drive plane, and the method further comprises making an adjustment of the sample or the platform, the adjustment making the sample more parallel with the lateral drive plane.

3. A method according claim 2, wherein the adjustment is an adjustment of a tip / tilt angle of the platform relative to the lateral drive plane.

4. A method according to claim 3, wherein the platform is mounted on an XY stage, and the tip / tilt angle of the platform is adjusted by moving the platform relative to the XY stage.

5. A method according claim 2, wherein the adjustment comprises a deformation of the sample.

6. A method according claim 2, wherein the adjustment comprises clamping the sample to the platform, the clamping causing the sample to deform and / or move relative to the platform.

7. A method according claim 6, wherein the adjustment comprises clamping the sample to the platform by applying a vacuum or electrostatic force between the sample and the platform.

8. A method according to any preceding claim, further comprising: obtaining height information by measuring heights of an array of test sites of a test item on the platform; and using the height information to check that a range of the fine Z-actuator is sufficient to avoid the probe clashing with the sample as it is laterally aligned with a new imaging region during a transit phase.

9. A method according to claim 8, wherein the scanning probe microscope further comprises a proximity sensor, and the height information is obtained by measuring the heights of the array of test sites of the test item on the platform with the proximity sensor.

10. A method according to claim 9 wherein the moving part of the coarse Z-actuator moves the proximity sensor as it moves from the first position to the second position.

11. A method according to any preceding claim, wherein the scanning probe microscope further comprises a detector (such as an interferometer) and an objective lens, the fine Z-actuator is positioned between the objective lens and the sample, and monitoring the probe to obtain an image of the imaging region comprises: illuminating the probe with a detection beam via the objective lens, wherein the detection beam is reflected by the probe to generate a reflected beam; collecting the reflected beam from the probe with the objective lens; directing the reflected beam from the objective lens to the detector; and monitoring the reflected beam with the detector.

12. A method according to claim 11 , wherein the moving part of the coarse Z-actuator moves the objective lens as it moves from the first position to the second position.

13. A method according to claim 11 or 12, wherein the objective lens has a working distance which is less than 30 mm.

14. A method according to any preceding claim, wherein the fine Z-actuator comprises a piezoelectric actuator.

15. A method according to any preceding claim, wherein for each imaging region the fine Z-actuator expands and contracts over a respective imaging range to react to topographic changes of the sample across the imaging region, and during each transit phase the fine Z-actuator contracts outside the imaging range of the immediately preceding imaging phase.

16. A method according to any preceding claim, wherein the lateral scanning motion moves the probe.

17. A method according to any preceding claim, further comprising: in the set-up phase, operating the coarse lateral alignment system to generate a lateral registration motion between the probe and the sample so that the probe becomes laterally aligned with a first imaging region of the sample, wherein the first image of the sequence is an image of the first imaging region.

18. A method according to claim 17, wherein the probe crosses a peripheral edge of the sample during the lateral registration motion.

19. A method according to any preceding claim, wherein the sample comprises a semiconductor wafer.

20. A method according to any preceding claim, wherein the fine Z-actuator is contracted to a limit of its range as it retracts the probe away from the sample.

21. A scanning probe microscope comprising: a platform; a probe comprising a probe mount, a cantilever extending from the probe mount, and a probe tip extending from the cantilever; a coarse lateral alignment system; a fine lateral actuation system; a coarse Z-actuator comprising a static part and a moving part; a fine Z- actuator; and a control system configured to operate the scanning probe microscope to image a sample by a method according to any preceding claim.

22. A method of imaging a sample with a scanning probe microscope, the scanning probe microscope comprising: a platform; a probe comprising a probe mount, a cantilever extending from the probe mount, and a probe tip extending from the cantilever; a lateral alignment system; a lateral actuation system; and a Z-actuation system, the method comprising: obtaining height information by measuring heights of an array of test sites of a test item on the platform;obtaining a sequence of two or more images of a sample on the platform, each image being obtained in a respective imaging phase by: generating a lateral scanning motion between the probe and the platform with the lateral actuation system so that the probe tip interacts with the sample across a respective imaging region, operating the Z-actuation system so that the probe moves towards and away from the platform to react to topographic changes of the sample across the imaging region, and monitoring the probe to obtain an image of the imaging region; and in a transit phase between each pair of imaging phases which are adjacent in the sequence, generating a relative transit motion between the probe and the platform, the relative transit motion following a trajectory in which the probe is retracted from the sample by the Z-actuation system, laterally aligned with a new imaging region by the lateral actuation system, and returned to the sample at the new imaging region by the Z-actuation system; wherein the height information is used to determine the trajectory of the relative transit motion of at least one of the transit phases.

23. A method of imaging a sample with a scanning probe microscope, the scanning probe microscope comprising: a platform; a probe comprising a probe mount, a cantilever extending from the probe mount, and a probe tip extending from the cantilever; a lateral alignment system; a lateral actuation system; and a Z-actuation system, the method comprising: obtaining height information by measuring heights of an array of test sites of the test item on the platform; obtaining a sequence of three or more images of a sample on the platform, each image being obtained in a respective imaging phase by: generating a lateral scanning motion between the probe and the platform with the lateral actuation system so that the probe tip interacts with the sample across a respective imaging region, operating the Z-actuation system so that the probe moves towards and away from the platform to react to topographic changes of the sample across the imaging region, and monitoring the probe to obtain an image of the imaging region; and in a transit phase between each pair of imaging phases which are adjacent in the sequence, generating a relative transit motion between the probe and the platform, the relative transit motion following a trajectory in which the probe is retracted from the sample by the Z-actuation system, laterally aligned with a new imaging regionby the lateral actuation system, and returned to the sample at the new imaging region by the Z-actuation system; wherein the height information is used to determine an order in which the sequence of three or more images is obtained.

24. A method according to claim 22 or 23, wherein the height information is obtained by: generating a lateral mapping motion between a proximity sensor and the test item so that the proximity sensor becomes sequentially aligned with the test sites; and at each test site, operating the proximity sensor to obtain the height information by measuring a proximity of the test item.

25. A method according to claim 24, wherein at least a component of the relative transit motion is driven by a lateral actuator of the lateral alignment system; and at least a component of the lateral mapping motion is driven by the same lateral actuator.

26. A method according to any preceding method claim, wherein the platform comprises a ceramic material which contacts the sample.

27. A method according to claim 26, wherein the ceramic material is silicon carbide.

28. A scanning probe microscope according to claim 21, wherein the platform comprises a ceramic material which is configured to contact the sample.

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