Scanning probe microscope

Abstract

A scanning probe microscope comprising: a sample stage configured to carry a sample; a probe stage configured to carry a probe; an objective lens with an objective lens focal plane; and an interferometer configured to direct a detection light beam onto the objective lens The objective lens is configured to focus the detection light beam onto the objective lens focal plane and the interferometer is configured to receive a return light beam returning from the objective lens focal plane via the objective lens, combine the return light beam with a reference light beam to produce interference, and measure the interference. An image sensor is provided; and an imaging lens configured to receive image light from a vision focal plane via the objective lens and focus the image light onto the image sensor. The vision focal plane is offset from the objective lens focal plane and the objective lens focal plane is positioned between the objective lens and the vision focal plane.

Description

[0001] SCANNING PROBE MICROSCOPE

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a scanning probe microscope, and associated method.

[0004] BACKGROUND OF THE INVENTION

[0005] When interacting with a sample using a scanning probe microscope, it is often important to know the location of the probe relative to features of the sample so that a user can navigate to a known position of the sample to perform a scan. During a typical measurement, the sample is loaded onto a support structure and positioned by aligning a point on the sample with a point on a sample stage. For example, the sample may be a wafer with a distinct notch which can be manually aligned with the support structure. This form of alignment can position the sample with only limited accuracy.

[0006] A known optical beam positioning unit is disclosed in US2016 / 0313368 Al. An optical light beam positioning system enables the combination of two or more light beams of different wavelengths to be focused onto a probe or sample of a scientific instrument, such as an atomic force microscope, for a number of specific uses typical to AFMs, like measuring the deflection or oscillation of the probe and illuminating an object for optical imaging, and less traditional ones like photothermal excitation of the probe, photothermal activated changes in the sample, photothermal cleaning of the probe and photochemical, photovoltaic, photothermal and other light beam induced changes in the sample. The focused light beams may be independently positioned relative to each other.

[0007] SUMMARY OF THE INVENTION

[0008] A first aspect of the invention provides a scanning probe microscope according to claim 1.

[0009] Optionally an offset between the objective lens focal plane and the detection focal plane is either zero, or less than the offset between the objective lens focal plane and the vision focal plane.

[0010] Optionally an offset between the objective lens focal plane and the detection focal plane is either zero, or less than 1 pm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm. Optionally the objective lens focal plane and the detection focal plane are substantially coplanar.

[0011] Optionally the scanning probe microscope further comprises: a probe attached to the probe stage, wherein the probe is at the detection focal plane, the return light beam is reflected from the probe, and the measurement of the interference provides an indication of a height of the probe.

[0012] Optionally the probe stage and / or the probe appears in a field of view of the image sensor.

[0013] Optionally the probe stage and / or the probe appears off centre in a field of view of the image sensor.

[0014] Optionally the interferometer is configured so that the return light beam and the reference light beam are each substantially collimated, and optionally parallel with each other, where they combine to produce the interference.

[0015] Optionally the scanning probe microscope further comprises: an actuation system configured to drive the probe towards and away from the sample surface.

[0016] Optionally the offset between the vision focal plane and the detection focal plane is greater than 2 pm, or greater than 5 pm, or greater than 10 pm, or greater than 20 pm.

[0017] Optonally the offset between the vision focal plane and the objective lens focal plane is greater than 2 pm, or greater than 5 pm, or greater than 10 pm, or greater than 20 pm.

[0018] Optionally the offset between the vision focal plane and the detection focal plane is less than 100 pm or less than 50 pm.

[0019] Optionally the offset between the vision focal plane and the objective lens focal plane is less than 100 pm or less than 50 pm.

[0020] Optionally the scanning probe microscope further comprises a scanning system configured to generate a relative scanning motion between the probe stage and the sample stage, in a direction substantially perpendicular to an optical axis of the objective lens. Optionally the interferometer comprises a light source for generating the detection light beam.

[0021] Optionally the scanning probe microscope further comprises an illumination system comprising an illumination light source configured to illuminate a sample in the vision focal plane via the objective lens, optionally by Kohler illumination.

[0022] Optionally the scanning probe microscope further comprises a drive system configured to generate a relative drive motion between the probe stage and the sample stage in a direction substantially perpendicular to the optical axis of the objective lens, wherein a range of motion of the drive system is greater than a range of motion of the scanning system.

[0023] Optionally the image sensor comprises a sensor array with an aspect ratio greater than 1 or greater than 1.5.

[0024] A further aspect of the invention provides a method of operating a scanning probe microscope according to claim 15.

[0025] The marker typically lies in the vision focal plane and hence it is in focus in the image of the marker. A surface of the probe typically lies in the detection focal plane, and not in the vision focal plane. An unfocussed image of the probe may lie in the field of view of the image of the marker. Alternatively the probe may not lie in the field of view of the image of the marker.

[0026] Optionally an offset between the objective lens focal plane and the detection focal plane is either zero, or less than the offset between the objective lens focal plane and the vision focal plane.

[0027] Optionally an offset between the objective lens focal plane and the detection focal plane is either zero, or less than 1 pm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm.

[0028] Optionally the objective lens focal plane and the detection focal plane are substantially coplanar. Optionally the identified location of the marker indicates an offset of the marker from an expected location in the field of view of the image.

[0029] Optionally the marker is closer to a centre of the sample than an edge of the sample.

[0030] Optionally the sample further comprises a second marker and the method further comprises: acquiring an image of the second marker using the optical vision system which receives light from the vision focal plane via the objective lens and the imaging lens; analysing the image of the second marker to identify a location of the second marker within a field of view of the image, and thereby improve the estimate of the unknown placement error.

[0031] Optionally the second marker is closer to an edge of the sample than a centre of the sample.

[0032] Optionally the marker is a fiducial marker.

[0033] Optionally the return light beam and the reference light beam are each substantially collimated, and optionally parallel with each other, where they combine to produce the interference.

[0034] Optionally the method further comprises: obtaining a topographical image based on the height of the probe determined by the interferometer as the probe interacts with the sample.

[0035] BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Embodiments of the invention will now be described, strictly by way of example only, with reference to the accompanying drawings, in which:

[0037] Figure 1 shows various parts of a scanning probe microscope according to an aspect of the invention;

[0038] Figure 2 shows other parts of the scanning probe microscope of Figure 1;

[0039] Figure 3 shows the offset between the vision focal plane and the objective lens focal plane;

[0040] Figure 4 shows details of the detection system; Figure 5a shows a preferred, infinity-corrected, optical arrangement of the detection system;

[0041] Figure 5b shows an optical arrangement in which the detection system is not perfectly infinity-corrected;

[0042] Figure 6 shows the non-infinity-corrected optical arrangement of the optical imaging system;

[0043] Figure 7 shows a sample;

[0044] Figure 8 shows an image of a first fiducial marker at a centre of the sample;

[0045] Figure 9 shows an image of the first fiducial marker after calibration of the drive system;

[0046] Figure 10 shows an image of a second fiducial marker; and

[0047] Figure 11 shows a method of operating the scanning probe microseopce.

[0048] DETAILED DESCRIPTION

[0049] A scanning probe microscope 1 according to an embodiment of the invention is shown in Figures 1-3. Certain elements of the microscope 1 are shown in all three Figures, and other elements are shown in only one or two Figures.

[0050] The scanning probe microscope 1 comprises a probe stage 4 configured to carry a probe 2, 3; an objective lens 32 shown in Figures 2 and 3; and a sample stage I la.

[0051] The objective lens 32 has an objective lens focal plane 60 shown in Figures 3, 5a, 5b and 6. The objective lens focal plane 60 is defined in a conventional manner: it is a plane where parallel light rays focused by the objective lens 32 meet at a point.

[0052] Figures 1 and 3 show the microscope 1 in use, with a probe 2, 3 attached to the probe stage 4 and a sample 7 mounted on the sample stage Ila, and Figure 2 shows various elements of the microscope 1 as it is supplied to an end user, before the probe and sample have been introduced. The scanning probe microscope 1 may be used to measure properties of the sample 7 (eg, material properties such as its surface topology, elasticity or composition). Alternatively, the scanning probe microscope 1 may be used to remove or deposit material on the sample 7, or to modify the sample in some other way.

[0053] As shown in Figure 1, the probe comprises a cantilever 2 and probe tip 3. The cantilever

[0054] 2 is carried by the probe stage 4 and extends from a proximal end at the probe stage to a distal end remote from the probe stage. The probe tip 3 is at the distal end of the cantilever 2.

[0055] The probe tip 3 comprises a conical or pyramidal structure that tapers from its base to a point at its distal end that is its closest point of interaction with a sample 7 on the sample stage Ila. The cantilever 2 may comprise a single beam with a rectangular profile. The cantilever 2 may have a length of about 20 micron, a width of about 10 micron, and a thickness of about 200nm.

[0056] In this example the probe tip 3 tapers to a point, but in other embodiments the probe tip

[0057] 3 may be specially adapted for measuring sidewalls. For instance the probe tip 3 may have a flared shape.

[0058] The cantilever 2 is a thermal bimorph structure composed of two (or more) materials, with differing thermal expansion coefficients - typically a silicon or silicon nitride base with a gold or aluminium coating. The coating extends the length of the cantilever and covers the reverse side from the tip 3. An actuation system (in the form of a laser 30) under the control of a photothermal (PT) drive signal 33 is arranged to illuminate the cantilever on its upper coated side with an intensity-modulated radiation spot.

[0059] The actuation system 30 is configured drive the probe towards and away from the sample surface, in this case by bending and unbending the cantilever 2.

[0060] The cantilever 2 may be formed from a monolithic structure with uniform thickness. For example the monolithic structure may be formed by selectively etching a thin film of SiC>2 or SiN4as described in Albrecht T., Akamine, S., Carver, T.E., Quate, C.F. J., Microfabrication of cantilever styli for the atomic force microscope, Vac. Sci. Technol. A 1990, 8, 3386 (hereinafter referred to as "Albrecht et al."). The tip 3 may be formed integrally with the cantilever, as described in Albrecht et al., it may be formed by an additive process such as electron beam deposition, or it may be formed separately and attached by adhesive or some other attachment method. The wavelength of the collimated actuation beam 35 output by the laser 30 is selected for good absorption by the coating, so that the cantilever 2 bends along its length and moves the probe tip 3. A suitable wavelength is 808nm although other wavelengths may be used. The actuation beam 35 is reflected towards the sample by a hot mirror 36 shown in Figure 2 which reflects the infrared light of the actuation beam 35 but allows visible light to pass.

[0061] In this example the cantilever coating is on the reverse side from the sample so the cantilever 2 bends down towards the sample when heated, but alternatively the coating may be on the same side as the sample so the cantilever 2 bends away from the sample when heated.

[0062] A height actuation system is configured to adjust a height of the proximal end of the cantilever 2 by moving the probe stage 4 vertically. The height actuation system may comprise a piezoelectric actuator 14, shown in Figures 1-3, driven by a piezo drive signal 5 shown in Figure 1.

[0063] A probe scanning system 13 shown in Figures 2 and 3, such as a pair of piezoelectric actuators, is configured to generate a relative scanning motion between the probe stage 4 and the sample stage Ila in a horizontal (XY) plane which is substantially perpendicular to an optical axis of the objective lens 32. In this case the relative scanning motion is achieved by motion of the probe stage 4, but in other embodiments the relative scanning motion may be achieved by motion of the sample stage Ila.

[0064] A sample stage drive system 15 shown in Figures 2 and 3 is configured to drive the sample stage Ila in a horizontal (XY) plane which is substantially perpendicular to the optical axis of the objective lens 32. In this case the drive motion is achieved by motion of the sample stage Ila, but in other embodiments the drive motion may achieved by motion of the probe stage 4. The range of motion of the sample stage drive system 15 is higher than the range of motion of the probe scanning system 13.

[0065] A detection system 80 is configured to direct a collimated detection light beam 103 shown in Figures 2 and 4 onto the objective lens 32 via a dichroic beam splitter 37 and a tip track mirror 26.

[0066] The objective lens 32 is configured to focus the collimated detection light beam 103 onto the objective lens focal plane 60 and the detection system 80 is configured to receive a collimated return light beam 31 returning from the objective lens focal plane 60 via the objective lens 32.

[0067] The detection system 80 is arranged to detect a height and angle of the free end of the cantilever 2 directly opposite to the probe tip 3. The detection system 80 includes an interferometer 81 configured to determine a height of the probe, and a quadrant photodiode (QPD) 108 configured to determine an angle of the probe.

[0068] Figures 1 and 2 show the detection system 80 schematically and Figure 4 gives a more detailed view. As shown in Figure 2, the detection system 80 is configured to receive the reflected return light beam 31 returning from the objective lens focal plane via the objective lens 32, the tracking mirror 26 and the dichroic beam splitter 37. As shown in Figure 4, the interferometer 81 combines the return light beam 31 with a reference light beam 104 to produce interference, and measures the interference with photodetectors 121, 122.

[0069] Collimated light 100 from a laser 101 is split by a beam splitter 102 into the detection light beam 103 and the reference light beam 104. The reference light beam 104 is directed onto a suitably positioned retro-reflector 120 and thereafter back to the beam splitter 102. The retro-reflector 120 is aligned such that it provides a fixed optical path length relative to the vertical (Z) position of the sample 7. The beam splitter 102 has an energy absorbing coating and splits both the detection light beam 103 and the reference light 104 beam to produce first and second interferograms with a relative phase shift of 90 degrees. The two interfero rams are detected respectively at the photodetectors 121, 122.

[0070] Ideally, the outputs from the photodetectors 121, 122 are complementary sine and cosine signals with a phase difference of 90 degrees. Further, they should have no de offset, have equal amplitudes and only depend on the position of the cantilever and wavelength of the laser 101. Known methods are used to monitor the outputs of the photodetectors 121, 122 while changing the optical path difference in order to determine and to apply corrections for errors arising as a result of the two photodetector outputs not being perfectly harmonic, with equal amplitude and in phase quadrature. Similarly, de offset levels are also corrected in accordance with methods known in the art.

[0071] These photodetector outputs are suitable for use with a conventional interferometer reversible fringe counting apparatus and fringe subdividing apparatus 123, which may be provided as dedicated hardware, FPGA, DSP or as a programmed computer. Phase quadrature fringe counting apparatus is capable of measuring displacements in the position of the cantilever to an accuracy of A / 8. That is, to 66 nm for 532 nm light.

[0072] Known fringe subdividing techniques, based on the arc tangent of the signals, permit an improvement in accuracy to the nanometre scale or less. In the embodiment described above, the reference light beam 104 is arranged to have a fixed optical path length relative to the Z position of the sample 7. It could accordingly be reflected from the surface of the sample stage Ila or from a retro-reflector whose position is linked to that of the sample stage. The reference path length may be greater than or smaller than the length of the path followed by the detection light beam 103. Alternatively, the relationship between reflector and sample Z position does not have to be fixed. In such an embodiment the reference light beam 104 may be reflected from a fixed point, the fixed point having a known (but varying) relationship with the Z position of the sample. The height of the tip is therefore deduced from the interferometrically measured path difference and the Z position of the sample with respect to the fixed point.

[0073] In the preferred optical arrangement shown in Figure 4, the interferometer 81 is configured so that the return light beam 31 and the reference light beam 104 are each substantially collimated, and optionally parallel with each other, where they combine at the photodetectors 121, 122 to produce the interference.

[0074] In an alternative optical arrangement, which is a less preferred embodiment of the present invention, the return light beam 31 and / or the reference light beam 104 may not be collimated where they combine at the photodetectors 121, 122. For instance the collimated beams 31, 104 may be focused onto the photodetectors 121, 122 by an additional lens.

[0075] The infinity-corrected optical arrangement of Figure 4 is preferred because the collimated beams 31, 104 have planar wavefronts so that the path length of the rays within the beam does not vary across the cross-section of the beam. It also enables the beams 31, 104 to be relatively large in cross-section which enables them to be easily overlapped with each other at the photodetectors 121, 122.

[0076] The reflected return light beam 31 is also split by a beam splitter 106 into first and second components 107, 110. The first component 107 is directed to the segmented quadrant photodiode 108 via a lens 109, and the second component 110 is split by the beam splitter 102 and directed to the photodiodes 121, 122 for generation of the height signal 20. The photodiode 108 generates an angle signal 124 which is indicative of the position of the first component 107 of the reflected return light beam 31 on the photodiode 108 and varies in accordance with the angle of inclination of the cantilever relative to the sensing beam 103.

[0077] The outputs of the detection system 80 are a height signal 20 and an angle signal 124 which each may be monitored to obtain a series of topography measurements indicative of a topography of the sample 7, which can be used to generate a topographical image of the sample 7.

[0078] As the tip track mirror 26 tilts, the detection light beam 103 and the actuation beam 35 deflect and retain their positions on the cantilever 2.

[0079] In taking an image of the sample, the scanning probe microscope 1 may be operated in a number of different modes. In one non-limiting example it may be operated in dynamic mode as follows. The cantilever is set into resonant oscillatory motion by the laser 30 and, using the Z-actuator 14, the probe tip is first brought into intermittent contact with the sample.

[0080] The probe stage 4 is lowered by the piezoelectric actuator 14, moving the probe tip 3 towards the sample whilst, in this embodiment, the detection system 80 monitors the oscillating and time-varying deflection of the cantilever. When the amplitude of deflection oscillation reaches a predetermined level the movement of the probe stage 4 is stopped.

[0081] As the raster scan progresses, the probe tip moves up and down as surface height / interaction force varies. Superimposed on this surface-induced motion is a higher frequency component arising from the oscillatory motion of the cantilever 2. The amplitude, phase and frequency of this oscillatory component of the probe tip's motion will all be dependent on both the oscillator settings and the interaction force between surface and probe tip. Similarly, the height of the probe tip is a superposition of a component arising from interaction with the sample surface and a second component due to probe oscillations. The amplitude, phase and frequency of the second component are affected by the probe tip's position with respect to the surface. As the scan progresses, the amplitude of the AC component of the height measurement signal is monitored.

[0082] It will be appreciated that the height signal 20 contains accurate information relating to the height of the probe tip 3, but this must be related to features of the sample surface in order for useful information to be obtained. In theory, the lowest point of each oscillation cycle represents the true height of the surface. Accordingly the probe microscope may be set up in order to be able to find the lowest height measurement for each period of probe oscillation and this is then output as a topography measurement signal that can be used to form a topographical image. That is, the lowest point in each oscillation cycle is representative of the position of the sample surface.

[0083] In this example, an image is constructed on the basis of the topography measurement and an XY position signal which indicates the current XY position of the probe tip 3. In other embodiments of the invention, rather than generating an image, the scanning probe microscope 1 may use the topography measurement in some other way - for instance to measure a critical dimension of the sample (such as a wall height, wall angle, trench width, etc.). In this case a series of topography measurements may be measured with only a single line scan (for instance across a trench) rather than scanning in two- dimensions to generate a topographical image.

[0084] An optical vision system 50 shown in Figure 2 is configured to generate an image of a small field of view of the sample 7. The optical vision system 50 comprises an image sensor 52, such as a two-dimensional CCD array; and an imaging lens 54 (conventionally known as a tube lens). The imaging lens 54 is configured to receive image light 55 from a vision focal plane 62 (shown in Figure 3) via the objective lens 32 and a periscope 82 and focus the image light 55 onto the CCD array of the image sensor 52 via a right angle prism 84 and a mirror. As shown in Figure 3, the vision focal plane 62 is offset behind the objective lens focal plane 60, and the objective lens focal plane 60 is positioned between the objective lens 32 and the vision focal plane 62. The vision focal plane 62 and the objective lens focal plane 60 are parallel with each other.

[0085] The image light 55 originates at an illumination system 40 which comprises an illumination light source 41 and an illumination system lens 42. Optionally the illumination system 40 illuminates the sample 7 via the objective lens 32 by Kohler illumination in order to generate an even illumination of a large area of the sample.

[0086] The light to the optical vision system 50 and the light from the illumination system 40 each travel via the periscope 82 which comprises a pair of mirrors and a beamsplitter 83.

[0087] As shown in Figures 5a / 5b and 6, the detection system 80 and the optical vision system

[0088] 50 have different optical arrangements: the optical arrangement of the detection system 80 being infinity-corrected (or approximately infinity-corrected) and the optical arrangement of the optical vision system 50 being non-infinity-corrected.

[0089] Figure 5a is a schematic view which demonstrates a preferred infinity-corrected optical arrangement for the detection system 80. The detection light beam 103 is collimated as it enters the objective lens 32 so it is focused onto the top surface of the probe 2 which lies in the objective lens focal plane 60. The reflected return light beam 31 is collimated as it exits the objective lens 32 and is input into the detection system 80. This infinity- corrected optical arrangement is beneficial because it enables the detection system 80 to be positioned at any optical distance from the objective lens. It also enables the return light beam 31 to be collimated where the interferometer 81 combines it with the reference light beam 104, giving the accuracy benefits discussed above.

[0090] Figure 5b shows an alternative optical arrangement where the detection light beam 103 is substantially collimated, but not perfectly collimated, as it enters the objective lens 32. As a result it is focused onto a detection focal plane 61 which is slightly offset from the objective lens focal plane 60. In this case the surface of the probe 2 lies in the detection focal plane 61, although it could lie in the objective lens focal plane 60 or between the planes 60, 61.

[0091] The interferometer is configured to receive the reflected return light beam 31 returning from the detection focal plane 61, and in the case of Figure 5b the reflected return light beam 31 is not perfectly collimated as it exits the objective lens 32 and is input into the detection system 80. Hence in the perfectly infinity-corrected optical arrangement of Figure 5a the offset between the objective lens focal plane 60 and the detection focal plane 61 is zero (in other words the objective lens focal plane 60 and the detection focal plane 61 are substantially coplanar) whereas in the approximately infinity-corrected case of Figure 5b there is a small offset between the objective lens focal plane 60 and the detection focal plane 61.

[0092] As shown in Figure 6 the vision focal plane 62 is offset from the objective lens focal plane 60, and also offset from the detection focal plane 61 in the case of Figure 5b. Consequently the image light 55 which is focused onto the image sensor 52 is not collimated as it exits the objective lens 32. Hence the optical arrangement of the optical vision system 50 is non-infinity-corrected.

[0093] The offset between the vision focal plane 62 and the objective lens focal plane 60 (and the detection focal plane 61, if they are different) may be in the range of 10 pm to 50 m, for example of the order of 30 pm. In more general terms, the offset between the vision focal plane 62 and the objective lens focal plane 60 (and the detection focal plane 61) may be greater than 2 pm, or greater than 5 pm, or greater than 10 pm, or greater than 20 pm. The offset between the vision focal plane 62 and the objective lens focal plane 60 (and the detection focal plane 61) may be less than 100 pm or less than 50 pm.

[0094] Any offset between the objective lens focal plane 60 and the detection focal plane 61 (as shown in Figure 5b) is as small as possible, and less than the offset between the objective lens focal plane 60 and the vision focal plane 60 (as shown in Figure 6). For example any offset between the objective lens focal plane 60 and the detection focal plane 61 may be less than 1 pm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm.

[0095] Figure 5b gives an example in which the objective lens focal plane 60 is offset behind the detection focal plane 61, but in other embodiments the objective lens focal plane 60 may be offset in front of the detection focal plane 61 instead.

[0096] Figure 7 shows an exemplary sample 7 for imaging with the scanning probe microscope 1. The sample 7 may be a circular semiconductor wafer.

[0097] The probe scanning system 13 may be used to perform a raster scan of the sample 7 across a small area (of the order of microns) to obtain an image of a specific sample feature, such as a trench, at a known location on the wafer. Before the raster scan is performed, the sample 7 must be driven by the sample stage drive system 15 so that the probe tip 3 is very accurately positioned at the specific location of the sample feature, typically with an accuracy of the order of 1 or 2 pm. If the sample 7 could be positioned and oriented relative to the sample stage Ila with high accuracy, then driving the probe tip 3 to the specific location of the sample feature would simply be a case of applying appropriate X and Y drive signals to the sample stage drive system 15. However, sufficiently accurate placement of the sample 7 on the sample stage I la may not be possible.

[0098] The sample 7 may comprise an alignment notch 72 which is used to coarsely align the wafer relative to the sample stage Ila at a desired position and orientation. However, the alignment achieved via the alignment notch 72 will only be roughly accurate, so there will be an unknown placement error (X-offset) in the X-position of the sample, an unknown placement error (Y-offset) in the Y-position of the sample, and an unknown placement error (9-offset) in the angular orientation of the sample. The X and Y-offset values may be of the order of hundreds of microns, and the 9-offset value may be of the order of 0.1 degrees.

[0099] Figure 11 shows an exemplary method of operating the scanning probe microscope 1 to account for such placement errors.

[0100] At step 82 the sample 7 is placed onto the sample stage I la with a robotic arm or other device. In a pre-alignment step 83, the sample 7 is approximately aligned using a device which detects the alignment notch 72 and uses this notch 72 as a reference point to align the sample 7 with an accuracy of approximately 100 pm in XY offset and 0.1 degrees in rotational offset.

[0101] In the next step 84, the probe stage 4 is driven by the scanning system so that the probe tip 3 is positioned at a first location 74a at the centre of the sample 7 as shown in Figure 7.

[0102] In the next step 85, an image is acquired by the image sensor 52. As noted above, during step 85 the image sensor 52 is arranged to receive light from the vision focal plane 62 via the objective lens 32 and the imaging lens 54.

[0103] Figure 8 is a schematic view of the image acquired in the first occurrence of step 85.

[0104] The image sensor 52 comprises a sensor array with X rows of pixels and Y columns of pixels. The aspect ratio of the array (Y / X) may be greater than 1 - for instance in a range of 1.5-2 (in this example it is about 1.8). This aspect ratio enables the probe 2 and / or the probe stage 4 to be positioned off-centre in the field of view of the image sensor, as shown in Figure 8.

[0105] By way of example, the width (Dx) and / or the length (Dy) of the field of view may be on the order of 500 pm.

[0106] Note that the probe 2 and probe stage 4 will be unfocussed in the image, because they lie out of the vision focal plane 62, but they are shown in Figure 8 to indicate their positions in the field of view. The major axis of the CCD array may be aligned with the Y-axis of the probe, i.e. it may be aligned with the length of the cantilever as shown in Figure 8. A fiducial marker 65 is carried by the sample 7 and positioned at a dead centre of the sample 7, at the centre of the first location 74a. The fiducial marker 65 lies in the vision focal plane 62, so it is in focus in the image.

[0107] The centre of the field of view of the image sensor 52 is indicated by a pair of crosshairs in Figure 8. Such crosshairs may not be included in the image, and they are shown in Figure 8 to indicate the centre of the field of view which is used as a reference point. In this example the probe tip 3 is accurately positioned with a known X and Y offset relative to the centre of the field of view.

[0108] If the sample 7 was positioned perfectly accurately on the sample stage Ila, then the fiducial marker 65 would be perfectly centred on the centre of the field of view of Figure 8. However, due to the placement errors mentioned above there is an error in the X- direction (X-error) and an error in the Y-direction (Y-error).

[0109] Note that by taking the first image at the centre of the wafer, the fiducial marker 65 will lie in the field of view regardless of the magnitude or sign of the placement error (9- offset) in the angular orientation of the sample.

[0110] At step 86, the image of the fiducial marker 65 is analysed, for instance using an automated optical recognition system, to identify a location of the fiducial marker 65 within the field of view of the image. In this analysis step the centre of the fiducial marker 65 is identified and the error values X-error and Y-error are measured. The error values are then stored in step 87, to record the location of the fiducial marker 65. These error values identify the location of the fiducial marker 65 by indicating an offset of the fiducial marker 65 from an expected location in the field of view of the image, and thereby provide a coarse estimate of the placement errors X-offset and Y-offset.

[0111] In this example the error values (X-error and Y-error) are determined on the basis of a cartesian coordinate system. In other embodiments the offset of the fiducial marker 65 from an expected location in the image may be recorded in a polar coordinate system using error values R-error and 9-error.

[0112] In the next step 88, the error values (X-error and Y-error) are used to calibrate the sample stage drive system 15 with associated translational calibration values (X-cal and Y-cal). Hence the drive system 15 is calibrated on a basis of the coarse estimate of the placement errors X-offset and Y-offset represented by these error values (X-error and Y- error). Optionally, as part of step 88 the calibrated sample stage drive system 15 may also be operated to adjust the position of the probe stage I la (based on X-cal and Y-cal) so the fiducial marker 65 moves to the expected location in the image as shown in Figure 9.

[0113] Note that the fiducial marker 65 is oriented at an oblique angle to the crosshairs due to the placement error (9-offset) in the angular orientation of the sample. This angle is exaggerated in Figure 9 and typically not measured because it is too small.

[0114] In the next repeat of step 84, the sample stage Ila is driven so that the probe tip is positioned at the next location in the sequence 74a-e. Steps 85-88 are then repeated at each location.

[0115] In the first repeat, the probe tip is positioned at a second location 74b only slightly away from the centre of the sample 7, which carries a second fiducial marker 66 shown in Figure 10. This second location 74b is chosen for the first repeat because it is less sensitive to angular mis-orientation of the sample 7 than locations towards the outer circumference of the wafer and hence it can be guaranteed that the second fiducial marker 66 will lie in the field of view, regardless of the magnitude or sign of the placement error (0-offset) in the angular orientation of the sample 7.

[0116] In the first repeat of step 88, the error values (X-error and Y-error) may be used to calibrate the sample stage drive system 15 with an associated angular calibration value (9-cal).

[0117] In the subsequent repeats of step 84, the sample stage is driven so that the probe tip is at peripheral locations 74c, 74d, 74e towards the outer circumference of the wafer, each carrying a respective fiducial marker. At each repeat, the calibration values X-cal, Y-cal, 9-cal are adjusted further based on a more accurate estimation of the unknown placement error.

[0118] Each fiducial marker 65, 66 is closer to a centre of the sample than an edge of the sample. The fiducial markers at the peripheral locations 74c, 74d, 74e are each closer to the edge of the sample than the centre of the sample.

[0119] The process is repeated until the calibration values are sufficiently accurate that the probe tip can be precisely driven to a known location with sufficient accuracy to ensure that a small area covered by a raster scan contains a sample feature of interest. At the end of calibration, the calibration values (X-cal, Y-cal, 9-cal) provide accurate estimations of the unknown errors (X-offset, Y-offset, 9-offset) in the placement of the sample on the stage.

[0120] When the calibration is finished, the calibrated drive system 15 is operated in step 89 to drive the sample 7 so the probe and the sample are positioned with the probe tip 3 aligned with a sample feature at a known location, which typically does not contain a fiducial marker.

[0121] Finally, in step 90 a raster scan of the sample feature is performed, for instance with the microscope operated in dynamic mode as described above, during which the probe tip 3 interacts with the sample feature. During the scanning step 90, the height and / or angle of the probe is determined using the detection system 80 and a topographical image of the sample feature may be obtained.

[0122] In the examples above the probe interacts with the sample to measure some property of the sample (eg, a material property such as its surface topology, elasticity or composition). Alternatively, the probe may be used to remove or deposit material on the sample, or to modify the sample in some other way.

[0123] If the detection system 80 and the optical vision system 50 both had infinity-corrected optical arrangements then it would be necessary to move the objective lens 32 up before step 90 in order to bring the probe into focus for the scanning of step 90.

[0124] As described above, the probe is at the objective lens focal plane 60 which is offset from the imaging focal plane 62. Hence it is not necessary to move the objective lens 32 before step 90 so the process of Figure 11 can be performed more quickly.

Claims

CLAIMS1. A scanning probe microscope comprising : a sample stage configured to carry a sample; a probe stage configured to carry a probe; an objective lens with an objective lens focal plane; an interferometer configured to direct a detection light beam onto the objective lens, wherein the objective lens is configured to focus the detection light beam onto a detection focal plane and the interferometer is configured to receive a return light beam returning from the detection focal plane via the objective lens, combine the return light beam with a reference light beam to produce interference, and measure the interference; an image sensor; and an imaging lens configured to receive image light from a vision focal plane via the objective lens and focus the image light onto the image sensor, wherein the vision focal plane is offset from the objective lens focal plane and offset from the detection focal plane, and wherein the objective lens focal plane and the detection focal plane are positioned between the objective lens and the vision focal plane.

2. The scanning microscope according to claim 1, wherein an offset between the objective lens focal plane and the detection focal plane is either zero, or less than the offset between the objective lens focal plane and the vision focal plane.

3. The scanning microscope according to claim 2, wherein an offset between the objective lens focal plane and the detection focal plane is either zero, or less than 1 pm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm.

4. The scanning microscope according to any preceding claim, wherein the objective lens focal plane and the detection focal plane are substantially coplanar.

5. The scanning microscope according to any preceding claim, further comprising : a probe attached to the probe stage, wherein the probe is at the detection focal plane, the return light beam is reflected from the probe, and the measurement of the interference provides an indication of a height of the probe.

6. The scanning probe microscope according to claim 5, wherein the probe appears off centre in a field of view of the image sensor.

7. The scanning probe microscope according to any preceding claim, wherein the interferometer is configured so that the return light beam and the reference lightbeam are each substantially collimated, and optionally parallel with each other, where they combine to produce the interference.

8. The scanning probe microscope according to any preceding claim, further comprising: an actuation system configured to drive the probe towards and away from the sample surface.

9. The scanning probe microscope according to any preceding claim, wherein the offset between the vision focal plane and the detection focal plane is greater than 2 pm, or greater than 5 pm, or greater than 10 pm, or greater than 20 pm; and the offset between the vision focal plane and the objective lens focal plane is greater than 2 pm, or greater than 5 pm, or greater than 10 pm, or greater than 20 pm.

10. The scanning probe microscope according to any preceding claim, wherein the offset between the vision focal plane and the detection focal plane is less than 100 pm or less than 50 pm; and the offset between the vision focal plane and the objective lens focal plane is less than 100 pm or less than 50 pm.

11. The scanning probe microscope according to any preceding claim, further comprising a scanning system configured to generate a relative scanning motion between the probe stage and the sample stage, in a direction substantially perpendicular to an optical axis of the objective lens.

12. The scanning probe microscope according to claim 11, further comprising a drive system configured to generate a relative drive motion between the probe stage and the sample stage in a direction substantially perpendicular to the optical axis of the objective lens, wherein a range of motion of the drive system is greater than a range of motion of the scanning system.

13. The scanning probe microscope according to any preceding claim, wherein the image sensor comprises a sensor array with an aspect ratio greater than 1 or greater than 1.5.

14. The scanning probe microscope according to any preceding claim, wherein the interferometer comprises a light source for generating the detection light beam; and the scanning probe microscope further comprises an illumination system comprising an illumination light source configured to illuminate a sample in the vision focal plane via the objective lens, optionally by Kohler illumination.

15. A method of operating a scanning probe microscope comprising: placing a sample with a marker onto a sample stage, wherein the sample is placed with an unknown placement error; providing a probe;acquiring an image of the marker using an optical vision system which receives image light from a vision focal plane via an objective lens and an imaging lens, the objective lens having an objective lens focal plane, wherein the imaging lens focusses the image light onto an image sensor, the vision focal plane is offset from the objective lens focal plane, and the objective lens focal plane is positioned between the objective lens and the vision focal plane; analysing the image of the marker to identify a location of the marker within a field of view of the image, and thereby estimate the unknown placement error; calibrating a drive system on a basis of the estimate; operating the calibrated drive system to drive the sample or the probe so the probe is aligned with a sample feature; and determining a height of the probe as it interacts with the sample feature using an interferometer, wherein the interferometer directs a detection light beam onto the objective lens, the detection light beam is focused by the objective lens onto the probe at a detection focal plane, the detection light beam is reflected by the probe to produce a return light beam, and the interferometer receives the return light beam via the objective lens, combines the return light beam with a reference light beam to produce interference, and measures the interference to determine the height of the probe, wherein the vision focal plane is offset from the detection focal plane, and the detection focal plane is positioned between the objective lens and the vision focal plane.

16. The method according to claim 15, wherein an offset between the objective lens focal plane and the detection focal plane is either zero, or less than the offset between the objective lens focal plane and the vision focal plane.

17. The method according to claim 16, wherein an offset between the objective lens focal plane and the detection focal plane is either zero, or less than 1 pm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm.

18. The method according to any of claims 15 to 17, wherein the objective lens focal plane and the detection focal plane are substantially coplanar.

19. The method according to any of claims 15 to 18, wherein the identified location of the marker indicates an offset of the marker from an expected location in the field of view of the image.

20. The method according to any of claims 15 to 19, wherein the marker is closer to a centre of the sample than an edge of the sample.

21. The method according to any of claims 15 to 20, wherein the sample further comprises a second marker and the method further comprises: acquiring an image of the second marker using the optical vision system; analysing the image of the second marker to identify a location of the second marker within a field of view of the image, and thereby improve the estimate of the unknown placement error.

22. The method according to claim 21, wherein the second marker is closer to an edge of the sample than a centre of the sample.

23. The method according to any of claims 15 to 22, wherein the marker is a fiducial marker.

24. The method according to any of claims 15 to 23, wherein the return light beam and the reference light beam are each substantially collimated, and optionally parallel with each other, where they combine to produce the interference.

25. The method according to any of claims 15 to 24, further comprising obtaining a topographical image based on the height of the probe determined by the interferometer as the probe interacts with the sample feature.

Images