Surface imaging method using a scanning probe microscope and corresponding system using the method

Abstract

A surface imaging method using a scanning probe microscope comprising a probe having a cantilever extending from a base support (2) to a free end, a probe tip carried by the free end of the cantilever, and a turning mirror (13); the method comprising: laterally scanning the probe across a surface such that the probe makes a scanning motion across the surface; turning a detection beam through the turning mirror onto the probe, the detection beam reflecting from the probe in the form of a return beam; moving the turning mirror such that the detection beam makes a tracking motion synchronised with the scanning motion and the detection beam is kept turned onto the probe by the turning mirror; obtaining image measurements using the return beam, each image measurement representing a measured height of a respective point on the surface; obtaining a related height error measurement for each point on the surface by comparing a target value for the turning mirror position with an actual position of the turning mirror, each height error measurement representing a respective error in the measured height; and, correcting the image measurements using the height error measurements to generate corrected image measurements.

Description

TECHNICAL FIELD

[0001] The present invention relates to a surface imaging method using a scanning probe microscope and a scanning probe microscope. BACKGROUND

[0002] US2015 / 0020244 discloses a scanning probe microscope. A lens is arranged to receive a light beam and focus the light beam onto a probe. A scanning system varies the angle of incidence of the light beam into the lens relative to the optical axis of the lens over time. The scanning system is typically arranged to move the light beam so as to track movement of the probe, thereby maintaining the focused position of the light beam on the probe. The scanning system can include a beam steering mirror to reflect the light beam towards the lens; and a mirror actuator to rotate the beam steering mirror.

[0003] US2007 / 0195333 discloses an atomic force microscope. A surface shape of a member to be measured is measured by reflecting a measurement light on a reflecting surface of a probe using an atomic force applied between the probe and the member to be measured. In addition to a first scanner for driving the probe, a second scanner for moving a focal position of an optical system is provided. Position conversion data representing a correlation between control amounts of the first scanner and the second scanner is obtained in advance. By driving the first scanner and the second scanner synchronously, the focal position of the optical system is caused to follow the probe, thereby improving measurement accuracy.

[0004] WO2019 / 00870 provides a scanning probe microscope comprising: a first actuator for moving a feature to cause the feature to perform a scanning motion; a steering element; a vision system for capturing light from a field of view to generate image data, wherein the field of view includes the feature and the light from the field of view travels from the feature to the vision system via the steering element; a tracking control system for generating one or more tracking drive signals in dependence on stored reference data; a second actuator for receiving the one or more tracking drive signals and moving the steering element based on the one or more tracking drive signals, thereby causing the field of view to perform a tracking motion synchronised with the scanning motion and causing the feature to remain within the field of view; an image analysis system for analysing the image data from the vision system to identify the feature and measure an apparent motion of the feature relative to the field of view; and, a calibration system for adjusting the stored reference data based on the apparent motion measured by the image analysis system. SUMMARY

[0005] A first aspect of the invention provides a method of surface imaging using a scanning probe microscope, the scanning probe microscope comprising a probe having a cantilever extending from a base to a free end, a probe tip carried by the cantilever free end, and a turning mirror; the method comprising: laterally scanning the probe across a surface such that the probe performs a scanning motion across the surface; turning a detection beam onto the probe via the turning mirror, the detection beam reflecting from the probe in the form of a return beam; moving the turning mirror such that the detection beam performs a tracking motion synchronised with the scanning motion and the detection beam is kept turned onto the probe by the turning mirror; obtaining image measurements using the return beam, each image measurement representing a measured height of a respective point on the surface; obtaining a related height error measurement for each point on the surface, each height error measurement representing a respective error in the measured height; and, correcting the image measurements using the height error measurements to generate corrected image measurements.

[0006] Scanning the probe typically comprises performing the scan to image / analyse a plurality of points on the surface during the scan. The turning mirror can be moved based on one or more scan command signals representing an intended position or orientation of the turning mirror.

[0007] Obtaining the related height error measurement can comprise determining an intended position or orientation of the turning mirror based on the one or more scan command signals; measuring an actual position or orientation of the turning mirror; and, calculating a mirror error measurement representing a difference between the intended position or orientation and the actual position or orientation, wherein the related height error measurement is based on the mirror error measurement.

[0008] Obtaining the related error measurement can further comprise applying a sensitivity parameter to the mirror error measurement.

[0009] Applying the sensitivity parameter to the difference can comprise multiplying or dividing the mirror error measurement by the sensitivity parameter.

[0010] The sensitivity parameter can be determined by: moving the turning mirror by a known amount; determining an observed change in the measured height; and, dividing the known amount by the observed change, or vice versa.

[0011] The image measurements and the related error measurements can be obtained simultaneously.

[0012] A plurality of height error measurements are obtained during the scan and used to correct their related image measurements. Further image measurements can also be made during the scan, which are uncorrected and / or do not have a related height error measurement, but more preferably all image measurements generated during the scan are corrected.

[0013] The step of using the height error measurements to correct the image measurements can be performed in post-processing after the scan has been completed. Alternatively, the image measurements can be corrected in real-time as the scan is being performed.

[0014] The correlated height error measurements can be obtained by filtering the mirror error measurements, for example using a low-pass filter, typically in post-processing after the scan has been completed and all mirror error measurements have been calculated. Alternatively, the filtering can be performed in real-time as the scan is occurring.

[0015] Moving the steering mirror can comprise translating and / or rotating the steering mirror.

[0016] A second aspect of the application provides a scanning probe microscope comprising: a probe having a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever; a scanner for laterally scanning the probe across a surface so that the probe performs a scan motion across the surface; a steering mirror for steering a detection beam onto the probe, the detection beam being reflected from the probe in the form of a return beam; one or more actuators for moving the steering mirror so that the detection beam performs a tracking motion synchronised with the scan motion and the detection beam is kept steered onto the probe by the steering mirror; and a processing system for: using the return beam to obtain an image measurement of a point on the surface, the image measurement representing a measured height of the point on the surface; obtaining a correlated height error measurement of the point on the surface, the height error measurement representing an error in the measured height; and using the height error measurement to correct the image measurement to generate a corrected measurement.

[0017] In one embodiment, the processing system comprises a tracking controller and an image acquisition processor. The processing system can be implemented in software or in dedicated hardware such as one or more digital signal processors.

[0018] Probe microscopes can have a variety of applications including (but not limited to): materials science and biological research, industrial inspection, semiconductor wafer and mask inspection and review; biosensing to detect a variety of biomarkers; nanolithography, for example dip-pen nanolithography in which a scanning probe deposits a compound onto a substrate; or data storage in which the probe has a heater which allows the temperature to be raised independently to melt a polymer substrate, followed by a pressing action by the probe to create indentations representing binary digits.

[0019] Generally, a scanning probe microscope is operable to use a probe to acquire information from a sample. The information acquired from the sample can be topographical information or any other type of information (e.g. chemical and mechanical information about the sample or the surface of the sample). The probe and microscope can further be adapted to measure other sample properties via suitable interaction forces, such as magnetic or electric fields. Alternatively, a scanning probe microscope is operable to manipulate or modify a sample with a probe, for example by removing or adding material (e.g. depositing a compound on the sample), or storing data on the sample. BRIEF DESCRIPTION OF DRAWINGS

[0020] Embodiments of the application will now be described, with reference to the accompanying drawings, in which:

[0021] Figure 1 Parts of the scanning probe microscope used in calibration mode are shown;

[0022] Figure 2 Further details of the scanning probe microscope operating in calibration mode are shown;

[0023] Figure 3 is a view of the second actuator and the steering mirror;

[0024] Figure 4 The field of view of the vision system is shown;

[0025] Figure 5 The field of view of the vision system after movement of the steering mirror and probe holder is shown;

[0026] Figure 6 The detection system is shown in detail;

[0027] Figure 7 is a flow diagram of the calibration process;

[0028] Figure 8 Parts of the scanning probe microscope operating in imaging mode are shown;

[0029] Figure 9 Light sources for the vision system and probe actuation are shown;

[0030] Figure 10 Various other elements used by the microscope when operating in imaging mode are shown;

[0031] Figure 11 The steering mirror tracking the scanning probe in two positions is shown;

[0032] Figure 12 The return beam offset caused by cantilever deflection is shown;

[0033] Figure 13shows return beam offset caused by pure rotation of the turning mirror if the turning mirror is offset from the back focal plane of the objective;

[0034] Figure 14 shows the turning mirror offset from the back focal plane of the objective and translated and rotated to avoid return beam offset;

[0035] Figure 15 shows the objective rotated to turn the detection beam and field of view;

[0036] Figure 16 shows the objective translated to turn the detection beam and field of view;

[0037] Figure 17 shows the objective and turning mirror translated in series to turn the detection beam and field of view;

[0038] Figure 18 shows alternative positions of the turning mirror;

[0039] Figure 19 shows the turning mirror translated in the alternative positions to turn the detection beam and field of view;

[0040] Figure 20 shows an alternative arrangement with a pair of relay lenses that produce an image of the back focal plane of the objective;

[0041] Figure 21 shows multiple elements of an alternative scanning probe microscope in which the detection beam is directed onto a scanning split photodiode;

[0042] Figure 22a and Figure 22b shows a method of operating a scanning probe microscope in an alternative calibration mode;

[0043] Figure 23a shows an array of positions of optical features during a scanning phase in the alternative calibration mode;

[0044] Figure 23b shows an array of apparent positions of optical features observed by a vision system during a scanning phase in the alternative calibration mode;

[0045] Figure 24 shows an array of apparent positions of optical features observed by a vision system during a tracking phase in the alternative calibration mode; and

[0046] Figure 25a and Figure 25b shows a method of operating a scanning probe microscope in another alternative calibration mode. DETAILED DESCRIPTION

[0047] Figure 1 A scanning probe microscope 1 is shown. The microscope includes a probe holder 2 supported by a first actuator 3, which is typically a piezoelectric actuator. The first actuator 3 is used to generate scanning motion in a horizontal (X, Y) plane. The X component of the scanning motion is generated by a scanner drive signal P. x The Y component of the scanning motion is driven by the scanner drive signal P. y Driven. The first actuator 3 can also generate motion in the vertical (Z) direction.

[0048] Microscope 1 can be used without a probe. Figure 1 , Figure 2 and Figure 6 Operate in the calibration mode shown, or when using a probe. Figure 8 and Figure 10 The imaging mode shown is in operation. The calibration mode will be described first.

[0049] Scanner sensor 4, such as an interferometer, capacitive sensor, or LVDT sensor, detects the position of the first actuator 3 to provide a scanner position feedback signal—one of the signals S. x Indicating the sensed X position, another signal S y This indicates the sensed Y position.

[0050] The scanning position system 5 generates scanning command signals X and Y: one of the scanning command signals X represents the expected X position of the first actuator 3, and the other scanning command signal Y represents the expected Y position of the first actuator 3. The scanner controller 6 bases its signals on the scanning command signals X and Y and the scanner position feedback signal S. x S y Generate scanner drive signal P x P y This feedback loop ensures that the first actuator 3 drives the probe holder 2 to the desired position.

[0051] The vision system 10 is arranged to acquire light 11 from the field of view to generate image data. For example, the vision system 10 may include a CCD array. Figure 1 , Figure 2 and Figure 6 The field of view in the calibration mode shown is centered on probe holder 2. Light 11 from the field of view originates from probe holder 2, passes through objective lens 12, steering mirror 13, and... Figure 2 and Figure 9 It is shown in the middle but Figure 1 The various other optical elements omitted in the text travel to the vision system 10.

[0052] The steering mirror 13 is mounted to the second actuator 14. The second actuator 14 is used for tracking drive signal P. A P B, P C The second actuator 14 is configured to move the steering mirror 13 so that the field of view of the vision system 10 performs a tracking motion in synchronism with the scanning motion, and the probe holder 2 remains centered within the field of view.

[0053] The second actuator 14 comprises Figure 2 three piezoelectric actuator legs 14a-c as shown in Fig. 2. As Figure 3 shown, the legs 14a-c are distributed around the center of the steering mirror 13 at 120° intervals. The lengths of the three legs 14a-c can be adjusted together to translate the mirror 13, or at different rates to cause the mirror 13 to rotate about two orthogonal axes.

[0054] Each of the legs 14a-c is extended and retracted based on a respective tracking drive signal. Thus leg 14a is driven by tracking drive signal P A , leg 14b is driven by tracking drive signal P B , and leg 14c is driven by tracking drive signal P C .

[0055] As explained in detail below, the tracking control system 15, 16 is configured to generate the tracking drive signals P A , P B , P C from stored reference data 17 and scanning command signals X, Y. The tracking control system comprises a tracking controller 15 and a transformation system 16.

[0056] Each of the legs 14a-c has an associated strain gauge that generates a strain gauge feedback signal S A , S B , S C . The tracking controller 15 generates the tracking drive signals P A , P B , P C from a set of tracking command signals A, B, C and the strain gauge feedback signals S A , S B , S C . This feedback loop ensures that the second actuator 14 drives the steering mirror 13 to the intended position determined by the tracking command signals A, B, C.

[0057] Alternatively, the feedback signals S A , S B , S C may be provided by interferometers, capacitive sensors or LVDT sensors, rather than by strain gauges.

[0058] The tracking command signals A, B, and C are generated by the transformation system 16, which converts the scan command signals X and Y in the X, Y coordinates of the scan position system 5 into the A, B, C coordinates required to drive the second actuator 14. The transformation system 16 performs this transformation based on reference data 17 stored in memory. This reference data 17 is typically in the form of a reference value matrix.

[0059] Figure 1 The various functional elements of the microscope shown (i.e., elements 5, 6, 15, 16, 20 and 21) can be implemented in computer software or dedicated hardware running on one or more computer processors.

[0060] Figure 4 This is a schematic diagram of the field of view 17 of the vision system 10, in which the probe holder 2 is shown. The probe holder 2 has markings on its upper surface, in... Figure 4 and Figure 5 In Chinese, this mark is represented by a plus sign (+). Figure 1 The image analysis system 20 shown is used to analyze image data from the vision system 10 to identify a marker and measure the apparent motion of the marker relative to the field of view 17. For example, by measuring the marker's position from... Figure 4 The positions x1 and y1 in the middle are transformed to Figure 5 The positions x2 and y2 are used to illustrate the apparent motion relative to the field of view 17. The offset between the two positions is represented by 18. In other words, the marked image has moved from one pixel to another in the CCD array of the vision system 10.

[0061] Back Figure 1 The stored reference data 17 is adjusted by programming the calibration system 21 to minimize the apparent motion measured by the image analysis system 20 when driving the first and second actuators. The reference data 17 is initially based solely on the known geometry of the individual components; therefore, the tracking motion of the field of view of the vision system 10 will not precisely follow the scanning motion of the probe holder 2, resulting in... Figure 4 and Figure 5 The apparent motion of the marker is shown over a relatively large range. The calibration system 21 performs an iterative optimization process, in which it adjusts the reference data 17 until the apparent motion of the marker is reduced to an acceptable amount. Ideally, the apparent motion will be reduced to zero, so that the image of the marker does not change at all within the field of view when scanning the probe holder. (See below for further details.) Figure 7 Describe the details of the iterative optimization process.

[0062] Figure 1 The detection system 26 is shown schematically, and Figure 6The detection system 26 is shown in more detail, and comprises a laser source 27 that generates a detection beam 40c that is turned onto the probe holder 2 by a turning mirror 13. The probe holder 2 reflects the detection beam 40c to generate a return beam 28 shown. Figure 6

[0063] The detection beam 40c is reflected from a fixed mirror 52 to a right-angle block 54. The right-angle block is oriented so that the detection beam 40c is incident perpendicularly onto the entrance face. The detection beam 40c travels to the turning mirror 13 and is reflected to the objective lens 12. As the turning mirror 13 is tilted, the reflected detection beam 40c is rotated so that the angle and point of incidence of the detection beam 40c into the objective lens 12 changes. The angle of the turning mirror 13 is synchronised with the scan pattern made by the probe holder 2 when driven by the first actuator 3, which means that the position of the detection beam 40c on the probe holder 2 is maintained.

[0064] Light from the laser source 27 is split by a beam splitter 102 into the detection beam 40c and a reference beam 29. The return beam 28 is directed into the detection system 26 by a second beam splitter 103. The return beam 28 is then split by a third beam splitter 107 into a first component that falls on a split photodiode 104 and a second component that is directed into an interferometer 105.

[0065] The split photodiode 104 is a position sensitive detector that produces an output 117 dependent on the offset of the first component of the return beam 28 from the split photodiode 104. Since this output 117 represents the deflection angle of the cantilever when the microscope is operating in an imaging mode that will be described later, the output 117 is referred to as the deflection signal 117 hereafter. Typically, the split photodiode 104 is divided into four quadrants, and the ratio between the signals from the four quadrants represents the offset of the first component of the return beam 28 from the split photodiode 104 in the X and Y directions.

[0066] Inside the interferometer 105, a beam splitter 106 splits the second component of the return beam 28. The reference beam 29 is directed onto a retroreflector 108 and then to the beam splitter 106. The retroreflector 108 is calibrated so that it provides a path length that is fixed with respect to the vertical (z) position of the probe holder 2 - in other words, the height of the probe holder 2. The beam splitter 106 has an energy absorbing coating, and the beam splitter 106 splits the return beam 28 and the reference beam 29 to produce first and second interference patterns with ~90° of relative phase shift. These two interference patterns are detected at photodetectors 112, 114 respectively.

[0067] ​Ideally, the photodetector signals are complementary sine and cosine signals with a 90° phase difference. Further, they should have no DC offset, equal amplitudes, and depend only on the position of the cantilever and the wavelength λ of the laser. Known methods are used to monitor the outputs of the photodetectors 112, 114 while varying the optical path difference in order to determine and correct for errors due to the two photodetector signals not being perfect harmonics with equal amplitudes and in phase quadrature. Similarly, the DC offset levels are corrected according to methods known in the art.

[0068] These photodetector signals are suitable for use with conventional interferometer reversible fringe counting and fringe subdivision apparatus, which can be provided as dedicated hardware or as programmed computers. Phase quadrature fringe counting apparatus is capable of measuring displacements of the cantilever position to an accuracy of λ / 8. That is, for light of 532 nm, the accuracy is to 66 nm. Known fringe subdivision techniques based on the signal arctangent are capable of increasing the accuracy to the nanometre scale or less. Interferometric methods of extracting the path difference between the two coherent beams are well known in the art and will not be described in detail. Figure 6 A processor 115 is shown which receives the signals from the photodetectors 112, 114 and performs the above described fringe counting and subdivision to generate an output 116. Since the output 116 represents the height of the probe when the microscope is operating in imaging mode as described in further detail below, the output 116 will be referred to hereafter as the height signal 116.

[0069] As described above, the calibration system 21 is programmed to adjust the stored reference data 17 to minimise changes in the apparent motion measured by the image analysis system 20 - such apparent motion representing errors in the scanning motion imparted to the steering mirror 13. It has been recognised that errors in the scanning motion will also result in changes in the height signal 116 from the interferometer and the deflection signal 117 from the split photodiode 104. Accordingly, the calibration system 21 is also used to adjust the stored reference data 17 to minimise changes in this height signal 116 and / or deflection signal 117.

[0070] Figure 7 The steps of the calibration procedure are shown in detail. In step 202, the optical feature is aligned in the field of view of the vision system 10. For example, with reference to Figure 2 The optical feature can be a marker (+) and the steering mirror 13 is moved until the marker is in the centre of the field of view 17.

[0071] In step 204, the detection system 26 is calibrated so that the interferometer 105 is focused on the surface of the probe holder 2. This is achieved by repeatedly moving the probe holder 2 up and down in the Z direction using the first actuator 3 and monitoring the output of the interferometer 105 as the lens driver 49 (as described above) is moved up and down. The output of the interferometer 105 is monitored by the processor 115 and the position of the lens driver 49 is adjusted until the output of the interferometer 105 is at a minimum. This is shown in Figure 4. The minimum output of the interferometer 105 is achieved when the lens driver 49 is focused on the surface of the probe holder 2. This is the calibration point for the detection system 26. Figure 2The lens driver 49, which is typically a motor, is controlled to move the objective lens 12 in the Z direction gradually (as indicated in the middle) and the amount of change in the intensity of the signals from the photodetectors 112, 114 is measured. This amount of change in intensity is indicative of the level of interferometer contrast and when the interferometer contrast is maximised, the objective lens 12 is in the correct Z position and the interferometer 105 is in focus on the surface of the probe holder 2.

[0072] In step 208, the scanning position system 5 causes the scanning command signals X, Y to change by ΔX and ΔY respectively. This causes the first actuator 3 to cause a step change in the position of the probe holder 2 by (ΔX, ΔY) and the second actuator 14 to cause a step change in the position or orientation of the tracking mirror 13 by (ΔA, ΔB, ΔC). It should be noted that these two step changes (ΔX, ΔY) and (ΔA, ΔB, ΔC) can be made simultaneously or one after the other.

[0073] The tracking command signals (A, B, C) used in step 208 of the calibration mode are generated by transforming the scanning command signals (X, Y) used in step 208 of the calibration mode in accordance with the provisional reference data 17.

[0074] In an ideal situation, the provisional reference data 17 is completely accurate so that the apparent position of the probe holder 2 and any other features moved by the first actuator 3 does not change for the vision system 10. In step 209, the apparent motion of the marker relative to the field of view 17 of the vision system 10 caused by the step changes (ΔX, ΔY) and (ΔA, ΔB, ΔC) is determined by the image analysis system 20. For example, by the marker position changing from x1, y1 in Figure 4 to x2, y2 in Figure 5 The error in the X direction caused by the offset 18 is represented by x(error) in Figure 1 , Figure 5 and Figure 7 The error in the Y direction is represented by y(error) in Figure 1 , Figure 5 and Figure 7 .

[0075] In step 210, the change in the height signal 116 from the interferometer 105 or the change in the deflection angle signal 117 from the split photodiode 104 is determined - this is represented as z(error) in Figure 1 and Figure 7 .

[0076] If in step 212, x(error), y(error) and z(error) are all determined to be below acceptable thresholds, the calibration process ends. If one or more of these errors are too high, new reference data is determined in step 214, and the reference data 17 is adjusted by applying the new reference data in step 216.

[0077] The error values x(error), y(error) for X, Y are used to change the rotational movement of the steering mirror 13, indicated by arrow 75 in Figure 1 ; the error value z(error) for Z is used to change the translational movement of the steering mirror 13, indicated by arrow 76 in Figure 1 .

[0078] Step 208 is then repeated - either repeating the previous step changes (ΔX, ΔY), or reversing them (-ΔX, -ΔY) to return the probe holder 2 to its previous position.

[0079] In the calibration process of Figure 7 , the apparent movement of the probe holder 2 is caused by step changes (ΔX, ΔY) and (ΔA, ΔB, ΔC), which can be performed simultaneously or not, and measurements and reductions of errors x(error), y(error) and z(error) are made. In an alternative calibration process, the scan command signals X, Y can be varied continuously during the calibration process, rather than step-by-step. Snapshots of the probe holder 2 are taken by the vision system 10 and analysed by the image analysis system 20 to determine the apparent movement in the X and Y directions (giving a series of error values x(error), y(error)), and the associated series of z(error) values are obtained from the detection system 26.

[0080] As shown in Figure 7 , when the scanning probe microscope has been calibrated, it can be used in imaging mode, in which case as shown in Figure 8 and Figure 10 , probes 30, 31 are mounted to the probe holder 2. Figure 7 The calibration process of Figure 1 may be performed with probes 30, 31 mounted to the probe holder 2, or without probes 30, 31 mounted to the probe holder 2. If probes are present during the calibration process, the calibration process described above can use the probes 30, 31 rather than the probe holder 2 as the scanning feature observed during the calibration process. As shown in Figure 2 , Figure 6 and Figure 8 , if probes are not present during the calibration process, once the calibration process has been completed, the probes 30, 31 can be mounted to the probe holder 2, and the scanning probe microscope can be used in imaging mode.When the probe is shown mounted to the probe holder, the first actuator 3 is then driven to move the probe 30, 31 to the position previously occupied by the probe holder 2 so that the probe is centred in the field of view 17 of the vision system and so that the detection beam 40c is incident on the probe rather than the probe holder 2 during the imaging mode.

[0081] The probe comprises a cantilever 30 and a tip 31. The tip 31 tapers to a point and is located at the distal end of the cantilever 30. The other (proximal) end of the cantilever is fixed to the probe holder 2. The cantilever 30 is a type of cantilever known as a thermal bimorph. That is, the cantilever 30 is constructed from two (or more) materials having different coefficients of thermal expansion. Typically, it will be a silicon or silicon nitride substrate with a gold or aluminium coating. The coating extends along the length of the cantilever and covers the back of the tip.

[0082] As shown in Figure 1, the microscope also comprises a first laser 22a and a second laser 22b. The first laser 22a is arranged to emit a first beam of intensity modulated radiation 40a and the second laser 22b is arranged to emit a second beam of intensity modulated radiation 40b. The first and second lasers 22a, b are arranged to direct the beams 40a, b towards the cantilever 30. Figure 9 As shown in Figure 1, the microscope also comprises a first laser 22a and a second laser 22b. The first laser 22a is arranged to emit a first beam of intensity modulated radiation 40a and the second laser 22b is arranged to emit a second beam of intensity modulated radiation 40b. The first and second lasers 22a, b are arranged to direct the beams 40a, b towards the cantilever 30. Figure 8 As shown in Figure 1, the microscope also comprises a first laser 22a and a second laser 22b. The first laser 22a is arranged to emit a first beam of intensity modulated radiation 40a and the second laser 22b is arranged to emit a second beam of intensity modulated radiation 40b. The first and second lasers 22a, b are arranged to direct the beams 40a, b towards the cantilever 30.

[0083] The cantilever 30 is a thermal bimorph structure, the materials of which expand differentially when heated. In one embodiment, the cantilever 30 is made of silicon nitride with an aluminium coating. The first and second lasers 22a, b emit one or more wavelengths of light at which there is a maximum or peak in the absorption spectrum of the particular coating. For example, the wavelength can be about 810 nm, around the absorption peak of aluminium. Other coating / wavelength combinations can be used, for example gold has a higher absorption for light of less than 500 nm. When such light is incident on the coated side of the cantilever 30, the aluminium expands more than the silicon nitride, causing the cantilever to bend, thereby causing the tip to move downwards towards the surface of the sample 32. If the intensity of the illumination is increased, the tip 31 is thereby brought closer to the sample 32. Conversely, if the intensity is decreased, the degree of bending is reduced, thereby causing the tip 31 to move away from the sample. Clearly, other arrangements of coatings and substrates can produce varying degrees of bending in the same or opposite directions in response to illumination.

[0084] The microscope also has a lens driver 49 (typically a motor) attached to the lens 12 which can move the lens 12 in the Z direction as well as the X and Y directions.

[0085] The beam 40a from the first laser 22a is horizontally polarised and the beam 40b from the second laser 22b is vertically polarised. The output intensity from both lasers 22a, b is independently controlled.

[0086] The polarizing beam combiner 42 is arranged so that the polarized light 40a, 40b from the two lasers 22a, b is incident at a respective face and is directed to exit at a single output face. In particular, the beam combiner 42 has an internal mirror arranged at 45° to the light beam 40a which reflects the S-polarized component of the light beam 40b while transmitting the P-polarized component of the light beam 40a.

[0087] The polarization of the light beams 40b as S-polarized and 40a as P-polarized is not to be taken as limiting. Either polarization can be used for either light beam, or indeed circular polarization. It is only required that they are different polarizations to enable them to be combined in the beam combiner 42.

[0088] The first motorized mirror 44a can be steered by the mirror actuator 44c about two orthogonal axes of rotation to ensure that the light 40b from the second laser 22b enters the beam combiner 42 at the required angle and position. Thus, the first mirror 44a can be adjusted so that the laser output beams 40a, 40b exit the combiner substantially parallel or at a small angle difference - the angle difference being more pronounced in Figure 8 .

[0089] The second motorized mirror 44b is oriented to direct the combined actuation light beams 40a, 40b to the mirror 52. The angle of the second mirror 44b can be adjusted by the mirror actuator 44d about two orthogonal axes so that the laser output beams 40a, b enter the tracking system at a small angle difference with respect to the detection light beam 40c so that the three light beams 40a, b, c fall on the mirror 52 at a small angle difference. As the first actuator 3 scans the probes 30, 31 across the surface of the sample 32, the steering mirror 13 is similarly scanned to ensure that the three light beams 40a, b, c follow the probe xy position and still maintain their relative displacement.

[0090] After being reflected by the steering mirror 13, the light beams 40a, b, c are simultaneously focused by the objective lens 12 to the back of the cantilever 30. As the light beams 40a, b, c enter the lens 12 at different angles, they are focused at positions on the cantilever 30 which are displaced laterally by a corresponding amount.

[0091] It will be appreciated by the skilled person that the system can be adapted to allow four, five or more beams of light to track a moving cantilever.

[0092] Figure 10Various other elements of the microscope 1 for the imaging mode are shown. The output of the detection system 26, i.e. the height signal 116 and / or the deflection angle signal 117, is analyzed by a processor 28 to extract information about the probe oscillation amplitude. The processor 28 can alternatively be operated to extract other parameters representative of the probe position, e.g. the deflection amount or the phase. In response to the measured oscillation amplitude, a feedback controller 30 outputs a feedback signal 31 for instructing an adjustment of the probe height Pz in order to maintain a constant oscillation amplitude while adjusting the probe (sample separation). This adjustment of the probe position is hereinafter referred to as z-position feedback. An image acquisition processor 33 receives the feedback signal 31 from the feedback controller 30, which represents information about the sample surface obtained by the microscope 1, which is used to construct a map of the sample 32 together with information of the xy-scan pattern. Typically, the feedback signal and thus the map provide height information about the sample surface.

[0093] Figures 11 to 13 is a schematic diagram showing that it is desired to arrange the rotation axis of the turning mirror 13 at the focal point of the objective lens 12 and in the plane of the turning mirror 13. It should be noted that, for simplification of the drawing, only the detection beam 40c is shown in Figures 11 to 20 . However, the probe can also be illuminated with an actuation beam as shown in Figure 8 and light from a vision light source 60 as shown in Figure 9 .

[0094] In Figures 11 to 13 , the cantilever 30 is located in the front focal plane 50 of the lens 12. In Figure 11 and Figure 12 , the mirror 13 and its rotation axis are both located at the back focal point of the lens 12, i.e. the point where the optical axis 51 of the lens 12 intersects with its back focal plane 52.

[0095] In Figure 11 it can be seen that if the detection beam 40c is coaxial with the optical axis 51 of the lens 12 and impinges the reflective upper surface of the cantilever 30 at a right angle, the reflected return beam follows the same optical path 80 back as it did before being reflected by the mirror 13. As indicated by the dashed line in Figure 11 , if the turning mirror 13 is rotated to track the probe such that the detection beam 40c is no longer coaxial with the optical axis of the lens 12 or no longer impinges the reflective upper surface of the cantilever 30 at a right angle, the return beam remains coaxial with the detection beam after being reflected by the turning mirror 13. Thus in Figure 11 , in order to track the probe 30, 31, the probe holder 2 or any other feature that is moved in a scan motion, the mirror 13 can be moved by pure rotation (no translation).

[0096] Figure 12 is shown in Figure 11The same optical arrangement, but viewed from a different direction, so the cantilever 30 is viewed from the side. The probe is shown in dotted lines and the cantilever 30 in solid lines in an elevated position in the front focal plane 50 of the lens 12 after the cantilever 30 has been deflected downwards by the heating of the cantilever 30 by the actuation beam 40a, b. As a result of the deflection of the cantilever 30, the detection beam 40c is no longer at right angles to the reflective upper surface of the cantilever 30, so the return beam 28 is no longer coaxial with the detection beam 40c and there is a misalignment 55 between the return beam 28 and the detection beam 40c. The deflection signal 117 from the split photodiode 104 measures this misalignment 55.

[0097] As mentioned above, in Figure 11 and Figure 12 the mirror 13 and its axis of rotation are both located at the back focal point of the lens 12 (i.e. the point at which the optical axis 51 of the lens 12 intersects its back focal plane 52). In Figure 13 the case, the mirror and its axis of rotation are offset from the back focal plane 52 by a mirror offset distance 53.

[0098] It can be seen from Figure 13 that if the detection beam is coaxial with the optical axis of the lens 12 and impinges on the probe at right angles, the reflected beam returns along the same path 80 before and after reflection by the mirror 13. However, if the mirror 13 is rotated to the position shown in dotted lines to track the movement of the probe, the return beam 28 is offset from the path 80 by a return beam misalignment 56, despite the deflection angle of the cantilever 30 not changing. The return beam misalignment 56 is undesirable because it causes an error in the deflection signal 117. In other words, the return beam misalignment 56 gives a false indication that the deflection angle of the cantilever 30 has changed, when in fact it has not.

[0099] In some cases, the construction of the objective lens 12 or other spatial constraints can make it impossible to place the mirror 13 in the back focal plane 52 of the objective lens 12. In such cases, as shown in Figure 14 the mirror 13 can be translated and rotated by the second actuator 14 so that the return beam does not drift from the path 80 as a result of the movement of the mirror 13.

[0100] In the above embodiments, the beam steering is achieved by movement of the steering mirror 13. Figure 15 and Figure 16 shows an alternative arrangement in which the steering mirror 13 is replaced by a fixed mirror 302 which is held stationary, and the beam steering is instead achieved by movement of the objective lens 12.

[0101] Figure 15An arrangement is shown in which beam steering is achieved by rotation of objective lens 12. The probe light beam 40c is directed along path 80. When lens 12 is in the position shown by the solid line, the optical axis 51 of lens 12 is in line with path 80 and at right angles to the reflective upper surface of cantilever 30, and then the return light beam follows the same path 80 after being reflected by mirror 302. When the probe is moved to the right hand position, lens 12 is rotated to the position shown by the dashed line so that probe light beam 40c continues to be steered by lens 12 onto cantilever 30. This arrangement suffers from the same problem as the arrangement of Figure 13 , namely that there is a return light beam offset 56.

[0102] Figure 16 An arrangement is shown in which beam steering is achieved by translation of objective lens 12. The probe light beam 40c is directed along path 80. When lens 12 is in the position shown by the solid line, the optical axis 51 of lens 12 is in line with path 80 and then the return light beam follows the same path 80 after being reflected by mirror 13. When the probe is moved to the right hand position, lens 12 is translated to the position shown by the dashed line so that probe light beam 40c continues to be steered by lens 12 onto cantilever 30. This arrangement also suffers from the same problem as the arrangement of Figure 13 , namely that there is a return light beam offset 56.

[0103] Figure 17 An arrangement is shown in which beam steering is achieved by translation of objective lens 12 and steering mirror 13. The probe light beam 40c is directed along path 80. When lens 12 is in the position shown by the solid line, the optical axis 51 of lens 12 is in line with path 80 and then the return light beam follows the same path 80 after being reflected by mirror 13. When the probe is moved to the right hand position by a distance ΔX, both lens 12 and mirror 13 are translated by the same distance ΔX to the position shown by the dashed line so that probe light beam 40c continues to be steered by lens 12 onto cantilever 30. Unlike Figure 15 and Figure 16 , there is no return light beam offset 56 here, so that after being reflected by steering mirror 13, the return light beam follows the same path 80 as the probe light beam. A second steering mirror (not shown) is also provided which directs the probe light beam to steering mirror 13 and also receives the return light beam reflected from steering mirror 13. This second steering mirror can be translated in the Y direction in order to track movement of the cantilever in the Y direction (into and out of the plane of Figure 17

[0104] Figure 18 ​An arrangement is shown in which beam steering is achieved by rotation of a beam steering mirror 13a located in the optical path between the probes 30, 31 and the objective 12. The detection beam 40c is directed onto the mirror 13a by the objective 12 along a path 81. When the mirror 13a is in the position shown by the solid line, the return beam follows the same path 81 after being reflected by the mirror 13a. When the probe is moved to the right hand position, the mirror 13a is rotated to the position shown by the dashed line so that the detection beam 40c continues to be steered by the mirror 13a onto the cantilever 30. This arrangement suffers from the same problem as the arrangement of Figure 13 , namely there is a return beam offset.

[0105] Figure 19 An arrangement is shown in which beam steering is achieved by translation of a beam steering mirror 13a between the probes 30, 31 and the objective 12. When the probe is moved to the right hand position, the mirror 13a is translated to the position shown by the dashed line so that the detection beam 40c continues to be steered by the mirror 13a onto the cantilever 30. Unlike Figure 15 and Figure 16 , there is no return beam offset here so that after being reflected by the steering mirror 13a, the return beam follows the same path 81 as the detection beam. A second steering mirror (not shown) is also provided which directs the detection beam to the steering mirror 13a and also receives the return beam reflected from the steering mirror 13a. This second steering mirror can be translated in the Y direction in order to track the movement of the cantilever in the Y direction (into and out of the plane of Figure 19

[0106] Figure 20 An improvement to the microscope 1 is shown in the case where the back focal plane 52 of the objective is not available. Apart from the objective 12 being replaced by an objective 12a, a back focal plane 52a is located within the spatial envelope of the objective 12a, Figure 20 , the microscope is the same as the microscope of Figure 1 , so it is not possible to place the beam steering mirror 13 at the back focal plane 52a.

[0107] A set of relay lenses 100 is located in the optical path between the objective 12a and the steering mirror 13. The relay lenses 100 are used to create an image 52b of the back focal plane 52a of the objective. As Figure 20 shown, since the image 52b of the back focal plane is outside the spatial envelope of the objective 12a, the mirror 13 can be located at the image 52b of the back focal plane 52a. As with Figure 1 the embodiment of , the mirror 13 is used to steer the detection beam onto the probe so that the probe reflects the detection beam to produce the return beam. The mirror 13 is positioned so that the detection beam travels from the mirror 13 to the probe via the relay lenses 100 and the objective 12a.

[0108] As Figure 20 shown, unlikeFigure 1 As in the embodiment of Fig. 1, the second actuator is used to rotate the mirror 13 so that the detection beam performs a tracking motion that is synchronized with the scanning motion and keeps the detection beam steered onto the probe via the mirror 13. The mirror 13 and its axis of rotation are both located at the point where the optical axis 51a of the relay lens 100 intersects the image 52b of the back focal plane 52a. As in the embodiment of Fig. 1, the return beam remains coaxial with the detection beam along the optical path 80 after being reflected by the steering mirror 13, so the mirror 13 can track the probe by pure rotational (no translational) movement. Figure 11 As in the embodiment of Fig. 1, the return beam remains coaxial with the detection beam along the optical path 80 after being reflected by the steering mirror 13, so the mirror 13 can track the probe by pure rotational (no translational) movement.

[0109] Figure 21 A scanning probe microscope la similar to the microscope 1 is shown, and identical features are given identical reference numerals, so will not be described again. The position sensitive detector 70 is mounted on the probe holder 2 so that the first actuator 3 moves the probe holder 2 and the position sensitive detector 70 together to perform the scanning motion.

[0110] The steering mirror 13 is used to steer the detection beam 40c onto the position sensitive detector 70, and the position sensitive detector 70 is used to generate an output 71 as a function of the offset of the detection beam 40c in the X and Y directions relative to the center of the position sensitive detector 70. The offset relative to the center of the position sensitive detector 70 is denoted as x(error) and y(error) in Figure 21 and is equivalent to the apparent motion measured by the image analysis system 20 in the embodiment of Fig. 1. It should be noted that Figure 1 the detection system 26 is omitted in Fig. 2, because in this case there is no return beam reflected by the position sensitive detector 70. Figure 21

[0111] Typically, the position sensitive detector 70 is a split photodiode that is divided into four quadrants, and the ratio between the signals from the four quadrants represents the offset of the detection beam 40c in the X and Y directions relative to the center of the split photodiode.

[0112] The second actuator 14 receives tracking drive signals P A , P B , P C and moves the steering mirror based on the tracking drive signals P A , P B , P C so that the detection beam 40c performs a tracking motion that is synchronized with the scanning motion and keeps the detection beam 40c steered onto the position sensitive detector 70. The calibration system 21 is used to adjust the stored reference data 17 based on the output 71 of the position sensitive detector 70 using a calibration procedure similar to Figure 7 but omitting step 210.

[0113] Figure 22a and​Figure 22b An alternative calibration procedure is shown. In Figure 7 the calibration procedure, both the first and second actuators are moved in step 208, but in Figure 22a the alternative calibration procedure, they are moved in different steps. Figure 22b

[0114] Figure 22a Some of the steps in Figure 22b are the same as the corresponding steps in Figure 7 , so these steps will not be described in detail. In step 202, the optical feature is aligned in the field of view of the vision system, and in step 204, the interferometer 105 is focused on the surface of the probe holder 2.

[0115] In step 408, the scanning position system 5 changes the scanning command signals X, Y by a known offset. This causes the first actuator 3 to produce a step change in the position of the optical feature. Unlike Figure 7 , the second actuator 14 is not moved during step 408.

[0116] Figure 23a The array of positions of the optical feature during the scanning phase of steps 408 and 409 is shown. During the initial steps 202 and 204, the probe holder is located at the central position marked 0,0 in Figure 23a . In the first instance of step 408, the optical feature is moved to one other position in the array (for example the position marked X,0). This causes the optical feature to move a known offset X to the right.

[0117] Figure 23b The apparent position of the optical feature as observed by the vision system 10 during the scanning phase of steps 408 and 409 is shown. After the first instance of step 408, the apparent position of the optical feature has moved from 0',0' to X',0'. In step 409, the x offset (X') and y offset (0') of the optical feature in the vision system is measured and stored.

[0118] Steps 408 and 409 are then repeated for the entire array of Figure 23a . Note that Figure 23a is a schematic diagram, in practice there will be many more points in the array.

[0119] In step 410, the scanner coordinate system (x',y') relative to the vision system (x,y) is determined by analysis of the known offsets of the scanner and the measured offsets observed by the vision system in step 409. Figure 23b Two sets of axes are shown: a set of orthogonal axes (x,y) and a set of non-orthogonal axes (x',y'). They define x and y rotations (θx,θy).​Figure 23b The scale factors Cxand Cyshown in are also determined by analyzing the measurement offsets observed by the vision system.

[0120] In step 411, the scanning position system 5 changes the scanning command signals X, Y to cause the second actuator 14 to produce a step change (ΔA, ΔB, ΔC) in the position or orientation of the tracking mirror 13. The first actuator 3 and the optical feature do not move during step 411. The tracking command signals A, B, C used in step 411 are generated by transforming the scanning command signals X, Y according to the temporary reference data 17.

[0121] Figure 24 The apparent position of the optical feature as observed by the vision system 10 during the tracking phase of steps 411 and 412 is shown. After the first instance of step 411, the apparent position of the optical feature has moved from 0”, 0” to X”, 0” using the temporary reference data 17. In step 412, the x offset (X”) and y offset (0”) of the optical feature in the vision system is measured and stored.

[0122] In step 210, the change in the height signal 116 from the interferometer 105 or the change in the deflection angle signal 117 from the split photodiode 104 is determined - this change is denoted as z (error).

[0123] As Figure 24 shown, in step 413, the measurement offsets observed by the vision system are analyzed to determine the tracker coordinate system (x”, y”) with x rotation (θx”), y rotation (θy”), scale factors Cx”, Cy”.

[0124] The reference data 17 represents the correlation between the scanning command signals X, Y and the tracking command signals A, B, C. If the temporary reference data 17 is completely accurate, then Figure 24 the apparent position shown in Figure 23b will be the same as the apparent position shown in

[0125] Any difference between these parameters is recorded as the measurement offsets observed by the vision system in Figure 22bThe errors calculated in step 414 (denoted as θx(error), θy(error), Cx(error), Cy(error)) are used. In step 415, the reference data 17 is adjusted based on θx(error), θy(error), Cx(error), Cy(error), and z(error) to provide new, improved reference data 17. The adjustment in step 415 improves the accuracy of the reference data 17 so that the offset applied by the tracking system does not cause a change in the measured height and produces the same offset as observed by the equivalent offset scanner.

[0126] After step 415, execute Figure 7 Steps 208-210 of the process serve as cross-checks. If the error is unacceptable, the process is repeated via return path 416.

[0127] Figure 25a and Figure 25b Another alternative calibration procedure is shown. Figure 7 and Figure 22a A vision system was used during the calibration process of / b. Figure 25a and Figure 25b No vision system was used during the calibration process. Therefore Figure 25a and Figure 25b The process can be performed using a microscope without a vision system.

[0128] Figure 25a and Figure 25b Some steps in the process and Figure 22a and Figure 22b The corresponding steps are the same as those in the previous section, so these steps will not be described in detail again.

[0129] In step 501, the detection system 26 is aligned with the cantilever 30 such that the detection beam 40c is focused and placed at a known position on the cantilever 30. The detection beam 40c is placed at the known position on the cantilever 30 as follows: First, the beam 40c scans across the cantilever 30 (or vice versa), and the detection system 26 detects two step changes in the detection signal as the detection beam 40c passes through the edge of the cantilever. The midpoint between these step changes forms the centerline of the cantilever. Next, the detection beam 40c scans longitudinally along the cantilever 30 (or vice versa) until a step change is detected when it passes through the far end of the cantilever. The longitudinal scan movement is then reversed by a known amount, such that the beam is now positioned on the centerline of the cantilever at a known distance from the far end of the cantilever.

[0130] In step 502, the scan position system 5 causes the scan command signals X and Y to change by a known offset. This causes the first actuator 3 to produce a step change in the cantilever position.

[0131] In step 503, the detection beam is offset by using the tracking control system 15, 16 so that the detection system is realigned with the cantilever 30. More specifically, the scan command signals X, Y are transformed according to the reference data 17 to generate tracking command signals A, B, C which are used to drive the mirror 13 until the detection beam is brought back to a known position of the cantilever (i.e. on the midline of the cantilever at a known distance from the distal end of the cantilever) using the above described procedure.

[0132] In step 504, the strain gauge feedback signals S A , B , C S A , B , C are recorded which are indicative of the position of the three struts of the second actuator 14.

[0133] In step 210, the change in the height signal 116 from the interferometer 105 or the change in the deflection angle signal 117 from the split photodiode 104 is determined - this change is denoted as z (error).

[0134] In step 505, improved new reference data 17 is determined based on the tracking actuator positions S A , B , C S measured in step 504 and the change in height z (error) measured in step 210. This improved new reference data 17 is indicative of the correlation between the scan command signals X, Y and the tracking command signals A, B, C and is subsequently used in the scanning or imaging mode.

[0135] Steps 502-504 and step 210 are then repeated for a series of different offsets.

[0136] The procedure can then end, or optionally can continue Figure 25b with a cross-check procedure similar to Figure 23b the cross-check procedure described above, and so will not be described again.

[0137] In summary, in Figure 7 , Figure 22a / b and Figure 25aIn step / b, the scanning probe microscope operates in calibration mode as follows: in step 208 / 408 / 502, a feature (optical feature or cantilever) is moved by a first actuator based on one or more scanning command signals X, Y; in step 208 / 401 / 503, a steering element 13 is moved by a second actuator based on tracking command signals A, B, C; in step 214 / 415 / 505, reference data 17 representing the correlation between the scanning command signals X, Y and the tracking command signals A, B, C is determined; and the reference data 17 is stored.

[0138] Reference data 17 has already been used as follows Figure 7 , Figure 22a / b and Figure 25a After the calibration mode shown in / b is stored, the scanning probe microscope will be used as follows: Figures 8-10 The microscope operates in the scanning mode shown. In the scanning mode, the microscope operates as follows: the detection beam 40c is directed onto the probe 30 by the steering element 13, causing the probe to reflect the detection beam to generate a return beam 28; the return beam 28 is received at the detection system 26, which generates a height signal 116 as output; the probe is moved by the first actuator 3 based on the scan command signals X and Y, causing the probe to perform a scanning motion across the sample 32; one or more scan command signals X and Y are transformed according to reference data 17 to generate tracking command signals A, B, and C; and the steering element 13 is moved by the second actuator 14 based on the tracking command signals A, B, and C, causing the detection beam 40c to perform a tracking motion synchronized with the scanning motion, and the steering element 13 keeps the detection beam directed onto the probe.

[0139] The probe performs a transverse scan across the surface of sample 32, enabling it to perform a cross-surface scanning motion. This cross-surface scanning motion allows the probe to perform a grating scan motion in the xy plane across the surface. The detection beam 40c is directed onto the probe via the steering mirror 13, and the detection beam is reflected from the probe as a return beam 28. Moving the steering mirror 13 causes the detection beam 40c to perform a tracking motion synchronized with the scanning motion, and keeps the detection beam 40c directed onto the probe via the steering mirror 13.

[0140] For each point on the surface, the image acquisition processor 33 obtains an image measurement value based on the output of the detection system 26. It also obtains a related height error measurement value for each image measurement value. The image measurement value represents the measured height of the point on the surface, and the height error measurement value represents the error in the measured height. Both the image measurement value and its related height error measurement value are obtained simultaneously.

[0141] The height error measurement is obtained based on the mirror error measurement from the tracking controller 15. The second actuator 14 includes three piezoelectric actuator struts 14a-c, which are used to move the steering mirror 13, thus obtaining three relevant mirror error measurements for each image measurement, i.e., one for each piezoelectric actuator strut. These three mirror error measurements are combined into a single height error measurement, for example, by linearly combining the three mirror error measurements.

[0142] As described above, each of the supports 14a-c extends and retracts based on a corresponding tracking drive signal. Therefore, support 14a is driven by the tracking drive signal P. A Driven by the tracking drive signal P, the support column 14b is driven by the tracking drive signal P. B Driven by the tracking drive signal P, the strut 14c is driven by the tracking drive signal P. C drive.

[0143] Tracking control systems 15 and 16 are used to generate tracking drive signals P based on stored reference data 17 and scan command signals X and Y. A P B P C The tracking control system includes a tracking controller 15 and a switching system 16.

[0144] Each of the supports 14a-c has an associated strain gauge that generates a strain gauge feedback signal S. A S B S C The tracking controller 15 generates a tracking drive signal P based on a set of tracking command signals A, B, and C. A P B P C The strain gauge feedback signal S is generated by operating the feedback loop using a proportional-integral (PI) or proportional-integral-derivative (PID) controller 15a. A S B S C ,as well as Figure 1 The mirror error signal 34 is shown in the diagram. The purpose of this feedback loop is to enable the second actuator 14 to precisely drive the steering mirror 13 to the expected position determined by the tracking command signals A, B, and C, but the actual position may exhibit errors as described below.

[0145] Each mirror error measurement represents the difference between the expected position of the steering mirror 13 (i.e., the position to which the steering mirror 13 is moved by the scanning command signals X, Y and the tracking command signals A, B, C) and the actual position of the steering mirror 13 (i.e., the actual position to which the steering mirror has moved). This difference is caused by the tracking drive signal P A P B P Ccaused by non-ideal operation in the feedback loop sending to the steering mirror 13. The tracking drive signal P A B C is sent to the steering mirror 13 to cause it to make a tracking motion that is synchronized with the scanning motion of the probe across the surface. The detection beam 40c can fall behind its intended position, and so the resulting surface image can have a double-peaked error or some other systematic error on the image.

[0146] The tracking controller 15 uses signals S A B C from strain gauges coupled to the piezoelectric actuator legs 14a-c of the steering mirror to obtain the actual position of the steering mirror 13.

[0147] In view of the fact that each mirror error measurement represents a difference between positions of the steering mirror 13, it is necessary to apply a conversion factor to the mirror error measurement to make it represent an error in the measured height of a point on the sample surface. This conversion factor is the sensitivity, and it is obtained by considering a point on the surface; observing the initial measured height; leaving two of the actuator legs 14b-c at constant extension and extending the third actuator leg 14a by a known amount; and determining the corresponding observed change in measured height. This determination is made by the interferometer 105. The sensitivity is then the conversion factor between the known extension of the actuator and the amount of observed change in height - that is, the sensitivity can be obtained by dividing the known extension of the actuator by the observed change in height, or by dividing the observed change in height by the known extension of the actuator. For example, if the actuator leg 14a is extended by 10 nm and a corresponding change in height of 1 nm is observed, then the sensitivity can be 1 nm / 10 nm = 0.1. The same operation can be performed for each of the remaining two actuator legs 14b-c to obtain their respective sensitivities. This sensitivity determination is typically performed as a calibration step prior to scanning. Alternatively, the sensitivity determination can be performed once (e.g., as part of an original system calibration) or periodically. The sensitivity determination can also be performed without the probe, and by detecting the light beam from the surface rather than from the probe. The reflected light beam is captured by the interferometer.

[0148] Each of the three sensitivities (corresponding to each of the three actuator legs 14a-c) can then be multiplied by or divided into (depending on the particular index used to obtain the sensitivity) the mirror error to obtain three height errors. These height errors can then be added together to obtain a total height error for the height measurement.

[0149] Figure 10 The above-described process is illustrated schematically. The tracking controller 15 Figure 10 ​​​​The mirror errors 34 (part of the control system shown in the figures) are output, these errors are combined and converted at 35, then low pass filtered at 36 to provide height error measurements which are input into the image acquisition processor 33. The image acquisition processor 33 then makes the necessary height corrections based on the height error measurements.

[0150] All image measurements and associated height error measurements are obtained before the image acquisition processor 33 corrects the image measurements, and the sensitivity factor is applied at 35. Thus, the image measurements can not be corrected until the end of the scan. This is because the image measurements and associated error measurements can be reduced in noise by the low pass filter 36 before the image measurements are corrected. After the scan is complete, the noise can be filtered more effectively, just as the patterns in the x and y axes can be distinguished.

[0151] In summary, the sensitivity factor is applied at 35 after the scan is complete, followed by the low pass filter 36. Each height error measurement is then used to correct its associated image measurement to generate a series of corrected image measurements. These corrected image measurements together form a corrected image.

[0152] Each electronic element shown in the figures and described in the text can be implemented in hardware, software, or any other manner, for example including any combination of hardware and software. For example, all electronic elements, or each electronic element, can be implemented by a single field programmable gate array (FPGA) or digital signal processor (DSP), or multiple FPGAs or DSPs, or by a dedicated FPGA or DSP for each electronic element, or any combination of FPGAs or DSPs.

[0153] While the application has been described above with reference to one or more preferred embodiments, it will be understood that various modifications or improvements can be made without departing from the scope of the application as defined in the appended claims.

Claims

1. A method of surface imaging using a scanning probe microscope, the scanning probe microscope comprising a probe having a cantilever extending from a base to a free end, a probe tip carried by the free end of the cantilever, and a turning mirror; the method comprising: laterally scanning the probe across a surface such that the probe makes a scan motion across the surface; turning a detection beam onto the probe via the turning mirror, the detection beam reflecting from the probe in the form of a return beam; moving the turning mirror such that the detection beam makes a tracking motion that is synchronized with the scan motion and the detection beam is kept turned onto the probe by the turning mirror; obtaining image measurements using the return beam, each image measurement representing a measured height of a respective point on the surface; obtaining a related height error measurement for each point on the surface, each height error measurement representing a respective error in the measured height; and correcting the image measurements using the height error measurements to generate corrected image measurements; wherein the image measurements and related error measurements are obtained simultaneously.

2. The method of claim 1, wherein, Obtaining a related height error measurement comprises determining an expected position or orientation of the turning mirror based on a scan command signal, measuring an actual position or orientation of the turning mirror, and calculating a mirror error measurement representing a difference between the expected position or orientation and the actual position or orientation, wherein the related height error measurement is derived based on the mirror error measurement.

3. The method of claim 2, wherein, Obtaining a related height error measurement further comprises applying a sensitivity parameter to the mirror error measurement.

4. The method of claim 3, wherein, Applying the sensitivity parameter to the mirror error measurement comprises multiplying or dividing the mirror error measurement by the sensitivity parameter.

5. The method of claim 4, wherein, The sensitivity parameter is determined by moving the turning mirror by a known amount, determining an observed change in measured height, and dividing the known amount by the observed change or dividing the observed change by the known amount.

6. The method of any one of claims 2 to 5, wherein, The related height error measurement is obtained by filtering the mirror error measurement.

7. The method of claim 6, wherein, Filtering the mirror error measurement occurs after all mirror error measurements have been calculated.

8. The method of any of the preceding claims, wherein, The step of correcting each image measurement using the height error measurement occurs after the scan is complete.

9. The method according to any of the preceding claims, wherein, Moving the turning mirror comprises translating and / or rotating the turning mirror.

10. A scanning probe microscope comprising: a probe having a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever; a scanner for laterally scanning the probe across a surface such that the probe makes a scan motion across the surface; a turning mirror for turning a detection beam onto the probe, the detection beam reflecting from the probe in the form of a return beam; one or more actuators for moving the turning mirror such that the detection beam makes a tracking motion that is synchronized with the scan motion and the detection beam is kept turned onto the probe by the turning mirror; and a processing system for: obtaining an image measurement of a point on the surface using the return light beam, the image measurement representing a measured height value of the point on the surface; obtaining a related height error measurement of the point on the surface, the height error measurement representing an error in the measured height; and correcting the image measurement using the height error measurement to generate a corrected measurement; wherein the image measurement and related error measurement are obtained simultaneously.

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