Scanning probe microscope

Abstract

A scanning probe microscope configured to measure a topography of a sample. The scanning probe microscope comprises: a probe holder; a cantilever carried by the probe holder, the cantilever extending from a proximal end located at the probe holder to a distal end distal to the probe holder; a probe tip located at the distal end of the cantilever; a scanning system configured to generate a relative scanning motion between the probe holder and the sample; an interferometer configured to measure a height of the distal end of the cantilever to generate a height measurement signal; a signal processor configured to monitor the height measurement signal to obtain a plurality of topography measurement values characterizing the topography of the sample; a height actuation system configured to adjust the height of the proximal end of the cantilever by moving the probe holder under control of a height control signal; a photothermal actuation system configured to bend the cantilever by irradiating the cantilever with an actuation beam under control of a photothermal deflection signal and an oscillation signal; an oscillation signal generator configured to generate the oscillation signal; and a control system configured to adjust the photothermal deflection signal and the height control signal based on the height measurement signal.

Description

Technical Field

[0001] This application relates to a scanning probe microscope and related methods. Background Technology

[0002] WO2010067129A1 discloses a dynamic probe detection system used in conjunction with a scanning probe microscope, which includes a probe that can be repeatedly moved toward and away from a sample surface. When scanning the sample surface, an interferometer generates an output height signal characterizing the optical path difference between light reflected from the probe and a height reference beam. A signal processing device monitors this height signal and derives a measurement characterizing the probe height in each oscillation cycle. Thus, a measurement representing the sample height can be extracted without averaging or filtering, which can be used to form an image of the sample. The detection system may also include a feedback mechanism to maintain the average value of feedback parameters at a set level.

[0003] US6189374 discloses an atomic force microscope (AFM) that combines an AFM Z-position actuator with a self-driven Z-position cantilever (both of which can operate in cyclic and contact modes) and is equipped with appropriate nested feedback control circuitry to achieve high-speed imaging and accurate Z-position measurement. In a preferred embodiment, the AFM for analyzing sample surfaces includes a self-driven cantilever with an integrated Z-positioning element and an oscillator that causes the self-driven cantilever to oscillate at a frequency approximately equal to the resonant frequency of the self-driven cantilever and an oscillation amplitude approximately equal to a set value. The AFM includes a first feedback circuit nested within a second feedback circuit, wherein, during scanning operations, the first feedback circuit generates a cantilever control signal in response to the vertical displacement of the self-driven cantilever, and the second feedback circuit generates a position control signal in response to the cantilever control signal. The second feedback circuit also includes a Z-position actuator that positions the sample in response to the position control signal. Preferably, in operation, only the cantilever control signal is sufficient to characterize the morphology of the sample surface.

[0004] Non-patent literature (NPL) [1] combines the fast response of photothermal cantilever excitation with traditional piezoelectric actuators to achieve a fast force clamp with both high precision and large working stroke.

[0005] NPL[2] uses intensity-modulated infrared laser to induce photothermal bending of the cantilever to control the tip-to-surface distance in atomic force microscopy. The slow response of the photothermal expansion effect is eliminated by inverse transfer function compensation. The tip-to-sample distance is controlled by adjusting the laser power and the cantilever deflection; since the resonant frequency of the small cantilever is significantly higher, the imaging speed is much faster than that of the traditional z-scanner based on piezoelectric actuators.

[0006] In NPL[3], a flexible system for increasing the throughput of atomic force microscopy without sacrificing imaging range was proposed. The system is based on a nested feedback loop that controls a micromechanical cantilever that integrates both piezoelectric and thermal actuators.

[0007] NPL [1]: Stahl.SW, Puchner.EM, Gobb.HE. Photothermal cantilever drive technology for rapid single-molecule force spectroscopy. Scientific Instruments Review. July 2009; 80(7): 073702. DOI: 10.1063 / 1.3157466, PMID: 19655951.

[0008] NPL [2]: Yamashita.H, Kodera.N, Miyagi.A, Uchihashi.T, Yamamoto.D, Ando.T. Tip-sample distance control in high-speed atomic force microscopy using photothermal drive of a small cantilever. Scientific Instrument Review. Aug 2007; 78(8): 083702. DOI: 10.1063 / 1.2766825, PMID: 17764324.

[0009] NPL [3]: T. Salczek, SC. ​​Minne, JD. Adams, DA. Fletcher, A. Atara, CF. Quett, DM. Adelton; Dual integrated actuators for extended-range high-speed atomic force microscopy. Applied Physics Letters. 13 Sep 1999; 75(11): 1637–1639, https: / / doi.org / 10.1063 / 1.124779. Summary of the Invention

[0010] In a first aspect, this application provides a scanning probe microscope configured to measure the morphology of a sample, the scanning probe microscope comprising: a probe holder; a cantilever supported by the probe holder, the cantilever extending from a proximal end of the probe holder to a distal end remote from the probe holder; a probe tip located at the distal end of the cantilever; a scanning system configured to generate relative scanning motion between the probe holder and the sample; an interferometer configured to measure the height of the distal end of the cantilever to generate a height measurement signal; a signal processor configured to monitor the height measurement signal to obtain a series of morphology measurements characterizing the morphology of the sample; a height actuation system configured to adjust the height of the proximal end of the cantilever by moving the probe holder under the control of a height control signal; a photothermal actuation system configured to bend the cantilever by irradiating it with an actuation beam under the control of a photothermal offset signal and an oscillation signal; an oscillation signal generator configured to generate an oscillation signal; and a control system configured to adjust the photothermal offset signal and the height control signal based on the height measurement signal.

[0011] Optionally, the control system is a feedback control system. Alternatively, the control system is a feedforward control system. Alternatively, the control system may combine feedforward control and feedback control, or employ other control methods. Optionally, the control system includes an oscillation error detector configured to monitor the height measurement signal to obtain oscillation parameters, and generate an oscillation error signal based on the oscillation parameters and oscillation parameter setpoints; and the control system is configured to adjust the photothermal offset signal and the height control signal based on the oscillation error signal.

[0012] Optionally, the control system includes: a first controller (e.g., a feedback controller or a feedforward controller) configured to adjust the photothermal offset signal; and a second controller (e.g., a feedback controller or a feedforward controller) configured to adjust the height control signal.

[0013] Optionally, the second controller may operate at a slower speed than the first controller, or the second controller may have a longer response time than the first controller, or the second controller may have a lower bandwidth than the first controller.

[0014] Optionally, the first controller and the second controller operate in non-overlapping frequency bands.

[0015] Optionally, the control system includes: a first feedback controller configured to adjust the photothermal offset signal to bring the input of the first feedback controller to zero; and a second feedback controller configured to adjust the height control signal to bring the input of the second feedback controller to zero.

[0016] Optionally, the input to the first feedback controller includes an oscillation error signal.

[0017] Optionally, the control system further includes a photothermal error detector configured to generate a photothermal error signal based on the photothermal offset signal and a photothermal offset setpoint; and the input of the second feedback controller includes the photothermal error signal.

[0018] Optionally, the photothermal offset setting is located at the midpoint of the photothermal offset signal's operating range.

[0019] Optionally, the input to the second feedback controller includes an oscillation error signal.

[0020] Optionally, the oscillation error signal is fed in parallel to the first feedback controller and the second feedback controller.

[0021] Optionally, the photothermal offset signal and the oscillation signal are different signals, wherein the intensity of the actuation beam is based on the sum (or other combination) of the photothermal offset signal and the oscillation signal. The photothermal offset signal and the oscillation signal can be output by different components: for example, the photothermal offset signal can be output by a first feedback controller, while the oscillation signal can be output by an oscillation signal generator.

[0022] Optionally, the control system is configured to adjust the photothermal offset signal in response to changes in the morphology of the sample.

[0023] Optionally, the scanning probe microscope also includes a combiner configured to combine the oscillation signal and the photothermal offset signal to generate a combined signal, wherein the photothermal actuation system is configured to irradiate the cantilever with an actuation beam under the control of the combined signal.

[0024] Optionally, the combiner is an adder that combines the oscillation signal and the photothermal offset signal by adding the oscillation signal and the photothermal offset signal.

[0025] Optionally, the oscillation signal causes the cantilever to bend in a series of oscillation cycles, and the signal processor is configured to acquire morphology measurements characterizing the morphology of the sample in each oscillation cycle.

[0026] Optionally, the signal processor is configured to extract data from positions that meet predetermined measurement criteria within each oscillation cycle.

[0027] Optionally, the position within each oscillation cycle is the extreme position.

[0028] Optionally, the signal processor is configured to monitor the height signal and derive a measurement characterizing the probe height in each oscillation cycle.

[0029] Alternatively, the bandwidth of the photothermal actuation system is higher than that of the high-amplification system.

[0030] Optionally, the range of motion of the high-amplification system is greater than that of the photothermal actuation system.

[0031] Optionally, the oscillation signal is basically a sine wave, but it may also have non-sine waves such as triangular waves or sawtooth waves.

[0032] The frequency of the oscillating signal can be the resonant frequency of the cantilever, or any other frequency.

[0033] Typically, an oscillating signal varies back and forth within a series of oscillation cycles. An oscillating signal can be periodic, meaning each oscillation cycle is the same, or it can be non-periodic, meaning each oscillation cycle is different.

[0034] Optionally, the high-amplification system includes a piezoelectric actuator.

[0035] On the other hand, this application provides a method for measuring the morphology of a sample using a scanning probe microscope according to the first aspect, the method comprising: generating relative scanning motion between a probe holder and a sample using a scanning system; measuring the height of the distal end of a cantilever using an interferometer to generate a height measurement signal; monitoring the height signal using a signal processor to obtain a series of morphology measurements characterizing the morphology of the sample; adjusting the height of the proximal end of the cantilever using a height actuation system by moving the probe holder under the control of a height control signal; generating an oscillation signal using an oscillation signal generator; bending the cantilever using a photothermal actuation system by irradiating the cantilever with an actuation beam under the control of a photothermal offset signal and the oscillation signal; and adjusting the photothermal offset signal and the height control signal using a control system based on the height measurement signal.

[0036] Optionally, the photothermal shift signal is adjusted in response to changes in the morphology of the sample.

[0037] Optionally, the cantilever is irradiated by: generating an oscillation signal and a photothermal offset signal respectively; combining the oscillation signal and the photothermal offset signal to generate a combined signal; and irradiating the cantilever with an actuation beam under the control of the combined signal.

[0038] Optionally, combining the oscillation signal and the photothermal offset signal includes adding the oscillation signal to the photothermal offset signal.

[0039] Optionally, the rate of change of the photothermal offset signal is lower than that of the oscillation signal. Attached Figure Description

[0040] Embodiments of this application will now be described with reference to the accompanying drawings, in which: Figure 1 A scanning probe microscope according to a first embodiment of this application is shown; Figure 2 A detection system is shown; Figure 3 The morphology of a sample is shown, along with the associated photothermal offset signal and height control signal; Figure 4 The first comparative example is shown; Figure 5 A second comparative example is shown; Figure 6 A third comparative example is shown; Figure 7 A scanning probe microscope according to a second embodiment of this application is shown; and Figure 8 A scanning probe microscope 1e according to another optional embodiment of this application is shown. Detailed Implementation

[0041] Figure 1 The scanning probe microscope 1 shown is configured to measure the morphology of sample 2 on sample stage 3.

[0042] Figure 1 The various functional elements of the microscope shown can be implemented as computer software running on one or more computer processors, or as dedicated hardware.

[0043] The scanning probe microscope 1 includes a probe holder 4 and a probe comprising a cantilever 5 and a probe tip 6. The cantilever 5 is supported by the probe holder 4 and extends from a proximal end of the probe holder to a distal end away from the probe holder. The probe tip 6 is located at the distal end of the cantilever 5.

[0044] The scanning systems 10 and 11 are configured to generate relative scanning motion between the probe holder 4 and the sample 2 on a horizontal plane (X, Y). In this case, the relative scanning motion is achieved by the movement of the probe holder 4; in other embodiments, the relative scanning motion can also be achieved by the movement of the sample stage 3.

[0045] The height actuation system 20 is configured to adjust the height of the proximal end of the cantilever by moving the probe holder 4 under the control of the height control signal 21.

[0046] In this case, the height drive system includes a Z-actuator 20, such as a piezoelectric actuator, which is configured to cause the probe holder 4 to translate vertically in the substantially vertical (Z) direction based on the height control signal 21.

[0047] In this configuration, the scanning system includes an XY scanner 10, such as a pair of piezoelectric actuators, each of which moves the Z actuator 20 and the probe holder 4 in their respective horizontal directions (X or Y). The XY scanner 10 is driven by a scan drive signal 12 output from the scan controller 11.

[0048] Figure 1 The detection system 30 is schematically shown, and in Figure 2 The detection system 30 will be described in more detail below.

[0049] Laser source 31 generates detection beam 32, which is guided to the distal end of cantilever 5 by steering mirror 33. The distal end of cantilever 5 reflects detection beam 32, thereby generating return beam 34.

[0050] The detection beam 32 is reflected from the fixed mirror 35 to the right-angle block 36. The orientation of the right-angle block 36 ensures that the detection beam 32 is incident perpendicularly onto its incident surface. The detection beam 32 propagates to the steering mirror 33 and is reflected to the objective lens 37. As the steering mirror 33 tilts, the reflected detection beam 32 deflects, causing changes in the angle and point of incidence of the detection beam 32 onto the objective lens 37. By synchronously controlling the tilt angle of the steering mirror 33 with the XY scanning trajectory formed by the XY scanner 10 driving the probe holder 4, the detection beam 32 can be kept at a constant position at the distal end of the cantilever 5.

[0051] The light emitted by the laser source 31 is split into a detection beam 32 and a reference beam 39 by a beam splitter 102. The return beam 34 is guided into the detection system 30 by a second beam splitter 103. Subsequently, the return beam 34 is split into a first component and a second component by a third beam splitter 107: the first component is projected onto a split photodiode 104, and the second component is guided into an interferometer 105.

[0052] The split photodiode 104 is a position-sensitive detector that generates a deflection signal 117 based on the offset of the first component of the returned beam 34 relative to the split photodiode 104. The deflection signal 117 characterizes the deflection angle of the cantilever. Typically, the split photodiode 104 is divided into four quadrants, and the ratio between the signals of the four quadrants represents the offset of the first component of the returned beam 34 relative to the split photodiode 104 in the X and Y directions.

[0053] Inside interferometer 105, the second component of the returned beam 34 is split by beam splitter 106. Reference beam 39 is guided to retroreflector 108 and then to beam splitter 106. Beam splitter 106 has an energy-absorbing coating and splits the returned beam 34 and reference beam 39 separately to generate a first interferogram and a second interferogram with a relative phase shift of approximately 90°. The two interferograms are detected by photodetectors 112 and 114, respectively.

[0054] Ideally, the photodetector signals are complementary sine and cosine signals with a 90° phase difference. Furthermore, these signals should have no DC offset, equal amplitude, and depend only on the cantilever position and the laser wavelength λ. Using known methods, the outputs of photodetectors 112 and 114 are monitored while the optical path difference is varied to identify and correct errors caused by factors such as the two photodetector signals not being ideal harmonics, having unequal amplitudes, and being non-phase orthogonal. Similarly, the DC offset has also been corrected according to methods known in the art.

[0055] These photodetector signals are suitable for conventional interferometric reversible fringe counting and fringe subdivision devices, which can be implemented as dedicated hardware or a programmable computer. Phase-orthogonal fringe counting devices can measure the displacement of the cantilever position with an accuracy of λ / 8. That is, for light with a wavelength of 532 nm, the accuracy can reach 66 nm. Known fringe subdivision techniques based on the arctangent of the signal can further improve the accuracy to the nanometer level or smaller. Interferometric methods for extracting the optical path difference between two coherent beams are well known in the art and will not be described further. Figure 2 A processor 115 is shown, which receives signals output from photodetectors 112 and 114 and performs the above-described stripe counting and subdivision functions to generate a height measurement signal 116.

[0056] In summary, interferometer 105 is configured to detect the optical path difference between the light 34 reflected from the distal end of the cantilever and the height reference beam 39. The height measurement signal 116 characterizes this optical path difference, and therefore also characterizes the height of the distal end of the cantilever 5.

[0057] Continue to refer to Figure 1 The signal processor 50 is configured to monitor the height measurement signal 116 to obtain a series of morphology measurements characterizing the morphology of the sample. These morphology measurements can be output as morphology measurement signal 51 to the image acquisition module 52.

[0058] A photothermal actuation system 60, such as a laser, is configured to bend the cantilever by irradiating it with an actuation beam 61, under the control of a photothermal offset signal 62 and an oscillation signal 63. An oscillation signal generator 64 is configured to generate the oscillation signal 63. The oscillation signal 63 is essentially sinusoidal, with a frequency equal to the resonant frequency of the cantilever 5. Typically, the resonant frequency of the cantilever is several hundred kilohertz, but it may also exceed 1 MHz.

[0059] The oscillation signal 63 can be a periodic signal that repeats at regular intervals, and its frequency is usually higher than 1 MHz. The photothermal offset signal 62 can be a non-periodic signal and is independent of the oscillation signal 63. The photothermal offset signal 62 is adjusted in response to changes in the morphology of the sample. Generally, the rate of change of the photothermal offset signal 62 is lower than that of the oscillation signal 63, that is, the frequency of the photothermal offset signal 62 is lower than the resonant frequency of the cantilever 5.

[0060] In this example, the oscillation signal 63 is essentially a sinusoidal waveform, but in other embodiments, the oscillation signal 63 may have a non-sinusoidal waveform, such as a triangular waveform or a sawtooth waveform.

[0061] The oscillation signal 63 changes back and forth in a series of oscillation cycles. Each oscillation cycle may be the same, or the oscillation cycles may be different from each other (for example, the cycle lengths may be different between cycles).

[0062] Generally speaking, the oscillation signal 63 is usually an AC signal (which usually changes continuously), while the photothermal offset signal 62 is usually a DC signal (which is constant or changes slowly).

[0063] The cantilever 5 is irradiated by generating an oscillation signal 63 and a photothermal offset signal 62, respectively; combining the oscillation signal and the photothermal offset signal to generate a combined signal 66; and irradiating the cantilever with an actuation beam 61 under the control of the combined signal 66. That is, the photothermal actuation system 60 adjusts the intensity of the actuation beam 61 based on the combined signal 66.

[0064] In this example, the oscillation signal 63 and the photothermal offset signal 62 are combined by adding them together using adder 67. Alternatively, the oscillation signal 63 and the photothermal offset signal 62 can be combined using other types of combiners.

[0065] The feedback control system, configured to generate a photothermal offset signal 62, is described in detail below. The photothermal offset signal 62 is independent of the oscillation signal 63.

[0066] The cantilever 5 has a thermally bicrystalline structure, meaning its material undergoes thermal expansion upon heating. In one embodiment, the cantilever 5 is made of silicon nitride and coated with an aluminum coating. The photothermal actuation system 60 includes a laser that emits an actuation beam 61, the wavelength of which is located at a position with a maximum or peak in the absorption spectrum of a particular coating. For example, this wavelength could be approximately 810 nm, the peak absorption wavelength of aluminum. Other coating / wavelength combinations can also be used, such as gold, which has a higher absorption rate at wavelengths below 500 nm. When light is incident on the coated side of the cantilever 5, aluminum expands more than silicon nitride, causing the cantilever to bend and move the tip downward toward the sample. If the irradiation intensity increases, the tip moves closer to the sample surface. Conversely, if the intensity decreases, the degree of bending decreases, and the tip moves away from the sample. With other arrangements of coating and substrate materials, different degrees of bending may occur due to irradiation in the same or opposite directions.

[0067] As mentioned above, Figure 2 The detection beam 32 is directed toward the cantilever via the steering mirror 33 and the objective lens 37. When the steering mirror 33 is tilted, the reflected detection beam 32 is deflected and remains at the distal end of the cantilever 5.

[0068] The actuation beam 61 is also directed toward the cantilever via the steering mirror 33 and the objective lens 37. When the steering mirror 33 is tilted, the actuation beam 61 is deflected and maintains its position on the cantilever 5 (in this case, toward the proximal end of the cantilever).

[0069] Therefore, the steering mirror 33 maintains the position of the beams 32 and 61 on the cantilever 5. The steering mirror 33 is driven by the steering signal 65 output by the scan controller 11.

[0070] The feedback control systems 70, 80, and 81 are configured to adjust the photothermal offset signal 62 and the height control signal 21 based on the height measurement signal 116, so that the probe tip follows the surface morphology of the sample 2.

[0071] The feedback control system includes an oscillation error detector configured to monitor the height measurement signal 116 to obtain oscillation parameters 71, and generate an oscillation error signal 73 based on the oscillation parameters and the oscillation parameter setpoint.

[0072] In this example, the oscillation parameter is the amplitude 71 measured by the amplitude detector 70, and the oscillation parameter setpoint is the amplitude setpoint. In other examples, the oscillation parameter can be the phase or the frequency.

[0073] The amplitude setpoint is subtracted from the amplitude 71 to generate an oscillation error signal, which is then the amplitude error signal 73. The feedback control system is configured to adjust the height control signal 21 and the photothermal offset signal 62 based on the amplitude error signal 73, as described below.

[0074] The feedback control system includes a first feedback controller 80, which is configured to adjust the photothermal offset signal 62 to bring the input of the first feedback controller 80 to zero.

[0075] In this embodiment, the input to the first feedback controller 80 includes an amplitude error signal 73. Therefore, the first feedback controller 80 is configured to adjust the photothermal offset signal 62 to maintain the amplitude 71 at a set amplitude value. Thus, when characteristics of the sample cause the amplitude error signal 73 to have a non-zero value, the photothermal offset signal 62 responds to changes in the sample's morphology.

[0076] For example, the first feedback controller 80 may include a proportional-integral (PI) feedback controller or a proportional-integral-differential (PID) feedback controller.

[0077] The second feedback controller 81 is configured to adjust the height control signal 21 so that the input of the second feedback controller 81 is zeroed.

[0078] The feedback control system includes a photothermal error detector configured to generate a photothermal error signal 82 based on a photothermal offset signal 62 and a photothermal offset setpoint; and the input of the second feedback controller 81 includes the photothermal error signal 82.

[0079] The photothermal offset setting value can be subtracted from the photothermal offset signal 62 to generate the photothermal error signal 82. The photothermal offset setting value can be set at the midpoint of the operating range of the photothermal offset signal 62. Therefore, the second feedback controller 81 is configured to adjust the height control signal 21 to keep the photothermal offset signal 62 at the midpoint of its operating range.

[0080] When capturing sample images, the scanning probe microscope operates in dynamic mode as follows: The cantilever 5 is set to resonant oscillation motion via an oscillation signal generator 64. First, the probe tip 6 is brought into intermittent contact with the sample 2 using the Z-actuator 20. In AFM terminology, it is generally considered that the probe tip 6 contacts the sample 2 when the interatomic forces are in a repulsive state. The probe holder 4 descends, moving the probe tip toward the sample 1, while in this embodiment, the detection system 30 monitors the oscillation and time-varying deflection of the cantilever. When the amplitude of the deflection oscillation reaches a predetermined level, the probe holder 4 stops moving. This predetermined amplitude level is the amplitude setpoint of the feedback controller 80.

[0081] As the scan progresses, the probe tip 6 moves up and down in response to changes in surface height / interaction force. Superimposed on this surface-induced motion is a higher-frequency component generated by the oscillating motion of the cantilever driven by the oscillation signal 63. The amplitude, phase, and frequency of the oscillating component of the probe tip motion depend on the oscillator settings and the interaction force between the surface and the probe tip. Similarly, the height of the probe tip is the superposition of two components: one originates from the interaction with the sample surface, and the second from the oscillation of the probe. The amplitude, phase, and frequency of the second component are affected by the position of the probe tip relative to the surface. As the scan progresses, the amplitude of the AC component of the height measurement signal 116 is monitored.

[0082] It should be understood that the height measurement signal 116 contains accurate information related to the height of the probe tip, but in order to obtain useful information, this information must be correlated with the characteristics of the sample surface. Theoretically, the lowest point of each oscillation cycle represents the true height of the surface. Therefore, the signal processor 50 can be configured to find the lowest height measurement value of each probe oscillation cycle and then output it as the topography measurement signal 51, which the image acquisition module 52 can use to form an image. That is, the lowest point of each oscillation cycle represents the position of the sample surface.

[0083] Other points on the high-frequency oscillation component of the signal can also be used to provide an indicator that effectively reflects the position of the sample surface. For example, the minimum velocity point in the second half of the oscillation period can be extracted using a data processing system. When imaging a flexible surface, the probe's velocity, or the rate of change of its equivalent height signal, decreases as the probe contacts the surface and begins to deform. In this case, the measured value of the sample height can be extracted from the point where the rate of change of the probe height is observed to change during each height oscillation.

[0084] Signal processor 50 is preferably configured to extract data from locations that meet predetermined measurement criteria within each oscillation cycle of height measurement signal 116. This location can be where the cycle indicates an extreme optical path difference, a minimum rate of change of optical path difference, or other suitable indicator. This ensures that the measurement point is extracted from the location most likely to reflect the true height of the sample. For example, at the minimum (or maximum) optical path difference, it can be inferred that the probe tip is in contact with the sample surface. This improves the accuracy of the information extracted by detection system 30, which image acquisition module 52 can then use to generate an image of the sample surface. This image can reflect the surface height or any other morphological features of the sample.

[0085] Alternatively, the signal processor 50 can use the average value of the height measurement signal 116 to generate the topography measurement signal 51, and the image acquisition module 52 can use the topography measurement signal 51 to form an image.

[0086] In this example, the image acquisition module 52 constructs an image based on the topography measurement signal 51 and the XY position signal 55 output by the scanning controller 11, wherein the XY position signal 55 represents the current XY position of the probe tip.

[0087] In other embodiments of this application, the scanning probe microscope 1 may not generate images through the image acquisition module 52, but instead utilize the topography measurement signal 51 in other ways—for example, for measuring key dimensions of the sample (such as wall height, wall angle, groove width, etc.). In this case, a series of topography measurements can be completed by a single-line scan (e.g., across the groove) without the need for a two-dimensional scan to generate a topography image.

[0088] Figure 3 An example of the surface morphology of sample 2 is shown. In this example, the sample surface has steep sidewalls 90 with a height of H. This type of sidewall is common in semiconductor samples, such as on the sidewalls of trenches or other features.

[0089] Figure 4 A scanning probe microscope 1a according to a comparative example, rather than that of this application, is shown. In this example, the resonant oscillation is driven by a piezoelectric actuator 60a, rather than by... Figure 2 The photothermal actuation system 60 drives the system. (And...) Figure 2Similar to the Z-actuator 20 in the above, the height actuation system 20a is configured to adjust the height of the proximal end of the cantilever by moving the probe holder under the control of the height control signal output by the PID controller. The scanning probe microscope 1a is similar to the scanning probe microscope described in WO2010067129A1.

[0090] Figure 4 The first problem with the scanning probe microscope 1a shown is the low bandwidth of the piezoelectric actuator 60a. Therefore, in order to accurately image the steep sidewalls 90, the probe must be scanned slowly in the XY plane. The problems caused by slow scanning are particularly prominent in semiconductor sample analysis, such as in process monitoring and control in semiconductor manufacturing plants or other large-scale manufacturing facilities.

[0091] The second problem is that the piezoelectric actuator 60a may respond to the oscillating signal with its own resonance, which may lead to undesirable characteristics in cantilever oscillation.

[0092] Figure 5 A scanning probe microscope 1b according to a second comparative example, rather than this application, is shown. In this example, the vertical movement of the probe is driven by a laser 60b via photothermal means. The bandwidth of the laser 60b is greater than that of the piezoelectric actuator 60a (typically 10 times or more, or 100 times or more), thus allowing the probe tip to be driven downward to the bottom of the sidewall 90 more quickly.

[0093] Figure 5 The problem with the scanning probe microscope 1b is that the photothermal actuation system 60b has a relatively small range, possibly smaller than the height H of the sidewall 90. Furthermore, Figure 5 The scanning probe microscope 1b also faces the problem of poor resonance in the piezoelectric actuator.

[0094] Figure 6 A scanning probe microscope 1c is shown according to a third comparative example, rather than this application.

[0095] In this example, the photothermal actuator 60c drives both the oscillation of the probe (via the oscillation signal output by the oscillation signal generator 64c) and the DC offset of the probe according to the contour of the sample.

[0096] Figure 6 Scanning probe microscope 1c and Figure 5 The scanning probe microscope 1b has the same range problem, but compared with the scanning probe microscope 1b, the scanning probe microscope 1c has several advantages, namely: the number of actuators is reduced (one actuator instead of two); and the direct drive of the cantilever makes the excitation of the cantilever purer, thereby avoiding the undesirable oscillation characteristics caused by the piezoelectric actuator 60a.

[0097] The range problem can be solved by increasing the length of the cantilever, but this increases its thermal mass, which in turn makes it respond more slowly to photothermal heating (in other words, it reduces the bandwidth of the photothermal actuation system).

[0098] Figure 1 The scanning probe microscope 1 in the middle has Figure 6 It has the advantages of the scanning probe microscope 1c, but solves the range problem without reducing bandwidth.

[0099] Figure 3 The photothermal offset signal 62 is shown as the probe scans the sidewall 90 from left to right. When the probe reaches sidewall 90, the photothermal offset signal 62 rapidly jumps from 50% to 80% at a step point 91. These percentages represent the amount of the photothermal actuation system at full scale: 0% indicates that the cantilever 5 is not bent, and 100% indicates that the cantilever 5 is fully bent downwards. Therefore, in this example, the height H of the sidewall represents 30% of the dynamic range of the photothermal actuation system. Full scale is typically less than 1 micrometer, for example, approximately 100 nanometers.

[0100] The Z-actuator 20 has a low bandwidth, thus it can only respond slowly to sudden changes in height at sidewall 90. Therefore, the height control signal 21 gradually increases from 50% to 51% on the ascending ramp 92. These percentages represent the amount of full-scale of the Z-actuator 20: 0% indicates that the Z-actuator 20 is not extended, and 100% indicates that the Z-actuator 20 is fully extended. In this example, the height H of the sidewall represents only 1% of the dynamic range of the Z-actuator 20. Full-scale range is typically on the order of 10-15 micrometers, which is more than ten times larger than the range of a photothermal actuation system.

[0101] As the height control signal 21 gradually increases from 50% to 51% on the ascending ramp 92, the first feedback controller 80 causes the photothermal offset signal 62 to gradually decrease on the descending ramp 93, counteracting the effect of the ascending ramp 92 until the photothermal offset signal 62 returns to its set value (50% in this case). Therefore, the feedback control system of the scanning probe microscope 1 solves the range problem by ensuring that the photothermal offset signal 62 remains close to its set value.

[0102] Figure 7 A scanning probe microscope 1d according to another embodiment of this application is shown. Most of the elements of the scanning probe microscope 1d are similar to those in the present application. Figure 1 The scanning probe microscope 1 is identical to the one in the series, and these elements are assigned the same reference numbers.

[0103] In the scanning probe microscope 1d, the input to the second feedback controller 81 includes an amplitude error signal 73, which is fed in parallel to the first and second feedback controllers 80 and 81. In the scanning probe microscope 1d, the second feedback controller 81 operates at a lower rate than the first feedback controller 80.

[0104] Operating the second feedback controller 81 at a lower rate than the first feedback controller 80 (or equivalently, with a longer response time or lower bandwidth) can avoid unnecessary interactions between controllers 80 and 81.

[0105] A preferred method to avoid unwanted interactions between feedback controllers 80 and 81 is to operate them in non-overlapping frequency bands. For example, the second feedback controller 81 may operate in a frequency band from DC to 1 kHz, while the first feedback controller 80 may operate in a frequency band from several kilohertz to several megahertz.

[0106] Another way to avoid unwanted interactions between controllers 80 and 81 is to operate them using different control methods. For example, the second feedback controller 81 can be replaced with a feedforward controller.

[0107] exist Figure 7 In the alternative embodiment shown, the second feedback controller 81 receives and processes the amplitude error signal 73 to provide the height control signal 21. Therefore, in this embodiment, it is not necessary to input the photothermal offset signal 62 into the second feedback controller 81. The advantage of this embodiment is that the height control signal 21 is generated more directly based on the amplitude error signal 73, which is obtained directly from the height measurement signal 116. Furthermore, the height control signal 21 can be generated without using the first feedback controller 80, thus making the system more robust.

[0108] Figure 8 A scanning probe microscope 1e according to another alternative embodiment of this application is shown. Most of the elements of the scanning probe microscope 1e are similar to those in the present application. Figure 1 The scanning probe microscope 1d is identical to the one in the series, and these elements are assigned the same reference number.

[0109] exist Figure 8 In the detection system 30a, a differential interferometer is included, which is connected to... Figure 2The interferometer 105 in this example is similar in that the steering mirror 33 directs the detection beam 32 to the distal end of the cantilever 5 to illuminate the probe. In this example, the reference beam 39 is directed by the steering mirror 33 to the proximal end of the cantilever 5 (or probe holder 4) to produce a reflected reference return beam, which combines with the reflected detection return beam from the distal end of the cantilever. Therefore, the differential interferometer produces an output characterizing the height difference between the proximal end (or probe holder 4) and the distal end of the cantilever. Thus, the output of the detection system 30a is signal 116a, which characterizes the height of the distal end of the cantilever (relative to the proximal end, not a fixed reference frame) and the deflection of the probe.

[0110] The signal 116a output by the detection system 30a is combined with the height control signal 21 at the signal processor 50 to determine the sample height, thereby providing morphology measurement values.

[0111] In summary, the embodiments of this application described above provide a feedback control system including a pair of feedback controllers 80 and 81. Each controller is configured to adjust a photothermal offset signal and a height control signal based on their respective inputs, which are obtained (directly or indirectly) based on height measurement signals 116 and 116a. The first feedback controller 80 can be configured to adjust the photothermal offset signal based on an oscillation error signal, which is directly obtained based on the height measurement signals 116 and 116a. The second feedback controller 81 can be configured to adjust the height control signal based on the photothermal error signal (obtained indirectly based on the height measurement signals 116 and 116a) or based on the oscillation error signal (obtained directly based on the height measurement signals 116 and 116a).

[0112] In other embodiments of this application (not shown), the first feedback controller 80 may be replaced by a first feedforward controller that operates using feedforward information from the previous line scan and in conjunction with feedback from the amplitude signal or sample interaction signal. The second feedback controller 81 may be replaced by a second feedforward controller that operates entirely based on feedforward information from the previous line scan or in other combinations thereof.

[0113] Although the present application has been described above with reference to one or more preferred embodiments, it should be understood that various changes or modifications may be made without departing from the scope of the present application as defined by the appended claims.

Claims

1. A scanning probe microscope configured to measure the morphology of a sample, characterized in that, The scanning probe microscope includes: a probe holder; a cantilever supported by the probe holder, the cantilever extending from a proximal end of the probe holder to a distal end away from the probe holder; a probe tip located at the distal end of the cantilever; a scanning system configured to generate relative scanning motion between the probe holder and the sample; an interferometer configured to measure the height of the distal end of the cantilever to generate a height measurement signal; a signal processor configured to monitor the height measurement signal to obtain a series of morphology measurements characterizing the morphology of the sample; a height actuation system configured to adjust the height of the proximal end of the cantilever by moving the probe holder under the control of a height control signal; a photothermal actuation system configured to bend the cantilever by irradiating it with an actuation beam under the control of a photothermal offset signal and an oscillation signal; an oscillation signal generator configured to generate the oscillation signal; and a control system configured to adjust the photothermal offset signal and the height control signal based on the height measurement signal.

2. The scanning probe microscope according to claim 1, characterized in that, The control system includes an oscillation error detector configured to monitor the height measurement signal to obtain oscillation parameters and generate an oscillation error signal based on the oscillation parameters and a setpoint for the oscillation parameters; and the control system is configured to adjust the photothermal offset signal and the height control signal based on the oscillation error signal.

3. The scanning probe microscope according to any of the preceding claims, characterized in that, The control system includes: a first feedback controller configured to adjust the photothermal offset signal to bring the input of the first feedback controller to zero; and a second feedback controller configured to adjust the height control signal to bring the input of the second feedback controller to zero.

4. The scanning probe microscope according to claims 2 and 3, characterized in that, The input to the first feedback controller includes the oscillation error signal.

5. The scanning probe microscope according to claim 3 or claim 4, characterized in that, The control system further includes a photothermal error detector, which is configured to generate a photothermal error signal based on the photothermal offset signal and a photothermal offset setpoint; and the input of the second feedback controller includes the photothermal error signal.

6. The scanning probe microscope according to claim 5, characterized in that, The photothermal offset setting value is located at the midpoint of the working range of the photothermal offset signal.

7. The scanning probe microscope according to claim 3 or claim 4, characterized in that, The input to the second feedback controller includes the oscillation error signal.

8. The scanning probe microscope according to claim 4, characterized in that, The input to the second feedback controller includes the oscillation error signal.

9. The scanning probe microscope according to any of the preceding claims, characterized in that, The oscillation signal causes the cantilever to bend in a series of oscillation cycles, and the signal processor is configured to acquire morphology measurements characterizing the morphology of the sample in each oscillation cycle.

10. The scanning probe microscope according to claim 9, characterized in that, The signal processor is configured to extract data from positions that meet predetermined measurement criteria within each oscillation cycle.

11. The scanning probe microscope according to claim 10, characterized in that, The position within each oscillation cycle is the extreme position.

12. The scanning probe microscope according to any of the preceding claims, characterized in that, The signal processor is configured to monitor the height signal and derive a measurement characterizing the height of the probe in each oscillation cycle.

13. The scanning probe microscope according to any of the preceding claims, characterized in that, The bandwidth of the photothermal actuation system is higher than that of the height actuation system.

14. The scanning probe microscope according to any of the preceding claims, characterized in that, The range of motion of the height-actuated system is greater than that of the photothermal-actuated system.

15. The scanning probe microscope according to any of the preceding claims, characterized in that, The oscillation signal is basically sinusoidal.

16. The scanning probe microscope according to any of the preceding claims, characterized in that, The control system includes: a first controller configured to adjust the photothermal offset signal; and a second controller configured to adjust the height control signal.

17. The scanning probe microscope according to claim 16, characterized in that, The second controller operates at a lower speed than the first controller, or the second controller has a longer response time than the first controller, or the second controller has a lower bandwidth than the first controller.

18. The scanning probe microscope according to any of the preceding claims, characterized in that, The control system is configured to adjust the photothermal offset signal in response to changes in the morphology of the sample.

19. The scanning probe microscope according to any of the preceding claims, characterized in that, The scanning probe microscope further includes a combiner configured to combine the oscillation signal and the photothermal offset signal to generate a combined signal, wherein the photothermal actuation system is configured to cause the cantilever to be irradiated by the actuated beam under the control of the combined signal.

20. The scanning probe microscope according to claim 19, characterized in that, The combiner is an adder that combines the oscillation signal and the photothermal offset signal by adding the oscillation signal to the photothermal offset signal.

21. A method for measuring the morphology of a sample using a scanning probe microscope according to any of the preceding claims, characterized in that, The method includes: generating relative scanning motion between the probe holder and the sample using the scanning system; measuring the height of the distal end of the cantilever using the interferometer to generate a height measurement signal; monitoring the height signal using the signal processor to obtain a series of morphology measurements characterizing the morphology of the sample; adjusting the height of the proximal end of the cantilever using the height actuation system by moving the probe holder under the control of a height control signal; generating an oscillation signal using the oscillation signal generator; bending the cantilever using the photothermal actuation system by irradiating the cantilever with an actuation beam under the control of a photothermal offset signal and the oscillation signal; and adjusting the photothermal offset signal and the height control signal using the control system based on the height measurement signal.

22. The method according to claim 21, characterized in that, The photothermal offset signal is adjusted in response to changes in the morphology of the sample.

23. The method according to claim 21 or 22, characterized in that, The cantilever is irradiated by: generating the oscillation signal and the photothermal offset signal respectively; combining the oscillation signal and the photothermal offset signal to generate a combined signal; and irradiating the cantilever with the actuation beam under the control of the combined signal.

24. The method according to claim 23, characterized in that, Combining the oscillation signal and the photothermal offset signal includes adding the oscillation signal and the photothermal offset signal.

25. The method according to any one of claims 21 to 24, characterized in that, The rate of change of the photothermal offset signal is lower than that of the oscillation signal, or the bandwidth of the photothermal offset signal is lower than that of the oscillation signal.

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