Scanning probe microscope

Abstract

A scanning probe microscope comprises: a sample stage configured to support a sample; a probe stage configured to support a probe; an objective lens having an objective lens focal plane; and an interferometer configured to guide a detection light beam onto the objective lens. The objective lens is configured to focus the detection light beam onto the objective lens focal plane, and the interferometer is configured to receive a return light beam returning from the objective lens focal plane through the objective lens, combine the return light beam with a reference light beam to create interference, and measure the interference. An image sensor is provided; and an imaging lens is configured to receive image light from a vision focal plane through the objective lens and focus the image light onto the image sensor. The vision focal plane is offset from the objective lens focal plane, and the objective lens focal plane is located between the objective lens and the vision focal plane.

Description

Technology Field

[0001] The present invention relates to a scanning probe microscope and an associated method. Background Technology

[0002] When interacting with a sample using a scanning probe microscope, it is typically important to know the position of the probe relative to the sample's features so that the user can locate a known position on the sample to perform a scan. During a typical measurement, the sample is loaded onto a support structure and positioned by aligning a point on the sample with a point on the sample stage. For example, the sample may be a wafer with an identifiable notch that can be manually aligned with the support structure. This type of alignment can position the sample with only limited accuracy.

[0003] A known optical beam positioning unit is disclosed in U.S. Patent Publication US2016 / 0313368 A1. An optical beam positioning system enables a combination of two or more light beams of different wavelengths to be focused onto a probe or sample of a scientific instrument such as an atomic force microscope (AFM), which is used for a number of specific applications typical of an AFM, such as measuring the deflection or vibration of a probe and illuminating an object for optical imaging, as well as for atypical applications such as photothermal excitation of a probe, photothermal activation changes of a sample, photothermal cleaning of a probe, and photochemical, photovoltaic, photothermal, and other light beam-induced changes of a sample. The focused light beams can be positioned independently of each other.

[0004] A first aspect of the present invention provides a scanning probe microscope according to claim 1. Optionally, the offset between the objective lens focal plane and the detection focal plane is zero or smaller than the offset between the objective lens focal plane and the vision focal plane.

[0005] Optionally, the offset between the objective lens focal plane and the detection focal plane is zero, or less than 1 μm, less than 500 nm, less than 100 nm, less than 50 nm, or less than 20 nm.

[0006] Optionally, the objective lens focal plane and the detection focal plane are substantially coplanar.

[0007] Optionally, the scanning probe microscope further includes a probe attached to a probe stage, wherein the probe is in the detection focal plane, the return light beam is reflected from the probe, and the measurement of the interference provides an indication of the height of the probe.

[0008] Optionally, the probe stage and / or probe appear within the field of view of the image sensor.

[0009] Optionally, the probe stage and / or probe appear out of center within the image sensor's field of view.

[0010] Optionally, the interferometer is configured such that the return light beam and the reference light beam are each substantially collimated and optionally parallel to each other, whereby these light beams combine to produce interference.

[0011] Optionally, the scanning probe microscope further includes an actuation system configured to drive the probe toward and away from the sample surface.

[0012] Optionally, the offset between the vision focal plane and the detection focal plane is greater than 2 µm, greater than 5 µm, greater than 10 µm, or greater than 20 µm.

[0013] Optionally, the offset between the vision focal plane and the objective lens focal plane is greater than 2 µm, greater than 5 µm, greater than 10 µm, or greater than 20 µm.

[0014] Optionally, the offset between the vision focal plane and the detection focal plane is less than 100 µm or less than 50 µm.

[0015] Optionally, the offset between the vision focal plane and the objective lens focal plane is less than 100 µm or less than 50 µm.

[0016] Optionally, the scanning probe microscope further includes a scanning system configured to generate relative scanning motion between the probe stage and the sample stage in a direction substantially perpendicular to the optical axis of the objective lens.

[0017] Optionally, the interferometer includes a light source that generates a detection light beam.

[0018] Optionally, the scanning probe microscope further includes an illumination system comprising an illumination light source configured to illuminate a sample within the vision focal plane through an objective lens, optionally by Kohler illumination.

[0019] Optionally, the scanning probe microscope further includes a drive system configured to generate relative drive motion between the probe stage and the sample stage in a direction substantially perpendicular to the optical axis of the objective lens, wherein the motion range of the drive system is greater than the motion range of the scanning system.

[0020] Optionally, the image sensor includes a sensor array with an aspect ratio greater than 1 or greater than 1.5.

[0021] An additional aspect of the present invention provides a method for operating a scanning probe microscope according to claim 15.

[0022] Since the marker is typically placed in the vision focal plane, it is focused in the image of the marker. The surface of the probe is typically placed in the detection focal plane and not in the vision focal plane. The non-focused image of the probe may be placed within the field of view of the marker image. Alternatively, the probe may not be placed within the field of view of the marker image.

[0023] Optionally, the offset between the objective lens focal plane and the detection focal plane is zero, or smaller than the offset between the objective lens focal plane and the vision focal plane.

[0024] Optionally, the offset between the objective lens focal plane and the detection focal plane is zero, or less than 1 μm, less than 500 nm, less than 100 nm, less than 50 nm, or less than 20 nm.

[0025] Optionally, the objective lens focal plane and the detection focal plane are substantially coplanar.

[0026] Optionally, the identified location of the marker indicates the offset of the marker from the expected location within the field of view of the image.

[0027] Optionally, the marker is closer to the center of the sample than to the edge of the sample.

[0028] Optionally, the sample further comprises a second marker, and the method further comprises: a step of acquiring an image of the second marker using an optical vision system that receives light from a vision focal plane through an objective lens and an imaging lens; and a step of analyzing the image of the second marker to identify the position of the second marker within the field of view of the image and thereby improving the estimate of an unknown placement error.

[0029] Optionally, the second marker is closer to the edge of the sample than to the center of the sample.

[0030] Optionally, the marker is a fiducial marker.

[0031] Optionally, the return light beam and the reference light beam are each substantially collimated and optionally parallel to each other, where these light beams combine to produce interference.

[0032] Optionally, this method further includes the step of acquiring a topographical image based on the height of the probe determined by an interferometer when the probe interacts with the sample. Brief explanation of the drawing

[0033] Now, with reference to the attached drawings, embodiments of the present invention are described solely by example. FIG. 1 illustrates various parts of a scanning probe microscope according to one embodiment of the present invention. Figure 2 shows another part of the scanning probe microscope of Figure 1. Figure 3 illustrates the offset between the vision focal plane and the objective lens focal plane. Figure 4 illustrates details of the detection system. FIG. 5a illustrates a preferred infinity-corrected optical array of a detection system. Figure 5b illustrates an optical array in which the detection system is not a complete infinite correction type. Figure 6 illustrates a non-infinity-corrected optical array of an optical imaging system. Figure 7 illustrates a sample. Figure 8 shows an image of the first reference marker at the center of the sample. Figure 9 shows an image of the first reference marker after calibration of the driving system. Figure 10 shows an image of the second reference marker. Figure 11 illustrates a method of operating a scanning probe microscope. Specific details for implementing the invention

[0034] A scanning probe microscope (1) according to one embodiment of the present invention is illustrated in FIGS. 1 to 3. Some elements of the microscope (1) are illustrated in all three figures, while other elements are illustrated in only one or two figures.

[0035] The scanning probe microscope (1) includes a probe stage (4) configured to support probes (2, 3); an objective lens (32) shown in FIGS. 2 and 3; and a sample stage (11a).

[0036] The objective lens (32) has an objective lens focal plane (60) as shown in FIG. 3, FIG. 5a, FIG. 5b and FIG. 6. The objective lens focal plane (60) is defined in a conventional way: it is a plane where parallel rays focused by the objective lens (32) meet at one point.

[0037] FIGS. 1 and FIGS. 3 illustrate a microscope (1) in use with a probe (2, 3) attached to a probe stage (4) and a sample (7) mounted on a sample stage (11a), and FIG. 2 illustrates various elements of the microscope (1) in a state supplied to the end user before the probe and sample are introduced.

[0038] A scanning probe microscope (1) can be used to measure the characteristics of a sample (7) (e.g., material properties such as surface topology, elasticity, or composition). Alternatively, the scanning probe microscope (1) can be used to remove or deposit material on the sample (7), or to modify the sample in some other way.

[0039] As illustrated in FIG. 1, the probe comprises a cantilever (2) and a probe tip (3). The cantilever (2) is supported by a probe stage (4) and extends from a proximal end on the probe stage to a distal end away from the probe stage. The probe tip (3) is located at the distal end of the cantilever (2).

[0040] The probe tip (3) includes a conical or pyramidal structure that tapers from the base to a point at the distal end, which is the closest point to interact with the sample (7) on the sample stage (11a). The cantilever (2) may include a single beam having a rectangular profile. The cantilever (2) may have a length of about 20 microns, a width of about 10 microns, and a thickness of about 200 nm.

[0041] In this example, the probe tip (3) tapers to one point, but in other embodiments, the probe tip (3) may be particularly suitable for measuring sidewalls. For example, the probe tip (3) may have a flared shape.

[0042] The cantilever (2) is a thermal bimorph structure composed of two (or more) materials having different coefficients of thermal expansion (typically a silicon or silicon nitride base coated with gold or aluminum). The coating extends along the length of the cantilever and covers the opposite side from the tip (3). An actuation system (in the form of a laser (30)) under the control of a photothermal (PT) driving signal (33) is arranged to illuminate the coated upper side of the cantilever with an intensity-modulated radiation spot.

[0043] The actuation system (30) is configured to drive the probe toward and away from the sample surface by bending and straightening the cantilever (2) in this case.

[0044] The cantilever (2) can be formed as a monolithic structure having a uniform thickness. For example, the monolithic structure can be formed by selectively etching a thin film of SiO2 or SiN4 as described in the literature “Albrecht T., Akamine, S., Carver, TE, Quate, CFJ, Microfabrication of cantilever styli for the atomic force microscope, Vac. Sci. Technol. A 1990, 8, 3386 (hereinafter referred to as “Albrecht et al.”). The tip (3) can be formed integrally with the cantilever as described in Albrecht et al., can be formed by an additive process such as electron beam deposition, or can be formed separately and attached by an adhesive or other attachment method.

[0045] The wavelength of the collimated actuation beam (35) output by the laser (30) is selected for good absorption by the coating, so that the cantilever (2) bends along its length to move the probe tip (3). A suitable wavelength is 808 nm, but other wavelengths may be used. The actuation beam (35) is reflected toward the sample by the hot mirror (36) shown in FIG. 2, which reflects the infrared light of the actuation beam (35) but allows visible light to pass through.

[0046] In this example, the cantilever (2) has a cantilever coating on the opposite side of the sample so that it bends downward toward the sample when heated, but alternatively, the coating may be on the same side of the sample so that the cantilever (2) bends away from the sample when heated.

[0047] The height actuation system is configured to adjust the height of the proximal end of the cantilever (2) by moving the probe stage (4) vertically. The height actuation system may include a piezoelectric actuator (14) shown in FIGS. 1 to 3, which is driven by a piezo driving signal (5) shown in FIG. 1.

[0048] The probe scanning system (13) (e.g., a pair of piezoelectric actuators) illustrated in FIGS. 2 and 3 is configured to generate relative scanning motion between the probe stage (4) and the sample stage (11a) in a horizontal (XY) plane substantially perpendicular to the optical axis of the objective lens (32). In this case, the relative scanning motion is achieved by the motion of the probe stage (4), but in other embodiments, the relative scanning motion may be achieved by the motion of the sample stage (11a).

[0049] The sample stage driving system (15) illustrated in FIGS. 2 and 3 is configured to drive the sample stage (11a) in a horizontal (XY) plane substantially perpendicular to the optical axis of the objective lens (32). In this case, the driving motion is achieved by the motion of the sample stage (11a), but in other embodiments, the driving motion may be achieved by the motion of the probe stage (4). The motion range of the sample stage driving system (15) is greater than the motion range of the probe scanning system (13).

[0050] The detection system (80) is configured to guide the collimated detection light beam (103) shown in FIGS. 2 and 4 onto the objective lens (32) through a dichroic beam splitter (37) and a tip track mirror (26).

[0051] The objective lens (32) is configured to focus the collimated detection light beam (103) onto the objective lens focal plane (60), and the detection system (80) is configured to receive the collimated return light beam (31) returning from the objective lens focal plane (60) through the objective lens (32).

[0052] The detection system (80) is arranged to detect the height and angle of the free end of the cantilever (2) located immediately opposite the probe tip (3). The detection system (80) includes an interferometer (81) configured to determine the height of the probe, and a quadrant photodiode (QPD) (108) configured to determine the angle of the probe.

[0053] FIGS. 1 and 2 schematically illustrate a detection system (80), and FIG. 4 provides a more detailed drawing. As shown in FIG. 2, the detection system (80) is configured to receive a reflected return light beam (31) returning from the focal plane of an objective lens through an objective lens (32), a tracking mirror (26), and a dichroic beam splitter (37). As shown in FIG. 4, the interferometer (81) combines the return light beam (31) with a reference light beam (104) to create interference, and measures the interference using photodetectors (121, 122).

[0054] Collimated light (100) from a laser (101) is split by a beam splitter (102) into a detection light beam (103) and a reference light beam (104). The reference light beam (104) is guided onto a properly positioned retro-reflector (120) and then guided back to the beam splitter (102). The retro-reflector (120) is aligned to provide a fixed optical path length for the vertical (Z) position of the sample (7). The beam splitter (102) has an energy-absorbing coating and splits both the detection light beam (103) and the reference light beam (104) to generate first and second interferograms having a relative phase shift of 90 degrees. The two interferograms are each detected by photodetectors (121, 122).

[0055] Ideally, the outputs from the photodetectors (121, 122) are complementary sine and cosine signals with a 90-degree phase difference. Additionally, they must have no DC offset, have the same amplitude, and depend only on the position of the cantilever and the wavelength of the laser (101). Known methods are used to monitor the outputs of the photodetectors (121, 122) while changing the optical path difference to determine the error resulting from the two photodetector outputs not being perfect harmonics with the same amplitude and orthogonal phase, and to apply a correction thereto. Similarly, the DC offset level is also corrected according to methods known in the art.

[0056] These photodetector outputs are suitable for use with a fringe subdivision device (123) and a conventional interferometric reversible fringe counting device, which can be provided as dedicated hardware, an FPGA, a DSP, or a programmed computer. An orthogonal phase fringe counting device can measure the displacement of the cantilever position with an accuracy of λ / 8, which is 66 nm for 532 nm light.

[0057] Known fringe segmentation techniques based on the arc tangent of a signal can improve accuracy to a sub-nanometer scale. In the above-described embodiment, the reference light beam (104) is arranged to have a fixed optical path length with respect to the Z position of the sample (7). Thus, it may be reflected from the surface of the sample stage (11a) or from a retroreflector whose position is associated with the position of the sample stage. The reference path length may be greater or smaller than the path length followed by the detection light beam (103). Alternatively, the relationship between the reflector and the sample Z position does not need to be fixed. In this embodiment, the reference light beam (104) may be reflected from a fixed point, and the fixed point has a known (but variable) relationship with the Z position of the sample. Thus, the tip height is inferred from the path difference measured by an interferometer and the Z position of the sample with respect to the fixed point.

[0058] In the preferred optical arrangement shown in FIG. 4, the interferometer (81) is configured such that the return light beam (31) and the reference light beam (104) are each substantially collimated and optionally parallel to each other, where these light beams are combined at photodetectors (121, 122) to produce interference.

[0059] In an alternative optical arrangement, which is a less preferred embodiment of the present invention, the return light beam (31) and / or reference light beam (104) may not be collimated at the location where they are combined at the photodetectors (121, 122). For example, the collimated beam (31, 104) may be focused onto the photodetectors (121, 122) by an additional lens.

[0060] The infinite correction optical array of FIG. 4 is desirable because the collimated beam (31, 104) has a planar wavefront, so the path length of the light rays within the beam does not change over the cross-section of the beam. In addition, this makes the beam (31, 104) have a relatively large cross-section, allowing them to easily overlap each other at the photodetectors (121, 122).

[0061] Additionally, the reflected return light beam (31) is split into first and second components (107, 110) by a beam splitter (106). The first component (107) is guided to a quadrant photodiode (108) split through a lens (109), and the second component (110) is split by a beam splitter (102) and guided to photodiodes (121, 122) to generate a height signal (20). The photodiode (108) indicates the position of the first component (107) of the reflected return light beam (31) on the photodiode (108) and generates an angle signal (124) that varies according to the angle of inclination of the cantilever relative to the detection beam (103).

[0062] The output of the detection system (80) is a height signal (20) and an angle signal (124), each of which can be monitored to obtain a series of topographic measurements representing the topography of the sample (7), which can be used to generate a topographic image of the sample (7).

[0063] As the tip track mirror (26) is tilted, the detection light beam (103) and the actuation beam (35) are deflected and maintain their positions on the cantilever (2).

[0064] When capturing an image of a sample, the scanning probe microscope (1) can be operated in a number of different modes. In one non-limiting example, it can be operated in the following dynamic modes. The cantilever is oscillated by a laser (30), and using a Z-actuator (14), the probe tip first comes into intermittent contact with the sample.

[0065] The probe stage (4) is lowered by a piezoelectric actuator (14) to move the probe tip (3) toward the sample, while in this embodiment, the detection system (80) monitors the deflection of the cantilever as it vibrates and changes over time. When the amplitude of the deflection vibration reaches a predetermined level, the movement of the probe stage (4) is stopped.

[0066] As the raster scan progresses, the probe tip moves up and down as the surface height / interaction force changes. A higher frequency component resulting from the vibration motion of the cantilever (2) is superimposed on this surface-induced motion. The amplitude, phase, and frequency of the vibration component of this probe tip motion all depend on the oscillator settings and the interaction force between the surface and the probe tip. Likewise, the height of the probe tip is a superposition of the component resulting from the interaction with the sample surface and the second component resulting from the probe vibration. The amplitude, phase, and frequency of the second component are influenced 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 is monitored.

[0067] The height signal (20) contains accurate information regarding the height of the probe tip (3), but it will be understood that for useful information to be obtained, this information must be related to the features of the sample surface. Theoretically, the lowest point of each vibration cycle represents the actual height of the surface. Thus, the probe microscope can be configured to find the lowest height measurement for each probe vibration cycle, which is output as a topographic measurement signal that can subsequently be used to form a topographic image. That is, the lowest point of each vibration cycle represents the position of the sample surface.

[0068] In this example, the image is constructed based on topographic measurements and an XY position signal indicating the current XY position of the probe tip (3). In another embodiment of the invention, instead of generating an image, the scanning probe microscope (1) may use the topographic measurements in some other way, for example, to measure the critical dimensions of a sample (e.g., wall height, wall angle, trench width, etc.). In this case, a series of topographic measurements may be measured by a single line scan (e.g., across a trench) instead of scanning in two dimensions to generate a topographic image.

[0069] The optical vision system (50) illustrated in FIG. 2 is configured to generate an image of a small field of view of a sample (7). The optical vision system (50) includes an image sensor (52), such as a two-dimensional CCD array; and an imaging lens (54) (commonly known as a tube lens). The imaging lens (54) is configured to receive image light (55) from a vision focal plane (62) (illustrated in FIG. 3) through an objective lens (32) and a periscope (82), and to focus the image light (55) onto the CCD array of the image sensor (52) through a right-angle prism (84) and a mirror. As illustrated in FIG. 3, the vision focal plane (62) is offset behind the objective lens focal plane (60), and the objective lens focal plane (60) is located between the objective lens (32) and the vision focal plane (62). The vision focal plane (62) and the objective lens focal plane (60) are parallel to each other.

[0070] Image light (55) is generated in a lighting system (40) comprising a lighting light source (41) and a lighting system lens (42). Optionally, the lighting system (40) illuminates the sample (7) through an objective lens (32) by Köhler illumination to produce uniform illumination over a wide area of ​​the sample.

[0071] Light from the light and lighting system (40) to the optical vision system (50) travels through a periscope (82) each comprising a pair of mirrors and a beam splitter (83).

[0072] As illustrated in FIGS. 5a / 5b and FIGS. 6, the detection system (80) and the optical vision system (50) have different optical arrays: the optical array of the detection system (80) is an infinite correction type (or approximately an infinite correction type), and the optical array of the optical vision system (50) is a non-infinite correction type.

[0073] FIG. 5a is a schematic diagram showing a preferred infinite correction optical array for a detection system (80). The detection light beam (103) is collimated when entering the objective lens (32) and focused onto the upper surface of the probe (2) placed on the focal plane (60) of the objective lens. The reflected return light beam (31) is collimated when exiting the objective lens (32) and is input into the detection system (80). This infinite correction optical array is advantageous because it allows the detection system (80) to be positioned at any optical distance from the objective lens. Additionally, this allows the return light beam (31) to be collimated at a position where the interferometer (81) combines the return light beam (31) with the reference light beam (104), thereby providing the aforementioned accuracy advantage.

[0074] FIG. 5b illustrates an alternative optical arrangement in which the detection light beam (103) is substantially collimated but not completely collimated when it enters the objective lens (32). As a result, it is focused on a detection focal plane (61) that is slightly offset from the objective lens focal plane (60). In this case, the surface of the probe (2) is placed on the detection focal plane (61), but may be placed on the objective lens focal plane (60) or between these planes (60, 61).

[0075] The interferometer is configured to receive a reflected return light beam (31) returning from the detection focal plane (61), and in the case of FIG. 5b, the reflected return light beam (31) is not fully collimated when it exits the objective lens (32) and enters the detection system (80). Thus, in the full infinite correction optical array of FIG. 5a, the offset between the objective lens focal plane (60) and the detection focal plane (61) is zero (i.e., the objective lens focal plane (60) and the detection focal plane (61) are substantially on the same plane), whereas in the case of the approximate infinite correction of FIG. 5b, a small offset exists between the objective lens focal plane (60) and the detection focal plane (61).

[0076] As illustrated in FIG. 6, the vision focal plane (62) is offset from the objective lens focal plane (60), and in the case of FIG. 5b, it is also offset from the detection focal plane (61). Consequently, the image light (55) focused on the image sensor (52) is not collimated when it exits the objective lens (32). Therefore, the optical array of the optical vision system (50) is a non-infinite correction type.

[0077] The offset between the vision focal plane (62) and the objective lens focal plane (60) (and the detection focal plane (61), where they are different) may be in the range of 10 µm to 50 µm, for example, about 30 µm. More generally, the offset between the vision focal plane (62) and the objective lens focal plane (60) (and the detection focal plane (61)) may be greater than 2 µm, greater than 5 µm, greater than 10 µm, or greater than 20 µm. The offset between the vision focal plane (62) and the objective lens focal plane (60) (and the detection focal plane (61)) may be less than 100 µm or less than 50 µm.

[0078] Any offset (shown in FIG. 5b) between the objective lens focal plane (60) and the detection focal plane (61) is as small as possible and is smaller than the offset (shown in FIG. 6) between the objective lens focal plane (60) and the vision focal plane (62). For example, any offset between the objective lens focal plane (60) and the detection focal plane (61) may be smaller than 1 μm, smaller than 500 nm, smaller than 100 nm, smaller than 50 nm, or smaller than 20 nm.

[0079] FIG. 5b illustrates an example where the objective lens focal plane (60) is offset behind the detection focal plane (61), but instead in other embodiments, the objective lens focal plane (60) may be offset in front of the detection focal plane (61).

[0080] FIG. 7 illustrates an exemplary sample (7) for imaging with a scanning probe microscope (1). The sample (7) may be a circular semiconductor wafer.

[0081] The probe scanning system (13) may be used to perform a raster scan of the sample (7) over a small area (on the order of microns) to acquire an image of a specific sample feature, such as a trench, at a known location on the wafer. Before the raster scan is performed, the sample (7) must be driven by the sample stage driving system (15) so that the probe tip (3) is positioned very accurately at a specific location of the sample feature, typically with an accuracy of about 1 µm or 2 µm. If the sample (7) can be positioned and oriented with high accuracy relative to the sample stage (11a), then driving the probe tip (3) to a specific location of the sample feature would be merely applying appropriate X and Y driving signals to the sample stage driving system (15). However, it may not be possible to position the sample (7) sufficiently accurately on the sample stage (11a).

[0082] The sample (7) may include an alignment notch (72) used to roughly align the wafer with respect to the sample stage (11a) at a desired position and orientation. However, since the alignment achieved through the alignment notch (72) is only roughly accurate, there is an unknown placement error (X offset) at the X position of the sample, an unknown placement error (Y offset) at the Y position of the sample, and an unknown placement error (θ offset) at the angular orientation of the sample. The X and Y offset values ​​may be on the order of several hundred microns, and the θ offset value may be on the order of 0.1 degrees.

[0083] FIG. 11 illustrates an exemplary method of operating a scanning probe microscope (1) to account for such placement errors.

[0084] In step 82, a sample (7) is placed on the sample stage (11a) using a robot arm or other device. In pre-alignment step 83, the sample (7) is roughly aligned using a device that detects an alignment notch (72) and uses this notch (72) as a reference point to roughly align the sample (7) with an accuracy of 100 μm for the XY offset and 0.1 degree for the rotation offset.

[0085] In the next step 84, the probe stage (4) is driven by the scanning system so that the probe tip (3) is positioned at a first position (74a) at the center of the sample (7) as shown in FIG. 7.

[0086] In the next step 85, an image is acquired by the image sensor (52). As described above, during step 85, the image sensor (52) is arranged to receive light from the vision focal plane (62) through the objective lens (32) and the imaging lens (54).

[0087] FIG. 8 is a schematic diagram of an image obtained during the first execution of step 85. The image sensor (52) includes a sensor array having pixels in row X and pixels in column Y. The aspect ratio (Y / X) of the array may be greater than 1, for example, in the range of 1.5 to 2 (approximately 1.8 in this example). This aspect ratio allows the probe (2) and / or probe stage (4) to be positioned off-center from the image sensor's field of view, as shown in FIG. 8.

[0088] For example, the width (Dx) and / or length (Dy) of the field of view may be about 500 µm.

[0089] It should be noted that the probe (2) and the probe stage (4) are not focused in the image because they are placed outside the vision focal plane (62), but are shown in FIG. 8 to indicate their position within the field of view. The major axis of the CCD array can be aligned with the Y-axis of the probe, that is, aligned along the length of the cantilever as shown in FIG. 8.

[0090] The reference marker (65) is supported by the sample (7) and is positioned at the exact center of the sample (7), that is, at the center of the first position (74a). Since the reference marker (65) is placed on the vision focal plane (62), it is focused in the image.

[0091] The center of the field of view of the image sensor (52) is indicated by a pair of crosshairs in FIG. 8. These crosshairs may not be included in the image and are illustrated in FIG. 8 to represent the center of the field of view used as a reference point. In this example, the probe tip (3) is positioned precisely with a known X and Y offset relative to the center of the field of view.

[0092] If the sample (7) is positioned perfectly accurately on the sample stage (11a), the reference marker (65) will be perfectly centered at the center of the field of view of FIG. 8. However, due to the aforementioned positioning error, there is an error in the X direction (X error) and an error in the Y direction (Y error).

[0093] It should be noted that by taking the first image from the center of the wafer, the reference marker (65) is placed within the field of view regardless of the magnitude or sign of the placement error (θ offset) in the angular orientation of the sample.

[0094] In step 86, the image of the reference marker (65) is analyzed, for example, using an automatic optical recognition system to identify the position of the reference marker (65) within the field of view of the image. In this analysis step, the center of the reference marker (65) is identified, and error values ​​X error and Y error are measured. Then, in step 87, the error values ​​are stored to record the position of the reference marker (65). These error values ​​identify the position of the reference marker (65) by indicating the offset of the reference marker (65) from the expected position within the field of view of the image, thereby providing approximate estimates of the placement error X offset and Y offset.

[0095] In this example, the error values ​​(X error and Y error) are determined based on an orthogonal coordinate system. In another embodiment, the offset of the reference marker (65) from the expected position in the image can be recorded in a polar coordinate system using error values ​​R error and θ error.

[0096] In the next step 88, the error values ​​(X error and Y error) are used to correct the sample stage drive system (15) with the associated translational correction values ​​(X correction value and Y correction value). Thus, the drive system (15) is corrected based on approximate estimates of the placement error X offset and Y offset represented by these error values ​​(X error and Y error). Optionally, as part of step 88, the corrected sample stage drive system (15) may be operated to adjust the position of the probe stage (11a) (based on the X correction value and Y correction value) so that the reference marker (65) moves to an expected position within the image as shown in FIG. 9.

[0097] It should be noted that the reference marker (65) is oriented at an oblique angle to the crosshairs due to the placement error (θ offset) in the angular orientation of the sample. This angle is exaggerated in FIG. 9 and is typically too small to be measured.

[0098] In the next iteration of step 84, the sample stage (11a) is driven so that the probe tip is positioned at the next position of the sequence (74a-74e). Then, steps 85-88 are repeated at each position.

[0099] In the first iteration, the probe tip is positioned at a second position (74b) slightly away from the center of the sample (7), which supports the second reference marker (66) shown in FIG. 10. This second position (74b) is selected for the first iteration because it is less sensitive to angular orientation misalignment of the sample (7) than the position facing the outer perimeter of the wafer, and thus, the second reference marker (66) can be ensured to be placed within the field of view regardless of the magnitude or sign of the placement error (θ offset) in the angular orientation of the sample (7).

[0100] In the first iteration of step 88, the error values ​​(X error and Y error) can be used to correct the sample stage drive system (15) with the associated angle correction value (θ correction value).

[0101] In subsequent iterations of step 84, the sample stage is driven so that the probe tip is positioned at peripheral locations (74c, 74d, 74e) toward the outer periphery of the wafer (each location supports a respective reference marker). In each iteration, the correction values ​​X, Y, and θ are further adjusted based on a more accurate estimate of the unknown placement error.

[0102] Each reference marker (65, 66) is closer to the center of the sample than to the edge of the sample. The reference markers at the surrounding locations (74c, 74d, 74e) are each closer to the edge of the sample than to the center of the sample.

[0103] This process is repeated until the correction value becomes accurate enough that the probe tip can be driven precisely to a known position with sufficient accuracy to ensure that a small area covered by the raster scan contains the sample features of interest. Upon completion of the correction, the correction values ​​(X correction value, Y correction value, θ correction value) provide an accurate estimate of the unknown error (X offset, Y offset, θ offset) in the placement of samples on the stage.

[0104] When the calibration is complete, in step 89, the calibrated driving system (15) is operated to drive the sample (7), so that the probe and the sample are positioned such that the probe tip (3) is aligned with a sample feature at a known location, which typically does not include a reference marker.

[0105] Finally, in step 90, a raster scan of the sample features is performed using a microscope operating in dynamic mode as described above, for example, and during this step, the probe tip (3) interacts with the sample features. During the scanning step 90, the height and / or angle of the probe is determined using a detection system (80), and a topographic image of the sample features can be obtained.

[0106] In the example above, the probe interacts with the sample to measure some characteristics of the sample (material properties such as surface topology, elasticity, or composition). Alternatively, the probe can be used to remove or deposit material on the sample, or to modify the sample in some other way.

[0107] If both the detection system (80) and the optical vision system (50) have infinitely corrected optical arrays, it is necessary to move the objective lens (32) upward before step 90 to focus on the probe for scanning in step 90.

[0108] As described above, the probe is located at the objective lens focal plane (60) offset from the imaging focal plane (62). Therefore, since there is no need to move the objective lens (32) before step 90, the process of FIG. 11 can be performed more quickly.

Claims

Claim 1 A scanning probe microscope comprising: a sample stage configured to support a sample; a probe stage configured to support a probe; an objective lens having an objective lens focal plane; an interferometer configured to guide a detection light beam onto the objective lens—the objective lens being configured to focus the detection light beam onto the detection focal plane, and the interferometer being configured to receive a return light beam returning from the detection focal plane through the objective lens, combine the return light beam with a reference light beam to create interference, and measure the interference—; an image sensor; and an imaging lens configured to receive image light from a vision focal plane through the objective lens and focus the image light onto the image sensor, wherein the vision focal plane is offset from the objective lens focal plane and offset from the detection focal plane, and the objective lens focal plane and the detection focal plane are located between the objective lens and the vision focal plane. Claim 2 A scanning probe microscope according to claim 1, wherein the offset between the objective lens focal plane and the detection focal plane is zero or smaller than the offset between the objective lens focal plane and the vision focal plane. Claim 3 A scanning probe microscope according to claim 2, wherein the offset between the objective lens focal plane and the detection focal plane is zero, or less than 1 μm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm. Claim 4 A scanning probe microscope according to any one of claims 1 to 3, wherein the objective lens focal plane and the detection focal plane are substantially on the same plane. Claim 5 A scanning probe microscope according to any one of claims 1 to 4, further comprising a probe attached to the probe stage, wherein the probe is in the detection focal plane, the return light beam is reflected from the probe, and the measurement of the interference provides an indication of the height of the probe. Claim 6 A scanning probe microscope according to claim 5, wherein the probe appears out of center in the field of view of the image sensor. Claim 7 A scanning probe microscope according to any one of claims 1 to 6, wherein the interferometer is configured such that the return light beam and the reference light beam are each substantially collimated and optionally parallel to each other, wherein these light beams are combined to produce the interference. Claim 8 A scanning probe microscope according to any one of claims 1 to 7, further comprising an actuation system configured to drive the probe toward and away from the sample surface. Claim 9 A scanning probe microscope according to any one of claims 1 to 8, wherein the offset between the vision focal plane and the detection focal plane is greater than 2 μm, greater than 5 μm, greater than 10 μm, or greater than 20 μm; and the offset between the vision focal plane and the objective lens focal plane is greater than 2 μm, greater than 5 μm, greater than 10 μm, or greater than 20 μm. Claim 10 A scanning probe microscope according to any one of claims 1 to 9, wherein the offset between the vision focal plane and the detection focal plane is less than 100 μm or less than 50 μm; and the offset between the vision focal plane and the objective lens focal plane is less than 100 μm or less than 50 μm. Claim 11 A scanning probe microscope according to any one of claims 1 to 10, further comprising a scanning system configured to generate a relative scanning motion between the probe stage and the sample stage in a direction substantially perpendicular to the optical axis of the objective lens. Claim 12 A scanning probe microscope according to claim 11, further comprising a driving system configured to generate relative driving motion between the probe stage and the sample stage in a direction substantially perpendicular to the optical axis of the objective lens, wherein the motion range of the driving system is greater than the motion range of the scanning system. Claim 13 A scanning probe microscope according to any one of claims 1 to 12, wherein the image sensor comprises a sensor array with an aspect ratio greater than 1 or greater than 1.

5. Claim 14 A scanning probe microscope according to any one of claims 1 to 13, wherein the interferometer comprises a light source that generates the detection light beam; and the scanning probe microscope further comprises an illumination system comprising an illumination light source configured to illuminate a sample within the vision focal plane through the objective lens by optionally Köhler illumination. Claim 15 A method for operating a scanning probe microscope, comprising the steps of: placing a sample having a marker on a sample stage—the sample being placed with an unknown placement error—; providing a probe; acquiring an image of the marker using an optical vision system that receives image light from a vision focal plane through an objective lens and an imaging lens—the objective lens having an objective lens focal plane, the imaging lens focusing the image light onto an image sensor, the vision focal plane being offset from the objective lens focal plane, and the objective lens focal plane being located between the objective lens and the vision focal plane—; analyzing the image of the marker to identify the position of the marker within the field of view of the image and thereby estimating the unknown placement error; correcting a driving system based on the estimated value; and operating the corrected driving system to drive the sample or the probe so that the probe is aligned with the sample feature. A method comprising the step of determining the height of the probe when the probe interacts with the sample feature using an interferometer, wherein the interferometer guides a detection light beam onto the objective lens, the detection light beam is focused onto the probe at a detection focal plane by the objective lens, the detection light beam is reflected by the probe to generate a return light beam, the interferometer receives the return light beam through the objective lens, combines the return light beam with a reference light beam to create interference, measures the interference to determine the height of the probe, and wherein the vision focal plane is offset from the detection focal plane, and the detection focal plane is located between the objective lens and the vision focal plane. Claim 16 A method according to claim 15, wherein the offset between the objective lens focal plane and the detection focal plane is zero or smaller than the offset between the objective lens focal plane and the vision focal plane. Claim 17 The method of claim 16, wherein the offset between the objective lens focal plane and the detection focal plane is zero, or less than 1 μm, or less than 500 nm, or less than 100 nm, or less than 50 nm, or less than 20 nm. Claim 18 A method according to any one of claims 15 to 17, wherein the objective lens focal plane and the detection focal plane are substantially on the same plane. Claim 19 A method according to any one of claims 15 to 18, wherein the identified position of the marker represents an offset of the marker from an expected position within the field of view of the image. Claim 20 A method according to any one of claims 15 to 19, wherein the marker is closer to the center of the sample than to the edge of the sample. Claim 21 A method according to any one of claims 15 to 20, wherein the sample further comprises a second marker, and the method further comprises: a step of acquiring an image of the second marker using the optical vision system; and a step of analyzing the image of the second marker to identify the position of the second marker within the field of view of the image and thereby improving the estimated value of the unknown placement error. Claim 22 The method of claim 21, wherein the second marker is closer to the edge of the sample than to the center of the sample. Claim 23 A method according to any one of claims 15 to 22, wherein the marker is a reference marker. Claim 24 A method according to any one of claims 15 to 23, wherein the return light beam and the reference light beam are each substantially collimated and optionally parallel to each other, wherein these light beams are combined to produce the interference. Claim 25 A method according to any one of claims 15 to 24, further comprising the step of acquiring a terrain image based on the height of the probe determined by the interferometer when the probe interacts with the sample feature.

Images