Atomic force microscope (AFM) device and its operation method

Abstract

Atomic force microscope (AFM) device and its operating method An atomic force microscope (AFM) apparatus 1 is disclosed that includes at least one scanning head for scanning a sample 9. A cantilever deflection detector 30 measures the deflection d of the probe relative to the scanning head during scanning and provides an output signal indicative of the measured deflection. A controller 40 of the AFM apparatus receives and analyzes the output signal from the cantilever deflection detector to measure the deflection of the probe, and automatically adjusts one or more imaging parameters during scanning in response to information about sample characteristics near the location of the sample currently being scanned. The information about the sample characteristics includes one or more of information from a specification of the sample, a recipe for manufacturing the sample.

Description

[Technical field]

[0001] The present invention relates to an Atomic Force Microscope (AFM) apparatus. The present invention further relates to a method of operating an AFM apparatus. [Background technology]

[0002] AFM devices are widely used, for example in the semiconductor industry, to measure the surface topography of (semi-finished) products. An AFM device comprises a scanning head with a tip that scans the surface of a sample along a scanning trajectory, and the motion of the AFM device is determined by various imaging parameters such as force threshold, baseline offset, approach profile, retraction profile, scanning speed, etc.

[0003] Actual samples for testing in the semiconductor industry typically include 3D samples with narrow, high aspect ratio features. Thus, when the surface topography is imaged by the AFM device, it may deviate from the actual surface topography of the sample due to various reasons. For example, if the tip is located on a relatively narrow feature, a relatively high contact force may cause the tip to slide off the top of the narrow feature and slide to a lower level (height). As a result, the height value shown in the imaged surface topography for a lateral position x,y is smaller than the actual height of the sample at that lateral position. It has also been found that deviations may occur, for example, when the tip is located at a lateral position of a narrow, relatively deep recess in the sample. An example of this is a feature with a characteristic lateral size that is moderately larger than the tip diameter, i.e., more than one time the tip diameter and less than ten times the tip diameter, and a depth that is substantially larger than the width, e.g., more than two times the diameter. Such features are, for example, holes whose diameter is the characteristic lateral size, or grooves whose width is the characteristic lateral size. In this case, misalignment may occur because the contact force of the tip is not sufficient to overcome the frictional force that the recess walls exert on the tip, and the tip does not protrude far enough into the recess. In this case, the height value shown in the imaged surface topography for a lateral position x,y will be larger than the actual height of the sample at that lateral position. Note that, in general, such misalignment will be smaller if the tip-cantilever combination is relatively stiff. The stiffness of the tip-cantilever combination is determined by the rotational stiffness of the cantilever and the bending stiffness of the tip. Other local device properties may also affect the imaged surface topography. For example, a feature of a relatively hard material may appear to be at a higher level in the imaged surface topography than a feature of a relatively soft material, even though the features are actually at the same level on the surface of the imaged 3D sample. Summary of the Invention [Problem to be solved by the invention]

[0004] It is a first object of the present disclosure to provide a method of operating an AFM apparatus that allows for more accurate imaging of 3D samples by mitigating deviations in the imaged surface topography from the actual device topography that depend on local device features, such as deviations in the presence of narrow, high aspect ratio features and deviations associated with differences in surface hardness.

[0005] A second object of the present disclosure is to provide an AFM apparatus that enables more accurate imaging of 3D samples by mitigating deviations in the imaged surface topography from the actual device topography that depend on local device features.

[0006] According to a first object, a method of operating an AFM device includes scanning a surface of a sample along a first lateral current scan line with a probe having a tip, measuring an interaction between the tip and the sample, and adjusting one or more imaging parameters, including a force threshold, during the scan based on prior information about the sample. The prior information about the sample includes information about the sample acquired prior to the scan. The prior information about the sample acquired prior to the scan may include, for example, information from a sample specification and / or information about the sample acquired during recipe creation. The sample specification defines the desired topography of the sample and the properties of the materials used for its features. The information about the sample acquired during recipe creation may indicate that the actual topography tends to deviate in a particular manner from the designed topography. The information acquired during recipe creation does not completely describe the actual topography, but is useful for predicting how the actual topography will deviate. For example, the information acquired during recipe creation may indicate that the width of the ends is narrower on average than the width originally specified. Prior information about the sample allows surface elements of the sample to be scanned at optimal settings, so that the output signals obtained at these settings more accurately indicate the actual height of the surface as a function of lateral position. Information about the sample obtained prior to scanning may further include information obtained from a previous device inspection method, for example information obtained from an electron microscope image of the sample.

[0007] The force threshold is the upper limit of the force that the tip can exert on the sample.

[0008] Based on information about the sample obtained prior to the scan, a relatively high force threshold is selected if information from the region of the current scan position indicates that the current scan point is expected to be within a relatively deep and narrow recess or at a bottom close to a steep wall, and a relatively low force threshold is selected if information from the region indicates a relatively wide recess or a wide plateau away from a relatively steep and high edge / wall.

[0009] Although the information about the sample acquired prior to scanning does not necessarily correspond exactly to the actual topology of the sample, this information makes it possible to optimally set the force threshold in order to more accurately determine the actual topology.

[0010] For example, surface features with a high level close to a low level are more accurately imaged by setting the force threshold relatively low, which reduces the risk of the tip sliding towards the low level. It also reduces the risk of the tip damaging the sample surface. As another example, relatively narrow and deep recesses in the sample surface are more accurately imaged by setting the force threshold relatively high, which allows the force on the tip to more easily overcome the forces exerted on the tip by the side walls of the recess, e.g., due to friction. This allows the depth of the recess to be more accurately represented in the imaged surface topography.

[0011] The information about the sample may further include information obtained from scanning a previous scan line in a direction substantially parallel to the current scan line.

[0012] The present invention is particularly suited to embodiments of trigger-based scanning, where the scan includes a scan cycle for each scan position, having the steps of: (a) approaching the tip to the sample at a lateral position on the sample surface, (b) retracting the tip from the sample, and (c) moving the tip towards a new lateral position. Interaction data indicative of the interaction of the tip with the sample is acquired during approach and / or retraction.

[0013] The approach to the sample in step (a) is determined by an approach profile. Examples of imaging parameters that may be adjusted include the initial height before approaching the sample surface and the speed of approaching the sample.

[0014] The retraction of the sample is determined by the retraction profile. Examples of imaging parameters that are adjusted include a force threshold, a displacement threshold, a retraction distance, and a speed. For example, retraction of the tip from the sample is initiated when the force exerted by the tip exceeds a predefined force threshold. As another example, retraction is initiated when the measured depth exceeds a depth threshold. As another example, retraction is initiated as soon as either of these conditions occurs, whichever occurs first, or when both conditions are met. Another imaging parameter related to the retraction phase is the retraction distance. In one example, the retraction distance is increased where the adhesion of the surface is determined to be relatively high. The retraction distance may also be adjusted depending on the local situation. That is, retraction may be interrupted and the surface may be approached again if it is detected that the resonance of the cantilever has sufficiently decreased even before the retraction of the predetermined distance is completed. This improves the measurement speed.

[0015] During operation, the cantilever is deflected due to forces acting between the tip and the sample surface. Electrostatic forces can also cause deflection of the cantilever at a distance from the sample. This introduces a bias that can be compensated for by taking into account the baseline offset. If necessary, the baseline offset may be updated every measurement cycle.

[0016] It should be noted that multiple imaging parameters may be adjusted during operation, for example both the baseline offset and the force threshold may be adjusted depending on prior knowledge of the sample at the lateral position being scanned, with the force threshold being selected relative to the baseline offset.

[0017] As mentioned above, the prior information about the sample may include information from the sample specification. This may include device design data and / or recipe data. In normal circumstances, it can be assumed that the topography of the measured sample is approximated by a topography derived from the prior information about the sample, e.g., the sample specification and / or information obtained during recipe creation. Adjusting one or more imaging parameters during scanning according to the expected topography can provide a significant improvement compared to using a uniform setting for imaging the entire sample, although the actual scanned position may deviate slightly from the desired scanned position and the topography of the imaged structure may actually deviate to some extent from the originally designed topography.

[0018] Also, as noted above, the prior information about the sample may include information obtained from scanning a previous scan line in a direction substantially parallel to the current scan line, again providing a significant improvement over using a uniform setting for imaging the entire sample, as the topography of the current scan line will be approximated by the topography of the previous scan line if the current and previous scan lines are sufficiently close together.

[0019] In some embodiments, subsequent scan lines are substantially parallel, i.e., in this case, subsequent scan lines are at a distance Δy from each other. In operation, scanning proceeds by moving the probe in the x direction along scan lines y, y+Δy, y+2Δy, etc. Alternatively, instead of increasing the coordinate y between subsequent scan lines stepwise, y may be increased gradually during the scan, which results in a triangular scan path.

[0020] Note that subsequent scan lines do not have to be exactly parallel to one another.

[0021] In some embodiments, imaging parameters are adjusted during the scan based on prior information about the sample, from prior information obtained from previous scan lines, thereby achieving further improvement.

[0022] In some embodiments, prior information about the sample is used in combination with information acquired during the scan: for example, if at a position x along a current scan line y the topography appears to deviate significantly from the topography at a position x along the previous scan line y-Δy, this deviation may be incorporated in preparing adjustments to the settings for a subsequent position x+Δx along the current scan line.

[0023] One embodiment further includes providing an acoustic signal to one or more of the probe, tip, or sample, and analyzing an output signal based on an interaction of the acoustic signal with the sample, the output signal being indicative of a subsurface characteristic of the sample, such that both topographical and subsurface information can be obtained in a common measurement, for example, for overlay and / or alignment analysis.

[0024] The force that the tip exerts on the sample is proportional to the deflection of the cantilever and the stiffness of the cantilever. In one embodiment, vibration of the probe is induced to change the effective force that the probe exerts on the sample. The vibration increases the stiffness of the cantilever, thereby increasing the effective force on the sample.

[0025] Although this approach is intended to avoid scanning the same location or scan line multiple times, analysis of image data acquired during a scan may indicate that it is desirable to perform one or more additional scans with different imaging parameter settings, which may then replace the original image data or may combine the original image data with new image data acquired from the one or more additional scans. [Brief description of the drawings]

[0026] [Figure 1] FIG. 1 shows a schematic of a portion of an exemplary improved AFM device and a sample scanned by the device. [Diagram 2] FIG. 2 illustrates aspects of a controller according to a first embodiment of an improved AFM device. [Diagram 3] FIG. 10 is a schematic diagram showing two graphs of the Z position signal of the Z actuator and the measured deflection signal from the probe tip in an embodiment of the present invention; [Figure 4] 4A-4C are schematic diagrams illustrating four probe deflection situations associated with particular positions in FIG. 3. [Diagram 5] FIG. 13 illustrates aspects of a controller according to a second embodiment of an improved AFM device. [Figure 6] FIG. 13 illustrates aspects of a controller according to a third embodiment of an improved AFM device. [Figure 7] FIG. 13 illustrates aspects of a controller according to a fourth embodiment of an improved AFM device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention will be further elucidated by describing specific embodiments with reference to the accompanying drawings. The detailed description provides possible examples of the present invention, but does not describe only the embodiments that fall within the scope of the present invention. The scope of the present invention is defined in the claims, and the following description should be considered as an example rather than a limitation on the present invention.

[0028] One embodiment of an improved atomic force microscope (AFM) apparatus 1 will now be described with reference to Figures 1 and 2. Figure 1 shows a schematic of part of an exemplary improved AFM apparatus and a sample 9 scanned by the apparatus. Figure 2 shows aspects of a controller according to a first embodiment of the improved AFM apparatus.

[0029] 1, the AFM apparatus 1 comprises at least one scanning head 10 for scanning a sample 9. The scanning head 10 includes a probe 11 comprising a cantilever 12 and a probe tip 13 disposed on the cantilever.

[0030] The AFM apparatus 1 further comprises a first actuator 20 which cooperates with the substrate holder 95 for relatively moving the probe tip 13 and the sample 9 in one or more directions x, y parallel to the sample surface 91 in order to scan the surface 91 of the sample with the probe tip. In other embodiments the first actuator cooperates with a scan head and in yet other embodiments a first actuator is provided which cooperates respectively with the scan head and the substrate holder in order to perform the scanning of the surface 91.

[0031] The AFM device 1 further comprises a cantilever deflection detector 30 configured to measure the relative movement / position of the probe tip 13 with respect to the scan head 10 and the sample during scanning. The detector 30 also produces an output signal S indicative of tip-sample interaction and cantilever deflection due to applied forces (in the static domain). out The device is configured to supply

[0032] The topography of the sample is derived from the height of the AFM head minus the tip-to-head distance as indicated by the deflection of the cantilever.

[0033] In the illustrated embodiment, a sense signal S indicative of the deformation of the tip 13 upon contact with the surface 91 of the sample 9 is tdf The AFM device 1 is also provided with a tip deformation sensor 15 that supplies an output signal S out 1, the controller 40 further comprises a controller 40 configured to receive and analyze position signals x,y indicative of the lateral position of the probe tip as indicated by control signals of the XY actuator 20 and / or displacement sensor 21, and a z position of the probe as indicated by a control signal of the Z actuator 60 and / or a displacement sensor 61. In the illustrated example, the controller 40 also receives a sense signal S indicative of the deformation of the tip 13. tdfThe controller 40 also receives a force threshold value indicating the tip deformation. The controller 40 is configured to lower the force threshold value if the indicated tip deformation exceeds a predetermined limit. Alternatively and / or in addition, the controller 40 may be configured to process the output signals obtained by the device to correct for deviations caused by deformation of the tip 13.

[0034] The AFM device is configured to perform, for each scan position, a scan cycle including the following steps:

[0035] In a first step, the tip 13 approaches the sample 9 according to an approach profile starting from a reference position z on the sample 9. The reference position z defined by the approach profile is, for example, a predefined z value higher than any z value on the surface 91 of the sample 9. Alternatively, the reference position z is a relative position at a predefined distance with respect to the expected z value of the surface at the abscissas x,y being approached. The approach profile further defines a speed with which the surface 91 is approached. This speed is, for example, constant. Alternatively, the speed is variable, for example starting at a relatively high speed and decreasing in steps or gradually as the approach progresses.

[0036] In the second phase, the tip 13 retracts from the sample surface 91. The transition from the first phase to the second phase may be determined by one or more of the following: One option is when the tip or probe has advanced a predetermined distance towards the sample. Another option is when the force exerted by the tip exceeds a threshold. Yet another option is for retraction to be initiated as soon as either of these conditions occurs, whichever occurs first, or when both conditions are met. Another imaging parameter related to the retraction phase is the speed of retraction. Operation in the second phase is determined by the speed at which retraction is performed. As with the approach profile, the speed may be a constant value or may vary during retraction.

[0037] In a third step, the tip is displaced toward the next lateral position. The magnitude of the lateral distance is an example of yet another imaging parameter.

[0038] During the first and / or second phase, interaction data indicative of the interaction of the tip with the sample is obtained, in one example, a tip-sample force curve is determined that defines the force between the tip and the sample as a function of the displacement of the tip towards and / or away from the sample.

[0039] As with many practical types of samples, the sample 9 shown in FIG. 1 has surface portions 91B, 91S, and 91T of different levels. In the example shown in FIG. 1, the tip 13 is located at the end of a shoulder 91S having a bottom 91B. A downward force acting on the tip 13 may cause deformation of the cantilever 12 and / or the tip, causing the tip 13 to slide towards the bottom 91B, which may result in the cantilever deflection detector 30 indicating a lower z value than the x,y position where the tip 13 was originally located. This phenomenon only occurs near the edge of the surface portion with a lower level surface portion, but the measurement error is larger for samples with surface features with high aspect ratios. When the tip 13 is located at position x, y of the bottom 91B, the wall 91BW of the bottom 91B close to the tip 13 exerts a force on the tip that may prevent the tip from approaching the bottom 91B sufficiently, so that the cantilever deflection detection unit 30 indicates a z value that is higher than the actual z value at position x, y of the tip 13. Furthermore, this type of measurement error becomes more pronounced as the aspect ratio, i.e., the ratio of the depth to the lateral dimension of the bottom 91B, increases.

[0040] The improved AFM device 1 disclosed herein mitigates these sources of error by configuring the controller 40 to automatically adjust one or more imaging parameters during scanning in response to information about the sample characteristics at the lateral position of the sample currently being scanned. The information about the sample characteristics includes one or more of information from the sample specifications, the recipe for manufacturing the sample, and information obtained from scanning previous scan lines in a direction substantially parallel to the current scan line. Additionally, information obtained during the scanning of the trajectory may be used to make additional adjustments depending on the local conditions. The adjustments locally optimize the imaging parameters for proper Z measurements. As an example, a relatively low force threshold is selected at an x,y location predicted to be a location on a surface with a relatively high level close to an edge with a lower level surface. As another example, a relatively high force threshold is selected at a lateral (x,y) location predicted to be part of the bottom close to a wall. A further example includes adjusting the baseline offset depending on the predicted height at the lateral position. Yet another example includes adjusting the recession profile depending on the estimated adhesion force of the sample surface at the lateral position of the tip.

[0041] 1, the AFM apparatus may include acoustic signal generators 51a, 51b, 51c that provide acoustic signals to one or more of the probe 11, the tip 13, and the sample 9, and a signal analysis module 52 that analyzes the output signal based on the interaction of the acoustic signal with the sample 9. In this case, the output signal S of the tip deflection sensor out further reveals subsurface properties of the sample, allowing both topographical and subsurface information to be obtained in a common measurement, e.g., for overlay and / or alignment analysis.

[0042] Additionally or alternatively, the AFM device may comprise a signal source, e.g. signal source 51a, for inducing vibrations in the probe 11 to increase the effective stiffness of the cantilever 12. The vibrations increase the stiffness of the cantilever 12 and therefore the effective force on the sample. In the absence of induced vibrations, the stiffness of the cantilever is determined by its static spring constant. By applying vibrations, the stiffness is increased. Since the deflection of the cantilever 12 is proportional to the ratio of the force exerted by the probe 11 on the sample 9 via the tip 13 divided by the stiffness of the cantilever, the effective force on the sample can be calculated, e.g. using a calculation module 53, from the deflection of the cantilever and its effective stiffness as a function of the induced vibrations.

[0043] In the example shown in FIG. 1, the AFM device generates a tip deformation signal S tdf The AFM device is provided with a tip deformation sensor 15 that provides a force threshold value. In this example, the controller 40 of the AFM device is configured to lower the force threshold value if the indicated tip deformation exceeds a predefined limit. Alternatively, or in addition, the AFM device may be configured to correct the reconstructed image for this deformation. This optional functionality may be provided as an alternative to, or in combination with, other measures taken to control the settings of the AFM device.

[0044] For clarity of illustration, the interconnections between the controller 40 and the components controlled by the controller, and the connections to the cantilever deflection detection unit 30 and the tip deformation sensor 15 are omitted. More detailed configurations are shown in subsequent figures.

[0045] FIG. 2 shows a first embodiment of the improved AFM device 1 in more detail. In the illustrated example, the controller 40 comprises a signal analysis unit 41, an image memory 42, and a motion profile generator 45. As further shown in FIG. 2, the probe / cantilever deflection detection unit 30 comprises a light detection element 31 (here, a four-quadrant light detection element) and a deflection signal processing unit 32. The deflection signal processing unit 32 calculates the deflection of the probe from the signal S31 emitted by the light detection element 31 and supplies the signal to the motion profile generator 45. The motion profile generator 45 generates a control signal Cz for controlling the Z actuator 60 and a control signal Cxy for controlling the XY actuator 20. The motion profile generator 45 further supplies an input signal to the image memory 42 indicating the z value determined for each pixel of the horizontal coordinates x, y. For simplicity, in this example, it is assumed that the coordinates x, y, z stored in the image memory 42 are the coordinates specified by the motion profile generator 45. Typically, the z-value is defined as the vertical "tip position" at the moment when a certain predefined force threshold is reached. In practice, there may be deviations between the specified and actual coordinates. To take this into account, position sensors (21, 61, see FIG. 1) are usually provided that sense the lateral position (x,y) of the sample 9 and the vertical position z of the probe 11. The sensing signals from these sensors are fed as feedback signals to the motion profile generator 45. Once the scanning process is completed, the topography Z(x,y) of the surface 91 of the sample 9 can be read out from the image memory 42.

[0046] As shown in FIG. 2, the signal processing unit 41 calculates a force threshold setting Ft(x,y) for lateral position x,y based on the topography data z(x,y-Δy) from the previous scan line.

[0047] The embodiment of FIG. 2 will be further described with reference to FIG. 3 and FIG. 4. FIG. 3 shows a schematic graph of the probe deflection signal d51 and the Z-position actuator signal z41 during the non-resonant imaging mode in the embodiment of FIG. 2. The lower graph shows the Z-position actuator signal Cz that drives the probe 11 towards and away from the surface 91 of the sample 9. The horizontal axis shows time t, and the level of the axis a43 corresponds to the Z-level where the probe tip 13 just touches the surface 91. As the probe 11 is further extended in the negative Z direction towards the surface 91, the reaction force of the surface 91 on the probe tip 13 increases. Thus, at Z-levels below the axis a43, a positive probe-sample interaction force acts on the probe tip 13. The upper graph shows a schematic of the deflection signal d51 of the probe tip 13 over time. The deflection signal d51 is represented by the parameter d. Starting from a retracted position at the remote Z level, as shown in the lower graph g40, the Z position actuator signal z41 drives the probe 11 (and the probe tip 13) further towards the surface 91. Vertical lines t45, t46, t47, t48, and t49 indicate characteristic times described below. As the Z position signal z41 indicates, when the probe 11 is extended towards the surface 91 of the sample 9, at time t45 the probe tip 13 first contacts the surface 91 of the sample 9. This corresponds to the point where the signal z41 crosses the axis a43. In the deflection signal d51, in a first phase A of the deflection signal d51, where the probe 11 approaches the surface 91 before time t45, no deflection of the probe tip 13 is measured, as shown by the flat portion of the deflection signal d51 at this phase. In FIG. 4, phase A is shown diagrammatically in situation s60, which represents the position of the probe 11 and the probe tip 13 in an undeflected state on the surface 91.

[0048] Immediately after contact, at time t45, the probe tip 13 is momentarily attracted to the surface 91 as indicated by the drop in the deflection signal d51. The Z position signal d41 indicates that the probe 11 is being extended further towards the surface 91, increasing the probe-sample interaction force. This is illustrated by the negative Z position indicated by the z44 portion of the Z position signal z41. The extension of the probe 11 towards the surface 91 continues until the threshold level Ft(x,y) of the probe-sample interaction force is reached at time t46. Then, between t46 and t47, the probe 11 retracts again and the Z position signal z41 rises. The period between times t45 and t47, during which the Z position signal z41 is negative, corresponds to phase B of the deflection signal. Phase B consists of a first part and a second part, which correspond to the part before the maximum of the deflection signal d51 before time t46 and the part after the maximum between times t46 and t47. The situation at stage B is shown diagrammatically by situation s61 in Figure 4. As shown, the positive probe-sample interaction force causes the probe's cantilever 12 to bend backwards, thereby providing a positive deflection signal d51 between times t45 and t47 in Figure 3.

[0049] At time t47, the probe 11 is at a Z position corresponding to the level of axis a43 where the probe-sample interaction force is zero. The probe 11 is further retracted from the surface 91 until the probe tip 13 is released. However, before that, between times t47 and t48, the adhesion forces pull the probe tip 13 towards itself, exerting a negative force on the probe tip 13 to maintain contact with the sample surface 91 during probe retraction. This portion of the deflection signal is shown as phase C.

[0050] In FIG. 4, stage C is shown by situation s62, which represents a negative deflection of the probe tip 13 caused by adhesion forces between the surface 91 and the probe tip 13. At time t48, the probe 11 has retracted so far that the balance between the adhesion forces and the force exerted by the Z actuator 60 cannot be maintained. The probe tip 13 is now released from the surface 91 and begins to oscillate at its natural frequency. This stage D of the deflection signal d51 is called resonance and is shown diagrammatically by situation s63 in FIG. 4. The resonance continues until it disappears and after time t49 the probe is again in an undeflected state corresponding to stage A. The next extension to the surface can then be initiated. In some embodiments, the next extension is initiated before the cantilever is in an undeflected state if the resonance amplitude decreases below a predefined threshold level. In these embodiments, the cantilever is in an undeflected state before reaching the surface. Between time t48 and the next approach to the surface after time t49, the probe may move laterally relative to the sample 9 towards the next pixel of the image. This process is repeated until all pixels have been imaged. For each pixel x,y the value of z(x,y) is registered to obtain a topographical image of the surface 91 of the sample 9. The choice of the force threshold Ft(x,y) used by the motion profile generator 45 is determined by the signal processing unit 41.

[0051] The surface height profile z(x,y) is estimated from the head position and the cantilever deflection as described with reference to Figures 3 and 4. The motion profile generator retracts the probe if the deflection d indicates that the current force threshold Ft has been exceeded. The estimated height profile in the vicinity, e.g. z(x,y-Δy), is used as an indication of the height profile z(x,y) at position x,y, and with this information an optimal force threshold Ft(x,y) is selected by the signal processing unit 41. As mentioned above, the selection of the optimal force threshold Ft(x,y) may additionally or alternatively be determined by prior knowledge of the sample, e.g. specified topographical data and / or recipe creation data and / or analytical results obtained from the current scan line, i.e. from the vicinity z(x-Δx,y).

[0052] In one embodiment, the signal processing unit 41 determines the value of the force threshold Ft(x,y) based on image data acquired for a previous scan line y-Δy having a corresponding x coordinate or having an x ​​coordinate within the region of the currently scanned coordinate x. In some embodiments, not only the z value measured when the force threshold is reached is registered, but instead the complete force-distance curve defining the relationship between distance z and applied force is registered.

[0053] In one example, the force threshold value for the abscissa position Ft(x,y) is selected from at least a relatively low threshold value Ftlow and a relatively high threshold value Fthigh depending on the z value measured in the previous scan line (y-Δy) according to the following criteria:

number

[0054] For example, it is conceivable to use further information from earlier scan lines -2Δy, -3Δy, which is less predictive of the characteristics of the sample at the current position x,y, but may be weighted with an appropriate weighting factor. In another example, the selection of the force threshold depends on a region with width Δy defined by the distance to the previous scan line and the length dx of the recently scanned portion of the current scan line, as follows:

number

[0055] That is, if there is an indication that features with low z values ​​are expected based on knowledge of the area at the current scan position (x,y), then a relatively high force threshold Ft(x,y) is selected for that position. The relatively high force threshold Ft(x,y) facilitates tip protrusion to the bottom surface of such features. This selection therefore improves measurements of features with low z values.

[0056] In another example, the force threshold Ft(x,y) is determined as follows:

number

[0057] This means that if there is an indication that a feature with a high z value is expected based on knowledge of the area of ​​the current scan position (x,y), then a relatively low force threshold Ft(x,y) is selected for that position. The relatively low force threshold Ft(x,y) is intended to prevent the tip from slipping from a feature with a high z value to a lower position. This selection therefore improves the measurement of features with high z values.

[0058] Also, the width of the region may span several preceding scanlines.

[0059] In yet other embodiments, the threshold force is adjusted according to the material properties at the sample location x,y based on prior information about the sample and / or information obtained from the sample during scanning of the previous or current scan lines. In one example, a relatively low threshold force is used when the surface hardness at the scanned location is expected to be relatively low based on prior information about the sample and / or information obtained from the sample during scanning, and a relatively high threshold force is used when the surface hardness at the scanned location is expected to be relatively high based on prior information about the sample and / or information obtained from the sample during scanning.

[0060] In these examples, the force threshold Ft(x,y) is determined by selection from predefined thresholds. In other embodiments, the force threshold is determined as a continuous function from prior information about the sample, for example using a polynomial function, such as an interpolation function, for example a cubic interpolation function, to calculate an interpolated threshold at a location from a number of predefined thresholds known at nearby locations. In yet another example, a look-up table is used.

[0061] It should be noted that the determination of the force threshold Ft by the signal processing unit 41 may further be based on recently acquired imaging data from the current scan line y. For example, the signal processing unit 41 may invalidate a determination based on a previous scan line y-Δy if the z-value z(x-Δx,y) determined for a lateral position of coordinate x-Δx,y significantly deviates from the z-value z(x,y-Δy) determined for a lateral position of coordinate x,y-Δy.

[0062] Alternatively, the signal processing unit 41 may calculate the average value of a threshold value FtΔx(x,y) determined based on imaging data from a current scan line and a threshold value FtΔy(x,y) determined based on imaging data from a previous scan line using the following equation:

number

[0063] In the above formula, 0 x ,a y < 1, and a x +a y =1.

[0064] ​Fig. 5 shows another embodiment. In the example shown in Fig. 5, the signal processing unit 41 uses topographic information zs(x,y), which can be obtained from the specification or recipe of the sample 9 and is stored in the topography storage unit 43. The motion profile generator 45 and / or the displacement sensor 21 provide the abscissas x,y of the currently scanned position to the signal processing unit 41. The signal processing unit 41 can then read the expected z-values ​​of that position zs(x,y) and / or the expected z-values ​​of the adjacent positions from the topography storage unit 43 in a manner similar to that described for the embodiment of Fig. 2, and determine the force threshold Ft(x,y) to be used by the motion profile generator 45 for that lateral position x,y. In one embodiment, the process of determining the force threshold Ft(x,y) includes estimating the uncertainty of the values ​​involved in the determination of the threshold. Depending on the magnitude of this uncertainty, different approaches may be used to assign different weights to each estimate of the force threshold. Estimates with high uncertainty, i.e. less confidence, are assigned a lower weight than estimates with low uncertainty. In this case, the force threshold Ft(x,y) used by the motion profile generator 45 may be determined by a weighted average of the various force threshold estimates according to the assigned weights. As mentioned above, the thresholds may be estimated based on prior knowledge and topographical information obtained from previous scanlines and / or preceding positions on the current scanline.

[0065] Also, in this embodiment, the determination of the force threshold Ft by the signal processing unit 41 may be further based on recently acquired imaging data from the current scanline y, as described above for the embodiment of FIG.

[0066] In yet another embodiment, the signal processing unit 41 is configured to determine a force threshold Ft(x,y) for the positions of the scanned abscissas x,y based on a combination of the topography information zs(x,y) from the specification or recipe of the sample 9 described with reference to FIG. 5 and the topography information z(x,y−Δy) obtained from the previous scan line y−Δy described with reference to FIG. 2. In some of those examples, the signal processing unit 41 is also configured to further use the image data most recently obtained from the same scan line to determine the force threshold Ft(x,y) for the positions of the abscissas x,y.

[0067] As described above, the signal processing unit 41 does not need to calculate the threshold force based on information from a single position near the current scan point. Instead, it may be calculated based on information from the area of the current scan point x,y, for example, information from the area of the preceding scan line x−dx<x′<x+dx, or information from the area of the current scan point defined by the design data or recipe data, for example, the area x−dx<x′<x+dx, y−dy<y′<y+dy. In an exemplary embodiment, the signal processing unit 41 selects a relatively high force threshold if the information from the area indicates that the current scan point is expected to be in a narrow groove, and selects a relatively low force threshold if the information from the area indicates a wide groove or other relatively flat area. If the difference between the maximum surface level and the minimum surface level within a configurable distance (for example, 10 times the diameter of the tip) is less than a configurable factor (for example, 0.5 times the height of the tip), the current scan point is determined to be in a flat area. The exact values of the configurable distance and the configurable factor depend on the stiffness of the combination of the tip and the cantilever. If the stiffness of the combination of the tip and the cantilever is relatively high, the configurable distance may be selected to be relatively small and the configurable factor may be selected to be relatively high. If the stiffness of the combination of the tip and the cantilever is relatively low, the configurable distance may be selected to be relatively large and the configurable factor may be selected to be relatively small.

[0068] In the exemplary embodiment described above, the imaging parameter adjusted by the signal processing unit 41 is the force threshold Ft as a function of the abscissas x,y, while in other embodiments the imaging parameter adjusted is the baseline offset, the approach profile, or the retraction profile.

[0069] FIG. 6 shows an exemplary embodiment. In the example shown in FIG. 6, the controller 40 comprises an adhesion signal processing unit 47. The adhesion signal processing unit 47 determines the retraction distance of the retraction profile used by the motion profile generator 45, i.e. the control signal dr(x,y) indicative of the distance at which the motion profile generator 45 causes the Z actuator 60 to retract the probe 11 away from the sample 9. In the example shown, the deflection curve signal processing unit 46 receives the deflection curve data dz(x,y) for each scan position. The deflection curve data dz(x,y) indicative of the deflection d measured as a function of the movement of the probe z, in particular the deflection of the probe in the time interval from t47 to t48. As explained with reference to FIGS. 3 and 4, during this time interval the deflection of the probe 11 is substantially determined by the adhesion force between the tip 13 and the sample 9. Using this data, the deflection curve signal processing unit 46 estimates the magnitude of the adhesion force Adh(x,y) at the lateral position x,y of the sample and stores this value in the memory 44. The adhesion signal processing unit 47 reads the data acquired for the adhesion Adh(x,y-Δy) for the previous scan line x,y-Δy and calculates the optimal retraction distance of the retraction profile based on the assumption that Adh(x,y) is approximately equal to Adh(x,y-Δy). If a relatively high adhesion is expected at the current scan position, the retraction distance is set to a larger value than if a relatively low adhesion is expected. In another embodiment, the adhesion signal processing unit 47 performs the calculation based on adhesion information from the specification or recipe of the sample 9. In yet another embodiment, the adhesion signal processing unit 47 is configured to determine the retraction distance of the retraction profile for the position of the scanned abscissa x,y based on a combination of adhesion information from the specification or recipe of the sample 9 and adhesion information acquired from the previous scan line y-Δy. In some examples thereof, the adhesion signal processing unit 47 is also configured to determine the retraction distance for the position of the abscissa x,y using adhesion information recently acquired from the same scan line.

[0070] FIG. 7 shows yet another embodiment. As in the example shown in FIG. 6, the deflection curve signal processing unit 48 receives the deflection curve data dz(x,y) for each scan position. However, in this embodiment, the deflection curve signal processing unit 48 calculates force-distance data for the lateral positions (x,y) determined in the time interval in which the force is positive from t45 onwards until the probe retracts at t46. The force-distance data may include two or more distances z measured for each force. The signal processing unit 41 can access the force-distance data and based thereon, determines which threshold force Ft(x,y) to select for the lateral positions x,y based on two or more distances z1(x,y-Δy), z2(x,y-Δy) measured for each force stored for a lateral position with coordinate x' at or near x,y-Δy. In some embodiments, the signal processing unit 41 also performs a calculation of a threshold force Ft(x,y) based on force-distance data obtained from the current scan line, e.g., force-distance data obtained from a previous scan position x-Δx,y.

[0071] As described above, the imaging parameter settings may be determined based on a combination of input data from various sources, including two or more pieces of prior information about the sample, such as design information, recipe information, information obtained from imaging data of a previous scan line, and information obtained from recent imaging data of a current scan line. If the prior information used in the combination is consistent, it indicates that the information is reliable. Otherwise, in one approach, the values ​​of the input parameters used to predict the imaging parameters to be set may be calculated as an average, e.g., a weighted average, of the input data from the various sources. That is, the imaging parameter settings may be calculated based on each input data from the various sources, and the imaging parameter settings actually used may be calculated as an average, e.g., a weighted average, of the settings calculated based on the various input data. In another approach, the input parameter settings are the most conservative settings among the settings predicted individually based on each input data. For example, the lowest value setting is selected from multiple force threshold settings to minimize the risk of damage to the sample.

[0072] Although the present invention has been described in its specific embodiment, it is to be understood that the embodiments shown in the attached drawings and the specification are merely illustrative and are not intended to limit the present invention in any manner or means. The operation and construction of the present invention are believed to be apparent from the above description and the attached drawings. It will be apparent to those skilled in the art that the present invention is not limited to the embodiments described herein, but may be modified within the scope of the appended claims. Kinematic reversals are also inherently disclosed and are considered to be within the scope of the present invention.

[0073] Furthermore, any of the components or elements of the various disclosed embodiments may be combined or incorporated into other embodiments where considered necessary, desirable or preferred without departing from the scope of the invention as defined in the claims.

[0074] In the claims, reference signs should not be interpreted as limiting the scope of the claims. The terms "comprising" and "including" used in this specification or the appended claims should not be interpreted in an exclusive or exhaustive sense, but in an inclusive sense. Thus, the term "comprising" used in this specification does not exclude the presence of other elements or steps in addition to those recited in the claims. Also, the terms "a" and "an" should not be interpreted as being limited to "only one", but are used in the sense of "at least one" and do not exclude a plurality. Features not specifically or explicitly described or claimed may further be included in the structure of the invention within the scope of the invention. Expressions such as "means for ..." should be interpreted as "a component configured for ..." or "a member configured for ..." and should be interpreted to include equivalents of the disclosed structures. The use of expressions such as "important", "preferred", "particularly preferred", etc. are not intended to limit the invention. Additions, deletions, and modifications within the knowledge of a person skilled in the art can generally be made without departing from the spirit and scope of the invention as defined by the claims. This invention may be practiced other than as specifically described herein and is limited only by the scope of the appended claims.

Claims

1. scanning a surface of a sample along a first lateral current scan line with a probe having a tip; measuring an interaction between the tip and the sample; and adjusting one or more imaging parameters during said scan based on information about the sample, said method comprising: the information about the sample includes information about the sample obtained prior to the scan; the one or more imaging parameters that are adjusted during the scan based on the information obtained prior to the scan include a force threshold; if information from the region of the current scan position indicates that the current scan point is expected to be within a relatively deep and narrow recess or at the bottom near a steep wall, a relatively high force threshold is selected; A method according to claim 1, wherein a relatively low force threshold is selected if information from said region indicates a relatively wide recess or wide plateau away from a relatively steep and high edge / wall.

2. The method of claim 1 , wherein the information about the samples further comprises information obtained from scanning a previous scan line in a direction substantially parallel to the current scan line.

3. The method of claim 1 or 2, wherein the one or more imaging parameters that are adjusted further include a baseline offset.

4. The method of claim 1 , 2 or 3 , wherein the one or more imaging parameters that are adjusted further include an approach profile.

5. The method of claim 1 , 2, 3 or 4, wherein the one or more imaging parameters that are adjusted further include a recession profile.

6. The method of any one of claims 1 to 5, wherein the information further comprises one or more pieces of information selected from the approximate shape of the sample, a profile of a previous line, adhesion force, and information indicated by a lateral displacement sensor.

7. 10. The method of any preceding claim, further comprising adjusting one or more image parameter settings based on information about samples acquired for one or more recently scanned positions along the current scan line.

8. applying an acoustic signal to one or more of the probe, the tip, and the sample; 2. The method of any preceding claim, further comprising: analyzing an output signal based on an interaction of the acoustic signal with the sample.

9. 10. A method according to any preceding claim, comprising inducing vibration of the probe to change the effective stiffness of the cantilever (12).

10. obtaining a tip deformation signal indicative of deformation of the tip; and reducing the force threshold if the indicated deformation of the tip exceeds a predetermined limit, and / or processing an output signal obtained by the method to correct for deviations caused by deformation of the tip.

11. 1. An atomic force microscope (AFM) apparatus (1) comprising at least one scanning head for scanning a sample (9), said scanning head comprising a probe (11), said probe comprising a cantilever (12) and a probe tip (13) disposed on said cantilever, a first actuator (20) cooperating with the at least one scan head or substrate holder (95) for relatively moving the probe tip and the sample in one or more directions parallel to the surface (91) of the sample to scan the surface with the probe tip; a cantilever deflection detector (30) arranged to measure a deflection (d) of the cantilever (12) relative to the scan head during said scan and to provide an output signal indicative of said deflection; a controller (40) configured to receive and analyze the output signal from the cantilever deflection detector to measure deflection of the probe and to automatically adjust one or more imaging parameters during the scan in response to information about sample characteristics proximate the position of the sample currently being scanned; the information about the sample characteristics includes information about the sample obtained prior to the scanning; the one or more imaging parameters that are adjusted during the scan based on the information obtained prior to the scan include a force threshold; if information from the region of the current scan position indicates that the current scan point is expected to be within a relatively deep and narrow recess or at the bottom near a steep wall, a relatively high force threshold is selected; An atomic force microscope apparatus, characterized in that a relatively low force threshold is selected when information from said region indicates a relatively wide recess or wide plateau away from a relatively steep and high edge / wall.

12. 12. The atomic force microscope apparatus of claim 11, wherein the information about the sample characteristics further comprises information obtained from scanning a previous scan line in a direction substantially parallel to the current scan line.

13. the controller is configured to relatively move the probe tip (13) and the sample (9) by the first actuator (20) according to a subsequent scan line extending in a first direction (x); The AFM apparatus further comprises a memory (42) for storing information regarding the sample acquired during scanning of a current scan line; An atomic force microscope apparatus as described in claim 11 or 12, wherein the controller determines respective values ​​of the one or more imaging parameters at the scan position along the subsequent scan line based on information about the sample stored in the memory (42) for at least an area or position of the current scan line that corresponds to a scan position along the subsequent scan line.

14. an acoustic signal generator configured to provide an acoustic signal to one or more of the probe, the tip, and the sample; 14. The atomic force microscope apparatus of claim 11, 12 or 13, further comprising a signal analysis module that analyzes an output signal based on the interaction of the acoustic signal with the sample.

15. An atomic force microscope apparatus according to any one of claims 11 to 14, comprising a signal source for inducing vibration in the probe to increase the effective stiffness of the cantilever.

16. A tip deformation signal (S ｔｄｆ ), 16. An atomic force microscope apparatus according to any one of claims 11 to 15, wherein the AFM apparatus is configured to lower the force threshold if the indicated deformation of the tip exceeds a predetermined limit, and / or to process output signals obtained by the apparatus to correct for deviations caused by deformation of the tip.

Images