Scanning Probe System

Abstract

A method of scanning a sample with a scanning probe system including a probe including a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever, the method comprising: using the probe to measure an electrostatic interaction between the sample and the probe; and, after measuring the electrostatic interaction between the sample and the probe, scanning the sample with the probe while simultaneously applying a bias voltage to the scanning probe system that is based on the measured electrostatic interaction between the sample and the probe.

Description

[Technical field]

[0001] The present invention relates to a scanning probe system and a method for scanning a sample with a scanning probe system. [Background technology]

[0002] Some samples scanned by scanning probe microscopes develop an electric charge. This charge can seriously affect the measurements made by the scanning probe microscope; in minor cases, the images obtained are distorted and inaccurate due to interactions between the scanning probe and the charges on the sample. In more serious cases, the scanning probe cannot even approach the sample within a suitable scanning range due to interactions.

[0003] US Patent No. 8,011,230 discloses a scanning probe microscope capable of performing shape measurement that is not affected by the charge distribution of a sample. The charge state is monitored by detecting changes in the deflection or vibration state of the cantilever caused by electrostatic charge in synchronization with the relative scanning between the probe and the sample during measurement. This microscope adjusts the potential to offset the effect of the charge distribution, thus preventing damage to the probe or sample caused by discharge and realizing a reduction in measurement errors caused by the charge distribution.

[0004] A problem with the microscope of US Pat. No. 8,011,230 is that it monitors the charging state synchronously with the scan, and this monitoring can slow down the scan speed. Summary of the Invention

[0005] A first aspect of the invention provides a method of scanning a sample with a scanning probe system. The scanning probe system includes a probe including a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever. The method includes moving the probe to a measurement position where the sample exerts an electrostatic force on the probe, applying a measurement voltage to the scanning probe system at the measurement position, varying the measurement voltage applied to the scanning probe system and monitoring a response of the probe to the variation in the measurement voltage to measure a cancellation voltage value that reduces or nullifies the electrostatic force on the probe at the measurement position, and scanning the sample with the probe while applying a bias voltage to the scanning probe system based on the cancellation voltage value after the cancellation voltage value has been measured.

[0006] Another aspect of the invention provides a scanning probe system including a probe comprising a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever, the scanning probe system arranged to perform the method of the first aspect of the invention.

[0007] The bias voltage reduces, and ideally nullifies (i.e., reduces to zero) the electrostatic force between the sample and the probe when the sample is scanned with the probe.

[0008] Unlike US Patent No. 8,011,230, which monitors the charge state synchronously with the scan, the present invention scans the sample after the compensation voltage value is measured. This means that the sample can be scanned more quickly based on a predefined bias voltage (i.e., a set of predefined bias voltages). Typically, the bias voltage does not vary during the scanning of an area of ​​the sample, but in some embodiments it may vary, for example between a set of predefined bias voltages.

[0009] Only a single offset voltage value may be measured, or alternatively, multiple offset voltage values ​​may be measured before the sample is scanned.

[0010] Optionally, the method includes moving the probe to a number of measurement positions where the sample applies an electrostatic force to the probe, and at each measurement position, applying a measurement voltage to the scanning probe system, varying the measurement voltage applied to the scanning probe system and monitoring a response of the probe to the variation in the measurement voltage to measure respective cancellation voltage values ​​that reduce or neutralize the electrostatic force on the probe at the measurement position, and determining a bias voltage based on at least two of the cancellation voltage values.

[0011] The bias voltage may be determined without reference to the location of the area of ​​the sample being scanned (e.g., it may be calculated as an average of the cancellation voltage values). More commonly, an area of ​​the sample is scanned with the probe and the bias voltage is determined based on the location of the area of ​​the sample (e.g., its distance from the center of the sample or its X,Y coordinates). This allows the bias voltage to more precisely cancel electrostatic forces on the probe even as the charge state of the sample varies across its surface.

[0012] In some embodiments, a first bias voltage is determined based on a location of a first area of ​​the sample, the first bias voltage being applied to the scanning probe system during scanning of the first area of ​​the sample, and a second bias voltage is determined based on a location of a second area of ​​the sample, the second bias voltage being applied to the scanning probe system during scanning of the second area of ​​the sample. Typically, the first and second areas are not adjacent to one another.

[0013] Optionally, the or each bias voltage is determined from the offset voltage values ​​by interpolation, for example linear interpolation.

[0014] Optionally, the cancellation voltage value is determined at the or each measurement location with the probe tip spaced from the sample by a distance greater than 1 μm.

[0015] Optionally, the method further comprises storing the or each offset voltage value before the sample is scanned.

[0016] Optionally, an area of ​​the sample is scanned with the probe and a bias voltage applied to the scanning probe system does not vary during scanning of the area of ​​the sample. Alternatively, a bias voltage applied to the scanning probe system may vary during scanning of the area of ​​the sample.

[0017] Optionally, scanning the sample with the probe includes making a series of scan measurements of the sample at a series of scan positions across the sample, each scan measurement being made by moving the probe tip towards the sample, making a scan measurement at each of the scan positions, and then retracting the probe tip away from the sample. Each scan measurement may be used, for example, as a pixel of a two-dimensional image. A bias voltage applied to the scanning probe system need not vary from scan position to scan position.

[0018] Optionally, for each scanning measurement, the bias voltage applied to the scanning probe system remains approximately constant as the probe tip moves towards the sample, and as the probe tip retracts away from the sample.

[0019] Scanning the sample with the probe may include imaging the sample with the probe to generate an image having a plurality of pixels. Typically, the bias voltage applied to the scanning probe system is the same for each pixel of the image.

[0020] Scanning the sample with the probe can reveal the topography of the sample or some other characteristic of the sample. This may include measuring

[0021] Scanning the sample with the probe may include generating a lateral or horizontal scanning motion between the sample and the probe. The scanning motion may be generated by moving the probe and / or by moving the sample. The scanning motion may, for example, follow a raster pattern.

[0022] The bias voltage may have a magnitude and sign based on the offset voltage value.

[0023] Optionally, variations in the measured voltage deflect a probe, and the deflection of the probe is monitored to determine the cancellation voltage. The monitored deflection may be the angle of the cantilever relative to a fixed axis.

[0024] Alternatively, variations in the measured voltage may change a vibration parameter of the probe (eg, the phase, frequency or amplitude of the probe's vibration) and the vibration parameter is monitored to determine the cancellation voltage.

[0025] The measurement voltage and the bias voltage may be applied to the probe, most preferably they are applied to the cantilever.

[0026] Alternatively, the measurement voltage and the bias voltage may be applied to a conductor proximate to the sample. For example, the conductor proximate to the sample may be a conductive plate. The conductive plate may be provided on the sample. The conductive plate may be provided on the opposite side of the sample to the probe. The conductor may be a stage that carries the sample.

[0027] Applying a bias voltage to the scanning probe system may additionally or alternatively include applying a bias voltage directly to the sample.

[0028] Optionally, the cantilever includes a conductive layer separated from the probe tip by an electrically insulating barrier, and a bias voltage is applied to the conductive layer. The probe may be positioned or constructed such that there is no conductive path from the conductive layer to the probe tip. The electrically insulating barrier may be the body of the cantilever.

[0029] As the sample is scanned with the probe, the conductive layer may be illuminated to impart a periodic bending motion to the probe. [Brief description of the drawings]

[0030] Embodiments of the present invention will now be described with reference to the accompanying drawings.

[0031] [Figure 1]1 illustrates some elements of a scanning probe system according to an embodiment of the present invention. [Diagram 2] 2 illustrates a pre-scan operation of a scanning probe system according to the embodiment of FIG. 1; [Diagram 3] 6 illustrates the change in vibration amplitude over time for a scanning probe system according to another embodiment of the present invention. [Figure 4] The probe is shown in detail. [Diagram 5] 2 illustrates the scanning and imaging elements of the system of FIG. 1. [Figure 6] 4 illustrates some elements of a scanning probe system according to another embodiment of the present invention. [Figure 7] A wafer with two measurement locations is shown. [Figure 8] The offset voltage values ​​at the measurement positions in FIG. 7 are shown. [Figure 9] A wafer with four measurement locations is shown. [Figure 10] The offset voltage values ​​at the measurement positions in FIG. 9 are shown. [Figure 11] 1 shows a wafer with a coarse square grid of measurement locations. [Figure 12] 12 shows offset voltage values ​​at several measurement locations in FIG. 11 . DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] A scanning probe system 50 (e.g. a scanning probe microscope) according to a first embodiment of the invention is shown in Figure 1. Some scanning and imaging elements of the scanning probe system 50 are omitted from Figure 1 for clarity, but are shown in Figure 5. Similarly, some elements of the scanning probe system 50 are omitted from Figure 5 for clarity, but are shown in Figure 1.

[0033] The scanning probe system 50 includes a driver 4 and a probe including a cantilever 2 and a probe tip 3. The bottom part of the driver 4 provides support for the probe, and the cantilever 2 extends from a base 2a from the driver 4 to a free end 2b. The probe tip 3 is carried by the free end 2b of the cantilever 2.

[0034] The probe tip 3 comprises a three-dimensional, often conical or pyramidal structure. The probe tip tapers to a point which is its nearest point of interaction with the surface under investigation. The cantilever 2 is itself a beam, apart from the probe tip 3, which supports the probe tip 3 at its free end 2b and its base 2a which is held by a driver 4.

[0035] It should be noted that the cantilever 2 and probe tip 3 are not shown to scale in FIG. 1 or in any of the other drawings.

[0036] The probe is typically made of an insulator such as silicon, silicon oxide or silicon nitride. Typically the cantilever 2 is about 50-200 μm long, 20-50 μm wide and about 0.2-2 μm thick, although this size can of course be varied according to the application. The shape can also be varied, typically being rectangular or triangular, in the latter case the tip near its apex being triangular. The probe tip 3 is typically made of carbon and is typically 5 μm at its base, with a height of 3-10 μm and a tip curvature of 2-20 nm. In use, the fine tip at the end of the probe tip is oriented towards the sample 7. Alternatively the probe may have a cantilever with a length of about 5-20 μm and a width of about 3-10 μm, with a correspondingly smaller tip.

[0037] The driver 4 may be a piezoelectric actuator that expands and contracts up and down in the Z direction according to a drive signal. Typically, the driver 4 is mechanically guided by a flexure (not shown).

[0038] A detector 20 is provided which detects the angle of the probe and therefore its deflection by means of an optical lever. The detector 20 comprises a laser which directs a sensing beam 21 onto the cantilever 2, the reflected beam 22 falling on a segmented photodiode 23 which is split into a number of segments (usually four). If the angle of the cantilever 2 changes, the position of the reflected beam 22 on the photodiode 23 also changes. Thus the relative output of the segments of the segmented photodiode 23 gives an indication of the angle of the cantilever 2 with respect to the sensing beam 21. This is output as an angle signal on output line 24. The detector 20 may equally use an interferometer to determine the height of the probe tip 3.

[0039] The angle signal is received in a deflection processor 26 which analyses the angle signal output by the photodiode 23 and determines a cancellation voltage value (i.e. a potential difference which cancels the electrostatic force on the probe) to be applied to the scanning probe system based on the deflection angle of the cantilever 2. A signal indicative of the cancellation voltage value to be applied is transmitted along output line 28 and received by a voltage generator 30. Based on the signal indicative of the cancellation voltage value, the voltage generator 30 determines a bias voltage and applies this bias voltage to the scanning probe system. In the illustrated embodiment, the bias voltage is applied to a stage 32 that carries the sample 7.

[0040] After the offset voltage values ​​have been determined as described above, the sample is scanned with the probe while simultaneously applying a bias voltage to the scanning probe system. Figure 5 shows the various elements of a scanning probe system 50 required to scan a sample and acquire an image. A detailed description of the scanning and imaging elements of the scanning probe system 50 can be found in WO 2012 / 104625, the contents of which are incorporated herein by reference.

[0041] During the scanning process, the cantilever 2 is driven by a laser illuminator 45 which illuminates the cantilever with an activation beam, which heats the cantilever 2 and causes it to bend downwards along its length so that the probe tip 3 moves downwards relative to the base 2a of the cantilever. In this case, the cantilever 2 is preferably a silicon nitride beam 204 with a gold plating 202 on its upper surface as shown in FIG. 4. This gold plating thermally expands above the silicon nitride material when heated by the activation beam. Any modification of the intensity of the activation beam therefore causes the cantilever 2 to heat or cool, thereby bending it downwards or stretching it upwards relative to the driver 4.

[0042] The cantilever 2 bends downwards towards the sample when the activation beam is turned on, and extends upwards away from the sample when the activation beam is turned off, and the cantilever assumes a more relaxed state. However, it will be appreciated that the opposite arrangement can be deployed, i.e. the cantilever bends upwards away from the sample when the activation beam is turned on, and extends downwards towards the sample when the activation beam is turned off. This can be achieved by placing gold plating 202 on the underside of the cantilever rather than on the top side.

[0043] The illuminator 45 is arranged to move the cantilever 2 in accordance with a second drive signal at a second driver input. The second drive signal is generated by a waveform generator 54.

[0044] The scanning motion between the sample and the probe may be generated by a piezoelectric XY actuator 44 which moves the stage 32. Thus, the relative lateral or horizontal scanning motion between the probe and the sample in the XY plane is generated by the motion of the sample 7 rather than the motion of the probe. In an alternative embodiment, the relative scanning motion in the XY plane may instead be generated by the motion of the probe (the sample remains stationary).

[0045] The probe periodic bending motion during scanning of the sample is generated by a waveform generator 54 that controls the illuminator 45. The probe periodic motion is set to a frequency lower than the resonant frequency of the cantilever. The probe motion can be periodic or non-periodic, provided that it repeatedly brings the probe tip towards and then away from the sample surface. In other words, the cycles of reciprocating motion of the probe tip can be spaced in time by regular or non-regular periods, for example, to sample various points spaced at irregular intervals across the surface of the sample rather than sampling a regularly spaced grid of pixels.

[0046] During the course of a measurement, the probe is cycled by the illuminator 45 and moved towards the sample surface until the surface is detected, at which point the probe is retracted and its height recorded at the time of surface detection. An image of the surface height is then generated by repeating this process while moving the probe under control of the x,y driver 44, typically following a raster pattern, although any x,y sequence may be followed.

[0047] Driver 4 controls the height of the base of the probe above the surface and sets the probe periodic amplitude. The probe is operated to maintain a reference level, where the "probe cyclic amplitude" is the height difference per cycle between the fully retracted position and the point of surface detection. Furthermore, the driver 4 has a range well beyond the probe cyclic motion, thus allowing the entire sample tilt and feature to be adjusted.

[0048] The height of the probe is measured by an interferometric height detector 52. The signal obtained from the interferometric height detector 52 is subjected to two analytical calculations. A Field Programmable Gate Array (FPGA) is configured to provide the necessary processing power. As known in the art, alternative signal processing techniques such as Digital Signal Processing (DSP) or dedicated analog or digital electronic methods can be used.

[0049] The probe periodic motion typically has a frequency range of tens to hundreds of kHz, and the sampling frequency for data recording is in the region of 100 MHz. As a result, each cycle of the probe motion is sampled in the range of 1000 to 10,000 times, which is more than enough to analyze the height detector signal to obtain the required information. The two analysis calculations can be performed in a single FPGA, but for clarity are shown in Figure 5 as being performed by separate processing units.

[0050] A probe periodic amplitude processing unit 56 is used to extract data for use in the microscope's z-position adjustment system. The amplitude of the probe period in the height signal can be extracted and used as a parameter to determine the z adjustment. The output from the probe periodic amplitude processing unit 56, which indicates the adjustment of the driver 4, is input to a feedback controller 58, which moves the base of the probe to return the probe period amplitude to its set point.

[0051] The surface detection unit 60 performs a second analytical calculation that provides an indication of the point in the period where the probe tip interacts with the sample surface. The rate of change of the probe velocity, or equivalently the height signal, decreases as the probe tip impacts the surface and begins to interact with it. An indication of the surface position is therefore extracted from the point in each period where that rate of change is below a threshold level for a set period of time.

[0052] The output of the surface detection unit 60 is the height of the probe interacting with the surface within each period of the probe cycle. That is, this provides a measure of the surface height at the x,y position of the probe which can be used to construct an image. Each data point representing the measured surface height is output to the scan controller / image processor 46. The measurement is mapped to the scan x,y position in the image processor 46, thus forming a point or pixel on the image.

[0053] Once the point of surface detection is determined, the surface detection unit 60 sends a trigger signal to the waveform generator 54. In response, the waveform generator 54 modifies the signal sent to the illuminator 45 to retract the probe tip away from the sample.

[0054] A method of determining a cancellation voltage value according to an embodiment of the present invention will now be described with reference to Figures 2A-2C. Figure 2A shows a first time when the probe starts at a relatively far distance from the sample 7 but is moved towards the sample 7 by the driver 4. At the relatively far distance the cantilever 2 has a first angle or deflection value α.

[0055] The top surface of the sample 7 carries a charge. In this embodiment, the charge is positive. The stage 32 is made of a conductive material (such as aluminum) and is connected to the voltage generator 30 by wires 34 so that a voltage can be applied to the stage 32 by wires 34. At this first time, no voltage is applied to the stage 32.

[0056] 2B shows a subsequent time when the probe is moved closer to the sample 7 into a measurement position where the sample exerts an electrostatic force on the probe. Specifically, the positively charged sample induces a negative mirror charge on the probe tip, and the potential difference between the probe tip and the sample causes an attractive electrostatic force. This attractive electrostatic force causes the cantilever 2 to bend downward to a second angle or deflection value β.

[0057] Electrostatic interactions occur on relatively large length scales: in the measurement position of Figure 2B, where the sample exerts an electrostatic force on the probe, the probe and sample 7 are separated by a distance of the order of 1-10 μm.

[0058] With the probe in the measurement position of Figure 2B, a measurement voltage is applied by voltage generator 30 to stage 32. This measurement voltage is initially low and then increased, for example to +1V. As the measurement voltage increases, deflection processor 26 monitors the probe's response to the increase in measurement voltage. Because the sample is positively charged and the measurement voltage is also positive (+1V), an increase in the potential difference between the probe tip and the sample causes the probe to bend further downward. In other words, a positive measurement voltage increases rather than decreases the electrostatic force on the probe.

[0059] If this is determined to be the case, the measurement voltage is returned to zero and then reduced, for example, to -1V. Such a negative applied voltage reduces the electrostatic force on the probe at the measurement position, so that the probe begins to bend upward again. As the measurement voltage continues to decrease, the deflection processor 26 monitors the response of the probe until it bends upward again to its first angle or deflection value α. At this point, the measurement voltage has completely cancelled or nullified the electrostatic force exerted on the probe by the sample. The measurement voltage at this point is recorded in memory as the cancellation voltage value. Note that this cancellation voltage value has not only a magnitude (e.g., 30V) but also a sign (negative in this case).

[0060] This process may be accomplished by a feedback loop that varies the measured voltage back to a set point based on the first angle or deflection value α.

[0061] If the measurement voltage is first decreased (rather than increased), in this example, it becomes unnecessary to change its sign during the measurement process -- in other words, since the polarity of the charge on the sample is unknown, there is a 50% chance of initially increasing the measurement voltage in the correct direction.

[0062] In this case, a negative voltage is applied to stage 32 , but if a negative charge is present on the sample, a positive voltage will be applied to stage 32 .

[0063] The sample is then scanned with the probe while simultaneously applying a bias voltage to the scanning probe system. The voltage applied to the stage 32 is maintained while the sample is scanned with the probe. In order to scan the sample 7 with the probe, the probe must be brought into close proximity to the sample 7. This may involve moving the probe to within the nm rather than μm range of the sample. At these short distances, the bias voltage may not completely nullify the electrostatic force originating from the sample, but still at least partially oppose it.

[0064] In one embodiment, only a single voltage measurement is made and the magnitude and sign of the bias voltage is constant during the entire sample scan. If the sample is imaged to produce an image having multiple pixels, the applied bias voltage may be the same for each pixel.

[0065] In another embodiment, multiple voltage measurements may be made, each at a different point on the sample. For example, one offset voltage measurement may be made at the center of the sample and another at an edge of the sample. The offset voltage may be measured near the center of the sample. The applied bias voltage during scanning of the sample may then be selected accordingly. For example, if the offset voltage value is measured to be high in the center of the sample and low near the edge of the sample, the applied bias voltage may be varied accordingly depending on the area of ​​the sample being scanned.

[0066] Figure 3 shows an alternative method of using a probe to measure the cancellation voltage value. In the embodiment of Figures 1 and 2, the probe is not vibrating and the probe deflection angle is measured. In this embodiment, the probe is vibrating and the amplitude of the probe vibration is measured. Figure 3 shows how the amplitude of the probe vibration changes over time.

[0067] The probe oscillates with a first amplitude during a first period 102. As with the embodiment of Figure 2, the probe is initially positioned at a relatively large distance from the sample, which carries a charge.

[0068] At the time indicated by 103, the probe is moved towards the sample to a measurement position and its amplitude of oscillation is reduced due to electrostatic interaction between the probe and the charged sample. The probe oscillates at this lower amplitude for an entire period 104.

[0069] In this embodiment, the vibration of the probe is detected by an interferometer or segmented photodiode, which outputs the vibration signal to an amplitude processor, which analyzes the vibration signal to determine not only the amplitude of the vibration, but also the change in the amplitude of the vibration over time. After receiving a signal indicating a change in the vibration amplitude that requires correction (i.e., due to charges on the sample exerting an electrostatic force on the probe), the amplitude processor sends a signal to the voltage generator 30 to increase the bias voltage applied to the scanning probe system. As can be seen from period 106, the bias voltage applied to the scanning probe system is increased until the amplitude of the vibration is returned to its initial value. The amplitude processor works with the voltage generator in a feedback loop to monitor the amplitude of the probe vibration to determine when the amplitude has returned to its initial value.

[0070] The probe scans the sample while a bias voltage is simultaneously applied to the scanning probe system, the magnitude and sign of the bias voltage being based on the measured cancellation voltage value that caused the amplitude to return to its initial value.

[0071] In this case, the amplitude of the vibration is monitored to determine the cancellation voltage value, although in other examples the frequency or phase of the probe vibration may be monitored instead.

[0072] In the above described embodiment, the bias voltage is applied to the stage 32 of the scanning probe system which carries the sample, but the bias voltage may alternatively or additionally be applied to any other part of the scanning probe system. As shown in FIG. 4, the probe includes a gold layer 202 which coats the top surface of the cantilever beam 204. The bias voltage applied to the scanning probe system may therefore instead be applied to this gold layer 202. In this case, the bias voltage applied to the probe will be of the same polarity as the voltage of the sample (i.e. if the sample has a positive voltage, the probe will also be raised to a positive voltage and vice versa). This is in contrast to the previous embodiment, in which the stage 32 is applied with a voltage of opposite polarity to the sample.

[0073] Figure 6 shows a scanning probe system 50a which is similar to the scanning probe system 50 shown in Figure 1. Like features have like reference numbers and will not be described again. In this case the voltage generator 30 applies a voltage to the cantilever beam 2 rather than to the stage 32.

[0074] If the probe includes a gold layer 202 to which a bias voltage is applied by a voltage generator 30, the gold layer 202 should be separated from the probe tip 3 by an electrically insulating barrier. because if charges build up on the probe tip 3, damage to the probe tip or sample may occur due to current flowing between the probe tip and the sample. Typical insulators suitable for use as an electrically insulating barrier are silicon, silicon oxide or silicon nitride, and therefore, as mentioned before, it may be advantageous from a construction point of view to simply apply the gold layer 202 directly to the top side of the cantilever beam 204, since the cantilever beam 204 may already be made of said insulating material. Nevertheless, it is important that no conductive path exists between the gold layer 202 and the probe tip 3, for the reasons mentioned above.

[0075] The bias voltage applied to the stage 32 may completely nullify the electrostatic forces coming from the sample when the probe is in the measurement position (e.g., 5 μm from the sample), but becomes less effective as the probe tip moves closer to the sample. For this reason, applying a bias voltage to the probe, as shown in Figure 6, may be preferred as it should remain effective over a wide range of distances.

[0076] On the other hand, the arrangement of FIG. 1 may be preferable in other cases where it is difficult to apply a voltage to the probe.

[0077] 7-12 show various different ways of scanning a sample 150 with the scanning probe system described above. The sample is a circular wafer with a diameter of 300 mm.

[0078] In the simplest case, only a single cancellation voltage value is measured, for example at a central measurement location 151 or an edge measurement location 152 of the wafer. In this case, the bias voltage applied to the scanning probe system during a scan of the wafer is based only on this single cancellation voltage value. For example, the bias voltage applied to the scanning probe system during a scan of the wafer may have the same magnitude and sign as this single cancellation voltage value.

[0079] In the example of Figures 7 and 8, two cancellation voltage values ​​are measured: one cancellation voltage value (V1) at measurement location 151 in the center of the wafer, and one cancellation voltage value (V2) at measurement location 152 on the edge. Figure 8 shows these cancellation voltage values. Note that the voltage is lower at the center than at the edge.

[0080] After the two offset voltage values ​​of FIG. 8 are measured, first and second non-adjacent image areas 153, 154 of the sample are scanned.

[0081] Before the first imaging area 153 is scanned, a first bias voltage V3 is determined by linear interpolation based on the two offset voltage values ​​V1, V2 and the distance R1 of the first imaging area 153 from the center of the wafer, as shown in Figure 8. This first bias voltage V3 is applied to the scanning probe system throughout the scan of the first imaging area 153.

[0082] Similarly, before the second imaging area 154 is scanned, a second bias voltage V4 is determined by linear interpolation based on the two offset voltage values ​​V1, V2 and the distance R2 of the second imaging area 154 from the center of the wafer, as shown in Figure 8. This second bias voltage V4 is applied to the scanning probe system throughout the scan of the second imaging area 154.

[0083] Each imaging area 153, 154 is typically a square with sides on the order of 1-10 μm, and therefore FIG. 7 is not to scale. The size of the imaging areas 153, 154 is so small that it is assumed that the voltage on the sample does not vary significantly across the imaging area. Thus, the bias voltage does not vary during scanning of any of the imaging areas 153, 154.

[0084] Each imaging area 153, 154 is scanned by making a series of scan measurements of the sample at a series of scan positions across the area 153, 154, each scan measurement corresponding to a respective pixel. Each scan measurement is made by moving the probe tip towards the sample, making a scan measurement from the interferometry height detector 52 at a respective one of the scan positions, and then retracting the probe tip from the sample. For each imaging area 153, 154, the bias voltage applied to the scanning probe system does not vary from scan position to scan position. The bias voltage also remains approximately constant when the probe tip moves towards the sample before the scan measurement and when the probe tip is retracted away from the sample after the scan measurement.

[0085] In the example of Figure 9, four offset voltage values ​​are measured: one at measurement location 151 in the center of the wafer and another at measurement location 152 on the edge, as well as two at intermediate locations. Figure 10 shows these offset voltage values. In this case, the voltage is lower at the center than at the edge.

[0086] Before the first imaging area 153 is scanned, a first bias voltage V5 is determined by linear interpolation based on the two intermediate offset voltage values ​​and the distance R1 of the first imaging area 153 from the center of the wafer, as shown in Figure 9. This first bias voltage V5 is then applied to the scanning probe system throughout the scan of the first imaging area 153.

[0087] Similarly, before the second imaging area 154 is scanned, a second bias voltage V6 is determined by linear interpolation based on the two highest offset voltage values ​​and the distance R2 of the second imaging area 154 from the center of the wafer, as shown in Figure 10. This second bias voltage V6 is applied to the scanning probe system throughout the scan of the second imaging area 154.

[0088] 9 and 10 assume that the voltage varies in a radially symmetric manner across the sample 150. Figures 11 and 12 show an alternative method in which the voltage varies more non-uniformly across the sample 150.

[0089] In the example of Figure 11, the cancellation voltage values ​​are measured across the sample 150 on a coarse square grid (approximately 50 mm square). Figure 12 shows seven radially non-symmetric cancellation voltage values ​​measured across the center of the wafer.

[0090] Before an imaging area 155 having coordinates X,Y is scanned, a bias voltage is determined by linear interpolation based on offset voltage values ​​measured at four measurement locations 156 in the grid closest to the imaging area 155, based on the X,Y coordinates of the imaging area 155. This bias voltage is applied to the scanning probe system throughout the scan of the imaging area 155.

[0091] In the above example, the bias voltage values ​​are calculated by linear interpolation, however, other methods of determining the bias voltage from multiple measured cancellation voltage values ​​may be used, such as non-linear interpolation, fitting a polynomial curve to the cancellation voltage values, or taking the average of all of the cancellation voltage values.

[0092] In the above embodiments, reference has been made to scanning the sample with the probe. By "scanning" it is meant that there is relative horizontal motion between the probe and the sample. This may mean that the sample is kept stationary while the probe traverses its surface, or it may equivalently mean that the probe is held stationary while the sample is moved laterally.

[0093] Additionally, while in the above-described embodiment the offset voltage value is determined before the probe is brought into scanning proximity with the sample (i.e., within nm of the sample), the offset voltage value is determined after the probe is brought into scanning proximity with the sample. The applied voltage can be measured and applied evenly only when the sample is in close scanning proximity (i.e., within nm of the sample).

[0094] Each of the electronic elements shown in the figures and described herein (e.g., deflection processor 26; voltage generator 30; amplitude processor, etc.) may be implemented as hardware, software, or otherwise, including, for example, any combination of hardware and software. As an example, a single field programmable gate array (FPGA) or digital signal processor (DSP) or multiple FPGAs or DSPs may implement all of the electronic elements, or each electronic element may be implemented by a dedicated FPGA or DSP or any combination of FPGAs or DSPs.

[0095] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications can be made without departing from the scope of the invention, as defined in the appended claims.

Claims

1. 1. A method of scanning a sample with a scanning probe system, the scanning probe system including a probe including a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever, the method comprising: moving the probe to a measurement position where the sample applies an electrostatic force to the probe; applying a measurement voltage to the scanning probe system at the measurement location, varying the measurement voltage applied to the scanning probe system and monitoring a response of the probe to the variation in measurement voltage to measure a cancellation voltage value which reduces or neutralises the electrostatic force on the probe at the measurement location; after the offset voltage value is measured, scanning the sample with the probe while applying a bias voltage to the scanning probe system based on the offset voltage value; Including, The method, wherein scanning the sample with the probe includes measuring a topography of the sample.

2. moving the probe to a plurality of measurement positions where the sample applies an electrostatic force to the probe; applying a measurement voltage to the scanning probe system at each measurement location, varying the measurement voltage applied to the scanning probe system, and monitoring a response of the probe to the variation in measurement voltage to measure respective cancellation voltage values ​​which reduce or neutralize the electrostatic force on the probe at the measurement location; determining the bias voltage based on at least two of the offset voltage values; The method of claim 1 , comprising:

3. The method of claim 2 , wherein an area of ​​the sample is scanned with the probe and the bias voltage is determined based on a location of the area of ​​the sample.

4. The method of claim 3 , wherein the bias voltage does not vary during the scanning of the area of ​​the specimen.

5. A first bias voltage is determined based on a location of the first area of ​​the specimen, a bias voltage is applied to the scanning probe system during scanning of the first area of ​​the sample; 5. The method of claim 3 or 4, wherein a second bias voltage is determined based on a location of a second area of ​​the sample, and the second bias voltage is applied to the scanning probe system during scanning of the second area of ​​the sample.

6. A method as claimed in any one of claims 2 to 5, wherein the or each bias voltage is determined from the cancellation voltage values ​​by interpolation.

7. A method according to any preceding claim, wherein the cancellation voltage value is determined at the or each measurement location with the probe tip spaced from the sample by a distance greater than 1 μm.

8. A method according to any preceding claim, further comprising storing the or each cancellation voltage value before the sample is scanned.

9. A method according to any preceding claim, wherein an area of ​​the sample is scanned with the probe and the bias voltage applied to the scanning probe system does not vary during the scanning of the area of ​​the sample.

10. 10. The method of claim 1, wherein scanning the sample with the probe comprises taking a series of scan measurements of the sample at a series of scan positions across the sample, each scan measurement being taken by moving the probe tip towards the sample, taking a scan measurement at each of the scan positions and then retracting the probe tip away from the sample.

11. The method of claim 10 , wherein the bias voltage applied to the scanning probe system does not vary from scan position to scan position.

12. 12. The method of claim 10 or 11, wherein for each scanning measurement, the bias voltage applied to the scanning probe system remains approximately constant as the probe tip moves towards the sample and as the probe tip retracts away from the sample.

13. A method according to any preceding claim, wherein the variation in the measured voltage causes the probe to deflect, and the deflection of the probe is monitored to determine the cancellation voltage.

14. A method according to any preceding claim, wherein the variation in the measured voltage changes a vibration parameter of the probe, the vibration parameter being monitored to determine the cancellation voltage.

15. The method of claim 14 , wherein the vibration parameter is a phase, a frequency or an amplitude of vibration of the probe.

16. The method of any one of claims 1 to 15, wherein the measurement voltage and the bias voltage are applied to the probe.

17. The method of claim 16 , wherein the measurement voltage and the bias voltage are applied to the cantilever.

18. The cantilever includes a conductive layer separated from the probe tip by an electrically insulating barrier.

20. The method of claim 17, comprising: wherein the bias voltage is applied to the conductive layer.

19. The method of any one of claims 1 to 15, wherein the measurement voltage and the bias voltage are applied to conductors in close proximity to the sample.

20. A method according to any preceding claim, wherein scanning the sample with the probe comprises imaging the sample with the probe to generate an image having a plurality of pixels.

21. 21. The method of claim 20, wherein the bias voltage applied to the scanning probe system is the same for each pixel of the image.

22. 22. A scanning probe system comprising a probe including a cantilever extending from a base to a free end and a probe tip carried by the free end of the cantilever, the scanning probe system being arranged to perform a method according to any one of claims 1 to 21.

23. 23. A scanning probe system according to claim 22 arranged to apply the bias voltage to the probe.

24. 24. The scanning probe system of claim 23, wherein the cantilever includes a conductive layer separated from the probe tip by an electrically insulating barrier, and the scanning probe system is positioned to apply the bias voltage to the conductive layer.

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