Scanning probe microscope with probe immersed in liquid

Abstract

A method of analysing a sample using a scanning probe microscope. The scanning probe microscope comprises a lateral scanning system; and a probe head comprising: a transparent window; and a probe comprising a cantilever and a probe tip extending from the cantilever, wherein the probe is immersed in liquid. The method comprises: constraining an upper surface of the liquid with the transparent window; constraining a lower surface of the liquid with the sample; wherein the liquid has unconstrained sides which form a meniscus which meets the sample; and obtaining sample data by: generating a lateral scanning motion between the probe and the sample, by operation of the lateral scanning system, so that the probe tip interacts with the sample across a region of the sample, and monitoring the probe to obtain sample data from the region of the sample. The probe remains immersed in the liquid as the sample data is obtained; and the probe is monitored by: illuminating the cantilever with a detection beam via the transparent window and the liquid, and monitoring a reflection of the detection beam from the cantilever, wherein the reflection is received via the liquid and the transparent window.

Description

[0001] SCANNING PROBE MICROSCOPE WITH PROBE IMMERSED IN LIQUID

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method of analysing a sample using a scanning probe microscope; and a scanning probe microscope for performing such a method.

[0004] BACKGROUND OF THE INVENTION

[0005] WO2023 / 247654 discloses a method of measuring a feature with a probe microscope. The feature comprises a base, an entrance, and a pair of opposed side walls. The feature is filled with a liquid. The probe microscope comprises a cantilever and a probe tip extending from the cantilever. The method comprises: inserting the probe tip into the feature via the entrance; and performing a measurement of the feature by contacting the base of the feature with the probe tip. The probe; sample; and actuators are housed in a closed cell with a transparent window through which a detection beam and actuation beam can pass on the way to the probe. The closed cell can be filled with liquid via an inlet port and emptied of liquid via an outlet port.

[0006] US2014289910 discloses a sealed AFM cell including: a cantilever including a probe; a sample holder for fixing the sample; a scanner for moving the sample holder; a lid part which holds the cantilever so as to position the probe near a measurement surface of the sample; and a main body part which is a component for holding the scanner and positioned opposite the lid part with the sample in between. The lid part and the main body part are joined via a sealing liquid to seal the observation liquid inside a space formed by the lid part, the main body part, and the sealing liquid. The sealing liquid is different from the observation liquid and not in contact with the observation liquid.

[0007] The probe cells in WO2023 / 247654 and US2014289910 fully immerse the sample in liquid. Such a large volume of liquid may cause damage to the sample, which is problematic if the sample is a valuable item such as a semiconductor wafer. Also, fully immersing the sample in liquid may take a long time.

[0008] SUMMARY OF THE INVENTION A first aspect of the invention provides a method of analysing a sample using a scanning probe microscope, the sample comprising a first sample surface, and a second sample surface opposing the first sample surface, the scanning probe microscope comprising a lateral scanning system; and a probe head comprising: a transparent window; and a probe comprising a cantilever and a probe tip extending from the cantilever, wherein the probe is immersed in liquid, the method comprising: constraining a surface of the liquid with the transparent window; constraining a surface of the liquid with the first sample surface; wherein the liquid has unconstrained sides which form a meniscus which meets the first sample surface, a minor portion of the first sample surface is contacted by the liquid and a major portion of the first sample surface is not contacted by the liquid; and obtaining sample data by: generating a lateral scanning motion between the probe and the sample, by operation of the lateral scanning system, so that the probe tip interacts with the first sample surface across a region of the sample, and monitoring the probe to obtain sample data from the region of the sample, wherein the probe remains immersed in the liquid as the sample data is obtained; and wherein the probe is monitored by: illuminating the cantilever with a detection beam via the transparent window and the liquid, and monitoring a reflection of the detection beam from the cantilever, wherein the reflection is received via the liquid and the transparent window.

[0009] Optional features are set out in the dependent claims.

[0010] Optionally the first sample surface is substantially horizontal and substantially planar.

[0011] Optionally an area of the minor portion contacted by the liquid is less than 10% of a full area of the first sample surface, or less than 1% of a full area of the first sample surface, or less than 0.1% of a full area of the first sample surface.

[0012] Optionally an area of the minor portion contacted by the liquid is less than 30mm2or less than 20mm2or less than 15mm2or less than 10mm2or less than 5mm2.

[0013] Optionally the meniscus has an edge which forms a closed periphery of the meniscus, and a full circumference of the closed periphery meets the first sample surface. The edge may be a lower edge of the meniscus.

[0014] Optionally the liquid only contacts the first sample surface. Optionally the second sample surface contacts a sample stage, and the sample stage is not contacted by the liquid.

[0015] Optionally the first sample surface is an upper surface and the second sample surface is a lower surface.

[0016] A second aspect of the invention provides a scanning probe microscope according to claim 23. Optional features are set out in the dependent claims.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the invention will now be described with reference to the accompanying drawings, in which:

[0019] Figure 1 shows a scanning probe microscope;

[0020] Figure 2 shows a measurement system of the microscope;

[0021] Figure 3 shows a probe head of the microscope;

[0022] Figure 4 is a plan view of a fine scanning system;

[0023] Figure 5 is a first example of a probe cell;

[0024] Figure 6 shows the probe cell at an operating height;

[0025] Figure 7 shows the probe cell with a body of liquid;

[0026] Figure 8 shows a further example of a probe cell;

[0027] Figure 9 shows a further example of a probe cell;

[0028] Figure 10 shows a further example of a probe cell;

[0029] Figure 11 shows a further example of a probe cell; and

[0030] Figure 12 is a flow diagram showing steps in a method of obtaining sample data from a semiconductor wafer.

[0031] DETAILED DESCRIPTION OF EMBODIMENT(S)

[0032] A scanning probe microscopy system 1 according to an embodiment of the invention is shown in Figure 1 . The system comprises a piezoelectric driver 4 (referred to below as a Z-piezo 4) and a probe comprising a cantilever 2 and a probe tip 3. The bottom of the Z-piezo 4 provides a cantilever mount, with the cantilever 2 extending from the cantilever mount from a proximal end or base to a distal free end. The probe tip 3 is carried by the free end of the cantilever 2. The probe 2, 3 is mounted in a probe cell which is not shown in Figure 1 for purposes of clarity.

[0033] The probe tip 3 comprises a conical or pyramidal structure that tapers from its base to a point at its distal end that is its closest point of interaction with a sample 7 on a sample stage 11a (conventionally known as a chuck). The sample comprises a sample surface which defines a sample surface axis which is normal to the sample surface and in Figure 1 also extends vertically. The cantilever 2 comprises a single beam with a rectangular profile extending from the cantilever mount 13. The cantilever 2 has a length of about 20 micron, a width of about 10 micron, and a thickness of about 200nm.

[0034] The cantilever 2 is a thermal bimorph structure composed of two (or more) materials, with differing thermal expansion coefficients - typically a silicon or silicon nitride base with a gold or aluminium coating. The coating extends the length of the cantilever and covers the reverse side from the tip 3. An illumination system (in the form of a laser 30) under the control of photothermal (PT) drive 33 is arranged to illuminate the cantilever on its upper coated side with an intensity-modulated radiation spot from an actuation beam 35.

[0035] The cantilever 2 is formed from a monolithic structure with uniform thickness. For example the monolithic structure may be formed by selectively etching a thin film of SiC>2 or SiN4 as described in Albrecht T., Akamine, S., Carver, T.E., Quate, C.F. J., Microfabrication of cantilever styli for the atomic force microscope, Vac. Sci. Technol. A 1990, 8, 3386 (hereinafter referred to as "Albrecht et al."). The tip 3 may be formed integrally with the cantilever, as described in Albrecht et al., it may be formed by an additive process such as electron beam deposition, or it may be formed separately and attached by adhesive or some other attachment method.

[0036] The wavelength of the actuation beam 35 output by the laser 30 is selected for good absorption by the coating, so that the cantilever 2 bends along its length and moves the probe tip 3. In this example the coating is on the reverse side from the sample so the cantilever 2 bends down towards the sample when heated, but alternatively the coating may be on the same side as the sample so the cantilever 2 bends away from the sample 7 when heated. The Z-piezo 4 expands and contracts up and down in the Z-direction in accordance with a piezo drive signal 5 at a piezo driver input. As described further below, the piezo drive signal 5 may cause the Z-piezo 4 to move the probe repeatedly towards and away from the sample 7 in a series of cycles. The piezo drive signal 5 is generated by a piezo controller (not shown).

[0037] A measurement system 80 is arranged to detect a height of the free end of the cantilever 2 directly opposite to the probe tip 3. The measurement system 80 includes an interferometer and a quadrant photodiode (QPD). Figure 1 only shows the measurement system 80 schematically and Figure 2 gives a more detailed view. Light 100 from a laser 101 is split by a beam splitter 102 into a sensing beam 103 and a reference beam 104. The reference beam 104 is directed onto a suitably positioned retro-reflector 120 and thereafter back to the beam splitter 102. The retro-reflector 120 is aligned such that it provides a fixed optical path length relative to the vertical (Z) position of the sample 7. The beam splitter 102 has an energy absorbing coating and splits both the incident 103 and reference 104 beams to produce first and second interferograms with a relative phase shift of 90 degrees. The two interferograms are detected respectively at first 121 and second 122 photodetectors.

[0038] Ideally, the outputs from the photodetectors 121 , 122 are complementary sine and cosine signals with a phase difference of 90 degrees. Further, they should have no de offset, have equal amplitudes and only depend on the position of the cantilever and wavelength of the laser 101. Known methods are used to monitor the outputs of the photodetectors 121 , 122 while changing the optical path difference in order to determine and to apply corrections for errors arising as a result of the two photodetector outputs not being perfectly harmonic, with equal amplitude and in phase quadrature. Similarly, de offset levels are also corrected in accordance with methods known in the art.

[0039] These photodetector outputs are suitable for use with a conventional interferometer reversible fringe counting apparatus and fringe subdividing apparatus 123, which may be provided as dedicated hardware, FPGA, DSP or as a programmed computer. Phase quadrature fringe counting apparatus is capable of measuring displacements in the position of the cantilever to an accuracy of A / 8. That is, to 66 nm for 532 nm light. Known fringe subdividing techniques, based on the arc tangent of the signals, permit an improvement in accuracy to the nanometre scale or less. In the embodiment described above, the reference beam 104 is arranged to have a fixed optical path length relative to the Z position of the sample 7. It could accordingly be reflected from the surface of the stage 11a on which the sample 7 is mounted or from a retro-reflector whose position is linked to that of the stage. The reference path length may be greater than or smaller than the length of the path followed by the beam 103 reflected from the probe. Alternatively, the relationship between reflector and sample Z position does not have to be fixed. In such an embodiment the reference beam may be reflected from a fixed point, the fixed point having a known (but varying) relationship with the Z position of the sample. The height of the tip is therefore deduced from the interferometrically measured path difference and the Z position of the sample with respect to the fixed point.

[0040] The interferometer detector is one example of a homodyne system. The particular system described offers a number of advantages to this application. The use of two phase quadrature interferograms enables the measurement of cantilever displacement over multiple fringes, and hence over a large displacement range. Examples of an interferometer based on these principles are described in US6678056 and WO2010 / 067129. Alternative interferometer systems capable of measuring a change in optical path length may also be employed. A suitable homodyne polarisation interferometer is described in EP 1 892 727 and a suitable heterodyne interferometer is described in US 5 144 150.

[0041] Returning to Figure 1 , the output of the interferometer is a height signal on a height detection line 20 which is input to a surface height calculator (not shown) and a surface detection unit (not shown). The surface detection unit is arranged to generate a surface signal on a surface detector output line for each cycle when it detects an interaction of the probe tip 3 with the sample 7.

[0042] The reflected beam is also split by a beam splitter 106 into first and second components 107, 110. The first component 107 is directed to a segmented quadrant photodiode 108 via a lens 109, and the second component 110 is split by the beam splitter 102 and directed to the photodiodes 121 , 122 for generation of the height signal on the output line 20. The photodiode 108 generates angle data 124 which is indicative of the position of the first component 107 of the reflected beam on the photodiode 108, and varies in accordance with the angle of inclination of the cantilever relative to the sensing beam 103.

[0043] The angle data 124 comprises a deflection / bending signal which indicates a flexural angle of the cantilever - i.e. an angle which changes as the cantilever bends along its length. Thus the deflection / bending signal is indicative of the flexural shape of the cantilever. The deflection / bending signal may be determined in accordance with a difference between the signals from the top and bottom halves of the quadrant photodiode 108.

[0044] The angle data 124 also comprises a lateral / twisting signal which indicates a torsion angle of the cantilever - i.e. an angle which changes as the cantilever twists. Thus the lateral / twisting signal is indicative of the torsional shape of the cantilever. The lateral / twisting signal may be determined in accordance with a difference between the signals from the left and right halves of the quadrant photodiode 108.

[0045] In summary, the measurement system 80 monitors the probe 2, 3 to obtain sample data from a region of the sample 7 which is scanned by the probe. This sample data may be topographic data indicating a topographic profile of the sample (based on the angle data 124 and / or the height signal on the height detection line 20) or any other type of data about the sample. For example the probe 2, 3 may be scanned in a single line, so that the sample data relates to a one-dimensional region of the sample; or the probe 2, 3 may be scanned in a raster pattern so that the sample data relates to a two- dimensional region of the sample.

[0046] Figure 3 shows an example of a probe head 40 of the microscope of Figure 1. The probe head 40 comprises a chassis 41 which carries the objective lens 32; a fine scanning system 42 carried by the chassis 41 ; and a probe cell 43 carried by the fine scanning system 42.

[0047] Figure 4 is a plan view of the fine scanning system 42, which comprises a frame 45; the piezoelectric actuator (Z-piezo) 4 which carries the probe cell 43 and can be driven to move the probe cell 43 up and down in a Z-direction perpendicular to the sample 7; a piezoelectric actuator (X-piezo) 47 which can be driven to move the Z-piezo 4 and the probe cell 43 in a lateral X-direction; and a piezoelectric actuator (Y-piezo) 48 which can be driven to move the Z-piezo 4 and the probe cell 43 in a lateral Y-direction. The probe cell 43 may be attached to the frame 45 by flexures (not shown).

[0048] The X-piezo 47 and the Y-piezo 48 together provide a lateral scanning system which can be operated to generate a lateral scanning motion between the probe and the sample, in this case by motion of the probe cell 43. In other case the lateral scanning motion between the probe and the sample may be caused by motion of the chuck 11a and the sample 7.

[0049] The sample 7 comprises a first sample surface which is contacted by the probe and the liquid, and a second sample surface which opposes the first sample surface. In one arrangement, the first sample surface may be an upper surface of the sample and the second sample surface may be a lower surface of the sample. However, the arrangement may be inverted so that the sample is held above the probe meaning that the first sample surface may be a lower surface of the sample and the second sample surface may be an upper surface of the sample.

[0050] A coarse actuation system is configured to bring the probe cell 43 and the sample 7 together in the Z-direction. In this case the coarse actuation system comprises a Z- motor 50, schematically indicated in Figure 3, which moves the probe head 40 up and down. In other embodiments the coarse actuation system may instead move the sample 7 by moving the chuck 11a up and down.

[0051] Figure 3 shows the probe head 40 in a raised position before liquid has been injected. The probe head 40 comprises a proximity sensor 42 (such as a capacitance sensor) which is configured to generate an output indicative of a distance D1 between the proximity sensor 42 and the sample 7. In this case the proximity sensor 42 is mounted to the frame 41 of the probe head 40, although in other embodiments the proximity sensor 42 may be mounted to any other part of the probe head 40.

[0052] In another example, the proximity sensor 42 may be replaced by an optical proximity sensor (such as an interferometer or a confocal optical sensor) in the probe head 40 which measures a distance to the sample 7 using a detection beam which passes through the window and the liquid (and optionally also through the objective lens). The proximity sensor 42 measures a distance D1 from the proximity sensor 42 to the sample 7, for instance by a capacitive measurement or optical measurement. The range of motion of the Z-piezo 4 is quite small, so the distance D1 from the proximity sensor 42 to the sample 7 is also approximately indicative of the distance D2 between the probe cell 43 and the sample 7 (D1 and D2 being different by a fixed and known offset).

[0053] Figure 5 is a cross-sectional view showing the probe cell 43 in more detail, with other parts of the probe head 40 omitted. The probe cell 43 comprises a probe cell body 50 which carries the probe 2, 3; and an optically transparent window 53 which covers an opening in the top of the probe cell body 50.

[0054] The probe cell body 50 also comprises an inlet 51 and an outlet 52 formed by respective openings in the probe cell body 50. The inlet 51 is connected to a flexible inlet line 51a and the outlet 52 is connected to a flexible outlet line 52a.

[0055] The Z-motor 50 brings the probe cell 43 and the sample together by moving the probe head 40 down, and as it does so the proximity sensor 42 continuously measures the distance D1 which it inputs to an injection system 49.

[0056] The injection system 49 (shown schematically in Figures 3 and 6) is configured to wait until the measured distance D1 is sufficiently small, as in Figure 6, before injecting liquid 54 into a space 55 between the transparent window 53 and the sample 7 as shown in Figure 7, thereby immersing the probe 2,3 in the liquid 54. Note that both the cantilever 2 and the probe tip 3 are immersed in the liquid 54. In other words, the injection system 49 is configured to wait until the measured distance D1 reaches a predetermined threshold before injecting the liquid 54.

[0057] This predetermined threshold may be an operating distance, which is a distance reached when the probe 2, 3 contacts the sample 7. In this case, the movement of the probe cell is stopped when the measured distance reaches the operating distance, and then the liquid is injected into the space between the transparent window and the sample.

[0058] Alternatively the predetermined threshold may be greater than the operating distance, so the predetermined threshold is reached before the probe 2, 3 contacts the sample 7. Here the threshold may be set so that the probe 2,3 is sufficiently close to the sample 7 that liquid can be injected before the probe makes contact with the sample. Therefore, the injection system 49 may begin injecting liquid 54 as the probe is transitioning between imaging phases, meaning that the probe cell 43 may be moving towards the sample whilst the liquid 54 is being injected. Between imaging sites, an amount of liquid 54 may remain within the probe cell 43. Therefore, when moving between imaging sites, the probe cell 43 may not have to be fully refilled.

[0059] Alternatively, the probe cell 43 may be at least partially filled before the probe cell 43 and sample 7 are brought together. For example, the probe cell 43 may hold a body of liquid which forms a convex meniscus on the open edge of the probe cell 43. As the probe cell 43 and sample 7 are brought together, the proximity sensor 42 may continuously measure the distance to the sample 7 so that the Z-motor 50 of the coarse actuation system is stopped when the measured distance reaches a predetermined threshold, thereby stopping the movement which is bringing the probe cell and the sample together. This predetermined threshold may be set to correspond with the point at which the convex meniscus has reached the sample 7. Optionally additional liquid may also be injected into the body of liquid when the measured distance reaches the predetermined threshold.

[0060] By way of example, the liquid 54 may comprise an aqueous alcohol (for example dilute isopropyl alcohol).

[0061] In this example the injection system 49 is configured to immerse the probe by injecting the liquid via the flexible inlet line 51 a and the inlet 51 , but in other embodiments of the invention the liquid may be injected into the space 55 via a different route.

[0062] As shown in Figure 7, an upper surface of the liquid 54 is constrained by the transparent window 53; a lower surface of the liquid 54 is constrained by the sample 7; and the liquid 54 has unconstrained sides which form a meniscus 56 which meets the sample 7, typically at an oblique contact angle. The term “meniscus” is used herein to refer to a liquid surface that curves either out or in as a result of surface tension. In this example the meniscus 56 curves outwardly but in other cases the meniscus 56 may curve inwardly. The meniscus 56 comprises an interface between the liquid 54 and a gas 57. By waiting until the measured distance D1 is sufficiently small before injecting the liquid 54, the injection system 49 ensures that a small and controlled volume of liquid 54 is dispensed. The small volume of liquid minimizes the risk of damage to the sample 7 or contamination of the sample 7, and speeds up the injection process.

[0063] Optionally the probe is immersed in a body of liquid 54 with a volume which is less than 1ml, or less than 0.1 ml, or less than 0.01 ml, or less than 50 pL, or less than 10 pL, or less than 5 pL, or less than 1 pL.

[0064] A minor portion of the first (upper) sample surface is contacted by the liquid 54 and a major portion of the first (upper) sample surface is not contacted by the liquid 54. In other words, less than 50% of the area of the first sample surface is contacted by the liquid. Minimising the amount of area contacted by the liquid 54 is advantageous because it minimizes the risk of damage or contamination of the sample 7. Therefore, the liquid 54 may be present at the imaging site only, and not covering larger areas of the sample.

[0065] An area of the minor portion (i.e. the area contacted by the liquid) may be less than 10% of a full area of the first sample surface, or less than 1% of a full area of the first sample surface, or less than 0.1% of a full area of the first sample surface. The term “area” is used here to refer to a footprint or 2D projection within an outer periphery of the sample or liquid, rather than its area taking into account microscopic surface features.

[0066] By way of example, the liquid 54 may contact a circular area of the sample 7 with a diameter of about 5mm (area about 20mm2) and the sample may be a circular wafer with a diameter of about 300mm. In this case about 0.03% of the first sample surface is contacted by the liquid.

[0067] In other examples the liquid 54 may contact a smaller area of the sample, for instance less than 20mm2or less than 15mm2or less than 10mm2or less than 5mm2.

[0068] After the liquid 54 has been injected, the microscope is operated to obtain sample data from the sample 7. A lateral scanning motion is generated between the probe 2, 3 and the sample 7 by operation of the lateral scanning system 47, 48, so that the probe tip 3 interacts with the sample 7 across a region of the sample. For example the lateral scanning system 47, 48 may scan the probe 2, 3 with a raster scanning motion across a two-dimensional region of the sample 7. As it scans across the sample, the probe 2,

[0069] 3 is monitored by the measurement system 80 to obtain the sample data from the region of the sample 7. As mentioned above, the sample data may be indicative of a topography of the region of the sample.

[0070] In this example the lateral scanning system 47, 48 moves the probe cell 43 (including the probe 2, 3 and the transparent window 53) so there may be a certain amount of disruption of the liquid 54 caused by the lateral scanning motion. Similar disruption of the liquid 54 may also be caused if the lateral scanning motion is generated by moving the chuck 11a (rather than moving the probe 2, 3). In both cases it is expected that the degree of disruption of the liquid 54 caused by the lateral scanning motion is unlikely to break the meniscus 56. This ensures that the probe 2, 3 remains immersed in the liquid 54 as the sample data is obtained.

[0071] Similarly the probe cell 43 may be moved up and down in the Z-direction by the Z-piezo

[0072] 4 as the probe tip 3 scans across the region of the sample. This Z-motion moves the transparent window 53 and the probe 2, 3, so it may also cause a certain amount of disruption of the liquid 54 but it is expected that the degree of disruption of the liquid 54 caused by the Z-motion is unlikely to break the meniscus 56. The Z-motion is generated in this case by motion of the probe, but in other embodiments it may be caused by motion of the chuck 11a and the sample 7.

[0073] The flexibility of the inlet line 51a and outlet line 52a ensure that they can flex to accommodate the scanning motion of the probe cell 43, both laterally and in the Z- direction.

[0074] As shown in Figure 7, the probe is monitored by: illuminating the cantilever 2 with a detection beam 60 via the transparent window 53 and the liquid 54, and monitoring a reflection 62 of the detection beam from the cantilever. The reflection 62 is received via the liquid 54 and the transparent window 53.

[0075] As also shown in Figure 7, the detection beam 60 is focused onto the cantilever by the objective lens 34, and the reflection 62 of the detection beam is monitored by collecting the reflected beam 62 from the cantilever 2 with the objective lens 34; and directing the reflected beam 62 from the objective lens 34 to the measurement system 80. The measurement system 80 monitors the reflected beam 62 using the interferometer and / or the quadrant photodiode.

[0076] The liquid 54 is removed by the injection system 49 via the outlet 52 and outlet line 52a after the sample data has been obtained. After the liquid 54 has been removed via the outlet 52, the probe head 40 is lifted up by the Z-motor 50 and then a lateral transit motion between the probe 2, 3 and the sample 7 is generated so that the probe moves to a further region of the sample. This lateral transit motion may be caused by motion of the probe head 40, but more typically it is achieved by motion of the chuck 11a and the sample 7. The sample 7 may be a 300mm semiconductor wafer, so the range of lateral transmit motion may be high, for instance higher than 100mm.

[0077] When the further region of the sample is aligned with the probe, the probe head 40 is lowered by the Z-motor 50 until the measured distance D1 is sufficiently small. Then liquid is injected into the space between the transparent window 53 and the sample, thereby immersing the probe at the further region of the sample in the liquid. This liquid may be different to the liquid 54 used previously, or the liquid 54 may be re-used by the injection system 49.

[0078] The imaging process is then repeated at the further region of the sample: that is, by generating a lateral scanning motion between the probe 2, 3 and the sample 7; by operation of the lateral scanning system 47, 48, so that the probe tip 3 interacts with the sample across the further region of the sample 7; and by monitoring the probe 2, 3 to obtain further sample data from the further region of the sample 7.

[0079] Figure 8 is a cross-sectional view showing an alternative probe cell 43 which can be used in the probe head 40 of Figure 3. The probe cell of Figure 8 is similar to the probe cell of Figure 5, and the same reference numbers are used to indicate the same (or equivalent) features.

[0080] The probe cell has an immersion liquid inlet 51 , an immersion liquid outlet 52, a barrier liquid inlet 75 and a barrier liquid outlet 76. Immersion liquid 54a is injected via the immersion liquid inlet 51 and removed via the immersion liquid outlet 52. Barrier liquid 54b is injected via the barrier liquid inlet 75 and removed via the barrier liquid outlet 76. The immersion liquid 54a may comprise an aqueous alcohol (for example dilute isopropyl alcohol) and the barrier liquid 54b may comprise an oil such as a mineral oil.

[0081] Although the two liquids 54a, 54b are immiscible, they can be considered as a single body of liquid. Hence an upper surface of the body of liquid is constrained by the transparent window 53; a lower surface of the body of liquid is constrained by the sample; and the body of liquid has unconstrained sides which form a meniscus 56 which meets the sample 7, typically at an oblique angle. The meniscus 56 comprises an interface between the barrier liquid 54b and a gas 57. The arrangement may be inverted so that the sample is held above the probe meaning that the lower surface of the body of liquid is constrained by the transparent window 53 and the upper surface of the body of liquid is constrained by the sample.

[0082] Optionally the probe is immersed in a body of liquid 54a, 54b (bounded at its edge by the meniscus 56) with a volume which is less than 1ml, or less than 0.1 ml, or less than 0.01 ml, or less than 50 pL, or less than 10 pL, or less than 5 pL, or less than 1 pL.

[0083] Figure 9 is a cross-sectional view showing an alternative probe cell 43 which can be used in the probe head 40 of Figure 3. Here the probe cell 43 has no probe cell body 50, but instead the transparent window 53 is held at the end of an arm 70 which is fixed at the other end to the frame 41 , or another part of the probe head 40. Hence the transparent window 53 is fixed relative to the frame 41 , so that the transparent window 53 does not move during the lateral scanning motion.

[0084] The lateral scanning system moves the probe 2, 3 laterally relative to the transparent window 53, and a further meniscus 56a bridges the variable gap between the Z-piezo 4 and the transparent window 53.

[0085] The Z-piezo 4 also generates a Z-motion which moves the probe 2, 3 up and down relative to the transparent window 53, so that the probe mount 58 moves in and out of the body of liquid 54.

[0086] An advantage of the arrangement of Figure 9 is that the meniscus 56 contacting the sample 7 is less disrupted by the Z-motion and the lateral scanning motion. Note that the meniscus 56 in Figure 9 is illustrated with an inwardly curved shape which may also occur with the other embodiments. The probe cell 43 of Figure 9 has no probe cell body with an inlet and outlet. Hence the liquid 54 can either be deposited as a drop on the sample 7 before the probe is lowered into the drop, or the liquid 54 may be injected down via the gap between the Z-piezo 4 and the transparent window 53, or via any other route.

[0087] Figure 10 shows an alternative probe cell 43 which can be used in the probe head of Figure 3. The probe cell 43 has no inlet or outlet. Hence the liquid 54 can either be deposited as a drop on the sample 7 before the probe is lowered into the drop, or the liquid 54 may be injected by a nozzle sideways into the probe cell 43 via the gap between the probe cell 43 and the sample 7. Here the measurement system illuminates the distal end of the cantilever with the detection beam 62, and also illuminates the base or proximal end of the cantilever with a reference beam 60a. The reflection 60a of the reference beam 60a is then combined with a reflection 62a of the detection beam 62 at the interferometer, to obtain an interferometric measurement of the height distance between the proximal and distal ends of the cantilever 2.

[0088] Figure 11 shows the probe cell 43 of Figure 10, also showing the actuation beam 35 which illuminates the cantilever via the transparent window and the liquid 54. The actuation beam 35 is modulated to bend and un-bend the cantilever 2 during the lateral scanning motion.

[0089] Figure 12 shows the general method steps which are performed to image a sample 7, such as a semiconductor wafer. The wafer is driven to align a site with the probe in step 90, then optical alignment is performed at step 91 to align the probe with the detection and actuation beams. In step 92 the probe head 40 is lowered, with the proximity sensor 42 monitoring the distance. In an alternative arrangement, the probe head may be raised or the sample may be lowered.

[0090] In step 93 the probe head 40 is stopped at an operating height, and the immersion liquid 54 is introduced at step 94. In step 95 the wafer is imaged as described above, to obtain sample data from a region of the sample at the current site.

[0091] In step 96 the immersion liquid is removed and in step 97 the probe head 40 is raised up from the operating height, with the proximity sensor 42 monitoring. Steps 90-97 can then be repeated one or more times to obtain sample data at one or more further sites of the wafer.

[0092] WO2023 / 247654 gives an example of an imaging mode which can be performed by the microscope described above. The disclosure of WO2023 / 247654 is incorporated herein by reference. The probe tip is moved in and out of a high aspect ratio feature, and the liquid 54 prevents attractive forces from the side walls acting strongly on the probe tip. This makes it easier to insert the probe tip into the feature.

[0093] WO2024201024 gives another example of an imaging mode which can be performed by the microscope described above. The disclosure of W02024201024 is incorporated herein by reference. The piezo drive signal 5 causes the Z-piezo 4 to move the probe repeatedly towards and away from the sample 7 in a series of measurement cycles. During scanning the probe is made to approach and retract from the sample 7 in the series of measurement cycles, each approach and retract drive phase making up one cycle which involves taking a single measurement point when the probe contacts the sample surface. A dither signal may be applied to the probe during the first (approach) drive phase as a means of determining contact with the sample 7. The dither signal is applied using a signal from the photothermal drive 33 to illuminate the back of the cantilever 2 with the actuation beam 35. Using this actuation beam 35 it is possible to cause the probe to oscillate with a dither oscillation. For each measurement cycle, the dither oscillation, as measured by the interferometer or the quadrant photodiode, is monitored to detect contact of the probe with the sample 7. For example the phase or amplitude of the dither oscillation may change and this change may be detected to detect the contact.

[0094] Although the invention has been described above with reference to one or more preferred embodiments, it will be appreciated that various changes or modifications may be made without departing from the scope of the invention as defined in the appended claims.

Claims

CLAIMS1. A method of analysing a sample using a scanning probe microscope, the sample comprising a first sample surface, and a second sample surface opposing the first sample surface, the scanning probe microscope comprising a lateral scanning system; and a probe head comprising: a transparent window; and a probe comprising a cantilever and a probe tip extending from the cantilever, wherein the probe is immersed in liquid, the method comprising: constraining a surface of the liquid with the transparent window; constraining a surface of the liquid with the first sample surface; wherein the liquid has unconstrained sides which form a meniscus which meets the first sample surface, a minor portion of the first sample surface is contacted by the liquid and a major portion of the first sample surface is not contacted by the liquid; and obtaining sample data by: generating a lateral scanning motion between the probe and the sample, by operation of the lateral scanning system, so that the probe tip interacts with the first sample surface across a region of the sample, and monitoring the probe to obtain sample data from the region of the sample, wherein the probe remains immersed in the liquid as the sample data is obtained; and wherein the probe is monitored by: illuminating the cantilever with a detection beam via the transparent window and the liquid, and monitoring a reflection of the detection beam from the cantilever, wherein the reflection is received via the liquid and the transparent window.

2. A method according to claim 1 wherein the probe head further comprises a proximity sensor, and the method further comprises: bringing the probe cell and the sample together by moving the probe head and / or the sample; measuring a distance between the probe head and the sample based on an output of the proximity sensor as the probe head and the sample are brought together; monitoring the measured distance; and waiting until the measured distance is sufficiently small before injecting the liquid into a space between the transparent window and the sample, thereby immersing the probe in the liquid.

3. A method according to claim 2, wherein the movement of the probe cell and / or the sample is stopped when the measured distance reaches an operating distance, and then the liquid is injected into a space between the transparent window and the sample.

4. A method according to any preceding claim, wherein the lateral scanning system moves the probe.

5. A method according to any preceding claim, wherein the lateral scanning system moves the probe and the transparent window.

6. A method according to claim 5, wherein the lateral scanning system moves the probe relative to the transparent window.

7. A method according to any preceding claim, wherein the meniscus comprises an interface between the liquid and a gas.

8. A method according to any preceding claim, wherein the sample comprises a semiconductor wafer.

9. A method according to any preceding claim, wherein the scanning probe microscope further comprises a measurement system and an objective lens, the detection beam is focused onto the cantilever by the objective lens, and monitoring the reflection of the detection beam comprises: collecting the reflected beam from the cantilever with the objective lens; and directing the reflected beam from the objective lens to the measurement system, wherein the measurement system monitors the reflected beam.

10. A method according to claim 9, wherein the lateral scanning system moves the probe relative to the objective lens.

11. A method according to any preceding claim, wherein the probe cell comprises an inlet and the method further comprises injecting the liquid via the inlet into a space between the transparent window and the sample, thereby immersing the probe in the liquid.

12. A method according to claim 11 , wherein the inlet does not pass through the probe tip.

13. A method according to any preceding claim, wherein the sample data is indicative of a topography of the region of the sample.

14. A method according to any preceding claim, wherein the region of the sample is a two-dimensional region.

15. A method according to any preceding claim, wherein the probe cell comprises an outlet and the method further comprises removing the liquid via the outlet after the sample data has been obtained.

16. A method according to any preceding claim, further comprising, after the sample data has been obtained: generating a lateral transit motion between the probe and the sample so that the probe moves to a further region of the sample; then injecting liquid into a space between the transparent window and the sample, therebyimmersing the probe at the further region of the sample in the liquid; then generating a lateral scanning motion between the probe and the sample, by operation of the lateral scanning system, so that the probe tip interacts with the sample across the further region of the sample; and monitoring the probe to obtain further sample data from the further region of the sample.

17. A method according to any preceding claim, further comprising generating Z- motion between the probe and the sample in a Z-direction as the probe tip scans across the region of the sample.

18. A method according to claim 17, wherein the Z-motion moves the transparent window or the sample.

19. A method according to claim 17, wherein the Z-motion moves the probe relative to the transparent window.

20. A method according to any preceding claim, wherein the probe is immersed in a body of liquid with a volume which is less than 1ml, or less than 0.1 ml, or less than 0.01 ml, or less than 50 pL, or less than 10 pL, or less than 5 pL, or less than 1 pL.

21. A method according to any preceding claim, wherein the liquid comprises an immersion liquid and a barrier liquid which is immiscible with the barrier liquid, wherein the probe is immersed in the immersion liquid.

22. A method according to any preceding claim, wherein the probe head further comprises a proximity sensor, and the method further comprises: bringing the probe cell and the sample together by moving the probe head and / or the sample; measuring a distance between the probe head and the sample based on an output of the proximity sensor as the probe head and the sample are brought together; monitoring the measured distance; and when the measured distance reaches a predetermined threshold, injecting the liquid into a space between the transparent window and the sample and / or stopping the movement which is bringing the probe cell and the sample together.

23. A scanning probe microscope configured to analyse a sample by the method of any preceding claim, the scanning probe microscope comprising: a probe head comprising a transparent window, and a probe comprising a cantilever and a probe tip extending from the cantilever, wherein the probe head is configured to immerse the probe in liquid with unconstrained sides which form a meniscus which meets the sample; a lateral scanning system configured to generate a lateral scanning motion between the probe and a sample; and a measurement system configured to illuminate the cantilever with a detection beam via the transparent window and the liquid, andmonitor a reflection of the detection beam from the cantilever, wherein the reflection is received via the liquid and the transparent window.

24. A scanning probe microscope according to claim 23, further comprising: an actuation system configured to bring the probe cell and the sample together by moving the probe head and / or the sample; a proximity sensor configured to measure a distance between the probe head and the sample; and an injection system configured to wait until the measured distance is sufficiently small before injecting the liquid into a space between the transparent window and the sample, thereby immersing the probe in the liquid.

25. A scanning probe microscope according to claim 23 or 24, further comprising a probe cell body which carries the probe, wherein the transparent window covers an opening in the probe cell body; an inlet in the probe cell body; a flexible inlet line connected to the inlet; and an injection system configured to immerse the probe by injecting the liquid via the flexible inlet line and the inlet.

26. A scanning probe microscope according to any of claims 23 to 25, further comprising: an actuation system configured to bring the probe cell and the sample together by moving the probe head and / or the sample; a proximity sensor configured to measure a distance between the probe head and the sample; and an injection system configured to inject the liquid into a space between the transparent window and the sample, thereby immersing the probe in the liquid.

27. A scanning probe microscope according to claim 26, wherein the injection system is configured to inject a body liquid into the space between the transparent window and the sample, wherein the body of liquid has a volume which is less than 1ml, or less than 0.1 ml, or less than 0.01 ml, or less than 50 pL, or less than 10 pL, or less than 5 pL, or less than 1 pL.

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