device

Abstract

There is provided an alternating current compensation circuit (36) for a device (30). The alternating current compensation circuit (36) includes, or is coupleable to, a voltage or current source (34) so as to generate a compensation current to cancel or minimise a capacitive current formed by a junction capacitance during an operation of the device (30).

Description

[0001] DEVICE

[0002] This invention relates to an alternating current compensation circuit, a device comprising an alternating current compensation circuit, and a method of operating an alternating current compensation circuit. The device of the invention may be a mechanical device and / or an electrical device and / or an electronic device and / or a measurement device. The invention is applicable, but not limited, to scanning probe microscopes in general, and scanning tunnelling microscopes, atomic force microscopes, and scanning near field optical microscopes in particular.

[0003] It is known to use a scanning probe microscope to scan or measure surfaces at the atomic level.

[0004] According to a first aspect of the invention, there is provided an alternating current compensation circuit for a device, wherein the current compensation circuit includes, or is coupleable to, a voltage or current source so as to, in use, generate a compensation current to cancel or minimise a capacitive current formed by a junction capacitance during an operation of the device.

[0005] The alternating current compensation circuit may form part of an alternating current compensation device or an alternating current compensation apparatus.

[0006] The junction capacitance may be a capacitance between two surfaces, a tip-sample capacitance or a probe-sample capacitance, preferably of a scanning probe microscope.

[0007] The junction capacitance may be a tunnelling junction capacitance. The tunnelling junction capacitance may be a tunnelling junction capacitance of a scanning probe microscope or a tunnelling capacitance between two objects in close proximity, be it any combination of conductive, semi-conductive and / or non-conductive materials, e.g. as found in a chip.

[0008] The operation of the device may be, but is not limited to, an actuation operation, a motion operation, a sensing operation, a measurement operation, a scan operation, an imaging operation or a spectroscopy operation.

[0009] The compensation current may be phase-shifted relative to the capacitive current. In particular, the compensation current may be inverted relative to the capacitive current. That is to say, the compensation current may be either switched in sign, multiplied by minus one, or phase-shifted relative to the capacitive current by 180 degrees.

[0010] In embodiments of the invention, the alternating current compensation circuit may include first and second circuit branches. The first circuit branch may be for carrying a current flowing through the junction, e.g. between the tip / probe and the sample or through a tunnelling junction. The second circuit branch may include a compensation capacitance element that is configured to have the same, or substantially the same, capacitance as the junction capacitance. The alternating current compensation circuit may be configured to, in use, generate the compensation current in the second circuit branch.

[0011] The first and second circuit branches may be connected in parallel. This includes layout configurations such as a stub tuner.

[0012] The alternating current compensation circuit may include an electrical modification device (e.g. a voltage or current modification device) configured to, in use, modify a voltage or current from the voltage or current source so as to generate the compensation current in the second circuit branch.

[0013] The electrical modification device may include, but is not limited to, a balun and / or a coil and / or a transformer. The electrical modification device may include electronic passive and / or active components arranged in a circuit to generate an inversed or 180 degrees phase-shifted signal.

[0014] The electrical modification device may be arranged to, in use, electrically couple the voltage or current source to the first and second circuit branches. Alternatively the second circuit branch may include the electrical modification device, which may be a current or voltage inversion device. The current inversion device may include a voltage inversion device and a resistor.

[0015] In further embodiments of the invention, the alternating current compensation circuit may include a tuning element configured to, in use, tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance.

[0016] The tuning element may also be configured to be manually operable by a user. Alternatively the tuning element may be configured to, in use, automatically tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance. For example, the tuning element may be configured to, in use, automatically tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance responsive to a received measurement of the junction capacitance. The tuning element may include a phase- locked loop and / or a varactor diode. The tuning element may be configured to, in use, mechanically and / or electronically and / or piezoelectrically tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance.

[0017] In still further embodiments of the invention, the compensation capacitance element may be or may include a stub. Preferably the stub is a tuneable stub. A stub may be tuneable by, for example, changing its frequency, changing its dimensions (e.g. changing its cable length(s)) and / or changing its capacitance.

[0018] In embodiments of the invention, the alternating current compensation circuit may include an amplifying circuit configured to, in use, amplify a signal corresponding to a total current flowing through the junction and / or the compensation current. Preferably the amplifying circuit includes a mixer (e.g. an image rejection mixer), a phase / frequency-specific detection element, a lock-in amplifier, a pre-amplifier, a transimpedance amplifier or a combination of two or more of the preceding components.

[0019] The alternating current compensation circuit may include a phase shift regulator configured to, in use, shift a phase of the capacitive current (which may also shift a phase of a signal of interest) and / or a phase of the compensation current. The phase shift regulator may be arranged in the first circuit branch or the second circuit branch. The phase shift regulator may be tuned mechanically or electronically controlled.

[0020] The alternating current compensation circuit may include an amplitude regulator configured to, in use, modify an amplitude of the capacitive current (which may also modify an amplitude of a signal of interest) and / or an amplitude of the compensation current. The amplitude regulator may be arranged in the first circuit branch or the second circuit branch. The amplitude regulator may be a passive amplitude regulator (such as an attenuator) or an active amplitude regulator. Multiple amplitude regulators may be arranged in the first and second circuit branches respectively. According to a second aspect of the invention, there is provided a device comprising: an AC voltage source for providing an AC bias voltage across a junction; and an alternating current compensation circuit according to any one of the first aspect of the invention and its embodiments.

[0021] The device may further include a tip or probe that may be brought into proximity with a sample. The device may be a scanning probe microscope. The junction may be a tunnelling junction. It will be understood that the device may be manufactured with the alternating current compensation circuit or that the device may be retrofitted or otherwise combined with the alternating current compensation circuit.

[0022] According to a third aspect of the invention, there is provided a method of operating an alternating current compensation circuit according to any one of the first aspect of the invention and its embodiments, the method including the step of: by the alternating current compensation circuit, generating a compensation current to cancel or minimise a capacitive current formed by a junction capacitance during an operation of the device.

[0023] The method of the invention may include the steps of: measuring the capacitive current prior to or during electron tunnelling through the junction; and by the alternating current compensation circuit, using the capacitive current measurement to generate the compensation current.

[0024] It will be appreciated that the use of the terms "first" and "second", and the like, in this patent specification is merely intended to help distinguish between similar features, and is not intended to indicate the relative importance of one feature over another feature, unless otherwise specified.

[0025] Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, and the claims and / or the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and all features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. Preferred embodiments of the invention will now be described, by way of non-limiting examples, with reference to the accompanying drawings in which:

[0026] Figure 1 shows the general principle of a scanning tunnelling microscope according to the invention;

[0027] Figure 2 illustrates a tip-sample capacitance;

[0028] Figures 3 and 4 illustrate capacitive current compensation according to the invention;

[0029] Figures 5 and 6 illustrate a working principle of a balun;

[0030] Figures 7 to 15 show various embodiments and features of the scanning tunnelling microscope according to the invention;

[0031] Figures 16 and 17 are images showing the scanning performance of the scanning tunnelling microscope according to the invention;

[0032] Figures 18 and 19 are SEM images of a sample prepared for scanning; and

[0033] Figures 20 to 23 are further images showing the scanning performance of the scanning tunnelling microscope according to the invention.

[0034] The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic form in the interests of clarity and conciseness.

[0035] The following embodiments are described with reference to a scanning tunnelling microscope. It will however be appreciated that the following embodiments apply mutatis mutandis to other types of scanning probe microscopes and other types of junctions in which a capacitive current is formed by a junction capacitance.

[0036] A scanning tunnelling microscope according to an embodiment of the invention is shown in Figure 1 and is designated generally by the reference numeral 30. The scanning tunnelling microscope 30 comprises a tip 32, an AC voltage source 34 and an alternating current compensation circuit 36.

[0037] In use, the tip 32 is brought into proximity with a sample 38 (typically within a few atomic distances above a surface of the sample 38), and the AC voltage source 34 is operated to provide an AC bias voltage across the tip 32 and the sample 38. This causes electrons to tunnel through the space between the tip 32 and the surface of the sample 38. By using the tip 32 to laterally scan across the surface and monitoring the tunnelling current, it becomes possible to acquire information about the surface of the sample 38. Alternatively the tip 32 can be kept stationary to, e.g., perform spectroscopy or tunnelling junction measurements so as to measure the sample's electronic properties. In constant current mode, feedback electronics 72 (which may be used in combination with a lock-in amplifier 74 as shown in Figures 12, 13 and 14) is used to continuously adjust the height of the tip 32 above the sample 38 so that the tunnelling current, which is exponentially dependent on the distance between the tip 32 and the sample 38, is kept constant. It will be appreciated that the tip 32 and the sample 38 can be turned upside down so that the sample 38 is above the tip 32.

[0038] For an alternating current scanning tunnelling microscope (AC-STM), there are two ways of measuring the tunnelling current at high frequency, which are transmission mode and reflection mode. However, at low frequencies, almost the complete current flows between the tip 32 and the sample 38 and there is hardly any current through the tip-sample capacitance 44, which favours operation in AC-STM in transmission mode. In transmission mode and at low frequencies, the current can be measured using a pre-amplifier 42 at a terminal either on the tip side or the sample side. If the current is measured at the terminal where the voltage is also applied, then a floating pre-amplifier may be used to simultaneously deliver the voltage to the terminal and amplify the current for measurement.

[0039] Current measurement in reflection mode is possible at high frequencies because a high- frequency AC signal partially reflects at all boundaries corresponding to an impedance mismatch. Examples of such boundaries include connections between cables and, in particular, the tip 32 with the surface of the sample 38. If E denotes the energy sent towards the tip, the following equation holds:

[0040] E = R + T in which R stands for the reflected energy and T stands for the transmitted energy. By knowing E and measuring R, it becomes possible to determine the amount of transmitted energy from the tip 32 to the sample 38.

[0041] A problem arises when measuring the tunnelling current at high frequencies (e.g. to perform imaging or to perform spectroscopy so as to measure the sample's electronic properties) or perform measurements without any DC signal (e.g. to measure insulator samples or measure the sample's electronic properties). In particular, the signal of interest (the tunnelling current) is overshadowed by an undesired second signal (a capacitive current), which is a result of the tip 32 and sample 38 simultaneously forming a parallel tip-sample capacitance 44 as shown in Figure 2 where the capacitive current flows through the parallel capacitance.

[0042] This problem becomes increasingly severe with increasing frequency. This is because the complex resistance of the tip-sample capacitance 44 decreases proportionally to 1 / f (with f being the frequency). To illustrate this problem, we compare the tunnelling current against the capacitive current. When a tunnelling voltage of IV is applied, the imaging or measurement is typically carried out with a tunnelling current of only 0.1 nA or less to prevent atomic changes on the sample surface by a tip-sample interaction that is too high. Assuming a typical tip-sample capacitance 44 of 200 fF, the capacitive current is 1.3 nA at 1 kHz, is 1.3 pA at 1 MHz and is 1.3mA at 1GHz, which makes it very difficult, if not impossible, to measure the tunnelling current.

[0043] The inventors have devised a solution for overcoming the problem caused by the capacitive current flowing through the parallel tip-sample capacitance 44. In particular, the solution relates to the alternating current compensation circuit 36 which, in use, enables cancellation or minimisation of the capacitive current signal during an operation of the AC-STM 30 so that the signal of interest (the tunnelling current) can not only be obtained at high frequencies but also can be measured very accurately at high frequencies.

[0044] The alternating current compensation circuit 36 may be implemented in a number of different ways, non-limiting examples of which are described as follows.

[0045] The alternating current compensation circuit 36 comprises first and second circuit branches 46,48 which are connected in parallel. The first circuit branch 46 includes the tip-sample junction so that a current flowing between the tip 32 and the sample 38 flows in the first circuit branch 46. The second circuit branch 48 includes a compensation capacitance element 50. First ends of the circuit branches 46,48 are electrically coupled to the AC voltage source 34, which is also the same AC voltage source 34 for applying the AC bias voltage across the tip 32 and the sample 38. Second ends of the circuit branches 46,48 are electrically coupled to a current amplifier 42.

[0046] The principle behind the alternating current compensation circuit 36 is the generation of a compensation current in the second circuit branch 48. The alternating current compensation circuit 36 is configured so that the compensation current is inverted relative to the capacitive current (or phase-shifted relative to the capacitive current by 180 degrees), as schematically indicated by the inverter element 52 in Figure 1. That is to say, the compensative current is generated to be a mirror image of the capacitive current. In other embodiments, the compensation current may be phase-shifted relative to the capacitive current by a different amount of degrees.

[0047] The compensation capacitance element 50 may be tunable to modify its capacitance Ccomp to match, or substantially match, the tip-sample capacitance CSTM . In other embodiments, the compensation capacitance element 50 may have a fixed capacitance that matches, or substantially matches, CSTM. The match between Ccomp and CSTM may be at a specific frequency or at a limited number or range of frequencies.

[0048] The configuration of the first and second circuit branches 46,48 is based on a nullifying capacitance bridge, which can be tuned to match Ccomp to CSTM . One way of tuning the compensation capacitance element 50 is by bringing the tip 32 into proximity with the sample 38 but without initiating electron tunnelling conditions, using the current amplifier 42 to measure the capacitive current prior to electron tunnelling between the tip 32 and the sample 38, and tuning the compensation capacitance element 50 so that the measured capacitive current reduces to a minimum magnitude, which is when Ccomp is equal to CSTM . Figure 3 illustrates how an almost exact compensation of CSTM can be reached, as the minimum magnitude of the measured capacitive current is less than IpA.

[0049] Since the capacitive current increases with frequency, the precise tuning of the compensation capacitance element becomes increasingly important with higher frequency. The use of the nullifying capacitance bridge supports this precise tuning because both capacitive and compensation currents increase linearly with the frequency. In other words, the higher the frequency used for AC-STM scanning, the more precise one needs to tune Ccomp to match CSTM, which is possible because the remaining current from the mismatch between Ccomp and CSTM is amplified with the increase in frequency. The following equation holds (Icapacitive being the capacitive current; f being the frequency; UAC being the AC bias voltage):

[0050] ^capacitive ^AC ■ i2 tf(CSTM- ^comp)

[0051] Figure 4 shows the magnitude of the capacitive current as a function of both frequency and mismatch between Ccomp and CSTM . If Ccomp is set to zero, the graph can be used to show the magnitude of the undesired capacitive current. It becomes apparent from Figure 4 that: (1) The capacitive current increases linearly with frequency and can reach values above ~100 nA at 1 GHz, although the mismatch between Ccomp and CSTM is in the low aF regime; and (2) Measurement of tunnelling currents below O. lnA at only 100kHz requires the mismatch between Ccomp and CSTM to be in the low aF regime or lower. Indeed, performing an accurate AC-STM measurement with a tunnelling current below 30 nA at 1GHz requires the mismatch between Ccomp and CSTM to be as accurate as 1 aF. In contrast, typical tip-sample capacitances are in the order of 200 fF (i.e. 200000 aF).

[0052] A benefit of the nullifying capacitance bridge is that the use of an inverter 52 means that any imperfection in the AC signal, either generated by the AC voltage source or coupled into it, is automatically compensated. This is because the imperfection will be present in both circuit branches 46,48 and thereby will be cancelled out. The imperfection may be or may be due to, but is not limited to, a voltage spike, a glitch, noise, electronic cross talk, or deviations from a perfect sine AC signal.

[0053] For the best AC-STM performance, the inverter 52 must be ideal as possible. The inverter 52 can be made from, but are not limited to, passive electronic components (e.g. coils, transformers, inductances and capacitors), distributed elements, lumped elements, and / or active electronic components (e.g. operational amplifiers or field effect transistors).

[0054] One way of implementing the inverter 52 in the alternating current compensation circuit 36 is through use of a balun 54 which can have a broad operating band (e.g. from 200 kHz up to 3 GHz).

[0055] The balun 54 is a "balanced to unbalanced" transformer that either transforms a single input signal 56 into a signal 58 with the same polarity and a signal 60 with an inverted polarity (balanced to unbalanced; Figure 5a) or vice versa (unbalanced to balanced; Figure 5b). A balun 54 may be made in different ways. For example, a balun 54 may be made from one or two coils with twisted pairs cabling (e.g. an input coil coupled with output coils formed by a double twisted pair cable) so that the signal 60 with an inverted polarity is produced via a magnetic field.

[0056] In the alternating current compensation circuit 36 shown in Figure 6, the balun 54 comprises a first coil 62 connected between the AC voltage source 34 and ground, a second coil 64 connected in series with the tip-sample junction in the first circuit branch 46, and a third coil 66 connected in series with the compensation capacitance element 50. In this way, first ends of the circuit branches 46,48 are electrically coupled to the AC voltage source 34 via the balun 54. The second ends of the circuit branches 46,48 are electrically coupled to a current amplifier 42. The galvanic isolation of the balun 54 decouples the AC voltage source 34 from the current amplifier 42, which reduces noise in the current amplifier readout.

[0057] In use, the configuration of the balun 54 means that the compensation current generated in the second circuit branch 48 will be the inverse of the current generated in the first circuit branch 46 when there is no tunnelling current between the tip 32 and the sample 38.

[0058] The balun-based alternating current compensation circuit 36 enables AC-STM measurements in both transmission mode and reflection mode because, as mentioned above, a balun 54 can transform signals in both directions: from balanced to unbalanced and vice versa. In the embodiment shown in Figure 6, the measurement is carried out in transmission mode due to the location of the pre-amplifier 42. Current or energy measurements can be measured at a terminal either on the tip side or the sample side. Figure 7 shows a high-electron mobility transistor (HEMT) 68 connected in shunt between the balun 54 and the first coil 62. This enables AC-STM measurements in transmission mode. Figure 8 shows a HEMT 68 connected in shunt between the circuit branches' second sides and ground. This enables AC-STM measurements in reflection mode. It will be appreciated that other measurement devices or tools may be used in place of the HEMT 68.

[0059] Figure 9 shows a summing circuit 70 coupled to the first sides of the circuit branches 46,48. The summing circuit 70 may include at least one operational amplifier and / or at least one transistor (e.g. a field effect transistor, a high-electron mobility transistor). The summing circuit 70 is configured to combine the reflected signals in the circuit branches 46,48 and provide an output signal that is indicative of the tunnelling current and the mismatch between the capacitive and compensation currents. Figure 10 shows an example of a summing circuit comprising an operational amplifier. Figure 11 shows another example of a summing circuit comprising field effect transistors.

[0060] The alternating current compensation circuit may include a tuning element 76 for tuning Ccomp to match, or substantially match, CSTM . For example, see Figure 13.

[0061] The tuning element 76 may be configured to be manually operable by a user. The tuning element 76 may be in the form of a mechanical actuator that adjust a distance and / or an overlapping area between two electrodes of the compensation capacitance element 50. For example, the distance and / or overlapping area between the two electrodes may be adjusted using one or more micrometer screws. In another example, the one or more micrometer screws may be replaced by any type of mechanical actuator, e.g. a hydraulic actuator, a syringe pump. The tuning element 76 may include a drive (e.g. a motor, such as a stepper motor, preferably with gearboxes to provide sub-steps for higher accuracy) to control the mechanical actuator in order to automate the electrode distance and / or overlapping area adjustment. The stepper motor may be coupled to, or form part of, a feedback circuit 74,76 that measures a current flowing in the first circuit branch 46, so that the stepper motor can adjust the electrode distance and / or overlapping area to minimise the current flowing in the first circuit branch 46, which corresponds to a minimum value of the capacitive current.

[0062] It was found by the inventors that the mismatch between Ccomp and CSTM can be reduced to the low aF regime, which enables accurate high frequency AC-STM measurement. By way of illustration, the arrows in Figure 4 indicate an experimental AC-STM measurement that was performed at 100 kHz with a tunnelling current of 86 pA. The capacitive current was decreased by more than 80 dB.

[0063] It will be understood that the feedback circuit 74,76 may be used in combination with other types of tuning elements and compensation capacitance elements 36. The feedback circuit 74,76 may include an amplifier (e.g. a lock-in amplifier 74 as shown in Figures 12 and 13) to, in use, amplify a signal corresponding to a total current flowing between the tip 32 and the sample 38. The lock-in amplifier 74 enables measurement of a signal amplitude at a particular frequency. The feedback circuit 74,76 may include a phase-locked loop, which provides phase-locked loop feedback as shown in Figure 13. For example, the lock-in amplifier 74 may include an internal or embedded phase-locked loop. This allows for an automated tuning of Ccomp during a measurement, even if the height between the tip 32 and sample 38 changes due to, e.g., a different setpoint tunnelling current or applying a different voltage of the AC voltage source 34.

[0064] The balun-based current compensation circuit 36 can be operated up to the upper frequency of the bandwidth of the balun 54, which may be at least 3 GHz. However, as the wavelength at 3 GHz is only 10 cm, any difference in cable lengths (and impedances) of the two circuit branches 46,48 become increasingly important. Fine tuning to eliminate or further minimise the difference between the compensation current and the capacitive current may be performed by applying or connecting a phase shift regulator in either circuit branch 46,48, preferably in combination with at least one attenuator and an amplitude regulator or a tuneable attenuator. The phase shift regulator is configured to, in use, shift a phase of the capacitive current and / or a phase of the compensation current. Commercial phase shift regulators can cover up to 120 degrees of phase shift at 2 GHz, without significant loss of energy or signal. The amplitude regulator is configured to, in use, modify an amplitude of the capacitive current and / or an amplitude of the compensation current.

[0065] Figure 14 shows the use of image rejection mixers 80 for up and down frequency conversion of signals measured at the tip side and / or the sample side. The image rejection mixers 80 are coupled to the lock-in amplifier 74 so that signals are fed from the image rejection mixers 80 to the lock-in amplifier.

[0066] The compensation capacitance element 36 may be an electrically tunable compensation capacitance, e.g. a varactor diode, a capacitance element of another STM or a scanning probe microscope (SPM), or a combination of two or more of the preceding examples.

[0067] The cancellation or minimisation of the capacitive current by the compensation current can be achieved through, for example, at least one radio-frequency line (e.g. coplanar wave guides, coaxial cables, strip lines, other waveguides), at least one distributed circuit element, at least one lumped circuit element, at least one active electronic component, at least one passive electronic component, or a combination of two or more of the preceding examples.

[0068] Another way of implementing the inverter 52 in the alternating current compensation circuit 36 is through use of a tuneable stub 82 in the second circuit branch 48. The tuneable stub 82 (or otherwise known as a stub tuner) is a resonance circuit. The capacitance of the stub 82 can be tuned by adjusting an electrical length and / or a frequency of the stub 82. Figure 15 shows an exemplary implementation of the stub 82.

[0069] The second circuit branch 48 is connected in shunt with the first circuit branch 46 so as to form a shunt-stub circuit. The first ends of the circuit branches 46,48 are electrically coupled to the AC voltage source 34 and additionally to a signal measurement device or component, such as a vector network analyser (VNA) or an amplifier. The second ends of the circuit branches 46,48 are electrically coupled to ground.

[0070] The stub-based alternating current compensation circuit 36 enables AC-STM measurements in reflection or transmission mode. In particular, respective reflected signals from the STM 30 and the shunt-stub circuit are measured (e.g. by the VNA). At a particular frequency, reflected or transmitted signals corresponding to the capacitive and compensation currents nearly cancel each other out, which leads to a sharp minimum of the measured reflected or transmitted signal. This reduces the capacitance current and thereby increases the sensitivity to the signal of interest (tunnelling current).

[0071] It will be appreciated that the geometry of the electrodes (e.g. plates, disks, wires, tips) can be freely chosen to achieve the required capacitance regime. This includes plates, disks, wires, tips and any other desired form.

[0072] The invention therefore provides a solution to enable improved AC-STM measurements, especially at high frequencies. This includes, but is not limited to, measurement on insulator, biological, catalyst and metal-on-insulator samples, measurement of electronic properties (e.g. electronic states, noise), investigations of dynamic behaviour (e.g. chemical dynamic behaviour), and faster measurements (e.g. beyond video rate).

[0073] Typical STMs measure with frequencies up to a few kHz. Some STMs can measure at several 100s of kHz. On the contrary, the AC-STM 30 according to the invention has been shown to be capable of accurate measurement at frequencies of at least 10MHz.

[0074] Figure 16 shows L2R and R2L images of a highly (lll)-textured Au sample obtained using a conventional STM while Figure 17 shows L2R and R2L images of the same region obtained using an AC-STM 30 according to the invention. It was observed that the AC-STM images include more details of the scanned region and are clearer than the conventional STM images.

[0075] Conventionally STMs are unable to tunnel on oxides (such as glass) if the oxide thickness surpasses 1 to 2 nm.

[0076] Figures 18 and 19 show a 21nm SiO? nano-patterned sample deposited on polycrystalline Au(lll). The nano-pattern is in the form of an array of nanoholes etched in the SiO? layer. Each nanohole has a diameter of about 230nm. The spacing between the nanoholes range between 160nm to 230nm.

[0077] The AC-STM scanning was performed at 10 MHz with tunnelling currents of around InA, which requires the mismatch between Ccomp and CSTM to be in the range of around 3 to 10 aF or lower. Figures 20 to 23 show AC-STM images of the nano-patterned sample. Details of the Au at the bottom of the nanoholes, as well as the SiO? layer, can be observed clearly. Steps of atomic resolution were observed in the SiO? layer, which indicates crystalline behaviour in the SiOz.

[0078] The invention may be implemented as a new scanning probe microscope incorporating the alternating current compensation circuit. Alternatively the alternating current compensation circuit may be incorporated or otherwise combined with (i.e. retrofitted to) an existing scanning probe microscope.

[0079] It will be appreciated that the above numerical values are intended to illustrate the working of the invention and are not limiting on the scope of the invention.

[0080] The listing or discussion of an apparently prior-published document or apparently prior- published information in this specification should not necessarily be taken as an acknowledgement that the document or information is part of the state of the art or is common general knowledge.

[0081] Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention.

Claims

CLAIMS1. An alternating current compensation circuit for a device, wherein the alternating current compensation circuit includes, or is coupleable to, a voltage or current source so as to, in use, generate a compensation current to cancel or minimise a capacitive current formed by a junction capacitance during an operation of the device.

2. An alternating current compensation circuit according to Claim 1 wherein the junction capacitance is a capacitance between two surfaces, a tip-sample capacitance or a probe-sample capacitance.

3. An alternating current compensation circuit according to Claim 1 or Claim 2 wherein the junction capacitance is a tunnelling junction capacitance or a tunnelling capacitance between two objects in close proximity.

4. An alternating current compensation circuit according to any one of the preceding claims wherein the compensation current is inverted relative to the capacitive current.

5. An alternating current compensation circuit according to any one of the preceding claims wherein the compensation current is phase-shifted relative to the capacitive current.

6. An alternating current compensation circuit according to any one of the preceding claims including first and second circuit branches, the first circuit branch for carrying a current flowing through the junction, the second circuit branch including a compensation capacitance element that is configured to have the same, or substantially the same, capacitance as the junction capacitance, wherein the alternating current compensation circuit is configured to, in use, generate the compensation current in the second circuit branch.

7. An alternating current compensation circuit according to Claim 6 wherein the first and second circuit branches are connected in parallel.

8. An alternating current compensation circuit according to Claim 6 or Claim 7 wherein the alternating current compensation circuit includes an electrical modification device configured to, in use, modify a voltage or current from the voltage or current source so as to generate the compensation current in the second circuit branch.

9. An alternating current compensation circuit according to Claim 8 wherein the electrical modification device includes a balun.

10. An alternating current compensation circuit according to Claim 8 or Claim 9 wherein the electrical modification device includes a coil and / or a transformer.

11. An alternating current compensation circuit according to any one of Claims 8 to 10 wherein the electrical modification device is arranged to, in use, electrically couple the voltage or current source to the first and second circuit branches.

12. An alternating current compensation circuit according to any one of Claims 8 to 10 wherein the second circuit branch includes the electrical modification device.

13. An alternating current compensation circuit according to any one of Claims 6 to 12 including a tuning element configured to, in use, tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance.

14. An alternating current compensation circuit according to Claim 13 wherein the tuning element is configured to be manually operable by a user.

15. An alternating current compensation circuit according to Claim 13 wherein the tuning element is configured to, in use, automatically tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance.

16. An alternating current compensation circuit according to Claim 15 wherein the tuning element is configured to, in use, automatically tune a capacitance of the compensation capacitance element to match, or substantially match, the junction capacitance responsive to a received measurement of the junction capacitance.

17. An alternating current compensation circuit according to Claim 15 or Claim 16 wherein the tuning element includes a phase-locked loop.

18. An alternating current compensation circuit according to any one of Claims 6 to 17 wherein the compensation capacitance element is or includes a stub.

19. An alternating current compensation circuit according to Claim 18 wherein the stub is a tuneable stub.

20. An alternating current compensation circuit according to any one of the preceding claims including an amplifying circuit configured to, in use, amplify a signal corresponding to a total current flowing through the junction and / or the compensation current.

21. An alternating current compensation circuit according to any one of the preceding claims including a phase shift regulator configured to, in use, shift a phase of the capacitive current and / or a phase of the compensation current.

22. An alternating current compensation circuit according to any one of the preceding claims including an amplitude regulator configured to, in use, modify an amplitude of the capacitive current and / or an amplitude of the compensation current.

23. A device comprising: an AC voltage source for providing an AC bias voltage across a junction; and an alternating current compensation circuit according to any one of the preceding claims.

24. A device according to Claim 23 further including a tip or probe that may be brought into proximity with a sample, wherein the device is a scanning probe microscope.

25. A device according to Claim 23 or Claim 24 wherein the junction is a tunnelling junction.

26. A method of operating an alternating current compensation circuit according to any one of Claims 1 to 25, the method including the step of: by the alternating current compensation circuit, generating a compensation current to cancel or minimise a capacitive current formed by a junction capacitance during an operation of the device.

27. A method according to Claim 26 including the steps of: measuring the capacitive current prior to or during electron tunnelling through the junction; andby the alternating current compensation circuit, using the capacitive current measurement to generate the compensation current.

Images