Atomic force microscopy imaging device and method for Josephson probe microscope

Abstract

The present invention discloses an atomic force microscopy imaging device and method for a Josephson probe microscope, belonging to the field of atomic force microscopy imaging technology. The device includes a sample stage, a second displacement component, a first displacement component, a superconducting Josephson probe component, a first controller, a second controller, a lock-in amplifier, a current amplifier, and a high-speed data acquisition device. The superconducting Josephson probe component includes a probe module and a tuning fork module. The probe module includes a probe and a probe box. The tuning fork module includes a tuning fork wafer and a substrate. The probe is connected to the tuning fork wafer. The lock-in amplifier is connected to the tuning fork wafer to monitor the amplitude change of the tuning fork wafer in real time. The lock-in amplifier controls the second controller to drive the sample up and down to achieve closed-loop regulation of the distance between the probe and the sample. The present invention adjusts the distance between the probe and the sample surface in real time, ensuring high stability during imaging and further improving resolution. It can also effectively extend the service life of the probe and reduce the cost of use.

Description

Technical Field

[0001] The present invention belongs to the technical field of atomic force microscopy, and in particular relates to an atomic force microscopy device and method for a superconducting Josephson probe microscope. Background Art

[0002] In recent years, atomic force microscopy (AFM) technology has made significant progress in nanoscience and engineering, and is widely used in materials science, life sciences, the semiconductor industry, and other fields. By detecting the atomic force interactions between the probe and the sample surface, AFM can image surface topography with nanoscale or even atomic-level resolution.

[0003] The Josephson probe microscope system is a near-field electromagnetic imaging system built with a Josephson nanoprobe as its core device. A Josephson probe is a device with high response sensitivity in the microwave to terahertz frequency range, and its tip contains two parallel Josephson junctions. By utilizing the superconducting Josephson junction's sensitive electromagnetic response and ability to simultaneously measure the intensity, frequency, and even phase of high-frequency signals, the surface of the device under test is scanned point by point in two dimensions, and the spatial distribution of the frequency domain radiation spectrum is recorded. This enables real-time detection of the device under test with spatial resolution, frequency resolution, and radiation intensity resolution. However, currently, the lack of a method to stably control the probe-sample distance at the nanometer level prevents the probe from getting closer to the sample, limiting the resolution. As a result, Josephson probe microscopes are unable to achieve topographic imaging and are difficult to determine the probe's scanning area. Summary of the Invention

[0004] In response to the above-mentioned problems existing in the prior art, the technical problem to be solved by the present invention is to provide an atomic force microscopy imaging device and method for superconducting Josephson probe microscope, combining AFM technology with Josephson probe microscope to build a near-field probe microscope imaging system with higher spatial resolution.

[0005] Technical solution: In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0006] An atomic force microscopy imaging device for a Josephson probe microscope comprises a sample stage for fixing a sample, a second displacement assembly connected to the sample stage, a first displacement assembly connected to the second displacement assembly, a superconducting Josephson probe assembly for detecting the sample, a first controller electrically connected to the first displacement assembly, a second controller electrically connected to the second displacement assembly, a lock-in amplifier electrically connected to the superconducting Josephson probe assembly, a current amplifier electrically connected to the lock-in amplifier, and a high-speed data acquisition device electrically connected to both the lock-in amplifier and the second controller. The superconducting Josephson probe assembly comprises a probe module and a tuning fork module, the probe module comprising a probe and a probe box, the tuning fork module comprising a tuning fork wafer and a substrate, the probe being connected to the tuning fork wafer, the lock-in amplifier being connected to the tuning fork wafer to monitor amplitude changes of the tuning fork wafer in real time, and the lock-in amplifier achieving closed-loop adjustment of the distance between the probe and the sample by controlling the second controller to drive the sample to move up and down.

[0007] Preferably, the first displacement assembly includes a first displacement stage for realizing movement in the Z-axis direction, a second displacement stage for realizing movement in the Y-axis direction, and a third displacement stage for realizing movement in the X-axis direction; the second displacement assembly includes a fourth displacement stage for realizing movement in the Z-axis direction and a fifth displacement stage for realizing movement in the X-axis and Y-axis directions; the phase-locked amplifier adjusts the upper and lower positions of the sample in real time through the fourth displacement stage to keep the distance between the probe and the sample surface constant.

[0008] Preferably, the diameter of the top of the probe tip is less than 50 nm, and the top of the probe tip has two parallel superconducting Josephson junctions.

[0009] Preferably, the substrate is a square plate, the crystal oscillation direction of the tuning fork crystal is parallel to the substrate plane, and one side of the probe tip contacts the end of one arm of the tuning fork crystal.

[0010] The present invention also provides an atomic force microscopy imaging method for a Josephson probe microscope, using the above-mentioned atomic force microscopy imaging device for a Josephson probe microscope, comprising the following steps:

[0011] Step 1: Fix the sample on the sample stage and move the sample to the bottom of the probe under the microscope using the first displacement assembly. The initial distance between the probe and the sample surface is adjusted to the micrometer range.

[0012] Step 2: The first displacement stage of the first displacement assembly and the fourth displacement stage of the second displacement assembly drive the sample to move upward to complete the needle insertion operation, and control the distance between the probe and the sample to be within the range of the atomic force;

[0013] Step 3: The fifth translation stage of the second translation assembly controls the movement of the sample in the XY plane, so that the probe tip scans the sample surface. During the scanning process, the PID control function of the lock-in amplifier is used to adjust the position of the sample in the Z-axis direction in real time through the fourth translation stage to maintain a constant distance between the probe and the sample surface.

[0014] Step 4: During the scanning process, the voltage signal of the second displacement component is collected in real time by a high-speed data acquisition device, and converted into height information of the sample surface to generate a high-resolution imaging image of the sample surface.

[0015] Preferably, in step 2, a phase-locked amplifier is first used to apply voltage signals of different frequencies to the tuning fork chip to obtain a spectrum of the relationship between the vibration amplitude of the tuning fork chip and the frequency, and the frequency of the descending part of the resonance peak of the tuning fork chip is selected as the reference frequency for subsequent needle insertion operations.

[0016] Preferably, in step 2, during the needle insertion operation, the sample is driven upward by the fourth displacement stage of the second displacement assembly so that the sample surface is continuously moved toward the probe. The high-speed data acquisition device controls the fourth displacement stage by outputting a voltage to the second controller. The phase-locked amplifier collects the tuning fork chip amplitude signal and sends the amplitude signal to the high-speed data acquisition device. Whether the change in the amplitude signal of the tuning fork chip 43 reaches a predetermined change amount is used as a judgment condition. When the probe and the sample surface are close to the range of action of the atomic force, the change in the amplitude signal of the tuning fork chip reaches the predetermined change amount, and the judgment condition is met. The high-speed data acquisition device stops the rise of the fourth displacement stage through the second controller. If the change in the amplitude signal of the tuning fork chip does not reach the predetermined change amount after the fourth displacement stage reaches the maximum elongation, the judgment condition is not met. At this time, the elongation of the fourth displacement stage is reset to zero, and the first displacement stage of the first displacement assembly drives the sample upward for multiple steps, and then continues to drive the sample upward through the fourth displacement stage of the second displacement assembly so that the sample surface is continuously moved toward the probe until the change in the amplitude signal of the tuning fork chip reaches the predetermined change amount.

[0017] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0018] 1. This invention uses a high-precision superconducting Josephson probe with a nanometer-scale tip diameter. The integrated design of the probe and quartz tuning fork significantly improves the imaging resolution of complex surface topography. At the same time, a real-time height feedback mechanism is introduced. Through the PID control function of the lock-in amplifier, the distance between the probe and the sample surface is adjusted in real time, ensuring high stability during imaging and further improving resolution.

[0019] 2. The present invention adopts a non-contact probe contact mode. The probe has no direct contact with the sample surface. An adaptive PID control algorithm is introduced to dynamically adjust the probe height according to the interaction force between the probe and the sample surface, ensuring that the probe is always in the optimal contact state during the imaging process, effectively reducing the wear between the probe and the sample surface, which can effectively extend the service life of the probe and reduce the cost of use. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the structure of a displacement assembly according to an embodiment of the present invention;

[0021] Figure 2 is a schematic structural diagram of a superconducting Josephson probe assembly according to an embodiment;

[0022] Figure 3 1 is a schematic structural diagram of a probe module according to an embodiment;

[0023] Figure 4 1 is a schematic structural diagram of a tuning fork module according to an embodiment;

[0024] Figure 5 1 is a schematic diagram of the PCB structure of an embodiment;

[0025] Figure 6 Schematic diagram of the sample structure of the embodiment;

[0026] Figure 7 1. It is a schematic structural diagram of the working state of the displacement assembly and the superconducting Josephson probe assembly of the embodiment;

[0027] Figure 8 Schematic diagram of the circuit connection structure of the imaging process of the embodiment device;

[0028] Figure 9 2. It is a schematic diagram of the connection structure of the tuning fork wafer electrode circuit of the embodiment device;

[0029] Figure 10 is a spectrum showing the relationship between the vibration amplitude of the tuning fork wafer and the frequency of the device according to the embodiment;

[0030] Figure 11 This is a schematic diagram of the circuit connection of the high-speed data collector during the needle insertion process of the embodiment device;

[0031] Figure 12 1 is a schematic diagram of the circuit connection of the high-speed data collector during the imaging process of the embodiment device. DETAILED DESCRIPTION

[0032] The present invention will be further illustrated below with reference to specific examples. The examples are implemented based on the technical solutions of the present invention. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0033] like Figure 1 、 Figure 6 and Figure 7 As shown, an atomic force microscopy imaging device for a Josephson probe microscope includes a first displacement assembly 1, a second displacement assembly 2, a sample stage 3, a superconducting Josephson probe assembly 4, a first controller 51, a second controller 52, a lock-in amplifier 6, a current amplifier 7, and a high-speed data acquisition device 8. The first displacement assembly 1 is disposed on a base 93, the second displacement assembly 2 is connected to the first displacement assembly 1, and the sample stage 3 is connected to the second displacement assembly 2. The sample stage 3 is used to place a sample to be detected. The first displacement assembly 1 includes a first displacement stage 11 for achieving movement in the Z-axis direction, a second displacement stage 12 for achieving movement in the Y-axis direction, and a third displacement stage 13 for achieving movement in the X-axis direction. The second displacement assembly 2 includes a fourth displacement stage 21 for achieving movement in the Z-axis direction and a fifth displacement stage 22 for achieving movement in the X-axis and Y-axis directions. This example uses Attocube's translation stages. The first displacement component 1 is a coarse positioner. The first translation stage 11 is model ANPz51, the second translation stage is model 12ANPx51, and the third translation stage is model 13ANPy51. The ANP series linear positioners use closed-loop or open-loop control. The typical single-step displacement step size is: 50nm at room temperature 300K. The second displacement component 2 is a fine positioner (scanner). The fourth translation stage 21 is model ANSz50, and the fifth translation stage 22 is model ANSxy50. The ANS series scanning stages achieve large-scale fine scanning with a scanning accuracy better than 1nm. Figure 1 As shown, the stages are installed from bottom to top, aligned in the Z, Y, and X directions for the positioner and the X, Y, and Z directions for the scanner. The bottom of the first displacement assembly 1 is screwed to a disc, which is then connected to the base 93. The sample, an interdigital capacitor with a line width of 700 nm and a spacing of 1300 nm, is fixed to the center of a rectangular sample stage 3 with low-temperature adhesive. The sample stage 3 is screwed to the top of the second displacement assembly 2.

[0034] like Figure 1 、 Figure 2 、 Figure 3 、 Figure 4 、 Figure 5 and Figure 9As shown, a superconducting Josephson probe assembly 4 is positioned above the sample stage 3 and is used to probe the sample. The superconducting Josephson probe assembly 4 comprises a probe module, a tuning fork module, and a probe stage 45. The superconducting Josephson probe assembly 4 is connected to a base 93 via a first bracket 92. The probe module comprises a probe 41 and a probe box 42. In this embodiment, probe 41 is a superconducting Josephson probe having an inner diameter of 0.4 mm, an outer diameter of 1.2 mm, and a length of 25 mm. The diameter of the tip of probe 41 is less than 50 nm, and two parallel superconducting Josephson junctions are fabricated at the tip of probe 41. The probe box 42 comprises a box body 421 and a cover plate 422. The box body 421 has a groove therein. The probe 41 is placed within the groove of the probe box 42, with the tip of the probe 41 extending approximately 4 mm beyond the edge of the probe box 42. A small amount of low-temperature glue is dripped onto the end of probe 41, and probe 41 is gently pressed to allow the glue to penetrate toward the needle tip. The probe box 42 is then placed on a 90°C oven for 15 minutes to solidify the low-temperature glue and secure probe 41. Finally, the probe box cover 3 is installed, and the box body 421 and cover 422 are fixed with screws. The tuning fork module includes a tuning fork wafer 43 and a substrate 44. The tuning fork wafer 43 is attached to the substrate 44. In this embodiment, a quartz tuning fork measuring 3 mm x 8 mm and with a crystal oscillator frequency of 32.768 kHz is used. A file is used to separate the tuning fork shell and base ring, and a soldering iron is used to remove the solder, leaving the complete tuning fork wafer 43. The substrate 44 is a 5mm×5mm×0.5mm sapphire substrate with a small amount of epoxy glue applied at the midpoint of its edge. The tail of the tuning fork chip 43 is fixed on the substrate 44, ensuring that the crystal oscillation direction of the tuning fork chip 43 is parallel to the plane of the substrate 44. The tuning fork chip 43 and the substrate 44 are placed on a 120°C baking table and baked for 2 hours to make them firmly bonded. The probe box 42 equipped with the probe 41 and the substrate 44 equipped with the tuning fork chip 43 are respectively connected to the probe station 45, fixed with low-temperature glue and baked at 90°C for 15 minutes. The tuning fork chip 43 has two arms, and the probe 41 is tilted and extends from one side area of ​​the tuning fork chip 43 to the other side area of ​​the tuning fork chip 43. One side of the end where the needle tip of the probe 41 is located contacts the end of one arm of the tuning fork chip 43, and the needle tip protrudes from the tuning fork chip 43. The length of the protruding part of the needle tip of the probe 41 is 3-5μm. The installation method is as follows: Figure 2 As shown. Epoxy glue is applied at the contact point between the probe 41 and the tuning fork wafer 43, and baked at 120℃ for 2 hours to solidify, so that the probe 41 is connected to the tuning fork wafer 43. Electrodes are provided on both sides of the tail of the tuning fork wafer 43. A small amount of silver glue is applied to the electrode on one side of the tail of the tuning fork wafer 43, and a 50μm diameter gold wire is connected. The other side electrode is also led out to the substrate 44 through the silver glue and then connected to the gold wire. Bake at 90℃ for 20 minutes to solidify the silver glue. Figure 5The illustrated PCBs 46 are two in number. One end of each PCB has an electrode pad 461, and the other end has five through-holes 462 for soldering a connector. An MMCX-KE connector is soldered to through-holes 462, connecting it to the PCBs 46. The center pin of the MMCX-KE connector is connected to the electrode pad 461, where indium is soldered. Epoxy is applied to the probe station 45, securing the PCBs 46 and baking them at 120°C for two hours to connect the two PCBs 46 to the probe station 45. The two gold wires extending from the two electrodes on the tuning fork wafer 43 are respectively inserted into the indium on the electrode sheets 461 of the two PCB boards 46 to complete the electrode lead-out. The two connectors (MMCX-KE) are then connected to the voltage output port and voltage input port of the lock-in amplifier 6 via microwave lines, thus forming a loop. That is, the voltage output port of the lock-in amplifier 6 outputs a sinusoidal voltage signal to one of the electrodes of the tuning fork wafer 43, thereby causing the tuning fork wafer 43 to vibrate. The other electrode of the tuning fork wafer 43 is connected to the voltage input port of the lock-in amplifier 6 via the gold wire, PCB board 46, and microwave line. The lock-in amplifier 6 can collect signals such as the vibration amplitude and phase of the tuning fork wafer 43.

[0035] like Figure 8 、 Figure 11 and Figure 12As shown, a first controller 51 is electrically connected to the first displacement assembly 1 for controlling the operation of the first displacement assembly 1, and a second controller 52 is electrically connected to the second displacement assembly 2 for controlling the operation of the second displacement assembly 2. In this embodiment, the first controller 51 is a positioner controller ANC350, and the second controller 52 is a scanner controller ANC300. Both ANC300 and ANC350 are controller models from Attocube. The ANC300 has a Z-direction control module corresponding to the fourth displacement stage 21 and X- and Y-direction control modules corresponding to the fifth displacement stage 22. A lock-in amplifier 6 is electrically connected to the tuning fork wafer 43 of the superconducting Josephson probe assembly 4. A current amplifier 7 is electrically connected to the lock-in amplifier 6, and the current amplifier 7 is also electrically connected to the tuning fork wafer 43. A high-speed data acquisition device 8 is electrically connected to both the lock-in amplifier 6 and the second controller 52. In this embodiment, the lock-in amplifier 6 uses the Guoyi Quantum lock-in amplifier LIA001M, the current amplifier 7 uses the FEMTO current amplifier HQA-15M-10T, and the high-speed data acquisition device 8 uses a National Instruments high-speed data acquisition device DAQ. The two electrodes of the tuning fork crystal 43 are connected to two shielded wires: one is connected to the voltage output V+ of the lock-in amplifier 6, and the output mode is set to "single-ended" in the software (the host computer control software for the lock-in amplifier LIA001M, provided by Guoyi Quantum, is pre-installed on the host computer); the other is connected to the input of the current amplifier 7. The output of the current amplifier 7 is connected to the voltage input V+ of the lock-in amplifier 6 via a shielded wire, and the input mode is set to "single-ended" in the software. The Aux Output 1 (auxiliary output 1) of the lock-in amplifier 6 outputs the amplitude signal of the tuning fork crystal 43 and is connected to the voltage acquisition port AI0 of the high-speed data acquisition device 8, which collects the amplitude signal of the tuning fork crystal 43 in real time and transmits it to the computer of the high-speed data acquisition device 8. The voltage output port AO0 of the high-speed data collector 8 is connected to the DC input port of the Z-direction control module of the second controller 52 (ANC300). The two voltage output ports AO0 and AO1 of the high-speed data collector 8 are respectively connected to the DC input ports of the X-direction and Y-direction control modules of the second controller 52 (ANC300). The Aux Output port 2 (auxiliary output 2) of the lock-in amplifier 6 is connected to the DC input port of the Z-direction control module of ANC300. Figure 11 and Figure 12As shown, the high-speed data acquisition device 8 (DAQ) is a device capable of high-speed signal input and output. It can receive analog signals and convert them into digital signals, and can also output digital signals through an output port. The function of the high-speed data acquisition device 8 can be discussed in two stages: the needle insertion process and the imaging process. The needle insertion process is a precursor to the imaging process. It controls the displacement component by programming to move the sample toward the probe 41. This is a test of the sensitivity of the tuning fork-probe sensor. If the needle insertion process ensures that the distance between the probe 41 tip and the sample is at an appropriate position, the subsequent PID function can be used normally. 1. During the needle insertion process, the amplitude signal of the tuning fork chip 43 collected by the phase-locked amplifier 6 must be transmitted to the host computer in real time. It is compared with the initial amplitude signal of the tuning fork chip 43 to determine whether the difference between the two exceeds the set threshold. The phase-locked amplifier 6 is connected to the host computer by a network cable, and the speed of data transmission of the network cable is slow, which will affect the speed of needle insertion. The high-speed data collector 8 is connected to the host computer by a USB cable, and its internal data processing speed and external data transmission speed are both fast. The data inside the phase-locked amplifier 6 is transmitted to the high-speed data collector 8 through the output port via the microwave line, and output to the host computer after rapid processing, which can greatly improve the speed of needle insertion. At the same time, during the needle insertion process, the high-speed data collector 8 also plays a role in controlling the fourth displacement stage 2 through the Z direction control module of the second controller 52. 1. To achieve Z-axis movement of the sample, the Z-direction control module of the second controller 52 has a DC input control function. By outputting a voltage ranging from 0V to 4V via the output port of the high-speed data acquisition device 8 to the DC input port of the Z-direction control module of the second controller 52 (the second controller 52 amplifies the DC input voltage by 15 times and then outputs it to the second displacement assembly 2), the fourth translation stage 21 can be extended from 0 to 4.3μm in the Z direction. At this time, the DC input port of the Z-direction control module of the second controller 52 is connected to the output port of the high-speed data acquisition device 8 via a microwave line. 2. During imaging, the DC input port of the Z-direction control module of the second controller 52 is connected to the voltage output port of the lock-in amplifier 6. This voltage output port outputs the PID calculation result, directly controlling the extension and retraction of the fourth translation stage 21. Simultaneously, the tuning fork crystal 43 amplitude signal and the PID output result acquired by the lock-in amplifier 6 are collected and stored in the host computer via the input port of the high-speed data acquisition device 8. The phase-locked amplifier 6 is connected to the tuning fork chip 43 to monitor the amplitude changes of the tuning fork chip 43 in real time. During the imaging process, the phase-locked amplifier 6 controls the fourth displacement stage 21 of the second controller 52 to adjust the upper and lower positions of the sample in real time to achieve closed-loop adjustment of the distance between the probe 41 and the sample, so that the distance between the probe 41 and the sample surface is constant.

[0036] This embodiment also provides an atomic force microscopy imaging method for a Josephson probe microscope, using the above-mentioned atomic force microscopy imaging device for a Josephson probe microscope, comprising the following steps:

[0037] Step 1: Fix the sample on the sample stage 3 and move the sample to the bottom of the probe 41 under a microscope using the first displacement assembly 1. The initial distance between the probe 41 and the sample surface is adjusted to the micrometer range.

[0038] The sample is secured to the sample stage 3 with low-temperature adhesive. The device is placed under an optical microscope, and the focus is adjusted to ensure that the tip of probe 41 and the sample surface are clearly visible within the field of view. Using the Daisy control software of the first controller 51 (ANC350), the first displacement assembly 1 is manually adjusted in the X and Y directions (with the second and third stages 12 and 13 in operation) to position the area to be measured on the sample surface directly below the probe tip. Subsequently, the first displacement assembly 1 is controlled to slowly raise the sample in the Z direction (with the first stage 11 in operation) while observing the reflection of probe 41 on the sample surface. When the actual tip of probe 41 is almost in contact with the reflected tip, Z-direction movement is stopped. At this point, the actual distance between probe 41 and the sample is D, where 10 microns < D < 100 microns.

[0039] Step 2: First, use the phase-locked amplifier 6 to apply voltage signals of different frequencies to the tuning fork chip 43 to obtain the spectrum of the relationship between the vibration amplitude of the tuning fork chip 43 and the frequency, and select the frequency point at the highest point of the half-width of the resonance peak of the tuning fork chip 43 as the reference frequency for the subsequent needle insertion operation. The sample is driven to move upward by the first displacement stage 11 of the first displacement component 1 and the fourth displacement stage 21 of the second displacement component 2 to complete the needle insertion operation, and the distance between the probe 41 and the sample is controlled within the range of the atomic force. During the needle insertion operation, the sample is driven upward by the extension of the fourth displacement stage 21 of the second displacement component 2 so that the sample surface continuously moves toward the probe 41. The high-speed data acquisition device 8 controls the fourth displacement stage 21 by outputting a voltage to the second controller 52, and the phase-locked amplifier 6 collects the amplitude signal of the tuning fork chip 43 and sends the amplitude signal to the second controller 52. The high-speed data acquisition device 8 determines whether the change in the amplitude signal of the tuning fork wafer 43 reaches a predetermined amount. When the probe 41 and the sample surface approach the atomic force range, the amplitude signal of the tuning fork wafer 43 changes and reaches the predetermined amount, thus meeting the determination condition. The lock-in amplifier 6 stops the ascent of the fourth displacement stage 21 via the second controller 52. If the change in the amplitude signal of the tuning fork wafer 43 does not reach the predetermined amount after the fourth displacement stage 21 reaches its maximum extension, then the determination condition is not met. At this time, the extension of the fourth displacement stage 21 returns to zero, and the first displacement stage 11 of the first displacement assembly 1 drives the sample upward for multiple steps. The fourth displacement stage 21 of the second displacement assembly 2 then continues to drive the sample upward, causing the sample surface to continuously move toward the probe 41, until the change in the amplitude signal of the tuning fork wafer 43 reaches the predetermined amount.

[0040] In this embodiment, the output voltage of the excitation tuning fork crystal 43 is set to 0.04V, and the tuning fork signal in the range of 33.0-33.6kHz is swept using the sweep function of the lock-in amplifier 6 to obtain the resonance spectrum of the tuning fork crystal 43 equipped with the probe 41. The resonance frequency is 33.294kHz, and the resonance peak amplitude is 1.1V. The frequency of 33.298kHz, the peak of the resonance peak (the frequency at the peak of the half-width of the resonance peak), is selected as the needle insertion locking frequency ( Figure 10 As shown in Figure 4 ), the corresponding amplitude signal is 600 mV. As the sample gradually approaches the tip of probe 41, within the range of the atomic force, the atomic force interaction between the tip and the sample surface affects the tuning fork vibration, causing the amplitude signal at the same frequency to change. The determination condition is that the change in the amplitude signal reaches 100 mV.

[0041] Set the Z-axis control module of the second controller 52 (ANC300) to "DC input" mode. Connect the voltage output port AO0 of the high-speed data acquisition unit 8 to the DC input port of the Z-axis control module of the second controller 52 (ANC300). For the ANSz50 positioner, a voltage of 60V corresponds to a displacement of 4.3μm, meaning the maximum extension of the fourth translation stage 21 is 4.3μm. The second controller 52 amplifies the DC input voltage by 15 times before outputting it. For the Z-axis of the first positioner 1, a voltage of 100V corresponds to a displacement of 3.5μm. A Python program was written to set the Z-axis of the first displacement assembly 1 (positioner) to stepping mode with a 35V step size. The calculated theoretical maximum extension was approximately 1.2μm. This means that a 35V pulse signal applied to the Z-axis of the first displacement assembly 1 (first stage 11) should theoretically move it upward by 1.2μm. However, due to gravity, friction, and other factors, the actual measurement data shows that the 35V pulse signal caused the Z-axis of the first displacement assembly 1 to move by 400nm. The program controlled the high-speed data acquisition device 8 to cyclically output a 0-4V voltage, causing the Z-axis of the second displacement assembly 2 (scanner) (fourth stage 21) to extend from zero to 4.3μm. Simultaneously, the tuning fork amplitude signal was monitored in real time to see if the difference between the initial value and the initial value met the 100mV threshold. If the judgment condition is met, the program is stopped immediately and the output of the high-speed data collector 8 is reset to zero; if not, the loop is stopped, the output of the high-speed data collector 8 is reset to zero, and the Z axis of the first displacement component 1 (positioner) is raised by 10 steps at a step size of 35V, and the next loop is entered until the change in the amplitude signal of the tuning fork chip 43 meets the judgment condition. At this time, the distance between the tip of the probe 41 and the sample surface is within the range of action of the atomic force.Principle description: First, the first displacement component 1 is a coarse displacement controller. When the displacement stage in the first displacement component is controlled to move forward in a certain direction, the first controller 51 will apply a triangular waveform pulse signal to the displacement stage. Due to the piezoelectric effect of the piezoelectric material in the displacement stage, the pulse signal will cause the displacement stage to stretch a certain amount. When the pulse signal is withdrawn, the friction inside the displacement device causes the piezoelectric material to stay in the stretched position instead of retracting to its original position. The stretching amount in this process is large. For the Z axis of the first displacement component 1, applying a 35V pulse voltage to it will cause it to rise by 400nm, while the range of action of the atomic force is only about 100nm, so it is not possible to use only the first controller. The controller 51 controls the first displacement component 1 to advance the needle. Otherwise, the needle tip cannot be controlled to stop at a very close distance to the sample surface, and the needle tip is very likely to be damaged. On the other hand, the second displacement control component 2 is a fine-tuning displacement controller. When a constant DC voltage is applied to the displacement stage, the displacement stage will extend to a specific value and stabilize. Its control accuracy can reach below 1nm. By slowly approaching the Z axis of the second displacement control component 2, it can ensure that the needle tip detects signal changes within the range of the atomic force on the sample surface and protect the probe 41 from being damaged. However, its maximum extension is only 4.3μm, so it must be combined with the first displacement component 1 to advance the needle. The needle insertion process The complete description is as follows: First, the sample is moved to a position close to the probe by visual observation under a microscope. At this time, the distance between the probe and the sample is on the order of tens of microns. Then, the Z axis of the second displacement control component 2 (the fourth displacement stage 21) is controlled by the program to slowly extend upward in steps of 1 nm. During this process, each step is used to determine whether the amplitude signal of the tuning fork meets the judgment condition. If the judgment condition is met, it means that atomic force has been generated between the atoms on the sample surface and the atoms at the tip of the probe, that is, the sample and the probe are close enough, and the needle is stopped immediately. If the judgment condition is not met, the Z axis continues to rise until the Z axis reaches the maximum extension of 4.3 μm. At this time, the second position The displacement control assembly 2 can no longer rise, but the distance between the sample and the probe 41 is still very far. The Z-axis extension of the second displacement control assembly 2 is reset to zero. The distance between the probe 41 and the sample should now be at least 4.3 μm. Therefore, the Z-axis of the first displacement assembly 1 (the first stage 11) is moved upward by 10 steps at a 35V step length. As can be seen from the previous description, the first displacement assembly 1 has now moved the sample up by approximately 4 μm, a safe distance for the needle tip. At this height, the Z-axis of the second displacement control assembly 2 (the fourth stage 21) slowly rises again to test whether the needle tip is close enough to the sample within a range of 4.3 μm upward. This cycle repeats.

[0042] Step 3: The fifth displacement stage 22 of the second displacement assembly 2 controls the movement of the sample in the XY plane, so that the tip of the probe 41 scans the sample surface. During the scanning process, the PID control function of the lock-in amplifier is used to adjust the position of the sample in the Z-axis direction in real time through the fourth displacement stage 21 to maintain a constant distance between the probe 41 and the sample surface.

[0043] Since the two voltage output ports AO0 and AO1 of the high-speed data acquisition device 8 are connected to the DC input ports of the X- and Y-direction control modules of the second controller 52 (ANC300), respectively, the Aux Output port 2 of the lock-in amplifier 6 is connected to the DC input port of the Z-direction control module of the second controller 52 (ANC300). A Python program is written to set the PID parameters (the Python program is written on the host computer. The lock-in amplifier 6, ANC300, and ANC350 controllers are all connected to the host computer. Python can connect to these instruments and call functions to implement controller settings, output, and acquisition functions. PID is a built-in function of the lock-in amplifier 6, and its parameters are set through functions in the Python program): The preset amplitude value of the tuning fork crystal 43 is 740mV, the sampling rate is 1000kbps, the PID coefficients kp = 0.0046, ki = 0.05, and the differential function is disabled. The PID control signal is output through Aux Output port 2 of the lock-in amplifier 6, and the PID control switch is turned on to ensure a constant distance between the needle tip and the sample surface during scanning. Scan within a 30μm x 30μm range with a 30nm step size in the X direction and a 1μm step size in the Y direction. For the ANSxy50 displacement sensor, a voltage of 60V corresponds to a 30μm displacement. Create a nested loop program (written in Python on the host computer) to control the Y-direction movement from 0 to 30μm in 1μm steps. After each step, the X-direction performs a forward line scan from 0 to 30μm in 30nm steps, followed by a reverse line scan to 0. After the X-direction is reset to zero, the Y-direction moves for the next step. After the program terminates, turn off the PID control switch and reset the Aux Output port 2 of the lock-in amplifier 6 and the output of the high-speed data acquisition device 8 to zero.

[0044] Step 4: During the scanning process, the voltage signal of the second displacement component 2 is collected in real time by the high-speed data collector 8 and converted into height information of the sample surface to generate a high-resolution imaging image of the sample surface.

[0045] The Aux Output port 2 of the lock-in amplifier 6 is split into two paths: one connected to the DC input port of the Z-direction control module of the second controller 52, and the other connected to the voltage acquisition port AI1 of the high-speed data acquisition device 8. Before scanning begins, the PID switch of the lock-in amplifier 6 is turned on. The PID function calculates the difference between the current signal V of the tuning fork crystal 43 and the preset value V0 in real time, e(t) = V0 – V, and calculates the result according to the PID formula, as follows:

[0046] u(t) = kp * e(t) + (ki *∫e(t)dt)

[0047] Where u(t) is the output of the PID controller, kp is the proportional coefficient, ki is the integral coefficient, and ∫e(t)dt is the time integral of the difference e(t) between the current signal V and the preset value V0;

[0048] The PID outputs a DC voltage to the second displacement controller 52 through the Aux Output port 2 of the lock-in amplifier 6, which brings the probe 41 closer to the sample until the amplitude signal of the tuning fork is equal to the preset value. At this time, the PID output remains constant. During the scanning process, as the sample surface rises and falls, the external voltage input received by the second displacement controller 52 will change, thereby controlling the Z-direction height change of the second displacement assembly 2 to maintain a constant amplitude signal of the tuning fork crystal 43. When the sample surface convexes during the scanning process, the distance between the probe and the sample becomes closer, the atomic force between the probe tip and the sample surface becomes stronger, the amplitude signal V of the tuning fork crystal 43 increases, and the difference e(t) = V0–V between the current signal V of the tuning fork crystal 43 and the preset value V0 decreases, thereby reducing the voltage output from the Aux Output port 2 to the second displacement controller 52. Conversely, when the sample surface is concave, the distance between the probe 41 and the sample becomes farther, the atomic force between the probe tip 41 and the sample surface weakens, the amplitude signal V of the tuning fork crystal 43 decreases, and the difference e(t) = V0–V between the current signal V of the tuning fork crystal 43 and the preset value V0 increases, thereby increasing the voltage output from the Aux Output port 2 to the second displacement controller 52. The voltage acquisition port AI1 of the high-speed data acquisition device 8 records the external voltage changes obtained by the second displacement controller 52. Through the correspondence between the 4.3μm in the Z direction of the second displacement component and the DC input voltage of 4V, the height difference of the sample surface can be calculated, thereby obtaining high-resolution imaging of the sample.

[0049] After scanning, a bidirectional scan image measuring 30 μm × 30 μm with a resolution of 1000 × 30 was obtained (the 30 μm length was divided into 1000 and 30 steps, with each step being 30 nm in the X direction and 1 μm in the Y direction). The forward and reverse scan data were separated, and the complete forward scan image was extracted. The data was then skew-corrected and filtered using Origin software to obtain the final AFM image of the sample.

[0050] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. An atomic force microscopy imaging method for a Josephson probe microscope, using an atomic force microscopy imaging device for a Josephson probe microscope, characterized in that: The device comprises a sample stage (3) for fixing a sample, a second displacement assembly (2) connected to the sample stage (3), a first displacement assembly (1) connected to the second displacement assembly (2), a superconducting Josephson probe assembly (4) for detecting the sample, a first controller (51) electrically connected to the first displacement assembly (1), a second controller (52) electrically connected to the second displacement assembly (2), a phase-locked amplifier (6) electrically connected to the superconducting Josephson probe assembly (4), a current amplifier (7) electrically connected to the phase-locked amplifier (6), and a current amplifier (8) electrically connected to the phase-locked amplifier (6) and the second controller. The high-speed data acquisition device (8) is electrically connected to the superconducting Josephson probe assembly (4), the superconducting Josephson probe assembly (4) includes a probe module and a tuning fork module, the probe module includes a probe (41) and a probe box (42), the tuning fork module includes a tuning fork chip (43) and a substrate (44), the probe (41) is connected to the tuning fork chip (43), the phase-locked amplifier (6) is connected to the tuning fork chip (43) to monitor the amplitude change of the tuning fork chip (43) in real time, and the phase-locked amplifier (6) drives the sample to move up and down by controlling the second controller (52) to achieve closed-loop adjustment of the distance between the probe (41) and the sample; The first displacement assembly (1) includes a first displacement stage (11) for achieving movement in the Z-axis direction, a second displacement stage (12) for achieving movement in the Y-axis direction, and a third displacement stage (13) for achieving movement in the X-axis direction; the second displacement assembly (2) includes a fourth displacement stage (21) for achieving movement in the Z-axis direction and a fifth displacement stage (22) for achieving movement in the X-axis and Y-axis directions; the lock-in amplifier (6) adjusts the upper and lower positions of the sample in real time through the fourth displacement stage (21) so that the distance between the probe (41) and the sample surface is constant; The diameter of the tip of the probe (41) is less than 50 nm, and the tip of the probe (41) has two parallel superconducting Josephson junctions; The substrate (44) is a square plate, the crystal oscillation direction of the tuning fork wafer (43) is parallel to the plane of the substrate (44), and one side of the tip of the probe (41) contacts the end of one arm of the tuning fork wafer (43); The method comprises the following steps: Step 1: Fix the sample on the sample stage (3), and move the sample to below the probe (41) through the first displacement component (1) under a microscope, and adjust the initial distance between the probe (41) and the sample surface to a micrometer range; Step 2: The sample is driven to move upward by the first displacement stage (11) of the first displacement assembly (1) and the fourth displacement stage (21) of the second displacement assembly (2) to complete the needle insertion operation, and the distance between the probe (41) and the sample is controlled to be within the range of action of the atomic force; Step 3: The sample is controlled to move in the XY plane by the fifth displacement stage (22) of the second displacement assembly (2), so that the tip of the probe (41) scans the sample surface. During the scanning process, the position of the sample in the Z-axis direction is adjusted in real time by the fourth displacement stage (21) using the PID control function of the lock-in amplifier (6), so as to keep the distance between the probe (41) and the sample surface constant. Step 4: During the scanning process, the voltage signal of the second displacement component (2) is collected in real time by a high-speed data acquisition device (8), and converted into height information of the sample surface to generate a high-resolution imaging image of the sample surface; In step 2, a lock-in amplifier (6) is first used to apply voltage signals of different frequencies to the tuning fork chip (43), thereby obtaining a spectrum showing the relationship between the vibration amplitude of the tuning fork chip (43) and the frequency, and selecting the frequency of the descending portion of the resonance peak of the tuning fork chip (43) as a reference frequency for subsequent needle insertion operations.

2. The atomic force microscopy imaging method for a Josephson probe microscope according to claim 1, characterized in that: In the step 2, during the needle insertion operation, the fourth displacement stage (21) of the second displacement assembly (2) drives the sample upward so that the sample surface continuously moves toward the probe (41). The high-speed data acquisition device (8) controls the fourth displacement stage (21) by outputting a voltage to the second controller (52). The phase-locked amplifier (6) collects the amplitude signal of the tuning fork chip (43) and sends the amplitude signal to the high-speed data acquisition device (8). Whether the amplitude signal change of the tuning fork chip 43 reaches a predetermined change amount is used as a judgment condition. When the probe (41) and the sample surface are close to the range of the atomic force, the amplitude signal change of the tuning fork chip (43) reaches the predetermined change amount, then When the judgment condition is met, the high-speed data acquisition device (8) stops the rise of the fourth displacement stage (21) through the second controller (52); if the change in the amplitude signal of the tuning fork chip (43) does not reach the predetermined change amount after the fourth displacement stage (21) reaches the maximum extension, the judgment condition is not met. At this time, the extension of the fourth displacement stage (21) is reset to zero, and the first displacement stage (11) of the first displacement assembly (1) drives the sample to move upward for multiple steps, and then continues to drive the sample upward through the fourth displacement stage (21) of the second displacement assembly (2) so that the sample surface continuously moves toward the probe (41) until the change in the amplitude signal of the tuning fork chip (43) reaches the predetermined change amount.

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