Atomic force microscopic imaging device and method for Josephson probe microscope

Abstract

The invention discloses an atomic force microscopic imaging device and method for a Josephson probe microscope, and belongs to the technical field of atomic force microscopic imaging. The device comprises a sample table, a second displacement assembly, a first displacement assembly, a superconductive Josephson probe assembly, a first controller, a second controller, a lock-in amplifier, a current amplifier and a high-speed data collector, the superconductive Josephson probe assembly comprises a probe module and a tuning fork module, and the probe module comprises a probe and a probe box. The tuning fork module comprises a tuning fork wafer and a substrate, the probe is connected with the tuning fork wafer, the lock-in amplifier is connected with the tuning fork wafer to monitor the amplitude change of the tuning fork wafer in real time, and the lock-in amplifier drives the sample to move up and down by controlling the second controller to realize closed-loop adjustment of the distance between the probe and the sample. The distance between the probe and the surface of the sample is adjusted in real time, the high stability in the imaging process is ensured, and the resolution is further improved; the service life of the probe can be effectively prolonged, and the use cost is reduced.

Description

Technical Field

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

[0002] In recent years, atomic force microscope (AFM) technology has made remarkable progress in the field of nanoscience and engineering, and is widely used in fields such as materials science, life science, and semiconductor industry. By detecting the atomic force interaction between the probe and the sample surface, AFM can achieve surface topography imaging with nanoscale or even atomic-scale resolution.

[0003] The Josephson probe microscope system is a near-field electromagnetic imaging system built with a Josephson nanoscale probe as the core device. The Josephson probe is a device with high response sensitivity in the microwave to terahertz frequency band, and its tip contains two parallel Josephson junctions. By using the characteristics of the superconducting Josephson junction being sensitive to electromagnetic response and capable of simultaneously measuring the intensity, frequency, and even phase of high-frequency signals, performing a two-dimensional point-by-point scan on the surface of the device to be measured and recording the spatial distribution of the frequency-domain radiation spectrum, real-time detection with spatial resolution, frequency resolution, and radiation intensity resolution of the device to be measured can be achieved. However, currently, due to the lack of a method to stably control the probe-sample distance at the nanoscale, the probe is prevented from approaching the sample further, limiting the resolution. Therefore, the Josephson probe microscope cannot achieve topography imaging and it is difficult to determine the scanning area of the probe. Summary of the Invention

[0004] Aiming at the above 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 a superconducting Josephson probe microscope, which combines AFM technology with a Josephson probe microscope to build a near-field probe microscope imaging system with higher spatial resolution.

[0005] Technical Solution: 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, comprising 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 collector electrically connected to both the lock-in amplifier and the second controller. The superconducting Josephson probe assembly includes a probe module and a tuning fork module. The probe module includes a probe and a probe cartridge. 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 drives the sample to move up and down by controlling the second controller to achieve closed-loop adjustment of the distance between the probe and the sample.

[0007] Preferably, the first displacement assembly includes a first displacement stage for moving in the Z-axis direction, a second displacement stage for moving in the Y-axis direction, and a third displacement stage for moving in the X-axis direction. The second displacement assembly includes a fourth displacement stage for moving in the Z-axis direction and a fifth displacement stage for moving in the X-axis and Y-axis directions. The lock-in amplifier adjusts the vertical position 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 top diameter 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 wafer is parallel to the substrate plane, and one side of the probe tip contacts the end of one arm of the tuning fork wafer.

[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 under the probe through the first displacement assembly under the microscope, and adjust the initial distance between the probe and the sample surface to the micron range.

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

[0013] Step 3: Control the sample to move in the XY plane through the fifth displacement stage of the second displacement component, so that the probe tip scans the surface of the sample. During the scanning process, utilize the PID control function of the lock-in amplifier to adjust the position of the sample in the Z-axis direction in real time through the fourth displacement stage, and keep the distance between the probe and the sample surface constant;

[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 collector, and is converted into the height information of the sample surface to generate a high-resolution imaging map of the sample surface.

[0015] Preferably, in step 2, first apply voltage signals with different frequencies to the tuning fork wafer by using a lock-in amplifier, obtain the relationship spectrum of the vibration amplitude of the tuning fork wafer with respect to frequency, and select the frequency of the descending part of the resonance peak of the tuning fork wafer as the reference frequency for subsequent needle insertion operations.

[0016] Preferably, in step 2, during the needle insertion operation, drive the sample upward through the fourth displacement stage of the second displacement component to continuously move the sample surface towards the probe direction. The high-speed data collector controls the fourth displacement stage through the voltage output to the second controller. The lock-in amplifier collects the amplitude signal of the tuning fork wafer and sends the amplitude signal to the high-speed data collector. Whether the change in the amplitude signal of the tuning fork wafer 43 reaches a predetermined change amount is used as the determination condition. When the probe approaches the sample surface to the atomic force action range, the change in the amplitude signal of the tuning fork wafer reaches the predetermined change amount, then the determination condition is reached, and the high-speed data collector stops the upward movement of the fourth displacement stage through the second controller; if the change in the amplitude signal of the tuning fork wafer does not reach the predetermined change amount after the fourth displacement stage reaches the maximum elongation amount, then the determination condition is not reached. At this time, the elongation amount of the fourth displacement stage returns to zero, and the first displacement stage of the first displacement control component drives the sample to move upward in multiple steps, and then continues to drive the sample upward through the fourth displacement stage of the second displacement component to continuously move the sample surface towards the probe direction until the change in the amplitude signal of the tuning fork wafer reaches the predetermined change amount.

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

[0018] 1. The present invention adopts a high-precision superconducting Josephson probe, and the diameter of the probe tip is controlled at the nanometer level. The integrated design of the probe and the quartz tuning fork significantly improves the imaging resolution of complex surface topographies. 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 to ensure the height stability during the imaging process and further improve the resolution;

[0019] 2. The present invention adopts a non-contact probe contact mode, where 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 best contact state during imaging. This effectively reduces the wear between the probe and the sample surface, can effectively extend the service life of the probe, and reduce the usage cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 is a schematic structural diagram of the displacement component in an embodiment of the present invention;

[0021] Figure 2 is a schematic structural diagram of the superconducting Josephson probe component in the embodiment;

[0022] Figure 3 is a schematic structural diagram of the probe module in the embodiment;

[0023] Figure 4 is a schematic structural diagram of the tuning fork module in the embodiment;

[0024] Figure 5 is a schematic structural diagram of the PCB board in the embodiment;

[0025] Figure 6 is a schematic structural diagram of the sample in the embodiment;

[0026] Figure 7 is a schematic structural diagram of the working state of the displacement component and the superconducting Josephson probe component in the embodiment;

[0027] Figure 8 is a schematic circuit connection structure diagram of the imaging process of the device in the embodiment;

[0028] Figure 9 is a schematic circuit connection structure diagram of the tuning fork wafer electrodes of the device in the embodiment;

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

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

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

[0032] The following further clarifies the present invention in conjunction with specific embodiments. The embodiments are implemented on the premise of the technical solution of the present invention. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0033] AsFigure 1 , Figure 6 and Figure 7 As shown in Figure 1 , 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 collector 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. 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 moving in the Z-axis direction, a second displacement stage 12 for moving in the Y-axis direction, and a third displacement stage 13 for moving in the X-axis direction. The second displacement assembly 2 includes a fourth displacement stage 21 for moving in the Z-axis direction and a fifth displacement stage 22 for moving in the X- and Y-axis directions. In this example, displacement stages from Attocube Company are used. The first displacement assembly 1 is a coarse displacement adjuster (positioner). The model of the first displacement stage 11 is ANPz51, the model of the second displacement stage is 12ANPx51, and the model of the third displacement stage is 13ANPy51. The ANP series linear displacement adjusters use closed-loop or open-loop control. The typical single-step displacement step size is: 50 nm at room temperature of 300 K. The second displacement assembly 2 is a fine displacement scanner (scanner). The model of the fourth displacement stage 21 is ANSz50, and the model of the fifth displacement stage 22 is ANSxy50. The ANS series scanning stages achieve a large-range fine scanning function, and the scanning accuracy is better than 1 nm. As shown in Figure 1 , each displacement stage is installed from bottom to top in the Z, Y, X directions of the positioner and the XY, Z directions of the scanner. The bottom of the first displacement assembly 1 is fixed to a wafer by screws, and the wafer is then connected to the base 93. The sample is an interdigital capacitor sample with a line width of 700 nm and a spacing of 1300 nm, and is fixed in the center of the rectangular sample stage 3 with cryogenic glue. The sample stage 3 is installed on the top of the second displacement assembly 2 by screws. Figure 1 As shown in Figure 1 , each displacement stage is installed from bottom to top in the Z, Y, X directions of the positioner and the XY, Z directions of the scanner. The bottom of the first displacement assembly 1 is fixed to a wafer by screws, and the wafer is then connected to the base 93. The sample is an interdigital capacitor sample with a line width of 700 nm and a spacing of 1300 nm, and is fixed in the center of the rectangular sample stage 3 with cryogenic glue. The sample stage 3 is installed on the top of the second displacement assembly 2 by screws.

[0034] As Figure 1 , Figure 2 , Figure 3 , Figure 4 , Figure 5 and Figure 9As shown, the superconducting Josephson probe assembly 4 is located above the sample stage 3 for detecting the sample. The superconducting Josephson probe assembly 4 includes a probe module, a tuning fork module, and a probe stage 45. The superconducting Josephson probe assembly 4 is connected to the base 93 through a first bracket 92. The probe module includes a probe 41 and a probe box 42. In this embodiment, the probe 41 is a superconducting Josephson probe with 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 top of the probe 41 is less than 50 nm, and two parallel superconducting Josephson junctions are fabricated at the tip top of the probe 41. The probe box 42 includes a box body 421 and a cover plate 422. The box body 421 has a groove. The probe 41 is placed in the groove of the probe box 42, and the tip of the probe 41 extends about 4 mm beyond the edge of the probe box 42. A small amount of low-temperature glue is dropped at the tail of the probe 41, and the probe 41 is gently pressed to make the glue penetrate towards the tip direction. Subsequently, the probe box 42 is baked on a 90°C baking table for 15 minutes to solidify the low-temperature glue and fix the probe 41. Finally, the probe box cover plate 3 is installed, and the box body 421 and the cover plate 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 connected to the substrate 44. In this embodiment, a quartz tuning fork with a size of 3 mm × 8 mm and a crystal oscillation frequency of 32.768 kHz is selected. The tuning fork housing and the base ring are separated with a file, and then the solder is removed with a soldering iron to obtain the complete tuning fork wafer 43. The substrate 44 is a 5 mm × 5 mm × 0.5 mm sapphire substrate. A small amount of epoxy glue is applied at the midpoint of its edge, and the tail of the tuning fork wafer 43 is fixed to the substrate 44. Ensure that the crystal oscillation direction of the tuning fork wafer 43 is parallel to the plane of the substrate 44. The tuning fork wafer 43 and the substrate 44 are placed on a 120°C baking table and baked for 2 hours to firmly bond them. The probe box 42 containing the probe 41 and the substrate 44 containing the tuning fork wafer 43 are respectively connected to the probe stage 45, fixed with low-temperature glue, and baked at 90°C for 15 minutes. The tuning fork wafer 43 has two arms. The probe 41 is inclined and extends from one side area of the tuning fork wafer 43 to the other side area of the tuning fork wafer 43. One side of the end where the tip of the probe 41 is located contacts the end of one arm of the tuning fork wafer 43, and the tip protrudes from the tuning fork wafer 43. The length of the protruding part of the tip of the probe 41 is 3 - 5 μm, and the installation method is as Figure 2 shown. Epoxy glue is dot-applied at the contact between the probe 41 and the tuning fork wafer 43 and baked at 120°C for 2 hours to cure, so as to connect the probe 41 and the tuning fork wafer 43. Electrodes are respectively provided on both side surfaces of the tail of the tuning fork wafer 43. A small amount of silver glue is applied on the electrode on one side surface of the tail of the tuning fork wafer 43, and a gold wire with a diameter of 50 μm is connected. The electrode on the other side surface is also led out to the substrate 44 through silver glue and then connected to a gold wire. The silver glue is baked at 90°C for 20 minutes to solidify, and Figure 5The PCB board 46 shown has a quantity of 2. One end of the PCB board 46 has an electrode piece 461 reserved, and the other end has five through holes 462 for welding plugs. A connector of model MMCX-KE is welded to the through holes 462 in the figure to connect with the PCB board 46. The central pin of the MMCX-KE connector communicates with the electrode piece 461, and indium is welded at the electrode piece 461. Epoxy glue is applied on the probe station 45 to fix the PCB board 46 and bake it at 120 °C for 2 hours to connect the two PCB boards 46 with the probe station 45. Two gold wires led out from the two electrodes on the tuning fork wafer 43 are respectively inserted into the indium on the electrode pieces 461 of the two PCB boards 46 to complete the electrode lead-out. Then, the two connectors (MMCX-KE) are respectively connected out through microwave lines to the voltage output port and the voltage input port of the lock-in amplifier 6, thus forming a loop, that is, the voltage output port of the lock-in amplifier 6 outputs a sine voltage signal to one of the electrodes of the tuning fork wafer 43, so that the tuning fork wafer 43 generates vibration. The other electrode of the tuning fork wafer 43 is connected to the voltage input port of the lock-in amplifier 6 through the gold wire, the PCB board 46, and the microwave line. The lock-in amplifier 6 can collect signals such as the vibration amplitude and phase of the tuning fork wafer 43.

[0035] As Figure 8 , Figure 11 and Figure 12As shown, the first controller 51 is electrically connected to the first displacement component 1 to control the operation of the first displacement component 1, and the second controller 52 is electrically connected to the second displacement component 2 to control the operation of the second displacement component 2. In this embodiment, the first controller 51 uses a positioner controller ANC350, and the second controller 52 uses a scanner controller ANC300. Both ANC300 and ANC350 are controller models of Attocube Company. 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. The lock-in amplifier 6 is electrically connected to the tuning fork wafer 43 of the superconducting Josephson probe assembly 4. The current amplifier 7 is electrically connected to the lock-in amplifier 6 and is also electrically connected to the tuning fork wafer 43. The high-speed data collector 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 a lock-in amplifier LIA001M of Guoyi Quantum, the current amplifier 7 uses a current amplifier HQA-15M-10T of FEMTO, and the high-speed data collector 8 uses a high-speed data acquisition device DAQ of National Instruments. The two electrodes of the tuning fork wafer 43 are respectively 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 upper computer control software provided by Guoyi Quantum Company and adapted to the lock-in amplifier LIA001M, software name LIA001M, and the software is pre-installed in the computer acting as the upper computer). The other is connected to the input end of the current amplifier 7. The output end of the current amplifier 7 is connected to the voltage input V+ of the lock-in amplifier 6 through a shielded wire, and the input mode is set to "single-ended" in the software. The Aux Output port 1 (auxiliary output 1) of the lock-in amplifier 6 outputs the amplitude signal of the tuning fork wafer 43 and is connected to the voltage acquisition port AI0 of the high-speed data collector 8 to collect the amplitude signal of the tuning fork wafer 43 in real time to the computer of the high-speed data collector 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 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 that can achieve high-speed signal input and output. It can receive analog signals and convert them into digital signals, and can also output digital signals through the 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 prerequisite step for the imaging process. It controls the displacement component to drive the sample closer to the probe 41 by programming, which is a test of the sensitivity of the tuning fork-probe sensor. If the distance between the tip of the probe 41 and the sample stops at an appropriate position during the needle insertion process, the subsequent PID function can be used normally. 1. During the needle insertion process, the amplitude signal of the tuning fork wafer 43 collected by the lock-in amplifier 6 needs to be transmitted to the host computer in real time and compared with the initial amplitude signal of the tuning fork wafer 43 to determine whether the difference between the two exceeds the set threshold. The lock-in amplifier 6 and the host computer are connected by a network cable, and the data transmission speed of the network cable is slow, which will affect the needle insertion speed. The high-speed data acquisition device 8 and the host computer are connected by a USB cable, and its internal data processing speed and external data transmission speed are both fast. The data inside the lock-in amplifier 6 is transmitted to the high-speed data acquisition device 8 through the output port via a microwave cable, and after rapid processing, it is output to the host computer, which can greatly improve the needle insertion speed. At the same time, during the needle insertion process, the high-speed data acquisition device 8 also plays a role in controlling the Z-axis movement of the fourth displacement stage 21 through the Z-direction control module of the second controller 52. The Z-direction control module of the second controller 52 has a DC input control function. By outputting a voltage from 0V to 4V from 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 component 2), the Z-direction of the fourth displacement stage 21 can be extended from 0 to 4.3μm. 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 through a microwave cable. 2. During the imaging process, the DC input port of the Z-direction control module of the second controller 52 is switched to be connected to the voltage output port of the lock-in amplifier 6. The voltage output port of the lock-in amplifier 6 outputs the calculation result of the PID, which directly controls the expansion and contraction of the fourth displacement stage 21. At the same time, the amplitude signal of the tuning fork wafer 43 collected by the lock-in amplifier 6 and the output result of the PID are both collected into the host computer for storage through the input port of the high-speed data acquisition device 8. The lock-in amplifier 6 is connected to the tuning fork wafer 43 to monitor the amplitude change of the tuning fork wafer 43 in real time. During the imaging process, the lock-in amplifier 6 adjusts the up and down position of the sample in real time by controlling the fourth displacement stage 21 of the second controller 52 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, it includes the following steps:

[0037] Step 1: Fix the sample on the sample stage 3. Under the microscope, move the sample to below the probe 41 through the first displacement assembly 1, and adjust the initial distance between the probe 41 and the sample surface to the micron range;

[0038] The sample is fixed on the sample stage 3 with cryogenic glue. Place the device under an optical microscope, adjust the focal length to ensure that the tip of the probe 41 and the sample surface are clearly visible within the field of view. Manually adjust the first displacement assembly 1 through the control software Daisy of the first controller 51 (ANC350) to move in the XY direction (the second displacement stage 12 and the third displacement stage 13 work), so that the area to be measured on the sample surface is directly below the tip. Subsequently, control the first displacement assembly 1 to slowly lift the sample in the Z direction (the first displacement stage 11 works), and at the same time observe the reflection of the probe 41 on the sample surface. When the actual tip of the probe 41 and the tip of the probe in the reflection are almost in contact, stop the Z-direction movement. At this time, the actual distance between the probe 41 and the sample is D, where 10 microns < D < 100 microns.

[0039] Step 2: First, use a lock-in amplifier 6 to apply voltage signals of different frequencies to the tuning fork wafer 43, obtain the spectrum of the variation of the vibration amplitude of the tuning fork wafer 43 with frequency, and select the frequency point at the high point of the half-height width of the resonance peak of the tuning fork wafer 43 as the reference frequency for subsequent needle insertion operations. Drive the sample to move upward through 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 control the distance between the probe 41 and the sample within the range of atomic force. During the needle insertion operation, drive the sample to move upward by extending the fourth displacement stage 21 of the second displacement assembly 2, so that the surface of the sample continuously moves towards the probe 41. The high-speed data acquisition device 8 controls the fourth displacement stage 21 through the voltage output to the second controller 52. The lock-in amplifier 6 collects the amplitude signal of the tuning fork wafer 43 and sends the amplitude signal to the high-speed data acquisition device 8. Whether the change in the amplitude signal of the tuning fork wafer 43 reaches a predetermined change amount is used as the judgment condition. When the probe 41 approaches the surface of the sample to the range of atomic force, the amplitude signal of the tuning fork wafer 43 changes and reaches the predetermined change amount, then the judgment condition is met, and the lock-in amplifier 6 stops the upward movement of the fourth displacement stage 21 through the second controller 52. If the amplitude signal of the tuning fork wafer 43 does not reach the predetermined change amount after the fourth displacement stage 21 reaches the maximum extension amount, then the judgment condition is not met. At this time, the extension amount of the fourth displacement stage 21 is reset to zero, and the first displacement stage 11 of the first displacement control assembly 1 drives the sample to move upward in multiple steps, and then continues to drive the sample to move upward through the fourth displacement stage 21 of the second displacement assembly 2, so that the surface of the sample continuously moves towards the probe 41 until the change in the amplitude signal of the tuning fork wafer 43 reaches the predetermined change amount.

[0040] In this embodiment, the output voltage for exciting the tuning fork wafer 43 is set to 0.04V. Use the Sweep function of the lock-in amplifier 6 to perform frequency sweeping on the tuning fork signal in the range of 33.0 - 33.6 kHz to obtain the resonance spectrum of the tuning fork wafer 43 equipped with the probe 41. The resonance frequency is 33.294 kHz, and the amplitude of the resonance peak is 1.1V. Select 33.298 kHz at the descending part of the resonance peak (the frequency at the high point of the half-height width of the resonance peak) as the needle insertion locking frequency ( Figure 10 as shown), and the corresponding amplitude signal is 600 mV. When the sample gradually approaches the tip of the probe 41 to the range of atomic force, the atomic force interaction between the tip and the surface of the sample will affect the tuning fork vibration, resulting in a change in the amplitude signal at the same frequency. The change amount of the amplitude signal reaching 100 mV is used as the judgment condition;

[0041] Set the Z-direction control module of the second controller 52 (ANC300) to the "DC input" mode, and connect the voltage output port AO0 of the high-speed data collector 8 to the DC input port of the Z-direction control module of the second controller 52 (ANC300). For the ANSz50 positioner, a 60V voltage corresponds to a displacement of 4.3μm, that is, the maximum elongation of the fourth displacement stage 21 is 4.3μm, and the second controller 52 amplifies the DC input voltage by 15 times and then outputs it. For the Z-axis of the first displacement assembly 1 (positioner), a 100V voltage corresponds to a displacement of 3.5μm. Write a Python program to set the Z-axis of the first displacement assembly 1 (positioner) to the stepping mode with a step size of 35V. The calculated theoretical maximum elongation is about 1.2μm, that is, the 35V pulse signal applied to the Z-axis (the first displacement stage 11) of the first displacement assembly 1 should theoretically push it up by 1.2μm. However, due to gravity, friction, etc., the actual measured data shows that the 35V pulse signal causes the Z-axis of the first displacement assembly 1 to move 400nm. The program controls the high-speed data collector 8 to cyclically output a voltage of 0 - 4V, so that the Z-axis (the fourth displacement stage 21) of the second displacement assembly 2 (scanner) extends from zero to 4.3μm, and at the same time, it continuously monitors whether the difference between the tuning fork amplitude signal and the initial value reaches the judgment condition of 100mV. If the judgment condition is reached, immediately stop the program and set the output of the high-speed data collector 8 to zero; if not, stop the loop, set the output of the high-speed data collector 8 to zero, and make the Z-axis of the first displacement assembly 1 (positioner) rise by 10 steps with a step size of 35V, and enter the next loop until the change amount of the tuning fork wafer 43 amplitude signal reaches the judgment condition. At this time, the distance between the tip of the probe 41 and the sample surface is within the range of atomic force action.Principle description: First, the first displacement component 1 is a coarse displacement controller. When the displacement stage in the first displacement component moves forward one step 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 elongate by a certain amount. When the pulse signal is withdrawn, the piezoelectric material inside the displacement device stays at the elongated position without retracting to its original position through friction. The elongation amount in this process is relatively large. For the Z-axis of the first displacement component 1, applying a pulse voltage of 35V to it will cause it to rise by 400nm, while the action range of the atomic force is only about 100nm. Therefore, the first controller 51 alone cannot be used to control the first displacement component 1 to perform needle insertion. Otherwise, it will be impossible to control the tip to stop at a very close distance from the sample surface, and it is very likely to damage the tip. On the other hand, the second displacement control component 2 is a fine displacement controller. When a constant DC voltage is applied to the displacement stage in it, the displacement stage will elongate to a specific value and stabilize. Its control accuracy can reach below 1nm. By slowly approaching upward along the Z-axis of the second displacement control component 2, it can be ensured that the tip detects signal changes within the atomic force action range of the sample surface and can protect the probe 41 from being damaged. However, its maximum elongation amount is only 4.3μm. Therefore, it must be combined with the first displacement component 1 for needle insertion. The complete description of the needle insertion process is as follows: First, move the sample to a position close to the probe by visual observation under the microscope. At this time, the distance between the probe and the sample is in the order of dozens of micrometers. Then, use the program to control the Z-axis (the fourth displacement stage 21) of the second displacement control component 2 to slowly elongate upward in steps of 1nm. During this process, judge whether the amplitude signal of the tuning fork reaches the determination condition for each step. If it reaches the determination condition, it means that an 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 then stop the needle insertion. If the determination condition is not reached, the Z-axis continues to rise until the Z-axis reaches the maximum elongation amount of 4.3μm. At this time, the second displacement control component 2 can no longer rise and the distance between the sample and the probe 41 is still very far. At this time, set the elongation amount of the Z-axis of the second displacement control component 2 to zero. At this time, the distance between the probe 41 and the sample should be at least greater than 4.3μm. Then, make the Z-axis (the first displacement stage 11) of the first displacement control component 1 move upward 10 steps in steps of 35V. As can be obtained from the above, at this time, the first displacement control component 1 moves the sample up by about 4μm. This rising distance is safe for the tip. At this height, the Z-axis (the fourth displacement stage 21) of the second displacement controller 2 slowly rises again to test whether the tip is close enough to the sample within the range of 4.3μm upward from here. And so on in a cycle.

[0042] Step 3: Control the sample to move in the XY plane through the fifth displacement stage 22 of the second displacement component 2, so that the tip of the probe 41 scans on the surface of the sample. During the scanning process, utilize the PID control function of the lock-in amplifier, and adjust the position of the sample in the Z-axis direction in real time through the fourth displacement stage 21 to keep the distance between the probe 41 and the sample surface constant;

[0043] Since 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 and Y direction control modules of the second controller 52 (ANC300), and 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). Write a Python program to set the PID parameters (write a Python program 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 functions such as setting the controller, making the controller output and collecting. PID is a built-in function in the lock-in amplifier 6, and its parameter setting is achieved through functions in the Python program): The preset amplitude value of the tuning fork wafer 43 is 740 mV, the sampling rate is 1000 kbps, the PID coefficient kp = 0.0046, ki = 0.05, and the differential function is not enabled. Output the PID control signal through the Aux Output port 2 of the lock-in amplifier 6 and turn on the PID control switch to ensure that the distance between the tip and the sample surface is constant during the scanning process. Scan within a range of 30 μm × 30 μm, with a step size of 30 nm in the X direction and 1 μm in the Y direction. For the ANSxy50 displacement device, a 60V voltage corresponds to a 30 μm displacement. Write a nested loop program (write a Python program on the host computer) to control the Y direction to move from 0 to 30 μm in steps of 1 μm. After each movement, perform a forward line scan from 0 to 30 μm in steps of 30 nm in the X direction, and then a reverse line scan back to 0. After the X direction returns to zero, the Y direction moves to the next step. After the program ends, turn off the PID control switch and zero the output of the Aux Output port 2 of the lock-in amplifier 6 and the high-speed data collector 8.

[0044] Step 4: During the scanning process, the high-speed data collector 8 collects the voltage signal of the second displacement component 2 in real time, converts it into the height information of the sample surface, and generates a high-resolution imaging map of the sample surface.

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

[0046]

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

[0048] It is output to the second displacement controller 52 through the Aux Output port 2 of the lock-in amplifier 6. The PID outputs a DC voltage to bring 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 output of the PID remains constant. During the scan, as the sample surface undulates, the external voltage input obtained by the second displacement controller 52 will change, thereby controlling the height change of the second displacement assembly 2 in the Z direction to maintain the amplitude signal of the tuning fork wafer 43 constant. When the sample surface bulges during the scan, 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 wafer 43 becomes larger, and the difference e(t) = V0–V between the current signal V of the tuning fork wafer 43 and the preset value V0 becomes smaller, so the voltage output from the Aux Output port 2 to the second displacement controller 52 becomes smaller. On the contrary, when the sample surface is concave, the distance between the probe 41 and the sample becomes farther, the atomic force between the probe 41 tip and the sample surface becomes weaker, the amplitude signal V of the tuning fork wafer 43 becomes smaller, and the difference e(t) = V0–V between the current signal V of the tuning fork wafer 43 and the preset value V0 becomes larger, so the voltage output from the Aux Output port 2 to the second displacement controller 52 becomes larger. The voltage acquisition port AI1 of the high-speed data collector 8 records the change in the external voltage obtained by the second displacement controller 52. Through the corresponding relationship between 4.3 μm in the Z direction of the second displacement assembly and 4 V of the DC input voltage, the height difference of the sample surface can be calculated, thereby obtaining a high-resolution image of the sample.

[0049] After the scanning is completed, a bidirectional scanning image with a size of 30μm×30μm and a resolution of 1000×30 (the 30μm length is divided into 1000 steps and 30 steps, with a step of 30nm in the X direction and a step of 1μm in the Y direction) is obtained. Separate the forward and reverse scanning data, and extract the complete image of the forward scanning. Use Origin software to perform slope correction and filtering on the data, and finally obtain the AFM imaging result of the sample.

[0050] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. An atomic force microscopy imaging device for a Josephson probe microscope, characterized in that: The invention 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 lock-in amplifier (6) electrically connected to the superconducting Josephson probe assembly (4), a current amplifier (7) electrically connected to the lock-in amplifier (6), and a current amplifier (8) electrically connected to the lock-in amplifier (6) and the second controller. (52) are electrically connected to a high-speed data acquisition device (8), 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.

2. The atomic force microscopy imaging device for a Josephson probe microscope according to claim 1, characterized in that: The first displacement assembly (1) comprises 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) comprises 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.

3. The atomic force microscopy imaging device for a Josephson probe microscope according to claim 2, characterized in that: 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.

4. The atomic force microscopy imaging device for a Josephson probe microscope according to claim 2, characterized in that: The substrate (44) is a square plate, the crystal oscillation direction of the tuning fork crystal (43) is parallel to the plane of the substrate (44), and one side of the tip of the probe (41) is in contact with the end of one arm of the tuning fork crystal (43).

5. An atomic force microscopy imaging method for a Josephson probe microscope, using the atomic force microscopy imaging device for a Josephson probe microscope as claimed in any one of claims 2 to 4, characterized in that: The following steps are involved: Step 1: fix the sample on the sample stage (3), and move the sample to below the probe (41) under a microscope by using the first displacement component (1), 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 upwards 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 within the range of action of the atomic force; Step 3: Control the sample to move in the XY plane by means of the fifth displacement stage (22) of the second displacement assembly (2), so that the tip of the probe (41) scans the surface of the sample. During the scanning process, the position of the sample in the Z-axis direction is adjusted in real time by means of 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.

6. The atomic force microscopy imaging method for a Josephson probe microscope according to claim 5, characterized in that: In step 2, a phase-locked amplifier (6) is first used to apply voltage signals of different frequencies to the tuning fork chip (43), thereby obtaining a spectrum of the relationship between the vibration amplitude of the tuning fork chip (43) and the frequency, and the frequency of the falling part of the resonance peak of the tuning fork chip (43) is selected as a reference frequency for subsequent needle insertion operations.

7. The atomic force microscopy imaging method for a Josephson probe microscope according to claim 6, 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), and whether the change of the amplitude signal 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 action of the atomic force, the change of the amplitude signal of the tuning fork chip (43) reaches the predetermined change amount, then the value is reached. When the judgment condition is reached, 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, and at this time the extension amount of the fourth displacement stage (21) returns to zero, the first displacement stage (11) of the first displacement control component (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 component (2) so that the surface of the sample continuously moves toward the probe (41) until the change in the amplitude signal of the tuning fork chip (43) reaches the predetermined change amount.

Images