CN106370891B - Preparation method and control circuit of scanning tunneling microscope scanning probe - Google Patents

Abstract

The application provides a preparation method and a control circuit of a scanning probe of a scanning tunnel microscope, wherein the method comprises the following steps: performing oxide film removal treatment on the probe: sleeving a rubber ring on the bottom of the probe with the diameter of 0.5mm, wherein the bottom of the rubber ring is 2mm away from the bottom of the probe; putting the metal wire ring into KOH solution with the concentration of 2-3 mol/L, and inserting the bottom of the probe into the depth of 1mm of the liquid level; the probe is an anode, the metal wire ring is a cathode, and 5-12V of voltage is applied to the two ends; when the first voltage on the anode is equal to the second voltage on the fixed resistor of the control circuit, the control circuit cuts off the circuit, so that the bottom of the probe forms a needle point; the control circuit comprises a relay connected with the singlechip in series, a fixed resistor and a cathode and an anode which are connected by using electrolyte. The application uses rubber ring to control aspect ratio and increase gravity, uses control circuit to cut off power supply automatically and detect voltage in real time, so that the ideal probe tip with proper aspect ratio and clear scanning pattern can be produced by one-step corrosion method.

Description

Preparation method and control circuit of scanning tunneling microscope scanning probe

Technical field

The present invention relates to the field of preparation of scanning tunneling microscope (Scanning Tunneling Microscope, STM) scanning probes. In particular, it relates to a preparation method of STM scanning probes and a control circuit used in the preparation method.

Background technique

As the part of the scanning tunneling microscope that directly detects the sample surface, the scanning probe is undoubtedly closely related to the quality of STM scanning images. In fact, the size, geometry, chemical composition and purity of the probe will directly affect the tunneling current between the tip and the sample, and even the detection of the electron density of states on the sample surface. Therefore, the preparation quality of the tip is critical to the work of STM. Performance is critical. For example, if the STM probe has multiple tips, the stability of the tunnel current will be reduced; in addition, in order to increase the image acquisition rate of the probe, the probe tip should have a higher bending resonance frequency to reduce phase lag.

The scanning tip of STM is usually made of metal wires. Commonly used metals include tungsten (W), platinum-iridium alloy (Pt-Ir), gold (Au) and other metals. There are many ways to use these metal wires to prepare STM scanning probes. Usually The methods used include: grinding, shearing, ion bombardment, field evaporation or electrochemical etching. The purpose of using these methods is to make the tip of the needle tip particularly sharp, or even with only one atom at the tip, and vertically and horizontally. The ratio (the ratio of the length of the needle tip to the cross-sectional diameter) should be as small as possible to minimize the influence of the bending vibration of the needle tip during scanning, such as the instability of the scanned image state.

Tips with only one atom at the tip can be prepared using ultrahigh vacuum ion field microscopy, but this method is complex and expensive to produce. The mechanical shearing method for preparing STM probes is: generally use scissors to directly cut the metal wire into a shape with a certain angle. However, the shearing method has obvious defects, that is, the tip of the needle tip is rough, and the proportion of qualified needle tips is extremely low. And the repeatability is poor, and often has multiple needle points. The image scanned using this kind of needle tip is generally the superposition of images reflected by these micro multiple probes, with serious distortion and poor effect. The grinding method usually involves grinding a platinum wire with a diameter of about 0.1 mm using a grindstone into a conical shape with a vertex angle of 30°, 45°, 60° or 90°, and further grinding it with emery with a particle size of about 1 μm. After ultrasonic cleaning with ethanol and deionized water, if the shape of the needle tip cannot be distinguished under an optical microscope, it means that the needle tip is sharp enough. In contrast, the electrochemical etching method is not only simple to operate, low in cost, and reproducible, but the shape of the tip obtained is more stable and can reach the resolution limit of an optical microscope. In other words, the tip made by the electrochemical etching method can meet the requirements of STM. The basic application, so the electrochemical corrosion method is currently more commonly used.

However, in the actual preparation of electrochemical etching method, due to the limitations and effects of various process conditions, the prepared needle tips are relatively rough or have multiple needle tips, and the yield rate is low. The preparation process mainly includes one-step corrosion or two-step corrosion. Among them, the one-step corrosion method refers to the corrosion method that does not change the voltage during the corrosion process and uses the same voltage from beginning to end; the two-step corrosion method uses the same voltage first during the corrosion process. This is an etching method that uses a higher voltage to form a thick tip with a satisfactory aspect ratio, and then chooses the right time to change the high voltage to a low voltage for sharpening and finishing.

In the 2012 master's thesis "Automated Control of STM Tungsten Wire Tip Instrument by Electrochemical Erosion Method" written by Zhang Shuo of Jilin University, a one-step corrosion method using 12V voltage resulted in a very unsatisfactory tip aspect ratio. When the solution concentration remains unchanged, the electrolysis voltage must be reduced to obtain a sharper tip, but the electrolysis voltage must be further increased to obtain a smaller aspect ratio. Therefore, he believes that simply reducing the electrolysis voltage to improve the quality of the tip is not feasible, so the one-step corrosion method is unreasonable. Only a two-step method can be used to prepare the ideal needle tip. Niu Jianlong, Wu Xue and others from the School of Physical Sciences of Nankai University published the document "Preparation of STM Tungsten Tips" in Physical Experiments Volume 30 Issue 4 in April 2010. They also used coarse and fine etching processes to obtain the ideal tip.

In summary, the existing technology believes that there is no feasible technology to obtain an ideal needle tip in one-step etching method.

Contents of the invention

The purpose of the present invention is to provide a method for preparing an STM scanning probe and a control circuit used in the preparation method, so as to solve the technical problem that the one-step electrochemical etching method cannot obtain an ideal probe with a small aspect ratio and a tip of one atom.

In order to achieve the above object, the present invention provides a method for preparing a scanning tunneling microscope scanning probe, which includes the steps:

A. Remove the oxidation film on the probe: use sandpaper to remove the oxide film on the surface of the tungsten wire, then soak it in dilute HCL, and finally use acetone for ultrasonic cleaning;

B. Put a rubber ring on the bottom of the probe with a diameter of 0.5mm, and the vertical distance between the bottom of the rubber ring and the bottom of the probe is 2mm;

C. Put the metal wire ring into the KOH solution with a concentration of 2 to 3 mol/L, insert the probe vertically into the liquid surface of the beaker, and insert the bottom of the probe into the liquid surface to a depth of 1 mm; use the probe as the anode, and the metal wire ring As the cathode, a voltage of 5 to 12V is applied to both ends;

D. When the first voltage on the anode is equal to the second voltage on the fixed resistor of the control circuit, the control circuit cuts off the circuit so that a tip is formed at the bottom of the probe;

The control circuit includes relays, fixed resistors, and cathodes and anodes connected in series with the microcontroller using electrolyte.

Preferably, the length of the rubber ring is 2cm.

Preferably, the control circuit further includes a digital-to-analog conversion module and a liquid crystal display module connected to the microcontroller.

Preferably, the control circuit further includes a buzzer connected to the microcontroller to sound an alarm when the microcontroller cuts off the relay.

Preferably, the relay is a solid state relay.

Preferably, the probe is a tungsten wire, and the metal wire loop is a copper wire loop.

Preferably, in step C, the bottom of the probe is inserted into the center of the liquid surface.

This application also provides a control circuit used according to the above preparation method. The control circuit includes a relay connected in series with the microcontroller, a fixed resistor, and a cathode and anode connected using an electrolyte;

Among them, the probe is used as the anode and the metal wire ring is used as the cathode; the electrolyte is a KOH solution with a concentration of 2 to 3 mol/L, and the fixed resistance is 100-1500Ω.

The invention has the following beneficial effects:

1. Use the rubber band to control the aspect ratio:

When the bottom of the probe is inserted into the liquid surface, the surface tension of the liquid surface causes an upward arching meniscus to appear around the probe. If the height of the meniscus can be controlled, the length of the solution corrosion probe can be controlled, thereby controlling the needle tip. aspect ratio. In this application, a 2cm long rubber ring is placed 2mm away from the bottom of the probe, and the physical properties of the rubber are used to control the liquid level from continuing to rise and maintain a small aspect ratio.

2. Increase probe stability:

During the corrosion process, if the relative position between the probe and the solution moves relative to each other, the tip of the probe will appear stepped, affecting the quality. Putting a rubber ring on the bottom of the probe can increase the gravity on the probe and reduce the possibility of movement.

3. The circuit automatically controls the timing of power outage:

When the probe is dropped, the corrosion power supply is cut off in time, so that the tip formed in this way is sharper and meets the experimental requirements. If the corrosion is not cut off in time and the corrosion continues, the original tip will continue to corrode and become blunt.

During the corrosion process, the probe at the junction of the liquid level slowly becomes thinner, causing the resistance of the probe to become larger, the resistance of the entire circuit to become larger, the current to slowly decrease, and the partial pressure on the fixed resistor of the control circuit to slowly decrease. Small. When the probe is corroded and falls off, the system resistance suddenly changes from tens of ohms to thousands of ohms, the system current also changes suddenly, and the partial voltage on the fixed resistor also changes suddenly, that is, U1. When the partial pressure U2 on the electrolytic cell is equal to U1, it is the moment when the probe is corroded and falls off. The control circuit monitors U1 and U2 at the same time. When U1=U2, the circuit power is cut off to stop corroding the probe, and the ideal tip can be obtained.

Moreover, this application uses solid-state relays to increase the cutting rate to the μs level, which can cut off the circuit the moment the tungsten wire falls to prevent excessive corrosion.

4. Voltage detection is more accurate:

The current of the system is relatively small, and the detection accuracy of the detection components is not enough, which is prone to errors. The PCF8591 chip used in this application is easier to collect voltage signals and has better experimental results.

In addition to the objects, features and advantages described above, the present invention has other objects, features and advantages. The present invention will be described in further detail below with reference to the drawings.

Description of the drawings

The drawings forming a part of this application are used to provide a further understanding of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the attached picture:

Figure 1 is a schematic diagram of the control circuit structure of a preferred embodiment of the present invention;

Figure 2 is a picture of the needle tip produced in the preferred embodiment 1 of the present invention, magnified 100 times and 400 times respectively; Figure (a) is a picture of the needle tip magnified 100 times, Figure (a) is a picture of the needle tip magnified 400 times;

Figure 3 is a picture of the needle tip produced in the prior art that is magnified 100 times and 400 times respectively; Figure (a) is a picture of the needle tip magnified 100 times, and Figure (a) is a picture of the needle tip magnified 400 times;

Figure 4 is a scanning picture of the Si (111) surface of the needle tip produced by the preferred embodiment 1 of the present invention and the prior art. Figure (a) is the scanning picture of the needle tip produced by the prior art, and Figure (b) is the preferred embodiment of the present invention. Example 1: A scanned picture of a needle tip is obtained.

Detailed ways

The embodiments of the present invention are described in detail below with reference to the accompanying drawings, but the present invention can be implemented in many different ways as defined and covered by the claims.

The needle tip preparation process using high-purity tungsten wire with a diameter of 0.5mm as the probe raw material and a KOH solution with a concentration of 2 to 3 mol/L is as follows:

Before the experiment, the tungsten wire should be deoxidized, polished with sandpaper to remove the oxide film on the surface, soaked in dilute HCL for 10 minutes, and finally ultrasonically cleaned with acetone for 15 minutes.

Put the copper wire ring into 2~3mol/L KOH solution, insert the vertical tungsten wire into the center of the beaker liquid surface, then use the tungsten wire as the anode, the copper wire ring as the cathode, and add 5~12V corrosion to both ends voltage, the electrochemical corrosion reaction begins at this time. During the reaction process, the tungsten wire at the liquid surface slowly becomes thinner, and finally breaks due to the action of gravity. At this moment, the corrosion voltage is cut off by the control circuit. An ideal needle tip is available. Then, clean the obtained tip with acetone and deionized water as soon as possible to remove the residual KOH solution on the surface.

In the electrochemical corrosion process, the tungsten wire serves as the anode and the copper wire coil serves as the cathode. Its chemical reaction formula is as follows:

Anodic reaction: W+8OH ^- ——WO ₄ ^2- +4H ₂ O+6e ^-

Cathodic reaction: 6H ₂ O+6e ^- ——3H ₂ +6OH ^-

Total reaction: W+2OH ^- +2H ₂ O——WO ₄ ^2- +3H ₂

The purpose of using the control circuit is to cut off the corrosion power supply in time when the probe falls, so that the tip formed in this way is sharper and meets the experimental requirements; if the corrosion is not cut off in time and the corrosion continues, the original tip will continue to corrode, thus becomes dull and does not meet the experimental requirements. Therefore, the main purpose of controlling the circuit is to cut off the circuit in time when the probe corrodes and falls.

Figure 1 is the schematic diagram of the entire corrosion circuit system. The corrosion voltage is 5V or 12V, and a 2mol/L or 3mol/L KOH solution is used. During the corrosion process, the tungsten wire at the liquid surface interface slowly becomes thinner, making the The resistance of the tungsten wire becomes larger, the resistance of the entire circuit becomes larger, the current slowly decreases, and the partial voltage on the fixed resistor slowly decreases. When the tungsten wire is corroded and falls off, the system resistance changes from tens of ohms to several ohms. When the kilohm ground mutates, the system current also mutates, and the fixed voltage U1 also mutates. Therefore, there is a most suitable cut-off voltage U2 (which can be considered constant in a short period of time). At this time, the tungsten wire is just corroded and broken. Cut off the circuit in time at this time to obtain an ideal needle tip. U2 is the instant voltage between the anode tungsten wire and the cathode copper wire coil in the electrolytic cell.

The purpose of the fixed resistor in series is to amplify the value of the circuit system and facilitate detection. The resistance of the fixed resistor can be 50-1500Ω, and can be flexibly changed according to the depth of the probe inserted into the liquid level and the diameter, length and material of the probe.

Embodiment 1

Use sandpaper to remove the oxide film on the surface of a high-purity tungsten wire with a diameter of 0.5 mm, then soak it in dilute HCL for 10 minutes, and finally use acetone to ultrasonically clean it for 15 minutes.

The bottom of the tungsten wire is covered with a 2cm long rubber ring, and the distance between the bottom of the rubber ring and the bottom of the probe is 2mm.

Put the copper wire ring into the 2mol/L KOH solution, insert the vertical tungsten wire into the center of the beaker liquid level, and insert the bottom of the probe into the liquid level to a depth of 1mm; then use the tungsten wire as the anode and the copper wire ring as the cathode , add a corrosion voltage of 5V to both ends; use the probe as the anode and the metal wire ring as the cathode to perform the electrochemical corrosion reaction.

Figure 1 is also a schematic diagram of each module of the needle tip preparation control circuit. The control circuit is mainly composed of AT89C52 microcontroller, digital-to-analog conversion module PCF8591, relay, liquid crystal display module LCD1602, and buzzer. During the electrochemical corrosion reaction, PCF8591 is used to sample the voltage U1 of the fixed resistor (50Ω) and the instant voltage U2 between the anode tungsten wire and the cathode copper wire coil in the electrolytic cell. The voltage U1 < U2 at the beginning. The data can be sent to the microcontroller through the bus, and then the real-time voltage data is sent to LCD1602 for real-time display, and U2 is displayed as 2.8711V. As the experiment proceeds, U1 continues to decrease until it decreases to U1=U2. At this time, the microcontroller controlled by the software program makes a judgment and cuts off the relay. At the same time, the buzzer sounds an alarm, and the corrosion ends.

The needle tip obtained in this example is shown in Figure 2. The magnification in (a) is ×100. It can be seen that the entire position of the needle tip is offset. This should be caused by the tungsten wire not being straightened before the experiment. , or the cross-section of the rubber ring is inclined, causing the entire needle tip to tilt, but this does not affect the quality of the needle tip. (b) It can be seen from the picture that the diameter of the part close to the tip reaches 0.6um. Due to the limited magnification of the optical microscope, further accurate measurement requires SEM observation.

The tip produced in Example 1 was mounted on the STM to scan the Si (111) surface, and a scanning pattern as shown in Figure 4(b) was obtained. It can be seen that the image shown in Figure 4(b) is obviously more three-dimensional and clearer than Figure 4(a).

Figure 4(a) is a scanning picture of the Si(111) surface of the tip produced using the prior art. The corrosion conditions of the existing technology: the tungsten wire is not covered with a rubber ring, and is directly inserted into the 2mol/L KOH solution level, with a depth of 1mm, a corrosion voltage of 5V, and a corrosion time of 35 minutes. The pattern of the prepared needle tip is shown in Figure 3. The magnification in Figure 3(a) is ×100. It can be observed that the tip of the needle is in the center of the tungsten wire, which is relatively symmetrical. (b) The magnification in the picture is ×400, and it can be seen that the diameter of the part close to the top of the tip is about 4.33um, which is obviously quite far away from the nanoscale tip.

In summary, through technological improvements, this application uses rubber rings to control the aspect ratio and increase gravity, and uses a control circuit to automatically cut off the power supply and detect the voltage in real time, overcoming industry bias and breaking through industry problems, so that the one-step corrosion method can produce products with appropriate aspect ratios and Ideal probe tip for clear scan patterns.

The above descriptions are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention.

Claims (6)

1. A method for preparing a scanning tunneling microscope scanning probe, which is characterized by including the steps: A. Remove the oxidation film on the probe: use sandpaper to remove the oxide film on the surface of the tungsten wire, then soak it in dilute HCL, and finally use acetone for ultrasonic cleaning; B. Put a rubber ring on the bottom of the probe with a diameter of 0.5mm. The vertical distance between the bottom of the rubber ring and the bottom of the probe is 2mm. Control the aspect ratio of the needle tip; C. Put the metal wire ring into the KOH solution with a concentration of 2~3mol/L, insert the probe vertically into the liquid surface of the beaker, and insert the bottom of the probe into the liquid level to a depth of 1mm; use the probe as the anode, and the metal wire ring As the cathode, a voltage of 5~12V is applied to both ends; D. When U2, which is the instant voltage between the anode tungsten wire and the cathode copper wire coil in the electrolytic cell, is equal to the second voltage on the fixed resistor of the control circuit, the control circuit cuts off the circuit, causing a needle tip to form at the bottom of the probe; control circuit It includes relays, fixed resistors connected in series with the microcontroller, and cathodes and anodes connected using electrolyte; The probe is a tungsten wire, and the metal wire loop is a copper wire loop. 2. The preparation method according to claim 1, characterized in that the length of the rubber ring is 2cm. 3. The preparation method according to claim 1, characterized in that the control circuit further includes a digital-to-analog conversion module and a liquid crystal display module connected to a microcontroller. 4. The preparation method according to claim 1, characterized in that the control circuit further includes a buzzer connected to the microcontroller to sound an alarm when the microcontroller cuts off the relay. 5. The preparation method according to claim 1, characterized in that the relay is a solid state relay. 6. The preparation method according to claim 1, characterized in that, in step C, the bottom of the probe is inserted into the center of the liquid surface.