CN106443218A - Surface charge measurement method - Google Patents

Abstract

The invention belongs to the technical field of surface charge measurement, and particularly discloses a kelvin probe force microscope technology-based surface charge measurement method. According to the method, a change in the intensity of interference laser is measured to measure the offset of a probe and finish correcting the distance from the tip of the probe to a sample by virtue of a fiber optic interferometer of a kelvin probe force microscope, and reliable surface charge density is finally obtained by virtue of a relationship between the actual distance of the tip of the probe and the sample and electrostatic force; the method is significant for researches on material and device surface charges.

Description

A method for measuring surface charge

technical field

The invention belongs to the technical field of surface charge measurement, in particular to a surface charge measurement method using Kelvin probe force microscope technology.

technical background

The surface charge measurement method is the basis of surface charge research, and accurate measurement of surface charge is of great significance for the miniaturization research of various electronic devices.

At present, the surface charge measurement methods include dust map method, electrostatic probe method and optical measurement method based on Pockels effect. However, none of the above methods can quantify the surface charge, and the resolution is not high.

A more effective method for measuring surface charge in the prior art is the Kelvin probe force microscopy technique, which calculates the surface charge density by detecting the electrostatic force signal between the probe and the sample surface. The specific method is to use a Kelvin probe force microscope to measure the relationship curve between the electrostatic force between the probe and the sample and the moving distance of the scanning tube (that is, the "force-distance" curve), and then use the formula to simulate the "force-distance" curve. Then, the surface charge density of the sample can be calculated.

In the literature "Mapping and Quantifying Surface Charges on Clay Naoparticles" (Jun Liu; Ravi Gaikwad; Aharnish Hande; Siddhartha Das.Langmuir 2015, 31, 10469-10476), the author introduced a method of using Kelvin probe force microscopy to charge clay A method for quantifying the surface charge of a material. The steps of the method are as follows:

1) Fix the sample to be tested on the sample stage of the Kelvin probe force microscope, and adjust the parameters of the instrument;

2) Apply an AC voltage between the probe and the sample to cause the probe and its cantilever to resonate, move the probe to the area of the sample to be tested, and measure the relationship between the vibration frequency of the probe and the moving distance of the scanning tube;

3) via the formula The relationship curve was fitted to obtain the surface charge density σ of the sample.

The above-mentioned surface charge measurement method can realize the quantitative measurement of the surface charge at the nanometer scale. However, when the distance between the probe and the sample is less than 50nm, the influence of van der Waals force, electrostatic force and other forces on the probe will cause the deformation of the probe cantilever. At this time, the actual "force-distance" distance is not equal to the scanning tube movement The distance leads to a large error in the method of extracting the surface charge density by fitting the "force-distance" curve.

Therefore, the method of measuring surface charge using Kelvin probe force microscope technology in the prior art is not equal to the distance traveled by the scanning tube in the actual "force-distance", resulting in large errors in surface charge measurement and inaccurate measurement. Disadvantages of surface charge.

Contents of the invention

The object of the present invention is to provide a measurement method for accurately measuring surface charge using Kelvin probe force microscope technology.

In order to solve the above-mentioned technical problems, the technical solution adopted in the present invention is a method for measuring surface charge, comprising the steps of:

1) Fix the sample to be tested on the sample stage of the Kelvin probe force microscope, the laser emits laser light with a wavelength of λ, and applies a DC voltage V _dc on the piezoelectric sheet, so that the probe moves vertically, and the moving distance is greater than the laser wavelength λ; at the same time , record the relationship between the interference laser intensity and the moving distance of the probe, and calculate the sensitivity of the interferometer to measure the probe offset through the maximum value I _max and the minimum value I _min of the interference laser intensity

2) Apply an AC voltage between the probe and the sample to cause the probe and its cantilever to resonate, move the probe to the top of the sample to be measured, the scanning tube moves towards the probe, and use a lock-in amplifier to monitor the probe Vibration amplitude R and phase θ, obtain the relationship curve between probe vibration amplitude R and scanning tube moving distance Z, the relationship curve between phase θ and scanning tube moving distance Z, and output vibration amplitude R and phase θ to the computer; monitor the interference laser at the same time Intensity I, the intensity of the interference laser at the initial position is I ₀ , and the relationship curve between the intensity of the interference laser and the moving distance Z of the scanning tube is obtained;

3) Through the phase θ and vibration amplitude R collected by the computer, use the amplitude value of the probe at the initial position to normalize and calculate to obtain the normalized amplitude value R _N , and then calculate to obtain A=cosθ/R _N ;

4) Calculate the deformation of the cantilever ΔZ=D×ΔI according to the change value of the interference laser intensity during the moving process of the scanning tube ΔI=II ₀ , and correct the actual distance Z*=Z-ΔZ of the "needle tip-sample" to obtain AZ* curve relationship;

5) via the formula Fit the AZ*force curve to obtain the values of M and B, where M and B are constants;

6) Calculate the surface charge density:

Wherein S=π×r ² , r is the radius of the tip of the probe; k is the elastic coefficient of the tip, and ε ₀ is the vacuum dielectric constant.

In order to solve the problem of the deviation of the "tip-sample" distance caused by the probe offset and obtain reliable surface charge density data, the present invention uses the optical fiber interferometer that comes with the Kelvin probe force microscope to measure the change of the interference laser intensity To obtain the offset of the probe, complete the correction of the distance from the tip to the sample, and use the relationship between the actual distance of the "tip-sample" and the electrostatic force to finally obtain a reliable surface charge density.

Description of drawings

Figure 1 is a schematic diagram of the "tip-sample" distance;

Figure 2 is a schematic diagram of the system structure of the Kelvin probe force microscope for measuring surface charge;

Fig. 3 is the relationship curve of interference laser intensity and probe offset;

Figure 4 is the relationship curve between the probe offset and the moving distance of the scanning tube when the needle tip is close to the sample;

Fig. 5 is the force-distance curve and the corresponding fitting curve before and after the "needle tip-sample" distance correction.

detailed description

The present invention will be further described below in conjunction with accompanying drawing and example:

In this example, the elastic coefficient of the conductive probe used is k=2.18N/m, and the resonance frequency is 75kHz; S=1×10 ^-15 m ² ; vacuum dielectric constant ε ₀ =8.85×10 ^-12 ; the sample to be tested SiO ₂ , through the test system shown in Figure 2, the specific measurement steps are as follows:

Fix the SiO ₂ sample on the sample stage of the Kelvin probe force microscope, the laser emits a 40uw laser, and applies a DC voltage V _dc = 3V on the piezoelectric sheet to make the probe move vertically, and record the relationship between the intensity of the interference laser and the moving distance , as shown in Figure 3, calculate the probe offset sensitivity D=142.1nm/V that the interferometer measures by interfering laser intensity maximum value I _max and minimum value I _min ;

Apply an AC voltage between the probe and the sample to make the probe and its cantilever resonate, move the probe to the top of the sample to be measured, the scanning tube moves towards the probe, and use the lock-in amplifier to monitor the vibration amplitude of the probe R and phase θ, obtain the relationship curve between the probe vibration amplitude R and the moving distance Z of the scanning tube, the relationship curve between the phase θ and the moving distance Z of the scanning tube, output the vibration amplitude R and phase θ signals to the computer; monitor the interference laser intensity at the same time I, record the intensity of the interference laser at the initial position as I ₀ , and obtain the relationship curve between the intensity of the monitoring interference laser and the moving distance Z of the scanning tube.

Through the phase θ and vibration amplitude R collected by the computer, use the amplitude value of the probe at the initial position to normalize and calculate the normalized amplitude value R _N , and then calculate the force A=cosθ/R _N ; at this time, AZ can be obtained the curve;

According to the change value of the interference laser intensity when the probe approaches the sample surface ΔI=II ₀ , the deformation of the cantilever ΔZ=142.1×ΔI is calculated, and the relationship between the probe offset and the moving distance of the scanning tube is obtained as shown in Figure 4, and then calculated Get the real distance Z*=Z-ΔZ of "needle tip-sample", and get the AZ* curve relationship as shown in Figure 5;

by formula Fit the force curves before and after correction respectively to obtain the values of M and B; M and B are constants; then substitute M and B into the formula Calculated surface charge density; from the fitting situation in Figure 5, it can be seen that the calculated surface charge density σ before correction is 5.0uC·cm ^-2 , and the corrected surface charge density σ is 4.4uC·cm ^-2 . By comparison, it is found that the surface charge density value obtained before the "tip-sample" distance correction is increased by 10% compared with the value after correction.

Claims (1)

1. A surface charge measurement method, characterized in that comprising the steps: 1) Fix the sample to be tested on the sample stage of the Kelvin probe force microscope, the laser emits laser light with a wavelength of λ, and applies a DC voltage V _dc on the piezoelectric sheet, so that the probe moves vertically, and the moving distance is greater than the laser wavelength λ; at the same time , record the relationship between the interference laser intensity and the moving distance of the probe, and calculate the sensitivity of the interferometer to measure the probe offset through the maximum value I _max and the minimum value I _min of the interference laser intensity 2) Apply an AC voltage between the probe and the sample to make the probe and its cantilever resonate, move the probe to the top of the sample to be measured, the scanning tube moves towards the probe, and use the lock-in amplifier to monitor the probe. Vibration amplitude R and phase θ, obtain the relationship curve between probe vibration amplitude R and scanning tube moving distance Z, the relationship curve between phase θ and scanning tube moving distance Z, and output vibration amplitude R and phase θ to the computer; monitor the interference laser at the same time Intensity I, the intensity of the interference laser at the initial position is I ₀ , and the relationship curve between the intensity of the interference laser and the moving distance Z of the scanning tube is obtained; 3) Through the phase θ and vibration amplitude R collected by the computer, use the amplitude value of the probe at the initial position to normalize and calculate to obtain the normalized amplitude value R _N , and then calculate to obtain A=cosθ/R _N ; 4) Calculate the deformation of the cantilever ΔZ=D×ΔI according to the change value of the interference laser intensity during the moving process of the scanning tube ΔI=II ₀ , and correct the actual distance Z*=Z-ΔZ of the "needle tip-sample" to obtain AZ* curve relationship; 5) via the formula Fit the AZ*force curve to obtain the values of M and B, where M and B are constants; 6) Calculate the surface charge density: Wherein S=π×r ² , r is the radius of the tip of the probe; k is the elastic coefficient of the tip, and ε ₀ is the vacuum dielectric constant.