CN102507988A - Intermittent-contact-mode measuring method of Kelvin probe force microscope - Google Patents

Abstract

The invention provides an intermittent contact measurement method for a Kelvin probe force microscope, the Kelvin probe force microscope includes a scanning head, a probe, a probe position sensor, a piezoelectric exciter, a piezoelectric scanner, a low-frequency A voltage signal generator, a high-frequency voltage signal generator, a low-frequency vibration signal detector, a high-frequency vibration signal detector, a high-frequency voltage and compensation voltage signal superimposed device, a Kelvin feedback controller and a controller, on the piezoelectric scanner The sample is placed, and on the basis of the existing Kelvin probe force microscope, the present invention adopts the scanning mode in which the probe and the sample are intermittently contacted, and the morphology and local potential images can be obtained at the same time in one scan, the scanning speed can be doubled, and at the same time, the Improve the sensitivity of signal detection.

Description

An Intermittent Contact Measurement Method for Kelvin Probe Force Microscope

technical field

technical field

The invention belongs to the measurement field of Kelvin probe force microscope.

Background technique

Kelvin Probe Force Microscopy (KPFM) is a technique based on Atomic force microscopy (AFM), which can measure the local potential of a sample and its two-dimensional distribution. Kelvin probe force microscopy has become an important tool for characterizing the microstructure and properties of materials.

Like electrostatic force microscopy (EFM), Kelvin probe force microscopy is also based on remote electrostatic force imaging, and the surface morphology of samples is also obtained by means of atomic force microscopy. The main difference between the Kelvin probe force microscope and the electrostatic force microscope is that the Kelvin probe force microscope applies a compensation voltage on the probe or the sample, and adjusts the compensation voltage in real time through a dedicated Kelvin feedback controller to make the electrostatic force between the probe and the sample is zero, so as to quantitatively measure the local potential of the sample surface.

Kelvin probe force microscopes operating in an atmospheric environment are usually used in "raised mode". For example, the Chinese patent application number CN201010103017.1 applied for by the present inventor is an invention patent for a Kelvin probe force microscope and its measurement method. Its "lifting mode" is an imaging method in which each row of images is scanned twice: In the first pass, the usual atomic force microscope imaging method (the scanning mode in which the probe and the sample are in intermittent contact, that is, tapping mode) is used to obtain the surface topography by measuring the interatomic force; while in the second pass, the probe is lifted to a certain extent. (keep the probe and the sample not in contact), keep the probe-sample distance constant according to the topographical fluctuation information obtained before and scan while obtaining the distribution image of the local potential through the feedback and compensation of the remote electrostatic force.

Kelvin probe force microscopes and atomic force microscopes use microcantilever probes to measure force, and it is necessary to excite the intrinsic mechanical vibration modes of the probe to improve sensitivity. The microcantilever probe has a variety of intrinsic mechanical vibration modes, such as the first, second, and third vibration modes. The topography imaging in the Kelvin probe force microscope in the atmospheric environment usually uses the first vibration mode of the microcantilever probe; the local potential imaging in the Kelvin probe force microscope in the air environment usually uses the first vibration mode, but also Higher order vibration modes may be used, such as the second vibration mode.

Since the measurement process of the existing Kelvin probe force microscope adopts the imaging method of lifting the probe and scanning each row of images twice, there is room for improvement in sensitivity and scanning imaging speed.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art and provide an intermittent contact measurement method for a Kelvin probe force microscope that effectively improves sensitivity and scanning imaging speed.

In order to realize the object of the above invention, the technical solution adopted is as follows.

A kind of intermittent contact measurement method of Kelvin probe force microscope, described Kelvin probe force microscope comprises scanning head, probe, probe position sensor, piezoelectric exciter, piezoelectric scanner, low-frequency voltage signal generator , a high-frequency voltage signal generator, a low-frequency vibration signal detector, a high-frequency vibration signal detector, a high-frequency voltage and compensation voltage signal superimposed device, a Kelvin feedback controller and a controller, and a sample is placed on the piezoelectric scanner, The measurement method is to carry out the following operations at the same time:

Operation A. The low-frequency voltage signal generator generates a voltage signal corresponding to or close to the lower eigenfrequency of the probe, and applies it to the piezoelectric vibrator closely connected to the probe, thereby exciting the probe in the low-frequency vibration mode Vibration, the probe position sensor senses this vibration and transmits it to the low-frequency vibration signal detector to measure its amplitude and phase signal, and the controller controls the piezoelectric scanner to scan the probe on the sample to obtain the sample morphology picture;

Operation B. The high-frequency voltage signal generator generates a high-frequency voltage signal that is the same as or close to the higher eigenfrequency of the corresponding probe, and applies the electrostatic force interaction between the probe and the sample and between the probe and the sample to excite the probe in the Vibration on the high-frequency vibration mode, the probe position sensor senses this vibration and transmits it to the high-frequency vibration signal detector, measures its amplitude and phase signal, and sends it to the Kelvin feedback controller. The amplitude of frequency vibration automatically generates a compensation voltage signal, which is sent to the high-frequency voltage and compensation voltage signal superimposer, superimposed with the high-frequency voltage signal and applied between the probe and the sample, and the compensation voltage signal is also simultaneously is transmitted to the controller, which uses the signal to form an image of the local potential distribution on the sample surface.

The voltage signal generated by the high-frequency voltage signal generator includes a DC signal and an AC signal, and the DC signal and the AC signal can be applied to the probe or the sample separately or simultaneously.

In the operation B, multiple voltage signals can be simultaneously generated by the high-frequency voltage signal generator to excite multiple high-frequency vibration modes of the probe. Correspondingly, the high-frequency vibration signal detector also simultaneously detects multiple vibration signals and Imaging is controlled by a controller.

Based on the existing Kelvin probe force microscope, the present invention adopts a scanning mode in which the probe and the sample are intermittently contacted, and obtains morphology and local potential images at the same time in one scan, and the scanning speed can be doubled. The morphology imaging of the present invention adopts the first vibration mode of the probe, while the electrostatic force imaging adopts a higher vibration mode. Compared with the existing Kelvin probe force microscope in the lifting mode, this method has a smaller probe-sample distance during measurement and a higher sensitivity for electrostatic force detection. Therefore, the Kelvin probe force microscope in the present invention has the characteristics of high resolution. And compared with the Kelvin probe force microscope that adopts the "lifting mode", this working mode of intermittent contact between the probe and the sample makes the average distance between the probe and the sample greatly reduced, and the probe can probe to a stronger The electrostatic force signal of the Kelvin feedback controller makes the signal-to-noise ratio larger and the performance more stable. Therefore, a higher resolution local potential image can be obtained with this mode of operation.

Description of drawings

Fig. 1 is a structural schematic diagram of the Kelvin probe force microscope of the present invention.

Figure 2 is a comparison of scanning image results, in which (A) is a topographic image, (B) is a local potential image obtained by using the measurement method in the present invention, and (C) is obtained by using the existing "lift mode" Local potential image.

Detailed ways

The present invention will be further described below in conjunction with the accompanying drawings.

The structure of Kelvin probe force microscope of the present invention is as shown in accompanying drawing 1, comprises scanning head 1, probe 1-1, probe position sensor 1-2, piezoelectric exciter 1-3, piezoelectric scanner 1-5, low-frequency voltage signal generator 2, high-frequency voltage signal generator 3, high-frequency voltage signal detector 4, low-frequency vibration signal detector 5, high-frequency voltage and compensation voltage signal adder 6, Kelvin feedback controller 7 , controller 8.

The sample 1-4 is fixed on the piezoelectric scanner 1-5, and the piezoelectric scanner 1-5 drives the sample to change in the X, Y, Z three-dimensional space under the action of the output voltage of the controller 8, thereby controlling the sample 1-4 The position relative to the tip of probe 1-1. Topography scanning uses the first vibration mode of probe 1-1 (its eigenfrequency is lower than the high-order vibration mode), and the AC voltage signal that is the same as or close to the first eigenfrequency is generated by the low-frequency voltage signal The generator 2 is applied to the piezoelectric vibrator 1-3 closely connected with the probe 1-1, so as to excite the probe 1-1 to vibrate in the first vibration mode. Probe position sensor 1-2 senses this vibration and transmits it to low frequency vibration signal detector 5 and measures its amplitude and phase signal. The controller 8 uses the signal to control the piezoelectric scanner 1-5 to scan the probe 1-1 on the sample 1-4 to obtain the topography map of the sample.

The local potential image is acquired simultaneously with the topography scan. That is to say, the probe only needs to scan once to obtain both topography and electrostatic force images. This is different from the "lift mode" that needs to be scanned twice in the Kelvin probe force microscope in the usual atmospheric environment. When using the present invention to measure the local potential image, the probe and the sample are intermittently in contact.

The higher order vibrational mode of probe 1-1 (which has a higher eigenfrequency relative to the first vibrational mode) was used for surface potential image scanning. The sinusoidal AC voltage signal with the same or close to its eigenfrequency is generated by the high-frequency voltage signal generator 3, and applied between the probe 1-1 and the sample 1-4, so as to excite the comparison of the probe 1-1 through the interaction of electrostatic force. Higher order vibration mode. The probe position sensor 1-2 senses this vibration and transmits it to the high-frequency voltage signal detector 4 to measure its amplitude and phase signal, and then sends it to the Kelvin feedback controller 7, and the Kelvin feedback controller 7 according to the high-frequency The amplitude of the vibration signal automatically generates a compensation voltage signal, which is sent to the high-frequency voltage and compensation voltage signal superimposer 6, and then applied between the probe 1-1 and the sample 1-4 after being superimposed with the high-frequency voltage signal . Therefore, the compensation voltage can be used to change the effective potential difference between the probe and the sample, thereby changing the magnitude of the electrostatic force on the probe. The Kelvin feedback controller 7 automatically adjusts the magnitude and polarity of the compensation voltage according to a certain strategy so that the electrostatic force between the probe and the sample becomes zero or minimum, that is to say, the compensation voltage is just equal to the local potential of the sample surface. The compensation voltage signal is simultaneously sent to the controller 8, which uses the signal to form an image of the local potential distribution on the sample surface.

When measuring the local potential of the sample, multiple vibration modes of the probe 1-1 can also be simultaneously excited. At this time, the high-frequency voltage signal generator 3 simultaneously generates multiple excitation signals, and the high-frequency voltage signal detector 4 also simultaneously detects multiple signals and is imaged by the controller 8 .

During Kelvin probe force microscopy imaging, the first vibrational mode originally used for topography measurement is excited in the same way as for topography scanning alone; this vibration is recorded to form a topography image. Typically, the vibration amplitudes of the higher vibration modes used for localized potential imaging are much smaller compared to the mechanical vibration amplitudes of the first vibration modes, so electrostatic forces have little influence on the topography during measurements. At the same time, the excitation of the first vibration mode also plays a role in stabilizing the vibration of the probe, which is beneficial to the measurement of the local potential using the higher vibration mode.

The measurement principle of the present invention is as follows:

When a compensation and AC voltage signal is applied between the probe 1-1 and the sample 1-4, the electrostatic force can be expressed as:

              (1)

(1)

为探针-样品间距，

为探针和样品间等效电容对距离的梯度，

为在未施加直流和交流的电压信号时探针和样品之间的电势差，

为补偿电压信号，

为正弦交流信号的幅度，

为其频率，

为时间。 In the formula

is the probe-sample distance,

is the gradient of the equivalent capacitance versus distance between the probe and the sample,

is the potential difference between the probe and the sample when no DC and AC voltage signals are applied,

To compensate the voltage signal,

is the amplitude of the sinusoidal AC signal,

for its frequency,

for time.

上的分量为： Electrostatic force at frequency

The components on are:

           (2)

(2)

为0，则检测到的高频率信号的幅度

与探针-样品间的静电力成正比。在开尔文探针力显微镜中，开尔文反馈控制器7自动产生补偿电压信号

，使得在探针-样品间施加的补偿电压正好等于探针和样品之间的电势差

，即使

。用该补偿电压信号成像，就可得到样品的表面电势的实际测量值及其分布。通常的静电力显微镜基于这一原理工作。 If a compensation voltage signal is applied between the probe-sample

is 0, the amplitude of the detected high-frequency signal

Proportional to the electrostatic force between the probe and the sample. In a Kelvin probe force microscope, a Kelvin feedback controller 7 automatically generates a compensation voltage signal

, so that the compensation voltage applied between the probe and the sample is exactly equal to the potential difference between the probe and the sample

,even though

. By imaging with the compensation voltage signal, the actual measured value and distribution of the surface potential of the sample can be obtained. Common electrostatic force microscopes work on this principle.

较大（梯度

与探针和样品间距的平方成反比），因而公式（1）中的总静电力和公式（2）中的静电力分量均得到增大。因此，采用这种工作方式后探针可探测到更强的静电力信号使得灵敏度增加，从而可得到具有更高分辨率的局域电势图像。 In the Kelvin probe force microscope of the intermittent contact type that the present invention relates to, because the average spacing between the probe and the sample is small, the gradient of the equivalent capacitance to the distance between the probe and the sample

Larger (gradient

is inversely proportional to the square of the distance between the probe and the sample), so both the total electrostatic force in equation (1) and the electrostatic force component in equation (2) are increased. Therefore, after adopting this working method, the probe can detect a stronger electrostatic force signal to increase the sensitivity, so that a local potential image with higher resolution can be obtained.

The specific measuring method of the present invention is as follows.

1. Determine the eigenfrequency. Excite the vibration modes of the probe 1-1, such as the first, second, and third vibration modes, etc., to obtain the eigenfrequency of each vibration mode. Select the first eigenfrequency or a frequency near it as the excitation frequency for the topography scan, and select one or more higher eigenfrequencies as the excitation frequency for the surface potential measurement.

2. Two-dimensional image scanning. Use "tapping mode" to scan the probe over the sample. Record the vibration signal of the first vibration mode of the probe 1-1 to obtain the profile curve; at the same time, make the Kelvin feedback controller 7 work normally and output a compensation voltage signal equal to the absolute value of the local potential, and compare the compensation voltage signal with the A high-frequency AC signal is applied between the probe-sample; this compensation voltage signal (whose value represents the local potential) is recorded. Finally, a plurality of whole images are obtained by scanning line by line at the same time.

和

，第二次本征振动的频率和品质因数分别为：

和

。所用样品为石墨上的钒氧酞菁薄膜（HOPG-VOPC）。探针第一本振机械振动振幅 A₁=320 nm；在第二本征振动频率上所加的交流信号振幅 A₂=1.5 V。高频激发电压和开尔文反馈控制器输出的补偿电压均施加于样品上，探针接地。 Accompanying drawing 2 shows the sample topography and local potential map measured by the present invention and the existing Kelvin probe force microscope. The probe used is a rectangular silicon probe (MESP type, Veeco, USA), and the frequency and quality factor of the first eigenvibration are:

and

, the frequency and quality factor of the second eigenvibration are:

and

. The sample used is vanadyl phthalocyanine film on graphite (HOPG-VOPC). The mechanical vibration amplitude of the first local oscillator of the probe A ₁ =320 nm; the amplitude of the AC signal applied on the second natural vibration frequency A ₂ =1.5 V. Both the high-frequency excitation voltage and the compensation voltage output by the Kelvin feedback controller are applied to the sample, and the probe is grounded.

，扫描速度为。图（A）中的高度变化范围为0～7nm，图（B）和图（C）中的电势变化范围均为100～300meV。从图2的结果看，本发明能够同时测得样品的表面形貌图（A）和局域电势图（B）。从形貌图看，样品表面形貌的高低起伏在整个扫描范围内仅为几个纳米，较为平坦。采用传统的抬起模式所得到的局域电势图像（图C）的横向分辨率较低（其分辨率低于形貌图像）。而采用本发明中的 “间歇接触模式”后，所得到的局域电势图像（图B）的横向分辨率有了很大的提高（其分辨率与形貌图像差不多）。这说明用本发明可以提高了开尔文探针力显微镜的局域电势图像的横向分辨率。 In Fig. 2, image (A) is a topographic image, image (B) is a local potential image obtained by using the "intermittent contact mode" in the present invention, and image (C) is an image obtained by using an existing Kelvin probe force microscope "lift Local potential image obtained in "lift-off mode" (probe lift-off height is 30 nm). The image scanning range is

, the scanning speed is . The range of height change in picture (A) is 0-7nm, and the range of potential change in picture (B) and picture (C) is 100-300meV. From the results in Fig. 2, it can be seen that the present invention can simultaneously measure the surface topography map (A) and the local potential map (B) of the sample. From the topography diagram, the ups and downs of the surface topography of the sample are only a few nanometers in the entire scanning range, which is relatively flat. The localized potential image (Panel C) obtained with the traditional lift-off mode has a lower lateral resolution (it has lower resolution than the topographic image). However, after adopting the "intermittent contact mode" in the present invention, the lateral resolution of the obtained local potential image (Figure B) has been greatly improved (its resolution is almost the same as that of the topographic image). This shows that the lateral resolution of the local potential image of the Kelvin probe force microscope can be improved by using the present invention.

The present invention is based on atomic force microscopy in the atmosphere, by utilizing the first eigenvibration mode of probe 1-1 to measure the topography, while utilizing its high Using sub-intrinsic vibration mode to measure the surface potential, the topography map and local potential distribution map of the tested sample can be obtained at the same time. This method will not affect the original measurement function of the AFM, and can improve the lateral resolution and sensitivity of the Kelvin probe force microscope.

Claims (3)

1. An intermittent contact measurement method of a Kelvin probe force microscope, said Kelvin probe force microscope comprising a probe (1-1), a probe position sensor (1-2), a piezoelectric vibrator ( 1-3), scanning head (1) composed of piezoelectric scanner (1-5), low-frequency voltage signal generator (2), high-frequency voltage signal generator (3), low-frequency vibration signal detector (5 ), high-frequency vibration signal detector (4), high-frequency voltage and compensation voltage signal superimposer (6), Kelvin feedback controller (7) and controller (8), the piezoelectric scanner (1-5) A sample (1-4) is placed on it, characterized in that the measurement method simultaneously performs the following operations: Operation A. The low-frequency voltage signal generator (2) generates a voltage signal corresponding to or close to the lower eigenfrequency of the probe (1-1), and applies it to the piezoelectric excitation closely connected to the probe (1-1). vibrator (1-3), thereby exciting the probe (1-1) to vibrate in a low-frequency vibration mode, the probe position sensor (1-2) senses this vibration and transmits it to the low-frequency vibration signal detector (5), measure its amplitude and phase signal, the controller (8) controls the piezoelectric scanner (1-5) to make the probe (1-1) scan on the sample (1-4) to obtain the topography of the sample ; Operation B. The high-frequency voltage signal generator (3) generates a high-frequency voltage signal that is the same as or close to the higher eigenfrequency of the probe (1-1), and applies the probe (1-1) and sample (1-4) Between, the electrostatic force interaction between the probe (1-1) and the sample (1-4), excites the probe (1-1) to vibrate in a high-frequency vibration mode, the probe position sensor (1-2) This vibration is sensed and sent to the high-frequency vibration signal detector (4), and its amplitude and phase signals are measured, and sent to the Kelvin feedback controller (7), which according to the high-frequency vibration The amplitude automatically generates a compensation voltage signal, which is sent to the high-frequency voltage and compensation voltage signal superimposer (6), and applied to the probe (1-1) and sample (1-4) after being superimposed with the high-frequency voltage signal ), the compensation voltage signal is simultaneously transmitted to the controller (8), and the controller (8) utilizes the signal to form an image of the local potential distribution on the surface of the sample. 2. The intermittent contact measurement method of the Kelvin probe force microscope according to claim 1, characterized in that the voltage signal generated by the high-frequency voltage signal generator (3) includes a DC signal and an AC signal, and the DC signal and The AC signal can be applied to the probe (1-1) or the sample (1-4) separately or simultaneously. 3. The intermittent contact measurement method of the electrostatic force microscope according to claim 1, characterized in that in the operation B, multiple voltage signals can be simultaneously generated by the high-frequency voltage signal generator (3) to excite the probe (1-1) multiple high-frequency vibration modes, correspondingly, the high-frequency vibration signal detector (4) also simultaneously detects multiple vibration signals and controls the imaging by the controller (8).

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