CN101493397A - Electrostatic force microscope and measurement method thereof - Google Patents

Abstract

The invention provides an electrostatic microscope and a measurement method. The electrostatic microscope consists of a scan head, a low frequency voltage signal generator, a high frequency voltage signal generator, a low frequency vibration signal detector, a high frequency vibration signal detector and a controller; and the scan head comprises a probe, a probe location inductor, a piezoelectric vibration exciter and a piezoelectric scanner. Based on an existing electrostatic force microscope, a higher order eigenvibration mode of the probe is adopted to measure electrostatic force. As interaction of the probe and the sample is different in different vibration modes, the corresponding performance is different in aspects such as resolution, sensitivity, stability and the like. The electrostatic microscope can help significantly improve spatial resolution of the electrostatic force microscope in atmospheric environment.

Description

An electrostatic force microscope and its measuring method

technical field

The invention belongs to the field of electrostatic force microscopes, in particular to an electrostatic force microscope and a measuring method thereof.

technical background

Electrostatic force microscopy (EFM) is a technique based on atomic force microscopy (AFM), which can measure the electrostatic interaction force between two objects and its two-dimensional distribution. Electrostatic force microscopy has become an important means of characterizing the microstructure and properties of materials by obtaining the local distribution image of the charge or potential on the surface of the sample through the dynamic measurement of the electrostatic force.

Electrostatic force microscopy requires the use of atomic force microscopy to obtain surface topography images of samples. Electrostatic force microscopy and atomic force microscopy employ microcantilever probes to measure force. In the atmospheric environment, the electrostatic force microscope and atomic force microscope need to excite the intrinsic vibration mode of the probe to improve the sensitivity, and use the "amplitude modulation" method to measure the vibration amplitude of the probe. The electrostatic force microscope has a high resolution in a vacuum environment, but it has certain requirements for the sample, for example, it cannot be used to measure samples such as biomolecules and working devices. Electrostatic force microscopy in atmospheric environments does not have these limitations, but the spatial resolution achieved by current methods is relatively low, up to about 100-200 nanometers.

The microcantilever probe has a variety of intrinsic vibration modes, such as the first, second, and third vibration modes, and its intrinsic vibration frequency becomes higher in turn. The existing electrostatic force microscope only utilizes the first eigenvibration mode of the micro-cantilever probe, and does not utilize other higher-order vibration modes, resulting in relatively limited use.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies of the prior art, and provide an electrostatic force microscope that utilizes a higher-order vibration mode of a micro-cantilever probe to measure electrostatic force.

Another object of the present invention is to provide a measurement method of the electrostatic microscope.

In order to realize the purpose of the invention one, the technical scheme adopted is as follows:

An electrostatic force microscope is composed of a scanning head, a low-frequency voltage signal generator, a high-frequency voltage signal generator, a low-frequency vibration signal detector, a high-frequency vibration signal detector, and a controller. The scanning head includes a conductive micro-cantilever probe , a probe position sensor, a piezoelectric vibrator, and a piezoelectric scanner; the low-frequency voltage signal generator is connected to the piezoelectric vibrator and provides a voltage signal to excite the probe to vibrate, and the high-frequency voltage signal generator is connected to the piezoelectric vibrator The probe is connected to provide a voltage signal to it, and the probe position sensor is respectively connected with the low-frequency vibration signal detector and the high-frequency vibration signal detector to transmit the vibration information of the probe sensed by it, and the low-frequency vibration The signal detector and the high-frequency vibration signal detector are respectively connected with the controller, and the controller is connected with the piezoelectric scanner and controls the operation of the piezoelectric scanner through an output voltage signal.

In the above technical solution, the high-frequency voltage signal generator is respectively connected to the probe and the measurement sample on the piezoelectric scanner and provides voltage signals to them.

The voltage signal generated by the high-frequency voltage signal generator is an AC signal, or a DC signal plus an AC signal.

The probe is provided with a variety of vibration modes from low to high vibration frequencies, and the voltage signal of the vibration mode with the lowest frequency is provided by a low-frequency voltage signal generator, while the voltage signals of vibration modes of other frequencies are generated by a high-frequency voltage signal provided by the device.

Measuring principle of the present invention is as follows:

When DC and AC voltage signals are applied between the probe and the sample, the electrostatic force can be expressed as:

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; el</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; D.</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}^{<span\; class="notranslate">\; \&prime;</span>}{<span\; class="notranslate">\; [</span>\mathrm{<span\; class="notranslate">\; \&Delta;\&phi;</span>}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; dc</span>}}<span\; class="notranslate">\; -</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; ac</span>}}\mathrm{<span\; class="notranslate">\; sin</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;f</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\mathrm{<span\; class="notranslate">\; t</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; ]</span>}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the above formula (1), D is the distance between the probe and the sample, C' is the gradient of the equivalent capacitance between the probe and the sample to the distance, and Δφ is the potential difference between the probe and the sample when no DC and AC voltage signals are applied , U _dc represents the DC voltage signal, U _ac is the amplitude of the sinusoidal AC signal, f is its frequency, and t is time.

The component of electrostatic force at frequency f is:

F _f (D)＝-C′[(Δφ-U _dc )U _ac Sin(2πf·t)](2)

If the DC voltage signal U _dc applied between the probe-sample is 0, the amplitude F _f (D) of the detected high-frequency signal is proportional to the potential difference between the probe-sample, so the obtained image reflects the Surface potential distribution. If the DC voltage signal U _dc is applied between the probe and the sample, and the DC voltage is just equal to the potential difference Δφ between the probe and the sample through the 6-controller and imaged with this voltage, the measurement of the surface potential of the sample can be obtained value and its distribution.

The tip of the probe near the measurement sample can conduct electricity. According to the formula (1), the electrostatic force is proportional to the square of the surface potential difference between the probe and the sample. For non-conductive probe tips, the surface potential is determined by the accumulated electrostatic charge, which is difficult to control. In order to facilitate the measurement, the surface potential of the probe must be a definite value, otherwise the measured results have no physical meaning. In addition, for the non-conductive probe tip, the high-frequency excitation voltage signal used in the measurement cannot be applied, so the vibration of the probe cannot be excited. Therefore, in electrostatic force measurements, the probe tip must be conductive.

In order to realize the purpose of the invention two, the technical scheme adopted is as follows:

A measuring method of an electrostatic force microscope, comprising the steps of:

1) The scanning head, low-frequency voltage signal generator, low-frequency vibration signal detector, and controller operate in coordination to measure the topography of the sample through low-frequency voltage signals;

2) The scanning head, high-frequency voltage signal generator, high-frequency vibration signal detector, and controller operate in coordination, and the electrostatic force image of the sample is obtained by further measuring the topography image obtained in step 1) through the high-frequency voltage signal.

In the above-mentioned technical scheme, described step 1) is specifically as follows:

The sample is fixed on the piezoelectric scanner, and the piezoelectric scanner drives the sample to change in the X, Y, and Z three-dimensional space under the action of the voltage signal output by the controller, thereby controlling the relative position between the sample and the probe tip, and the low-frequency voltage signal The generator generates a low-frequency voltage signal and applies it to the piezoelectric vibrator to excite the first intrinsic vibration mode of the probe. The probe position sensor senses this vibration and transmits it to the low-frequency vibration signal detector , the amplitude and phase signals are measured, and finally the controller uses this signal to control the piezoelectric scanner to scan the probe on the sample to obtain the topography.

Described step 2) is specifically as follows:

A high-frequency voltage signal is generated by a high-frequency voltage signal generator, which is applied to the probe or the sample (or applied to the probe and the sample at the same time (remove the sentence in red)) to excite the higher-order locality of the probe. The probe position sensor senses this vibration and transmits it to the high-frequency vibration signal detector to measure its amplitude and phase signal, and finally the controller uses this signal to obtain the electrostatic force distribution of the sample. dimensional image.

The step 2) can simultaneously send multiple voltage signals of different frequencies by the high-frequency voltage signal generator, thereby simultaneously exciting multiple intrinsic vibration modes of the probe, and the high-frequency vibration signal detector also detects multiple corresponding Vibration signal, and finally the imaging is controlled by the controller.

The voltage signal generated by the high-frequency voltage signal generator (3) in step 2) is an AC signal, or a DC signal plus an AC signal.

The step 2) when performing electrostatic force image measurement, the probe is lifted to a certain height relative to the sample so that the probe and the sample are in a non-contact state, and the distance between the probe and the sample is kept constant to facilitate scanning to obtain a remote electrostatic force image .

On the basis of the existing electrostatic force microscope, the present invention uses the higher order vibration mode of the probe to measure the electrostatic force. Due to the different interaction between the probe and the sample in different vibration modes, the corresponding performance will also be different, such as in terms of resolution, sensitivity, and stability. The invention can significantly improve the spatial resolution of the electrostatic force microscope under the atmospheric environment.

When the present invention works normally, the environment where the probe and the sample are located is an atmospheric environment, a special atmosphere environment or a liquid environment. It overcomes the defect that the existing electrostatic force microscope cannot be used in an atmospheric environment and can only work in a vacuum because its shape scanning uses "frequency modulation", and the invention uses "amplitude modulation" because of its shape scanning, that is By measuring its amplitude and phase signals, the results can be accurately measured in the atmospheric environment.

Description of drawings

Fig. 1 is the structure diagram of electrostatic microscope of the present invention, wherein 1 is scanning head, 1-1 is conductive microcantilever probe, 1-2 is probe position sensor, 1-3 is piezoelectric exciter, 1-4 For measuring samples, 1-5 are piezoelectric scanners, 2 is a low-frequency voltage signal generator, 3 is a high-frequency voltage signal generator, 4 is a low-frequency vibration signal detector, 5 is a high-frequency vibration signal detector, 6 is a control device;

Fig. 2 is an image of gold particles dispersed on a silicon surface obtained simultaneously by the measurement method provided by the present invention, wherein (A) is a surface topography image, and (B) is an electrostatic force image.

Detailed ways

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

The schematic diagram of the structure of the present invention is shown in Figure 1, and the present invention does not affect the original measurement function of the static atomic force microscope. Before scanning the electrostatic force image, it is necessary to scan the surface topography of the sample with an atomic force microscope. The atomic force microscope adopts a progressive scanning method to obtain topography. The atomic force microscope works in the "intermittent contact" mode of the conductive microcantilever probe 1-1 and the sample 1-4. The sample 1-4 is fixed on the piezoelectric scanner 1-5, and the piezoelectric scanner 1-5 drives the position of the sample 1-4 to change in the X, Y, Z three-dimensional space under the action of the voltage signal output by the controller 6, thereby controlling The relative position of sample 1-4 and the tip of probe 1-1. The topography scan utilizes the first eigen-vibration mode of probe 1-1 (its eigenfrequency is low compared to the higher-order vibration mode). The AC voltage signal which is the same as or close to the first eigenfrequency is generated by the low-frequency voltage signal generator 2 and applied to the piezoelectric vibrator 1-3 closely connected to the probe 1-1, so that the probe 1- 1 vibrates on the first eigenmode of vibration. The probe position sensor 1-2 senses this vibration and transmits it to the low-frequency vibration signal detector 4 to measure its amplitude and phase signal. The controller 6 uses the signal to control the piezoelectric scanner 1-5 to scan the probe 1-1 on the sample 1-4 so as to obtain the topography map of the sample.

Electrostatic force microscopy uses a "lift-off mode". The so-called "lift-up mode" is an imaging method in which each row of images is scanned twice: in the first pass, the surface topography is obtained by measuring the force between atoms using the above-mentioned method of the atomic force microscope, and in the second pass, the probe is scanned. The needle 1-1 is raised to a certain height, and the distance between the probe and the sample is kept constant according to the previously obtained information on the fluctuation of the sample shape, and the remote electrostatic force image is obtained by scanning.

When performing electrostatic force image scanning, the higher-order eigenvibration mode of the conductive microcantilever probe 1-1 (its eigenfrequency is higher than that of the first-order vibration mode) is utilized. The AC voltage signal with the same or close to its eigenfrequency is generated by the high-frequency voltage signal generator 3 and applied to the probe 1-1, while the voltage applied to the sample 1-4 is zero. The voltage signal generated by the high-frequency voltage signal generator 3 may include a DC component in addition to the AC signal, and both the DC and AC signals are applied to the probe 1-1. Due to the electrostatic force interaction between the probe 1-1 and the sample 1-4, the vibration of the probe 1-1 in the higher order eigenvibration mode will be excited. The probe position sensor 1-2 senses this vibration and transmits it to the high-frequency vibration signal detector 5 to measure its amplitude and phase signal. The controller 6 uses the signal to obtain a two-dimensional image of the electrostatic force distribution of the sample.

During electrostatic force image scanning, multiple eigenvibrational modes of the probe can be simultaneously excited. These vibration modes can be of the second or higher order. At this time, the high-frequency voltage signal generator 3 simultaneously generates multiple excitation signals, and the high-frequency vibration signal detector 5 also simultaneously detects multiple corresponding vibration signals and is imaged by the controller 6 .

When the electrostatic force image is scanned or in the lifted state, the first vibration mode originally used for topography measurement can be simultaneously excited in the original way. The amplitude or phase of this vibration can be simultaneously recorded and imaged.

Concrete 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 eigenvibration modes, etc., to obtain the eigenfrequencies of each vibration mode. The first eigenfrequency or a frequency near it is selected as the excitation frequency of the topography scan, and one or more higher eigenfrequencies are selected as the excitation frequency of the electrostatic force measurement.

(2) Two-dimensional image scanning. Each row is scanned first to obtain the topography curve of the sample surface, and then the probe 1-1 is lifted to a certain height, and a higher frequency voltage signal for electrostatic force measurement is applied to it, and then this row of scanning is repeated to obtain the electrostatic force or surface potential The change curve, and finally scan progressively to obtain multiple whole images at the same time.

(3) Electrostatic force spectroscopy measurement. Select different measurement points on the topography diagram or electrostatic force diagram, lift the probe 1-1 to a certain height, and measure the relationship curve of the electrostatic force at the high-order eigenfrequency with the applied DC or AC voltage signal, or with the The relationship curve of probe lifting height.

Figure 2 shows the topography and surface potential of the sample measured by the above device. The probe used is a rectangular silicon microcantilever probe, and its first, second and third eigenfrequency are respectively: 23.096kHz, 119.565kHz and 146.711KHz. The sample used is gold (Au) particles dispersed on the surface of silicon (Si). The topography scan uses the first eigenfrequency, while the electrostatic force image scan uses the third eigenfrequency, with a lift-off height of 30 nm. The DC voltage applied between the sample and the probe is 0, and the amplitude of the third eigenfrequency voltage signal is 1V, which is applied to the probe. The scanning range is 1 μm×1 μm, and the scanning speed is 0.2 Hz.

From the results in Figure 2, this method can simultaneously measure the surface topography (A) and electrostatic force (B) of the sample. On the topography, there is no difference between the gold particles and the crystallized silicon particles. The electrostatic force image shows different details from the topography image. Several smaller dark areas appeared in the lower left corner of the electrostatic force image, and several larger dark areas appeared in the middle and lower right corners. These features did not appear in the topography image. These image features are caused by electrostatic forces due to differences in the surface potential of the gold and silicon particles, which can induce changes in electrostatic forces. These dark regions are likely to correspond to the gold grains. Judging by the size of these regions, the method produces electrostatic force images with a spatial resolution of at least 30 nanometers.

Claims (10)

1. An electrostatic force microscope, characterized in that it consists of a scanning head (1), a low-frequency voltage signal generator (2), a high-frequency voltage signal generator (3), a low-frequency vibration signal detector (4), a high-frequency vibration signal Detector (5), controller (6), described scanning head (1) comprises conductive microcantilever probe (1-1), probe position sensor (1-2), piezoelectric exciter (1 -3), a piezoelectric scanner (1-5); the low-frequency voltage signal generator (2) is connected to the piezoelectric vibrator (1-3) and provides a voltage signal to excite the probe (1-1) to vibrate, The high-frequency voltage signal generator (3) is respectively connected with the probe (1-1) and the measurement sample (1-4) located on the piezoelectric scanner (1-5) and provides voltage signals to them, The probe position sensor (1-2) is respectively connected with the low-frequency vibration signal detector (4) and the high-frequency vibration signal detector (5) to transmit the vibration information of the probe (1-1) it senses , the low-frequency vibration signal detector (4) and the high-frequency vibration signal detector (5) are connected with the controller (6) respectively, and the controller (6) is connected with the piezoelectric scanner (1-5) and passed The output voltage signal controls the operation of the piezoelectric scanner (1-5). 2. The electrostatic force microscope according to claim 1, characterized in that the high-frequency voltage signal generator (3) applies a voltage signal between the probe (1-1) and the measurement sample (1-4) to The probe (1-1) is excited to vibrate. 3. The electrostatic force microscope according to claim 1 or 2, characterized in that the voltage signal generated by the high-frequency voltage signal generator (3) is an AC signal, or a DC signal plus an AC signal. 4. The electrostatic force microscope according to claim 3, characterized in that the probe (1-1) is provided with a variety of eigenvibration modes from low to high vibration frequencies, and the eigenvibration mode with the lowest frequency is The voltage signal is provided by a low-frequency voltage signal generator (2), and the voltage signals of other frequency eigenvibration modes are provided by a high-frequency voltage signal generator (3). 5. The electrostatic force microscope according to claim 4, characterized in that the tip of the probe (1-1) on the side close to the measurement sample (1-4) can conduct electricity, so as to control the surface potential of the probe tip so as to Generates a measurable electrostatic force. 6. A measuring method using an electrostatic force microscope according to claim 1, characterized in that it comprises the steps of: 1), the scanning head (1), low-frequency voltage signal generator (2), low-frequency vibration signal detector (4), and controller (6) operate in coordination to measure the shape of the sample (1-4) through the low-frequency voltage signal appearance map; 2) Coordinated operation of the scanning head (1), high-frequency voltage signal generator (3), high-frequency vibration signal detector (5), and controller (6), the morphology obtained by the high-frequency voltage signal according to step 1) The figure further measures the electrostatic force images of the measurement samples (1-4). 7. The measuring method of electrostatic force microscope according to claim 6, characterized in that said step 1) is as follows: The sample (1-4) is fixed on the piezoelectric scanner (1-5), and the piezoelectric scanner (1-5) drives the sample (1-4) under the action of the voltage signal output by the controller (6) at X, The three-dimensional position of Y and Z changes, so as to control the relative position of the sample and the probe tip. The low-frequency voltage signal is generated by the low-frequency voltage signal generator (2), and applied to the piezoelectric vibrator (1-3), thereby exciting The first intrinsic vibration mode of the probe (1-1), the probe position sensor (1-2) senses this vibration and transmits it to the low-frequency vibration signal detector (4), and measures its amplitude and The phase signal is finally used by the controller (6) to control the piezoelectric scanner (1-5) to scan the probe (1-1) on the sample (1-4) to obtain a topography map. 8. The measuring method of electrostatic force microscope according to claim 6 or 7, characterized in that said step 2) is as follows: The high-frequency voltage signal is generated by the high-frequency voltage signal generator (3) and applied to the probe (1-1) or the sample (1-4) to excite the higher-order intrinsic vibration of the probe (1-1) mode, the probe position sensor (1-2) senses this vibration and transmits it to the high-frequency vibration signal detector (5), measures its amplitude and phase signal, and finally the controller (6) uses the The signals yield a two-dimensional image of the electrostatic force distribution of the samples (1-4). 9. The measurement method of electrostatic force microscope according to claim 8, characterized in that said step 2) can simultaneously send a plurality of voltage signals of different frequencies by the high-frequency voltage signal generator (3), thereby exciting the probe ( 1-1) multiple intrinsic vibration modes, and the high-frequency vibration signal detector (5) also detects multiple corresponding vibration signals at the same time, and the imaging is finally controlled by the controller (6). 10. The measurement method of electrostatic force microscope according to claim 6, characterized in that in step 2) when performing electrostatic force image measurement, the probe (1-1) is lifted to a certain height relative to the sample so that the probe without contact with the sample.

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