Scanning impedance microscope

Abstract

The present invention achieves both improvement of spatial resolution and a wide measurement frequency band in a scanning impedance microscope. This scanning impedance microscope 100 comprises: an AC voltage generation unit 10 that generates a sample AC voltage, applies the sample AC voltage to a sample 9, generates a probe AC voltage, and applies the probe AC voltage to a probe 21; and a signal measurement unit 20 that measures the amplitude and phase of the sample AC voltage inside the sample 9 by detecting a modulation component of a mixed signal S1 detected through the probe 21, the mixed signal S1 being generated by frequency mixing of the sample AC voltage and the probe AC voltage. The signal measurement unit 20 comprises a first lock-in amplifier 25 that synchronously detects a signal component of a frequency f1 from a sum frequency component of the mixed signal S1. The frequency fm of the probe AC voltage, primary and secondary resonance frequencies f0, f1 of the vibration of the probe 21, and the frequency fs of the sample AC voltage satisfy the relationship of fm=f1-f0-fs.

Description

Scanning impedance microscope

[0001] This invention relates to a scanning impedance microscope for measuring the distribution of AC voltage inside a sample.

[0002] Impedance measurement is commonly used to evaluate the performance of energy storage and power generation devices such as all-solid-state batteries and perovskite solar cells. Impedance measurement allows for the estimation of internal resistance factors. Impedance measurement is performed by applying an AC voltage to the sample under measurement, such as an energy storage or power generation device, and measuring the response current while sweeping the frequency. Measurements are generally performed at frequencies ranging from approximately 0.1 Hz to approximately 1 MHz, with a range of approximately 10⁻¹⁰ ７ It is performed across a wide frequency range of orders of magnitude.

[0003] Impedance measurements allow for the analysis of impedance components within a sample. However, it is difficult to pinpoint which part of the sample is responsible for each impedance component. For example, while it is known that an all-solid-state battery, including a positive electrode, solid electrolyte, and negative electrode, has three impedance components, it is difficult to determine which part of the positive electrode, solid electrolyte, or negative electrode is responsible for each component. However, if the distribution of the applied AC voltage within the sample can be understood, it becomes possible to correlate the impedance components with the internal structure of the sample. Scanning impedance microscopy (SIM) is a known technique that enables the measurement of such impedance distributions.

[0004] SIM is a type of scanning probe microscope (SPM), which measures the amplitude and phase distribution of an AC voltage within a sample by scanning the sample with a conductive probe while an AC voltage is applied to the sample. More specifically, SIM visualizes the distribution of the AC voltage within a sample by measuring the electrostatic force generated between the conductive probe and the sample.

[0005] In the initial development of SIMs, electrostatic force detection was performed using an amplitude modulation method (hereinafter simply referred to as the AM method) with a high upper limit on the measurement frequency (for example, Non-Patent Document 1). Figure 7 shows a schematic example of the configuration of SIM 90A using the AM method. In the figure, the AC voltage generation unit 91 applies an AC voltage to the sample 9. One end of the conductive probe 92 is directed toward the sample 9, and a piezoelectric element is provided at the other end. The probe excitation unit 93 excites the probe 92 at its resonant frequency. The laser 94 irradiates the excited probe 92 with laser light for measurement. The photodiode 95 converts the laser light reflected from the probe 92 into a current signal. The signals detected through the probe 92 and the photodiode 95 are input to the lock-in amplifier 96 for synchronous detection and input to the image generation unit 98. The image generation unit 98 generates an image of the AC voltage distribution inside the sample 9 based on the signal supplied from the lock-in amplifier 96. The sample 9 to be measured is, for example, a battery, and the potentiostat 101 applies a DC voltage to the sample 9 that is the same magnitude as the electromotive force of the battery. The AC voltage generated by the AC voltage generator 91 is added to the DC voltage output from the potentiostat 101 in the adder 102 and applied to the sample 9.

[0006] While AM-based SIM (Simulation Imaging) can detect electrostatic forces even at high measurement frequencies, it suffers from a drawback: low spatial resolution. This is because AM simultaneously detects not only the electrostatic force between the probe tip and the sample, but also the electrostatic force between the probe's cantilever and the sample. This superposition of electrostatic forces reduces the detection sensitivity and limits the spatial resolution of AM.

[0007] To solve these problems with the AM method, a SIM using frequency modulation (hereinafter also simply referred to as the FM method) was developed. Figure 8 shows a schematic example of the configuration of the FM method SIM 90B. In the figure, reference numeral 97 denotes an FM detector (demodulator). With the FM method, instead of directly detecting the electrostatic force component with the same frequency as the AC voltage applied to the sample, as with the AM method, the sideband component generated when the applied AC voltage modulates the probe vibration is detected using the FM detector 97. With the FM method, the spatial resolution that was limited in the AM method is improved (for example, Non-Patent Documents 2 and 3).

[0008] SV Kalinin and DA Bonnell. "Scanning impedance microscopy of electroactive interfaces." Applied Physics Letters 78.9 (2001) 1306-1308.T. Kimura, K. Kobayashi, H. Yamada. "Local impedance measurement of an electrode / single-pentacene-grain interface by frequency-modulation scanning impedance microscopy." Journal of Applied Physics 118.5 (2015).N. Ishida, "Local impedance measurement by direct detection of oscillating electrostatic potential using kelvin probe force microscopy." The Journal of Physical Chemistry C 126.41 (2022) 17627-17634.

[0009] In the FM method, a low-frequency AC voltage of several kHz is applied to the sample, and the resulting sideband component appearing near the primary resonant frequency of the probe vibration is detected by an FM detector, thereby achieving high spatial resolution. The FM method makes it possible to detect the local electrostatic force between the tip of the probe and the sample with greater precision. On the other hand, the FM method has the problem that the frequency of the AC voltage that can be applied is limited to the bandwidth that the FM detector can detect. Current general-purpose FM detectors can only detect signals up to a frequency of about 5 kHz. Therefore, the measurement of impedance distribution in frequency bands higher than about 5 kHz has not been achieved with the FM method (Non-Patent Literature 2, 3). Although the FM method can improve the low spatial resolution that was a drawback of the AM method, it loses the advantage of the AM method, which is the upper limit of the measurement frequency, and the measurement frequency is limited to a low frequency band. In impedance distribution measurement using SIM, there is a need to develop a new method that can achieve both improved spatial resolution and a wide measurement frequency band.

[0010] The present invention aims to achieve both improved spatial resolution and a wide measurement frequency range in a scanning impedance microscope.

[0011] The inventors of the present invention have been diligently working to solve the above problems and have discovered that in SIM measurement, the frequency of the AC voltage signal applied to the sample to be measured can be converted to a frequency detectable by the heterodyne method. Specifically, they have found that by generating a new AC voltage signal for SIM signal detection and applying it to the probe, the modulation component (sum frequency component) of the mixed signal that occurs when multiple AC voltage signals of different frequencies are applied to the sample and probe can be adjusted to a frequency component detectable by the heterodyne method.

[0012] That is, the present invention for solving the above problem includes, for example, the embodiments described below. (Item 1) A scanning impedance microscope that measures the distribution of said AC voltage inside a sample while scanning said sample with a conductive probe in a state where an AC voltage is applied to the sample, comprising: an AC voltage generator that generates a sample AC voltage and a first probe AC voltage, applies said sample AC voltage to said sample, and applies said first probe AC voltage to said probe; and a signal measurement unit that measures the amplitude and phase of said sample AC voltage inside said sample by detecting a modulation component of a mixed signal detected through said probe, said mixed signal being generated by frequency mixing of said sample AC voltage and said first probe AC voltage, wherein said signal measurement unit extracts a frequency f from a sum frequency component of said mixed signal １ comprises a first lock-in amplifier that performs synchronous detection on the signal component of, and the frequency f of said first probe AC voltage ｍ , the resonance frequency of vibration of said probe, and the frequency f of said sample AC voltage ｓ satisfy the relationship: f ｍ = f １ - f ０ - f ｓ , wherein f ０ is the primary resonance frequency of said probe, and f １ is the secondary resonance frequency of said probe. (Item 2) The scanning impedance microscope according to Item 1, further comprising an AC voltage distribution image generation unit that generates an AC voltage distribution image inside said sample based on the measured amplitude and phase of said sample AC voltage. (Item 3) The scanning impedance microscope according to Item 1 or 2, wherein said AC voltage generator further generates a second probe AC voltage for modulating said resonance frequency of said probe, adds the generated second probe AC voltage and said first probe AC voltage and applies the sum to said probe, and said signal measurement unit further comprises a KPFM measurement unit that measures the surface potential of said sample based on said mixed signal to which said second probe AC voltage is further added. (Item 4) The scanning impedance microscope according to any one of Items 1 to 3, wherein said KPFM measurement unit comprises: a demodulation unit that demodulates a sideband component of said mixed signal to which said second probe AC voltage is further added to generate a demodulated signal; and from said demodulated signal, the frequency f of said second probe AC voltageｋｐｆｍ A scanning impedance microscope according to item 3, comprising: a second lock-in amplifier for synchronously detecting the signal component of; and a bias feedback unit for feedback-controlling the potential of the probe to correspond to the surface potential of the sample based on the signal synchronously detected by the second lock-in amplifier. (Item 5) The bias feedback unit controls the frequency f of the AC voltage of the sample. ｓ A scanning impedance microscope according to item 4, wherein the potential of the probe is controlled to maintain a predetermined potential when the frequency is between 0.1 Hz and 100 Hz. (Item 6) A scanning impedance microscope according to any one of items 3 to 5, further comprising a surface potential distribution image generation unit that generates a distribution image of the surface potential of the sample based on the measured surface potential.

[0013] According to the present invention, it is possible to achieve both improved spatial resolution and a wide measurement frequency band in a scanning impedance microscope.

[0014] This figure shows the electrostatic force spectrum between the probe and the sample measured by a scanning impedance microscope (SIM) for different measurement methods. This figure shows a schematic configuration of a scanning impedance microscope (SIM) according to one embodiment of the present invention. (A) is a schematic diagram of the sample to be measured used in the example. (B) is a legend for the distribution images (surface potential, amplitude, phase) shown in Figures 4 and 5. This is a distribution image of AC voltage obtained in the measurement frequency range of 0.3 kHz to 5 kHz by measurement in Example 1. This is a distribution image of AC voltage obtained in the measurement frequency range of 10 kHz to 300 kHz by measurement in Example 1. This is a graph of the line profile extracted from the amplitude image obtained in Example 1 when the measurement frequency is 300 kHz. This figure shows an example of a schematic configuration of a scanning impedance microscope (SIM) using the amplitude modulation (AM) method. This figure shows an example of a schematic configuration of a scanning impedance microscope (SIM) using the frequency modulation (FM) method.

[0015] Embodiments of the present invention will be described in detail below with reference to the accompanying drawings. In the following description and drawings, the same reference numerals indicate the same or similar components, and therefore, redundant explanations of the same or similar components will be omitted.

[0016] [Measurement Principle] Figure 1 shows the electrostatic force spectra between the probe and the sample measured by a scanning impedance microscope (SIM) for different measurement methods. (A) is the spectrum obtained by the conventional AM method, and (B) is the spectrum obtained by the conventional FM method. (C) is the spectrum obtained by the heterodyne AM method of the present invention. In (A) to (C), f ０ is the primary resonant frequency of the probe, and f １ is the secondary resonant frequency of the probe. In (B), the shaded frequency range is the bandwidth that the FM detector can detect.

[0017] The measurement principle of SIM according to the present invention will be explained below with reference to Figure 1.

[0018] ・Introduction of the heterodyne method As shown in Figure 1(A), in the conventional AM method, a component f with the same frequency as the frequency of the AC voltage applied to the sample １ By directly detecting the electrostatic force, the spatial resolution is low, but the measurement bandwidth is wide. On the other hand, as shown in Figure 1(B), in the conventional FM method, the primary resonant frequency f of the probe is ０ frequency f ｍ The sideband component (f) generated by modulation with the AC voltage ０ +f ｍ By detecting (), electrostatic force is detected. This improves spatial resolution, but it has the problem of limiting the upper limit of the measurement frequency, resulting in a narrow measurement bandwidth.

[0019] In contrast, as shown in Figure 1(C), the present invention introduces a heterodyne method to the detection of electrostatic force by SIM, thereby achieving both improved spatial resolution and a wide measurement frequency band in SIM. Heterodyne is the process of generating a new frequency by combining or multiplying multiple vibration waveforms. According to the heterodyne method, the primary resonance frequency f of the probe ０To detect the electrostatic force of the sideband components, SIM measurements are made possible in a higher frequency band while maintaining high spatial resolution. In the heterodyne AM method shown in Figure 1(C) introduced in this invention, the primary resonance frequency of the probe is f ０ The frequency (f) of the AC voltage １ -f ０ ) The component f produced by modulation １ By detecting this, electrostatic force is detected.

[0020] - Introduction of a new AC voltage for SIM signal detection. In SIM measurements, the frequency of the AC voltage applied to the sample is set to several orders of magnitude (for example, about 10). ３ Measurements are performed while varying the voltage within the specified range. Therefore, it is difficult to directly detect the AC voltage signal applied to the sample using the heterodyne method.

[0021] In the heterodyne method introduced in this invention, the frequency of the AC voltage signal applied to the sample to be measured is converted to a frequency detectable by the heterodyne method. Specifically, a new AC voltage signal for SIM signal detection is generated and applied to the probe, thereby adjusting the modulation component (sum frequency component) of the mixed signal that occurs when multiple AC voltage signals of different frequencies are applied to the sample and probe so that it becomes a frequency component detectable by the heterodyne method.

[0022] - Model analysis of the heterodyne method: In SIM measurements, the electrostatic force F acting between the conductive probe and the sample ＥＳ This is expressed by equation 1.

[0023]

[0024] The right-hand side of Equation 1 represents the energy (CV) stored in the capacitance component between the probe and the sample. ２ This is the Z-direction derivative of (2). In equation 1, the voltage V on the right-hand side ａｌｌ When expressed by separating it into its voltage components, the electrostatic force F ＥＳ This is expressed by equation 2.

[0025]

[0026] Here, the first term V in parentheses ＤＣ This is the DC voltage applied to the probe. Term 2 VＣＰＤ This is the DC potential of the sample. Term 3 V ＫＰＦＭ cosω ｔ t is the AC voltage applied to the probe during KPFM measurement. KPFM measurement is optional. KPFM measurement will be described later. Section 4 V ｍ cosω ｍ t is a new AC voltage applied to the probe for SIM signal detection. (Section 5 V) ｓ cosω ｓ t is the AC voltage applied to the sample.

[0027] From Equation 2, the electrostatic force F ＥＳ The frequency components of this signal include a DC component and multiple AC components, and each AC component has a frequency ω ｔ , ω ｍ , ω ｓ , 2ω ｔ , 2ω ｍ , 2ω ｓ , ω ｔ ±ω ｍ , ω ｍ ±ω ｓ , ω ｔ ±ω ｓ It can be understood that each of these is included. Of these AC frequency components, the frequency component measured by the heterodyne method of the present invention is the frequency component f of the new AC voltage applied to the probe for SIM signal detection. ｍ The frequency component f of the AC voltage applied to the sample ｓ The sum frequency component corresponds to ω ｍ +ω ｓ That is the case.

[0028] Therefore, of the voltage components listed in parentheses in Equation 2, the components we want to measure are the cross-term components of the 4th and 5th terms. Extracting only these cross-term components that we want to measure from Equation 2 gives us Equation 3.

[0029]

[0030] Equation 3 contains the frequency f of the new AC voltage applied to the probe. ｍ The frequency f of the AC voltage applied to the sample ｓ The sum frequency component (f ｍ +f ｓ), the term cos(ω ｍ +ω ｓ )t appears. In the present invention, this sum frequency component (f ｍ +f ｓ ) is detected by a heterodyne method.

[0031] In the heterodyne AM method shown in FIG. 1(C), the sum frequency component (f ｍ +f ｓ ) of the mixed signal matches the frequency (f １ -f ０ ), the frequency f ｍ of a new alternating voltage applied to the probe is adjusted. That the sum frequency component (f ｍ +f ｓ ) of the mixed signal matches the frequency (f １ -f ０ ) means that f ｍ +f ｓ =f １ -f ０ , that is, f ｍ =f １ -f ０ -f ｓ is satisfied. Accordingly, the modulation component (sum frequency component) of the mixed signal becomes a frequency component detectable by the heterodyne method.

[0032] The electrostatic force acting on the probe includes the sum frequency component (f ｓ of the alternating voltage applied to the sample and the frequency f ｍ of the new alternating voltage applied to the probe, that is, the sum frequency component (f ｍ +f ｓ ), that is, the sum frequency component (f １ -f ０ ) is included. This sum frequency component (f １ -f ０ ) modulates the frequency f ０ of the primary resonance mode of the probe, and the sum frequency component appears at a frequency f １ that matches the secondary resonance mode of the probe. By detecting the signal of this sum frequency component with a lock-in amplifier, the amplitude and phase of the alternating voltage applied to the sample are extracted. According to this method, even if the frequency f ｓ of the alternating voltage applied to the sample changes greatly, the frequency f ｍ of the new alternating voltage applied to the probe changes greatlyBy adjusting, the sum frequency component matches the secondary resonance frequency f of the probe vibration １ Control can be performed so as to match the above, thereby enabling SIM measurement while maintaining constant measurement sensitivity over a wide measurement frequency band.

[0033] [Configuration of Apparatus] FIG. 2 is a diagram showing a schematic configuration of a scanning impedance microscope (SIM) according to an embodiment of the present invention. In the present embodiment, the distribution of AC voltage inside the sample 9 is visualized by measuring the electrostatic force generated between the conductive probe and the sample by the AM method.

[0034] The scanning impedance microscope 100 according to the present embodiment is a scanning impedance microscope that measures the AC voltage distribution inside the sample 9 while scanning the conductive probe 21 over the sample 9 in a state where an AC voltage is applied to the sample 9, comprising an AC voltage generation unit 10, a signal measurement unit 20, and an AC voltage distribution image generation unit 41.

[0035] The AC voltage generation unit 10 comprises a sample AC voltage generation unit 11 that generates an AC voltage to be applied to the sample 9, and a first probe AC voltage generation unit 12 that generates an AC voltage to be applied to the probe 21. The AC voltage generation unit 10 generates a sample AC voltage and a first probe AC voltage, applies the sample AC voltage to the sample 9, and applies the first probe AC voltage to the probe 21. In the present embodiment, the sample AC voltage generated by the sample AC voltage generation unit 11 is added to the DC voltage output from the potentiostat 101 in an adder 102, and then applied to the sample 9.

[0036] In order to perform SIM measurement by the heterodyne method of the present invention, the frequency of the AC voltage generated by the AC voltage generation unit 10 satisfies the predetermined relationship described in the above measurement principle. In the present embodiment in which SIM measurement is performed by the heterodyne AM method, the frequency f of the first probe AC voltage applied to the probe 21 ｍ , the resonance frequency of vibration of the probe 21, and the frequency f of the sample AC voltage applied to the sample 9 ｓ satisfy the following relationship: f ｍ =f １ −f ０ −f ｓ where, f ０f is the primary resonant frequency of the probe 21, and １ This is the secondary resonant frequency of the probe 21. For example, frequency f ｍ The frequency range is approximately 1 Hz to approximately 400 kHz. ｓ The frequency range is approximately 1 Hz to approximately 380 kHz. ０ For example, this is approximately 70 kHz, and the frequency f １ For example, this is approximately 450 kHz.

[0037] The signal measurement unit 20 measures the amplitude and phase of the sample AC voltage inside the sample 9 by detecting the modulation component (sum frequency component) of the mixed signal S1, which is detected through the probe 21 and is generated by frequency mixing of the sample AC voltage and the first probe AC ​​voltage.

[0038] The signal measurement unit 20, configured for detecting the mixed signal S1 through the probe 21, comprises a conductive probe 21, a probe excitation unit 22, a laser 23, and a photodiode 24. One end of the conductive probe 21 is directed toward the sample 9, and a piezoelectric element is provided at the other end. The probe excitation unit 22 excites the probe 21 at its resonant frequency. The laser 23 irradiates the excited probe 21 with laser light for measurement. The photodiode 24 converts the laser light reflected from the probe 21 into an electric current signal.

[0039] In this embodiment, which performs SIM measurement using the heterodyne AM method, the signal measurement unit 20 measures the sum frequency component (f) of the mixed signal S1. １ -f ０ ) from frequency f １ The system includes a first lock-in amplifier 25 that synchronously detects the signal components. The mixed signal S1 detected through the probe 21 and photodiode 24 is input to the first lock-in amplifier 25 and synchronously detected. In order to enable detection by heterodyne, the frequency f of the AC voltage applied to the probe 21 is ｍ is f ｍ = f １ -f ０ -f ｓ The relationship is satisfied, and the sum frequency component (f) of the mixed signal S1 is satisfied. ｍ +f ｓ ) is the sum frequency component (f １ -f ０)

[0040] The AC voltage distribution image generation unit 41 generates an AC voltage distribution image inside the sample 9 based on the amplitude and phase of the sample AC voltage measured by the signal measurement unit 20. In this embodiment, the AC voltage distribution image generation unit 41 generates an AC voltage distribution image inside the sample 9 based on the signal synchronously detected by the first lock-in amplifier 25.

[0041] ・Configuration for KPFM measurement In this embodiment, surface potential measurement using Kelvin Probe Force Microscopy (KPFM) (hereinafter also simply referred to as KPFM measurement) is performed simultaneously with SIM measurement. Measurement of surface potential by KPFM is optional. By performing KPFM measurement, unwanted electrostatic forces acting between the probe 21 and the sample 9 can be suppressed by feedback control so that the potential of the probe 21 is the same as the surface potential of the sample 9. This improves the accuracy of SIM measurement. The configuration for KPFM measurement will be described below.

[0042] For KPFM measurement, the AC voltage generator 10 further includes a second probe AC ​​voltage generator 13 that generates a second probe AC ​​voltage for modulating the resonant frequency of the probe 21, and an adder 14. The generated second probe AC ​​voltage and the first probe AC ​​voltage are added together and applied to the probe 21. The second probe AC ​​voltage is an AC voltage for KPFM measurement. By applying the second probe AC ​​voltage to the probe 21, the component of the second probe AC ​​voltage is further added to the mixed signal S1 detected through the probe 21, and the resonant frequency of the probe 21 is modulated. Exemplarily, the frequency f of the second probe AC ​​voltage ｋｐｆｍ It is approximately 2 kHz.

[0043] For KPFM measurement, the signal measurement unit 20 further comprises a KPFM measurement unit 30 that measures the surface potential of the sample 9 based on a mixed signal S1 in which a second probe AC ​​voltage is further added, and the scanning impedance microscope 100 further comprises a surface potential distribution image generation unit 42.

[0044] The KPFM measurement unit 30 includes a demodulation unit 31, a second lock-in amplifier 32, and a bias feedback unit 33. The demodulation unit 31 demodulates the sideband component of the mixed signal S1, which is further enhanced by the second probe AC ​​voltage, to generate a demodulated signal S2. The second lock-in amplifier 32 extracts the frequency f of the second probe AC ​​voltage from the demodulated signal S2. ｋｐｆｍ The signal component is synchronously detected. The bias feedback unit 33 feedback-controls the potential of the probe 21 to correspond to the surface potential of the sample 9 based on the signal synchronously detected by the second lock-in amplifier 32. The frequency f of the sample AC voltage applied to the sample 9 ｓ When the frequency is low, approximately 0.1 Hz to 100 Hz, preferably the bias feedback unit 33 controls the potential of the probe 21 to maintain it at a predetermined potential. This is because at low frequencies, the KPFM measurement affects the SIM measurement.

[0045] The surface potential distribution image generation unit 42 generates a surface potential distribution image of the sample 9 based on the surface potential measured by the KPFM measurement unit 30. In this embodiment, the surface potential distribution image generation unit 42 generates a surface potential distribution image of the sample 9 based on the signal input from the bias feedback unit 33.

[0046] As described above, according to the scanning impedance microscope 100 of one embodiment of the present invention, the first probe AC ​​voltage generation unit 12 generates a first probe AC ​​voltage as a new AC voltage for SIM signal detection and applies it to the probe 21. The frequency f of the first probe AC ​​voltage applied to the probe 21 ｍ The resonant frequency f of the vibration of the probe 21 ０ , f １ The frequency f of the AC voltage applied to sample 9 ｓ That is, f ｍ = f １ -f ０ -f ｓ This satisfies the following relationship. As a result, the modulation component (sum frequency component) of the mixed signal S1 detected through the probe 21 is adjusted to become a frequency component detectable by the heterodyne AM method, and the scanning impedance microscope 100 can achieve both improved spatial resolution and a wide measurement frequency band.

[0047] The following examples illustrate the features of the present invention.

[0048] In Example 1, SIM measurement using the heterodyne AM method according to the present invention was actually performed on a sample to generate an AC voltage distribution image within the sample. The AC voltage distribution image was created for both amplitude and phase. The spatial resolution of the generated distribution image (amplitude image) was also compared with an image obtained using a conventional method.

[0049] [Measurement Environment] ・A schematic diagram of the sample to be measured is shown in Figure 3(A). Glass substrate (SiO ２ A gap electrode with a thin Au film (thickness: approximately 100 nm) formed on a surface was used as the sample for measurement. The horizontal gap length between the Au electrodes in the figure was approximately 3 μm.

[0050] • Configuration of the Measurement Device The configuration of each component in the SIM device was as follows, and each component was connected by the wiring shown in Figure 2. The base SIM device used was a Bruker Dimension Icon AFM device. The probe used was a Budget Sensors Multi75E-G. The lock-in amplifier, FM detector, bias feedback, and AC voltage source were all MFLIs from Zurich Instruments. The potentiostat used was a Biologic VSP-300.

[0051] ・Measurement conditions The probe is set to primary resonance (frequency f ０ With the motor excited by the following, the frequency and amplitude of the AC voltage were set as follows.

[0052] AC voltage applied to the sample: frequency f ｓ = 300 kHz to 0.3 kHz, amplitude 300 mV AC voltage applied to the probe for heterodyne detection: frequency f ｍ = f １ -f ０ -f ｓ (Adjustable for each probe), frequency f applied to the probe for amplitude 1V kPFM measurement. ｋｐｆｍ =1kHz, amplitude 2V

[0053] [Measurement Results] ・AC Voltage Distribution Images Figures 4 and 5 show the AC voltage distribution images obtained from the measurement in Example 1. Figure 4 is a distribution image for measurement frequencies from 0.3 kHz to 5 kHz, and Figure 5 is a distribution image for measurement frequencies from 10 kHz to 300 kHz. In Figures 4 and 5, row (A) is a surface shape image representing the surface irregularities of the sample in the z direction. Row (B) is a surface potential distribution image obtained by KPFM measurement. Row (C) is an AC voltage amplitude distribution image obtained by SIM measurement. Row (D) is an AC voltage phase distribution image obtained by SIM measurement. The legend for these distribution images (surface potential, amplitude, phase) is shown in Figure 3(B).

[0054] As shown in rows (C) and (D) of Figure 5, it was confirmed that the SIM measurement using the heterodyne AM method according to the present invention can measure the AC voltage distribution image at 300 kHz, which is about the same as the measurement frequency band (approximately 450 kHz) of conventional AM method SIM measurement.

[0055] - Confirmation of spatial resolution and comparison with conventional methods: The spatial resolution of the AC voltage distribution image obtained by measurement in Example 1 was confirmed. For confirmation, a line profile graph extracted from the amplitude image when the measurement frequency was 300 kHz, as shown in Figure 6, was used.

[0056] In Figure 6, the horizontal axis represents the lateral dimensions of the sample being measured, as shown in Figure 3(A). To facilitate understanding, the upper part of the figure shows the corresponding sample composition (Au or SiO ２ The distinction between the two is indicated, and the boundary between the components of the sample (Au and SiO ２ A dashed line is drawn vertically in the diagram at the boundary between the two points.

[0057] As shown in Figure 6, in an Au gap electrode with a gap length of approximately 3 μm, SiO ２ We were able to confirm that the linear signal distribution on the transition region could be accurately measured.

[0058] The spatial resolution was compared with that of images obtained using a conventional method. The amplitude image of the AC voltage obtained by the measurement in Example 1 has a resolution of at least approximately 3 μm, as shown in Figure 6. In contrast, in the SIM measurement using the conventional AM method reported in Non-Patent Literature 1, the SIM image was found to be blurred on the order of 5 μm or more, and the measurement signal was found to be distorted on the order of approximately 5 μm. Thus, it was confirmed that the spatial resolution of the AC voltage distribution image obtained by the measurement in Example 1 is higher than that of images obtained using the conventional AM method.

[0059] • Discussion of Results Based on the measurement results in Figures 5 and 6, the SIM measurement of Example 1 according to the present invention was able to improve spatial resolution while maintaining a measurement frequency band comparable to that of conventional AM-based SIM measurements.

[0060] 9 Sample 10 AC voltage generation unit 11 Sample AC voltage generation unit 12 First probe AC ​​voltage generation unit 13 Second probe AC ​​voltage generation unit 14 Adder 20 Signal measurement unit 21 Conductive probe 22 Probe excitation unit 23 Laser 24 Photodiode 25 Lock-in amplifier 30 KPFM measurement unit 31 Demodulation unit 32 Lock-in amplifier 33 Bias feedback unit 41 AC voltage distribution image generation unit 42 Surface potential distribution image generation unit 91 AC voltage generation unit 92 Conductive probe 93 Probe excitation unit 94 Laser 95 Photodiode 96 Lock-in amplifier 97 FM detector 98 Image generation unit 100 Scanning impedance microscope 101 Potentiostat 102 Adder

Claims

1. A scanning impedance microscope that measures the distribution of said alternating voltage inside a sample while scanning a conductive probe across said sample in a state where an alternating voltage is applied to the sample, comprising: an alternating voltage generator that generates a sample alternating voltage and a first probe alternating voltage, applies said sample alternating voltage to said sample, and applies said first probe alternating voltage to said probe; and a signal measurement unit that measures the amplitude and phase of said sample alternating voltage inside said sample by detecting a modulation component of a mixed signal detected through said probe, said mixed signal being generated by frequency mixing of said sample alternating voltage and said first probe alternating voltage, wherein said signal measurement unit comprises a first lock-in amplifier that synchronously detects a signal component of frequency f １ from a sum frequency component of said mixed signal, and the frequency f of said first probe alternating voltage ｍ , the resonance frequency of vibration of said probe, and the frequency f of said sample alternating voltage ｓ satisfy the relationship f ｍ =f １ -f ０ -f ｓ , wherein f ０ is the primary resonance frequency of said probe, and f １ is the secondary resonance frequency of said probe. A scanning impedance microscope.

2. The scanning impedance microscope according to claim 1, further comprising an AC voltage distribution image generation unit that generates an AC voltage distribution image within the sample based on the amplitude and phase of the measured AC voltage of the sample.

3. The scanning impedance microscope according to claim 1 or 2, wherein the AC voltage generating unit further generates a second probe AC ​​voltage for modulating the resonant frequency of the probe, adds the generated second probe AC ​​voltage and the first probe AC ​​voltage and applies them to the probe, and the signal measuring unit further comprises a KPFM measuring unit that measures the surface potential of the sample based on the mixed signal into which the second probe AC ​​voltage is further added.

4. The scanning impedance microscope according to claim 3, wherein the KPFM measuring unit comprises: a demodulation unit that demodulates the sideband component of the mixed signal obtained by further adding the second probe AC ​​voltage and generates a demodulated signal; a second lock-in amplifier that synchronously detects the signal component of the frequency of the second probe AC ​​voltage from the demodulated signal; and a bias feedback unit that feedback-controls the potential of the probe to correspond to the surface potential of the sample based on the signal synchronously detected by the second lock-in amplifier.

5. The bias feedback unit controls the frequency f of the sample AC voltage. ｓ The scanning impedance microscope according to claim 4, wherein when the frequency is between 0.1 Hz and 100 Hz, the potential of the probe is controlled to be maintained at a predetermined potential.

6. The scanning impedance microscope according to claim 3, further comprising a surface potential distribution image generation unit that generates a surface potential distribution image of the sample based on the measured surface potential.

Images