**3. Microwave imaging for materials on nanometer‐scale**

## **3.1. Experimental setup**

Figure 16 schematically depicts the integrated test system of the M‐AFM [31]. In our M‐AFM system, the initial microwave signals, which are working at a frequency f = 16.66 GHz, are generated by a microwave generator. Next, the frequency of the microwave signals is extended by a six‐frequency multiplier, which results in a stable testing frequency f = 94 GHz. The microwave signals propagate through an isolator and a circulator and then propagate into the M‐AFM probe. The transmission line that connects the circulator and the probe changes from a rectangular waveguide into a coaxial line, which then changes into the parallel‐plate waveguide (in the M‐AFM probe). A detector is connected to the circulator, to measure the microwave signals that are received by the tip of the probe and indicate the voltage data that are converted from the reflected microwave signals. The measured signals are synchronized with positional information thatis obtained from the AFM scanner, which is then used to create a microwave image. At the same time, by evaluating the output voltage data, the electrical properties of the measured materials can be determined.

**Figure 16.** Diagram of the M-AFM system.

We prepared a sample for the scanning test of surface topography and microwave imaging. At first, a resist mask was patterned onto the glass substrate wafer by lithography. After developing the resist pattern, a 200‐nm thick Au layer was deposited on the glass substrate by electron beam (EB) evaporation. Finally, the unexposed photo‐resists were lifted off in acetone. The resulting Au and glass step structure is depicted in SEM image of Figure 17.

**Figure 17.** SEM image of measured sample.

**3. Microwave imaging for materials on nanometer‐scale**

properties of the measured materials can be determined.

**Figure 16.** Diagram of the M-AFM system.

Figure 16 schematically depicts the integrated test system of the M‐AFM [31]. In our M‐AFM system, the initial microwave signals, which are working at a frequency f = 16.66 GHz, are generated by a microwave generator. Next, the frequency of the microwave signals is extended by a six‐frequency multiplier, which results in a stable testing frequency f = 94 GHz. The microwave signals propagate through an isolator and a circulator and then propagate into the M‐AFM probe. The transmission line that connects the circulator and the probe changes from a rectangular waveguide into a coaxial line, which then changes into the parallel‐plate waveguide (in the M‐AFM probe). A detector is connected to the circulator, to measure the microwave signals that are received by the tip of the probe and indicate the voltage data that are converted from the reflected microwave signals. The measured signals are synchronized with positional information thatis obtained from the AFM scanner, which is then used to create a microwave image. At the same time, by evaluating the output voltage data, the electrical

Micro-Nano Mechatronics — New Trends in Material, Measurement, Control, Manufacturing and Their Applications in

We prepared a sample for the scanning test of surface topography and microwave imaging. At first, a resist mask was patterned onto the glass substrate wafer by lithography. After

**3.1. Experimental setup**

Biomedical Engineering

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### **3.2. Microwave image of Au/glass step sample**

Figures 18 and 19 depict the M‐AFM scanning results of the sample at the step area between the Au coating film and the glass wafer substrate. The measurements were performed in the air, and the M‐AFM worked in non‐contact mode, with a working environment temperature of 24.5 °C and a relative humidity of 38.4%. The resonance frequency of M‐AFM probe was 133 kHz and the *Q*‐value of it was 295. The scan area was 10×10 μm2 , scanning speed was 5 μm/s. Figure 18 depicts the surface topography ofthe M‐AFM‐measured sample.In this image, the left side represents the Au film, whereas the right side is the glass substrate. As can be seen in the scanning profile depicted in Figure 18, the thickness of the Au film was approximately 200 nm on average.

Figure 19 depicts the microwave image of the voltage that was converted from the measured microwave signals, which were simultaneously acquired by the M‐AFM probe at the corre‐ sponding position depicted in Figure 18. This experimental result demonstrates that the microwave image has two spatial phases. Because the standoff distance between the tip of the M‐AFM probe and the surfaces of the Au film and glass substrate is constant and controlled by the atomic force,thus,the response ofthe microwave signals were observed to change based on the different electrical characteristics ofthe measured materials. As per Figure 19,the output voltage over the glass area is larger than that over the Au area, because the scanning started from the Au area with the initial offset from the nulling operation.

> An analysis of the scanning profile depicted in Figure 19 demonstrates that the spatial resolution is higher than 120 nm, and that the output voltage measured over the Au and glass areas were 262.5 mV and 281.7 mV, respectively. The difference between the measured voltages between the Au and glass areas is 19.2 mV. Since, the stability of the measurement is high,this value is large enough for evaluating the electrical properties of other materials having the conductivity between Au and glass. As the results presented, the M‐AFM should allow us to scan the electrical conductivities of other conductor materials on a nanometer scale. In addition, based on the same principle, it can also be used to measure the permittivities of

Micro-Nano Materials Characterization and Inspection 259

**4. Quantitative measurement ofthe electrical properties of materials on the**

For quantitative measurement, the operating frequency of M‐AFM is set at 94 GHz. The high‐ frequency microwaves are easy to propagate in the waveguide and emit from the nano‐slit on the probe tip. Since the width of the nano‐slit is around 100 nm, the field of microwave interacting with the measured materials can be considered to be in 100 nm order. Thus, if the thickness of measured materials is larger than 100 nm, the reflection from the bottom surface of the sample can be neglected. Therefore, only the reflection from the top surface needs to be

Moreover, the diode detector works in a small signal range, where it is considered to be a square‐law detector. Therefore, while keeping the standoff distance between the tip of the M‐ AFM probe and samples constant, the output reflected voltage V, which varies only with the conductivity ofthe sample, has a relationship with the squaredabsolute value ofthe top surface

The two undetermined constants *k*<sup>0</sup> and *b*<sup>0</sup> can be calibrated with two samples whose conduc‐ tivities are known. For good conductors, which are used in this experiment, the surface

> 1 / 1 / *<sup>s</sup>*

 

 

 

where *ε*<sup>0</sup> and *σ* represent permittivity of free space and the conductivity of the measured material, respectively, and *ω* is the angular frequency of the microwave. For semiconductor

0 0

*j j*

<sup>2</sup> *Vk b* 0 0 *<sup>s</sup>* (1)

(2)

dielectric materials on a nanometer scale.

**nanometer‐scale**

considered.

reflection coefficient, |*Γ<sup>s</sup>* | <sup>2</sup> as

reflection coefficient |*Γ<sup>s</sup>* | can be written as [34]

**Figure 18.** AFM topography image of the Au/Glass step sample.

**Figure 19.** Microwave image of the output voltage that was converted from the measured microwave signals.

An analysis of the scanning profile depicted in Figure 19 demonstrates that the spatial resolution is higher than 120 nm, and that the output voltage measured over the Au and glass areas were 262.5 mV and 281.7 mV, respectively. The difference between the measured voltages between the Au and glass areas is 19.2 mV. Since, the stability of the measurement is high,this value is large enough for evaluating the electrical properties of other materials having the conductivity between Au and glass. As the results presented, the M‐AFM should allow us to scan the electrical conductivities of other conductor materials on a nanometer scale. In addition, based on the same principle, it can also be used to measure the permittivities of dielectric materials on a nanometer scale.
