**2. The principle, fabrication and evaluation of microwave AFM**

The microwave atomic force microscope (M‐AFM) is a combination of the principles of the scanning probe microscope and the microwave‐measurement technique [28‐32]. M‐AFM can maintain the constant stand‐off distance between the M‐AFM‐probe tip and scanned sample surface, by detecting the deflection of the atomic force between them, and measure the electrical properties of materials with nanometer scale spatial resolution.

Figure 3 shows the schematic diagram of the M‐AFM probe that was used to measure the electrical properties of materials in this study. Different with the normally commercial AFM probes, the M‐AFM probe is having a special structure. That is, a pair of metal films was fabricated on surfaces ofthe M‐AFM cantilever by EB (electron beam) vapor method. The metal films play a role of parallel‐board microwave‐signal waveguide, which can make the micro‐ wave signal propagate in the body of M‐AFM cantilever and emit at the probe‐tip apex. Then, by detecting the response of microwave signal reflected from the vicinity of the sample, the electrical properties can be evaluated on the nanometer scale.

**Figure 4.** Fabricated tip of GaAs probe.

be as very sharp as possible.

to <011> direction.

**Figure 5.** GaAs probe tip with high magnification of apex part. The key component in an AFM is the tip, which should

Micro-Nano Materials Characterization and Inspection 247

In our study, it was found that only the square resist pattern can form a sharp tip (Figure 4 and Figure 5). In the case of hexagonal pattern, the reason that tip was not formed well may be due to the side of the etching mask to be too short. The reason for triangular pattern may due to that there is no crystalline plane parallel to the side of the etching mask. In addition, it was also found that one side of the square mask being 45 to the <011> direction can form a tip with a higher aspectratio comparing with the case of one side ofthe resist pattern being parallel

**Figure 3.** Schematic diagram of the M-AFM probe that was used to measure the electrical properties of materials in this study.

### **2.1. Fabrication of M‐AFM probe**

#### *2.1.1. Fabricating the tip of M‐AFM probe*

To restrain the attenuation of microwave in the M‐AFM probe, GaAs was used as the substrate of the probe. On the other hand, to obtain the desired structure, wet etching was used to fabricate the tip of the probe. Different with the dry etching, a side‐etching will occur under the etching mask. Utilizing this property, a micro tip can be fabricated by etching a wafer, of which a small mask was introduced on the surface in advance. In the case of single crystalline wafer, such as Si and GaAs, the chemical activities are different for different crystalline planes, thereby, the etch rates are also different. Therefore, the side plane obtained at the side of the mask pattern is the most inactive plane (that is the plane having the most low etching speed) which is parallel to the side of the mask pattern. Consequently, the result of etching is strong affected by the direction of mask pattern. On the other hand, GaAs has a sphalerite structure that is more complex than that of Si, which has a similar structure as diamond. Therefore, the prediction of the etch effects is very difficult [28, 29].

**Figure 4.** Fabricated tip of GaAs probe.

Figure 3 shows the schematic diagram of the M‐AFM probe that was used to measure the electrical properties of materials in this study. Different with the normally commercial AFM probes, the M‐AFM probe is having a special structure. That is, a pair of metal films was fabricated on surfaces ofthe M‐AFM cantilever by EB (electron beam) vapor method. The metal films play a role of parallel‐board microwave‐signal waveguide, which can make the micro‐ wave signal propagate in the body of M‐AFM cantilever and emit at the probe‐tip apex. Then, by detecting the response of microwave signal reflected from the vicinity of the sample, the

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

**Figure 3.** Schematic diagram of the M-AFM probe that was used to measure the electrical properties of materials in

To restrain the attenuation of microwave in the M‐AFM probe, GaAs was used as the substrate of the probe. On the other hand, to obtain the desired structure, wet etching was used to fabricate the tip of the probe. Different with the dry etching, a side‐etching will occur under the etching mask. Utilizing this property, a micro tip can be fabricated by etching a wafer, of which a small mask was introduced on the surface in advance. In the case of single crystalline wafer, such as Si and GaAs, the chemical activities are different for different crystalline planes, thereby, the etch rates are also different. Therefore, the side plane obtained at the side of the mask pattern is the most inactive plane (that is the plane having the most low etching speed) which is parallel to the side of the mask pattern. Consequently, the result of etching is strong affected by the direction of mask pattern. On the other hand, GaAs has a sphalerite structure that is more complex than that of Si, which has a similar structure as diamond. Therefore, the

electrical properties can be evaluated on the nanometer scale.

this study.

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**2.1. Fabrication of M‐AFM probe**

*2.1.1. Fabricating the tip of M‐AFM probe*

prediction of the etch effects is very difficult [28, 29].

**Figure 5.** GaAs probe tip with high magnification of apex part. The key component in an AFM is the tip, which should be as very sharp as possible.

In our study, it was found that only the square resist pattern can form a sharp tip (Figure 4 and Figure 5). In the case of hexagonal pattern, the reason that tip was not formed well may be due to the side of the etching mask to be too short. The reason for triangular pattern may due to that there is no crystalline plane parallel to the side of the etching mask. In addition, it was also found that one side of the square mask being 45 to the <011> direction can form a tip with a higher aspectratio comparing with the case of one side ofthe resist pattern being parallel to <011> direction.

### *2.1.2. Fabrication of M‐AFM probe*

The process of probe fabrication is shown in Figure 6 in details: (a) Patterning the etching mask for tip generation; (b) Forming the tips by wet etching; (c) Patterning the stencil mask for the waveguide and evaporating the metal film; (d) Removing resist and film; (e) Patterning the etching mask for the beam of cantilever; (f) Forming the beam of cantilever by wet etching; (g) Patterning the etching mask on back side for the fabrication of the holder; (h) Forming the holder; (i) Evaporation of metal film on the back side; (j) Introducing slit aperture at the tip of the probe.

In the experiment, no doped semi‐insulted GaAs wafer having (100) oriented surface and 350 μm thickness was used. At first, the tips were formed by etching the wafer for 100 seconds to reach the etching depth of 7.7 μm. After that, Au film used to construct the waveguide was evaporated on the substrate. The film thickness was about 50 nm (Figure 6(c)). After the deposition, the pattern of the waveguide was formed by lift‐off process, where the film on the resist mask corresponding to the area without waveguide pattern was removed (Figure 6(d)). Then, in order to form the beam of the cantilever, the beam etching mask was patterned. Here, by considering the chemical activities at different crystalline planes, the length direction of the etching mask was patterned along the <011> direction. In consequence, the side‐etching occurred under the resist mask, and mesa type planes appeared at the both sides of the beam (45º inclined plane). On the other hand, inverse‐mesa type plane was formed at the end of the beam (60‐75º inclined plane). Etching depth of the beam was about 20 μm.

In the same conditions as the beam fabrication process, holder was formed by back side etching (Figure 6(f)). Here, the etching mask was patterned on the bottom surface, and etching was carried out until the substrate was penetrated. The stirring was performed by magnetic stirrer in orderto etch the sampleuniformly.In the step(i) as shown in Figure 6,Aufilm wasdeposited on bottom surface of the probe to propagate a microwave signal in the probe. The thickness of the film was 50 nm, which is the same as that on the top surface of the probe. Both plane surfaces of the waveguide which were evaporated Au film are connected at the end of the beam. However, there is no Au film on the sides of the beam, since the formed inclined planes at the beam sides are not face to the direction of the evaporation. Finally, by using FIB fabrication, a slit at the tip of probe was formed to open the connection of the Au film on the two surfaces ofthe probe. Consequently, a homogeneous parallel plate waveguide was formed and microwaves are able to propagate along the probe and emit at the tip apex of the M‐AFM probe.

**Figure 6.** Fabrication processes of the M-AFM probe. (a) Patterning the etching mask for the generation of tip. (b) Forming the tip by wet etching. (c) Patterning the resist mask for the waveguide. (d) Evaporating the metal film. (e) Removing the resist and metal film. (f) Patterning the etching mask for the beam of cantilever. (g) Forming the beam of cantilever by wet etching. (h) Patterning the etching mask on back side for fabrication of the holder. (i) Forming the

Micro-Nano Materials Characterization and Inspection 249

The SEM images of the fabricated M‐AFM probes are depicted in Figure 7 to Figure 10. Figure 7 shows the SEM photograph ofthe fabricated M‐AFM probes. There 44 probes were fabricated in one process for one substrate. Figure 8 shows the as‐fabricated cantilever of the M‐AFM probe. The dimensions of the M‐AFM probe depend on several small variations of experi‐ mental parameters, including the developing time of the resist pattern, the wet etching rate, and the EB evaporation rate. The average dimensions of the cantilever and the body of the M‐ AFM probes are typically 252×31×14 μm and 2742×723×339 μm, respectively. Thus, the characteristic impedance of the M‐AFM probes is, on average, 49.3 Ω. Figure 9 depicts an SEM photograph of the FIB‐fabricated nano‐slit that has been patterned across the cantilever through the center of the probe tip. The observed tip is located near the front edge of the cantilever. As can be observed in Figure 10, the tip is approximately 7 μm high, and the nano‐

holder. (j) Evaporation of metal film on the back side. (k) Introducing the micro slit at the tip of probe.

**2.2. SEM observation for fabricated M‐AFM probes**

slit is approximately 100 nm in width.

It should be mentioned that dimensions of the GaAs substrate and the Au films of the M‐ AFM probe decide the characteristic impedance of the waveguide, in order to make certainly that microwave signals can propagate properly in the M‐AFM probe for maxi‐ mum sensitivity, the waveguide should have a characteristic impedance of 50 Ω (to match the characteristic impedance of a coaxial transmission line). Thus, the cantilever and the body of the M‐AFM probe were designed with the dimensions of 250×30×15 μm and 2740×720×340 μm respectively.

**Figure 6.** Fabrication processes of the M-AFM probe. (a) Patterning the etching mask for the generation of tip. (b) Forming the tip by wet etching. (c) Patterning the resist mask for the waveguide. (d) Evaporating the metal film. (e) Removing the resist and metal film. (f) Patterning the etching mask for the beam of cantilever. (g) Forming the beam of cantilever by wet etching. (h) Patterning the etching mask on back side for fabrication of the holder. (i) Forming the holder. (j) Evaporation of metal film on the back side. (k) Introducing the micro slit at the tip of probe.

#### **2.2. SEM observation for fabricated M‐AFM probes**

*2.1.2. Fabrication of M‐AFM probe*

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the probe.

probe.

2740×720×340 μm respectively.

The process of probe fabrication is shown in Figure 6 in details: (a) Patterning the etching mask for tip generation; (b) Forming the tips by wet etching; (c) Patterning the stencil mask for the waveguide and evaporating the metal film; (d) Removing resist and film; (e) Patterning the etching mask for the beam of cantilever; (f) Forming the beam of cantilever by wet etching; (g) Patterning the etching mask on back side for the fabrication of the holder; (h) Forming the holder; (i) Evaporation of metal film on the back side; (j) Introducing slit aperture at the tip of

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

In the experiment, no doped semi‐insulted GaAs wafer having (100) oriented surface and 350 μm thickness was used. At first, the tips were formed by etching the wafer for 100 seconds to reach the etching depth of 7.7 μm. After that, Au film used to construct the waveguide was evaporated on the substrate. The film thickness was about 50 nm (Figure 6(c)). After the deposition, the pattern of the waveguide was formed by lift‐off process, where the film on the resist mask corresponding to the area without waveguide pattern was removed (Figure 6(d)). Then, in order to form the beam of the cantilever, the beam etching mask was patterned. Here, by considering the chemical activities at different crystalline planes, the length direction of the etching mask was patterned along the <011> direction. In consequence, the side‐etching occurred under the resist mask, and mesa type planes appeared at the both sides of the beam (45º inclined plane). On the other hand, inverse‐mesa type plane was formed at the end of the

In the same conditions as the beam fabrication process, holder was formed by back side etching (Figure 6(f)). Here, the etching mask was patterned on the bottom surface, and etching was carried out until the substrate was penetrated. The stirring was performed by magnetic stirrer in orderto etch the sampleuniformly.In the step(i) as shown in Figure 6,Aufilm wasdeposited on bottom surface of the probe to propagate a microwave signal in the probe. The thickness of the film was 50 nm, which is the same as that on the top surface of the probe. Both plane surfaces of the waveguide which were evaporated Au film are connected at the end of the beam. However, there is no Au film on the sides of the beam, since the formed inclined planes at the beam sides are not face to the direction of the evaporation. Finally, by using FIB fabrication, a slit at the tip of probe was formed to open the connection of the Au film on the two surfaces ofthe probe. Consequently, a homogeneous parallel plate waveguide was formed and microwaves are able to propagate along the probe and emit at the tip apex of the M‐AFM

It should be mentioned that dimensions of the GaAs substrate and the Au films of the M‐ AFM probe decide the characteristic impedance of the waveguide, in order to make certainly that microwave signals can propagate properly in the M‐AFM probe for maxi‐ mum sensitivity, the waveguide should have a characteristic impedance of 50 Ω (to match the characteristic impedance of a coaxial transmission line). Thus, the cantilever and the body of the M‐AFM probe were designed with the dimensions of 250×30×15 μm and

beam (60‐75º inclined plane). Etching depth of the beam was about 20 μm.

The SEM images of the fabricated M‐AFM probes are depicted in Figure 7 to Figure 10. Figure 7 shows the SEM photograph ofthe fabricated M‐AFM probes. There 44 probes were fabricated in one process for one substrate. Figure 8 shows the as‐fabricated cantilever of the M‐AFM probe. The dimensions of the M‐AFM probe depend on several small variations of experi‐ mental parameters, including the developing time of the resist pattern, the wet etching rate, and the EB evaporation rate. The average dimensions of the cantilever and the body of the M‐ AFM probes are typically 252×31×14 μm and 2742×723×339 μm, respectively. Thus, the characteristic impedance of the M‐AFM probes is, on average, 49.3 Ω. Figure 9 depicts an SEM photograph of the FIB‐fabricated nano‐slit that has been patterned across the cantilever through the center of the probe tip. The observed tip is located near the front edge of the cantilever. As can be observed in Figure 10, the tip is approximately 7 μm high, and the nano‐ slit is approximately 100 nm in width.

**Figure 7.** The fabricated M-AFM probes.

**Figure 9.** The 100-nm-wide-FIB-fabricated nano-slit that is across the cantilever and through the center of the tip.

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**Figure 10.** The high-magnification image of the M-AFM-probe tip.

**Figure 8.** The cantilever of the M-AFM probe.

**Figure 9.** The 100-nm-wide-FIB-fabricated nano-slit that is across the cantilever and through the center of the tip.

**Figure 10.** The high-magnification image of the M-AFM-probe tip.

**Figure 7.** The fabricated M-AFM probes.

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**Figure 8.** The cantilever of the M-AFM probe.

## **2.3. Measuring topography by M‐AFM probe**

In order to confirm the spatial resolution of the fabricated M‐AFM probes, the AFM topogra‐ phy of two grating samples having 2000 line/mm and 17.9 nm step height were measured by a commercial Si AFM probe, a GaAs probe without nano‐slit and a M‐AFM probe with nano‐ slit, respectively.


**Table 1.** The properties of AFM probes in the atmosphere.

A JSPM‐5400 was used for measurement of the sample under the noncontact mode (frequency modulation (FM) mode). The properties of three kinds of probe are given in Table 1, the resonance frequency was swept and the *Q* value was defined by the follow‐ ing relation, *Q*=*f*0/(*f*+‐*f*‐ ), where *f*<sup>0</sup> is the peak frequency, *f*<sup>+</sup> and *f*‐ the shifted frequency from *f*<sup>0</sup> at 70.7% of peak intensity. The *Q* value indicates a resonance sharpness of the cantile‐ ver; the higher the *Q* value, the better stabilization of the oscillation.

**Figure 11.** Surface topography of the grating sample obtained by the commercial Si probe.

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**Figure 12.** Surface topography of the grating sample obtained by the GaAs probe without the nano-slit.

Figures 11 to 13 show the topographies of the standard sample having 2000 lines/mm obtained by the commercial Si probe, GaAs probe without the nano‐slit, and M‐AFM probe with the nano‐slit underthe non‐contact mode,respectively. The measurements were performed in the air, and the AFM worked in non‐contact mode, with a working environment temperature of 25.0 °C anda relative humidity of 50%.The resonance frequency of Siprobe andM‐AFMprobes (without nano‐slit and with nano‐slit) were 262 kHz, 185 kHz and 201 kHz, respectively, and the *Q*‐value of them were 370, 510 and 333. The scan area was 2×2 μm2 , scanning speed was 3 μm/s, and the white spots in these figures are due to micro‐dust on the sample surface. Even though the *Q*‐value is lowerthan that of the GaAs probe without the nano‐slit, the commercial Si probe still can obtain a little higher resolution topography due to the higher aspect ratio of the tip.

Comparing the obtained images of Figure 12, Figure 13 with the ones in Figure 11, the results illustrate that M‐AFM probe has a similar capability for sensing surface topography of materials as that of commercial AFM probes.

**Figure 11.** Surface topography of the grating sample obtained by the commercial Si probe.

**2.3. Measuring topography by M‐AFM probe**

**Probe The resonance frequency**

**Table 1.** The properties of AFM probes in the atmosphere.

materials as that of commercial AFM probes.

slit, respectively.

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A GaAs probe without the

A M-AFM probe with the

ing relation, *Q*=*f*0/(*f*+‐*f*‐

nano-slit

nano-slit

the tip.

In order to confirm the spatial resolution of the fabricated M‐AFM probes, the AFM topogra‐ phy of two grating samples having 2000 line/mm and 17.9 nm step height were measured by a commercial Si AFM probe, a GaAs probe without nano‐slit and a M‐AFM probe with nano‐

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

A commercial Si probe 262 370 Typical value: 42

A JSPM‐5400 was used for measurement of the sample under the noncontact mode (frequency modulation (FM) mode). The properties of three kinds of probe are given in Table 1, the resonance frequency was swept and the *Q* value was defined by the follow‐

*f*<sup>0</sup> at 70.7% of peak intensity. The *Q* value indicates a resonance sharpness of the cantile‐

Figures 11 to 13 show the topographies of the standard sample having 2000 lines/mm obtained by the commercial Si probe, GaAs probe without the nano‐slit, and M‐AFM probe with the nano‐slit underthe non‐contact mode,respectively. The measurements were performed in the air, and the AFM worked in non‐contact mode, with a working environment temperature of 25.0 °C anda relative humidity of 50%.The resonance frequency of Siprobe andM‐AFMprobes (without nano‐slit and with nano‐slit) were 262 kHz, 185 kHz and 201 kHz, respectively, and

μm/s, and the white spots in these figures are due to micro‐dust on the sample surface. Even though the *Q*‐value is lowerthan that of the GaAs probe without the nano‐slit, the commercial Si probe still can obtain a little higher resolution topography due to the higher aspect ratio of

Comparing the obtained images of Figure 12, Figure 13 with the ones in Figure 11, the results illustrate that M‐AFM probe has a similar capability for sensing surface topography of

ver; the higher the *Q* value, the better stabilization of the oscillation.

the *Q*‐value of them were 370, 510 and 333. The scan area was 2×2 μm2

**(kHz) Q-value Spring constant (N/m)**

185 510 Typical value: 134

201 333 Typical value: 134

), where *f*<sup>0</sup> is the peak frequency, *f*<sup>+</sup> and *f*‐ the shifted frequency from

, scanning speed was 3

**Figure 12.** Surface topography of the grating sample obtained by the GaAs probe without the nano-slit.

**Figure 13.** Surface topography of the grating sample obtained by the M-AFM probe with the nano-slit.

**Figure 15.** Topography of the grating sample obtained by the commercial Si probe.

M‐AFM probe.

In order to evaluate the accuracy of height measurement, a grating sample having 17.9 nm ±1nm step height was measured by using the commercial Si probe and the M‐AFM probe, respectively. Figures 14 and 15 show the AFM topographies and the cross‐section profiles of the grating sample obtained by the commercial Si probe and the fabricated M‐AFM probe, respectively. From the slope of the step of the cross‐section profile in the figures, it is confirmed that the fabricated M‐AFM probes have the capability to catch the AFM topography with the resolution of nanometer order. The height of the step of the grating sample obtained by each probe was 19.17 nm and 19.67 nm, respectively. The fabricated M‐AFM probe has also high resolution although the resolution was inferior as compared to the commercial Si probe. The reason why the resolution degraded with the fabricated probe is that the tip of the probes was cut by FIB fabrication. From these results, it is considered that the control of the standoff distance between the probe and the sample with high precision was achieved by the fabricated

Micro-Nano Materials Characterization and Inspection 255

**Figure 14.** Topography of the grating sample obtained by the commercial Si probe.

**Figure 15.** Topography of the grating sample obtained by the commercial Si probe.

**Figure 13.** Surface topography of the grating sample obtained by the M-AFM probe with the nano-slit.

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**Figure 14.** Topography of the grating sample obtained by the commercial Si probe.

In order to evaluate the accuracy of height measurement, a grating sample having 17.9 nm ±1nm step height was measured by using the commercial Si probe and the M‐AFM probe, respectively. Figures 14 and 15 show the AFM topographies and the cross‐section profiles of the grating sample obtained by the commercial Si probe and the fabricated M‐AFM probe, respectively. From the slope of the step of the cross‐section profile in the figures, it is confirmed that the fabricated M‐AFM probes have the capability to catch the AFM topography with the resolution of nanometer order. The height of the step of the grating sample obtained by each probe was 19.17 nm and 19.67 nm, respectively. The fabricated M‐AFM probe has also high resolution although the resolution was inferior as compared to the commercial Si probe. The reason why the resolution degraded with the fabricated probe is that the tip of the probes was cut by FIB fabrication. From these results, it is considered that the control of the standoff distance between the probe and the sample with high precision was achieved by the fabricated M‐AFM probe.

> 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.

> 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

> 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

> 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

, scanning speed was 5

Micro-Nano Materials Characterization and Inspection 257

133 kHz and the *Q*‐value of it was 295. The scan area was 10×10 μm2

from the Au area with the initial offset from the nulling operation.

The resulting Au and glass step structure is depicted in SEM image of Figure 17.

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

200 nm on average.

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