**1. Introduction**

The individual surface atoms of flat samples could be made visible in real space until the introduction of the Scanning Tunneling Microscope (STM) in 1981 by Binnig, Rohrer, Gerber, and Weibel [1]. This powerful instrument has provided a breakthrough in our possibilities to investigate matter on the atomic scale. Within one year of its invention, the STM has helped to solve one of the most intriguing problems in surface science: the structure of the Si surface. Because of theirfabulous contribution, G. Binnig and H. Rohrer were rewarded with the Nobel Prize in physics in 1986. A huge number of conductors and semiconductors have been investigated on the atomic scale and marvelous images of this world of atoms have been created within the first few years after the inception of the STM. Today, the STM is an invaluable asset in the surface scientist's toolbox.

Despite the great success of the STM, it has a serious limitation. The STM requires electrical conduction of the sample material, because the STM needs the tunneling current which is flowing between a pin contact with or very nearing the sample. Thus, the STM can only image electrically conductive samples, which limits its application to imaging metals and semiconductors. But even conductors except for a few special materials, like highly oriented pyrolytic graphite can not be studied in ambient conditions by STM but have to be investigated in an ultra‐high vacuum (UHV). In ambient conditions, the surface layer of solids constantly changes by adsorption and desorption of atoms and molecules. UHV is required for clean and well defined surfaces. Taking the above condition into account, Binnig speculated the atomic force between the tip and sample, the Atomic Force Micro‐ scope (AFM) [2, 3] was invented by him in 1986. Because electrical conductivity of the sample is not required in AFM, the AFM can image virtually any solid surface without the need for surface preparation. Consequently, thousands of AFMs are in use in universi‐ ties, public and industrial research laboratories all over the world.

#### **1.1. Principle of atomic force microscope**

The AFM consists of a cantilever with a sharp probe‐tip at its end that is used to scan the specimen surface (see Figure 1). The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hookeʹs law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, etc. Along with force, additional quantities may simulta‐ neously be measured through the use of specialized types of probe. The deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. *1.2.1. Reflection method*

sample loading.

often specially designed.

*1.2.2. Near‐field scanning probe*

In a reflection method, the properties of a sample are obtained from the reflection due to the impedance discontinuity caused by the presence of the sample in a transmission structure. The reflection method is a type of non‐resonant method. From the view of transmission line, in a reflection method, the sample under test is introduced into a certain position of a transmission line, and so the impedance loading to the transmission line is changed. The properties of the sample are derived from the reflection due to the impedance discontinuity caused by the

In a reflection method (see Figure 2), the measurement fixture made from a transmission line is usually called measurement probe or sensor. In order to increase the measurement accuracy and sensitivity, or to satisfy special measurement requirements, the measurement probes are

Sample Incident

wave

**Figure 2.** Boundary condition for material characterization using a non-resonant method.

are obtained from the reflectivity due to the presence of the sample.

In the reflection methods, there is a special method named near‐field scanning probe should be introduced. Scanning techniques for local characterization of conducting and insulating films are attracting much interest. Many efforts have been made on developing microwave near‐field scanning techniques, and various types of near‐filed microwave microscopes have been developed for different purposed. Underthe reflection method,the properties of a sample

In principle, any type of transmission lines can be used to develop near‐field microwave microscopes. In a near‐field microwave microscope developed form parallel‐board wave‐ guide, the most important part is an aperture in the form of a narrow slit (the following

Reflected wave

> Transmitted wave

Micro-Nano Materials Characterization and Inspection 243

**Figure 1.** Atomic Force Microscope.

#### **1.2. Microwave technique for materials characterization**

The microwave methods for materials characterization generally fall into resonant methods and non‐resonant methods. Resonant methods are used to get knowledge of dielectric properties at single frequency or several discrete frequencies, while non‐resonant methods are often used to get a general knowledge of electromagnetic properties over a frequency range. By modifying the general knowledge of electrical properties over a certain frequency range obtained from non‐resonant methods with the accurate knowledge of electrical properties at several discrete frequencies obtained from resonant methods, accurate knowledge of materials properties over a frequency range can be obtained.

### *1.2.1. Reflection method*

**1.1. Principle of atomic force microscope**

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242

**Figure 1.** Atomic Force Microscope.

**1.2. Microwave technique for materials characterization**

properties over a frequency range can be obtained.

The microwave methods for materials characterization generally fall into resonant methods and non‐resonant methods. Resonant methods are used to get knowledge of dielectric properties at single frequency or several discrete frequencies, while non‐resonant methods are often used to get a general knowledge of electromagnetic properties over a frequency range. By modifying the general knowledge of electrical properties over a certain frequency range obtained from non‐resonant methods with the accurate knowledge of electrical properties at several discrete frequencies obtained from resonant methods, accurate knowledge of materials

The AFM consists of a cantilever with a sharp probe‐tip at its end that is used to scan the specimen surface (see Figure 1). The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hookeʹs law. Depending on the situation, forces that are measured in AFM include mechanical contact force, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, etc. Along with force, additional quantities may simulta‐ neously be measured through the use of specialized types of probe. The deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.

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

In a reflection method, the properties of a sample are obtained from the reflection due to the impedance discontinuity caused by the presence of the sample in a transmission structure. The reflection method is a type of non‐resonant method. From the view of transmission line, in a reflection method, the sample under test is introduced into a certain position of a transmission line, and so the impedance loading to the transmission line is changed. The properties of the sample are derived from the reflection due to the impedance discontinuity caused by the sample loading.

In a reflection method (see Figure 2), the measurement fixture made from a transmission line is usually called measurement probe or sensor. In order to increase the measurement accuracy and sensitivity, or to satisfy special measurement requirements, the measurement probes are often specially designed.

**Figure 2.** Boundary condition for material characterization using a non-resonant method.

### *1.2.2. Near‐field scanning probe*

In the reflection methods, there is a special method named near‐field scanning probe should be introduced. Scanning techniques for local characterization of conducting and insulating films are attracting much interest. Many efforts have been made on developing microwave near‐field scanning techniques, and various types of near‐filed microwave microscopes have been developed for different purposed. Underthe reflection method,the properties of a sample are obtained from the reflectivity due to the presence of the sample.

In principle, any type of transmission lines can be used to develop near‐field microwave microscopes. In a near‐field microwave microscope developed form parallel‐board wave‐ guide, the most important part is an aperture in the form of a narrow slit (the following mentioned nano‐slit plays this role in M‐AFM probe). When a sample surface is in the near‐ filed zone of the slit, the microwave is reflected mostly from the region under the slit. Since reflection from a sample surface is determined by the resistivity, by measuring the amplitude and phase of the reflected wave while raster scanning the surface, it is possible to map the microwave resistivity of the surface. For conductive layer with thicknesses much larger that the skin depth, we can get surface impedance, while for thin layer, we can get sheet resistance. In the determination of microwave resistivity, it is necessary to measure layer thickness independently.

the electrical properties of the material [17]. Thus, to obtain the microscopic electrical infor‐ mation, a variety of microwave microscopes have been developed [18, 19]. Steinhauer et al. developed a non‐destructive and non‐invasive near‐field scanning microwave microscope (NSMM), which can image the local permittivity and tenability of a dielectric thin film with a spatial resolution of 1 μm [20]. Zhang and co‐authors improved the NSMM to investigate the local perpendicular dielectric information of single‐phase multi‐ferroic thin films and single crystal materials [21]. Ferd Duewer et al. introduced scanning evanescent microwave micro‐ scopy (SEMM) [22, 23], which measures the changes of the tip‐sample capacitance at the resonant frequency and the quality factor of microwave absorption. They succeeded in imaging the topography and surface resistance of metallic samples. However, to evaluate the electrical properties of materials using microwaves, it is necessary to keep the stand‐off distance between the microwave probe and the sample constant because microwave signals in the near‐field are extremely sensitive to this distance. Otherwise, it would be difficult to distinguish whether the changes in the signal are due to the difference of the material prop‐ erties or the variation of the stand‐off distance. In particular, to evaluate the electrical proper‐ ties of materials with high resolution on the nanometer scale, it is indispensable to control the

Micro-Nano Materials Characterization and Inspection 245

Recently, to solve the problem of how microwave signals are affected by the stand‐off distance, a technique of combining AFM with microwave microscopy has been studied [24‐27]. K. Lai et al. invented a microwave impedance microscope (MIM) [24, 25], which fed a microwave signal to a silicon nitride cantilever with a Pt tip that was used to investigate the nano‐scale dielectric inhomogeneity in a non‐invasive manner. The Weide group combined an NSMM with an AFM (NSMM‐AFM) [26, 27] by adding a microwave signal to a commercial probe. The NSMM‐AFM can measure the topography and dielectric constant of thin film simultane‐ ously. However, it is noted that MIM and NSMM‐AFM do not use matched probes or cantilevers as the microwave‐guide connected with the source of microwave signals. Thus, the microwave signals may not propagate along the probe and emit from the tip apex of the probe. Therefore, these techniques can only measure the changes of the probe‐sample system

To summarize, these AFM‐based methodologies and microwave microscopy techniques can only image relative electrical properties, rather than the absolute values of the intrinsic electrical properties, such as the conductivity, permittivity, and permeability. Thus, the need remains for a microscopy technique that can provide a simultaneous measurement of topog‐

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

impedance but not the intrinsic electrical properties of the measured materials.

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

electrical properties of materials with nanometer scale spatial resolution.

stand‐off distance precisely on the order of nanometers.

raphy and electrical properties on the nanometer scale.

### **1.3. Developed AFM‐based and microwave technique for measuring the electrical properties**

Electrical properties are the most significant intrinsic characteristics of substances; they strongly affect the work functions of different materials, especially in nanometer‐scale materials and devices. Thus, measuring electrical properties has become an urgent need in many areas of modern technology. For instance, in the electronics industry, critical feature sizes are becoming smaller, and it is necessary to evaluate the electrical properties of the materials with the spatialresolution on a nanometer scale to establish the knowledge to predict the behavior of materials in real devices. In addition, newly developed materials, such as conducting plastic thin films and biomaterials, which may possibly have some uncertain physical properties, will be important in the field of surface science and biological applications. Despite being intensely studied for years, their electrical properties, especially their conduc‐ tivity and permittivity, are still poorly understood.

As the first section of this chapter saying, atomic force microscopy (AFM) has played an important role in nano‐scale science and technology because it is one of the most versatile instruments available for imaging and manipulating structures on the nanometer scale [4‐7]. Several attempts based on atomic force microscopy have been made to characterize the electrical information of materials on the nanometer scale, such as conducting atomic force microscopy (C‐AFM) [8, 9], scanning capacitance microscopy (SCM) [10, 11] and electrostatic force microscopy (EFM) [12, 13]. Although C‐AFM can produce a nano‐scale electrical characterization of thin‐films, the AFM tip must contact the conducting substrate to apply a current, so during the probing process, the AFM tip will scratch the surface of the sample. SCM can characterize electrical information by measuring the capacitance between the tip of the probe and the sample. However, it suffers from a limited spatial resolution and is sensitive to the thickness of the specimen. EFM, including Kelvin probe force microscopy (KFM) [14], scanning surface potential microscopy (SSPM) [15] and scanning Maxwell‐stress microscope (SMM) [16], can measure the surface electrical potential of materials by detecting the electro‐ static force between the probe tip and the sample. However, the van der Waals forces and chemical bonding forces, as well as the electrostatic forces are included in the measured data. Hence, the sample surface chemistry and atmospheric conditions greatly impact the measured electrical potential.

On the other hand, microwave measurements have been of great interest to many researchers because microwaves can propagate easily in air, and the sample response is directly related to the electrical properties of the material [17]. Thus, to obtain the microscopic electrical infor‐ mation, a variety of microwave microscopes have been developed [18, 19]. Steinhauer et al. developed a non‐destructive and non‐invasive near‐field scanning microwave microscope (NSMM), which can image the local permittivity and tenability of a dielectric thin film with a spatial resolution of 1 μm [20]. Zhang and co‐authors improved the NSMM to investigate the local perpendicular dielectric information of single‐phase multi‐ferroic thin films and single crystal materials [21]. Ferd Duewer et al. introduced scanning evanescent microwave micro‐ scopy (SEMM) [22, 23], which measures the changes of the tip‐sample capacitance at the resonant frequency and the quality factor of microwave absorption. They succeeded in imaging the topography and surface resistance of metallic samples. However, to evaluate the electrical properties of materials using microwaves, it is necessary to keep the stand‐off distance between the microwave probe and the sample constant because microwave signals in the near‐field are extremely sensitive to this distance. Otherwise, it would be difficult to distinguish whether the changes in the signal are due to the difference of the material prop‐ erties or the variation of the stand‐off distance. In particular, to evaluate the electrical proper‐ ties of materials with high resolution on the nanometer scale, it is indispensable to control the stand‐off distance precisely on the order of nanometers.

mentioned nano‐slit plays this role in M‐AFM probe). When a sample surface is in the near‐ filed zone of the slit, the microwave is reflected mostly from the region under the slit. Since reflection from a sample surface is determined by the resistivity, by measuring the amplitude and phase of the reflected wave while raster scanning the surface, it is possible to map the microwave resistivity of the surface. For conductive layer with thicknesses much larger that the skin depth, we can get surface impedance, while for thin layer, we can get sheet resistance. In the determination of microwave resistivity, it is necessary to measure layer thickness

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

**1.3. Developed AFM‐based and microwave technique for measuring the electrical**

tivity and permittivity, are still poorly understood.

Electrical properties are the most significant intrinsic characteristics of substances; they strongly affect the work functions of different materials, especially in nanometer‐scale materials and devices. Thus, measuring electrical properties has become an urgent need in many areas of modern technology. For instance, in the electronics industry, critical feature sizes are becoming smaller, and it is necessary to evaluate the electrical properties of the materials with the spatialresolution on a nanometer scale to establish the knowledge to predict the behavior of materials in real devices. In addition, newly developed materials, such as conducting plastic thin films and biomaterials, which may possibly have some uncertain physical properties, will be important in the field of surface science and biological applications. Despite being intensely studied for years, their electrical properties, especially their conduc‐

As the first section of this chapter saying, atomic force microscopy (AFM) has played an important role in nano‐scale science and technology because it is one of the most versatile instruments available for imaging and manipulating structures on the nanometer scale [4‐7]. Several attempts based on atomic force microscopy have been made to characterize the electrical information of materials on the nanometer scale, such as conducting atomic force microscopy (C‐AFM) [8, 9], scanning capacitance microscopy (SCM) [10, 11] and electrostatic force microscopy (EFM) [12, 13]. Although C‐AFM can produce a nano‐scale electrical characterization of thin‐films, the AFM tip must contact the conducting substrate to apply a current, so during the probing process, the AFM tip will scratch the surface of the sample. SCM can characterize electrical information by measuring the capacitance between the tip of the probe and the sample. However, it suffers from a limited spatial resolution and is sensitive to the thickness of the specimen. EFM, including Kelvin probe force microscopy (KFM) [14], scanning surface potential microscopy (SSPM) [15] and scanning Maxwell‐stress microscope (SMM) [16], can measure the surface electrical potential of materials by detecting the electro‐ static force between the probe tip and the sample. However, the van der Waals forces and chemical bonding forces, as well as the electrostatic forces are included in the measured data. Hence, the sample surface chemistry and atmospheric conditions greatly impact the measured

On the other hand, microwave measurements have been of great interest to many researchers because microwaves can propagate easily in air, and the sample response is directly related to

independently.

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244

electrical potential.

**properties**

Recently, to solve the problem of how microwave signals are affected by the stand‐off distance, a technique of combining AFM with microwave microscopy has been studied [24‐27]. K. Lai et al. invented a microwave impedance microscope (MIM) [24, 25], which fed a microwave signal to a silicon nitride cantilever with a Pt tip that was used to investigate the nano‐scale dielectric inhomogeneity in a non‐invasive manner. The Weide group combined an NSMM with an AFM (NSMM‐AFM) [26, 27] by adding a microwave signal to a commercial probe. The NSMM‐AFM can measure the topography and dielectric constant of thin film simultane‐ ously. However, it is noted that MIM and NSMM‐AFM do not use matched probes or cantilevers as the microwave‐guide connected with the source of microwave signals. Thus, the microwave signals may not propagate along the probe and emit from the tip apex of the probe. Therefore, these techniques can only measure the changes of the probe‐sample system impedance but not the intrinsic electrical properties of the measured materials.

To summarize, these AFM‐based methodologies and microwave microscopy techniques can only image relative electrical properties, rather than the absolute values of the intrinsic electrical properties, such as the conductivity, permittivity, and permeability. Thus, the need remains for a microscopy technique that can provide a simultaneous measurement of topog‐ raphy and electrical properties on the nanometer scale.
