Handbook of Modern Coating Technologies

Operating principles of the microwave microscope

Scanning microwave microscopes are used to explore materials at microwave frequencies and to measure changes in their resistivity [182—187].
Fig. 5—22A presents a schematic diagram of a microwave microscope with MTs built around a coaxial line segment, and Fig. 5—22B shows that of an SMWM with a coaxial resonator-based MT. A microwave signal from the source enters the linear resonator with a coaxial transmission line bounded by a decoupling capacitor on the one side, and an open coaxial probe on the other. Due to multiple reflections in the coaxial resonator (with Q~102—103), the signal-to-noise ratio is significantly improved and the accuracy of measure-ments enhanced. However, measurements at different frequencies require resonator para-meters to be readjusted.
The dependence of the reflected signal on the distance to the sample is analyzed to con¬trol the probe/sample spacing. The sample has a capacitive coupling with the system. The central conductor of the coaxial probe forms one capacitor plate, while the other plate forms a sample if it is metal. A decrease in the resonance frequency of the coaxial resonator occurs because the capacitance Cx increases as the distance between the sample and the probe decreases (see the inset in Fig. 5—22A) [187]. The transmission line has only matching 
impedance and is open due to capacitance with an edge in one extreme case when the probe is very far from the sample (Cx and C0 in Fig. 5—22A). The system is a half-wave reso¬nator with a resonant frequency f 0 in this case. Corbino contact (short circuit) occurs when a metal sample is in contact with the probe (second limiting case) [188,190]. In this case, the system becomes a quarter-wave resonator (Fig. 5—22B) with a resonant frequency decreased by
1 c/2L
Afmax 5 2 j"/T (5.10)
2 £r
where sr is the relative dielectric constant of the coaxial cable, L is the length of the coaxial resonator, and c is the speed of light. For a typical final probe/sample separation, the fre¬quency shift values are between Af and 0. 
Table 5.3 Applications of microwave microscopy in nanotechnologies.
Fields of SMWM applications Peculiarities of an SMWM
• Established: • Multifunctionality
- Technologies of high-temperature superconductors • Possibility of additional manipulation of the
- Visualization of surface resistance distribution sample using:
- Biology and medicine: • constant electric field
- visualization of the structure of biological objects • magnetic field
- studies and visualization of tumors • mircowave field
• mechanical force field
• Potential: • Possibility of studying the properties of
- Semiconductor micro- and nanoelectronics subsurface layers
- Multiparametric studies of surface and near-surface layers and
nanoclusters
- Topological structure of electrophysical parameters of
materiasls
- Possibility of local nonthermal modification of surface and
near-surface layers
SMWM, Scanning microwave microscopy.

An increase in radiation losses and additional dissipation are caused by currents flowing in the sample. The resistance Rx between the ground and the probe/sample capacitor can be modeled amplified scattering (see inset to Fig. 5—22A). The quality factor of the coaxial resonator decreases due to resistance. The quality factor of the resonator and the resonant frequency change due to the local resistance of the plate and the capacitive gap, which change as the probe moves. The scanning mode is determined by the feedback loop, which will keep the microwave source attached to a certain resonant frequency of the coaxial reso¬nator [191].
The authors of Ref. [184] used two microscopes, one operating at room temperature, the other a cryogenic microwave. The former is shown in Fig. 5—22C. A sample is attached to the manual movement step along the Z-axis. The step is equipped with a biaxial leveling unit. With a step of 0.1 p,m along the X-axis, all equipment is mounted on the motorized cascade. At the motorized stage of movement along the Z-axis, a probe is held above the sample. The motorized step is mounted on a rigid frame. A binocular microscope and a digital camera are used to monitor the probe/sample separation scan during scanning. To obtain an image with submicron resolution, a microscope is used located on a vibration- insulated table.
Based on the SQUID design, the cryogenic microscope consists of a system containing a holder of a z-coaxial probe with a variable height and a cryogenic xy-slider that transfers the sample with an accuracy of ~1 p,m. Using computer-controlled stepper motors, x—y move-ment is achieved. Stepper motors through hypothermic hermetic seals transmit movement to the cryogenic slider and operate at room temperature. The temperature range of the microscope is from 4.2 to 300K. 

Layer resistance (П/sq)
FIGURE 5-22 (A) Schematic diagram of a microwave microscope with an MT based on a coaxial line segment.
(B) Schematic diagram of a microwave microscope with a coaxial resonator-based MT. (C) Near-field microwave microscope scanning at room temperature. The probe is at the stage of movement along the Z-axis suspended on the frame, and the sample is located at the xy-stage of movement on the right. decoupling on the optical table is controlled by a coaxial resonator [188,189].