Handbook of Modern Coating Technologies

Characteristics of the microwave microscope

5.6.2.1 Spatial resolution
Two linear scales set the spatial resolution of the microscope (which must be significantly smaller than the wavelength for a given frequency) and may be modulated in accordance with the objective of the study. The first scale is the characteristic probe size [192,193]. As fol¬lows from Figs. 5—17C and 5—20A and C, it is defined by the radius of curvature of the
protuberances of the probe gap rod or the diameter of the central conductor of the coaxial probe. This length scale can be significantly reduced by localizing the electromagnetic fields more strongly using a center conductor with a sharp tip. It should be noted that sometimes, as required by the construction, the probe is to be defined by two linear parameters. The dis¬tance between the sample and the probe is the second important length scale. In the system being considered, the separation is less than the radiation wavelength in a free space. The probe should be much closer to the surface compared with the characteristic size of the probe to obtain maximum spatial resolution (Figs. 5—17C and 5—20A—C). In principle, one can use an atomic force microscope operating in the contact mode [189] or set the scanning tunneling mode to maintain a fixed height [194].
We note that near-field microwave microscopy exhibits a two-order-of-magnitude lower absolute spatial resolution than near-field optical microscopy. Nevertheless, it shows a much better multiplicity in overcoming the diffraction limit (up to the factor this multiplicity can be expressed as the wavelength-to-resolution ratio). If, for example, the resolution of optical near-field microscopy ranges (0.01—0.1) A, the multiplicity in overcoming the diffraction limit for scanning near-field microscopy can reach 104 A or more (depending on the frequency).
The best resolution was obtained when a sharp tip was used instead of the central con-ductor of the coaxial probe when working in contact mode. The initial central conductor was replaced by a hypothermic tube with the probe tip placed inside (see Fig. 5—23A) [191 — 193]. Using this configuration, it was possible to visualize a series of lines 0.5 mm wide in an aluminum film deposited on Mylar (see the photograph in Fig. 5—23B at
7.5 GHz). The lines were located at a distance of 2 pm from each other. Fig. 5—23C shows a frequency shift scan along the dashed line shown in Fig. 5—23B. A noticeable frequency drop coincides with the Al lines and has a frequency of 2 mm. Due to intermittent contact during the movement of the probe through the sample, the signal intensity changes. This indicates that the spatial resolution of the microwave microscope in this configuration is >2 pm.
The wide dynamic range of spatial resolution is one of the advantages of this microscope. Using very fine structures or a coaxial probe with an internal conductor diameter of 480 pm (as shown below), the entire plate can be imaged. Other probe sizes can also be used with a central conductor 200, 100, or 12 pm in diameter, which makes it possible to choose the spa¬tial scale [189] and to increase the scanning rate when very high resolution is not required.
5.6.2.2 Frequency bandwidth
The system can operate in a wide passband and does not depend on the spatial resolution of the microscope on the measurement frequency. This means that maps of the surface distri-bution of material properties can be obtained at exactly the same frequency at which the material will be used. In the case of a microscope with a coaxial resonator length L = 2 m, overtones for image formation are available for all integers that are multiples of 50 MHz, since the frequency of the main mode is ~50 MHz. The working bandwidth of the electron¬ics determines the upper limit of the frequency of the microscope. A microwave source has

an upper frequency limit of ~50 GHz in most practical situations. However, a coaxial cable, a detector, and a microwave directional coil (coupler) can also limit the bandwidth of the microscope. However, it is possible to construct a microscope with the continuous possibility of acquiring images at thirty frequencies between 50 MHz and 50 GHz.
5.6.3 Images
Surface resistance of thin YBa2Cu3O7-