Nondestructive testing (NDT) technology is used to inspect the integrity, surface condition, and cracks of workpieces and structures at manufacturing sites without destroying them[1]. This technology is crucial in modern industries, owing to which many studies have been conducted on various physical phenomena to develop practical NDT methods[2-5]. Among existing technologies, microwave imaging techniques have received considerable attention because of their unique advantages, such as low cost, strong penetrability for dielectric materials, nanometer-scale spatial resolution, and noncontact measurement capability[6-8].

So far, studies on the NDT technology based on microwave imaging have mainly relied on the scanning method that uses microwave antenna probes. However, owing to its extremely low measurement throughput, this scanning method has only been employed in academic research. Recently, a new method termed thermoelastic optical indicator microscopy (TEOIM) that realizes scan-free microwave imaging has been reported. Because TEOIM is based on an optical microscopic system that uses a charge-coupled device (CCD) camera, it shows the advantages offered by optical microscopes, such as a high spatial resolution of a few micrometers, wide field of view, and high measurement throughput. TEOIM has successfully been applied to various NDT applications, such as the monitoring of electronic devices during operations, characterization and crack detection of conductive thin films, inspection of conductive particles embedded in opaque dielectric materials, and noninvasive measurement of liquid concentrations.

Although considerable research has been conducted on TEOIM applications recently, investigations focusing on improving its measurement sensitivity have not been reported. Because the measurement sensitivity of TEOIM depends on the optical indicator that converts the microwave electromagnetic field into an optical signal, it can be improved by adjusting the physical properties of the optical indicator. For instance, the microwave magnetic-field sensitivity of TEOIM can be improved by increasing the microwave magnetic heating efficiency of metal thin films of optical indicators. In particular, because the microwave magnetic heating of metal thin films depends on the film thickness, the measurement sensitivity can be improved by adjusting the metal thin film thickness of the optical indicator. Herein, the microwave heating of metal thin films was investigated based on the determination of its optimal thickness for optical indicators for microwave magnetic-field imaging. Optical indicators coated with metal thin films of various thicknesses were prepared, and the microwave magnetic heating distribution of the prepared optical indicators was visualized using TEOIM. Measurement results showed that as the thickness of the metal thin film of the optical indicator decreased, the microwave magnetic heating efficiency increased, thereby enhancing the microwave magnetic-field sensitivity of the optical indicator.

1. Measurement principle

Figure 1 (a) presents the measurement principle of TEOIM. The optical indicator comprises a glass substrate coated with a thin film heated using a microwave electromagnetic field. When the optical indicator is placed in the microwave near-field region, the thin film is heated using the electric or magnetic field component of the microwave near-field depending on the loss property of the thin film. Thus, the heat generated in the thin film diffuses into the glass substrate, inducing thermal stress on the glass substrate. The thermal stress distribution can be visualized by measuring the linear birefringence distributionof the glass substrate induced by the photoelastic effect, which can be performed using the conventional polarizing microscopic system. Based on the birefringence distribution, the heat source distribution that induces the thermal stress can be calculated using the following equation:[9]:

Figure 1. (Color online) (a) Measurement principle of TEOIM. (b) Measurement setup of TEOIM. (c) Illustration of microwave signal generation system. (d) Illustration of microstrip transmission structure.

where q denotes the heat source density, β₁ and β₂ represent the birefringence related to normal and shear stress components, respectively, and C is a constant related to the wavelength of incident light and physical properties of the optical indicator. The heat source distribution is identical to the microwave magnetic-field distribution for indicators coated with a conductive thin film. Therefore, the microwave magnetic-field distribution can be determined by calculating the heat source distribution from the measured birefringence distribution using Eq. (1).

2. Optical system of TEOIM

Figure 1 (b) shows the optical system of TEOIM. Here, a light-emitting diode (LED; λ = 530 nm) was used as the light source and the polarization of the incident light was modulated into left and right circularly polarized states using a linear polarizer and a liquid crystal modulator. The modulated incident light propagates to the optical indicator and is reflected to a second linear polarizer (analyzer). Therefore, the intensity of the light passing through the analyzer is measured using a CCD camera. The β₁ and β₂ distributions were calculated based on the measured intensity using the following equations[9]:

where φ denotes the polarization angle of the analyzer, S denotes the retardation of the liquid crystal modulator, and I represents the measured intensity.

3. Microwave system

Figure 1 (c) presents the microwave radiation system used in the experiment. A 50 Ω-matched microstrip transmission line was used as the microwave source. The microstrip transmission line was placed at an interval of 1 mm behind the optical indicator, and the signal line of the microstrip faced the optical indicator. To apply microwave signals to the microstrip transmission line, a microwave signal generator connected to a microwave power amplifier and a variable microwave attenuator was used. The microwave signal emitted from the generator was transmitted to the microwave power amplifier, which amplified it to 30 dBm. The power of the amplified microwave signal was modulated from 0 to 30 dBm using the variable microwave attenuator. Finally, the modulated microwave signal was transmitted to the microstrip transmission line.

4. Preparation of optical indicators

The optical indicator was fabricated by depositing an aluminum thin film on a 0.8–mm-thick soda–lime glass substrate. The aluminum thin films with thicknesses of 10, 25, 50, 75, and 100 nm were deposited on the glass substrate by thermal evaporation. During deposition, the chamber pressure and deposition rate were maintained at 10^-5 - 10^-6 torr and 1 Å/s, respectively. After deposition, the thickness of the aluminum thin films was verified using an optical scanning interferometer.

Figure 2(a - c) show the microwave magnetic-field distribution of the microstrip transmission line measured using the optical indicator coated with an aluminum thin film with different thicknesses. A strong microwave magnetic field was detected at the signal of the microstrip transmission line, consistent with the results reported in a previous study[9]. Furthermore, the overall spatial structure of the measured microwave magnetic-field distribution for a given microwave frequency remained unchanged even when the aluminum thin film thickness of the optical indicator varied. This result indicates that the near-field distribution of the microwave magnetic field remained unchanged under varying thicknesses of the aluminum thin film of the optical indicator.

Figure 2. (Color online) Microwave magnetic field images of microstrips measured with indicators with various metal thin film thicknesses: (a) 10 nm; (b) 50 nm; (c) 100 nm. The input microwave power was 30 dBm.

Although the field structure was unaffected by the change in the thickness of the aluminum thin film, the field intensity strongly depended on the thickness of the thin film. Based on Fig. 2(a - c), the maximum measured magnetic-field intensity was obtained when the thickness of the aluminum thin film of the optical indicator was 10 nm. Moreover, the intensity gradually decreased as the thickness of the thin film increased, indicating that the microwave magnetic-field measurement sensitivity of the optical indi-cator increased with a decrease in the thickness of the aluminum thin film of the optical indicator.

To quantitatively analyze the measurement sensitivity of the optical indicator based on the aluminum thin film thickness of the optical indicator, the microwave magnetic-field intensity was measured based on the input microwave power. Figure 3(a) shows the microwave magnetic-field intensity per input microwave power (I/P) measured using optical indicators with different thicknesses of the aluminum thin films. When the thickness increased from 10 to 25 nm, the I/P decreased rapidly. Moreover, at a thickness exceeding 25 nm, the I/P gradually decreased with increasing thickness. These findings show that the microwave magnetic-field measurement sensitivity of the optical indicator can be enhanced considerably by reducing the thickness of the thin film to 10 nm or less.

Figure 3. (Color online) (a) Measured microwave magnetic field intensity per input microwave power (I/P) as a function of the aluminum thin film thickness of the optical indicator. (b) Measured electrical resistance according to the aluminum thin film thickness. The electrical resistance was measured using the two-point probe method with a distance of 2 mm between the probes. (c) Calculated microwave absorption according to aluminum thin film thickness.

The enhanced measurement sensitivity of the optical indicator can be explained by the inversion relation between the electrical resistance of the metal thin film and the film thickness:

where R and ρ represent the electrical resistance and re-sistivity of the aluminum thin film, respectively, and A and L represents the area and length of the aluminum thin film, respectively. The microwave heating of metal thin films is attributed to the electric current induced by the microwave magnetic field and increases with an increase in the electrical resistance of the thin film. Because the electrical resistance of the metal thin film increases with the film thickness, the measurement sensitivity improves as the film thickness decreases. This can be confirmed by the change in the measured electrical resistance of the aluminum thin film based on the film thickness (Fig. 3(b)). Thus, the measured electrical resistance of the aluminum thin film increased rapidly when the film thickness decreased from 25 to 10 nm. Notably, the change trend of the electrical resistance of the aluminum thin film is the same as that of the measured microwave magnetic-field intensity. Therefore, the increased measurement sensitivity of the optical indicator is ascribed to the increased electrical resistance of the aluminum thin film.

This simple approach based on the resistance of aluminum thin films is consistent with previous studies on microwave absorption in metal thin films[14,15]. For microwave absorption using a metal thin film, the absorbed microwave power can be calculated using the following equation [14]:

where A denotes the absorbed microwave power, L denotes the thickness of the metal thin film, and s represents the scale length. Further, s depends on the impedance of the surrounding medium (Z₀) and conductivity of the metal thin film (σ), which can be calculated using the following equation[15]

By using equation (4), we calculated the microwave power absorption according to the thickness of the aluminum thin film as shown in Fig. 3 (c), where the aluminum conductivity was 3.5 × 10⁷ S/m and the Z₀ was 377 Ω. From the calculated results, one can see that calculated microwave absorbance showed the same behavior as the indicator sensitivity according to the thickness of the aluminum thin film. Therefore, it can be concluded that the improvement of the microwave magnetic field sensitivity of the optical indicator is because the electric resistance increases as the thickness of the aluminum thin film decreases, thereby improving microwave magnetic heating.

In this study, optical indicators using aluminum thin films of various thicknesses (10-100 nm) were prepared and the microwave magnetic-field sensitivity of the prepared optical indicators was investigated. The measurement results showed that the sensitivity of the optical indicator increased with a decrease in the thickness of the aluminum thin film. Based on theoretical analysis, a decrease in the thickness of the aluminum thin film rapidly increased the resistance, thereby enhancing the microwave heating efficiency and improving the measurement sensitivity of the optical indicator. In conclusion, the thickness of the metal thin film of the optical indicator is a critical factor for improving the microwave magnetic-field sensitivity of TEOIM.

This research was supported by the 2021 scientific promotion program funded by Jeju National University.