Photoacoustic technology, first observed by A.G. Bell at the end of the 19th century, refers to the phenomenon where an intensity-modulated light beam entering a closed medium generates acoustic waves. At that time, due to limited technological capabilities, the study of this effect was not widely pursued. However, as the understanding of light-matter interactions deepened, researchers began to explore how the absorption of light at the sample surface leads to heat generation and subsequent thermal diffusion. By solving the thermal diffusion equation in such a system, scientists derived expressions for the photoacoustic signal generated when periodic light with modulation frequency ω is incident on the material.
The formula includes parameters such as the incident light intensity (I₀), ambient temperature (T₀), the absorption coefficient of the sample (α), the conversion efficiency of light energy into heat (Z), the cavity pressure (P₀), the heat capacity ratio (V), and several dimensionless ratios involving thermal and optical properties of the materials involved. This theoretical framework allows for the estimation of both the amplitude and phase of the photoacoustic signal from condensed samples.
When the cavity size is much smaller than the wavelength of the acoustic wave, the strength of the signal becomes proportional to the absorbed optical power and inversely proportional to the cavity volume. It also depends on the thermal, optical properties of the sample and the modulation frequency. Thus, using these equations, it's possible to directly determine the thermal and optical characteristics of the medium.
A typical photoacoustic detection system consists of three main components: a radiation source, a photoacoustic cell, and a signal processing and display unit. The excitation sources can include incandescent lamps, lasers, electron beams, ion beams, molecular beams, and even x-rays or neutron fluxes.
In the field of electronic devices, photoacoustic technology has found numerous applications. One key area is the measurement of thermal diffusivity in thin film materials such as metal-coated semiconductors, diamond films, and others. As space technology, optoelectronics, and high-temperature material demands have grown, the need to study the thermal properties of thin films has become increasingly urgent. The photoacoustic phase method offers a feasible approach to measuring thermal diffusivity.
For example, British researchers Adams and Kirkbright used a 20W tungsten halogen lamp to measure the thermal diffusivity of a 20nm thick copper sheet, achieving promising results. Their work highlighted the potential of the back-illumination method for studying thin-layer materials. However, overcoming the after-effect during measurements is crucial for improving data accuracy. Other methods, such as lateral illumination and front incidence, are also used, particularly for multi-layer thin films, offering great potential in semiconductor heterojunction research.
Additionally, the AC photothermal method is another effective technique for determining thermal diffusivity across a wide range of materials, including polymer films and artificial diamond films. While this method has shown good results, eliminating systematic errors remains a key challenge.
Photoacoustic signals can also carry information about surface defects in integrated circuits. By analyzing these signals, researchers can detect and identify unknown defects through comparison with standard spectra. Laser ultrasonic inspection is one of the most studied techniques for detecting micron-level defects, though its theory and experimental methods still require further development.
In terms of microscopic electrical properties, photoacoustic spectroscopy plays a vital role in analyzing surface states, charge transfer processes, and other electrical characteristics of materials. Surface photovoltaic technology (SPS) and infrared photoacoustic spectroscopy are two well-established methods for studying these properties. Researchers like F. McClland have used PAS to analyze the surface state of Ge crystals, revealing important insights into semiconductor annealing processes.
Moreover, photoacoustic techniques are being used to measure electrical conductivity, electron mobility, and carrier concentration in heavily doped materials. For instance, the measurement of electron mobility in silicon carbide (SiC) has been challenging, but PAS shows promise in improving accuracy.
Photoacoustic technology also aids in determining the doping distribution and bandgap width in semiconductors. Ion implantation imaging, combined with electroacoustic microscopy, provides non-destructive analysis of impurity distribution, making it a powerful tool for semiconductor characterization.
Beyond material properties, photoacoustic methods are used to assess crystal dislocation density, which is closely related to the mechanical and electrical performance of semiconductor materials. Non-destructive testing of dislocation distribution is therefore highly valuable.
Another application is in measuring the thickness of semiconductor thin layers or metal coatings. Low-frequency incident beams allow deeper penetration, enabling accurate thickness determination. Techniques like PAS interference are commonly used for this purpose, especially for materials deposited on opaque substrates.
In the field of nanomaterials, photoacoustic technology has proven useful in analyzing sound velocity, pressure, and temperature relationships during synthesis. Studies on nano-metals like Zn and Ag have demonstrated the effectiveness of laser ultrasound in characterizing their acoustic properties. However, more research is needed to fully understand the relationship between acoustic behavior and microstructure in various nanomaterials.
Photoacoustic technology is also being applied in the development of micro-silicon beam resonant sensors. When modulated laser light is incident on a micro-silicon beam, the resulting photothermal effect induces oscillations, leading to the creation of highly sensitive, safe, and explosion-proof sensors.
In conclusion, as photoacoustic technology continues to advance, its applications are becoming more widespread. Future research may focus on low-temperature applications, where the high signal-to-noise ratio offers significant advantages. For electronic devices, non-destructive subsurface testing will remain one of the most promising areas, particularly in the study of integrated circuits, semiconductor films, and superconducting materials.
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