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The perfect blend of laser and sound waves: How does picosecond ultrasound change measurement technology?
Picosecond ultrasonic technology, which uses ultra-high-frequency sound waves generated by ultra-short light pulses, is opening a new chapter in measurement technology. This non-destructive technique effectively penetrates films or nanostructures to reveal internal features such as film thickness, cracks, delaminations and voids. With the advancement of science and technology, picosecond ultrasonic wave has gradually become a mainstream method in the semiconductor processing industry, and has demonstrated its high-precision detection capabilities in many fields.
Technical principles
When an ultrashort light pulse called a pump pulse is focused onto the film, the absorption of light energy causes thermal expansion, which in turn produces a pulse of elastic strain.
In this process, the strain pulse generated is mainly composed of longitudinal phonons, which propagate directly into the film as coordinated pulses. After reflection at the film-substrate interface, the strain pulse returns to the film surface and can therefore be detected by delayed optical detection pulses, by light reflection or, in the case of extremely thin films, by exploiting changes in transmission.
Generation and detection process
Generate
Absorbed optical pump pulses generate localized thermal stresses near the sample surface, which push elastic strain pulses into the interior of the sample.
The depth of this process is affected by material properties and optical pump wavelength. For example, in metals and semiconductors, ultra-short periods of heat and carrier diffusion increase the depth of initial heating. Generally speaking, the transit time between the duration of the generated acoustic pulse and the depth of heating is equal.
Detection
The strain pulse returned to the surface generates a series of echoes from which important parameters such as film thickness, ultrasonic attenuation and ultrasonic dispersion can be derived.
Initially, the detection mechanism of picosecond ultrasonic waves was based on the photoelectric effect. The echoed strain pulse interferes with the refractive index and extinction coefficient near the sample surface, which changes the reflection or transmission of light. As technology advances, techniques for detecting displacements involving surfaces have also been introduced, such as ultrafast beam deflection and interferometer techniques.
Application areas and future challenges
With the widespread application of picosecond ultrasonic technology, this technology has been successfully used in the analysis of various materials, whether solid or liquid. Especially in the detection of nanostructures, such as submicron thin films, multilayer structures, quantum wells and semiconductor heterostructures, their application value has become increasingly prominent. In addition, scientists are beginning to use this technique to probe the mechanical properties of individual biological cells.
Conclusion
With in-depth research on picosecond ultrasonic technology, this technology may achieve greater breakthroughs in measurement accuracy, range and application fields in the future. In the face of ever-changing technological needs, can we expect picosecond ultrasound to exert a revolutionary influence in a wider range of fields?
