Principle of the Scanning Acoustic Microscope

In this application, instead of a pulse wave, a tone-burst wave is used (see Fig. 1), and the frequency domain is such that the wavelengths of the ultrasound (i.e., in water range from 15.0 to 1.5 p,m see Table 1). The penetration depth of the waves is limited by attenuation. The SAM is used for penetrations substantially up to 300 pm. 

An electrical signal is generated by an RF tone-burst source. In the beginning, a Colpitts oscillator, or the like, was used as the RF tone-burst source. Nowadays, however, burst waves are gated out from continuous waves by a single pole double throw switch for frequency stability. The output (i.e., voltage) of the source is approximately 10 V 

The electrical signal is transmitted to a piezoelectric transducer located on the top of a buffer rod through a circulator (or the single pole double throw switch). The electrical signal is converted to an acoustic signal (i.e., ultrasonic plane wave) at the transducer. The 

To form a 2D acoustic image, an acoustic lens and/or an X-Y stage is mechanically scanned across a given area of the specimen. 

The acoustic lens is able to translate axially along the z direction by variation of the distance between the specimen and the lens for subsurface visualization. That is, when the surface of the specimen is visualized, the acoustic lens is focused on the specimen (we denote z = 0 pm), and when a subsurface of the specimen is visualized, the acoustic lens is mechanically defocused toward the specimen (we denote z = -x pm, where x is the defocused distance). 

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