Ultrasonic wave type and transducer type

Ultrasonic wave type and transducer type

If the vibration direction of the particles inside the wafer is perpendicular to the wafer plane, then the wafer emits ultrasonic waves outward. The propagation of ultrasonic waves in the medium can have different forms, it depends on what kind of force the medium can bear and how to excite the ultrasonic waves on the medium. There are usually three types as follows:

Longitudinal wave mode: When the vibration direction of the particle in the medium is consistent with the propagation direction of the ultrasonic wave, the ultrasonic wave is the longitudinal wave mode. Any solid medium can generate longitudinal waves when its volume changes alternately.

Shear wave mode: When the vibration direction of the particles in the medium is perpendicular to the ultrasonic wave, this ultrasonic wave is a transverse wave mode. In addition to being able to withstand volumetric deformation, solid media can also withstand shear deformation. Shear waves can be generated when shear forces alternately act on solid media. Shear waves can only propagate in solid media.

Surface wave mode: It is a wave with dual properties of longitudinal wave and transverse wave propagating along the solid surface. The surface wave can be seen as a combination of a longitudinal wave parallel to the surface and a transverse wave perpendicular to the transverse wave. The trajectory of the vibrating particle is an ellipse, with the strongest amplitude at a depth of 1/4 wavelength from the surface. In fact, the amplitude of the particle vibration is very weak at a wavelength above the surface.

Commonly used ultrasonic probes include straight probes and inclined probes, and their structures are shown in Figure 1-3. The probe emits ultrasonic waves outward through a protective film or inclined wedge; the function of the absorption backing is to absorb the sound waves emitted by the wafer toward the back to reduce clutter; the function of matching the inductance is to adjust the waveform of the pulse wave. Generally, straight probes generate longitudinal waves, and inclined probes generate transverse or surface waves. Another kind of variable angle probe is shown in Figure 1-4. The probe core can be rotated, and the oblique probes with different refraction angles can be obtained by changing the incident angle q of the probe. When q = 0, it becomes a straight probe.