Ultrasonic transducer is a very critical component of scanning  acoustic microscope. In various scanning microscopic inspections, operators select appropriate ultrasonic scanning probes according to the corresponding parts size and customer needs. According to different user requirements, ultrasonic scanning microscopes use ultrasonic transducers of various specifications and performances to meet the inspection application scenarios of different material devices. Shanghai HIWAVE has more than 10 years of experience in developing ultrasonic transducers and has developed a variety of ultrasonic probes, including different frequencies, chip diameters, connector types, etc. These performance indicators directly affect the penetration depth and scanning resolution of ultrasonic scanning microscopes.

High-frequency electronic pulse signals into ultrasonic signals in ultrasonic scanning microscopes.The component that converts electrical energy into mechanical energy, is called an ultrasonic transducer. It can also convert ultrasonic signals into electrical signals and input them into computers for processing. The ultrasonic transducer contains a piezoelectric ceramic crystal. When a voltage pulse is applied to the piezoelectric ceramic crystal, the crystal will deform slightly under the action of the voltage and generate ultrasonic waves. Ultrasonic transducers are designed in various shapes to generate ultrasonic waves of various specifications to meet the testing needs of different samples of ultrasonic scanning microscopes.

The ultrasonic waves emitted by the ultrasonic scanning microscope probe have an effective area for detecting material devices, which we call the “transducer’s sound field”. The sound field of the ultrasonic scanning microscope can be divided into the near field (Near field) and the far field (Far field), and their positions are shown in the figure. The near field refers to the area close to the probe. In this area, the sound pressure reaches the maximum and minimum values repeatedly several times. The terminal of this area is the position on the axis where the maximum sound pressure value appears for the last time. The distance from this position to the probe surface is expressed as N, that is, the near field distance. The near field distance N represents the natural focal length of the probe.

The far field of the ultrasonic scanning microscope probe is the area outside the near field distance (N value). In this area, the sound pressure gradually decreases to zero as the beam diameter expands and the sound energy dissipates. The near field distance is a function of the interaction between the probe frequency, the wafer diameter and the sound velocity of the material being tested. The following formula can be used to calculate this function value for the most commonly used ring wafer in ultrasonic defect detection:

Calculation formulas for near-field and far-field of ultrasonic scanning microscope probe：

N=D2f/4c or N=D2/4λ

N = Near field length

D = wafer diameter

f = frequency

C = material sound velocity

λ = wavelength (c/f)

Because of the acoustic pressure differences in the near field of the SAM probe, it is difficult to accurately evaluate defects using amplitude-based techniques (although thickness measurements in the near field are not a problem). In addition, the N value represents the maximum distance that the probe beam can be focused by acoustic lensing or phase adjustment techniques. The use of acoustic lenses can focus the SAM immersion probe to produce an hourglass-shaped beam that gradually narrows until it becomes a tiny focus, and then spreads out again from this focus to propagate farther. Some delay line type probes can also be focused. Beam focusing is very useful when testing small diameter tubes or other test pieces with sharp corners because it can concentrate the acoustic energy in a smaller area and also improve the echo response.