A Scanning Acoustic Microscope (SAM) requires water (usually deionized or purified water) as the coupling medium during operation, mainly for the following reasons:

1. Essential Medium for Ultrasonic Propagation

Ultrasound is essentially a mechanical wave that relies on media (solid, liquid or gas) to propagate. In air, ultrasonic waves suffer rapid attenuation, slow propagation speed and massive energy loss, making it impossible to effectively transmit waves into the interior of tested objects.

With its density and molecular structure, water serves as an ideal medium for efficient ultrasonic transmission. Its acoustic impedance (density × sound velocity) is between that of air and most solid materials, which greatly reduces ultrasonic reflection loss at the interface between the probe and the sample, and improves acoustic wave penetration efficiency.

2. Avoid Ultrasonic Attenuation in Air

Ultrasonic energy decays rapidly when traveling through air, resulting in insufficient signal strength and failure of precise imaging. Water medium effectively maintains acoustic energy, ensuring ultrasonic waves penetrate the sample and return to the detector to form clear internal defect images.

3. Realize Non-contact Testing and Operational Convenience

By adopting the water immersion method (both probe and sample immersed in water), the probe does not need direct contact with the sample surface. This avoids surface scratches or coupling agent residue caused by contact testing.

The water medium allows a fixed distance between the probe and the sample. It is suitable for all-round scanning of complex curved workpieces or fragile materials such as semiconductor chips, improving testing flexibility and repeatability.

4. Optimize Imaging Accuracy and Signal Stability

The high uniformity of water reduces acoustic wave scattering, stabilizes reflected and refracted signals, and facilitates accurate identification of the location and size of internal material defects such as cracks, voids and delaminations.

The purity of deionized water prevents impurities from interfering with ultrasonic waves and ensures the reliability of test results.

Summary of Core Functions

In SAM, water acts as both a bridge for acoustic energy transmission and a key carrier for protecting samples and improving imaging quality. Its physical properties (density, sound velocity, low attenuation) make it the most practical coupling medium at present, widely used in high-precision non-destructive testing in semiconductors, aerospace, new energy and other fields。