Acoustic microscopes are non-destructive microscopes that utilize a high frequency ultrasound in order to penetrate solid materials. By doing this, the microscope is able to form an image of the internal features of the material, such as defects like cracks. The ultrasound frequency emitted ranges from 5 MHz to 400 MHz, making it capable of producing images the size of mere micrometers and an ideal technique for investigating semiconductor devices.
There are three basic types of acoustic microscopes, all three of which have only just been developed in the last 50 years. These are the scanning acoustic microscope (SAM), the C-mode scanning acoustic microscope (C-SAM) and the scanning laser acoustic microscope (SLAM). Most microscopes in the industry fall under C-SAM due to their efficiency and capabilities.
Acoustic microscopes work by emitting an ultrasound frequency that are literally able to see into the material. This ultrasound technology disturbs the sample before absorbing, reflecting or scattering the inner material, with only the reflected ultrasound that has traveled throughout the sample creating an image. This works because the technique used by the microscope is the same technique that allows us to perceive light. Low frequency ultrasound waves penetrate deeply, but high frequency ultrasound waves produce a greater resolution. Therefore, the frequency at which the acoustic microscope views an image relies on the geometric pattern of the object itself, with an appropriate amount of depth and resolution.
The process of working an acoustic microscope starts with the ultrasonic transducer. Once it has begun scanning the surface of the object, thousands upon thousands of pulses will enter the material being scanned. These pulses then either absorb or scatter as they pass through the homogeneous parts of the object. It is then that some of this pulse gets reflected at the ultrasonic transducer. From there, the image can be processed from the amplitude of the wave.
Acoustic microscopes are excellent for being able to look into materials for defects and faults, especially major problems like cracks or delaminations. These microscopes are manufactured for the purpose of looking into one or more solid materials with a flat surface. There are several other applications outside of electronics for acoustic microscopes, including medical products, ceramic materials, and even blood analysis. Utilizing ultrasound technology, the image is processed through the frequency and amplitude of the waves. Being able to detect these unseen defects allows for semiconductors and other products to be corrected, even when it seems like there are no problems on the surface.