Acoustic microscopy

Acoustic microscopy is the technique of using sound waves to visualize structures too small to be resolved by the human eye. Frequencies up to several gigahertz are used in optical microscopes. The reflection and diffraction of sound waves from microscopic structures can yield information not available with light. Acoustic microscopy is microscopy that employs very high or ultra high frequency ultrasound. Acoustic microscopes operate nondestructively and penetrate most solid materials to make visible images of internal features, including defects such as cracks, delaminations and voids.Types of acoustic microscopes In the half-century since the first experiments directly leading to the development of acoustic microscopes, at least three basic types of acoustic microscope have been developed. These are the scanning acoustic microscope (SAM), scanning laser acoustic microscope (SLAM), and C-mode scanning acoustic microscope (C-SAM).[1] For details of development see the History section below. Since the vast majority of acoustic microscopes in use today are C-SAM type instruments, this discussion will be limited to these instruments. [edit]Behavior of ultrasound in materials Ultrasound is broadly defined as any sound having a frequency above 20 kHz, which is approximately the highest frequency that can be detected by the human ear. However, the acoustic microscopes emit ultrasound ranging from 5 MHz to beyond 400 MHz so that micrometre size resolution can be achieved. The ultrasound that penetrates a sample may be scattered, absorbed or reflected by the internal features or the material itself. These actions are analogous to the behavior of light. Ultrasound that is reflected from an internal feature, or ( n some applications) that has traveled through the entire thickness of the sample, is used to make acoustic images. [edit]Sample types and preparation Samples need no special treatment before acoustic imaging, but they should be able to withstand at least brief exposure to water or to another fluid, since air is a very poor transmitter of high frequency acoustic energy from the transducer. The sample may be completely immersed in the water, or scanned with a narrow stream of water. Alternately, alcohols and other fluids can be used so as to not contaminate the sample. Samples typically have at least one flat surface that can be scanned, although cylindrical and spherical samples can also be scanned with the proper fixtures. In the following paragraphs, the sample being described is a plastic-encapsulated integrated circuit. [edit]Ultrasonic frequencies The ultrasonic frequencies pulsed into samples by the transducers of acoustic microscopes range from a low of 10 MHz (rarely, 5 MHz) to a high of 400 MHz or more. Across this spectrum of frequencies there is a trade-off of penetration and resolution. Ultrasound at low frequencies such as 10 MHz penetrates deeper into materials than ultrasound at higher frequencies, but the spatial resolution of the acoustic image is less. On the other hand, ultrasound at very high frequencies do not penetrate deeply, but provide acoustic images having very high resolution. The frequency chosen to image a particular sample will depend on the geometry of the part and on the materials involved. The acoustic image of the plastic-encapsulated IC below was made using a 30 MHz transducer because this frequency provides a good compromise between penetration and image resolution.