Applications of high frequency ultrasonic inspection in microelectronics manufacturing

The problems involved in generating, detecting and quantifying ultrasound increase as the frequency increases, but the technology has kept pace and a range of sophisticated ultrasonic imaging equipment is now available. The techniques used can be broadly divided into two frequency ranges; namely 20-100 MHz and 100 MHz-1 GHz or more. The term `scanning acousting microscopy´ (SAM) is often used to describe imaging with high frequency ultrasound but purists may argue that this term should be applied only to a specific method of imaging, usually using frequencies of over 100 MHz. As the frequency increases the penetration falls and the resolution increases. In the 20-100 MHz range features of around 20-30 μm can be resolved, with penetration of up to 10 mms or so in some materials. At the top of the 100 MHz-1 GHz range the resolution can be sub-micrometre, but a penetration of only a few micrometres is possible. The applications described here are all in the 20-75 MHz range, where a variety of techniques and measurement methods can be employed