Image Processing for an Amplitude and Phase Acoustic Microscope
We have built a scanning acoustic microscope (SAM) operating in the 3 to 10 MHz range that measures both amplitude and phase reliably and accurately. It has been primarily used in non-destructive evaluation (NDE) of machined parts, composite materials, etc. for the presence of cracks, voids, and delaminations. Most SAMs measure amplitude only; by measuring phase as well, we can carry out quantitative NDE and image processing that can not be done wid1 amplitude or phase alone. For example, the obscuring effect of surface reflections, particularly from a rough surface, can be greatly reduced to give dramatically better detection of subsurface defects. We have demonstrated several image processing applications that use amplitude and phase measurements; such as transducer characterization, material reflectance function measurements using V(z) inversion, and thin film/delamination thickness measurements. The transverse and depth resolution of the microscope can be enhanced by numerically post processing the digitized images. The transverse resolution is increased by approximately 15%, and with an F2 transducer, the depth resolution was doubled. For lower f-number transducers, the percentage increase in depth resolution is not as large but is still significant. The enhanced depth resolution has been applied to measuring the profile of deep trenches (2 mm wide by 5 mm deep) designed as scale models of integrated circuit capacitors (2.4 um by 6 um). This technique could also be applied to a system scaled down in wavelength to characterize ICs. Such a system would use an acoustic microscope operating at higher frequencies or a confocal scanning optical microscope (CSOM) that measures amplitude and phase. Even better depth resolution can be obtained by numerically combining images taken at several different frequencies. The resulting image has a greater range of coverage in the spatial frequency domain in the depth direction than does a single frequency image. This technique could also be implemented in a CSOM using illumination from lasers operating at different wavelengths. The method has the potential of sharpening tine depth response of an optical microscope by a factor of two from what is currently possible. It is also possible to improve the depth resolution of a low aperture microscope so it is as good as with a wide aperture system. This could be very useful for imaging objects with high relief, where the use of a wide aperture lens could degrade the images of deep features. The ability of the acoustic microscope to measure phase allows a novel application for acoustic microscopes: the measurement of capillary waves. Capillary waves are short wavelength ripples on the surface of water. The presence of capillary waves initiates the build up of larger waves via wind interaction. A measurement of their damping could thus give an indication of weather changes. They can also be used to characterize the marine microlayer. Up to this time, no satisfactory method for measuring capillary waves in situ had existed. The phase measured in the acoustic microscope is used to determine the height of a controlled capillary wave packet. The packet is excited using acoustic radiation pressure from another nearby transducer. Properties such as surface tension and viscosity can be determined from the measured dispersion curve. This method is non-contacting and thus has the advantage of not disturbing the surface film during a measurement.
