Developement of improved imaging techniques and new, structured, high frequency ultrasound transducers for the acoustic microscopy

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The progress in packaging of integrated circuits, with more miniaturisation, high wiring densities and mounting of flat components requires techniques of non-destructive testing with increasing capabilities. Scanning acoustic microscopy (SAM) is especially well applicable to reconstruct the inner structure of objects. It allows to detect voids like air inclusions or cracks or to test boundary layers to localise delaminations. An ultrasonic image displays such voids much better than an X-ray image. To obtain an Ultrasonic image a focussing single probe scans the specimen. Ultrasound microscopes work in a frequency range of 10 up to 230 MHz, plot the amplitudes of the reflected signals and reach a resolution in the range of microns for flat structures. The resulting image is incorrect, if the sound velocities of any part of the structure are unknown. SAM fails if a structure with inclined or curved interfaces has to be tested. SAM needs a lot of time because of the necessary adjustment and the individual test of each layer. Therefore, SAM is not suitable for an application on online monitoring of industrial production. The goals of the project are an improvement of image quality, the expansion of test scenarios and a reduction of the necessary measuring time for the scanning acoustic microscopy. To reach this, new measuring techniques and hardware, especially structured phased arrays, which allow focusing and steering the sound field, shall be provided. The new measurement techniques allows a simultaneous testing in different depths and under inclined or curved surfaces as well as a determination of sound velocity in each part of the structure. Therefore a multichannel scanning acoustic microscope and segmented annular arrays up to 40 MHz will be developed and constructed. The number of elements of these arrays should be as low as possible whereas the resolution of conventional probes shall be reached or beaten. Because commercial composites for the construction of these arrays are not available, the soft mold technique shall be advanced, to obtain fine scaled composites up to 40 MHz with a spherical curvature. The advanced soft mold technique enhances the degree of freedom for structuring phased arrays and allows the development of probes which exactly match the problem of testing for this project and beyond.