Simultaneous determination of layer thickness and sound velocity for multifocal ultrasound microscopy

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The aim of this project is to provide a multifocal ultrasound microscopy system for simultaneous investigation at different depths for analysis frequencies from 100 to approx. 250 MHz for a broad spectrum of research disciplines. Besides the application in the inspection of electronics, e.g. the inspection of multilayer electronic components, the microscope provides numerous innovation impulses for microfluidics and biotechnology. Due to the greatly reduced measuring time, for example, in-situ observation of biological processes is within reach.The advantages of a multifocal microscope consist in the possibility of varying the focal point and thus focusing to different depths while maintaining a constant distance to the studied object, as well as in an increased accuracy with considerably less expenditure of time compared to a single-channel microscope. If the material composition and structure of the layered test object is unknown, further development and application of the method for simultaneous determination of sound velocity and distance is necessary. The method enables the optimization of the delay times for focusing and thus the achievement of the maximum possible resolution. Because of the high analysis frequencies in combination with a multi-channel technique, considerable challenges arise with regard to: (i) the sound field-based design of the system consisting of annular array and aspherical lens, (ii) the necessary high-precision focusing electronics, (iii) the provision of algorithms for processing the high-frequency signals and (iv) ensuring high signal energy. The requirements for the pre-focused annular array to be developed consist in a very small number of elements combined with excellent focusing properties and an exact matching of array curvature and aspherical lens, so that the extension of focus area in the wavelength range can be achieved at any depth of the test object. The fabrication of the arrays requires the use of latest technology and manufacturing technologies in lens production, the thin-film application of the oscillator elements and electrode structuring. The electronics must allow quasi-simultaneous control of the individual array elements in order to place the focus in the desired plane and to reduce its extension extremely, as well as to record, digitize and arbitrarily process high-frequency signals. The analysis software delivers high-resolution quasi-3D images.The solution of these tasks requires close cooperation with a practical partner who has already developed electronics and evaluation software for single-channel microscopy systems and has mastered the manufacturing technology for high-frequency transducers.