Vacuum deposition of mass-spring matching layers for high-frequency ultrasound transducers

We have developed a technique of applying multiple mass-spring type matching layers to high-frequency (>20MHz) imaging transducers, by using carefully controlled vacuum deposition. A vacuum deposited matching layer design has significant advantages over traditional quarter wave matching for high frequency transducers, because thin uniform layers with no adhesion layer can be used and the materials used with vacuum deposition exhibit lower acoustic losses than materials typically used for high frequency quarter wave matching. Two different 3 mm diameter 45 MHz planar lithium niobate transducers and one geometrically curved 3 mm lithium niobate transducer were designed and fabricated using this matching layer approach with copper as the 'mass' layer and parylene-C as the 'spring' layer. The first planar lithium niobate transducer used a single mass-spring matching network and the second planar lithium niobate transducer used a single mass-spring network, to approximate the first layer in a dual quarter wavelength matching layer system in addition to a conventional quarter wavelength layer as the second matching layer. The curved lithium niobate was press focused and used a similar mass-spring + quarter wavelength matching layer network. These transducers were then compared with identical transducers with no matching layers and the performance improvement was quantified. The bandwidth of the lithium niobate transducer with the single mass-spring layer was measured to be 46 % and the insertion loss was measured to be -21.9 dB. The bandwidth and insertion loss of the lithium niobate transducer with the mass-spring network plus quarter wavelength matching were measured to be 59 % and -18.2 dB respectively. These values were compared to the unmatched transducer, which had a bandwidth and insertion loss of 28 % and -34.1 dB. The bandwidth and insertion loss of the curved lithium niobate transducer with the mass-spring plus quarter wavelength matching layer combination were measured to be 68% and -26 dB respectively. This compared to the measured unmatched bandwidth and insertion loss of 35 % and -37 dB. All experimentally measured values were in excellent agreement with theoretical models.