Progress towards wafer-scale fabrication based on gel casting technique for 1-3 randomised piezocomposite μUS linear array

Microultrasound (mu US) linear arrays operating at frequencies over 25 MHz have applications in high resolution biomedical imaging. 1-3 connectively piezoceramic - polymer composite (piezocomposite) material is attractive for fabrication of these devices due to its high effective electromechanical coupling coefficient and low acoustic impedance for better acoustic matching between transducer and tissue. However, a major concern with this type of material comes from interference between the fundamental thickness-mode resonance and spurious modes, which is usually generated by wave propagation and reflection within the repetitive and symmetrical structure of classical piezocomposite. In general, a fine spatial scale is required of the material structure to suppress the spurious modes; however, the fabrication process is challenging using standard dice-and-fill methods at the fine scales required for high frequencies. A promising way to overcome this challenge is to manipulate the lateral geometry and spacing of the piezoceramic pillars with a random distribution. In this work, gel casting in association with a micromoulding technique has been developed for manufacturing 1-3 randomised piezocomposite active material for mu US linear arrays. 48 vol% solid loading of piezoceramic powder with 30 wt% Hydantoin resin content was employed to prepare a low viscosity aqueous suspension. Through varying powder size, it was found that the suspension with 1.22 mu m powder had the highest viscosity, similar to 0.47 Pa.s, and a short gelation time, similar to 10 mins. However, all suspensions had viscosities less than 1 Pa.s at a shear rate of 100 s 1, indicating that they had good flowability. The green body samples showed mean flexural strength 49.7 +/- 2.49 MPa. After piezocomposite fabrication with randomised pillars, surface planarisation was used to obtain reliable edge definition of photolithographically-defined electrodes. 20-element arrays with 50-mu m element pitch were configured using a bilayer lift-off process. The 1-3 randomised piezocomposite demonstrated its capability to minimise the effects of spurious modes in the thickness mode frequency range, while the thickness resonances provided k33 = 0.67. Without a matching layer, the array produced a 6 dB bandwidth of 38.4%- and 20-dB pulse length of 0.26 mu s. These results show that 1-3 randomised piezocomposite fabricated from gel-casting associated with a micromoulding technique is feasible for fabrication of mu US linear arrays and may offer a route to small wafer-scale production.

Automated summary

This article introduces a preparation method for 1-3 randomised piezocomposite used in the fabrication of micro-ultrasound linear arrays. Through gel casting and micromoulding techniques, the structure of the material was successfully controlled, allowing for effective suppression of spurious modes while maintaining good flowability. The array made from this material demonstrated a lower resonant ability in the thickness mode frequency range, with a wide bandwidth and short pulse length.