Numerical and experimental investigation of kerf depth effect on high-frequency phased array transducer

High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the structure design and fabrication of the kerfed ultrasonic array is quite challenging when very high frequency (≥100MHz) is required.

Here we investigate the effect of kerf depth on the performances of array transducers. A finite element tool, COMSOL, is employed to simulate the properties of acoustic field and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. Furthermore, Inductively Coupled Plasma (ICP) deep etching process is used to etch 36°/Y-cut lithium niobate (LiNbO(3)) crystals and the limitation of etching aspect ratio is studied. Several arrays with different profiles are realized under optimized processes. At last, arrays with a pitch of 25μm and 40μm are fabricated and characterized by a network analyzer.

Kerf depth plays an important role in the performance of the transducer array. The crosstalk is proportional to kerf depth. When kerf depth is more than 13μm, the array with crosstalk less than -20dB, which is acceptable for the real application, could provide a desired resolution. Compared to beam focusing, kerf depth exhibits more effect on the beam steering/focusing. The lateral pressure distribution is quantitatively summarized for four types of arrays with different kerf depth. The results of half-cut array are similar to those of the full-cut one in both cases of focusing and steering/focusing. The Full-Width-at-Half-Maximum (FWHM) is 55μm for the half-cut array, and is 42μm for the full-cut one. The 5-μm-cut array, suffering from severe undesired lobes, demonstrates similar behaviors with the no-cut one. ICP process is used to etch the 36°/Y-cut LiNbO(3) film. The aspect ratio of etching profile increases with the kerf width decreasing till it stops by forming a V-shaped groove, and the positive tapered profile angle ranges between 62° and 80°. If the mask selectivity does not limit the process in terms of achievable depth, the aspect ratio is limited to values around 1.3. The measurement shows the electrical impedance and crosstalk are consistent with the numerical calculation.

The numerical results indicate that half-cut array is a promising alternative for the fabrication of high-frequency ultrasonic linear arrays. In fact, the minimum pitch that could be obtained is around 25μm, equivalent to a pitch of 1.6λ, with a kerf depth of 16μm under the optimized ICP parameters.