Fabrication of 20 MHz convex array transducers for high frequency ophthalmic imaging

20 MHz 192-element convex array transducers have been designed, fabricated and tested. Major design parameters are 111 μm pitch, 24 mm radius of curvature, 52° maximum view angle, 7 mm elevation width, and 30 mm geometric focus. A 1-3 composite was fabricated with a ceramic post width of 28 μm and a polymer kerf width of 9 μm. The ceramic volume fraction was approximately 57%. Three ceramic posts formed one element in the azimuth direction. A custom designed flexible circuit was aligned and attached to a scratch-diced composite and 75 Ω micro-coaxial cables. A single epoxy matching layer and a backing block were used for acoustic matching. The pulse-echo response for a typical element acquired at the axial peak, 28.0 mm, matched with a designed geometrical focusing at 30 mm, had a center frequency of 24.4 MHz and 64% of -6 dB fractional bandwidth. The estimated insertion loss at 24 MHz after compensating for attenuation and reflection was 35 dB and the maximum crosstalk was -17 dB and -22 dB for the nearest and for the next-nearest element respectively.