Ba0.7Sr0.3TiO3 (BST) capacitors for high frequency circuits have been fabricated at 175 degrees C on epoxy-based copper-clad printed circuit boards, having a glass transition temperature of approximate to 200(degrees)C by hydrothermal processing of dried BST acetate-based sol-gel powder slurries. The film thickness was 2-10m. BST powders were prepared respectively as ceramic derived powders processed from oxide mixtures fired to 1100 degrees C, sol-gel derived powders fired to 750 and 1100 degrees C, and a low temperature solution process dried at 500 degrees C. The performance of the capacitor was determined by both the dielectric properties of the powder and the interaction between the crystallizing gel and the powder surface. High relative permittivity is obtained for sol-gel derived powders fired to 750 degrees C, while high voltage tunability is a characteristic of the ceramic derived powders. Low temperature powders are ineffective. The difference is attributed to the nature of the crystalline interface formed between the surface of the powder particles and the hydrothermally crystallized BST formed from the dried gel within the composite. The optimum molar concentration of the hydrothermal solution was 0.1M with a Ba content larger than that of the powder. Capacitor composites having a relative permittivity of up to 300 at 20MHz were demonstrated.

Fabricating arrays for high-frequency image applications such as ophthalmic imaging, intravascular imaging, and small animal imaging is challenging. For example, an array for intravascular imaging must be small enough to fit within the lumen of a catheter and inexpensive enough to be discarded after a single use. This article presents a new method for fabricating high-frequency annular arrays that is simple and inexpensive. The annular array elements are defined by the electrode pattern on a back surface of a polyimide quarter-wavelength matching layer that is glued to the front face of a ceramic transducer substrate (PZT5H). Electrical losses associated with bonding the matching layer to the transducer substrate are reduced by fabricating a second set of electrodes on the transducer substrate and then bonding the substrates using an anisotropic conductive epoxy. The feasibility of this technique was established by fabricating a seven-element, 20-MHz, 5-mm diameter annular array. The prototype array produced a pulse with a −6-dB factional bandwidth of 50%, an insertion loss of 22 dB, and secondary lobes in the radiation pattern at f/2 that decreased to −65 dB with respect to the main lobe with a peak amplitude of −53 dB.

A 9-element annular array is presented that employs a newly-proposed interconnection scheme that simplifies the fabrication process. The fabricated array is a hybrid transducer structure incorporating both a piezoelectric layer and a silicon substrate in the same device. The interconnection scheme consists of a set of 9 equal area Cr/Au electrodes with a 2 mm aperture and 17 μm kerfs patterned on the surface of the silicon substrate using photolithography. A grid of Cr/Au electrodes was patterned on the surface of the piezoelectric layer, and the two layers were connected using an anisotropic conductive adhesive. To avoid the severe alignment restrictions that would result if the two electrode patterns were identical, a grid-pattern of square electrodes was substituted on the piezoelectric layer with a smaller diagonal dimension than the spaces between the silicon electrodes. A Tungsten-loaded epoxy backing layer was added to the acoustic stack and an impedance plot was measured for a single array element. Both 22 MHz and 40 MHz arrays were manufactured and the impedance plots show good correspondence with KLM modeling. A pulse-echo response was generated for the 22 MHz array, showing no degradation due to the silicon layer.