An easy and inexpensive method for fabricating high frequency annular arrays

We have developed a new method for fabricating high frequency annular arrays based on ceramic or single crystal transducer substrates. 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 the transducer substrate. Losses associated with exciting the array elements though the bonding layer are eliminated by fabricating a second set of electrodes on the transducer substrate and the two substrates are then bonded using an anisotropic conductive epoxy. The alignment problem that would normally be associated with this operation is avoided by using a rectangular grid of square electrodes on the transducer substrate in place of a matching annular array pattern. Provided the diagonal length of the square electrodes is chosen to be smaller than the spaces between the annular array elements on the polyimide, electrical contact between the two substrates can be established, independent of the orientation of the substrates, while avoiding the risk of shorting adjacent annular array elements. A 12 element, 22 MHz, 6 mm diameter annular array was fabricated using a PZT5H transducer substrate. The annular array electrode pattern was defined using a 25 micron flex circuit bonded to the front face of the PZT using Monopox AC 256. The thickness of the polyimide substrate was selected to be approximately 1/4 wavelength at 22 MHz. The array was mounted in a stainless steel housing and backed using tungsten loaded epoxy (EPOFIX, EMS, PA). The pulse echo response of the array was measured in a water bath by recording the reflection from a point source at the focal region. An excellent pulse shape with a -6dB factional bandwidth of 50% was obtained.