用于高频微加工超声换能器的硅衬底上 PMN-PT 单晶厚膜

In this work, a novel high-frequency ultrasonic transducer structure is realized by using PMNPT-onsilicon technology and silicon micromachining. To prepare the single crystalline PMNPT-on-silicon wafers, a hybrid processing method involving wafer bonding, mechanical lapping and wet chemical thinning is successfully developed. In the transducer structure, the active element is fixed within the stainless steel needle housing. The measured center frequency and −6 dB bandwidth of the transducer are 35 MHz and 34%, respectively. Owing to the superior electromechanical coupling coefficient (kt ) and high piezoelectric constant (d33) of PMNPT film, the transducer shows a good energy conversion performance with a very low insertion loss down to 8.3 dB at the center frequency.

High-frequency ultrasonic imaging has many clinical applications because of its improved image resolution (less than 10 μm) and is receiving acceptance as a clinical tool for the examination of skin, the arterial walls  and anterior chamber of the eye . The development of high-frequency transducers has pushed the limits of ultrasonic imaging technology, giving diagnostic quality information about microscopic structures in living tissue. However, with operating frequency increasing up to 30 MHz, conventional transducer fabrication techniques are facing many difficulties in handling the miniaturized element and inter-element dimensions . Piezoelectric micromachined ultrasound transducers (pMUTs) have thus been investigated as a promising new approach. By employing micromachining technologies, pMUTs offer advantages such as size reduction, batch production with high precision, repeatability and yield, low cost and possible realization of complete systems-on-achip.

In this work, we report the development of 〈001〉-orientated PMN-30%PT thin film on-silicon wafer by using a hybrid method involving wet etching and micromachining. For highfrequency operation, thicknesses of PMN-PT layers are in the range from several microns to several tens of microns. With the PMNPT-on-Si wafer, a prototype pMUT with a center frequency of 35 MHz was fabricated and characterized.

For making of prototype transducer, the silicon die was housed in stainless steel tubing with outer diameter of 2.5 mm. An electrical connector was fixed to the conductive backing using a conductive epoxy. A Cr/Au layer was sputtered across the PMN-PT layer and the stainless steel housing to form the ground plane connection. Finally, a layer of vapor deposited parylene with a thickness of 10 μm was coated on the transducer surface as an acoustic matching layer, which is used to improve ultrasonic energy transmission between the piezoelectric material and the load medium. The acoustic impedance of the matching layer is 2.59 MRayl.

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This paper describes the development and fabrication of a high-frequency (35 MHz) ultrasonic transducer with an aperture size of 0.7 × 0.7 mm2  by micromachining of single crystalline PMN-PT on silicon substrate. The PMN-PT thin layer exhibits excellent electromechanical properties which are comparable to those of bulk PMN-PT single crystals. The transducer has a −6 dB bandwidth of 34% and a minimum two-way insertion loss of 8.3 dB at the center frequency. This insertion loss is significantly better than what can be obtained using devices with the conventional piezoelectric materials (such as PZT and PVDF). A further improvement in both bandwidth and insertion could be achieved through the use of a double-matching layers scheme.