Development of high-frequency (~100mhz) PZT thick-film ultrasound transducers and arrays

Unrestricted Fabrication of high-frequency (30 MHz -50 MHz) ultrasonic linear arrays is still a challenge. The task is even more difficult to build arrays at a frequency higher than 100 MHz, which has the potential to provide more detail skin texture for early diagnosis of melanoma or to image small objects such as stem cells. Integrating of PZT films into MEMS micro-machined is a potential solution to such high-frequency applications. This thesis presents the development of high-frequency (~100 MHz) PZT thick-film transducers and arrays.; PZT thick films thicker than 10 µm were fabricated with a composite spin-coating method. The functional properties of the films were improved by optimization of composite solution ball-milling time, PZT sol-gel to powder mass ratio and sol-gel infiltration process. Films suitable for high frequency ultrasonic transducer applications were produced and characterized. The films were found to have a remanent polarization of 37 µC/cm2, a dielectric constant of 1250 (at 1 kHz), and electromechanical coupling coefficient (kt) of 0.34.; High-frequency (~100 MHz) single-element transducers and linear arrays were afterwards fabricated with the PZT films. The single-element transducer was constructed by transferring the film from silicon substrate to E-solder backing material. The finished transducer was found to have a center frequency of 120 MHz and a bandwidth of 60% with a layer of parylene. Ultrasonic images of the porcine eyeball and normal human skin were also successfully acquired with the transducer. 32-element kerfless high-frequency linear array was fabricated with a 12-µm PZT film by photolithography. The array had a center frequency of 120 MHz, a bandwidth of 60% with parylene matching and an insertion loss of 41 dB. Performance of the array was compared to a PZT-5H ceramic sheet kerfless array fabricated with an identical array pattern.; The development of 32-element kerfed arrays was also presented. DRIE dry-etching technique was investigated to produce an array with sharp element edges (profile angle > 85º) and narrow kerf ( ~12 µm). The kerfed array shows similar performance with kerfless one, except the crosstalk is ~5 dB lower.

高频（30 MHz～50 MHz）超声线阵的制备仍是一项技术挑战。而制备频率高于100 MHz的超声线阵则难度更甚，此类阵列可实现更精细的皮肤纹理成像，用于黑色素瘤的早期诊断，或是对干细胞等微小目标进行成像。
将锆钛酸铅（PZT）薄膜集成于微机电系统（MEMS）微加工流程中，是解决这类高频应用需求的潜在方案。
本论文围绕高频（约100 MHz）PZT厚膜换能器及线阵的研发展开工作；研究采用复合旋涂法制备了厚度大于10 μm的PZT厚膜，通过优化复合溶液的球磨时长、PZT溶胶-凝胶与粉体的质量配比，以及溶胶-凝胶渗透工艺，提升了薄膜的功能特性，最终制备得到适用于高频超声换能器的PZT厚膜并完成表征。测试结果显示，该薄膜的剩余极化强度为37 μC/cm²，1 kHz下的介电常数为1250，机电耦合系数（kt）达0.34。
随后，基于所制备的PZT厚膜，研制了高频（约100 MHz）单阵元换能器及线阵；单阵元换能器通过将薄膜从硅衬底转移至E-solder背衬材料制备而成，搭载聚对二甲苯（parylene）匹配层的成品换能器中心频率为120 MHz，相对带宽达60%。该换能器还成功采集到了猪眼球与正常人体皮肤的超声图像。
采用光刻工艺，以12 μm厚的PZT薄膜制备了32阵元无开槽（kerfless）高频超声线阵，该线阵中心频率为120 MHz，搭载聚对二甲苯匹配层时相对带宽为60%，插入损耗为41 dB。研究将该线阵的性能与采用相同阵元布局制备的PZT-5H陶瓷片无开槽线阵进行了对比。
此外，本论文还开展了32阵元有开槽（kerfed）线阵的研发工作，研究采用深反应离子刻蚀（DRIE）干法刻蚀工艺，制备了阵元边缘锐利（轮廓角＞85°）、开槽宽度窄（约12 μm）的线阵。该有开槽线阵的性能与无开槽线阵基本一致，仅串扰低约5 dB。