ASIC and System State Key Lab, Department of Microelectronics, Fudan University, 200433 Shanghai, China.
Ultrasonics. 2012 Feb;52(2):223-9. doi: 10.1016/j.ultras.2011.08.006. Epub 2011 Aug 24.
High-frequency ultrasonic transducer arrays are essential for high resolution imaging in clinical analysis and Non-Destructive Evaluation (NDE). However, the structure design and fabrication of the kerfed ultrasonic array is quite challenging when very high frequency (≥100MHz) is required.
Here we investigate the effect of kerf depth on the performances of array transducers. A finite element tool, COMSOL, is employed to simulate the properties of acoustic field and to calculate the electrical properties of the arrays, including crosstalk effect and electrical impedance. Furthermore, Inductively Coupled Plasma (ICP) deep etching process is used to etch 36°/Y-cut lithium niobate (LiNbO(3)) crystals and the limitation of etching aspect ratio is studied. Several arrays with different profiles are realized under optimized processes. At last, arrays with a pitch of 25μm and 40μm are fabricated and characterized by a network analyzer.
Kerf depth plays an important role in the performance of the transducer array. The crosstalk is proportional to kerf depth. When kerf depth is more than 13μm, the array with crosstalk less than -20dB, which is acceptable for the real application, could provide a desired resolution. Compared to beam focusing, kerf depth exhibits more effect on the beam steering/focusing. The lateral pressure distribution is quantitatively summarized for four types of arrays with different kerf depth. The results of half-cut array are similar to those of the full-cut one in both cases of focusing and steering/focusing. The Full-Width-at-Half-Maximum (FWHM) is 55μm for the half-cut array, and is 42μm for the full-cut one. The 5-μm-cut array, suffering from severe undesired lobes, demonstrates similar behaviors with the no-cut one. ICP process is used to etch the 36°/Y-cut LiNbO(3) film. The aspect ratio of etching profile increases with the kerf width decreasing till it stops by forming a V-shaped groove, and the positive tapered profile angle ranges between 62° and 80°. If the mask selectivity does not limit the process in terms of achievable depth, the aspect ratio is limited to values around 1.3. The measurement shows the electrical impedance and crosstalk are consistent with the numerical calculation.
The numerical results indicate that half-cut array is a promising alternative for the fabrication of high-frequency ultrasonic linear arrays. In fact, the minimum pitch that could be obtained is around 25μm, equivalent to a pitch of 1.6λ, with a kerf depth of 16μm under the optimized ICP parameters.
高频超声换能器阵列对于临床分析和无损评估（NDE）中的高分辨率成像是必不可少的。然而，当需要非常高的频率（≥100MHz）时，开槽超声阵列的结构设计和制造极具挑战性。
在这里，我们研究了开槽深度对阵列换能器性能的影响。采用有限元工具 COMSOL 模拟声场特性，并计算阵列的电特性，包括串扰效应和电阻抗。此外，采用感应耦合等离子体（ICP）深蚀刻工艺来蚀刻 36°/Y 切铌酸锂（LiNbO(3)）晶体，并研究了蚀刻纵横比的限制。在优化的工艺下实现了具有不同轮廓的多个阵列。最后，制作了具有 25μm 和 40μm 节距的阵列，并通过网络分析仪进行了表征。
开槽深度对换能器阵列的性能起着重要作用。串扰与开槽深度成正比。当开槽深度大于 13μm 时，串扰小于-20dB 的阵列可以提供期望的分辨率，这对于实际应用是可以接受的。与波束聚焦相比，开槽深度对波束转向/聚焦的影响更大。定量总结了具有不同开槽深度的四种类型的阵列的横向压力分布。半切阵列的结果在聚焦和转向/聚焦两种情况下均与全切阵列相似。半切阵列的半峰全宽（FWHM）为 55μm，全切阵列为 42μm。由于存在严重的不期望的旁瓣，5μm 切槽阵列的行为与无切槽阵列相似。采用 ICP 工艺刻蚀 36°/Y 切铌酸锂（LiNbO(3)）薄膜。随着开槽宽度的减小，刻蚀轮廓的纵横比增加，直到形成 V 形槽，并且正锥形轮廓角度在 62°和 80°之间。如果掩模选择性在可实现的深度方面不限制工艺，则纵横比限制在约 1.3 的值。测量结果表明，电特性和串扰与数值计算一致。
数值结果表明，半切阵列是制造高频超声线性阵列的一种很有前途的替代方法。实际上，在优化的 ICP 参数下，获得的最小节距约为 25μm，相当于 1.6λ的节距，开槽深度为 16μm。
