Micromachined Ultrasonic Transducer Arrays with Embedded MEMS T/R Switches

 描述（由申请人提供）：超声成像系统在传输使命模式下需要换能器阵列的高压（> 100 Vpp）激励，在接收模式下需要回波信号的低噪声放大。在成像期间，系统在发射和接收模式之间来回切换。目前可用的T/R开关实现基于电子组件，如二极管和场效应晶体管偏离理想的开/关切换行为。超声前端中的T/R开关的常见实现是基于使用二极管桥或交叉耦合二极管对，这是不理想的，因为它们在接收路径中引入噪声和失真，并且不能将接收器输入与发射脉冲完全隔离，从而导致近场中的一些扩展死区。MEMS开关的使用可以显著地帮助，因为MEMS开关在打开状态下提供高隔离并且在闭合状态下提供低插入损耗。然而，这些MEMS开关通常是为特定的RF应用而设计的，其中不需要快速的开关速度。对于超声前端，需要数百纳秒的切换时间。 我们提出开发一种快速开关（<1 µs），低控制电压（<10 V），直流接触模式的微机电开关集成和共同制造的一维电容式微机械超声换能器（CMUT）阵列。具有将微机电开关与换能器阵列集成在同一衬底上的能力还将使得能够在标准低电压CMOS工艺中实现低噪声MEMS电路。这种前端信号放大对于保持带宽和信号完整性至关重要，特别是当长电缆加载小型换能器元件时。我们在这项研究中的两个具体目标是：具体目标1：开发所提出的微机电开关结构的详细3D有限元模型，并基于该模型最终确定开关结构的尺寸，以实现< 1 µs的开关速度和< 10 V的控制电压。制造和测试在特定目标1中开发的设计，作为一个独立的可测试单元，以及使用我们在NCSU纳米制造设施开发的基于晶片键合的制造工艺与1D CMUT阵列相结合。

 DESCRIPTION (provided by applicant): Ultrasound imaging systems require high-voltage (> 100 Vpp) excitation of transducer arrays in the trans- mission mode and low-noise amplification of echo signals in the reception mode. The system is switched back and forth between the transmission and reception modes during imaging. The currently available T/R switch implementations based on electronic components such as diodes and field-effect transistors deviate from the ideal on/off switching behavior. The common implementations of the T/R switch in ultrasound frontends are based on using a diode bridge or cross-coupled diode pair, which are not ideal as they introduces noise and distortion in the receive path and fail to completely isolate the receiver input from the transmit pulse causing some extended dead zone in the near field. The use of a MEMS switch can significantly help as MEMS switches provide a high isolation in the open state and low insertion loss in the closed state. However, these MEMS switches are often designed for specific RF applications, in which fast switching speed is not a requirement. For ultrasound frontends switching times of a few hundreds of nanoseconds are desired. We are proposing to develop a fast-switching (<1 µs), low control voltage (<10 V), DC-contact mode micro-electro-mechanical switch integrated and co-fabricated with a 1D capacitive micromachined ultrasonic transducer (CMUT) array. Having the ability to integrate the micro-electro-mechanical switches on the same substrate with the transducer array will also enable implementation of the low-noise preamplifier circuitry in a standard low-voltage CMOS process. This frontend signal amplification is essential to preserve the bandwidth and signal integrity, especially when small transducer elements are loaded by a long cable. Our two specific aims in this study are: Specific Aim 1: Develop a detailed 3D finite element model of the proposed micro-electro-mechanical switch structure and based on this model finalize the dimensions of the switch structure to achieve a switching speed of < 1 µs and a control voltage of < 10 V. Specific Aim 2: Fabricate and test the design developed in Specific Aim 1 as a standalone testable unit as well as in combination with a 1D CMUT array using the wafer bonding based fabrication process we developed at the NCSU Nanofabrication Facility.