CMOS-based 電容式微機電超音波換能器操作於崩潰模態之開發

Abstract

本研究利用TSMC 0.35μm 2P4M CMOS-MEMS製程製作電容式微機電超音波換能器（Capacitive Micromachined Ultrasonic Transducers，簡稱CMUT），並將CMUT操作於大於崩潰電壓的模態（稱為崩潰模態）。然而本團隊製作CMUT的良率需要提昇，因此本研究第一部分著重於尋找濕蝕刻製程上影響良率的關鍵步驟，發現烤乾後的晶片可以確保良率，目前製作CMUT的良率到達100%；且CMUT在崩潰模態時必須在水中偏壓高達90V的直流電壓，防水鍍膜強度不足使57%的薄膜損壞，且水中漏電現象明顯使電路板氧化，然而提高防水鍍膜厚度使元件特性下降，因此本研究第二部分將防水薄膜厚度最佳化，以及電路板的防水設計。 本研究利用共軛焦顯微鏡呈現崩潰模態時的表面輪廓，量測出CMUT的崩潰電壓在50~60V區間；也呈現CMUT操作於崩潰模態的特性。由於直流偏壓大於崩潰電壓，因此CMUT的靈敏度變好，且薄膜觸底將使中心頻率變化。本研究利用商用Pulser 5077PR與商用探頭發射超音波，CMUT在崩潰模態的接收效率為傳統薄態的4倍，且中心頻率由2.7MHz轉換到7.9MHz；而CMUT做自發自收時，傳統模態的電壓峰對峰值達2.625V，比例頻寬132%，然而由於致動元件的交流電壓遠大於直流偏壓，元件會操作於深崩潰模態，在崩潰模態的自發自收電壓峰對峰值可達3.427V，但比例頻寬下降至40~45%。

In this work, experimental results of Capacitive Micromachined Ultrasonic Transducers (CMUTs) operated in collapse-mode is presented. The device is implemented with the TSMC 0.35μm CMOS-MEMS process. Through the surface profile measured by a confocal microscope, the collapsed voltage is measured at approximately 50 ~ 60V. With a bias voltage over the collapse voltage, the receiving sensitivity of CMUTs is increased 4 times larger and the center frequency is shifted from 2.7MHz to 7.9MHz. Driven by a commercial pulser, the received echo signal using our device in traditional mode is 2.625Vpp and the fractional bandwidth is 132%. Affected by the deep-collapse operation, the largest received echo signal reaches 3.427Vpp and the fractional bandwidth reduced to approximately 40 ~ 45%. The CMUT center frequency shifts from 3.12MHz to 9.12MHz. We also present an improved wet etch process to significantly increase the yield to almost 100%. Depositing thicker sealing membrane material ensures the robustness of CMUTs. Our developed CMUTs operated in DC bias of 90V in a water tank for over half an hour was demonstrated.