CMOS-MEMS電容式微機電系統超音波換能器製作與開發

Abstract

本研究利用TSMC 0.35μm 2P4M CMOS-MEMS製程開發電容式超音波換能器(Capacitance Micromachined Ultrasonic Transducer ,簡稱CMUT)，由於元件不耐高壓、介電層的充電效應，以及種種降低元件可靠度的因素，過去CMUT只能當作很好的超音波接收器，但發射超音波訊號並非易事。本研究藉由增厚絕緣層和由脈衝發射接收器(Pulser) 取代傳統上DC加上AC的CMUT發射驅動模式，使元件得以有效的發射的超音波訊號。此外，我們發現可以利用介電層易累積電荷的現象，改採Pulser來致動元件，使得致動元件時滯留的電荷能提供足夠的靜電作用力。在此操作狀況下，元件並不需要加偏壓，即可有良好的元件的靈敏度以及換能器效能。量測的結果證實，我們所開發出來的CMOS-MEMS CMUT其中心頻率大概7~8MHz，比例頻寬(Fractional Bandwidth)大約75%~85%，靈敏度為2.894mV/kPa，由商用Pulser PR5900(Panametrics Inc., Waltham, MA, USA)致動，其發射聲場大約為760kPa。此外我們也利用本團隊研發之CMUT進行超音波的應用實驗，像是非破換性檢測和線仿體掃描，都獲得不錯的應用成果。與目前其他團隊所開發出來的CMOS-MEMS CMUT比較，此元件除了具有較大的發射聲場具有較好的靈敏度，也有較佳的比例頻寬。而不需加偏壓的操作模式，也開啟了CMUT在超音波應用上另一個世代。

In this study, a zero-bias CMOS-based Capacitive Micromachined Ultrasonic Transducers (CMUTs) with high sensitivity is developed. The device is implemented with the TSMC 0.35μm CMOS-MEMS process. Driven by a commercial pulser, it is speculated that charges were injected into the charge traps in the CMUT and these temporarily stored charges provided a built-in electric field comparable to that created by an external DC bias. Therefore, our CMUTs can achieve great sensitivity without an external DC bias. When driven by a commercial pulser, the CMUTs generated ultrasound signals with a center frequency of 7-8 MHz, fractional bandwidth of 75-85%, and the acoustic filed of 760kPa. The sensitivity at receive was 2.9mV/kPa. It is successfully demonstrated that our developed CMUT-based system was able to image a coin. Compared with other CMOS-based CMUTs, our device generally has good sensitivity and wide bandwidth. To our knowledge, it is the first zero-bias CMUT device using commercial CMOS process reported in the literature. It is believed that several applications can be explored with our developed CMUTs.