互補式金氧半微機電技術零偏壓電容式微型超音波換能器元件理論開發及其應用

Abstract

根據CMUT (電容式微機電超音波換能器Capacitive Micromachined Ultrasond Transduser) 運作的理論，在實際操作時，CMUT需要加上接近崩潰電壓的直流偏壓，使薄膜緊繃，儀器才能到達較高的靈敏度。但是本實驗室製作的CMOS based 低崩潰電壓 CMUT在過去的實際操作上，就算沒有偏壓到接近崩潰電壓，甚至是沒有添加任何偏壓，只接受AC訊號造成震盪，也能正常進行訊號的收發。 過去本團隊在論文中推論，這是CMUT因為長期使用所造成的電荷累積，導致元件中形成內建電廠，所以不用外加任何偏壓也能有效的進行訊號的收發。在其他人的論文中也有提出簡化的經驗模型，用來描述CMUT內建電場的現象，甚至有設計後端的回饋電路，讓CMUT即使在內建電廠產生崩潰電壓偏移時，晶片也會提供額外電壓進行補償。然而，諸多論文中對於電荷累積的機制往往停留在經驗法則，或是只考慮高電場下的Fowler–Nordheim tunneling，對於本實驗室製作的低崩潰電壓CMUT而言，這些解釋都無法恰當的描述CMUT在低電場(<4MV/cm)下的電荷累積。 本研究藉由MIM (Metal-Insulator-Metal)理論，以及數種發生在低偏壓下的電流穿隧與電荷累積模型，以及量測CMUT的I-V特徵曲線變化，試圖用穿隧電流的先後變化來解釋CMUT中內建電荷累積的現象，也期望在了解電荷累積的機制後，能有助於以後後端電路的設計與應用。 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。第一部分著力於元件結構的設計、製程的製作，與良率的改善。第二部分則著重於電荷累積模型的建立，測量其電流-電壓之特徵曲線變化，並驗證自發自收操作之可行性。

According to the fundamental principle of CMUT (Capacitive Micromachined Ultrasound), DC bias must be applied on CMUT for better energy transfer efficiency. In 2013, our team fabricated low collapse voltage CMUTs device and found out its sensitivity was larger than expected. Even without applied DC bias, the CMUTs could receive pulse-echo signal with acceptable sensitivity. Our team deduced there was charges accumulated in CMUTs device. Thus, the build-in electric field enhanced the performance of the CMUTs device. There are also empirical model of CMUT charge injection in other teams’ research. However, all those researches lacks the explanation of the charging mechanism, or they only discuss charge injection via Fowler-Nordheim tunneling in high voltage region. Most charging models cannot explain the charging effect in our CMUTs which were operated under relatively low gate voltage. Based on the theory of MIM (Metal-Insulator-Metal), we strive to build a more all-around CMUTs charging model under various bias. And we analyze the I-V characteristic change of the CMUTs device to verify our charging model. Our research utilizes low collapse voltage CMUTs fabricated by TSMC CMOS 0.35 process and etching post-process. The former part focused on the fabrication of CMUTs and the yield rate improvements. The latter part focused on the modeling of CMUT charging effect, the analysis of I-V characteristic and the verification of pulse-echo operation of the pre-charged CMUTs.