CMOS製程相容之熱驅動、壓阻式微感測振盪器設計與製作

Abstract

本論文藉由CMOS製程，設計製做出以電熱驅動之微振盪器，並以壓阻材料作為感測元件，此結構可藉著吸附預量測物質，改變結構的共振頻，經由壓阻材料的電阻值將質量變化轉換為電訊號，計算所吸附物質的重量。此元件可應用於量測氣體與流體中物質含量，在生醫工程領域，可依吸附材料不同感測物質或細胞之密度。 為設計出高效能之微振盪器，研究中使用電熱轉換來驅動元件，並對於驅動端經由有限元素模擬將尺寸及形狀作最佳化設計。另外，藉由研究CMOS製程中壓阻材料-polysilicon的壓阻性質，了解影響壓組材料特性的各種因素，並建立其物理模型。將結果應用於提升感測性能的穩定性與準確度。為使元件有良好的穩定度與高感測效能，以橋樑式結構作為元件的形狀(如下圖)，此設計可將電熱驅動與感測元件分開，避免溫度改變壓阻材料之特性，且維持感測元件的位移量。經由相關量測，得到元件在0.5mW的功率下，共振頻振幅可達0.6μm，並且在給予負載情況下，能藉由前後共振頻變化估計負載質量。

Micromachined resonant sensors have the advantages of robust measurement and quasi-digital output and thus have been applied to various applications such as: pressure measurement, bio-molecular detection, and etc. Realizing these devices with industrial IC CMOS processes is attractive for low cost. However, it can be very challenging for engineers to optimize the device performance. This study intends to develop a resonant sensor using commercially available CMOS process (TSMC, 2P4M) and one post-process step. This resonant sensor is excited by the electro-thermal actuations and the motions are measured by the piezoresistive sensing. The design procedures are discussed in details to indicate that this could be an optimal design with this fabrication process. Besides, different to most sensor applications that acquired piezoresistivity of a film by experiments, this study intend to develop theoretical models to predict it from the film conditions. The existing models cannot be applied because they were all developed for P-type silicon with low doping concentrations. However, it is highly doped, N-type polysilicon in this process.