機械耦合式CMOS-MEMS濾波器之設計與特性探討

Design and Characterization of Mechanically-Coupled CMOS-MEMS Filters

陳昭瑜(Chao-Yu Chen)

指導教授 ： 李昇憲

國立清華大學/工學院/奈米工程與微系統研究所/碩士(2013年)

摘要

本研究將針對機械耦合式CMOS-MEMS濾波器進行設計及特性探討。為了於頻譜上實現訊號濾波功能，文中所使用的共振器皆以雙端自由樑來實現，並以特殊設計之機械耦合樑，將一對共振器單元組成雙自由度系統，其中耦合樑之幾何形狀與座落位置將決定濾波通帶之頻寬大小。
建立在上述基礎下，我們利用CMOS結構中現有之金屬堆疊層進行濾波器製作，過程中為了使電容式元件具備較低的運動阻抗值，將使用陣列式結構設計以及間隙縮減技術，藉此獲得大範圍之感測面積與次微米換能間隙。量測上將使用De-Embedding技術去除外界寄生效應的干擾，並搭配適當端點阻抗，使平坦之濾波通帶順利於頻譜上實現。值得一提的是，於量測過程中，亦發現使用複合材料將有效提升系統共振頻之溫度穩定度。
而文中亦提出利用氧化物複合結構所開發的濾波元件，相較於上述之金屬複合結構，雖說元件後製作過程相對複雜，但理想上可獲得較為優異之機電轉換係數與品質因數表現。此外，鑑於傳統雙埠量測架設將為元件帶來可觀的寄生效應，因此，本研究使用差分驅動架構進行濾波元件的操作，在不使用De-Embedding技巧下，成功移除雜散電容對濾波頻帶所帶來的影響。
本文採用標準CMOS製程來實現中頻機械耦合式帶通濾波器，希冀能藉由其高度電路整合特性，進一步改善傳統應用於無線收發系統內之離散式或高功率消耗之濾波元件。

關鍵字

金氧半導體微機電系統 ； 機械耦合 ； 電容式元件 ； 微機械共振器 ； 微機械濾波器 ； 窄頻應用 ； 阻抗匹配 ； 雙埠量測 ； 差分驅動

並列摘要

This work reports on the design and characterization of a mechanically-coupled CMOS-MEMS filter centered at several MHz with a narrow bandwidth and reasonable insertion loss after filter termination performed in a 4-port network analyzer. To implement a bandpass filter, the proposed filter structure produces two physical resonance modes, therefore forming a filter passband with a desired bandwidth by the use of the mechanical coupler at proper coupling locations.
By the use of a conventional filter design, the metal stacking layers in a foundry-oriented CMOS platform were used to fabricate the device structures, making great progress aligned with existing fabrication lines. To further reduce the motional impedance, the high-velocity coupled array and gap-reduction mechanism were adopted to create larger transduction areas and tiny gap spacing for capacitive transducers, respectively. However, due to the existence of undesired parasitics from a typical two-port configuration, the resonance response would be dwarfed and masked by the background feedthrough floor. Furthermore, the matching condition is also limited by shunt capacitance at the I/O ports for filter transmission, that significantly barricades the proper filter termination. To solve this issue, a pure motional response of the proposed filter can be extracted once the de-embedding scheme is carried out.
To attain the lower material loss, the silicon dioxide structure was developed in this work as well. Ideally, the SiO2 can provide better electromechanical coupling and higher quality factor as compared with the metal stacking counterparts. Based on the electrical isolation of the oxide structure, the filter devices can be operated in differential configuration via the balun function on the network analyzer. With that, the feedthrough parasitic can be evidently alleviated without any post-data processing, thus creating 30dB noise-floor improvement in filter performance.

參考文獻

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