具低運動阻抗與高功率負載特性之微機械共振器應用於單晶片CMOS-MEMS振盪器電路

A Monolithic CMOS-MEMS Oscillator Based on A Low Motional Impedance and High Power Handling Resonator

蘇煥鈞(Su, Huan Chun)

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(2015年)

摘要

本論文利用標準CMOS 0.35 µm製程中之Poly-2層蝕刻以及Contact陣列結構設計製作出一具有微小等效換能間隙之微機械共振器陣列，並搭配全差動式轉阻放大器實現單晶片CMOS-MEMS振盪器電路架構。文中亦探討了高剛性驅動機制對本研究所設計共振器之最大功率負載的影響。
由於一般使用CMOS製程製作的電容式微機械共振器的運動阻抗(Rm)大約在0.1~10 MΩ的等級，使得將其應用於振盪器的設計變得非常困難，同時也降低了相位雜訊的表現。本論文嘗試在CMOS 0.35 µm製程中製作具有微小等效換能間隙之微機械共振器，其低運動阻抗的特性有利於振盪器電路的設計。我們同時也採用了「陣列結構」與「高剛性驅動」設計，在進一步降低運動阻抗的同時盡可能地提高其最大功率負載能力。另外在振盪器製作方面，我們設計一全差動式轉阻放大器並配合另一個「虛設」共振器(Dummy Resonator)用以在振盪器迴路中實現「Feedthrough Cancellation」的技巧。
本研究所設計之微機械共振器在真空環境的量測結果可以發現，在給定直流偏壓(VP)為30 V的條件下其Q值約為1,000，共振頻率為4.15 MHz，Rm經粗略計算為11 kΩ左右。我們同時也進行了高剛性與低剛性驅動機制的特性量測，並觀察到在兩種狀況下對於共振器之最大功率負載的明顯差異。而振盪器閉迴路的量測結果顯示了其工作頻率為4.22 MHz，且於1-kHz頻率偏移下的相位雜訊表現為-90 dBc/Hz，1-MHz頻率偏移下的相位雜訊為-121 dBc/Hz。

關鍵字

高剛性驅動 ； 運動阻抗 ； 單晶片整合 ； 電容式微機械共振器 ； 轉阻放大器 ； CMOS-MEMS振盪器 ； 相位雜訊

並列摘要

This work reports the design of a monolithic oscillator based on a low motional impedance (Rm) CMOS-MEMS resonator array with high-stiffness driving scheme in a standard 0.35 μm CMOS. Combined with polysilicon release process and the proposed “contact-array-assisted” gap spacing design, a tiny equivalent transducer’s gap (deff) of only 190 nm is successfully attained. We also discussed the effectiveness of the high-stiffness driving scheme to the proposed MEMS resonator.
Traditional capacitive CMOS-MEMS resonators often exhibit high motional impedance (Rm) in a range of 0.1 to 10 MΩ owing to their large gap spacing and insufficient transduction areas. Such a high Rm not only makes oscillator design very difficult, but also introduces additional thermal-mechanical noise in the oscillation spectrum. This work attempted to fabricate a MEMS resonator with deep-submicron gaps in the CMOS process, and the feature of the small motional impedance of the resonator makes the implementation of the oscillator easier. To address the nonlinearity issue for the narrow-gap resonators, the designed resonator is formed by multiple high-velocity coupled clamped-clamped beams with a high-stiffness driving scheme, thus greatly improving power handling and reducing motional impedance of the resonator. The dummy resonator and fully-differential transimpedance amplifier (FD-TIA) are also employed for active feedthrough cancellation.
Based on this feature, a low Rm of 10 kΩ is achieved under a medium bias voltage (VP) of only 30 V for a 4.15-MHz resonator in vacuum with the measured Q of 1,000. The combination of the mechanically coupled array and high-stiffness driving scheme significantly enhances oscillator performance. The 4.22-MHz single-chip CMOS-MEMS oscillator exhibits the phase noise of -90 dBc/Hz at 1-kHz offset and -121 dBc/Hz at 1-MHz offset, respectively.

並列關鍵字

High-stiffness Driving ； Motional Impedance ； Monolithic ； Capacitive Micromechanical Resonators ； Transimpedance Amplifier ； CMOS-MEMS Oscillators ； Phase Noise

參考文獻

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