Design of Piezoelectric MEMS Resonators and Oscillators

壓電式微機電共振器與振盪器設計

嘉佳(Gayathri Pillai)

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

This work focuses on the design and analysis of different Super High Frequency AlN-SOI FBAR resonators. The main idea used in this work to reduce the energy loss is the apodization principle. Various polygons and irregular shaped FBAR resonators are studied. The behavior of composite Thin film Piezoelectric on Substrate (TPoS) is examined in detail.
The AlN SOI MEMS - CMOS devices are fabricated using InvenSense Inc. platform. COMSOL simulations for the energy distribution of various FBAR designs are compared and are discussed in the contents of the thesis. In order to have a precise frequency reference for the consumer electronic products, quality factor and electromechanical coupling coefficient should have sufficiently high values. Operation frequency of the resonators in this work is around 3GHz and its electromechanical coupling factor is about 2.5%. A detailed description of the calibration and de-embedding techniques using on chip test keys are described in the thesis. Since the resonators operates in the Giga Hertz range, these resonators are suitable in wireless RF MEMS systems or, when divided down to the MHz-range, as frequency references. To realize a low power RF oscillator design, it is desirable to have a high quality factor (Q) as the power efficiency is determined by the magnitude of the tank impedance at resonance. The resonators reported in this thesis have Q around 2000.
Printed Circuit Board (PCB) using high frequency dielectric substrate is designed to realize Pierce oscillator. An RF NPN BJT is used in the amplifier section and InvenSense's FBAR is used as the resonant tank. Three different circuit topologies are simulated. Agilent's ADS is used to carry out the simulations. Oscillations are observed around 3GHz.
This work also focuses on Lithium Tantalate resonators. The anisotropic nature of the crystal is used to find an optimum cut for temperature independent operation of resonators. Different modes of operations were studied and it was observed that for length extension mode resonator zero Temperature Coefficient of Frequency (TCF) and maximum bandwidth of 2.67% can be achieved when the material axis is oriented along Euler angle (90⁰, 41⁰, 60⁰) and (90⁰, 44⁰, 60⁰) respectively. Close-to-zero TCF resonators can be a strong candidate to make frequency reference devices as the frequency drift would be minimum.

關鍵字

AlN-SOI ； TPoS ； 品質因子 ； 射頻微機電 ； 功率負載能力 ； 切趾法共振器 ； 非等向性 ； 零頻率溫度係數

並列摘要

本研究專注於設計與分析不同的超高頻AlN-SOI FBAR共振器，並且透過切趾法(apodization)降低共振器的能量損失。在本研究中亦設計了不同多邊形以及不規則形狀之FBAR共振器，並且針對在絕緣體上之複合薄膜壓電材料(Thin-film Piezoelectric on Silicon, TPoS)之行為進行詳細的探討。
本研究所設計之AlN SOI MEMS-CMOS元件是透過InvenSense Inc.的平台所製作。為了達到精準的時脈基準並應用在商用電子產品中，其共振器需要有足夠高的品質因子以及機電耦合係數，因此本論文透過COMSOL模擬比較不同FBAR設計下之能量分布。本研究所設計之共振器的操作頻率約在3GHz而其機電耦合係數為2.5%。在論文中亦詳述在量測時使用到晶片上所設計之測試元件進行校準以及de-embedding。由於共振器操作於GHz的範圍，因此適合應用於無線射頻微機電系統中或者是當經過除法器到MHz的範圍時可以使用為時脈基準。為了實現低功率的射頻振盪器設計，通常會希望共振器擁有高品質因子，亦即所消耗的功率會受到在共振頻時的阻抗影響，而本論文中所設計共振器之品質因子約為2000。相較於SAW共振器，FBAR形式之共振器有著更好的功率負載能力。
在本研究中透過高頻專用之PCB板，並使用RF NPN BJT當作放大器配合研究中設計之FBAR以實現Pierce振盪器。透過Agilent’s ADS電路模擬三種不同電路設計，從模擬中可以發現其振盪頻率為所設計之3GHz。
在研究中亦著力於Lithium Tantalate共振器，並且利用其非等向性的特性取得對溫度最不敏感的切面。從不同操作模態中可以發現以軸向伸展模態的共振器而言，當材料軸轉向於Euler角度(90⁰, 41⁰, 60⁰)時可以觀察到約為零的頻率溫度係數(Temperature Coefficient of Frequency, TCF)以及2.67%的最大頻寬。由於在此條件下頻率漂移將會為最低，因此共振器將適合使用於商用電子元件中之時脈基準。

並列關鍵字

AlN-SOI ； TPoS ； Quality Factor ； RF-MEMS ； Power Handling ； Apodized Resonator ； Anisotropic ； Zero Temperature Coefficient of Frequency

參考文獻

[27] U. Madhow, “Introduction to Communication Systems,” University of California, Santa Barbara.

[1] W. C. Tang, T.-C. H. Nguyen, M. W. Judy, and R. T. Howe, “Electrostatic-comb drive of lateral polysilicon resonators,” Sensors and Actuators, A21-A23, pp. 328-331, 1990.

[4] S. Gong, G. Piazza, “Design and analysis of lithium-niobate-based high electromechanical coupling RF-MEMS resonators for wideband filtering,” IEEE Transactions on Microwave Theory and Techniques, pp.403-414, 2013.

[5] A. Schaufelbühl, U. Münch, C. Menolfi, O. Brand, H. Baltes, and Q. Huang, “256-pixel CMOS-integrated thermoelectric infrared sensor array,” Proceedings, 14th IEEE International Conference on Micro Electro Mechanical Systems (MEMS’01), pp. 200- 203, 2001.

[7] S. Tadigadapa and K. Mateti, “Piezoelectric MEMS sensors: state-of-the-art and perspectives,” Measurement Science and Technology, vol. 20, pp.1-30, 2009.