Design and fabrication of a surface micromachined frequency tunable film bulk acoustic resonator with an extended electrostatic tuning range
Design and fabrication of a surface micromachined frequency tunable film bulk acoustic resonator with an extended electrostatic tuning range
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具有扩展静电调谐范围的表面微机械频率可调谐薄膜体声谐振器的设计和制造
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发表时间：
2005
期刊：
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通讯作者：
H. Tilmans
中科院分区：
文献类型：
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作者：
W. Pan;P. Soussan;B. Nauwelaers;H. Tilmans
集成了可调谐串联电容器和压电谐振层的表面微机械薄膜体声谐振器（FBAR）的频率调谐已在先前的出版物中得到证明。在所提出的结构中，一个可移动的压电悬臂悬挂在底部电极的上方。悬臂和底部电极之间的气隙既可以作为可调谐电容器又可以作为隔声。然而，在静电驱动的情况下，连续调谐被限制在悬臂梁拉入之前的范围内，并且在现有技术中非常小。本文提出了一种改进的设计方案。通过改变可调谐电容器与压电谐振膜之间的有效面积比，在约6.3GHz的谐振频率下实现了更大的连续调谐范围24.5MHz（0.39%）。薄膜体声谐振器（fbar）是传统压电谐振器（如石英晶体）的微加工版本，具有体积小、频率高、质量因数高和CMOS兼容等优点。为了补偿来自不同来源的漂移，如老化、温度变化和制造不均匀性，需要小范围的频率调谐。在早期的工作(1)、(2)中，已经设计并演示了通过静电驱动对表面微机械FBAR进行频率调谐，其中具有顶部电极的垂直可移动压电层悬浮在底部电极之上，如图1所示。压电层和底部电极之间的气隙既可作为可调谐电容器，又可作为声学隔离。频率调谐是通过在上下电极之间施加直流电压，使悬浮的压电层静电位移，从而改变气隙电容和器件的频率响应来实现的。然而，由于静电驱动悬臂的拉入，这种设计只能显示非常小的连续调谐范围。在早期的工作(2)中，在拉合发生之前，仅实现了1.9MHz (~ 0.03%)o的连续调谐。因此，有必要修改设计以满足更大（> 0.2%）连续调谐范围的需求。2。调谐原理FBAR在谐振基模附近的电特性可以用如图2(a)所示的集总元件电路来近似，其中串联RF信号+直流电压
The frequency tuning of a surface micromachined film bulk acoustic resonator (FBAR), which integrates the tunable series capacitor and the piezoelectric resonating layer, had been demonstrated in previous publications. In the proposed structure, a movable piezoelectric cantilever is suspended above a bottom electrode. The air gap between the cantilever and the bottom electrode functions both as a tunable capacitor and an acoustic isolation. However, in case of electrostatic actuation, continuous tuning is limited in the range before the pull-in of the cantilever and is very small in prior art. In this paper, an improved design is suggested. By changing the the effective area ratio between the tunable capacitor and the piezoelectric resonating film, a larger continuous tuning range of 24.5MHz (0.39%) at a resonating frequncy of about 6.3GHz is realized. I. INTRODUCTION Film bulk acoustic resonators (FBARs) are microma- chined versions of the conventional piezoelectric resonators such as quartz crystals, with the advantages of small size, high frequency capability, high quality factor and CMOS- compatibility. In order to compensate for drifts from different origins such as aging, temperature change and fabricational inhomogeneities, a small range of frequency tuning is desired. Frequency tuning of a surface micromachined FBAR by electrostatic actuation has been designed and demonstrated in earlier works (1), (2), in which a vertically movable piezoelectric layer with a top electrode is suspended above a bottom electrode, as shown in Fig. 1. The air gap between the piezoelectric layer and the bottom electrode serves both as a tunable capacitor and as an acoustic isolation. Frequency tuning is realized by applying a DC voltage between the top and bottom electrode to electrostatically displace the suspended piezoelectric layer and thus change the air gap capacitance and the frequency response of the device. This design however only displays a very small continuous tuning range because of the pull-in of the cantilever by electrostatic actuation. In earlier works (2), only 1.9MHz (∼0.03% )o f continuous tuning was achieved before pull-in takes place. It is thus necessary to revise the design to meet the demand of a larger (> 0.2%) continuous tuning range. II. THE PRINCIPLE OF TUNING The electrical characteristics of an FBAR near its funda- mental mode of resonance can be approximated by a lumped- element circuit as shown in Fig. 2(a), in which the series RF signal+DC voltage