A micromachined thermally compensated thin film Lamb wave resonator for frequency control and sensing applications

Abstract

Micromachined thin film plate acoustic wave resonators (FPARs) utilizing the lowest order symmetric Lamb wave (S0) propagating in highly textured 2 μm thick aluminium nitride (AlN) membranes have been successfully demonstrated (Yantchev and Katardjiev 2007 IEEE Trans. Ultrason. Ferroelectr. Freq. Control 54 87-95). The proposed devices have a SAW-based design and exhibit Q factors of up to 3000 at a frequency around 900 MHz as well as design flexibility with respect to the required motional resistance. However, a notable drawback of the proposed devices is the non-zero temperature coefficient of frequency (TCF) which lies in the range −20 ppm K −1 to −25 ppm K −1 . Thus, despite the promising features demonstrated, further device optimization is required. In this work temperature compensation of thin AlN film Lamb wave resonators is studied and experimentally demonstrated. Temperature compensation while retaining at the same time the device electromechanical coupling is experimentally demonstrated. The zero TCF Lamb wave resonators are fabricated onto composite AlN/SiO 2 membranes. Q factors of around 1400 have been measured at a frequency of around 755 MHz. Finally, the impact of technological issues on the device performance is discussed in view of improving the device performance.

Thermally compensated aluminum nitride Lamb wave resonators for high temperature applications

“…The thermal compensation technique using a compensating layer of SiO 2 to a device structure has been applied to many different kinds of resonators. [7][8][9][10][11][12][13] This present work demonstrates the thermal compensation for AlN Lamb wave resonators operating at high temperature using an AlN/SiO 2 composite structure. By designing composite structures with different normalized AlN thickness ͑h AlN / ͒ and normalized SiO 2 thickness ͑h SiO 2 / ͒, Lamb wave resonators with a zero first-order temperature coefficient of frequency ͑TCF͒ at 214°C, 430°C, and 542°C are experimentally demonstrated.…”

“…The metalized interface significantly enhances the electromechanical coupling coefficient because a strong electric field can be induced between the IDT electrodes and the metalized interface. [11][12][13][14] Aluminum ͑Al͒ is often used as the electrode material for the bottom electrode and the IDT. However, the Lamb wave resonators are designed to operate at high temperature up to 700°C so Al is not preferred due to its low melting point of 660°C.…”

Flexible Surface Acoustic Wave Device with AlN Film on Polymer Substrate

“…Since the resonance frequency has the largest total stored energy, the SAW resonator with the wavelength of 7.128 μm has the resonance frequency of 1.415 GHz, corresponding to the acoustic wave velocity of 10086 m/s, and that with Polymer substrates have a very low acoustic impedance (2 Mrayls) which is much smaller than that of the AlN layer (36 Mrayls). The Al underlay enhances the electromechanical coupling coefficient as an electric field can be induced between the IDT electrodes and the Al underlay [10]. Moreover, the c-axis-oriented thin AlN films have relatively large thicknesses at about d = 0.2λ, where λ is the acoustic wavelength; therefore, the detected acoustic waves are the symmetrical S 0 Lamb waves which theoretically have the acoustic wave velocity near 10,000 m/s [11][12][13] in AlN.…”

Lamb waves propagation in functionally graded piezoelectric materials by Peano-series method

“…The obtained numerical results can be used to design different sensors with 36 high performance working at different frequency ranges by adjusting the extent of the gradient property. Lamb waves in piezoelectric materials or layered piezoelectric 56 structures have been of a major concern due to their high 57 performance and simple particle motion in technological 58 applications such as surface acoustic wave (SAW) sensors [1][2][3][4][5][6]. 59 Important advantages of SAW sensors are obtained by utilizing 60 the lowest-order antisymmetric Lamb wave mode (A 0 ) propagating 61 in plates that are only a few micrometers thick.…”

“…Lamb waves are 64 also useful for measuring the elastic properties of thin films [1]. 65 Recently, resonant Lamb wave geometries based on c-oriented alu-66 minum nitride (AlN) thin film membranes are studied [3,5,6]. 67 Attention has been focused on the lowest order symmetric (S 0 ) 68 Lamb mode because of its extremely high velocity compared to 69 the (A 0 ) mode, thus being suitable for high frequency applications 70 [4][5][6] which were homogeneous within each layer [5,8].…”