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ScienceDirect Procedia Engineering 87 (2014) 1152 – 1155

EUROSENSORS 2014, the XXVIII edition of the conference series

Aluminum nitride SOI Lamb-wave resonators towards multifrequency, multi-sensitive temperature sensor platform Margarita Narduccia,*, Marco Ferrarib, Vittorio Ferrarib, Humberto Campanellaa a

Sensors, Actuators and Microsystems Program, Institute of Microelectronics A*STAR, Singapore 117685, Singapore b Department of Information Engineering, University of Brescia, Via Branze 38, 25123, Brescia, Italy

Abstract We report a resonant sensor platform based on multi-band Lamb acoustic wave resonators and multiple thermal sensitivities. The demonstrated devices are enabled by a powerful platform which uses aluminum nitride (AlN) on silicon-on-insulator (SOI) substrate that offers process robustness, design flexibility to provide devices with multiple frequencies within the same process, and multiple temperature coefficients of frequency (TCF) that can be tailored to meet the application needs. Symmetric-mode, zeroth-order resonators (S0) with frequencies between 218 MHz and 1.9 GHz, quality factors (Q) up to 2200, and TCFs from 11.4ppm/°C to -2.5ppm/°C are some of the remarkable features of the platform. © 2014 The Authors. Published Elsevier Ltd.access article under the CC BY-NC-ND license 2014 Published by Elsevier Ltd. by This is an open Peer-review under responsibility of the scientific committee of Eurosensors 2014. (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: Acoustic resonators; Lamb wave resonators; AlN SOI

1. Introduction AlN resonators have been investigated as an alternative to quartz for timing applications. However, the main drawback of AlN-based devices is their poor temperature stability. To overcome this issue, different temperature compensation methods can be implemented. A first approach is a passive temperature compensation achieved by designing the resonator with multilayers of opposite coefficients of frequency (TCF) or p-n doping or by engineering the device geometry, however these methods are limited by non-linearities of the material properties. A second approach is to use an external reference device to measure temperature with a readout circuitry to perform real-time temperature compensation. However this increases the physical size of the sensor. Another approach is to use self-temperature sensing, by exciting and combining two different modes coming from the same resonator, or two tones produced by two resonators. The frequency modes or tones are assumed to have different TCFs values to produce a temperature-dependent beat frequency that is inherent to the resonators, and thus eliminates any spatial and thermal lag between the two devices in the case of active temperature compensation –resonator and temperature

1877-7058 © 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.370

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sensor (in a single-device system, resonator and sensor are spatially co-located; therefore exposed to the same environmental conditions) [1, 2, 3]. This latter scheme provides single-frequency temperature feedback to the control system of the actuator. Such scheme offers a level of resolution and sensing range that might be improved with the realization of multiple feedback paths, each path delivering information on different frequencies and sensitivities. Towards a multiplefeedback and highly accurate MEMS temperature sensor, miniaturized devices with multiple thermal behavior and high Q are required. AlN Lamb wave resonators are ideal candidates for resonant sensors as they combine high acoustic velocity (~10,000m/s) and high frequency sensitivity thereof, and as a difference to merely bulk acoustic wave (BAW) resonators, they can be implemented on single-chip platforms to obtain multiple frequencies, each one with a different sensitivity to temperature. AlN resonators with [002] orientation offer TCFs up to -25 ppm/°C, which is suitable for temperature sensing applications [4-5]. 2. Lamb wave resonator design and process As test experiment for sensor implementation, a set of Lamb wave resonators was designed and fabricated by leveraging on AlN SOI platform. The key features of our Lamb-wave resonators are: (1) Lithography-based dimension control in the same chip using the CMOS compatible Mo-AlN-Mo-SiO2 platform to demonstrate resonance frequencies from 218 MHz up to 1.9 GHz and (2) Tailored temperature compensation that can adjust the TCF from -11.4 ppm/°C down to -2.5 ppm/°C on a device-basis and process-basis. The resonator is a suspended structure that consists of an array of interdigitated top Mo electrodes (checkerpatterned), a thin film of AlN, and one grounded bottom Mo electrode (Fig. 1). The stack consists of 300 nm SiO2 deposited on the Si device layer, 20-nm AlN seed layer, 250-nm bottom Mo electrode and 1-um AlN acoustic layer, another 250 nm of top Mo electrode to implement the top interdigitated electrodes, and lastly SiO2 is deposited to passivate and customize the TCF of the resonator. Devices are released by XeF2 –based isotropic Si etching. By applying an electric field between adjacent top electrodes (Lateral field excitation - LFE) and setting the bottom electrode as ground, two intersecting acoustic waves –one traveling along X direction and the other along Y direction– are simultaneously excited. The resonators were designed to operate at their lowest symmetric wave propagation mode (S0), where the energy conversion efficiency is the highest amongst the other possible Lamb and lateral-field modes. The S0 mode is also less sensitive to fabrication process variation, i.e. thin film thickness variation, which enables better frequency control [6-7]. λ λ

10um

(a)

(b)

Fig. 1. AlN SOI Lamb wave resonator process: (a) Conceptual cross section showing a 1um AlN layered that is sandwiched between patterned top and bottom Mo electrodes. Another thin AlN layer serves as seed layer for good crystal growing. Passivation of top Mo is provided by a SiO2 layer. Openings at pad locations are provided to contact Mo electrodes with Al pads; (b) Scanning electron microscope image of the top view of one example resonator showing the checkered structure of top Mo. Inset: Detail of top Mo patterning.

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3. Characterization The frequency response and thermal performance of resonators were extracted from scattering (S) parameters data taken by an Agilent E5071B network analyzer in a setup using a Cascade Microtech’s probe station with temperature and vacuum control. Figure 2 plots the magnitude and phase frequency response of four Lamb wave resonators with various wavelengths λ that deliver frequencies from 200 MHz to 1.9 GHz. The quality factors obtained are suitable for integration with a control circuit. On the other hand the high frequency improves the overall sensitivity of the system, which can be further increased as we discuss in the next section.

Fig. 2. Measured frequency characteristic of Lamb wave resonators with various wavelengths λ.

Figure 3 shows the measured fractional frequency variation vs. temperature for different resonators. Each point in the plot was taken by sweeping the chamber’s temperature with a fixed pressure from -25 °C to +75 °C, and measuring the corresponding resonance frequency. Temperature was changed in 25 ºC steps with at least 30 minutes of temperature stabilization between S-parameter measurements. Part (a) shows how TCF values change with frequency, whereas part (b) shows that TCFs can be customized by controlling the etching time (T etch), which tailors the passivation SiO2 thickness, thus enabling customizable TCF range. From the figures we observe a direct dependence of the TCF with frequency, which enables thermometric beat frequency systems. Also, the TCF customization for a given frequency allows fine tuning of the application requirements, which is especially useful in thermometric beat frequency sensors.

Fig. 3. Temperature coefficient of frequency (TCF) of devices fabricated within the platform: (a) Measured fractional frequency variation vs. Temperature for Lamb wave resonators from 200 to 1900 MHz; (b) Measured fractional frequency variation vs. Temperature as a function of processing time. TCF is extracted from the curve fitting of experimental data. TCF can be customized as a function of the thickness of the oxide layer on top of resonators.

Margarita Narducci et al. / Procedia Engineering 87 (2014) 1152 – 1155

4. Discussion on temperature sensing applications Thermometric beat frequency is one example of the application of our resonator platform in sensing systems. In such application the difference between the TCFs of two different frequencies is used to improve the overall thermal sensitivity [8]. To accomplish this purpose a circuit controller uses this difference and creates a beat frequency with multiplied sensitivity that is proportional to the differences of the first and second order coefficients of the TCF. Using as examples resonators R1 (215 MHz) and R2 (900 MHz) in Figure 3(a), which have 1 st –order TCFs of -2.5 °C and -8.4 ppm/°C, respectively, and 2nd –order TCFs of -0.05 ppm/°C and -0.02 ppm/°C, respectively, we could provide a 1st-order sensitivity of 127 ppm/°C. This represents an improvement of two orders of magnitude in the thermal sensitivity.

5. Conclusions and future work We have presented an AlN SOI MEMS resonator platform with multiple-frequency and multiple-TCF capabilities. As one conclusion, our platform shows to be suitable for multi-sensitivity temperature sensor applications. The high Q and adjustable TCF capabilities can be exploited to provide general purpose temperature sensors with high accuracy and customizable sensitivity ranges. Future developments of this work will consider ASIC implementations of thermometric beat frequency sensors based on multiple frequencies provided by two or more MEMS resonators. Moving forward, we propose an extension of the single-path thermometric system to multiple-feedback paths. To do so, we will need to study the behavior of more variables involved in the resonator thermal performance, like the hysteresis of the TCF. In this line of study, compensation mechanisms to reduce the hysteresis might be useful to optimize the response time of the sensor.

References [1] M.J. Dalal., J.L. Fu and F. Ayazi, Simultaneous dual-mode excitation of piezo-on-silicon micromechanical oscillator for self-temperature sensing, IEEE 24th International Conference on Micro Electro Mechanical Systems (MEMS), pp. 489-492, 2011. [2] C.M. Jha., G. Bahl., R. Melamud., S.A. Chandorkar., M.A. Hopcroft., B. Kim., M. Agarwal., J. Salvia., H. Mehta and T.W. Kenny, CMOSCompatible dual-resonator MEMS temperature sensor with milli-degree accuracy, Transducer, pp. 229-232, 2007. [3] C. Zuo., J.V. Der Spiegel and G. Piazza, Dual-mode resonator and switchless reconfigurable oscillator based on piezoelectric AlN MEMS technology, IEEE Transactions on electron devices, Vol. 58, No. 10, 2011. [4] G. Wingqvist, L. Arapan, V. Yantchev and I. Katardjiev, A micromachined thermally compensated thin film lamb wave resonator for frequency control and sensing applications, J. Micromech. Microeng, 19 (2009) 035018. [5] C.M. Jha., G. Bahl., R. Melamud., S.A. Chandorkar., M.A. Hopcroft., B. Kim., M. Agarwal., J. Salvia., H. Mehta and T.W. Kenny, CMOSCompatible dual-resonator MEMS temperature sensor with milli-degree accuracy, Transducer, pp. 229-232, 2007. [6] H. Campanella, L. Khine and J. Tsai, Aluminum Nitride Lamb-wave resonators for high-power high-frequency Applications, in IEEE Electron Device Letters, vol. 34, no. 2, pp.316-318, 2013. [7] C-M. Lin, Y-J. Lai, J-C. Hsu, Y-Y. Chen, D. G. Senesky and A. Pisano, High-Q aluminum nitride Lamb wave resonator with biconvex edges, in Applied Physics Letters, 99,143501, 2011. [8] S. S. Schodowski, Resonator self-temperature-sensing using a dual-harmonic-mode crystal oscillator, in Proc. 43rd Annual Symposium on Frequency Control 1989, pp. 1-7.

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