Email: {rezaa,ayazi}gece.gatech.edu. Abstract-This paper reports on the fabrication and measurement of ZnO-on-diamond length-extensional resonant mass ...
IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006
A
Temperature-Compensated ZnO-on-Diamond Resonant Mass Sensor Reza Abdolvand, Zhili Hao*, and Farrokh Ayazi School of Electrical and Computer Engineering, Georgia Institute of Technology Atlanta, GA, 30332 Email: {rezaa,ayazi}gece.gatech.edu
Abstract-This paper reports on the fabrication and measurement of ZnO-on-diamond length-extensional resonant mass sensors. Improved mass sensitivity due to the higher frequency of operation as well as lower motional impedance compared to capacitively-transduced sensors of the same type is demonstrated. Measured mass sensitivity of I-kHz/pg is shown at -39MHz operation frequency. Moreover, a small temperature coefficient of frequency (TCF) of -6ppm/°C is achieved by incorporating a layer of silicon dioxide in the resonator structure. I.
INTRODUCTION
Quartz crystal microbalance (QCM) mass sensors have found many applications in chemical and biological sensors [1]. However, their relatively large size can limit the extent in which QCM sensors are used in microsystems to detect small traces of chemical or biochemical agents. Specifically, in applications for which an array of mass sensitive sensors is required to distinguish between various types of molecules, QCM sensors fail to offer a compact and cost effective solution. In recent years, micromachined resonant mass sensors with much smaller form-factor have attracted a lot of attention to fill in the gap for arrayed and/or implantable mass sensors. Cantilever beams [2] and thin film bulk acoustic resonators [3] are amongst the more successful realizations of micromachined mass sensors. Higher frequency of operation can potentially improve the sensitivity of these devices compared to QCM sensors, if high quality factors are maintained [4]. Sensitivity to environmental parameters (e.g., temperature) is also an issue that needs to be addressed in order to facilitate robust operation of micromachined mass sensors. Complex actuation and readout mechanism [2] is another bottleneck hindering widespread use of these devices in microsystems. Capacitively-transduced lateral bulk acoustic resonant sensors were introduced in our group as an effort to address some of the issues associated with micromachined mass sensors [5]. These devices demonstrated relatively high Q values in air at -12MHz while minimizing the change in the effective stiffness of the structure imposed by absorbed mass. However, to increase the sensitivity, the device
*
dimension needed to be scaled down, resulting in a reduced capacitive transduction area. Therefore, motional impedance of the device will increase, which translates to high power consumption and higher phase noise when interfaced with an oscillator circuit [6]. More importantly, small capacitive air gaps are prone to blockage and squeeze film damping when exposed to environment. In this paper, piezoelectric transducers are employed for actuation and read-out of the sensor structure introduced in [5]. In this way, the motional impedance of the device is reduced by orders of magnitude to a range that can be comfortably interfaced with the most generic oscillator circuits. Moreover, application of polycrystalline diamond as the resonator structural material is investigated. High elastic modulus of diamond (highest in nature) increases the frequency of operation, resulting in higher sensor sensitivity while sustaining relaxed lithographic requirement. On the other hand, TCF of a diamond resonator is a relatively small value (-12ppm/°C) [7] that can be compensated by incorporating a thin layer of silicon dioxide in the resonator structure. The class of devices introduced in this work can promise extremely compact assembly of fully temperaturecompensated array of mass sensors in the future. DEVICE OPERATION PRINCIPLE As shown in Fig. 1, the resonant structure consists of a central block and two annexed sensing platforms attached to it via separation beams. This design is utilized to minimize the change in the effective stiffness of the resonator when an extra mass is absorbed to the sensing platform [5]. II.
Connection
to the bottom electrode
ZnO -i
Figure 1. The piezoelectrically-transduced lateral resonant mass sensor.
Dr. Hao is now an assistant professor at the Old Dominion University.
1-4244-0376-6/06/$20.00
}2006 IEEE
Connection to the top electrode
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IEEE SENSORS 2006, EXCO, Daegu, Korea / October 22-25, 2006
The structural material (polycrystalline diamond in this case) is coated with a thin piezoelectric layer (ZnO). The piezoelectric layer covering the central block is sandwiched between two metal electrodes. The top electrode can be split into two separate electrodes for two-port operation of the device. However, by doing so, the available actuation/sense area is reduced to half, increasing the motional impedance of the resonator. The ZnO layer can be either removed or kept intact on the sensing platform depending on the application and the receptor coating. An alternating electrical field applied across the ZnO layer introduces an in-plane strain field through the d3l piezoelectric coefficient of ZnO. When the frequency of the applied electrical signal matches the lateral bulk-extensional resonance mode of the structure, the vibration amplitude and the current passing through the electrodes are maximized. Figure 2 is a Butterworth-Van Dyke (BVD) equivalent electrical model of the device.
Figure 2. BVD equivalent electrical model of the piezoelectricallytransduced lateral resonant mass sensor.
Cf is the static capacitance of the device created between the electrodes. Lm, Cm and R,, are called motional inductance, capacitance and resistance of the resonator respectively. The resonance frequency is calculated from: 2z=
2z LmCm
Meff
(1)
At resonance the motional inductance and capacitance of the device cancel out each other and the motional impedance of the device is reduced to Rm [8]: o 2 . RmRccI
By increasing the stiffness of the resonator structural material (using diamond) while keeping the effective mass almost constant, the natural frequency and hence the sensitivity of the device are both increased. However, this may not result in a better mass detection resolution since other factors such as Q of the resonator and electrical noise of the interface circuit will also contribute to the overall resolution of the sensor [4]. Improvement in the mass resolution can only be claimed after performing a systemlevel measurement, where an oscillator circuit is included. III. FBRICATION PROCESS Figure 3 is a schematic process flow for fabrication of ZnO-on-diamond resonators. The process is a lowtemperature 4-mask process. The starting substrate is a highresistivity 4" silicon wafer coated with 2,um nanocrystalline diamond film. The surface roughness of the diamond film is in the order of a few tens of nanometer. This surface roughness, even though small, is not suitable for deposition of high quality ZnO film with oriented grains normal to the plane. Figure 4 is an SEM picture from the cross section of a ZnO-coated diamond film. The surface roughness of the diamond film is directly projected to the ZnO film. Therefore, as verified by XRD, the crystallographic axis of the grains is widely scattered, resulting in a weak piezoelectric coefficient. Due to extremely high hardness of diamond, polishing of the diamond surface is not a trivial solution for reducing the roughness. Instead, an intermediate PECVD oxide layer is deposited on the wafer and then polished down. This polished layer of oxide has dual functionality. First, it provides a smooth surface for sputtering high quality ZnO films; and second, it can compensate the temperature sensitivity of the sensor. Oxide has a positive TCF (-85ppm/°C) causing the overall composite stack of material consisting of diamond, oxide and ZnO to have small (ideally near zero) TCF.
(2)
d31
Metal
Lower motional impedance at resonance translates to higher output current, which eases the design ofthe oscillator
Zinc Oxide
circuit. When some amount of mass (AM) is absorbed to the sensing platform the change in the frequency can be approximated by Sauerbrey equation (assuming K/Keff
