Hydrogen sensor based on Pd-functionalized film bulk acoustic resonator

doi:10.1016/j.snb.2011.06.078

Sensors and Actuators B: Chemical

Volume 159, Issue 1, 28 November 2011, Pages 234-237

https://doi.org/10.1016/j.snb.2011.06.078 Get rights and content

Abstract

We present an application of the Pd-functionalized film bulk acoustic resonator for hydrogen detection. The resonator consisted of an AlN piezoelectric stack and a Bragg reflector has a working resonance near 2.23 GHz and a high performance. A 50 nm thick Pd film was coated on the top electrode as a specific layer to capture hydrogen. The absorption of hydrogen can trigger changes of elastic properties of the Pd layer, which affects the resonance frequency of the resonator. The experimental results show that the Pd-functionalized film bulk acoustic resonator can yield a rapid, sensitive, reversible and reproducible response to hydrogen in the concentrations from 0.3% to 2%. The advantages of this sensor, including the simple fabrication process, ease of detection method, rapid response and high sensitivity and working at room temperature, make this promising in the early alarm of hydrogen leaking.

Introduction

Hydrogen is one of the most attractive and ultimate candidate for future fuel and energy carrier. However, hydrogen is highly explosive in air at concentrations as low as 4%, which is a major problem in the wide application of hydrogen energy. Hence sensors are required to detect the hydrogen leaking to warn of explosion hazards. At present, a wide range of technologies, including mental oxides devices [1], microcantilevers [2], [3], [4], carbon nanotubes based devices [5], [6], optical fibers [7] and surface acoustic wave (SAW) devices [8], [9], [10] has been used as platforms for hydrogen detection. Among these sensors, Pd, Pt, mental oxides and conducting polymers are widely used as sensitive layer.

In recent years, film bulk acoustic resonator (FBAR) has been attracted great attentions because of its promising applications in radio frequency integrated circuits and mass loading measurements. With a working resonance in 2–10 GHz, FBAR has a minimum detectable mass change in the range of ∼10 ng/cm² [11]. In addition, due to the small size of FBAR they can be mass produced, and arrayed for analysis of multiple targets simultaneously. To date, many attempts to use FBAR for chemical and biological sensor applications have been made with remarkable results, including liquid sensing [12], [13], gas sensing [14], [15] and biological detection [16], [17]. In the recent paper, we also reported an application of FBAR for trace nerve gas sensing [18].

In this paper, we demonstrate a Pd-functionalized FBAR based hydrogen sensor. This sensor works at room temperature without any heater, which is very necessary for the hydrogen detection from the standpoint of the safety. The sensing characteristics of hydrogen detection of the proposed sensor are evaluated. The advantages of the FBAR based sensor, including the rapid response and high sensitivity, low energy consumption, simple fabrication process and easy to be implanted into microelectronic IC, make it promising in the early alarm of hydrogen leaking.

Section snippets

Device structure and sensing theory

Fig. 1 shows the basic configuration of the hydrogen sensor device. The FBAR is consisted of an AlN piezoelectric stack (t_AlN = 2 μm) and an all-metal Bragg reflector (Ti/Mo with the thickness of λ/4). The bottom electrode is incorporated into the conductive Bragg reflector, which is grounded to excite the acoustic wave of thickness extension mode in AlN film. In order to detect hydrogen, a 50 nm thick Pd film is coated on the top Au electrode (t_Au = 100 nm) as the sensitive layer.

The behavior of a

Experimental

The fabrication process of the FBAR device has been reported in other paper [21]. In this experiment, the sensitive layer of Pd was sputtered and pattern on the center area of the Au surface by means of a lift-off process. Gas testing for the sensor was carried out in a chamber by exposing the sensor to various concentration of hydrogen gas in air at 20 °C. In operation, the FBAR sensor was packaged in a PCB 3-pin case, which was connected to a network analyzer (Agilent 8722D). The frequency

Results and discussion

Fig. 4 shows the measured admittance curves (conductance and susceptance) obtained with the Pd-functionalized FBAR in air. The resonance frequency f_R is found at 2.23 GHz. The Q-factor is 625 calculated from the admittance curves using the following equation: $Q = {|\frac{f}{FWHM}|}_{f = f_{R}}$ where FWHM is the full wave at half maximum of the conductance curve at resonance frequency. The measured resonance frequency is slight smaller than the simulated one as shown in Fig. 3, since the latter did not take into account

Conclusions

In summary, we have demonstrated a Pd-functionalized FBAR and applied it for hydrogen detection. The resonance frequency shifts can be observed due to the changes of the elastic properties of the Pd layer after hydrogenation. The sensor shows a quite stable and reversible performance with fast response and recovery time at room temperature. On the basis of these results, it is expected that the Pd-functionalized FBAR could be further developed as a handheld sensor for early alarm of hydrogen

Acknowledgement

This work was partially supported by the Research Project of SDUST Spring Bud under Grant No. 2010AZZ071.

Da Chen was born in 1980 in China. He received his Ph.D. degree in Shanghai Jiao Tong University, China, in 2009. He then joined in Department of Applied Physics, Shandong University of Science and Technology, in China. His research interests include semiconductor thin film, piezoelectric materials and nano-materials. His current researches focus on physical properties and sensing application of thin film bulk acoustic resonators.

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Short communicationHydrogen sensor based on Pd-functionalized film bulk acoustic resonator

Abstract

Introduction

Section snippets

Device structure and sensing theory

Experimental

Results and discussion

Conclusions

Acknowledgement

Int. J. Hydrogen Energy

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Short communication
Hydrogen sensor based on Pd-functionalized film bulk acoustic resonator