Tfbar Sensors for Detection of Explosives Substances
Authors: Moldovan Carmen , Anghelescu Adrian , Firtat Bogdan , Muscalu George , Dinulescu Silviu , Brasoveanu Costin , Savin Mihaela , Gartner Mariuca 
Published: May 2020

Introduction

The development of a portable, miniaturized and cheap microsystem based on TFBAR (thin-film bulk acoustic resonator) sensors for detection of explosive substances for anti-terrorism control in public or private places, appeared over the last years like a big challenge of our days. The research work focused on a microsystem consisting of an array of TFBAR sensors, each provided with a specific biochemical layer. TFBAR sensors were built on Si wafers and formed from two gold metal electrodes with a piezoelectric layer between them. The AlN piezoelectric layer allows resonator operation at high frequencies of 1-10 GHz and a high quality factor[1]. In this way, tiny mass shifts at low vapor concentrations in the ppb-ppt range can be detected and quantified by the resonance frequency (fR) shift. Mass detection of a vapor-explosive substance is performed according to an biomolecule immobilization algorithm, with the help of antibodies specifically prepared for each explosive substance. When explosive vapors come into contact with the TFBAR resonant sensor, they bind to the sensor antibodies causing the change of the resonance frequency. We focused in this paper on TNT (trinitrotoluene) detection.

Design

The TFBAR structure (Fig.1a) will be made from silicon on which a Si3N4 membrane (0.6-1.0um thickness) will be patterned. The membrane will be the support for the deposition of the piezoelectric layer AlN of 1um between two metal electrodes (Cr-Au) with thicknesses of 80-100 nm.
A Ti/Pt resistor has been design for heating a small cavity around the sensor with the purpose of enhancing the TNT vapour quantity (Fig1.b). The TNT explosive molecule will be reacting antibodies deposited on the gold surface changing the resonance frequency of the sensor
To obtain a dense, uniform piezoelectric layer and high piezoelectric coefficients, we must resort to the physical methods of deposition of thin layers such as sputtering deposition. The thickness of the piezoelectric layer should be within the 0.6-1.2um range. A higher thickness would be favorable to increase the piezoelectric coefficients but will simultaneously lead to a decrease of the resonance frequency and implicitly to a decrease of sensor sensitivity [2]. The thickness of silicon nitride and metallic electrodes also influence the resonance frequency of the sensor in sense of decreasing the resonance frequency.

Surface Functionalization

The explosive substance for which the surface functionalization and deposition of biomaterial was trinitrotoluene (TNT). At present, there are different techniques for detecting traces of explosive substances[3]. The sensor for explosive substances must be sensitive, selective and easily used in field applications. The selectivity of the sensor was achieved by specific interaction between the sensor’s sensitive layer and the explosive analyte. TFBAR structure with a biolayer has been considered the best choice for explosive substances detection. A change in a measurable property, such as the mass, translated in the change of resonant frequency was then used to determine the interaction of TNT with sensor active area. A gold surface functionalization method has been developed and optimized for deposition of biomaterial consisting of anti-TNT antibodies through Protein A / G and was chosen for tests with the TNT antigen, the protein that bound most on anti-TNT antibodies to the surface.
The most efficient immobilization of anti-TNT antibodies on the gold surfaces was made by a chemical linker:11-mercapto-undecanoic-acid (11-MUA, SAMs) and a biomolecular linker: G protein, since the antibody alone has no affinity to the metal electrode.
Electrochemical impedance characterization showed that anti-TNT antibodies immobilization through Protein G resulted in greater gold surface isolation (with a 40 kΩ resistance compared to 17 kΩ obtained by Protein A).
Obtained results of TNT concentrations, in the range of 50-1000 ppm, showed a saturation of the gold surface at 1000 ppm and further testing should be done to determine the detection limit of the device and of biofunctionalization method sensitivity to concentrations lower than 50 ppm. For 10 ng TNT molecules on the active sensor area of 200x200 um2, the resonance frequency shifts for 4.9 GHz to 4.75 GHz.
The quality factor was found Q = 300.

Results and Conclusions

A new sensor based on MEMS technology associated with the fabrication of a thin piezoelectric layer which allows resonator operation at high frequencies of 1-10 GHz and with a high quality factor has been developed. The system consists of an array of TFBAR sensors, each provided with a specific biochemical layer. This MEMS system will operate on a GHz frequency band with a mass detection sensitivity directly proportional to the resonance frequency of the sensor. Bioimmobilization of anti-TNT antibodies was obtained through Protein G and after testing a readable shift of fR was reached. Further testing should be done in order to determine the detection limit of the device and also for other explosive substances such as RDX. Next to this work, XDX (trinitro-triazinano). NG (nitroglycerin) and HMTD (Hexamethylene triperoxide diamine) sensors will be developed following the same TFBAR structure and resonant frequency shift principle.

References

[1] Akiyama et al., Influence of growth temperature and scandium concentration on piezoelectric response of scandium aluminum nitride alloy thin films, Appl.Phys. Lett. 95, 162107 (2009), https://doi.org/10.1063/1.3251072
[2] Ali M. Niknejad, MEMS Reference Oscillators, EECS 242: Lecture 25, http://rfic.eecs.berkeley.edu/~niknejad/ee242/pdf/eecs242_lect25_xtal.pdf.
[3] R. P. Kengne-Momo, Daniel, F. Lagarde, Y. L. Jeyachandran, J. F. Pilard, M. J. Durand-Thouand, and G. Thouand, Protein Interactions Investigated by the Raman Spectroscopy for Biosensor Applications, International Journal of Spectroscopy, Volume 2012; http://dx.doi.org/10.1155/2012/462901

Figure 1