QI Jing, WEN Yumei, LI Ping

(1.College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044,P.R.China;2.Chongqing Key Lab of Mobile Communications Technology, Chongqing University of Posts and Communications,Chongqing, 400044,P.R. China)

Research on key technologies of 3D nano clustersensitive film based on SAW resonator

QI Jing1, 2, WEN Yumei1, LI Ping1

(1.College of Optoelectronic Engineering, Chongqing University, Chongqing, 400044,P.R.China;2.Chongqing Key Lab of Mobile Communications Technology, Chongqing University of Posts and Communications,Chongqing, 400044,P.R. China)

This research attempts to design a new type of surface acoustic wave (SAW) gas sensor that integrates both the three-dimensional nanostructure technology and the SAW technology and thereby makes use of their advantages in sensitivity and specific surface area, respectively. By taking advantage of the resonant surface acoustic wave(SAWR)’s high Q-factor and low insertion loss characteristic, this paper mainly focuses on the design of the SAW chip and modification of the nanowires structure. Finally，through pumping the Sarin and HD gas into the sample pool of the 3D nano-SAW gas sensor with Back-Propagation Network recognize algorithm, As shown by the experiment results, the overall distinguishable rate is more than 90%, and the 3D nano-SAW gas sensor is superior to traditional sensing devices concerning the sensitivity, identification accuracy, and response speed.

surface acoustic wave (SAW)； nanowires； gas sensor；BP-ANN

1 Introduction

The surface acoustic wave(SAW) sensor refers to a new type of transducer that can measure frequency’s change, and it includes some environmental parameters, such as temperature and pressure. The SAW gas sensor detects gas by measuring the amplitude and frequency shifts of SAW when the target gas moves over the surface of the gas-sensitive materials[1]. One-dimensional metals or transition metal oxides, such as ZnO, SnO2, TiO2, and In2O3, are most widely applied in SAW sensor design. Besides, the nanowires or nanotubes materials, which are extensively utilized in gas sensing and detecting devices, have captured the attention of multiple researchers. Additionally, the three-dimensional (3D) gas-sensitive nanowires have a high specific surface area. When they integrate with the highly sensitive and fast responding SAW devices, they can create a new type of SAW gas sensor which will overcome the limits of traditional two-dimensional (2D) membrane sensors[2-8].

As ZnO has a faster reaction rate, and can react with gas at ambient temperature. Then, based on the method of preparing the ZnO nanowires, this paper researches the SAW technology, the aim of which is to create a new type of SAW gas sensor that can quickly detect gas at low concentration.

This new type of gas sensor takes advantage of the 3D nanostructure technology as well as the SAW technology, and it can be acutely sensitive, highly integrated, small, lightweight, and inexpensive. With micro, integrated and digitalized features, the 3D nano-SAW gas sensor can not only detect gas rapidly and reliably but also operate at room temperature.

2 Working principle

In essence, the SAW gas sensor is a SAW device coated with a film of gas-sensitive material. When the target gas moves over the surface of the SAW sensor, the gas-sensitive material will selectively absorb the molecules of the target gas, which results in the changes in its mass per unit area[9]. This can affect the properties of SAW in two ways: firstly, it will not only reduce the amplitude of SAW but also exacerbate its insertion loss. Secondly, it will cause a change in the base frequency of SAW, which can be represented as Δfs. The correlation between them is

(1)

In the function,cmrefers to the parameters of SAW substrate; Δ(m/A) refers to the increase in mass per unit area in the sensitive material;f0refers to baseband signal frequency.

When the SAW substrate is given, there is a linear relationship between the increase in mass per unit area of the sensitive material and the square of the SAW operational frequency. Therefore, to increase the Δfs, the sensitivity per unit area of the sensitive material and the operational frequency of the SAW sensor shall be increased. Sensitivity per unit area is closely related to both the sensitive material itself and the techniques, by which the SAW chip and the sensitive material can be combined[10]. Fig.1 shows the working principle of the SAW gas sensor.

Fig.1 Working principle of the SAW gas sensor

Fig.2 Two-dimensional sensitive membrane

Fig.3 Three-dimensional nanowire clusters

According to the comparison between the specific surface area of 2D sensitive membrane and 3D nanowires clusters, it can be seen that 2D sensitive membrane area is

(2)

The 3D sensitive membrane area can be expressed as follows:

(3)

(4)

Ast=2 μm,h=5 μm,d=0.5 μm, ifs=1 mm2, thenS1=1.000 mm2,S2=8.85 mm2. The specific surface area of the 3D nano structure is more than 9 times larger than that of the traditional 2D structure. Correspondingly, the sensitivity per unit area of the 3D nano structure will be more than nine times as much as that of the 2D nano structure. All threshold values are held equal, and the increase in sensitivity will be translated into a faster response time.

Fig.4 illustrates the 3D SAW gas sensor under discussion, which uses ZnO nanowires as the sensitive material[11].

Fig.4 New type of 3D nano-SAW sensor

3 Design of the SAW device

In general, there are two types of SAW gas sensors, including delay surface acoustic wave(SAWD) and resonant surface acoustic wave(SAWR). SAWR is generally characterized with highQ-factor and low insertion loss. As a device affected by frequency, SAWR is not only easy to resonate but also has good frequency stability. The SAW gas sensor studied here refers to a SAW resonator.

Through the Agilent’s Advanced design system platform simulation , selected parameters are as follows: IDT(inter digital transducer) number is 50, aperture width as 100λ, IDT period as 6.154 4 μm, other parameters keep default.

For SAWR, the location of the resonator can significantly influence its sensitivity. Traditionally, signals are processed using a two-port SAW resonator, which, however, is not the optimal choice for a SAW gas sensor because the distance between these two IDTs is too small. Fig.5 illustrates how the varying distance between the IDTs affects the properties of the resonator. When the distance is 100 wavelengths, there is the maximum harmonic suppression.

Fig.5 Impact of the distance between the IDTs on the properties of the resonator

4 Formation and modification of three-dimensional nanowires

There are different methods to grow the desired pattern of the 3D nanowires, such as hydrothermal synthesis, template synthesis, and electrochemical precipitation. The hydrothermal synthesis stands out with its low-cost and low-temperature characteristics as well as its ability to be repeated and mass produced in a non-vacuum environment. Using this method, nanowires can grow on a photolithographic substrate coated with photoresist (such as a SAW device). Its operating temperature is in the range of tens of degrees Celsius, which will not affect the IDTs on the SAW piezoelectric substrate. Furthermore, the method is easy-to-conduct and suitable for the mass production.

Fig.6 and Fig.7 show the growth process of nanowires and the crystal layers using hydrothermal synthesis and template synthesis. Firstly, the SAW device will be spin-coated or sputtered with ZnO nano particles. As s result, it can be used as the seed layer for the growth of ZnO nanowires. To grow ZnO nanowires that are spaced in a particular way, the entire SAW device will be firstly filmed with PMMA (polymethyl methacrylate). Then, it will be etched by electron beams according to the particular distance and size. The SAW device, which has been coated with PMMA, has gone through the photolithographic process. Then, it will be dipped into a mixed liquor for 18 hours for hydrothermal synthesis[12-13].

Fig.6 Growth process of ZnO nanowires on the SAW device

Fig.7 ZnO crystal layer on the surface of the SAW device.

From Fig.6 and Fig.7, it can be found that the structure and density of nanowires are not easy to control. However, these flaws can be overcome using template synthesis, by which the position and density of the nanowires will be controlled. Moreover, nanowires produced in this way usually stand more upright. The PMMA template, which has undergone the photolithographic process, will not be damaged in the low-temperature hydrothermal process. The pattern of the nanowires is of critical importance to their subsequent modification as well as the fabrication of the gas sensor.

Fig.8 Modification effect of ZnO nanowires

The formation and modification of the nanowires on the surface of the SAW device will lead to greater insertion loss as -29.30 dB , and a shift in its resonance frequencies as 300 MHz, through the Agilent vector analyzer experimental, the results are shown in Fig.9.

Fig.9 Display of SAW property after the nanowires have been formed and modified

5 Test results of 3D SAW gas sensor and the analysis

Sarin and HD gas are pumped into the sample pool of the 3D nano-SAW gas sensor whose SAW resonators are coated with SE-30 (Silicone), PTFP (a hydrogen bond acidic polysiloxane), BSP3 (a hydrogen bond acidic polysiloxane) or PECH (a poly propylene oxide). Besides, the typical response of the SAW gas sensor to the mix of Sarin and HD gas can be captured in Fig.10.

Fig.10 Responsive curves of the SAW sensor

By using Back-Propagation Network algorithm[10,14], the parameters are listed as follows:

? the neural network transfer function: tansig

? training function: trainscg

? output function: purelin

? BP network structure: 4-20-20-2

3.1.1 前列腺解剖 復習系統(tǒng)、局部解剖，結(jié)合實時超聲圖像，讓學生熟悉前列腺不同區(qū)帶及相鄰的精囊、尿道等結(jié)構(gòu)，前列腺體積測量等。

Then, 58 groups of experimental data were selected as the training sample data. The part of the data is displayed in Tab.1.

Tab.1 Training data for quantitative analysis of mixed components

The test results are shown in the Fig.11. and Fig.12. The ‘o’ indicates the value of the gas concentration in the calibration, and the ‘+’ indicates the predicted gas concentration.

Fig.11 Results of mixed qualitative judgment by test data

Fig.12 Test data errors of mixed gas qualitative judgment

As shown by the test results, the 3D nano-SAW gas sensor has high targeting capacity and disturbance resistance. Besides, comprehensive recognition rate of training sample and test sample data，the overall distinguishable rate is more than 90%.

6 Conclusions

By integrating the nanostructure technology and the SAW technology, this paper attempts to design a new type of three-dimensional nano-SAW gas sensor by integrating the nanostructure technology and the SAW technology. The research has comprehensively have been discussed in detail: the design of SAW chipas well as the growth and modification of nanowires. Finally, by using back-propagation network algorithm to recognize the mixed gas with HD and Sarin, the experiment shows that SAW gas sensor can gain a high degree of detecting capacity and disturbance resistance.

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Biographies:

QI Jing(1983-) was born in Jiangxi Province, China. Electrical & Information Engineering from Chongqing University of Posts and Telecommunications in 2005, and the ME degree in Electrical & Information Engineering from Chongqing University in 2008. Now, he is working towards his PHD in School of Chongqing University. His research interests include sensors and instrumentation, energy harvesting circuit, short distance wireless communication, sensor search. E-mail: qijing@cqupt.edu.cn.

WEN Yumei(1964-) was born in Chongqing, China. She received the BE degree in electrical engineering from Beijing Aeronautic and Astronautic University in 1984, the ME degree in electrical engineering from China Academy of Launch Vehicle Technology in 1987, and the PhD degree in instrumentation engineering from Chongqing University in 1997. She has been a professor at College of Optoelectronic Engineering in Chongqing University since 1998. Her research interests include sensors and instrumentation, signal processing, energy harvesting devices, and LED lighting.

(編輯：魏琴芳)

2016-10-23

2017-05-09 通訊作者：漆 晶 qijing@cqupt.edu.cn

基于聲表面波諧振器的三維納米團簇敏感薄膜關(guān)鍵技術(shù)研究

漆 晶1,2，文玉梅1，李 平1

(1.重慶大學 光電工程學院，重慶 400044; 2.重慶郵電大學 移動通信重點實驗室，重慶 400060)

設(shè)計了一種新型基于聲表面波技術(shù)的氣體傳感器，理論分析了三維納米線結(jié)構(gòu)的比表面積大、靈敏度高等優(yōu)點，采用具備高Q值和低插損的諧振型聲表面波器件結(jié)構(gòu)，制備了三維敏感膜結(jié)構(gòu)的聲表面波氣體傳感器。在此基礎(chǔ)上，為提高吸附效應(yīng)，對三維納米線簇進行了修飾改進。通過將沙林氣和芥子氣注入放置了聲表面波的氣體傳感器密閉腔體內(nèi)，經(jīng)過神經(jīng)網(wǎng)絡(luò)識別系統(tǒng)進行定性識別。實驗結(jié)果表明，基于修飾改進后的納米線簇敏感膜制備的聲表面波氣體傳感器對給定毒氣混合氣體的整理識別率大于90%，能夠滿足通用的毒氣定性檢測要求。并且三維納米聲表面波氣敏傳感器的靈敏度和響應(yīng)速度優(yōu)于傳統(tǒng)的傳感裝置，在識別系統(tǒng)加大樣本數(shù)據(jù)量時，能夠進一步提高識別精度。

聲表面波；納米線簇；氣體傳感器；神經(jīng)網(wǎng)絡(luò)識別算法

10.3979/j.issn.1673-825X.2017.04.011

TN65; TM93 Document code：A

1673-825X(2017)04-0494-06

The Science and Technology Project Affiliated to the Education Department of Chongqing Municipality(KJ1500433)