Dr Aamir F Malik

Time Dependent Effects of Corroded Fe Delay Line for a SAW Device Aamir F. Malik*, Z A. Burhanudin*1, V. Jeoti* and U. Hashim** Abstract —in this paper, designing and fabrication of surface acoustic waves (SAW) delay line to monitor corrosion in real time is presented. The electrical and acoustic shielding of Inter-Digital Transducers (IDT) for SAW delay lines (DL) to monitor corrosion is also discussed in detail. Initially, the designing and fabrication of SAW delay lines is presented. The frequency response of fabricated SAW devices is then observed. A simple and cost effective method is then introduced to electrically and acoustically protect IDTs using thin Piezo-PVDF sheet, prior to immerse in liquids. The transfer characteristics of protected SAW DL is measured before dipping in water, after dipping in water and after dipping in corrosive solution. It is observed that SAW signal is completely attenuated inside water and corrosive solution, however it is successfully retrieved back after device is taken out of water and dried. Further, a micro liter (μL) droplet of corrosive solution is dispensed on the surface of iron and frequency response is measured in real time. The results show that SAW DL can be used to monitor corrosion. Keywords-component; SAW, IDT, LiNbO3, Delay Line, Corrosion I. INTRODUCTION Almost all materials deteriorate when interact with environment. The process is called corrosion. It occurs in various industries, like shipping and airlines as well as oil and gas, causing a severe damage to their assets by millions of dollars [1-3]. Corrosion being a natural process, cannot be completely stopped but can be reduced with continuous monitoring and proper prevention measures. Therefore it is utmost important to monitor corrosion on a regular basis and take significant measures to minimize this huge loss as much as possible. Some of the well-established corrosion monitoring techniques include electrical resistance (ER), linear polarization resistance (LPR), corrosion coupons and electrochemical impedance spectroscopy (EIS) [4]. In addition, bulk acoustic waves (BAW) and surface acoustic waves (SAW) are also used to monitor corrosion of copper [5, 6]. However, in most of the industries, pipelines are made of either iron or carbon steel. To the best of author’s knowledge, there is no work done on corrosion monitoring of these materials using SAW. SAW devices become essential part of today’s electronic systems due to their flexibility, reliability and robustness. The SAW devices are not only used as filters [7] and resonators [8, 9] in telecommunications but also used as sensors for several applications like temperature [10, 11], pressure [12, 13], traces of vapour [14, 15] and gases [16, 17]. Inter-digital transducers (IDTs) are sets of finger-like metal electrodes, separated by some distance, deposited on the surface of piezoelectric substrate and are used to generate and detect SAW. SAW device can be used as sensor by depositing a metal, called sensing element in between the IDTs. The central frequency or phase of SAW sensor significantly alters due to any physical change on the surface of sensing element e.g. loading or unloading of mass. These variations in frequency or phase of SAW may be utilized to monitor corrosion of iron or carbon steel [18, 19]. However, IDTs of SAW sensors must be electrically and acoustically protected prior to soak in corrosive solution. In this work, a SAW delay line (DL) on 500 μm thick LiNbO3 (LNB) with iron (Fe) as sensing element is investigated for corrosion monitoring in real time. Initially, design parameters of SAW DL is calculated using Impulse-Response (IR) [20] model. The device is then fabricated using UV lithography process. This is followed by measurement of frequency response using vector network analyzer. Next a unique and cost effective method is proposed to electrically and acoustically protect the IDTs, prior to immerse in corrosive solution. Further, the frequency response of protected SAW DL is measured in corrosive and noncorrosive environments. SAW DL is dipped in water and in corrosive solution and frequency response is observed. Finally, a micro-liter droplet of corrosive solution is dispensed on the surface of iron and frequency response is measured in real time. The corrosion of iron as sensing element suggest that SAW delay line can be used to monitor corrosion. II. METHODOLOGY A. Designing of SAW Delay Line using IR Model SAW delay line comprises of IDTin and IDTout as shown in Fig. 1(a). IDTs are used to generate and detect the SAW on the piezoelectric substrate. The important design parameters are shown in Fig. 1 (b). These parameters include finger width (Wf), spacing (Sf), wavelength (λ0), central frequency (fo), bandwidth (BW), delay between the two IDTs (D) and suitable piezoelectric substrate. In this work, a 500-µm thick single crystal Y cut 1280 rotated LiNbO3 (LNB) is used to generate SAW. The piezoelectric substrate decides very important parameters i.e. surface acoustic velocity (Vo), piezoelectric coefficient (K2) and capacitance per unit finger pair (Cs). 1 The period of IDT is determined by P = Wf + S f (1) where Wf and Sf are width and spacing between the fingers respectively [21]. The wavelength of IDT is then calculated by (2) o = 2  P Finally, central frequency of SAW is computed by V fo = o (3) o B. Fabrication of SAW Delay Lines A non-metallized piezoelectric substrates LiNbO3 is used to fabricate SAW delay lines using UV lithography. The fabrication process is shown in Fig. 2. Initially, acetone and Isopropyl alcohol (IPA) is used to clean LiNbO3 substrate with the help of ultrasonic bath. The substrate is then dried using blower and 400-500 μm thick aluminum (Al) is deposited using thermal evaporator. The defects in metalized substrate are minimized by annealing of substrate at 100 0C for 90 sec. PR1-2000A photoresist is coated on the metalized LiNbO3 using spin coater and substrate is baked at 100 0 C for 90 sec. Next, IDTs are patterned by exposing photoresist to UV light. The development of photoresist is done in RD6 developer for 20-25 sec. The Al metal is then etched by dipping LiNbO3 in alum etcher for 56 min, leaving behind Al metallic fingers covered with photoresist. Finally, the acetone is used to strip off the photoresist on metallic fingers. Figure 1. Schematic of (a) Reference (without sensing element) and Sensing (with sensing element) SAW delay lines (b) Important design parameters of IDT, i.e. finger width (Wf), finger spacing (Ws), period of IDTs (P), wavelength (λ0), finger length (Lf), finger overlap (Ha) and bus bar height (B). The designed parameters calculated by IR model are summarized in Table I. TABLE I DESIGN PARAMETERS OF SAW DELAY LINE SAW Chosen Parameters Device Name A B C Finger width (Wf) μm 5 8 11 Finger spacing (Sf) 5 8 11 μm Wavelength (λ0) μm 20 32 44 Central frequency (fo- MHz Parameters based on 1280 YX-LiNbO3 Substrate Thickness 500 μm Acoustic Velocity (Vs) Piezoelectric Coefficient (K2) Capacitance/ finger pair/ m (Cs) D- 3997 m/s 0.056 5.0 pF/cm Table. 1 shows a variety of SAW devices that are designed at different finger width/spacing, wavelength and therefore operate at different central frequencies. It can be noted that the variation in central frequencies depends on wavelength of device Figure 2. Fabrication process of SAW delay lines using UV lithography. C. Protection of IDTs using Piezo-PVDF Sheet In order to measure the frequency response of SAW DL in corrosive solution, IDTs must be protected to avoid electrical shorting as well as acoustic wave attenuation in liquids. This goal is achieved with the help of Piezo-PVDF. A Piezo-polymer (PVDF) strip, equal to the size of IDTs, is placed carefully on the IDTs such that it completely covers the IDTs while the pads of SAW DL are kept uncovered so that the connection will be made afterwards as shown in Fig. 3(a). The substrate is then placed on a hot plate and temperature is raised from room temperature (25 C0) to 50 0C for 3-4 min. The PVDF is started melting and the temperature is further increased to 70 0C for 2-3 min. At this stage, PVDF is attached to IDTs and LNB substrate. 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