2011 International Conference on Electronic Devices, Systems & Applications (ICEDSA)
Design and Simulation of SAW Delay Line for Corrosion Detection Aamir F. Malik, Asif Iqbal, and Zainal A. Burhanudin 1 Department of Electrical & Electronic Engineering, Universiti Teknologi Petronas (UTP) Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia 1
[email protected]
Abstract— Surface acoustic wave (SAW) delay line with two Interdigital Transducers (IDTs) and an intermediate metal sensing element is proposed to detect and monitor corrosion. Generation and detection of SAW on ST cut Quartz piezoelectric substrate are made using platinum (Pt) IDTs. The SAW, then travels in between the IDTs underneath a sensing element made of iron (Fe). The device parameters like central frequency, number of finger pairs, finger width and spacing are designed and simulated using impulse response model. The frequency response of the device is then simulated and analysed using coupling-of-mode model. It is found that, by thinning the sensing element, the phase shift increases and become more significant when the thickness of sensing element is below 300 nm. Since corrosion can be regarded as thinning of a metallic element, the results suggest that it is feasible to develop a SAW based sensor for corrosion detection. Keywords — IDT, SAW, Sensor, Corrosion, COM Model, Phase shift
pipeline located tens of meter above ground. Therefore, there is a need to find an alternative and proven technique for this purpose. In this work, SAW delay line with two IDTs and an intermediate metal sensing element is proposed to detect and monitor corrosion. Platinum and iron are chosen as IDTs and sensing element, respectively. The design of the delay line and the effects of thinning of the sensing elements on the phase and frequency of the detected SAW are simulated using impulse-response and coupling-of-mode models. II. DESIGN OF SAW SENSOR A SAW delay line consists of a pair of IDTs, IDTin generates the SAW whereas IDTout detects the propagating SAW. To distinguish the effect of corrosion, two delay lines will be considered. The first delay line is the reference delay line without the sensing element as shown in top portion of Fig. 1.
I. INTRODUCTION Surface acoustic wave (SAW) devices are one of the important devices in electronics. It has been used as sensors for temperature, pressure, traces of vapor and gases [1-6] as well as filters and resonators in communication systems [7]. Typically the SAW is generated and detected with inter-digital transducers (IDT) on piezoelectric substrate. For sensing application, sensing elements are placed in between the IDTs. Physical changes, e.g., increase in mass due to absorption in the sensing elements, alter the frequency or phase of the detected SAW. Similar technique may also be applied for corrosion monitoring as demonstrated in previous work using either bulk or surface acoustic wave with copper as the sensing elements [8-9]. For industrial application like monitoring corrosion in pipelines, iron or carbon steel as sensing elements is preferred although no work on it has been reported. Corrosion is a natural phenomenon that cannot be Fig. 1 Schematic diagram of a SAW delay line with and without eliminated but can be minimized with constant monitoring and sensing element between IDTin & IDTout. Important design appropriate prevention measures. It exists in various industries, parameters such as number of fingers Np, finger width & spacing like shipping, airlines, oil and gas, damaging their assets by w, finger length, L, finger overlap Ha and bus bar height B, are also millions of dollars [10-13]. For corrosion monitoring purposes, shown. techniques utilizing corrosion coupons, electrical resistance The second delay line is the ‘sensor’ delay line with a (ER), linear polarization resistance (LPR) and electrochemical sensing element placed in between the IDTs as in the bottom impedance spectroscopy (EIS) are typically used [14]. These portion of Fig. 1. Only this delay line will be exposed to the techniques, however, may not be suitable for complex corrosive environment. surfaces located in difficult to access locations like bend This work is supported by UTP Graduate Assistantship Scheme.
2011 International Conference on Electronic Devices, Systems & Applications (ICEDSA)
In order to design a SAW delay line, impulse response (IR) model is used [15]. It calculates important design parameters including central or synchronous frequency fo, number of IDT fingers, Np, finger width and spacing between them, W, wavelength, λo, acoustic aperture, Ha and total capacitance, CT. The governing equations that determine the central frequency and number of fingers are
f0 =
Vs
is best described by its admittance [17]. As a result, the mixed matrix represents the whole IDT. The transmission matrix for IDT is given in Fig. 3. The subscripts ‘1’ and ‘2’ in the transmission matrix refer to the acoustic ports and subscript ‘3’ refers to the electrical port.
(1)
λ0
2 f0 NBW (2) where Vs is acoustic velocity in media, and NBW is null band width. The design parameters of SAW delay line calculated by IR model are given in table 1. Np =
TABLE 1
Fig. 2
Schematic of an IDT with two acoustic ports and one electrical port.
DESIGN PARAMETERS OF SAW DELAY LINE
Substrate Material
ST Cut Quartz
Acoustic Velocity (Vs)
3158 m/s
Piezoelectric Coefficient (k)
0.04
Capacitance/ finger pair/ m (Cs)
0.5033 pF/cm
Bus Bar Height (B)
500 µm
Synchronous frequency (fo)
50 MHz
Delay length (D)
20 λ
Null band Width (NBW)
2.0 MHz
Finger length (L)
2650 μm
Finger width/spacing
15.79 μm
Finger thickness
900 nm
Number of finger pairs (Np)
50
Wavelength (λo)
63.16 μm
Acoustic aperture (Ha)
2525.1368 μm
III. ANALYSIS OF SAW DELAY LINE The response of SAW delay line can be simulated and analyzed using coupling-of-modes (COM) model. The COM equations are usually represented by P-matrix, also called transmission matrix developed by Tobolka [16]. In the following sub-section, conventional P-Matrix model will be initially reviewed. Then, the modified P-Matrix model used in this work will be introduced. A. Conventional P-Matrix Model In this approach each IDT is represented with two acoustic ports and one electrical port as shown in Fig. 2. Two acoustic ports have incident and reflected waves represented by Ui-1 and Ui, the ‘+’ and ‘-’ signs indicating the forward and backward traveling directions, respectively. From this schematic it is obvious that the acoustic ports are best described using scattering parameters while the electrical port
Fig. 3
Conventional P-matrix of a SAW delay line
The above left sub-matrix defines the scattering of the incident waves. The coefficients t11 = t22 are reflection and t12 = t21 are transmission coefficients respectively. The remaining elements of the P-matrix represent the electrical properties of the device; t13 and t23 describe the electroacoustic transfer function of the IDT. The components t31 and t32 determine the current generated in the IDT by the arriving waves. The t33, the admittance term, which relates the generated current to the applied voltage [16]. B. Modified P-matrix Model In this approach each IDT is divided into two sections, each of length ½λ0. It is then further divided into two un-metallized or free regions (1/8λ0) and one metalized region (1/4λ0) as shown in fig. 4. Each subsection is modeled by a transmission matrix [18].
2011 International Conference on Electronic Devices, Systems & Applications (ICEDSA)
length of sensing element and Ds2 is the distance between sensing element & IDTout. From equation (7) and (8), the phase difference between the SAW traveling in two different paths can be calculated using ⎛ 1 1⎞ Δφ = φ1 − φ2 = 2πfDs ⎜⎜ − ⎟⎟ (9) ⎝ Vm Vs ⎠
Fig. 4
Schematic of a single finger in the IDT used in the modified P-matrix model.
IV. RESULTS AND DISCUSSION Fig. 5 shows the frequency response of a SAW delay line without sensing element calculated using the modified P-matrix model. It shows that the central frequency designed and simulated with the IR model is exactly at 50 MHz.
The value of the velocity for metalized region is given by,
250 200
Frequency vs. Phase of Reference Delay Line (Without Sensing Element) Phase without SE
Phase (degree)
⎞ ⎛ 150 ⎟ ⎜ 1 ⎛ λ0 ⎞⎜ ⎟ 100 Vm = ⎜ ⎟ (3) 50 ⎝ 4 ⎠⎜ 1 − 1 ⎟ ⎟ ⎜ 2f 0 4 f a ⎠ ⎝ 0 -50 where fa is the average shifted center frequency and is given -100 by, -150 ⎛ Va ⎞ ⎛ Vs (1 − k111 ) ⎞ -200 ⎜ ⎟ ⎟ ⎜ fa = ⎜ ⎟ = (4) ⎜ ⎟ λ0 -250 ⎝ λ0 ⎠ ⎝ ⎠ 47 48 49 50 51 52 53 / In equation 4, k 11 is the normalized self coupling coefficient f (MHz) and Va is the average shifted velocity due to metallization [18]. Fig. 5 Frequency response of SAW delay line without sensing When a sensing element is placed in between the two IDTs, element. the surface acoustic velocity Vs is changed to Va due to which central frequency fo is also changed to fa as given in (3) & (4). This shift in frequency depends on normalized coupling In Fig. 6, the frequency response of a SAW delay line with coefficient k/11 which is given as, a 900-nm thick sensing element (circular symbol) is k /11 = k /11 p + k /11m + k /11s (5) superimposed on the frequency response of SAW delay line without sensing element (triangular symbol).
In the case of ST Quartz as piezoelectric substrate, k/11 is given by [19], h h k /11 = 0.004 + 0.002( ) + 7.9( ) 2 (6)
250
where h is the thickness of sensing element.
100
200
λ0
150 Phase (degree)
λ0
Frequency vs. Phase without & with Sensing Element (900 nm thick) without SE with SE
50 0
C. Phase Measurement -50 -100 The phase of the SAW traveling through path D (see top of -150 Fig. 1), i.e., without sensing element between the two IDTs, is -200 given by -250 D 47 48 49 50 51 52 53 φ1 = 2πf . ( 7) Vs f (MHz) The phase of the SAW traveling through path Ds1 + Ds + Fig. 6 Frequency response of SAW delay lines with a 900-nm thin Ds2 (see bottom of Fig. 1), i.e., in the presence of a sensing sensing element and without sensing element. Distinct phase shift element in between the two IDTs can be expressed as, can be observed between them. Ds1 Ds Ds 2 φ2 = 2πf + 2πf + 2πf (8) Vs Vm Vs From the figure, a distinct shift in phase of approximately 0.3 rad can be seen. where Vm is the phase velocity of SAW in the sensing element, Ds1 is the distance between IDTin & sensing element, Ds is the
2011 International Conference on Electronic Devices, Systems & Applications (ICEDSA)
When a sensing element is placed in between the two IDTs, the surface acoustic velocity, Vs, is changed to Va which contributes to the change in the phase velocity of metalized region Vm. This change in the phase velocity is strongly dependent on self coupling coefficient and hence on the thickness of sensing element as given in equations (3) to (6). Therefore, when the thickness of sensing element is varied manually from 900 nm to 100 nm, the theoretical phase difference between the reference and the sensing delay lines can be extracted. This phase difference is plotted against thickness in Fig. 7. Thickness vs Phase
3.0 2.8 2.5 2.3
ΔΦ (rad)
2.0 1.8 1.5 1.3 1.0 0.8 0.5 0.3 0.0
0
100 200 300 400 500 600 700 800 900 1000 Thickness (nm)
Fig. 7
Thickness of sensing element varying from 900 nm to 100 nm vs. Phase difference (∆Φ)
From Fig. 7, it is found that during the change in thickness of Fe from 900 nm to 300 nm, phase shift is varied only from 0 to 0.9 rad. However, below 300 nm, the phase difference increases significantly from 0.9 rad to 2.8 rad. Since thinning of Fe represents corrosion, it is therefore possible to detect corrosion using SAW sensor, particularly when the thickness is below 300 nm. V. CONCLUSION Based on the results, the thinning of the sensing element, i.e., corrosion can be detected by calculating the phase differences of the SAW. Sensing element with thickness less than 300 nm shows high phase shift indicating that the corrosion can easily be detected. For sensing element thicker than 300 nm, however, linearization technique may need to be applied to the phase shift and thickness relationship in order to determine the corrosion. This work together with the actual fabrication and characterization of the SAW delay line will be carried out in the near future.
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