Process-tolerant Surface Acoustic Wave Delay LinesAamir F. Malik, Zainal A. Burhanudin1, V. Jeoti and

Adz J. Jamali Department of Electrical and Electronic Engineering

Universiti Teknologi PETRONAS, 31750, Tronoh, Perak, Malaysia 1zainabh@patronas.com.my

U. Hashim and K.L. Foo Institute of Nanoelectronic Engineering

Universiti Malaysia Perlis, 01000, Kangar, Perlis, Malaysia

Abstract — Surface Acoustic Wave (SAW) delay lines micro-fabrication is outlined and its characteristics due to wet chemical etching process variation are investigated. The SAW delay lines consist of Al inter-digital transducers (IDT) on LiNbO3 substrate. It is designed with specific IDT feature size to produce SAW at predetermined central frequency, fo. The effect of the etching process onto the IDT, in particular the feature size, is investigated using field emission secondary electron microscope (FESEM). The effect is then translated into transfer characteristics of the SAW delay lines by measuring its transmission and reflection coefficient using network analyzer. It is found that even with a ~5 μm IDT feature size variation due to the wet chemical etching process, the fabricated delay lines can still produce SAW at the designed fo, and hence a process-tolerant SAW delay lines.

Keywords-component; SAW, IDT, LiNbO3, Delay Line, Sensor

I. INTRODUCTION Surface acoustic wave (SAW) devices have attracted much

attention due to its versatility in electronic systems. It has been used as filters [1], resonators [2, 3] as well as sensors for temperature [4, 5], pressure [6, 7], traces of vapor [8, 9] and gases [10, 11]. SAW is generated and detected by finger-like metal electrodes called inter-digital transducers (IDTs) fabricated on the surface of a piezoelectric substrate. The central frequency (fo) of the generated SAW is strongly dependent on the width and spacing of these IDTs. The choice of substrate also plays an important role in determining the central frequency of the generated SAW.

Application of mechanically flexible substrates in flexible electronics such as e-paper display [12] microprocessor [13], and radio frequency identification (RFID) tag [14, 15] are actively being researched and developed. Similar idea of generating SAW on such flexible piezoelectric substrate is also highly desirable as proposed in few sensor related articles [16]. This flexible piezoelectric polymer substrate, however, has some fundamental issue that needs to be addressed. The polymer needs to be chemically modified and poled in order to make it piezoelectric. It must also be able to retain the poles for a significant amount of time. Successful generation of SAW on polymer lies among other with the coupling coefficient of the polymer and velocity of the acoustic wave in the polymer.

Another important factor that has to be considered for a successful generation of a SAW is the feature size of the IDT itself. It determines the fo of the launched SAW. However, it is

well known that process variation can easily lead to deviation of fabricated feature size from the design ones. Although with a strict control of a fabrication process, it is still expected to have a certain margin of feature size variation. On laboratory scale, the margin can be significant if the process is not properly done.

In this work, the effect of a wet chemical etching process on IDT feature size, and consequently on the fo of a SAW launched on LiNbO3 is investigated. Initially, SAW delay line is designed using Impulse-Response (IR) and Coupling-of-mode (COM) models. Then the SAW delay line is fabricated. The average feature size of IDTs is then investigated using FESEM. This is followed by measurement of transfer characteristics of the device using network analyser. The results show minimal effect of IDT feature size on fo, and hence a process-tolerant SAW delay lines.

II. METHODOLOGY

A. Design of SAW Delay Lines The SAW delay line is schematically shown in Fig. 1(a). It

basically consists of IDTin which launches the SAW and IDTout which detects the propagating SAW on the piezoelectric substrate. Design of SAW delay lines begins by choosing the suitable central frequency (fo), null bandwidth (NBW), total delay length between the two IDTs (D) and suitable piezoelectric substrate. In this work, the targeted fo is chosen to be 55.5 MHz, NBW is 2 MHz, D is 3.2 mm, and the substrate is 500-μm thick single crystal Y cut 1280 rotated LiNbO3. The choice of piezoelectric substrate determines acoustic velocity (Vo), piezoelectric coefficient (K2) and capacitance per unit finger pair (Cs).

Next, important design feature size of an IDT as shown in Fig. 1(b) is calculated using Impulse Response model [17]. The governing equation that determines the period of IDT is given by

f fP W S= + (1)

where Wf and Sf are width and spacing between the fingers respectively [18]. The wavelength of IDT is then calculated by

2o Pλ = × (2)

Finally, central frequency of SAW is computed by

2012 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2012), December 11 - 13, 2012, Melaka, Malaysia

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TABLE I

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design paramTable I.

DESIGN PARAM

SAW Chose

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ameters based

Velocity (Vs)

ric Coefficient (

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esponse of SAWof the SAW

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ght (B).

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METERS OF SAW D

en Parameters

on 1280 YX-L

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(3)

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DELAY LINES

55.5 MHz

2 MHz

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LiNbO3

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2012 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2012), December 11 - 13, 2012, Melaka, Malaysia

188

D. A

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TABLE II C

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dth and spaci

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wires pasted nt 8712ET ve

asure transfer c

II. RESULTS

d feature sizeTable II. Basefeature size wNbO3 substrat

CALCULATED FEA

Calculated

r width (Wf)

r spacing (Sf)

d of IDT (P)

length of IDT (�

r thickness

stic aperture (Ha

images of From the fig

although withingers in Fig.ly due to the o

ure 4. SEM ima

the finger ws (D1, D2 ands are shown inat there is an aalculated ando noted that eing have bee

Delay Lines CB) is designefabricated SAWs as a ground

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ector network characteristics

S AND DISCUS

es of the SAd on IR and C

will ensure thate is at 55.5 M

ATURE SIZE OF SA

d parameters

18

18

36

�o) 72

90

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the fabricategure, well defih uneven edg 4 have ~2 μover etching o

ge of SAW Delay

width and spd D3) taken an Table III. Frapproximatelyd fabricated even though then reduced,

d and fabricatW delay lineplane to minctors are mor while the lines are conntive silver eanalyzer (VN

s of the SAW

SION W delay lineCOM model, at the fo of a MHz.

AW DELAY LINES

8 μm

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LE III DESIGNED

Parameter

Finger width (Wf)

Finger spacing (Ws)

Period of IDT (P)

Wavelength of IDT (�)

ypical simulaW delay line a

reflection coulated and mea

re 5. Measured aa) Transmission c

he simulatesmission and rummarized in

45

-140

-120

-100

-80

-60

-40

-20

0(a)

f0(Simuf0(Meas

S21

(dB)

45

-25

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-15

-10

-5

0

(b)

DeviceWf=18λ=72 μ

S11

(dB)

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D AND MEASUREFABRIC

Designed value (μm)

18

18

36

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ted and meaare shown in oefficients, itasured fo is at

and Simulated Frcoefficient (S21) a

d and mereflection coefn Table IV.

50

Device-D2Wf=18 μmλ=72 μm

lated)=55.5 MHz

sured)=54.6 MHz

Freq

50

e-D28 μmμm

Freq

MeasuredSimulated

ely the same w

ED PARAMETERS OCATED ON LINBO

Measured val(μm

Device D1

DeD

13.0 16

22.6 17

35.6 34

71.2 69

asured frequenFig 5. From

it can be ob~55.5 MHz.

requency Responsand (b) Reflection

easured cenfficients of the

55 60

q (MHz)

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q (MHz)

with the calcul

OF SAW DELAY LO3

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ency, lines

2012 IEEE Asia-Pacific Conference on Applied Electromagnetics (APACE 2012), December 11 - 13, 2012, Melaka, Malaysia

189

TABLE IV SUMMARY OF MEASURED PARAMETERS OF SAW DELAY LINES USING VNA

Parameter SAW Delay Lines D1 D2 D3

Central frequency f0 (MHz)

Simulated 55.5 55.5 55.5 Measured 55.7 54.6 54.5

Transmission S21 (dB)

Simulated -7 -7 -7 Measured -17 -7.4 -8

Reflection S11 (dB)

Simulated -18 -18 -18 Measured -0.4 -16 -19

From Table III, the fabricated device D1, for example, has IDT feature size of Wf = 13 μm, Ws = 22.6 μm and P = 35.6 μm. The feature size variation, i.e. the difference between the designed and fabricated ones, are ΔWf = – 5 μm, ΔWs = +4.6 μm and ΔP = – 0.4 μm. Using the period of the fabricated IDTs, as well as equations (1), (2) and (3), the wavelength of the IDT and the central frequency of the SAW, is recalculated to be 71.2 μm and 56.0 MHz, respectively. Therefore, even with feature size variations resulting from the wet chemical etching process, as long as the period of the IDT remains close to the calculated ones, the designed fo of a SAW could still be realized. This is validated by the data shown in Table IV.

Simulated and measured insertion loss for device D1, however, shows a significant 10 dB difference. The difference could have been due to lossy connecting wires and silver epoxy used in this work. Ohmic losses at high frequencies due to thin 900 nm electrode fingers could also contribute to the increase in insertion loss.

IV. CONCLUSION The paper outlined the micro-fabrication process of SAW

delay lines using UV lithography and investigated the effect of IDT feature size variation on the fo of the launched SAW. From this work, it can be concluded that even though etching process could lead IDT feature size variation, the designed fo could still be realized as long as the period of the IDT remains relatively close to the designed one.

ACKNOWLEDGMENT

This work is supported by Universiti Teknologi PETRONAS (UTP) Graduate Assistantship scheme.

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