In this work we report a systematic investigation of polymer overlayer thickness in a Love wave device working at a fundamental frequency of 110MHz and at the 330MHz harmonic. At both frequencies we observe the initial reduction in insertion loss associated with a Love wave device. Significantly, we also observe a series of resonant conditions as the layer thickness is further increased. The separation of these resonances is attributed to an increase in thickness of half of the acoustic wavelength in the polymer

Gizeli, E

Martin, F

McHale, G

Melzak, KA

Newton, MI

English

In this work, we report an investigation of polymer overlayer thickness in a Love wave device working at a fundamental frequency of 110 MHz and at the 330 MHz harmonic. At both frequencies, we observe the initial reduction in insertion loss associated with a Love wave device. Significantly, we also observe a series of resonant conditions as the layer thickness is further increased. The separation of these resonances is attributed to an increase in thickness of half of the acoustic wavelength in the polymer

McHale, Glen

Newton, Michael

Martin, Fran

Gizeli, Electra

Melzak, Kathryn

Northumbria Research Link

Resonant conditions for Love wave guiding layer thickness

Nottingham Trent Institutional Repository (IRep)

APPLIED PHYSICS LETTERS VOLUME 79, NUMBER 21 19 NOVEMBER 2001DownResonant conditions for Love wave guiding layer thicknessG. McHale, M. I. Newton,a) and F. MartinDepartment of Chemistry and Physics, The Nottingham Trent University, Clifton Lane, Nottingham,NG11 8NS, United KingdomE. Gizeli and K. A. MelzakInstitute of Biotechnology, University of Cambridge, Tennis Court Road, Cambridge, Cambridgeshire,CB2 1QT, United Kingdom~Received 29 March 2001; accepted for publication 17 September 2001!In this work, we report an investigation of polymer overlayer thickness in a Love wave deviceworking at a fundamental frequency of 110 MHz and at the 330 MHz harmonic. At bothfrequencies, we observe the initial reduction in insertion loss associated with a Love wave device.Significantly, we also observe a series of resonant conditions as the layer thickness is furtherincreased. The separation of these resonances is attributed to an increase in thickness of half of theacoustic wavelength in the polymer. © 2001 American Institute of Physics.@DOI: 10.1063/1.1420776#A Love wave is a type of shear horizontally polarizedsurface acoustic wave that can be produced in a surfaceskimming bulk wave ~SSBW! device when an overlayer withan acoustic shear velocity less than that in the bulk is depos-ited over the propagation path.1 The overlayer has the effectof confining energy to the surface and hence acting as aguiding layer producing a reduction in the insertion losscompared to that of an uncoated SSBW device. The use ofLove wave devices for biosensing applications was reportedin 19922 and has since attracted much attention.3–7 Severalreports have focused on establishing the optimum guidinglayer thickness for sensing applications7,8 and have thereforeconcentrated on determining the change in insertion loss andfrequency as an overlayer is built up. The maximum sensi-tivity has been reported as occurring for a thickness slightlygreater than that required for the minimum insertion loss.There have been no reports of the basic properties for de-vices with substantially thicker guiding layers. In this work,we report an investigation of the effect of overlayer thicknesson the insertion loss and shift in resonant frequency for aLove wave device operated at a 110 MHz fundamental fre-quency and at the 330 MHz harmonic.The SSBW devices used in our experiments consisted ofa split-finger ~double–double! interdigital transducer ~IDT!design. Devices were fabricated on ST-cut quartz with propa-gation orthogonal to the crystalline X direction, which isknown to support a SSBW, and designed for operation at afundamental frequency of 110 MHz. With an appropriateIDT metalization thickness, this split-finger design also reso-nates at the third harmonic. Coating the SSBW device with asuitable overlayer allows a Love wave to be excited at eitherthe fundamental or third-harmonic frequency.9 Each IDT wasof length 40l and aperture 65l where the wavelength l was45 mm, the finger widths were 6.75 mm and spacings were4.5 mm. The path length was 7 mm and the guiding layer,deposited directly onto the quartz surface, consisted of spin-a!Author to whom all correspondence should be addressed; electronic mail:michael.newton@ntu.ac.uk3540003-6951/2001/79(21)/3542/2/$18.00loaded 23 Sep 2011 to 152.71.223.129. Redistribution subject to AIP licoated Shipley S1813 photoresist. After deposition the resistfilms were soft baked at 110 °C for 10 min; film thickness forthe layers were confirmed by measurements using a VeecoDektak 3 Surface Analysis System. The insertion loss andresonant frequency of the devices were measured using anAgilent 8712ET network analyzer.In Fig. 1 we show the insertion loss ~squares! and reso-nant frequency ~triangles! as a function of overlayer thick-ness for operation at the fundamental frequency of 100 MHz.The Love wave guiding effect can clearly be seen with aninitially improving insertion loss and a reducing frequency ofoperation with increasing overlayer thickness. The lowest in-sertion loss, due to the guiding effect, occurs at around 1.5mm. Beyond this overlayer thickness, the insertion loss in-creases as previously reported in the literature. However, thefrequency also continues to decrease with increasing over-layer thickness until at around 2.5 mm a clear resonance isobserved corresponding to both a large frequency decreaseand a substantial insertion loss. This pattern is further re-peated in a periodic manner and we were able to observefour clear minima in the insertion loss and frequency beforethe noise level became too large. The average overlayerthickness between two successive minima was (5.2FIG. 1. Insertion loss ~squares! and resonant frequency ~triangle! for a de-vice operated at the fundamental frequency as a function of overlayer thick-ness.2 © 2001 American Institute of Physicscense or copyright; see http://apl.aip.org/about/rights_and_permissions3543Appl. Phys. Lett., Vol. 79, No. 21, 19 November 2001 McHale et al.Down60.1) mm and the thickness for the first minima was (2.760.1) mm.In Fig. 2, we show the insertion loss ~squares! and fre-quency ~triangles! as a function of overlayer thickness for theharmonic of the same device operated at 330 MHz. As thedevice is operating at three times the fundamental frequency,the SSBW wavelength is one third that of the fundamental.Thus, assuming the acoustic speed in the overlayer is inde-pendent of frequency, the acoustic wavelength in the over-layer will also be one third that occurring when the device isoperated at the fundamental frequency. This is reflected inFig. 2 in the positions of the resonances and the initial Lovepeak, which now occurs for an overlayer thickness of 0.5mm. The periodic pattern in the frequency shift is clearlyobservable with a period in the overlayer thickness of (1.760.1) mm compared to that of (5.260.1) mm at the funda-mental frequency. A series of related maxima and minimacan also be observed in the insertion loss although the exactshape is different to that observed at the fundamental.The solution of the dispersion equation from Love wavetheory10 predicts a series of modes with the first Love wavemode corresponding to a displacement with an antinode atFIG. 2. Insertion loss ~squares! and resonant frequency ~triangle! for a de-vice operated at the third harmonic as a function of overlayer thickness.loaded 23 Sep 2011 to 152.71.223.129. Redistribution subject to AIP lithe top surface of the layer and a single node located in thesubstrate close to the interface with the overlayer. The nexthigher order mode also has an antinode at the overlayer sur-face, but two nodes with one located within the layer and onelocated in the substrate close to the interface with the over-layer. Subsequent higher order modes introduce additionalnodes within the overlayer. Equating the period in our over-layer thickness to one half of an acoustic wavelength givesan estimated acoustic speed in the overlayer of (1120660) m/s and this is consistent with expectations from Lovewave theory.In conclusion, we have studied the effect of increasingthe overlayer thickness on a SSBW device. A series of reso-nance patterns have been observed in the insertion loss sepa-rated by a thickness consistent with a change of half anacoustic wavelength in the overlayer. This was observed foroperation at both the fundamental frequency and harmonicfor the same device.The authors gratefully acknowledge the BBSRC for fi-nancial support ~research Grant No. 301/E11140! and J. R.Middleton and J. Chauhan from the molecular-beam epitax-ity unit at Nottingham University for surface profile mea-surements.1 Y. V. Gulyaev, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 45, 935~1998!.2 E. Gizeli, A. C. Stevenson, N. J. Goddard, and C. R. Lowe, IEEE Trans.Ultrason. Ferroelectr. Freq. Control 39, 657 ~1992!.3 F. Hermann, D. Hahn, and S. Buttgenbach, Appl. Phys. Lett. 74, 3410~1999!.4 B. Jacoby and M. J. Vellekoop, Sens. Actuators A 68, 275 ~1998!.5 M. Weiss, W. Welsch, M. V. Schickfus, and S. Hunklinger, Anal. Chem.70, 2881 ~1998!.6 B. Jacoby and M. J. Vellekoop, IEEE Trans. Ultrason. Ferroelectr. Freq.Control 45, 1293 ~1998!.7 L. G. Harding and J. Du, Smart Mater. Struct. 6, 716 ~1997!.8 E. Gizeli, Smart Mater. Struct. 6, 700 ~1997!.9 M. I. Newton, F. Martin, K. Melzak, E. Gizeli, and G. McHale, Electron.Lett. 37, 340 ~2001!.10 D. P. Morgan, Surface-wave Devices for Signal Processing ~Elsevier, NewYork, 1991!, p. 27.cense or copyright; see http://apl.aip.org/about/rights_and_permissions

Resonant conditions for Love wave guiding layer thickness

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