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Department of Radiophysics, Vilnius University, Vilnius, LITHUANIA. ABSTRACT. The acoustoelectric interaction of leaky surface acoustic wave (LSAW) on 36o.
W3P.018

THIN FILM CONDUCTIVITY SENSOR BASED ON ACOUSTOELECTRIC INTERACTION IN 36O YX LITHIUM TANTALATE D. Čiplys, R. Rimeika, and A. Sereika Department of Radiophysics, Vilnius University, Vilnius, LITHUANIA The acoustoelectric interaction of leaky surface acoustic wave (LSAW) on 36o rotated Y-cut Xpropagation lithium tantalate was studied and the thin film conductivity sensor was implemented on this base. The technique of slow evaporation of thin copper film on the wave propagation surface with simultaneous measurements of the wave amplitude and phase was used. The observed strong response of the LSAW to the film conductivity is explained by the acoustoelectric attenuation and velocity change. The sheet conductivity extracted from the changes in wave amplitude and phase was compared with the results of DC resistance measurements. The potential of LSAWs in LiTaO3 for contactless monitoring of changes in thin film conductivity has been demonstrated.

KEYWORDS Leaky surface acoustic waves, lithium tantalate, acoustoelectric interaction, thin metal film, conductivity sensor.

INTRODUCTION Surface acoustic waves being very responsive to propagation surface properties are very attractive for applications in sensing technologies [1]. Amongst various wave types, leaky surface acoustic waves (LSAWs) deserve special attention. Their advantages, as compared to more common Rayleigh-type surface acoustic waves (SAWs), comprise higher coupling efficiency, operation at higher frequencies, and essentially lower propagation loss on solid-liquid interface [2]. The 36o rotated Y-cut Xpropagation LiTaO3 single crystals are amongst the most usable substrates supporting the LSAWs. In spite of numerous applications in telecommunications (primarily for RF filters), the acoustoelectric interaction involving leaky waves on these crystals has been studied insufficiently. It was the purpose of the present work to perform such investigations. We report on the direct measurements of the acoustoelectric attenuation and velocity change of the LSAW during thermal evaporation of a metal film on the crystal surface. We demonstrate that both the amplitude and phase of transmitted wave are very sensitive to the conductivity of the thin film and propose a sensor on this basis.

EXPERIMENTAL Sample preparation and characterization The method of in situ measurements of wave transmission parameters during evaporation of conductive metal film on the piezoelectric crystal surface (see e.g. [36]) was used in our experiments. A pair of identical interdigital transducers (IDTs) was fabricated on the surface of the 36o rotated Y-cut X-propagation single-

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crystal lithium tantalate substrate. The IDT period was 50 μm, the aperture was 1.75 mm, each IDT had 30 finger pairs, and the center-to-center spacing between the IDTs was 8 mm. The SAW device under investigation is shown in Fig. 1.

Figure 1: Leaky SAW device on LiTaO3 substrate. The signal transmission characteristics (parameter S21) of such a device measured using the radio-frequency network analyzer HP 8752A is shown in Fig. 2. The high peak at 83.6 MHz corresponds to the leaky SAW with the velocity 4180 m/s. A much weaker Rayleigh wave transmission is also observed at 62.6 MHz corresponding to the velocity 3130 m/s. We determine the wave velocities as V = Λf , where Λ is the acoustic wavelength (equal to the IDT period), and f is the relevant peak frequency. These velocity values are in excellent agreement with literature data [2]. -10 Leaky wave

Transmission (dB)

ABSTRACT

-20 Rayleigh wave -30

-40

60

70

80

Frequency (MHz)

Figure 2: SAW transmission versus frequency in 36 o rot. YX LiTaO3 substrate for IDTs with period 50 μm period.

Measurements during evaporation The sample was placed in the vacuum chamber, and the acoustic wave transmission parameters were measured during thermal evaporation of copper film onto the wave path on the crystal surface. According to calculations [7], the leaky wave attenuation for a given LiTaO3 orientation is very weak, implying that the energy leakage into the crystal bulk is negligibly small. Hence, like the Rayleigh wave, the leaky SAW remains confined to the propagation surface on its path between the IDTs. It follows that the metal should be deposited on the same surface on which

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Transducers 2013, Barcelona, SPAIN, 16-20 June 2013

the IDTs have been fabricated. This is different from the previously studied cases of strongly radiating leaky waves in YX-LiTaO3 when the metal should be evaporated onto the opposite surface [6]. The schematic of experiment is depicted in Fig. 3. The evaporation process lasted a few tens of seconds, and its rate could be varied by adjusting the heater current. To protect the transducers, the mask with rectangular window was located before the crystal to protect the transducers and to form the evaporated film into a shape of rectangle. Typically, the length of metalized region along the wave propagation direction was 1.5 mm, and the metal stretched across the entire substrate width in the perpendicular direction .

the piezoelectric substrate-conductive thin film structure.

THEORETICAL BACKGROUND In the structure, consisting of a piezoelectric substrate coated with a conducting thin film, the SAW attenuation α and velocity V depend on the film sheet conductivity σs as [8]:

α k

=

σs σm K2 , 2 1 + (σ s σ m )2

V −V f Vf

(1)

K 2 (σ s σ m ) , 2 1 + (σ s σ m )2 2

=−

(2)

where σ m = V f ε 0 (ε + 1) , Vf is the wave velocity on the free surface, k is the wave number, ε and K2 are the dielectric constant and electromechanical coupling coefficient, respectively, and ε0 is the vacuum permittivity. The dependencies of normalized attenuation α/k and relative velocity change ΔV/Vf (both in K2/2 units) on ratio σs/ σm calculated from Eqs.1,2 are plotted in Fig. 5. With varying sheet conductivity, the normalized attenuation α/k attains maximum value K2/4 at σ = σ m . When the sheet conductivity varies from σ > σ m , the velocity decreases by relative

Figure 3: Schematic of SAW parameters measurements during metal evaporation.

amount ΔV V f = K 2 2 . 0.6

(α/k)/(K2/2)

The time variations in transmitted leaky SAW amplitude and phase during Cu evaporation were measured with the network analyzer at frequency 83.6 MHz. They are plotted in Fig. 4. The zero of time scale corresponds to the beginning of evaporation process. As seen, the amplitude dependence on time exhibits a sharp minimum, and the phase decreases monotonically between the steady values corresponding to the start and end of the evaporation process.

0.4 0.2 0.0

2

(ΔV/V) / K /2

-0.2 -0.4 -0.6 -0.8 -1.0

0.01

0

2 -5

-90

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-180

-15

-270 0

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Phase change (deg)

Amplitude change (dB)

0

0.1

1

10

100

σs/σm

Figure 5: Dependencies of normalized attenuation (upper trace) and relative velocity change (bottom trace) on normalized sheet conductivity for surface acoustic wave in piezoelectric substrate- thin conducting film structure. Calculated using Eqs. 1, 2.

RESULTS AND DISCUSSION

40

Extraction of K2

Time (s)

Figure 4: Variation of LSAW amplitude (1) and phase (2) during Cu film deposition on 36o rot. YX LiTaO3 substrate. No substantial changes in SAW amplitude and phase were observed at times beyond 40 s when the evaporation process was terminated. We attribute the observed variations in wave amplitude and phase to the acoustoelectric interaction of the LSAW propagating in

As the Cu film thickness increased during evaporation, its conductivity varied in a wide range from the free-surface to the shorted-surface condition. The attenuation α and velocity change ΔV = V − Vf can be extracted from measured changes in transmitted wave amplitude ΔA and phase ΔΦ with respect to their freesurface values using the relations:

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ΔA(dB ) = 8.686 ⋅ 2π

wα , Λ k

(3)

ΔΦ (deg ) = 360 0

w ΔV , Λ Vf

(4)

where w is the length of the conducting film in the wave propagation direction, and Λ is the acoustic wavelength. As seen from Fig. 4, the amplitude change from initial to maximum-attenuation value is 15 dB. This yields K2 = 0.037. The determination of phase value for the shorted surface is somewhat ambiguous: the slow decrease at times beyond 26 s seems to be caused by mechanical loading rather than by acoustoelectric screening. Therefore, we take 237 deg as the phase span between the free-surface and shorted-surface values, resulting in K2 = 0.044. The average value 4 % (in percent units) of the electromechanical coupling coefficient is in good agreement with theoretical values available in literature [7]. It should be noted that it is comparable to most efficient orientations for Rayleigh waves, in particular, YZ and 128o rot. Y-cut LiNbO3 [2].

1. It follows that the phase and amplitude changes are sufficient in this case:

σs ΔV V ΔΦ (deg ) . =− = 0.1516 σm α k ΔA(dB )

The dependence of σs/σm on time during evaporation extracted using Eq.7 is shown in Fig. 6 by curve 3. As seen, the curves 1 and 3 of Fig. 6 exhibit saturation at the ending stage of evaporation process. This is related to the observed anomalous behavior of amplitude at times > 26 s (Fig. 4), which does not return to initial value as it could be expected from theory as predicted by Eq. 1 and shown by upper curve in Fig. 5. 12

2 8

σs / σm

Extraction of thin film conductivity

We made an attempt to extract the thin film conductivity from the measured variations in SAW propagation parameters. This could be done in three different ways: from attenuation measurements using Eq. 1, from velocity measurements using Eq. 2, or by combining both equations. From Eq. 1, it follows: 1 ⎤2

2

1

2 ⎤2 σs K w ⎡⎛ K 2w ⎞ ⎟⎟ − 1⎥ (5b) = 13.64 ± ⎢⎜⎜13.64 ΔAdB ⎢⎝ ΔAdB ⎠ σm ⎥⎦ ⎣ where sign minus must be taken if σ s σ m < 1 , and sign 2

plus is for σ s σ m > 1 . The extracted variation of σs/ σm with time during evaporation is plotted in Fig. 6 by curve 1. From Eq. 2, one obtains:

σ s ⎡ ⎛⎜ K 2 2 ⎞⎟⎤ = ⎢− 1 + ⎥ σ m ⎣⎢ ⎜⎝ ΔV V f ⎟⎠⎦⎥



1 2

,

(6a)

or, by substitution of Eq.4,

σ s ⎡ ⎛⎜ 180 K 2 w ⎞⎟⎤ = ⎢− 1 + ⎥ σ m ⎢⎣ ⎜⎝ ΔΦ deg Λ ⎟⎠⎥⎦



1 2

.

4

0 20

25

Figure 6: Variation in Cu film sheet conductivity during its evaporation extracted from measurements of LSAW amplitude (1), phase (2) and their ratio (3).

Comparison with dc measurements An alternative pulse-mode technique was used to measure amplitude changes during evaporation. The schematic of experiment is shown in Fig. 7. The LSAWs were excited by applying the 83.6 MHz radio-frequency pulses of 1.7 μs width to the transmitting IDT, and the received SAW pulses from the receiving IDT were recorded using the time-gate and the peak detector. The sheet conductivity variation extracted from these measurements was compared with the results of direct measurements of the film ohmic conductivity in dc mode. For this purpose, the strip Cu electrodes have been evaporated in advance at the sides of the wave path metallization area on the sample surface.

(6b)

U0

The expression in square brackets is positive since the relative velocity change ΔV V f is negative and its absolute value does not exceed K2/2. The time dependence of σs /σm during evaporation extracted from the phase measurements is shown in Fig. 6 by curve 2. When equations 5 or 6 are used, the electromechanical coupling constant and the length of metalized region must be known. We used K2=0.04 and w =1.5 mm. These parameters can be eliminated by division of Eq. 2 by Eq.

30

Time (s)

(5a)

or, substituting Eq. 3,

3 1

15

σ s 1 K 2 2 ⎡⎛ 1 K 2 2 ⎞ ⎟ − 1⎥ = ± ⎢⎜ σ m 2 α k ⎢⎜⎝ 2 α k ⎟⎠ ⎥⎦ ⎣

(7)

RF pulse generator

Peak detector Time gate

R1

Voltage meter

Figure 7: Schematic of pulse amplitude measurements

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The dc voltage U0 on the order of several volts was applied to the electrodes, and the voltage drop U1 across the connected in series resistance R1 was measured to determine the sheet conductivity σs of the metal film at any moment of evaporation as

σs =

U1 l , w (U 0 − U 1 ) R1

(8)

where l and w are the evaporated film dimensions across and along the wave propagation direction, respectively. The time dependencies of the SAW pulse amplitude received by the output IDT and of the voltage drop across the series resistor of dc measuring circuit are presented in Fig. 8. The amplitude behavior is similar to that observed in network-analyzer measurements. The amplitude passes through the minimum (attenuation maximum), and, again, does not return to the initial value.

As seen, the conductivity values extracted from SAW measurements are several times lower than those obtained by the DC measurements. The general trends of both dependencies are very similar, except the apparent saturation of curve 2 at the end of deposition, where the SAW amplitude change deviates from behavior predicted by Eq. 1.

CONCLUSION In conclusion, we have demonstrated the strong acoustoelectric response of the leaky surface acoustic wave in 36o rot. YX LiTaO3 crystal to the variation in the conductivity of a metal film during its deposition on the wave propagation surface. Our results reveal the potential of LSAWs for contactless monitoring of changes in thin film conductivity and implementation of various sensors on this base.

ACKNOWLEDGMENT Amplitude (V)

0.10

4

0.05

2

This work was supported by the Research Council of Lithuania under Project No. PRO-01/2012.

Series resistor voltage (V)

0.15

6

REFERENCES

0.00

0 0

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Time (s)

Figure 8: Time dependencies of LSAW amplitude measured by pulse technique and voltage drop across the series resistor of dc measuring circuit during Cu film deposition on the crystal surface. The sheet conductivity of the film was extracted with the help of Eq. 6. To find its absolute value, σm was calculated. With V = 4180 m/s and ε = 43 we obtain σm=1.63·10-6 S. The variations in Cu film sheet conductivity during evaporation, extracted from pulse amplitude measurements and from dc measurements are compared in Fig. 9.

Sheet conductivity (S)

1E-4 1E-5

1

1E-6

2 1E-7 1E-8 1E-9 20

40

60

80

[1] D.S. Ballantine, S.J. Martin, A.J. Ricco, G.C. Frye, E.T. Zellers, R.M. White, H. Wohltjen, Acoustic Wave Sensors: Theory, Design, and PhysicoChemical Applications, Academic, San Diego, 1997. [2] K. Hashimoto, Surface Acoustic Wave Devices in Telecommunications: Modelling and Simulation, Springer, 2000. [3] A. P. Sereika, E. P. Garska, Z. A. Milkevichene, A. I. Yutsis, “Electronic Absorbtion of a Surface Wave in a Piezoelectric-Metal Film Structure”, Soviet PhysicsSolid State, vol. 16, no. 8, p. 1572-1573, 1975. [4] D. Ciplys, R. Rimeika, “Electromechanical Coupling Coefficient for Surface Acoustic Waves in ProtonExchanged 128 Degrees-Rotated Y-Cut Lithium Niobate”, Appl. Phys. Lett., vol. 73, no. 17, p. 24172419, 1998. [5] R. Rimeika, D. Ciplys, M. S. Shur, R. Gaska, M. A. Khan, J. Yang, “Electromechanical Coupling Coefficient for Surface Acoustic Waves in GaN-onSapphire”, Phys. Stat. Solidi B, vol. 234, no. 3, p. 897-900, 2002. [6] R. Rimeika, A. Sereika, D. Čiplys, “Acoustoelectric Effects in Reflection of Leaky Acoustic Waves from LiTaO3 Crystal Surface Coated with Metal Film”, Appl. Phys. Lett., vol. 98, 052909 (1-3), 2011. [7] K. Yamanouchi, M. Takeuchi, “Applications for Piezoelectric Leaky Surface Waves”, in IEEE 1990 Ultrasonics Symposium Proceedings, Honolulu, December 4-7, 1990, pp. 11-18. [8] K. A. Ingebrigtsen, “Linear and Nonlinear Attenuation of Acoustic Surface Waves in a Piezoelectric Coated with a Semiconducting Film”, Journ. Appl. Phys., vol. 41, no. 2, pp. 454-459, 1970.

Time (s)

Figure 9: Change in sheet conductivity of the Cu film during its deposition on the crystal surface: (1) dc measurements, (2) SAW pulse amplitude measurements.

CONTACT *D.Čiplys, tel: +3705-236-6025; [email protected]

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