HIGH RESOLUTION DIFFERENTIAL TECHNIQUE FOR PHASE VELOCITY AND ATTENUATION MEASUREMENTS OF ULTRASONIC INTERFACE WAVES

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HIGH RESOLUTION DIFFERENTIAL TECHNIQUE

FOR PHASE VELOCITY AND ATTENUATION

MEASUREMENTS OF ULTRASONIC INTERFACE

WAVES

S. Rokhlin

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C10, suppl6ment au n012, Tome 46, dhcembre 1985 page C10-805

HIGH RESOLUTION DIFFERENTIAL TECHNIQUE FOR PHASE VELOCITY AND ATTENUATION MEASUREMENTS OF ULTRASONIC INTERFACE WAVES

Department of Materials ~ n g i n e e r i n g , Ben-Gurion u n i v e r s i t y of

t h e Negev, PO Box 6 5 3 , Beer-Sheva, 84105, Israef

Abstract

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A computerized system is described for measuring viscoelastic properties of thin interface layers using ultrasonic interface waves. This system includes an acoustic bridge for eliminating the temperature dependence of the transducers and t h e substrates. To check t h e accuracy of phase velocity measurements, an analog simulation of t h e measurement method using a highly stable frequency synthesizer was applied. In this way, the resolution of t h e electronic device f o r t i m e delay measurements was estimated to be less than 30 psec.

I

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INTRODUCTON

Recently the method of interface waves was suggested for measurements of viscoelastic properties of thin interface layers [I). By theoretical analysis and experimentally i t was shown t h a t t h e complex shear modulus of interface layer c a n be obtained from velocity and attenuation measurements of interface waves. This method was previously discussed[l-31 and reviewed [4]. More recent results we discuss in the accompanying paper in this volume [ S ] .

To measure t h e interface film properties a s function of temperature t h e differential method of measurement (ultrasonic bridge) was developed [3

1.

This makes i t possible t o eliminate t h e e f f e c t of t h e temperature dependences of t h e substrates and transducers from t h e measure- ments.

In this paper we described the further development of the techniques: modified ultrasonic bridge and high resolution computerized measurement system.

Although we discuss this measurement system in connection with t h e interface wave method, i t can be used for precise measurements of t h e phase velocity changes of any type of waves, in dispersive and nondispersive media.

I1

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ULTRASONIC BRIDGE AND TRANSDUCERS

Temperature changes modify t h e elastic moduli of the film and substrates and, by virtue of thermal expansion, change t h e density. This results in a change in t h e velocity of t h e interface wave. In addition, the thermal expansion of the substrate modifies the acoustical path length. To measure t h e properties of t h e film under nonisothermal conditions, i t is necessary t o eliminate the e f f e c t of temperature variations in the substrates and transducers. For this purpose we use a differential measuring arrangement, in t h e form of an acoustic bridge, shown in Figure 1. The middle transducer (transmitter) radiates a n ultrasonic surface wave symmetricaliy into two a r m s of t h e bridge. In t h e l e f t arm, t h e surface wave incident on t h e boundary between

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'On Sabbatical leave at the Ohio State University, Department of (Welding Engineering, Columbus, Ohio 43210, U.S.A.

JOURNAL DE PHYSIQUE

I

I

ADHERENT-I

MONOLITHIC SPECIMEN

TRANSMITTER

ADHESIVE -OFFSET BLOK

I

HEATER-II

1

Fig. 1 Schematic of the specimen for differential measurements

Fig. 2 Schematic of the comb and lower substrate in the form of a single monolitic specimen the f r e e surface of t h e lower substrate and t h e interface region, is transformed into an interface wave. After leaving the interface region, the interface wave is transformed into a surface wave, and is sensed by a receiver. A surfaqe wave propates along the entire right arm. An offset block which is identical t o substrate-2 is placed without bonding on substrate-1 in order t o equalize the thermal masses of both arms (Fig. 1). due to surface roughness there is'no acoustic contact between the substrate and the offset block and it does not affect the propagation of the surface wave.

If the dimensions of both arms of the bridge and of the transducers a r e the same, then the use of the differential arrangement, eliminates t h e effect of the thermal changes of the acoustical path. As shown in the figure, pressure may be applied t o the specimen. Strictly perpendicular force transmission is ensured by means of spherical contact.

In Fig. 1 is also shown the location of the heater. An additional heater is placed on the upper adherent, designated by 1 t o eliminate temperature gradients throuth t h e interface film thickness. The temperature is measured by means of miniature thermocouples, two of which a r e located near the interface in the upper and lower substrates of t h e left arms, and t h e third

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in

the right arm of the lowsr substrate. The temperature gradient in the course of

measurements did not exceed 0.5 C.

In this work we used comb transducers 1 3 1 for excitation and reception of surface waves. The comb transducer generates identical signals in mutually opposite directions, which makes i t possible t o perform the differential measurements. To increase the efficiency of excitation and reception, t h e height of t h e comb was taken to be such a s to provide resonance, i.e., equal t o one half or t o one longitudinal wavelength. The comb can be damped by adding an acoustic absorber in the grooves. In contrast to previous work [3]

,

the combs and the lower substrate were constructed in the form of a single monolithic speciman, a s is shown schematically in Fig. 2. Placement of transducers with a precision ensuring a difference in t h e lengths of arms of O.lmm (distance between the emitter and receiver) yields a time balance of the bridge not exceeding 2

-+

0 ,bo IJO 2k3 2:0 TEMPERATURE ( O C )

Fig. 3 Theoretical estimation of the unbalance of the bridge ( h r l ~ ) .

DU- W A W E L SCOPE PHASE-LOCK ~ p . , ~ ~ 3 ~

TRIGGER

1 I .

Fig. 4 The block diagram of t h e measuring arrangement. substrates change with temperature. The quantity AT characterizes the unbalance of t h e bridge upon changes in temperature and introduces a systematic error in measurements. The shear modulus of a polymer interface film drops significantly in the course of heating the specimen, which reduces t h e magnitude of t h e error. The change in AT

,

taking into consideration t h e

change in t h e shear modulus of the film upon heating, amounted t o a maximum of about 2 ns/cm for a temperature change of up t o 2 0 0 ~ ~ . The value of AT can be reduced by making a correction in velocity measurements [3].

111

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COMPUTERTIZED AUTOMATIC MEASURING ARRANGEMENT

C10-808 JOURNAL DE PHYSIQUE

To check the accuracy of the system, we performed an analog simulation of the measuring method. The measuring arrangement is shown schematically in Fig. 6. A reference and a working signal from a highly stable frequency synthesizer a r e fed t o the phase-lock trigger. The synthesizer's clock signal was used a s t h e reference. The working signal can be phase shifted relative t o the reference signal with an accuracy of 0.1'. At 10 MHz this yields a shift of the time delay by 28 psec. The synthesizer was synchronized by means of the highly-stable clock of an HP-5345A counter. The phase shift of the working-signal is preset by a microcomputer program and is measured by t h e phase-trigger and counter. The phase-lock trigger is synchronized through a divider with a synthesizer.

The difference between t h e measured synthesized and preset phase changes yields the absolute measuring error. The measurements show that this error is independent of the magnitude of the time interval (distance between the selecting- windows of 800 and 10 ps). Measured in this way the error due to the electronic system, namely, the phase-lock trigger and counter, does not exceed 30 ps. This value (30 ps) gives t h e resolution of t h e dlectronic system.

Another experimental probl'em is associated with the effect of the amplitude changes of the input signal on the measured phase shift. The error is due t o t h e f a c t that i t is impossible t o establish a zero comparison level by the zero-crossing comparator. The minimum comparison level (threshold level) results in shifting in the zero-crossing output upon a change in t h e signal amplitude.

This effect can be estimated by means of the unit described above. The amplitude of the synthesizer working signal is varied by a microcomputer. At each voltage setting t h e time delay is measured using the above arrangement. To improve the amplitude characteristic of the unit, an automatic gain control system (AGC) can be used.

Different examples of measurements performed with this system a r e given on the accompanying paper C5

3.

RF I N ZERDCRlSSNj O-ETUT I M G E R IN

n

I S E W DELAY 0.5-6ps MAIN DELAY @-

7

I 01-10Wps WlNDOU 1 W m - b r w m w 7 500s-05ur

ZERO CROSSNC. U S E CUT TRKiCER FOR START ICHbNNELAI OR STW ICHANML 61 TIME INTERVAL MEASURMNTS

Fig. 5 The pulse sequence of the phase-lock trigger. References HP- 3325A DIVIDER Y SCOPE HP-1743A PLOTTER

1

Fig. 6 The block diagram of the experimental set-up for performing analog simulation.

1. Rokhlin, S.I., Hefets, M. and Rosen, M., J. Appl. Phys.,

2

(1980) 3579.

2. Rokhlin, S.t, Hefets, M. and Rosen, M., J. Appl. Phys.,

52

(1981) 2847. 3. Rokhlin, S.I., 3. Acoust. Soc. Am., 73 (1983) 1619.

4. Rokhlin,

S.I.,

in "Adhesive Joints: m e i r Formation Characteristics and Testing,I1

K.L.

Mittal, ed., Plenum Press, N.Y. (1984).