Acoustic microscope-scanning version R. A. Lemons and C. F. Quate

Microwave Laboratory, Stanford University, Stanford, California 94305 (Received 22 October 1973)

This letter reports the development of a mechanically scanned acoustic microscope shOwing 100jJ.m resolution. Using single.surface lenses an acoustic beam is focused with negligible spherical aberration in a water cell. The image is formed by mechanically scanning an object through this focused beam in a raster pattern. Transmitted power is detected with a piezoelectric transducer, and this signal modulates the synchronized raster of a CRT display. By employing piezoelectric detection, sensitivities of 10-8 W!cm2 are obtained, yielding images of excellent clarity and contrast.

A microscope can be made with acoustic waves rather than optical waves. We have set out to explore the poten­tial of such an instrument and compare its performance with that of the optical version. The acoustic micro­scope would provide us with a new method for delineat­ing the detail of microscopic objects if it could be made with a resolution comparable to that of the optical in­strument. 1 In principle this can be done. With a fre­quencyof 1 GHz the wavelength for sound in water is near 1. 5 /J.m. The utility of such an instrument arises because the scattering of acoustic waves is dependent on the change in elastic properties, whereas it is the change in the index of refraction that determines the scattering of optical waves. This fundamental difference suggests that acoustic radiation, responding to struc­tures within the object, may allow one to resolve details which are different from those recorded with optical waves: Often, objects which are transparent to optical radiation show considerable acoustic contrast. This increase in contrast will allow us to view structural de­tails within an object without chemical staining. More­over, the optically opaque objects frequently allow acoustic transmission, enabling internal structure to be observed. This is the basis for our interest in an acoustic microscope.

Such an instrument has not yet been fully exploited and we would like to report here on our progress toward a scanning version. This system may have some advan-

TRANSMITTER ROD

WATER CELL

OBJECT

RECEIVER ROD

PIEZOELECTRIC TRANSDUCER

MECHANICAL 'r

FIG. 1. Schematic showing the lens configuration.

163 Applied Physics letters, Vol. 24, No.4, 15 February 1974

tage over the two other systems which have been pre­viously reported. 2-4 We have determined that it is pos­sible with a most Simple lens to focus an acoustic beam with negligible spherical aberration. Thus the potential resolution is limited only by the acoustic wavelength. This is in contrast to the previous two systems wherein the recording process itself can influence and degrade the ultimate resolution. In addition, the detector in the scanning version is a piezoelectriC, which is the most sensitive detector available for acoustic radiation. We can therefore work with average sound intensity levels of 10-8 W Icm2, monitoring either variations of acoustic

(a) 200/-lm

(b) 200 j.lm

FIG. 2. Comparison of the acoustic (a) and optical (b) images of a 200-mesh copper electron microscope grid.

Copyright © 1974 American Institute of Physics 163 Downloaded 27 Sep 2011 to 143.89.145.54. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

164 R.A. lemons and C.F. Quate: Acoustic microscope-scanning version 164

(a) 300 p.m

(b) 300 p.m

FIG. 3. Comparison of the acoustic (a) and optical (b) images of onion cells.

intensity or variations of acoustic phase. This level is well below the damage threshold for biological speci­mens. Finally, the scanning rate is sufficiently high that image formation time is reduced to less than 1 sec.

The heart of our system is a pair of single-surface acoustic lenses. Each is a polished concave spherical surface ground into the end of a crystal rod. These two lenses are positioned as mirror images with a water cell filling the space between them (see Fig. 1). The water serves the dual function of providing a slower refracting medium and of making acoustic contact be­tween the lens surfaces and the object to be imaged. On the end of each rod, opposite the lens surface, a piezo­electric transducer is applied. One of these transducers acts as the transmitter, generating a plane acoustic wave. This acoustic power is then focused by the lens into the water cell. The specimen itself is placed at the minimum waist of this beam. Since the two lenses are spaced so that their fOCi are cOincident, the power transmitted by the specimen is recollimated by the second lens. Thus, all the transmitted power from a specimen pOint is detected with the second piezoelectric

Appl. Phys. lett., Vol. 24, No.4, 15 February 1974

transducer. Accordingly, this tranSmission geometry has the dual advantage of eliminating spurious signals while fully utilizing the sensitivity of piezoelectric detection.

To produce an image with this system the specimen is mechanically translated through the beam in a raster pattern. This motion is synchronized with a raster on a CRT display, and the output of the detector modulates the display beam current. In this way, modulation of the acoustic beam by the object is converted directly into a brightness pattern or image.

The specimen is attached to a small piston which rides in a cylinder that constrains both lateral and vertical motions to less than 1 /lm. The horizontal com­ponent of the mechanical motion is then achieved by affixing the free end of the piston to the cone of a dy­namic speaker. In practice this speaker is driven sinus­oidally and can provide up to 3-mm displacement with scan rates of 300 lines/sec. Since the voltage which drives the speaker also drives the X axis of the display, synchronization is simplified. The vertical motion is provided by mounting the speaker assembly on a motor­driven stage. This motor also drives a potentiometer, providing the Y-axis deflection for the display.

Clearly, the resolution of this system is determined by the diameter of the focused beam in the object plane. If an optical system of the same design were construct­ed, one would find that the spherical aberration inherent to a single spherical surface would severely limit the resolution. However, the preCision with which the beam is focused depends crucially upon the ratio of the prop­agation velOCities at the crystal-water interface. By maximizing this ratiO, the spherical aberration can be greatly reduced. This can be understood in a qualitative way by a simple application of Snell's law (sinl1a = Cal Cl sinOl). As the ratio of propagation velocities (Ca/Cl) is reduced, the angle l1a between the refracted ray and the intersecting radius is likewise reduced. In the limit Ca/Cl - 0 all rays would converge on the center of cur­vature, and the spherical aberration would be exactly zero. A more quantitative calculation based on first­order aberration theory shows that the spherical aber­ration is roughly proportional to (Ca/Cl)a. Fortunately, in acoustics available materials allow much smaller values of Ca/Cl than one is accustomed to in optics. For a sapphire-water system Ca/Cl = 0.134. This means an f/0.8 lens can be diffraction limited down to a resolu­tion of 1 /lm.

In our present system the lenses are ground with a 1. 59-mm radius of curvature and have an f number of 0.92. The piezoelectric transducers are 35 Y-cut LiNb03 platelets operated in their fifth harmonic at a frequency of 160 MHz. Accordingly, the acoustic wave­length in the water is 9.4 /lm, and the diffraction-limit­ed resolution of the lens is - 10 /lm.

The image-forming capabilities of this system are illustrated in Figs. 2 and 3. In each case the acoustic image can be compared with the corresponding optical image. In Fig. 2 the acoustic image of a 200-mesh copper electron microscope grid is shown. The excel­lent contrast and definition seen in the acoustic image

Downloaded 27 Sep 2011 to 143.89.145.54. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

165 R.A. Lemons and C. F. Quate: Acoustic microscope-scanning version 165

is one of the chief advantages of this microscope. In Fig. 3 the acoustic image of a layer of onion cells is shown. The most prominent details here are the individ­ual cell walls. Both the marked contrast of their struc­ture and the appearance of internal cell detail suggests the potential useMness of acoustic microscopy. In ad­dition, analysis of these and other images indicates that the resolution of our system is very near the 10-IJ.m diffraction limit.

These results are suffiCient to allow us to comment on the improved resolution that can be attained by mov­ing to higher frequencies. We will be limited by the acoustic attenuation in water which increases as the square of the frequency. We can partially compensate for this increase by shortening the liquid path length and this in turn will require a lens with a smaller radius of curvature. We have successfully fabricated and tested a pair of lenses with a focal length of 0.46 mm, and we are confident that these can be operated at 400 MHz with a resulting resolution of 3 IJ.m. At 1000 MHz the acous-

tic attenuation in water is apprOximately 145 dB/mm (at 35 °C)5 and lenses with O. 1-mm radius of curvature would be needed. The resolution of an acoustic micro­scope at this frequency would be equal to 1. 2 p.m.

The authors would like to acknowledge their debt to Dr. W. L. Bond for his guidance in the design and fabri­cation of this instrument and to the John A. Hartford Foundation, Inc., for their generous financial support of this research.

1This was pointed out to us by Professor R. Kompfner who was the first to realize the potential of acoustic microscopy.

2L.W. Kessler, P.R. Palermo, andA. Korpel, Acoustic Holography, Vol. 4 (Plenum, New York, 1972), p. 51.

3J.A. Cunningham and C. F. Quate, J. Phys. Colloq. C-6 Suppl 33, 42 (1972).

4J.A. Cunningham and C. F. Quate, Acoustical Holography, Vol. 5 (Plenum, New York, to be published).

5J. M. M. Pinkerton, Nature 160, 128 (1947).

Bulk wave generation by surface interdigital transducers operating near resonance *

P. J. Vella, W. S. Goruk, and G. I. Stegeman

Department of Physics, University of Toronto, Toronto, Ontario M5S IA 7, Canada (Received 29 October 1973)

The fraction of electrical power converted into spurious bulk waves at a surface interdigital transducer operating on and near surface wave resonance was measured. Bulk wave generation on y-z lithium niobate was found to be strong below, at, and above resonance and to increase with decreasing number of finger pairs.

Theories and experiments on the generation of sur­face acoustic waves (SAW) by interdigital transducers1

generally neglect the possibility of appreciable bulk acoustic wave (BAW) generation at and near resonance. However, recent experiments by Schmidt2 and Goruk et al. 3 using optical probing and by Williamson4 using an electrostatic probe have shown that significant bulk wave generation occurs even on resonance. In this paper we report quantitative measurements of electrical power conversion into spurious BAW at interdigital transducers operating at and near surface wave resonance.

Interdigital transducers deposited on y -z lithium niobate were excited electrically near their resonance frequency of 105 MHz which corresponded to a SAW wavelength of 0.033 mm. The metallized fingers were fabricated of either gold or aluminum, normally 1200 A thick, and had an equal mark-space ratio. The prop­agation surface and the crystal sides were polished flat to -1000 A and the crystal edges were imbedded in absorbing tape to eliminate SAW reflections.

The power converted by the transducers into BAW was determined from the difference between the total

Applied Physics Letters, Vol. 24, No.4, 15 February 1974

electrical power absorbed and the SAW power generated. Standard5 techniques utilizing a vector voltmeter and a dual directional coupler were used to evaluate accurate­ly the electrical power PH absorbed by the transducer. The SAW power generated by the transducer was mea­sured by high -resolution spectroscopy. 6 Since other loss mechanisms, such as electrode reSistance, are negligible in this experiment, the BAW power is deter­mined directly from the difference.

The optical measurements utilized a single-frequency Ar+ laser for the light source and a 50-cm confocal Fabry-Perot interferometer to frequency analyze the first-order reflected diffraction spot. It is well known6

that the SAW power is given by PA = CdI+1/Io, where d is the effective width of the SAW wavefront and 1+1/10 is the ratio of the first-order diffraction component arising from SAW leaving the transducer to the undif­fracted reflected light intensity. The constant C de­pends on the crystal cut, SAW frequency, and optical frequency, and also takes into account the transducer bidi recti onality . In order to determine d and 1+1/10 ac­curately' it was necessary to measure the SAW radia­tion diffraction pattern (typical width -1 .2 mm) with a

Copyright © 1974 American Institute of Physics

Downloaded 27 Sep 2011 to 143.89.145.54. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions