Acoustic microscopy: present and futureH.K. Wickramasinghe, B.Sc.(Eng.), Ph.D.

Indexing terms: Acoustics, Microscopes, Acoustic microscopy

Abstract: After nearly ten years of research and development, the acoustic microscope has now reached thepoint where its resolution is far superior to that of the optical microscope. Several commercial firms are now inthe process of manufacturing scanning acoustic microscopes, and it will not be long before these are availableon the open market. In the paper, we review the basic operating principles of the microscope and describe someof the achievements made over the past decade. The paper is concluded by a discussion of possible areas forfuture advance.

1 Introduction

Mechanically scanned acoustic microscopy (SAM) wasfirst demonstrated in 1974 by C.F. Quate and R.A. Lemonsof Stanford University [1]. After nearly ten years ofresearch and development, the instrument is now becom-ing a commercial reality. It is therefore appropriate toreview progress made over the past decade and commenton possible future advances.

Experience has shown that whenever a microscopebased on a new class of radiation is developed, our under-standing of the microscopic structure of nature hasimproved. The introduction of acoustic radiation tomicroscopy can be expected to have a similar impact. Con-trast in acoustic micrographs derives from changes inmechanical properties (such as density, elasticity andviscosity) within the sample; for this reason, the imagesobtained with an acoustic microscope are fundamentallydifferent to those obtained with other techniques such aslight, X-rays or electron microscopy.

The basic idea of an acoustic microscope is not new; itwas first put forward by the Russian scientist S. YaSokolov, who realised that acoustic waves in water at giga-hertz frequencies had wavelengths comparable to that ofvisible light. In the years before the Second World War, hedescribed his investigations on an 'acoustic microscope'which was based on the use of a high-resolution electronprobe to scan the acoustic field. Although he mentionedthe possibility of operating at acoustic frequencies of a fewgigahertz, the technology to generate acoustic waves effi-ciently at such high frequencies did not exist at the time,and his work culminated with the demonstration of asystem working at frequencies around 1 MHz [2]. Interestin acoustic microscopy was revived during the latter partof the 1960s, following the development of techniques fordepositing efficient thin-film acoustic transducers [3]. Overthe past decade, several novel acoustic microscopes havebeen demonstrated [4, 5, 6]. In this paper, however, weshall be exclusively concerned with a review of the mecha-nically scanned acoustic microscope [7] as invented byQuate and Lemons.

The following Section describes the basic operating prin-ciples of the SAM both in reflection and transmission.Many of the imaging modes available to us in opticalmicroscopy, such as phase-contrast, differential-phase-contrast, dark-field and stereo imaging, have their acousticcounterparts; these will be discussed in Section 3. Tech-

Paper 3182A(E1), received 14th December 1983The author was formerly with the Department of Electrical Engineering, UniversityCollege London, London WC1, England. He is now with International BusinessMachines, Thomas J. Watson Research Centre, PO Box 218, Yorktown Heights,NY 10598, USA

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niques for measuring the elastic properties of a sample ona microscopic scale will be discussed in Section 4; suchmeasurements are invaluable for materials character-isation. Section 5 presents some examples of high-resolution acoustic imaging as applied to biology andmaterials science. Acoustic waves are capable of penetrat-ing substantial distances into most solids; a unique appli-cation is therefore interior imaging of objects opaque tooptical radiation. Problems associated with interiorimaging and some solutions will be discussed in Section 6.Methods for improving the resolving power of the SAMare presented in Section 7. Finally, in Section 8, we shallbriefly discuss possible future advances.

2 Principles of operation

The schematic of a transmission SAM is shown in Fig. 1.Two identical lens assemblies are rigidly mounted andseparated by a coupling fluid. On the back surface of eachsapphire lens crystal is a piezoelectric transducer; at fre-quencies below 150 MHz this would typically be a slab oflithium niobate bonded via a thin indium film, while athigher frequencies a zinc-oxide film is sputtered directly onto a gold electrode deposited on the sapphire surface. AnRF input signal is applied to the transmitting lens trans-ducer, which, in turn, excites longitudinal acoustic waves

piezoelectrictransducer

i . itransmitterrod

watercell

object

receiver rod

piezoelectrictransducer

Fig. 1 Transmission scanning acoustic microscope configuration

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in the sapphire rod. These propagate (without significantloss) to the far side of the rod, where they encounter thelens: a simple spherical depression ground into the sap-phire. Because of the small ratio between the acoustic velo-city in the coupling fluid (in the case of water, 1500 ms"1)and that in sapphire (11100 ms"1), the sound waves arestrongly refracted by the lens. Rays incident upon the lensemerge close to the direction of the lens surface normal,and are focused to a diffraction-limited spot a small dis-tance in front of the lens centre of curvature.

The receiving lens, which is confocal with the transmit-ter, collects and recollimates the acoustic energy, and asecond transducer converts it back into an electrical signal.This signal is then amplified and detected using conven-tional RF electronics. The specimen is mounted on a thinmylar film placed at the focal plane of the two lenses.Thus, the detected output is proportional to the transmit-tance of the specimen averaged over the narrow focalregion of illumination. In order to form a complete image,the specimen is mechanically scanned in a raster fashion ina plane normal to the lens axis, while the detected signal isused to brightness modulate a scan-synchronised storagedisplay. Alternatively, the image may be directly recordedby a digital frame store and displayed on a VDU.

The resolving power of the acoustic microscope is pri-marily determined by the width of the acoustic-beam focus.In an on-axis configuration such as this, the only lensdefect present is spherical aberration. It can be shown [1]that the spherical aberration is proportional to the squareof the velocity ratio between the coupling fluid and sap-phire, and is, in this case, negligible compared with diffrac-tion effects. Thus for a well designed lens with wideaperture (//0.7 or smaller) the spot diameter approachesone acoustic wavelength and the resolution can be shown[8] to be approximately 0.7 X.

Because of the large difference in acoustic velocitybetween the sapphire and the coupling fluid, there is gener-ally a large difference in acoustic impedance (product ofdensity and velocity). This, in turn, causes much of thesound energy to be reflected at the lens surface. The reflec-tion loss can be reduced by using a quarter-wavelength [9]matching layer at the lens/coupling fluid interface. In thecase of water coupling, a typical matching material mightbe glass [10], while, for liquid helium, carbon layers havebeen found to be more satisfactory [11].

Even with acoustic matching layers, reverberationwithin the lenses is a significant factor. For this reason, themicroscope is generally used with pulsed rather than CWexcitation. Time gating is then employed to select thewanted signal. Because sharply focused sound beams areused, the depth of focus is approximately double the trans-verse resolution and is unaffected by the pulse length (sincethe latter is usually much greater).

The transmission geometry shown in Fig. 1 requiresprecise alignment of the focal points of the two lenses, aswell as a thin specimen. The alignment becomes prohibi-tive for focal spot sizes below 1 nm, and so the morecommon arrangement for the highest resolution instru-ments is the reflection SAM (Fig. 2). The single lens trans-mits a short pulse at the desired acoustic frequency andreceives the energy reflected by the specimen. A circulator(or microwave switch) separates the transmission andreception paths, and an image is scanned in the normalway. The pulses must be sufficiently short to discriminatethe energy reflected at the lens interface from the delayedenergy reflected by the specimen. Thus there is a minimumlens radius depending upon the transducer bandwidth andacoustic velocity of the coupling fluid. Although in Fig. 2

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the scan control is shown attached to the object, in somedesigns the lens can be scanned, with the object either heldstationary or scanned just in one dimension.

input RFpulse

i nPu t output

electricalmatchingnetwork

circulator

transducer

to outputelectronicsand display

lens

anti reflectioncoating

reflecting objectmechanically scanned

Fig. 2 Basic lens geometry for reflection acoustic microscopyRadius of lens r0 = 40 /zmFocal length of l e n s / = 1.13r0

Radius of lens aperture R — 0.7ro

Finally, magnification of the SAM is simply the ratio ofthe scan size to that of the display. Since the scan mecha-nism is usually electromechanical, the magnification iselectronically variable. Similarly, there is no inherent limitto the field of view, which can be as large as the scannerpermits.

3 Imaging modes

Many of the imaging modes available to us in opticalmicroscopy have their acoustic counterparts. Since thepiezoelectric transducers used in the SAM generate coher-ent acoustic waves when excited by an RF signal, in thereceive mode the acoustic waves are converted back intocoherent RF signals. At each pixel point, it is thereforepossible to measure two parameters: the amplitude andphase of the received signal. As we shall see in Section 4,this additional information is of considerable help indeducing the elastic properties of the specimen.

Initial experiments [12] used an electrically derived ref-erence signal, suitably delayed by an acoustic delay line ina phase bridge configuration to determine the phase of thesignal (Fig. 3). The acoustic delay line enables one toequalise the delays in the signal and reference arms,thereby relaxing the need for a high-stability oscillator.Such a system can be used to make absolute phase mea-surements [13]. Recent work has shown that the sensitivitycan be improved further by resorting to differential phasesystems [14, 15], where phase gradients in the region of thefocus are recorded; spurious phase shifts such as thosecaused by temperature fluctuations in the coupling fluid ormechanical instabilities in the scanner appear equally onthe signal and reference arms, and therefore do not con-tribute to the final signal.

Fig. 4 shows one embodiment of a differential phasesystem [15]. Here the transducer is driven at two differentfrequencies which are locked to one another; in this casethe fundamental is at 3.57 MHz and its third harmonic isat 10.71 MHz. The diffraction-limited performance of thelens ensures that the third harmonic is focused to a spotsize which is one-third that of the fundamental. On recep-tion, the fundamental is tripled in frequency and a phase

283

comparison is made. The output is proportional to thephase gradient of the object in the focal region being illu-

Both dark-field and stereo microscopy [16] have beendemonstrated with the acoustic microscope. Dark-field

microscope

1MHz filter30kHzbandwidth

zero-voltscomparator

bistable

30 kHzlowpassfilter

1MHz filter30 kHzbandwidth

zero-voltscomparator

Iphase output

Fig. 3 Experimental system for recording phase-only acoustic micro-graphs using electronic reference

minated. Fig. 5 shows a surface of a coin imaged using thissystem. An image of the same area taken using a phasemicroscope employing an electronic reference is shown inFig. 6. The broad fringes running across Fig. 6 are due to awarping of the coin surface, while the fine zig-zag detail isdue to a scanning defect which caused the object to movetowards the lens every other scan line. As we can see, thesedetails are not present in the differential phase image ofFig. 5.

Fig. 5 Acoustic differential phase contrast image of a coin using thesystem illustrated in Fig. 4

artzthird-harmonic

X

ty

digital framestore and scancontrol

TV

Fig. 4 Complete electronics for differential phase contrast acousticmicroscopy at 3.57 MHz and 10.71 MHz

Fig. 6 Acoustic phase contrast image of the coin as in Fig. 5 taken at10 MHz using an electronic reference

284 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 4, JUNE 1984

imaging is achieved by tilting the receive lens shown inFig. 1 while ensuring that the system remains confocal.With no object present, the tilt angle is increased until thereceived signal diminishes to a negligible value. With thescanned object in place, diffracting features then show upas bright areas on a dark background. Stereo imaging hasbeen achieved by taking two images where the scan planeis tilted by ± 0 ($ ~ 7°) relative to the normal scan posi-tion shown in Fig. 1. Dark-field microscopy has also beendemonstrated by replacing the transmitting lens by aplane-wave transducer and using a receive lens with azero-order stop at its aperture centre [17].

Another form of imaging available to us is nonlinearacoustic microscopy. For low levels of acoustic powerdensity, Hooke's law is obeyed, i.e. stress is proportional tostrain. However, for large acoustic power densities, thissimple relationship between stress and strain no longerholds; in this situation an incident acoustic beam at fre-quency co can generate harmonics at 2co, 3a> etc. within theobject. Harmonic imaging has been demonstrated wherethe receiver is tuned to a harmonic of the incident fre-quency [18]. Sources of contrast in such images have beenstudied both experimentally and theoretically [19, 20].

Changes in a specimen that occur as a function of timecan be observed by difference microscopy [21]. In thistechnique the object scanner is computer controlled, andthe amplitude or phase of the object is recorded on com-puter during the first raster scan at time t = 0. A secondscan is made over precisely the same area at time t = tuand the difference, which shows the change that hasoccurred during the time interval tu is displayed on theVDU.

A new form of imaging in the acoustic microscope is thephotoacoustic mode [22]. In this method the transmittinglens shown in Fig. 1 is replaced by a microscope objective;the optical input is then the output from a pulsed laser(mode-locked and Q-factor-switched YAG laser in the firstdemonstration). Transient heating at the sample surfaceexcites acoustic waves which are detected by the receiveacoustic lens, as shown in Fig. 7. Contrast in photoacous-tic microscopy is due to the optical, thermal and acoustic

object

optical lens,

opticalinput = =

acousticlens

acousticoutput

Fig. 7 Configuration for photoacoustic microscopy

properties of the specimen. Variants of this theme such aselectron-acoustic microscopy [23, 24] have generatedmuch interest in recent times.

4 Quantitative methods in acoustic microscopy

Acoustic micrographs record the variations in elasticproperties across a sample surface. For some applicationsit is sufficient simply to study these acoustic variations in aqualitative fashion and interpret the contrast. In others(such as in materials evaluation) it might be necessary toperform a more detailed, quantitative study at a chosenpixel point within the image plane. Several techniques have

been proposed for performing such an analysis in the SAMoperating in a nonscanning mode. Perhaps the mostfamous of these techniques is the so-called V(z) method[25, 26]. If one stops the raster scan and plots the detectedacoustic signal V as a function of defocus distance z (asmeasured from the geometrical focus), we find that eachmaterial surface produces a characteristic response. In par-ticular, nulls in the detected signal are observed spacedregularly along the defocus direction. This effect has beentheoretically analysed by a number of authors [27, 28, 29].For the case of smooth solid surfaces the ray mode [29]due to Parmon and Bertoni provides the clearest physicalpicture and accurately predicts the null spacing in V(z).The basic model can be understood with reference to Fig.8. Ray (i) is incident at normal incidence, while ray (ii) isincident at the Rayleigh critical angle. Ray (ii) excites aleaky Rayleigh wave at the liquid/solid interface, which re-excites a bulk wave within the liquid, as shown in Fig. 8.

liquid solid

Fig. 8 Ray diagram interpretation of the V(z) response

Only the ray which phase matches into the lens will con-tribute to the detected voltage. For the same reason, otherrays incident on the lens surface from the transducer donot significantly contribute to a received signal as theobject is moved away from the geometrical focus. A nullresponse in the detected signal will therefore be obtainedwhenever the phase difference between rays (i) and (ii) is anodd multiple of n. Application of this simple principleleads to an expression for the null spacing Az in the V(z)response:

Az =XR (\ + cos

sin 6R(1)

where XR is the Rayleigh wavelength and 6R is the Rayleighcritical angle. This relationship has been used to measureRayleigh wave velocities on isotropic [30] and anisotropic[31] substrates, with an accuracy approaching 0.1%.

Quate [32] has pointed out that by choosing a couplingliquid such as gallium, one would be operating in a regimewhere the shear velocity in most solids is less than thelongitudinal velocity in gallium. In this situation Rayleighwaves cannot be excited, and the null spacing in V(z) canbe related to the longitudinal velocity in the solid beingexamined. Furthermore, one can determine the acousticimpedance by measuring the maximum value of V(z).These two parameters will then give both the density andlongitudinal velocity of the material being studied.

The Parmon-Bertoni model provides an accuratedescription of the physical situation in the case of smoothsolid surfaces. In the case of multilayered surfaces,however, the situation becomes much more complex, andit is necessary to resort to a wave approach in order tocalculate V(z) [27, 28]. By decomposing the lens focal dis-

IEE PROCEEDINGS, Vol. 131, Pt. A, No. 4, JUNE 1984 285

tribution into an angular spectrum of plane waves, it ispossible to arrive at an integral 'expression for V(z) interms of the 'acoustic reflectivity' of the sample beingimaged:

V(z)=

x expO'2/cz.yi - p2X2)2np dp (2)

U(p) is the acoustic reflectivity of the sample correspondingto a plane wave having a spatial frequency p, P is the lenspupil function, R is the lens radius and k = 2n/A is theacoustic propagation constant in the liquid. U representsthe lens input distribution and permits us to simulate theeffects of nonuniform lens illumination. Eqn. 2 applies toan isotropically layered substrate; its extension to aniso-tropic situations is straightforward and will not be con-sidered here.

The reflectivity curve U(p) contains information aboutthe critical angles of the object; thus, if this curve can bemeasured or deduced in some way, its features can be usedto determine not only the shear, Rayleigh and longitudinalvelocities, but also the Rayleigh wave attenuation in thesample. Several techniques [33, 34, 35] for measuring R(p)have been proposed and demonstrated. The measurementaccuracies reached for the velocities in these experimentsare of the order of a few per cent. Another approach mightbe to adopt a 'forward optimisation' technique to deter-mine U(p) from a measured V(z) curve. The method wouldinvolve using eqn. 2 and varying U(p) until the differencebetween the calculated and measured V(z) curves is lessthan some prescribed value. Such a technique would,however, require accurate knowledge of the lens pupilfunction and input distribution, or a separate measurementwith a known reflector which can then be used to normal-ise these quantities. Initial experiments along these lines,primarily aimed at determining Rayleigh wave attenuationin steel, have been reported in the literature [36]. Yetanother proposition for determining U(p) has been putforward which involves inverting the V(z) eqn. 2. For alens with axial symmetry, a flat isotropic specimen andsmall z, the V(z) equation can be cast into the form of aFourier transform of the reflectivity function U(p). U(p) canthen be recovered by inverse transforming V(z) [37].However, to date, no experimental results have beenpublished.

In the case of thin transmissive samples such as bio-logical tissue, Bennet [38] has pointed out that one coulddetermine the longitudinal velocity and impedance bysimply measuring the amplitude and phase of the acousticsignal in transmission provided we know the thickness ofthe sample and have approximate estimates for the velo-city and impedance.

In addition to the forward optimisation scheme men-tioned earlier for measuring Rayleigh wave attenuation,two further schemes have been demonstrated. The first[39] relies on the fact that the V(z) response is a manifesta-tion of the interference of two sets of waves as shown inFig. 8; the direct contribution (i) and the Rayleigh wavecontribution (ii). Any change in the Rayleigh wave attenu-ation would result in a corresponding change in the 'depthof modulation' of the V(z) fringes. This 'depth of modula-tion' can be related to the Rayleigh wave attenuation coef-ficient. In the second approach [40] an annular transduceris used, so that the only contribution to the V(z) measure-ment is due to Rayleigh waves; the direct contribution issuppressed. By measuring the slope of the resulting V(z)

curve, the Rayleigh wave attenuation coefficient can bededuced.

Finally, our work on Rayleigh wave attenuation [40]coupled with the work of Farnell et al. [41] on a novelacoustic lens using Rayleigh compressional mode conver-sion has led us to suggest a new form of microscopy basedon focused Rayleigh waves [42]; Fig. 9 illustrates the basicprinciple. It relies on the fact that when the acoustic micro-scope is operating in a defocused condition (as shown in

input V(z)

semicircularbisec tedtransducer

focused SAW beamon specimen

couplingliquid

conical beamof longitudinalwaves incidentat SAW criticalangle

Fig. 9 Geometry for reflective confocal SAW microscopyThe bisected lens excites a circular source of SAW, and these focus on the specimen.Back-scattered SAW form the scanned image

Fig. 8), the detected transducer voltage is due to the super-position of the direct contribution (i) and the Rayleighcontribution (ii). By using a semicircular transducer asshown in Fig. 9 in conjunction with very short pulses, it ispossible to gate out just the Rayleigh wave contribution.These waves, which are caused by a conical beam of longi-tudinal waves incident on the object at the Rayleigh wavecritical angle, have semicircular wavefronts on the objectsurface and are therefore focused to a diffraction limitedspot; a reflection image is thus capable of showing up veryfine surface features on the sample. Furthermore, the direc-tionality of the Rayleigh beam can be used to measure theorientation of features such as cracks on the samplesurface.

5 Application to biology and the near-surfaceimaging of solids

Early experiments with the acoustic microscope werebased on a transmission system and concentrated largelyon biological specimens [43]. The very high intrinsicacoustic contrast in biological specimens makes the SAMa very attractive tool in biology. Fig. 10 shows an acousticimage of red blood cells from human bone marrow takenat 1300 MHz using a transmission instrument, and illus-trates the high contrast; the acoustic wavelength used hereis close to one micrometre.

As mentioned in Section 2, if one seeks to improve theresolution below one micrometre, reflection instrumentsare generally favoured. It is also possible to image bio-logical specimens in the reflection SAM; one simply

286 1EE PROCEEDINGS, Vol. 131, Pt. A, No. 4, JUNE 1984

attaches the specimen on to a strong acoustic reflector asshown in Fig. 11; the acoustic beam then traverses twicethrough the object to be imaged.

Fig. 10 Transmission image of red blood cells at 1300 MHz fromhuman bone marrow

-w-input

° outputto amplifierand display

transducer

lens

7 7reflectingobject

Fig. 11 Geometry for recording images of biological cells in the reflec-tion acoustic microscope

The very high intrinsic acoustic contrast in biologicalspecimens without the need for staining makes it possibleto study living cells by replacing the water couplingmedium in the SAM with a suitable cell culture medium.Fig. 12 shows a reflection acoustic micrograph of a livingchick heart fibroblast [44] on a quartz substrate taken at1700 MHz (k acoustic = 0.9 nm) by J. Hilderbrand and D.Rugar; the temperature of the cell culture medium wasmaintained at 37°C for these experiments. The outerregion of the cell appears bright, while the thick interiorappears dark. Two nuclei are visible in the dark region,while both bright and dark particles are observed in a ringaround the nuclei. Dark streaks are observed along the cellmargin, which may represent sites of cell-substrate attach-ment.

[D. Rugar and J. Hilderbrand]

Fig. 12 Acoustic micrograph of living chick heart fibroblast on a quartzsubstrate taken at 1700 MHz

Perhaps the first commercial application of acousticmicroscopy can be considered to be the 'ultrasonic opthal-moscope', invented by Luukkala and Merilainen [45].Although this is a relatively low-resolution instrument (theacoustic lens operates at 6 MHz and the lateral resolutionis around 0-5 mm) in the B-scan mode, it shows a cross-section of the human eye and is currently used by opthal-mologists to image detached retinas. The lens is focusedthrough the eyelid to a depth of 24 mm below the surfaceand mechanically sector scanned in order to record animage. Fig. 13 shows a B-scan image of a human eye in

[M. Luukkala]

Fig. 13 B-scan image of human eye at 6 MHz showing detached retina

vivo; the detachment of the retina R from the back surfaceof the eye containing the optic nerve N is clearly visible.Some weak echoes from the vitreous humor V are alsovisible and are associated with aging of the collagen-fibrenetwork. For the purpose of comparison, Fig. 14 shows animage from a healthy human eye where the retinal detach-ment is clearly absent.

Reflection acoustic microscopy has found many applica-tions in the study of material surfaces. Grain structure ofpolished metal samples, surface cracks, mechanical proper-ties of coal, defects in integrated circuits, polymers etc.have all been studied [46, 47, 37]. Here, we shall onlypresent one example; a reflection image taken by Hollisand Hammer [48] of a silicon integrated circuit at800 MHz in water. The device consisted of a silicon sub-strate, on top of which was grown a 0.1 /mi thermal oxidelayer followed by a 2.3 /mi Al-Cu circuit pattern; the wholedevice was then protected by a 3.8 /mi overlay of passivat-ing oxide, except in the regions where a solder connectionhad to be made to an Al-Cu pad, in which case a contact

[M. Luukkala]

Fig. 14 B-scan image of healthy eye at 6 MHz

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hole was etched through the oxide passivation to reachthis pad. In this experiment, a device where the solder ballconnections had already been made was taken and thesolder connections etched, leaving behind an opticallyopaque intermetallic layer on the surface where the solderballs resided. Fig. 15 shows an acoustic image of the

[«. Hollis]Fig. 15 Acoustic reflection image of a silicon test chip at 800 MHzshowing structure invisible to optical radiation

resulting structure. The small circular holes within eachAl-Cu pad are the contact holes, whilst the larger ellipticalregions surrounding them are the intermetallic layerswhich show the areas where the solder balls were sitting.The edge of the Al-Cu pad is clearly seen through the opti-cally opaque intermetallic, demonstrating the ability ofacoustic waves to penetrate through such structures.

6 Interior imaging

The basic problem that one faces when attempting toimage the interior of solids can be understood by referringto the ray diagram shown in Fig. 16. Due to the large velo-city ratio between solids and liquids, converging sphericalwaves are seriously distorted at the solid/liquid interface;paraxial rays come to a sharp focus at/, while rays inci-dent at large angles come to a focus closer to the surface.Due to this effect, lenses that provide maximum refractionangles greater than 30° within the solid suffer from severespherical aberration, and the resulting resolution is not dif-fraction limited.

Several approaches have been suggested for overcomingthis problem, the simplest of these being to reduce the lensnumerical aperture to the point where the maximum re-fraction angle within the solid is restricted to about 30°[49, 50]. Calculations show that, in this case, the focal spotis diffraction limited and its diameter is approximately twoacoustic wavelengths within the solid. Another possibilityis to use a suitably shaped, wide-aperture aspheric lens[50]; such a technique, however, will only work at a fixeddepth beneath the surface of the solid, a new aspheric lensbeing required in order to image at a different depth. Oneof the first solutions to the problem of interior imagingwas suggested by Jipson [51]. He chose the coupling liquidin such a way that the longitudinal velocity in the liquidclosely matches the shear velocity within the solid; thereduced velocity ratio minimises the spherical aberration,and the shear waves come to a diffraction limited focuswithin the solid. This technique only works well in thepulsed mode, as it is necessary to resort to short pulses inorder to detect the mode converted shear waves in the pre-sence of longitudinal waves within the solid. Jipson wasable to record acoustic images at 1 GHz through 75 fim offused quartz using gallium as the coupling liquid; the mea-sured resolution was 2.5 /mi. Jipson's basic idea has alsobeen demonstrated with mercury as the coupling medium[52, 53]. Finally, a mechanically scanned B-scan systemhas been proposed which utilises a second spherical surfacein order to reduce spherical aberration [54]. This secondconvex spherical surface is ground on to a couplingelement (preferably made of the same material as thesample), which is in acoustic contact with the sample to beimaged as shown in Fig. 17. Such a system has been suc-cessfully applied to image holes drilled in a cylindrical rod[55].

So far, we have only discussed one problem associatedwith interior imaging: spherical aberration. The secondproblem stems from the high impedance discontinuity atthe coupling fluid/object interface; this greatly reduces thefraction of acoustic energy transmitted into the solid and,consequently, the signal/noise ratio of the imaging signal.Furthermore, in most practical situations, the defect to beimaged is not very far below the surface of the solid, andtherefore the image pulse arrives very shortly after theinterface pulse. In order to discriminate against this inter-face pulse, one has to resort to broad-bandwidth trans-ducers capable of transmitting very short acoustic pulses;for a fixed peak power in the acoustic input pulse (which,in practice, is determined by system parameters such as

ou tput

couplingpiece C2

thin liquid layer

object C2

I iquid solid

Fig. 16 Ray diagram showing the aberrations introduced on to a spher-ical wave focused inside a solid sample

x , y scan

Fig. 17 Mechanically B-scanned acoustic microscope for interiorimaging

288 IEE PROCEEDINGS, Vol. 131, Pt. A, No. 4, JUNE 1984

transducer breakdown and saturation effects) this furtherreduces the available signal/noise ratio in the image. Atechnique for improving the signal/noise in such situationshas been described which utilises coded pulses for trans-mission followed by a matched filter for reception [56, 57].

The basic idea is illustrated in Fig. 18. An impulseapplied to an SAW expander excites a wide, broad-

putser SAW expander

1 1(1object

SAW compressor

to receive- arid-display electronics

Fig. 18 Pulse-compression imaging electronics used in the reflectionacoustic microscope

bandwidth chirp pulse, which, in turn, is applied to thetransducer. The received echoes are applied to a SAWcompressor, where they are compressed to very narrowpulses; noise, which is random in nature is, however, notcompressed. After time gating and amplification, the signalis directed to conventional display electronics to record animage. Due to the wide input pulses used on transmission,it is possible to supply much more energy per pulse usingsuch a system as compared with conventional systems; inan ideal situation, the improvement in signal/noise ratio(or 'processing gain') can be shown to be the product of thepulse duration time T and bandwidth B. To date, a pro-cessing gain of 12 dB has been demonstrated at 750 MHz[58]. Fig. 19 shows a subsurface image taken using a pulsecompression system at 60 MHz. The image is of a diffusionbond region between a copper plate 0-53 mm thick and aWC-Co composite [59]. Areas of poor bonding show upas bright patches in a dark background.

[M. Nikoonahad]Fig. 19 Pulse-compression image of a tungsten carbide/cobolt: copperdiffusion bond taken at 60 MHz through a 0.53 mm copper plate

IEE PROCEEDINGS, Vol. 131, Pt. A, No. 4, JUNE 1984

7 Towards higher resolution

By stretching the existing technology to its limits, thereflection SAM has been operated in hot water at3.75 GHz, with a corresponding wavelength of 400 nm.*The major obstacle to improving the resolution is the highvalue of absorption of sound in water: at 3.75 GHz and60°C it is 2690 dB/mm. For this reason, one has to resortto very small radius lenses; at 3.75 GHz, the lens radiusused is typically 18 /mi. Generally, for most liquids, theacoustic absorption increases as the square of the fre-quency. Thus, to obtain a wavelength below 400 nm, onemust find a fluid which has a lower velocity, a lowerabsorption coefficient, or preferably both. One possibilityis to use cryogenic liquids such as argon and helium [11].

Liquid argon has a velocity which is approximately halfthat of water and comparable attenuation, so that it iscapable of improving the resolution by more than a factorof two. Images have been taken at a wavelength of 400 nmin liquid argon at 2 GHz [60]. In liquid helium at1.25 GHz and T = 0-4 K, the measured acoustic attenu-ation is only 1.5 dB/mm [61]; below this temperature, theattenuation decreases as T4. Furthermore, the loss scaleslinearly with frequency, at least until the phonon energybecomes comparable to the thermal energy [62]. Recently,Foster and Rugar [63] have demonstrated an SAM oper-ating in liquid helium at 0.1 K and 4.2 GHz with a wave-length of 500 A. Fig. 20 shows an image of an integratedcircuit obtained using this instrument. The aluminium linesshown here are 2 /im wide, and the detail in this image isconsistent with a resolution below 500 A; this result rep-resents the highest resolution achieved so far in any SAM.

Fig. 20 Acoustic micrograph of a transistor on a silicon integratedcircuit taken at 4.2 GHz in super fluid helium (T = 0.1 K)Aluminium lines are 2 fim in width, and the resolution is estimated to be better than500 A

In order to achieve the highest resolutions, cryogenicliquids such as helium are clearly the best candidates.However, the use of cryogenic liquids involves severalinstrumental complexities. An alternative class of fluid isgases at high pressure. It is well known that the velocity ofsound in gases is five to ten times lower than in mostliquids, although the acoustic absorption is typically 100-1000 times higher. It has been shown that the acousticabsorption, at least in the case of monoatomic gases suchas argon and xenon, varies inversely with pressure. It istherefore, in principle, possible to exceed the resolutionlimit in water using gases at elevated pressures. An acous-tic microscope operating in argon at 40 MHz and 30 barshowing a resolution of 7 /*m has already been demon-

* QUATE, C.F., private communication.

289

strated [64]. Achieving higher resolutions with gases pro-vides the basis for current research.

To assess the quality of liquids for acoustic microscopy,we can define a coefficient of merit M by calculating theminimum wavelength that can be achieved for a fixed lossand transit time within the fluid and relating it to the cor-responding value for water [65]:

1/2 3/2

(3)

where C is the velocity of sound in the fluid, a is theattenuation coefficient normalised with respect to thesquare of the frequency/and the subscript w refers to cor-responding quantities in water. In Table 1 we present therelevant acoustic properties of some fluids and the corre-sponding coefficient of merit M.

Table 1 : Relevant acoustic properties of some fluids used inhigh-resolution acoustic microscopy

Fluid Temperature, Velocity, Absorption MK m/s a / f 2 x 1 0 1 5 ,

s2/m

H20H2Ocs2

GaHgAr (40 bars)Ar (250 bars)Xe (40 bars)ArN2

HeHeHe

298.0333.0298.0303.0296.8293.0293.0293.0

85.077.0

4.21.90.2

14951550131028701450

323323178840850183227238

22.010.210.1

1.65.8

412.083.0

953.015.213.8

226.370.2

0.8

1.01.41.81.42.02.35.13.72.93.07.39.5

82.5

Both gases and cryogenic liquids (such as helium) sufferfrom a common difficulty: they have a very low acousticimpedance. As a result, first, very little energy is coupledfrom the lens rod into the fluid, and secondly (for the samereason) very little energy is coupled into a typical object;most of the sound is reflected off the object surface. Thefirst problem can largely be overcome by applying aquarter-wave matching layer of carbon [11] on to the lenssurface, which results in a transmission coefficient of 0.85into liquid helium. Although the second problem can also,in principle, be overcome in the same way, this wouldinvolve modifying the object by depositing carbon on to itssurface and might not be acceptable in most nondestruc-tive testing situations. Thus, the contrast in cryogenic andgas medium microscopy can be expected to stem primarilyfrom topographic changes on the sample.

8 Conclusion

In this paper we have attempted to cover some of theadvances made in scanning acoustic microscopy over thepast decade. Unfortunately, due to reasons of spacerestriction, it has proved necessary to leave out many areasof interesting research. However, even from a restrictedreview such as this, it should be clear that acoustic micros-copy is here to stay. Its potential for imaging through opti-cally opaque material has been clearly demonstrated by anumber of workers. Techniques for obtaining quantitativeinformation from acoustic micrographs are graduallybeing established, and it will not be long before routinemaps of density and stiffness are available on a micro-scopic scale. In the field of biology, the high intrinsic con-

trast opens up a number of interesting possibilities; theability to study tissue in vivo has already been demon-strated. Living cell populations have been imaged in vitro;these images have delineated areas of cell-substrate attach-ment. Such experiments, together with the work on elasticconstant determination, can form the basis for furtherwork on cell motion.

Finally, progress has been made in high-resolutionacoustic microscopy. Operating in liquid helium at 0.1 Kand 4.2 GHz, images have been obtained showing aresolution better than 500 A. The future will see furtherimprovements in this area. In liquid helium, the limitationis not likely to be the maximum operating frequency thatcan be achieved; incoherent acoustic phonons in the300 GHz frequency range (A = 8 A) have been propagatedand detected over a distance of several millimeters [66].The ultimate resolution is more likely to depend on inge-nious methods for scanning and focusing, progress in newtechniques for transduction in the multigigahertz rangeand the development of precision acoustic lenses.

9 Acknowledgements

The work performed at University College London waspartially supported by the Wolfson Unit for Micro-NDEand partially by the British Technology Group. Theauthor is grateful to Prof. E.A. Ash for encouragement andadvice.

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