PAST, PRESENT AND FUTURE OF SURFACE ELASTIC WAVES

HAL Id: jpa-00215159

https://hal.archives-ouvertes.fr/jpa-00215159

Submitted on 1 Jan 1972

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

PAST, PRESENT AND FUTURE OF SURFACE ELASTIC WAVES

J. de Klerk

To cite this version:

J. de Klerk. PAST, PRESENT AND FUTURE OF SURFACE ELASTIC WAVES. Journal de

Physique Colloques, 1972, 33 (C6), pp.C6-182-C6-188. �10.1051/jphyscol:1972641�. �jpa-00215159�

JOURNAL DE PHYSIQUE

Colloque C6, suppl6ment au no 11-12, Tome 33, Novembre-Dkcrmbre 1972, page 182

PAST, PRESENT AND FUTURE OF SURFACE ELASTIC WAVES

J. DE KLERK

Westinghouse Research Laboratory, Pittsburgh, Pennsylvania 15235, USA

RbumB. ^- Le

developpement de I'utilisation des ondes Clastiques de surface pour le traitement des signaux sera retrace.

Dans le passe les Ctudes furent principalement centrks sur la mise en evidence des effets de premier ordre.

Actuellement les efforts principaux sont consacres

a

la maitrise des effets de second ordre qui determinent si un procCdC doit rester une curiosite de laboratoire ou s'il peut Ctre utilise dans la pratique.

Des prkdictions sur I'avenir des ondes de surface seront faites a la lumihre des tendances actuelles.

Abstract. -

The development and growth in the use of surface elastic waves for signal pro- cessing of all kinds will be traced. The past was mainly concerned with first order effects in showing feasibility. The present is largely concerned with overcoming second order effects which will determine whether a device remains a laboratory curiosity or becomes a practical device in systems.

Predictions for the future of surface waves will be made in the light of present trends.

I .

Introduction. -

Although surface elastic waves were first studied by Lord Raleigh [ l ] nearly 100 years ago, interest in their potential use in devices for signal processing has only developed during the past decade. Perhaps the most significant dcvelopment which initiated this interest was the technique of launching and detecting surface elastic waves by means of interdigital grids [2]. As this comparatively simple technique was available to many universities and research laboratories, the feasibility of many different devices was rapidly demonstrated. These devices included delay lines, various types of filters and amplifiers. These dcmonstrations showed great potential for realizing practical devices which could be used in radar and com~iiunications systems.

Many new acoustic counterparts to existing electro- magnetic microwave devices were suggested [3].

However, very few of these suggestions have reached the operating device stage for various practical reasons.

Simple theories [4]-[5] attempting to explain the elastic behavior of these devices in terms of electrical analogies, were only able to predict the behavior to a first order approximation. These theories proved inadequate for designing devices capable of meeting the stringent characteristics required for operational radar, communications and ECM systems.

The greater part of the research being performed on surface waves today is being devoted to overcoming the second order effects not predicted by the early simple theories, but which second order effects are obvious when analyzed from elasticity theory [6], [9]

and which are observed experimentally. More accurate theories are being developed [7], [24] to account

for this behavior, with the hope that techniques will be developed to reduce to acceptable levels the second order effects which at present are mainly responsible for the failure of most surface wave devices to reach operational performances.

Attempts are also being made to improve the ancuracy and reliability of mass production tech- niques. Any inaccuracies or unreliability in techniques might be regarded as third order effects which could prevent workable laboratory devices from being mass produced to tolerances within their required characteristics.

2. Past and

present. -

One could regard the introduction of the interdigital surface wave trans- ducer, first reported by White and Voltmer [2], as the most significant technical advance towards realizing practical surface elastic wave devices for signal processing applications. This type of transducer is shown in figure 1. The reasons for the success of this structure will be appreciated when the physical nature of Rayleigh surface waves is understood.

The retrograde elliptical particle motion of a Rayleigh wave is due to two components of displacement always present, and 900 out of phase with one another, viz a shear displacement normal to the surface and a compressional displacement parallel to the surface.

Figure 2 shows the phase relationship between the two displacements. The two wave equations for this type of surface wave are

and

u,

=

U3 sin k(x, - v,

t) Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972641

PAST. PRESENT AND FUTURE OF SURFACE ELASTIC WAVES C6-183

FIG. 1 . - 1. D. grid on surface of piezoelectric material for launching a surface wave.

i'

_U3

₌

^niaximum _shear

d

is~lacernent

. -

u3

sin (kxl-vRt) XI

I

u, = U, cor (bx, v,t)

maximum

U1

=

conipresslonal

dispkernent FIG. 2.

-

Displacement components of a Rayleigh wave.

where U , and U , are respectively the maximum compressional and shear displacements, k is the propagation vector, v, is the Rayleigh wave velocity,

t

represents time and x, is the distance in direction of propagation.

Any transducer system which is capable of gene- rating the appropriate compressional and shear dis- placements with the phase relationship given in eq. (1) and (2) will readily launch Rayleigh waves.

As an example of the use of the interdigital trans- ducer on a piezoelectric material [9], such as quartz, let us consider the electric fields and elastic strains generated by such an interdigital grid, shown in figure 3. When this grid, shown in figure 3a is connect- ed to an R F generator, the electric field components shown in figure 36 and 3c are generated. Using the equations for the inverse piezoelectric effect, i. e.

FIG. 3. - Rayleigh wave generation on quartz using an inter- digital grid.

where

^E~ =

elastic strain,

dij =

piezoelectric modulus and Ei

⁼

electric field, the elastic strains, generated by the electric fields shown in figure 36, can be comput- ed as follows

:

combine to form the Rayleigh waves.

I n addition, several bulk waves and a Love type wave are also generated. Although these unwanted bulk modes are not part of the Rayleigh wave, any theory which attempts to predict the behavior of a surface wave device, must include them. These unwanted bulk modes are described by eq. (5) through (7)

:

and

E5 = d 2 5 E 2

c2

represents a bulk compressional wave which pro- pagates along

x2

and is independent of frequency.

c4

represents a bulk shear wave which propagates at 450 to

x2

and x,, and is independent of frequency.

e5

represents a bulk shear wave propagating along xi, with particle displacement along x,, at a frequency ft, determined by the velocity of this bulk shear mode, us,, viz

where 2 is the distance between adjacent -fingers of the same electrical polarity in figure 36.

13

C6-184 J. DE KLERK

That Rayleigh waves generated by structures simi- lar to that shown in figure 1 propagate equally in both directions normal to the comb fingers, led to designs for modified interdigital grids to recover the energy being propagated away from the desired direction. One example [Ill is shown in figure 4.

, Reflecting Driving I l i n t e r d g t a l ~ y l interdigital

I 1

trans ucer~

21

transducer

I

-

Forward direction

~ l e c t r i c a l ~ termination

9

^-Matching _network

source

-

D i r e c t i o n a l t r a n s d u c e r

FIG. 4. - Directional transduder.

This design reflects approximately 80 % of the inci- dent surface wave energy [9], the remainder being lost due to bulk mode conversation, reradiated sur- face waves in the backward direction and resistive losses. In this design an identical reflector grid is placed behind the launcher grid and spaced a quarter wavelength away from the latter, so that reflected surface wave energy will be in phase with the for- ward launched energy. The reflector is terminated with an inductance suitable for resonating with the I. D. reflector grid capacity at center frequency.

This type of unidirectional transducer is more sui- table for devices operating under cw conditions than under pulsed conditions. The reflector will add as many cycles to the pulse as there are finger pairs employed in the reflector. It should be noted that the 80 % reflectivity only occurs at the resonant frequency, at other frequencies, however, the reflec- tivity is reduced and falls away rapidly with increasing and decreasing frequency. The rate of loss in reflec- tivity is directly proportional to the bandwith or inversely proportional to the number of finger pairs employed. If this type of reflector is employed in a band pass filter in an attempt to reduce the overall electrical loss, and reduce undesired multiple transit signals both amplitude and phase characteristics as well as bandwidth are seriously degraded. More recently reported unidirectional transducers [8] use either two phase or three phase instead of a single phase [2] elec- trode and drive voltage systems. Although both repor-

ted unidirectional transducers have marked forward directivity, the extra insertion loss incurred far exceeds the 3 dB loss of the standard bidirectional transducer.

Let us now examine some of the devices reported in the literature. Delay lines were reported using materials such as LiNbO, 1121, CdS [13], SiO, [2], [17], Bi,,GeO,, [14] and PZT [15]. Of these materials LiNbO, suffered the lowest insertion loss and was thus soon regarded as the most desirable material for surface wave devices. This high electromechanical material, however, proved to be undesirable in ano- ther way, viz it exhibited a large reflection coefficient, resulting in an undesired triple transit echo [16]

only about 12 dB below the desired delayed signal as shown in figure 5. When the same material was

. . . _....

_d

. '.

^{T T S}

.J

. .... . ... .. .. ... ...- ..

^(After

Srnlth e t d

J

F R E Q U E N C Y - M H z

FIG. 5.

-

LiNbO3 delay line a) direct and b) triple transit measured signals after Smith et al. [ l l ] using 5 period trans-

ducers.

employed with directional transducers to suppress the high triple transit signal, the bandwidth was severely narrowed as shown in figure 6 .

0)

( 1 5 + 1 5 F l N G E R P A I R S )

_I

z 4 0 -

0 - B l D l R E C T l O N A 30 - T R A N S D U C E R a ( 5 F I N G E R

w P A I R S )

*

2 0 -

-

Z

S m i t h e t . al, l o -

R e f . 1 1 ) 0 _{6 0} I ₈₀ I I ₁₀₀ I I _I20 I I ₁₄ I ₀

I

F R E Q U E N C Y - M H z

FIG, 6 . - Passbands of LiNbOs delay lines using unidirectional (15 f 15 period) and bidirectional transducers (5 finger pairs).

Low coupling materials, such as SiO,, did not

suffer this disadvantage, but exhibited high insertion

losses [17]. However, a zero temperature [I81 cut

PAST, PRESENT AND FUTURE OF SURFACE ELASTIC WAVES C6-185

of quartz, viz ST-cut, has been reported. This cut of quartz has so far proved to be the, most suitable material for many elastic surface wave devices for signal processing applications.

Various filters have also been reported. Within a few months of each other three pulse compression filters [19], [20], [21] wave reported. These three devices readily proved the feasibility of using sur- face waves for pulse compression. Since Rayleigh waves on a free surface are intrinsically dispersionless, i. e., all frequencies propagate at the same velocity, dispersive surface wave filters must be constructed in such a way that the propagation distance varies with frequency as indicated in figure 7. The left I. D. grid

FIG. 7.

-

Dispersive filter.

structure is a mirror image of the right half. If all frequencies are launched simultaneously by applying an electrical impulse to the left, a linear frequency modulated expanded pulse will be detected by the right I. D. structure. The length of this pulse will be equal to the difference in propagation times for the low and the high frequencies. When the resultant FM pulse is applied to one of another identical pair of dispersive combs which have been reversed in frequency, all frequencies will arrive at their respec- tive sections of the second comb at the same time, resulting in a compressed pulse, the amplitude of which will be the sum of the detected signals from all the elements of the comb. An important charac- teristic of such dispersive filters is the change of phase with frequency. An ideal filter will have a linear phase characteristic. Early devices were constructed with fingers of uniform length and the total number of fingers required to provide the desired bandwidth.

One such device [I91 had very undesirable phase or group delay characteristics. Departures from linea- rity approached 10 % of the total group delay at each end of the spectrum and about 5 % at center frequency. Such large deviations cannot be tolerated in a practical device. By changing the finger length alone as a function of frequency in a sinusoidal manner such that the minimum finger lengths occurr- ed at the ends of the dispersive grids, deviations from linearity of the group delay were reduced to

less than 2 %. Considerable further improvement could be achieved by allowing for the <<back piezo- electric effect

^)>

[9]. This effect is due to the fact that both direct and inverse piezoelectric effects operate simultaneously, i. e., when an electric field is applied to an interdigital grid, the resultant elastic wave according to eq. (3) modifies the applied field by generating its own electric field of opposite electrical polarity. This generated field can be accurately calcu- lated from the equation for the direct piezoelectric effect,

where Pi

=

electric polarization and oj

=

elastic stress. This equation indicates that large back fields will be generated by strong coupling material, i. e., those which have large values of piezoelectric moduli.

This strong influence of a high electromechanical coupling coefficient on the characteristics of surface wave devices was readily demonstrated in another type of filter, viz., digital phase coded filters. The first of this type reported [22] were a 7 bit and a 13 bit Barker code filter [23], using Y-cut Z-propa- gating LiNbO,. This type of filter employs a binary code of a particular bit phase sequence. Barker codes can achieve a theoretical maximum possible corre- lation peak to side lobe ratio for the number of bits in the sequence. For example, a 13 bits Barker code employs a sequence of phased bits which have a main correlation peak to side lobe ratio of 13 or 22.3 dB and a compression ratio of 13

:

1 for corre- lation peak width to the total 13 bits sequence width.

In a practical device, the coupling between acoustic waves and electrical taps on the filter must be low to avoid deterioration of the acoustic signal as it propagates under the phase coded I. D. grid. If the coupling is strong, not only is energy extracted from the acoustic signal but reradiated antiphase acoustic energy, due to the back piezoelectric effect [9], modu- lates and hence degrades the propagating acoustic phase coded signal. The magnitude of this effect for a 13 bit Barker code filter using LiNbO, has been calculated [24] for LiNbO, as shown in figure 8.

This theoretical filter employed 6 cycles/bit in the electrically generated code, and in the main filter comb. Only 3 finger pairs were employed in the output I. D. grid.

The experimental correlated signal obtained for LiNbO, using 6 cycles/bit is shown in figure 9. The correlation peak to side lobe ratio for this filter was only about 15.3 dB. The loss of 7 dB below theoretical value of 22.3 dB is mainly due to the large piezoelec- tric coupling of this material. The peak to side lobe ratio was increased to approximately 18.5 dB when only 1 cycle per bit was used in the filter and 6 fin- ger pairs in the output grid, as shown in figure 10.

By contrast, it was found possible to obtain a near perfect correlation signal using ST-cut quartz [25]

for a 13 bit, 14 cycleslbit Barker code filter using

J. DE KLERK

FIG. 8. - Theoretical distortion of 13 bit Barker code filter using LiNb0,a due to back piezoelectric effect (after W. J. Jones et al. [24]).

FIG. 9.

-

13 bits-6 cyc!es/bit Barker code filter output for YZ LiNbO3 after W. S. Jones et al. [24], figure 14.

FIG. 10.

-

Experimental compressed pulse using 1 sample per bit in the main filter and 6 finger pairs in the output grid for YZ

lithium niobate (after W. S. Jones et al. 1241, Fig. 17).

7 finger pairs/bit in the filter I. D. grid and 7 finger pairs for the output grid. The correlation signal of this device, which has a peak to side lobe ratio > 21 dB, now being manufactured for U. S. Air Force Radar Type FPS 27, is shown in figure 11. No thermal compensation was required for operation between

-

60

^{O C}

and + 100

^{O C}

for this ST-cut quartz device.

Over this temperature range the maximum frequency deviation from the center frequency of 29.953 MHz was + 3.5 kHz. If this device had been fabricated on LiNbO,, the maximum frequency deviation [22]

over the same temperature would have been

$.

225 kHz.

Thus, it would have been necessary to control the temperature of the device to within

$.

2.5

^{O C .}

FIG. 11. - Correlation signal of 13 bits-14 cycles/bit Barker code filter on ST-cut quartz at 30 MHz (after R. L. Thomas et

d., ref. 1251).

Discretely variable delay lines with programmable taps, the polarity of which can be electrically switch- ed, have been reported [26]. These delay lines have been demonstrated to be useful as binary phase coded sequence generators and correlators. These devices lead the way in sophistication as they have attempted to integrate electronic and acoustic functions. Many other devices, such as band pass filters, amplifiers, convolvers, directional couplers, waveguides and mixers should be included in this survey but must be omitted due to lack of time and space.

3. Future. -

The switchable acoustic surface wave sequence generator described in reference [26] points the way to future refinements in surface wave devices.

This particular device, which employed hybrid circuit techniques, housed the complete switching circuitry on a separate substrate from the acoustic device.

Further refinements and miniaturization could be

achieved by employing the same substrate material

for both acoustic and electronic operations. The most

suitable material for this purpose would be silicon

with acoustic propagation along the (100) and/or (1

10)

directions. Acoustic surface wave energy would be

launched along the surface of the silicon substrate

by means of an interdigital ZnO transducer fabricated

on the silicon surface. The acoustic energy would

be sensed at any desired point along its path by means

of an integrated MOS transistor [27]. Surface wave

reflectors in the form of narrow vertical slots seiec-

tively etched into the substrate would be placed near

the edge of the silicon substrate at an angle of 450

to the direction of acoustic propagation, which would

PAST, PRESENT AND FUTURE

OF

SURFACE

ELASTIC

WAVES

C6-187

be say a (110) direction. The surface wave energy would then be reflected along a (100) type direction.

As these directions are pure mode directions for both Rayleigh wave displacement components, the surface wave energy would always propagate as a pure mode.

By this means it would be possible to propagate the surface wave along parallel folded paths -as suggested in figure 12. It would also be possible to amplify

FIG.

^13.

- MuItimegabit memory.

For any surface wave device to te incorporated in a radar, electronic countermeasures or communi- cations system it must be capable of mass production.

Present integrated circuit (IC) technology makes use of photolithography and photoresist techniques for mass production of IC devices. The same technology which is used for surface wave devices will limit their operating frequency to that dictated by the FIG.

12. -

Proposed silicon surface wave delay line

with

multiple folded paths.

the surface wave energy where needed to compensate for propagation losses by the method suggested in figure 13. Here some of the surface wave energy would be detected by a ZnO thin film interdigital transducer, electronically amplified and relaunched as surface wave energy from another ZnO interdigital transducer. The undetected surface wave energy would be absorbed to prevent unwanted signals.

maximum reliaby rhroduciable resolution and edge definition. For materials like quartz and lithium nio- bate future mass produced surface wave devices will be restricted to frequencies below 500 MHz. The corresponding upper frequency limit for devices fabri- cated on Bi,,GeO,, will be approximately 250 MHz.

Although devices have been made for operation above these frequencies, scanning electron microscope tech- niques were necessary. As no mass production methods for this technique have yet been developed, the prospect for gigahertz surface wave device mass production does not look very promising at this time.

References

[I] LORD RAYLEIGH, ((On Waves Propagated Along the

171

SMITH W. R., GERARD

H.

M. and JONES

W.

R.

Plane Surface of an Elastic Solid

)),

Proc. London

<<

Analysis and Design of Dispersive InterdigitaI Math. Soc.. 17 (1885). 4. Surface-Wave Transducers

^D, ZEEE Trans.,

MTT- [2] WHITE

R.

M. and

VOLTMER

F. M.,

(<

Direct Piezo-

²⁰

(1972) 458.

electric Coupling to Surface Elastic Waves

^D,

[81 WORLEY

J.

C. and MATTHEWS

^{H.9 ((}

Broadband

Appl. Phys. Lett.

7 (1965) 314. Unidirectional Surface Wave Transducer >>,

ZEEE

131 STERN E.,

^<(

Microsound Components, Circuits and

^Trans.

SU-18 (1971) 52.

Applications

>>,

Presented as a talk at the IEEE [9] DE KLERK J.,

<<

Materials for Elastic Surface Wave Ultrasonics Symposium, New York (1968)

;

also Applications

^{D, <<}

Invited

^>)

Proceedings 1970 published in

Ultrasonics

7 (1969) 227 and an Ultrasonics Symposium, IEEE Cat. No. 70

C

69 SU (1971) 94.

invited paper in

ZEEE Trans.,

MTT-17 (19691,835.

_{HnRTMANN C. S., JONES} W. S. and VoLLERs H.,

[4] SMITH W. R., GBRARD H. M., COLLINS

J.

H., REEDER

_<<

Wide Band Unidirectional Interdigital Surface T. M. and SHAW H. T.,

<<

Analysis of Interdigital Wave Transducers

^D, ZEEE Trans.

SU-19 (1972) Surface Wave Transducers by Use of Equivalent 378.

Circuit IEEE Trans. MTT-17

[ll] SMITH W. R., GERARD H. M., COLLINS J. H., REEDER

856. T. M. and SHAW H.

J., (<

Design of Surface Wave

[5] TANCREU H. R. and HOLLAND M. G., <<Acoustic Delay Lines with Interdigital Transducers),, Surface Wave Filters

>), ^<(

Invited

ⁿ

Proc. 1970

ZEEE Trans.

MTT-17 (1969) 865.

Ultrasonics Symposium, IEEE Cat. No. 70 C [12] COLLINS J. H., GERARD H. M. and SHAW H.

^J.,

69 SU (1971) 48.

<r

High Performance Lithium Niobate Acoustic [6] MUSGRAVE M. J. P.,

<<

Crystal Acoustics

>),

Holden-by Surface Wave Transducers and Delay Lines

>>,

(San Francisco 1970).

^Appl. Phys. Lett.

13 (1968) 312.

C6-188 J. DE KLERK

[13] WHITE R. M. and VOLTMER F. W.,

cr

Ultrasonic

Surface Wave Amplification in Cadmium Sul- fide

h, Appl. Phys. Lett. 8

(1966) 40.

[14] KRAUT E. A., TITTMANN B. R., GRAHAM L.

J.

and LIM T. C., (<Acoustic Surface Waves on Metal- lized and Unmetallized Bi12Ge020

>), Appl. Phys.

Lett. 17

(1970) 271.

[15] TODA K., KAWABATA A. and TANAKA T.,

^(<

Surface Wave Delay Lines with Interdigital Transducers on Unpolarized PZT Ceramic Plates

>>, Jap. J . Appl. Phys. 10

(1971) 671.

[16] See figures 5 and 6 of Reference [ll].

1171 COLLINS

J.

H., GERARD H. M., LAIKIN K. M., SHAW H.

J., cr

100 MHz Quartz Delay Line Utilizing Rayleigh Waves

B, Proc. ZEEE 56

(1968) 1635.

11 81 SCHULZ M. B., MATSINGER B.

J.

and HOLLAND M. G.,

((

Temperature Dependence of Surface Acoustic Wave Velocity on Quartz

>>, J. Appl. Phys. 41

(1970) 2755.

El91 TANCRELL R. H., SCHULZ M. B., BARRETT H. H., DAVIS L. jr. and HOLLAND M. G., <<Dispersive Delay Lines Using Ultrasonic Surface Waves

D, Proc. ZEEE 57

(1969) 1211.

1201 HARTMANN P. and DIEULESAINT E.,

^<(

Intrinsic Com- pensation of Side Lobes in a Dispersive Acoustic Delay Line

^D, Electr. Lett. 5 (1969) 219.

[21] TEODORI E.,

<<

Surface-Wave Matched Delay Lines for Pulse Compression >>,

Electr. Lett. 5

(1969) 334.

[22] JONES W. S., HARTMANN C. S., CLAIBORNE L. T.,

<(

Evaluation of Digitally Coded Acoustic Sur-

face-Wave Matched Filters

D, ZEEE Trans.

SU-18

(1972) 21.

[23] SKOLNIK M. I.,

<<

Radar Handbook

>),

McGraw- Hill (New York, 1970).

[24] JONES W. S., HARTMANN C. S., STURDIVANT T. D., ((Second Order Effects in Surface Wave Devi- ces

>>, ZEEE Trans. SU-19

(1972) 368.

[25] THOMAS R. L., VALE C. R. and FOSTER T. M.,

<<

Design and Fabrication of Precise and Repeatable Sur- face Wave Barker Code Correlators

>),

Paper

Q

5, 1971 Ultrasonics Symposium, Miami, December 1971,

ZEEE Trans. SU-19

(1972) 415

;

also

DE

KLERK J.,

⁽⁽

Surface Wave Devices >>, Physical Acoustics, vol. 10, W. P. Mason and R.

N.

Thurs- tan (Eds.), Academic Press (New York, 1973).

[26] O'CLOCK

G .

D. jr., GRASSE C. L., GANDOLFO D. A.,

<<

Switchable Acoustic Surface Wave Sequence

Generator

^B, Proc. ZEEE 59

(1971) 1536.

[27] CLAIBORNE L. T., STAPLES E.

J.,

HARRIS J. L. and MIZE J. P.,

<<

MOSFET Ultrasonic Surface-Wave Detectors for ProgrammabIe Matched Filters

D, Appl. Phys. Lett. 19

(1971) 5 8 .

DISCUSSION R. BEYER.

-

For the sake of history, could you

tell us who first used interdigital electrodes for sur- faces waves

?

J.

DE

KLERK. - The first published report of I. D.

grids was by

Voltmer

and

White. Appl.

Phys.

Lett.,

7 (1965), 314. However White maintained that he .suspected that the technique had been used at Bell Labs. before but was not publicized as the work was classified. The archives of U. S, Patent Dept. will have to be consulted to determine the original inventor.

M. MORIAMEZ.

-

Vers quelles frtquences

((

limite

D,

b docteur de Klerk estime-t-il possible Ia fabrication

et I'utilisation des dispositifs interdigitts, en parti- culier en tenant compte des phCnomBnes de produc- tion d'ondes parasites citts par lui

?

J.

DE

KLERK. - The frequency limit will be set by the technique used to fabricate I. D. grids, i. e. photo- lithographic and photoresist. The limit is set by opti- cal resolution i. e . about 1 micron. This determines the minimum line with and spacing. For LiNbO, and SiO, the upper frequency limit will be about 700 MHz, and for Bi,,GeO,, about 300 MHz.

Although I. D. grids have been fabricated with sub-

micron dimensions, no mass production techniques

for this are as yet available.