reflective array compressors using 180° reflecting metal dot arrays
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Reflective array compressors using 180° reflecting metal dot arraysR. C. Woods Citation: Journal of Applied Physics 54, 6240 (1983); doi: 10.1063/1.331941 View online: http://dx.doi.org/10.1063/1.331941 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/54/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Waveguide array-grating compressors Appl. Phys. Lett. 87, 131104 (2005); 10.1063/1.2056578 Surface acoustic wave reflections from a proton exchanged dispersive dot array Appl. Phys. Lett. 67, 1844 (1995); 10.1063/1.115422 Ion beam reflection in a 180° bending magnet system Rev. Sci. Instrum. 59, 2163 (1988); 10.1063/1.1139980 A SAW pulse compression filter using the reflective dot array (RDA) Appl. Phys. Lett. 31, 1 (1977); 10.1063/1.89484 Surface acoustic wave reflective dot array (RDA) Appl. Phys. Lett. 28, 420 (1976); 10.1063/1.88804
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Reflective array compressors using 1800 reflecting metal dot arrays R. c. Woods8
)
Department of Engineering Science, University of Oxford, Oxford, England
(Received 13 December 1982; accepted for publication 19 April 1983)
Recently, a reflective array compressor using 180· grooved reflectors and a 3-dB multistrip coupler has been described. This has the advantage over conventional 90· reflective devices that the surface waves undergo only one reflection instead of two, and so the insertion loss can be low. This paper reports a new device, using surface acoustic waves propagating on YZ LiNb03, in which the grooves are replaced by metal dot arrays, removing the need for ion-beam milling and-through variation of dot density-providing a simple method of weighting. The devices designed had a time-bandwidth product of 1000, dispersion time of 20 Jis, bandwidth of 50 MHz centered at 150 MHz, and a 4O-dB sidelobe six-term Taylor amplitude weighting. Compared with dot array devices using 90· reflectors the design is simplified since the velocity and scattering anisotropy, important for 90· reflectors, need not be considered. The surface wave velocity change under the metal dot array is potentially a source of phase error, but it is shown that a simple empirical correction in the mask design is sufficient to reduce this to around 5· rms. Deviation of the weighting characteristic from the ideal is typically of the order ± 1 dB.
PACS numbers: 43.35.Pt,85.50.Ly
I. INTRODUCTION
The Reflective Array Compressor (RAC) is a dispersive delay line using surface acoustic waves (SAW) designed for use in, for example, high performance pulse-compression systems. 1 Whilst the RAC is successful in overcoming some of the disadvantages of high time-bandwidth chirped transducer designs, it introduces its own problems since the surface waves undergo two reflections by 90· and, because of anisotropy, the reflector angle is not exactly 45·. More recently, the 'in-line' RAC has been introduced2 using reflection by grooves once through 180·. Its advantages are that a single direction of propagation is used, there is only a single reflection and so the insertion loss can be low, and there is greater design flexibility since rapid changes in weighting can be accommodated.
Both the 'in-line' and the conventional grooved RACs suffer from the disadvantage of requiring both photolithography and ion-beam milling for fabrication. Solie3 suggested the use of reflecting metal dot arrays to overcome this. The reflecting metal dots are produced simultaneously with the IDTs (inter-digital transducers) by photolithography; amplitude weighting of the passband response can be accomplished by varying the number density of dots on the surface. Solie has mainly used gold dot arrays, reflecting through 90·, retaining the attendant disadvantages of 90· reflectors. An additional disadvantage of using gold dots is that the reflection mechanism involves both electrical effects (through metallic shorting of the piezoelectric substrate, which also affects the SA W velocity and hence the phase response of the device) and "mass loading" effects, so that the thickness of the gold film is a design parameter which must be controlled.
This paper reports the development of the in-line dot RAe in which the surface waves are reflected through 180· by arrays of thin aluminum dots. There is then no need for
-) Present address: Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield SI 3JD, England.
ion-beam milling, and negligible variation of reflection coefficient with the metal film thickness.4 A remaining problem is the phase error introduced by the slowing effect of the metal dots, but it will be shown that the error can be reduced to an acceptable level by incorporating a correction at the mask-making stage.
II. DESIGN
The specification aimed at was a time-bandwidth product of 1000, dispersion time 20 Jis, bandwidth 50 MHz centered on 150 MHz, and a 4O-dB sidelobe six-term Taylor amplitude weighting.2 In addition, "cos-squared" extensions of2.5 MHz were added to the Taylor weighting characteristic at each end, so the nominal device response was from 122.5 to 177.5 MHz. All devices have been downchirps; all were fabricated on yz LiNb03 and using 1000A. Al metallization. This specification was drawn up as a "design exercise" to simulate a typical performance requirement. As a consequence of this, few steps were taken to reduce spuriae and electromagnetic breakthrough in the devices and all measurements took account of this (see Sec. III).
A diagram of the design is shown in Fig. 1. The 3-dB multistrip coupler acts as a hybrid coupler and splits the input signal equally (but with a 90· relative phase shift) between the two arrays. The reflected signals are recombined in the multistrip coupler with an additional 90· phase shift, and so all the power is directed to the output transducer.
The transducers were identical conventional IDTs, with 3~ finger pairs, centered at 150 MHz; they were unapodised, unchirped, and bidirectional. They were operated with a series matching inductor, and their aperture was 2 mm. The multistrip coupler had a stopband frequency of 200 MHz, and consisted of 86 strips (including 3 on each side which were Chebychev length weighted2
,5 to reduce spur-
6240 J. Appl. Phys. 54 (11), November 1983 0021-8979/83/116240-05$02.40 6240
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Multi-strip co up ler
Output
Dot In-Line RAe
ious reflections). The separation between the acoustic tracks was 1 mm.
The masks for the devices were produced by an Electromask step-and-repeat camera. The dots were rectangular, 10.5 f-lm in width (perpendicular to the SAW propagation) andA /4 in length (parallel to the SA W propagation) where A is the instantaneous SAW wavelength in the array: this was achieved by double exposure during mask-making. The two arrays were exposed simultaneously on the Electromask machine by using a reticle plate consisting of two "unit cells" of dots separated by the correct (magnified) distance to produce identical arrays 1 mm apart on the mask. Each unit cell was either a group offour dots (vertically arranged in one column with vertical separation on the mask 25.0 f-lm) or one dot. Amplitude weighting of the arrays was accomplished by varying the number of unit cells in a given vertical column. Maximum weighting corresponded to 20 unit cells across the array aperture; for smaller weightings, the number of unit cells was reduced in proportion, but always remained an integer. (The sidelobe level in an expansion-compression system was predicted to be degraded from 40 to 37 dB,6 and the frequency response was expected to show errors of less than ± 0.5 dB, due to this "quantization"). The frequency response of the final device is the product of the frequency responses of the IDTs and the multistrip coupler, the density of dots at the appropriate part of the arrays, and the inherent response of a thin metal dot array (proportional to frequency). The effects of diffraction and propagation loss were neglected. The transducers themselves contributed approximately 10 dB loss at the band edges, and the array was designed to give approximately a further 10 dB loss so as to result in the required Taylor weighting. Typically a mask required - 87 500 exposures of the reticle; alignment markers were included to confirm reliable operation of the machine.
The quantization in the array weighting function was smoothed by adding a random number (between - 0.5 and
6241 J. Appl. Phys., Vol. 54, No. 11, November 1983
Metal dot array
FIG. I. Diagram of the dot in-line RAe. The metal dot arrays cover the areas with dashed borders.
0.5) to the number of unit cells in a column before conversion to the nearest integer; also, the vertical positions of the unit cells were randomized. (A photomicrograph of a section from a mask is shown in Fig. 2).
The response of the IDTs and multistrip coupler alone was determined by measurement of the actual insertion loss of a delay line. This delay line was 12.9 f-ls long and incorporated IDTs and a multistrip coupler identical to those used in the RAe.
For column No. I, the instantaneous frequency f of the reflective array is given by
f = ~ (/;tart + 21 Ie) , ( 1)
where the chirp constant, c = - 0.4 f-ls/MHz, and the start frequency is Istart = 177.5 MHz. The position of each column (measured in the direction of SAW propagation) is
z = vo( f - Istart )c/2 - 8, (2)
where Vo is the free-surface Rayleigh velocity and 8 is a correction applied to z (required because of the slowing caused
------ Z (SAW propagation)
~-------~ 500Jlm
FIG. 2. Photomicrograph of part ofthe dot array from a device using 4 dots per unit cell, showing the randomization of position and variation of dot density.
R. C. Woods 6241
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(a)
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(b) 130 140 150 . Frequency (MHz)
(e) 130 140 150 Frequency (MHz)
J. Appl. Phys .• Vol. 54. No. 11. November 1983
160 170
160 170
FIG. 3. Performance of device. shown in Fig. 2. using 21 % maximum metallization. (a) Impulse response. Time axis extends 25 p.s in total. (b) Insertion loss: dashed line is expected Taylor weighting. solid line is experimental result. (c) Phase error after subtraction of bestfit quadratic; rms error = 8.6'.
R.C. Woods 6242
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by the dots). There were nominally 3301 columns in each array.
The correction tenn 8 was taken to be the linear interpolation rule 7
I
8 = L(~v)a A !2J, (3) o
where ~ v is the slowing of a SAW on a completely metallized surface, A is the proportion of area metallization, and a is an empirical constant used to improve the phase response. To first order, the phase error is given by the cubic and higher order contributions to
360.2!(Z)f{[V(Z')/VO ] - I} dz'!v(z),
and if Eq. (3) is assumed to hold then an iterated value of a
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(b) 130 140 150 160 Frequency (MHz)
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may be calculated from measurements of phase error. The value a = 0.6623 was used finally.
III. EXPERIMENTAL PERFORMANCE
Measurements of insertion loss were made by computer-controlled equipment using a programmable synthesizer, by comparison with standard attenuators. Measurements of phase error were made using the method of "21T points" and the results plotted after subtraction of the best-fit quadratic. All measurements were made using RF pulses of - 3 J-LS duration, gating out unwanted breakthrough and spuriae.
The midband insertion loss and the phase error are detennined by the peak dot density and the value of a; initial experiments8 indicated that a = 1 gave a large phase error
170
FIG. 4. Performance of a device using 5.25% maximum metallization. (a) Insertion loss: dashed line is expected Taylor weighting. sol· id line is experimental result. (b) Phase error after subtraction of best·fit quadratic; rms error = 4.48'.
R. C. Woods 6243
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( - 72° rms). Devices with 4 dots per unit cell (maximum 21 % metallization) and a = 0.7175 had a midband insertion loss of23 dB (of which 18.6 dB was due to the transducers). The phase errors, on the 4 devices tested, ranged from 8.3 to 16.7° rms. The performance of one of these devices is shown in Fig. 3. The impulse response is quite smooth and corresponds to the insertion loss plot. A further 4 devices were made from a mask designed using I dot per unit cell (maximum 5.25% metallization) and the value a = 0.6623. The phase error was reduced (at the expense of insertion loss) to the range 4.45-6.96° rms. Figure 4 shows the performance of one such device. (Small asymmetry of the frequency response could be corrected by slightly changing the value of the matching inductors in series with the IDTs).
Any mismatch in the arrays manifests itself in the overall response as the vector sum of the two individual responses. The efficiency of the muItistrip coupler, and the phase tracking between the two arrays, was determined by placing acoustic absorber between the multistrip coupler and one only of the arrays. The operation of the in-line RAC should be unaffected by this, apart from increased insertion loss. In most cases the insertion loss was increased by 6 dB as expected and the weighting and phase error curves were virtually unaltered. (The exception to this was when a defect caused identifiable differences between the arrays.) This indicates that lack of phase tracking between the arrays is not a problem; arrays which are identical for these purposes are readily obtainable using conventional photolithographic techniques.
IV. DISCUSSION
Early devices did not use randomization, and this produced wide departures from the required weighting function. However, when the dots are randomized, the amplitude response is within ± I dB of that expected, and small deviations from the ideal are accounted for by variations in the response of the IDTs. All initial devices were designed using an equivalent circuit model9 of the IDT frequency response, with the assumption that the muItistrip coupler made no contribution to the overall response; the responses obtained were not as close to the ideal as when the actual response of a delay line is used.
Array loss as low as 5 dB (at center frequency) has been demonstrated, but using such a high dot density that slightly degraded phase response was observed. Dispersing the dots to give an array loss of 18 dB, simultaneously with modifying the velocity correction, reduced the phase error significantly. In conventional RACs the phase error can be reduced by interposing a "phase plate" 10 between the two reflective arrays. This is a metal film of varying width which the surface waves must cross on passing between the arrays; because of the slowing of the waves due to the metal film, additional phase delay is introduced. This can vary with position down
6244 J. Appl. Phys., Vol. 54, No. 11, November 1983
the arrays and hence with frequency, so compensating for the phase error in the RAe. On an in-line RAC the surface waves do not pass between the arrays, and so a "phase plate" cannot be used. It is therefore important that phase errors are small, and for their accurate correction it is also important that the phase response is repeatable. This implies care in fabrication to ensure that dots of repeatable dimensions are produced; variations in the phase response were correlated very roughly with variations in dot size of order 0.5j1m, but the phase error is reduced to an acceptable level by using relatively low dot densities.
Recent theoretical work4 indicates that the reflectivity of the dots used in the present work departs from the model presented here only by ± 0.23 dB over the frequency band used, and so the dot design is already close to optimum (as originally intended).
V. CONCLUSIONS
The design and performance of a high-performance dot in-line RAC has been presented. This device offers significant advantages over both conventional grooved RACs and 90° reflecting dot RACs. Although there is no possibility of correcting the phase error on individual devices by using a phase plate, device performance is sufficiently repeatable that a simple empirical phase correction can be included in the mask design.
ACKNOWLEDGMENTS
It is a pleasure to acknowledge many helpful discussions with Professor E. G. S. Paige, and with Dr. A. G. Stove (Philips Research Laboratories, Redhill, England); mask manufacture was by Mr. A. Young and Mr. G. Gibbons (RSRE, Malvern, England). Most devices were fabricated by Mr. R. E. Chapman and Mr. R. Gibbs [Plessey Research (Caswell) Ltd .. Towcester, England]; two devices were made by Mr. R. Barton (Marconi Research Center, Chelmsford, England). This work was supported by CVD (UK Ministry of Defence).
I R. C. Williamson and H. I. Smith, IEEE Trans. Microwave Theory Tech. MTI-21, 195 (1973).
oR. E. Chapman, R. K. Chapman, D. P. Morgan, and E. G. S. Paige, Proc. IEEE Ult. Symp. 696 (IEEE, New York, 1979).
'L. P. Solie, Proc. IEEE Ult. Symp. 682 (IEEE, New York, 1979). 4F. Huang and E. G. S. Paige, Proc. IEEE UIt. Symp. (IEEE, New York, 1982).
'F. G. Marshall, RRE Internal Report (1973). 6R. K. Chapman (private communication). 7L. P. Solie, Proc. IEEE UIt. Symp. 579 (IEEE, New York, 1977). K A. G. Stove (private communication). "CO S. Hartmann, D. T. Bell, and R. C. Rosenfeld, IEEE Trans. Microwave, Theory Tech. MTI-21, 162 (1973).
lOT. A. Martin, IEEE Trans. Sonics Ultrason. SU-20, 104 (1973).
R. C. Woods 6244
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