Zinc oxide on silicon device for parallel in, serial out, discrete fourier transform

A material suitable for the propagation of acoustic waves on its surface prises a substrate of semiconductor material, of which there exists an oxide, excluding the class of piezoelectric materials, the substrate having at least one flat surface. A layer of thermally grown oxide of the semiconductor material, is disposed on the flat surface. A film of titanium, approximately 300 angstroms thick, is disposed on at least a part of the layer of oxide. A layer of vacuum-deposited metal is disposed on the film of titanium. A layer of a piezoelectrid vacuum-sputtered material is on the layer of vacuum-deposited metal and on the oxide. The semiconductor material may be silicon, the oxide may be silicon dioxide, the piezoelectric material may be zinc oxide, and the metal may be gold. The material further comprises an interdigitated electrode structure disposed upon the piezoelectric material, which, when an electrical signal is applied to it, can cause propagation of a surface acoustic wave (SAW) upon the surface of the piezoelectric material, the combination comprising a SAW transducer.

BACKGROUND OF THE INVENTION 
Modern signal processing systems call for a variety of complex operations, 
including correlating, matched filtering, and Fourier transforming. A 
number of different approaches have been successful in achieving these 
functions, one of which is a technology based on the propagation of 
high-frequency acoustic waves on the surface of piezoelectric crystals. 
The chief advantages of these surface acoustic wave (SAW) devices, 
compared with alternative technologies, are their simplicity, processing 
speed, and compactness. Already they have been incorporated into military 
communication systems, radars, and satellite systems. 
Work is currently being done to extend SAW techniques to silicon integrated 
circuits, thus combining real-time analog processing with complex 
peripheral circuitry on a single small chip of silicon. 
This invention relates to a device capable of executing a high-speed 
discrete Fourier transform of a signal for the situation in which the 
signal appears in a parallel format and is read out in a serial format. 
The device could be used for various signal processing functions, 
including beamforming, image data compression, etc. 
Only one prior art experimental device exists that is capable of performing 
the above function, and it requires the use of a lithium niobate delay 
line and a transducer structure to which is bonded an array of diodes 
fabricated on a silicon-on-sapphire substrate. This prior art embodiment 
is discussed by Reeder, T. M. and Gilden, M., "Convolution and Correlation 
by Nonlinear Interaction in a Diode-coupled Tapped Delay Line," Applied 
Physics Letters, Vol. 22, No. 1, Jan. 1, 1973, p. 8. The prior art device 
described therein requires separate complicated linearization circuitry to 
implement a linear discrete Fourier transform. 
Prior art which provides useful background information for both the 
material of this invention and the convolver implemented upon the material 
comprise the following: (1) Coldren, L. A., "Effect of bias field in a 
zinc-oxide-on-silicon acoustic convolver", Applied Physics Letters, Vol. 
25, No. 9, Nov. 1, 1974, Pp 473-475; (2) Coldren, L. A., "Zinc oxide on 
silicon memory cells scanned by acoustic surface waves", Applied Physics 
Letters, Vol. 26, No. 4, Feb. 15, 1975, Pp 137-139; (3) Davis, J. L., 
"Properties of the MZOS Surface Wave Convolver Configuration N", IEEE 
Transactions on Electron Devices, Vol. ED-23, No. June 1976, Pp 554-559; 
and (4) Davis, K. L., "S. A. W. Frequency Synthesis Using a Monolithic 
ZnO-on-Si Convolver", Electronic Letters, Vol. 12, Sept. 16, 1976, Pp 
487-488. 
SUMMARY OF THE INVENTION 
A material suitable for the propagation of acoustic waves on its surface 
comprises a substrate of semiconductor material of which there exists an 
oxide. The class of piezoelectric materials is excluded. The substrate has 
at least one flat surface. A layer of the oxide of the semiconductor 
material is disposed on the flat surface. A layer of metal is disposed on 
at least a part of the layer of oxide. A layer of piezoelectric material 
is disposed on the layer of metal and on the oxide. Typically, the 
semiconductor material may be silicon, the oxide may be silicon dioxide, 
and the piezoelectric material may be zinc oxide. The metal may be either 
aluminum or gold, as examples. An interdigitated electrode structure may 
be disposed upon the piezoelectric material. When an electrical signal is 
applied to the electrode structure a surface acoustic wave (SAW) is caused 
to propagate upon the surface of the piezoelectric material. The 
combination of material and electrode structure comprises a SAW 
transducer. The electrode structure comprises an input electrode structure 
and an output electrode structure, both so configured as to form a 
triple-product convolver. 
OBJECTS OF THE INVENTION 
An object of the invention is to provide a material having a 
non-piezoelectric surface, which nevertheless can propagate surface 
acoustic waves. 
Another object of the invention is to provide such a material with an 
interdigitated electrode structure. 
Yet another object of the invention is to provide an interdigitated 
electrode structure which is capable of convolving input signals. 
A still further object of the invention is to provide a transducer which 
can perform a high-speed discrete Fourier transform of an input signal in 
parallel format and can be read out in a serial format.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In typical SAW devices, 10 in FIG. 1, a high-frequency electrical signal is 
applied to a periodic metal pattern 14 and 16 deposited on the surface of 
a piezoelectric crystal 12. If the frequency of the electrical signal and 
the spacing of the metal patterns, 14 and 16, are adjusted so that their 
product equals the natural phase velocity of the crystal 12, a surface 
acoustic wave 18 will be generated and will propagate a long distance on 
the crystal with very little loss of amplitude. Since the energy in the 
wave 18 is confined to a region at the crystal surface, the wave may be 
tapped at any point in its path with another periodic metal pattern. 
Reference is now directed to the transducer 20 shown in FIG. 2. Silicon 
does not have piezoelectric properties. To launch surface acoustic waves 
onto silicon 22, therefore, a thin layer of piezoelectric material, 
usually zinc oxide 24, is deposited on top of the silicon to mechanically 
couple the acoustic energy in the layer to the silicon. The electrical 
signal generated by signal source 26 can be efficiently converted into 
acoustic waves in the silicon 22 if the geometry and the electrical 
conditions in the piezoelectric layer 24 are carefully designed. 
Surface acoustic wave transducers 28 have been built with several 
geometries, the most effective of which is shown in FIG. 2. In this case, 
the thermally grown silicon dioxide layer 28 is used to protect the 
silicon 22 surface from impurities and to electrically separate the 
silicon from the grounded metal plate 32 under the zinc oxide film 24. The 
aluminum ground plate 32 prevents electrical energy in the input signal 
from source 26, from being dissipated in the semiconducting silicon 22 
instead of in the piezoelectric conversion process in the zinc oxide 24. 
Transducer pairs of this sort have been built at 80 MHz with 24 dB 
insertion loss. Further work should decrease insertion losses to 10 dB. 
The ability to propagate SAWs efficiently on silicon, in addition to the 
advantages gained from using integrated circuit technology, also expands 
the signal processing capability of SAW devices into areas not easily 
available to standard SAW technology. In particular, two SAWs can be 
easily multiplied or mixed on silicon and their product scaled by a third 
electrical signal--a triple product. Since this triple product can be 
distributed over the entire surface wave propagation path, programmable 
correlating, matched filtering, and Fourier transforming can be done. A 
generalized embodiment 40 is shown in FIG. 3. An object of the invention 
was to build a monolithic delay line device 40 that could form a product 
from three input signals in real time at various positions along the delay 
path. It can be seen that two of the input signals, f.sub.1 (t) and 
f.sub.2 (t), are oppositely propagating waves in the delay lines, 42 and 
44, while the third input signal, g.sub.i, scales the product of the two 
propagating waves at discrete positions along the propagating path. 
The importance of such a device 40 lies in the fact that it can be used to 
perform several useful signal-processing functions, including electrically 
programmable correlation or convolution and real-time Fourier transforms 
where the input function g.sub.i is in a parallel format. In the case of 
the Fourier transform, the two oppositely propagating waves, f.sub.1 and 
f.sub.2, can be modulated with FM signals so that the distributed product 
along the delay path is a linear function of modulation frequency with 
distance. Thus the frequency of the resultant wave is a linear function of 
position. If the function to be transformed is now sampled at equal 
intervals and if each sample is used as the scaling signal at evenly 
spaced taps along the delay path, then the sum of the output products from 
all taps is S.sub.o (t), as shown in FIG. 3. A time-sampled version of 
this output signal is proportional to the discrete Fourier transform (DFT) 
of g(x.sub.i). 
A more specific device 50 has been built with a configuration similar to 
that shown in FIG. 4. The response of each of the center taps 52 can be 
electrically programmed by depleting the silicon 22 of free carriers at 
varying depths with a DC bias voltage, V.sub.1, V.sub.2, V.sub.3 and 
V.sub.4. Thus each tap 52 acts much like a bias-variable diode. 
Before discussing the convolver 60 shown in FIG. 5, operation of the device 
can be better understood by first considering the definition of the 
discrete Fourier Transform (DFT): 
##EQU1## 
But 
EQU 2mn=1/2[(n+m).sup.2 -(n-m).sup.2 ], (2) 
so that 
##EQU2## 
In this form, then, the DFT components G.sub.m are given by the 
convolution of two chirps weighted by the samples g.sub.n of the function 
to be transformed. 
A possible implementation of the above form of the DFT would consist of (a) 
modulating oppositely propagating surface acoustic waves (SAWs) of 
frequencies w.sub.1 and w.sub.2 with chirp waveforms; (b) detecting the 
overlapping and superimposed SAWs with an array of discrete taps; (c) 
performing a squaring operation at each tap; (d) linearly attentuating or 
amplifying the squared signal at each tap according to the sampled input 
waveform amplitudes; (e) summing the taps; and (f) extracting the sum or 
difference frequency component (w.sub.1 .+-.w.sub.2) with a bandpass 
filter. 
The device 60 shown in FIG. 5 is designed to perform the above operations 
in monolithic form. The SAWs are launched onto the SiO.sub.2 /Si surface 
with the gold and ZnO film transducer structures 62 and 64 at opposite 
ends of the device 60. This is possible since ZnO, 24, is a piezoelectric 
material. At each of the detecting taps 66 a depletion region is created 
with bias (not shown) applied to the tap. The SAWs propagate through the 
material 24 accompanied by electric fields exponentially decaying into the 
Si, 22, which upon entering the tap array 66 region, modulate the 
depletion width, and thereby cause a change in the Si, 22, surface 
potential. This change is approximately given by 
EQU .DELTA..phi.=a constant.times.[2E.DELTA.E+(.DELTA.E).sup.2 ](4) 
where E is the electric field at the Si, 22, surface. Consequently 
EQU .DELTA.V.sub.G .perspectiveto.constant.times.[(.DELTA.E).sup.2 + . . . 
,](5) 
where V.sub.G is the bias voltage on a tap. The MOS capacitors in depletion 
thus perform the needed squaring operation at each tap. 
The attenuation or amplification can be performed in a number of ways. One 
way that is simple for integration uses one MOSFET 70 at each tap 66 as a 
linear variable conductance, as shown in FIG. 5. These MOSFETs 70 are 
fabricated on the same chip as the taps 66, in an area free of the ZnO 
film 24, as shown in FIG. 5. The external chirp circuitry, not shown, is 
similar to that described by Reeder, T. M. and Swindal, J. L., "Variable 
Acoustic Surface Wave Correlator", United Aircraft Research Laboratories 
Report ECOM-73-0194-F, November 1974. 
The above device 60 is also capable of performing a convolution or a 
correlation operation with properly applied input signals, thus the 
general designation of "monolithic triple product convolver" is 
appropriate. 
To clarify the ideas behind the nonlinearity of a MOS capacitor tap, the 
structure 80 shown in FIG. 6 is instructive. When a negative bias is 
applied to the metal tap of the metal/oxide/n-type semiconductor sandwich, 
the free electrons contributed by the donor impurities are driven from the 
region of the semiconductor directly under the metal tap, leaving this 
region depleted of free carriers. This depleted region consists of a 
uniform distribution of uncovered immobile donor impurities, each having a 
positive charge. Poisson's equation with respect to the depleted region 
may be solved, and the potential at the surface of the silicon substrate 
84. 
If the static electric field is perturbed by a time-varying electric field 
carried by a propagating piezoelectric surface wave, the resulting small 
signal potential may be obtained. Therefore, the silicon surface potential 
will have a major component proportional to the square of the electric 
field carried by the surface wave. A plot 90 of the theoretical variation 
of the second harmonic component as a function of bias voltage, based on a 
more detailed analysis, is shown in FIG. 7. 
Regarding the manufacture of the convolver material, the first step in 
developing the triple-product convolver, 60 of FIG. 4, was to learn how to 
grow thin piezoelectrically active films for coupling acoustic energy into 
the silicon substrate 22. Zinc oxide (ZnO), 24, was chosen as the 
piezoelectric film because of its large piezoelectric coupling factor and 
because such films already had been grown successfully by a few 
laboratories for SAW applications. For example, see the article by 
Hickernell, F. S., and Brewer, J. W., "Surface-elastic-wave properties of 
dc-triode-sputtered zinc oxide films," Appl. Phys. Lett., vol. 21, no. 8, 
389 (1972). These films are usually RF sputtered. Since most RF sputtering 
systems are unique and since RF sputtering is largely an empirical 
technique, an experimental program was embarked upon to develop a 
repeatable ZnO process for the existing system being used. 
Referring now to FIG. 8, the major modifications to the system 100 were the 
installation of a high-purity ZnO target 102, the construction and 
installation of a temperature controlled substrate holder, 104, the 
installation of a laser film thickness monitor 106, and the addition of an 
80-percent argon/20-percent oxygen mixture 108 for the sputtering 
atmosphere. The substrate holder 104 was designed to be vacuum tight to 
prevent outgassing of the heater (not shown) into the vacuum chamber 112. 
The thickness monitor 106 was based on the idea of continuously observing 
the reflection of a laser 114 from the transparent ZnO film which forms on 
top of substrate 116, and noting where the interference nulls occurred as 
a function of time. If it is assumed that the film deposition is linear 
with time, a trial film deposition run allows calibration of the laser 
output record in angstroms per interference fringe. 
To launch surface acoustic waves efficiently onto a layered structure 
consisting of a piezoelectric film on a nonpiezoelectric substrate, it is 
necessary for the major piezoelectric axis of the film to be in a 
direction normal to the substrate. Since ZnO films have isotropic 
piezoelectric properties in a plane normal to the main piezoelectric axis, 
or Z axis, only polycrystalline films need be grown as long as the Z axes 
of the crystallites are vertical. Previous experiments by workers in the 
field have shown that ZnO films tend to orient with the Z axis in the 
direction of the incident atoms, if the deposition rate is low enough and 
the substrate hot enough to allow sufficient molecular mobility on the 
substrate after condensation. At temperatures in excess of 300.degree. C. 
and deposition rates around 200 angstroms per hour, the films can become 
epitaxial. This is discussed by Rozgonyi, G. A., and Polito, W. J., 
"Epitaxial thin films of ZnO on CdS and Sapphire", J. Vac. Sci. Tech., 
vol. 6, no. 1, 115 (1969). 
The first film deposition runs were made using glass microscope slides as 
substrates and adjusting the system parameters approximately to those used 
by Khuri-Yakub, B. T., Kino, G. S., and Galle, P., "Studies of the optimum 
conditions for growth of rf-sputtered ZnO films", J. Appl. Phys., vol. 46, 
no. 8, 3266 (1975). Checks on film orientation were made initially with 
the use of an X-ray diffraction system; the Z axis orientation was judged 
according to the relative magnitudes of the reflections from the three 
principal planes. It was found that for substrate temperatures in excess 
of approximately 120.degree. C., X-ray reflections from the (101) and 
(100) planes were indiscernable in the noise with a deposition rate of 150 
angstroms per min. 
FIG. 9 shows a graph 120 of X-ray data from a well oriented film of Zn) on 
glass. The reflections from the (101) plane at 36.5.degree. and the (100) 
plane at 31.5.degree. are absent. Similar experiments were carried out on 
gold, titanium, aluminum, and SiO.sub.2. 
Reference is directed back to FIG. 5A. Adherence to the gold 66 was a 
problem because of the inability of gold to form good chemical bonds with 
the ZnO, 24; however, a 200-angstrom top layer of titanium 68 between the 
gold substrate 66 and the ZnO film 24 cured the problem, with the 
additional advantage of less ZnO film surface roughness. It was found that 
well oriented films of ZnO could be grown on all the substrates, with a 
deposition rate of 150 angstroms per minute and a substrate temperature of 
200.degree. C. 
The bottom layer of titanium, also designated 68, was generally necessary 
if gold 66 were used as a layer on the silicon dioxide 28, to provide 
better adhesion, through an alloying action but would not generally be 
necessary for aluminum. 
Standard integrated-circuit photolithographic techniques normally are not 
suitable for fabricating devices with ZnO on silicon, since the etching 
required to delineate metal pattern in most cases affects the ZnO. 
Consequently, a photolithographic technique without etching had to be 
developed in the laboratory. The process chosen was patterned after one 
called "lift-off" which is used by some surface-wave and integrated-optics 
laboratories. For example, see Smith, H. I., Bachner, F. J., and Etremow, 
N., "A High-Yield Photolithographic Technique for Surface Wave Devices", 
J. Electrochem. Soc., Vol. 118, No. 5, 821 (1971). 
The basic process sequency 130 is pictured in FIG. 10, with the left column 
showing older methods and the right column showing the newer method. It 
consists of (1), FIG. 10B, spinning a layer of positive photoresist 134 
onto the substrate 132; (2), FIG. 10C, exposing the photoresist 136 with 
ultraviolet light through a prepared photographic mask; (3), FIG. 10C, 
developing, or removing, the regions exposed to the light; (4), FIG. 10D, 
depositing a thin layer of metal 138 over the developed photoresist 
pattern; and (5) FIG. 10E, "lifting-off" the metal 138 lying directly on 
the photoresist regions 136 by removing the photoresist with a solvent. 
Success of this technique depends basically on two factors: vertical or 
undercut photoresist edges after developing and good adhesion of the metal 
film 138 to the areas of the substrate 132 where the photoresist 134 has 
been removed. A large number of trials were run on glass and ZnO using 
gold, titanium, and aluminum metallization to establish the best 
parameters for use with the laboratory system and the best chemical 
procedure. 
The key elements to success with "lift-off" photolithography were found to 
be (1) a thorough cleaning of the substrates 132 in inorganic solvents 
when possible and boiling in a detergent solution followed by boiling in 
deionized water; (2) intimate contact between substrate and mask during 
exposure, achieved by using vacuum pulldown on the mask and evidenced by 
visible optical interference fringes at the interface; (3) exposure times 
in excess of 200 seconds to enhance undercutting; (4) the use of a 
300-angstrom titanium film between the substrate and metallization to 
assure good adhesion; and (5) pressures less than 5.times.10.sup.-8 torr 
in the vacuum chamber before metal deposition to thoroughly out-gas 
regions of the substrate 132 where photoresist 134 was removed. 
A summary of the lift-off photolithographic procedure follows: 
1. Clean substrate by swabbing with lab soap (Liquinox), rinsing in 
deionized water, and boiling in deionized water. 
2. Bake-out slides for 1 hour. 
3. Let substrate cool to room temperature. 
4. Flood substrate with filtered AZ 1350 B photoresist. 
5. Spin at 3000 rpm for 30 seconds. 
6. Bake-out at 90.degree. C. for 25 minutes. 
7. Cool for 10 minutes to room temperature. 
8. Expose through mask using vacuum pulldown for 220 seconds (11-inch 
mask-to-light separation). 
9. Develop for 35 seconds in MF-312 developer, 1:1 with deionized water at 
room temperature. 
10. Rinse for 1 minute in deionized water. 
11. Deposit 300 angstroms of Ti and about 1000 angstroms of other metal on 
photoresist pattern. Ultimate vacuum should be around 5.times.10.sup.-8 
torr. 
12. Lift-off with 50.degree. C. acetone accompanied by light swabbing with 
a cotton swab. Acetone may be decanted to eliminate large pieces of metal 
initially lifted off. 
13. Rinse in deionized water. 
To achieve adequate piezoelectric coupling to thin ZnO films, carrier 
frequencies in excess of 60 megahertz are required for the ZnO-on-silicon 
devices, which in turn requires high-resolution contact masks for device 
fabrication. The very small pattern dimensions rule out hand-prepared 
rubylith masters. Consequently, computer-generated masks had to be 
developed using Gerber plotting facilities. 
A general computer program was written to calculate automatically the mask 
coordinates of a variety of different kinds of SAW device geometries by 
entering the appropriate parameters into the program. The punched-card 
output from this program is used as input to the Gerber plotter, which 
photographically reproduces SAW device geometry onto a large sheet of film 
that serves as a master mask. This mask is photographically reduced by a 
factor of ten onto a high resolution photographic plate in a surface wave 
laboratory. The photoresist film in the substrate is directly exposed 
through the light and dark pattern on the plate as explained hereinabove. 
FIG. 11 is a schematic diagram of a convolver 140 which is electronically 
equivalent to the convolver 60 shown in FIG. 5. Another way of 
implementing the device 60 with a more limited linear dynamic range is to 
accomplish the attenuating operation by varying the DC potentials on the 
taps. This implementation 160 with connecting circuitry is shown in FIG. 
12. 
The primary advantages of the device 60 of FIG. 5 over previously built 
devices are (a) that a completely monolithic structure is used and (b) 
that no complicated linearizing circuits are needed. A major new feature 
is that an entirely different physical effect that is compatible with 
integrated circuit processing is used to generate the squaring operation. 
Obviously, many modifications and variations of the present invention are 
possible in the light of the above teachings, and, it is therefore 
understood that within the scope of the disclosed inventive concept, the 
invention may be practiced otherwise than as specifically described.