Title:
ELECTRONIC INTERACTION GUIDE STRUCTURE FOR ACOUSTIC SURFACE WAVES
United States Patent 3777274


Abstract:
Acoustic surface waves are produced in a lossy piezoelectric material and e guided therealong due to the interaction between the acoustic waves and a drift electron flow established in an adjacent semiconductor matrial; i.e., the drift electrons impart energy sufficient to sustain propagation of the acoustic surface waves only in the desired direction defined by the drift electron path or channel.



Inventors:
ABRAHAM D
Application Number:
05/218732
Publication Date:
12/04/1973
Filing Date:
01/18/1972
Assignee:
NAVY,US
Primary Class:
Other Classes:
333/150
International Classes:
H03F13/00; H03H9/02; H03H9/76; (IPC1-7): H03F3/04
Field of Search:
330/5.5 333
View Patent Images:
US Patent References:
3406358Ultrasonic surface waveguides1968-10-15Seidel et al.



Other References:

Collins et al., "Electronics," Dec. 8, 1969, pp. 102-111. .
White, "Proc. IEEE," Aug. 1970, p. 1238-1276..
Primary Examiner:
Lake, Roy
Assistant Examiner:
Hostetter, Darwin R.
Parent Case Data:


CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of my copending and commonly assigned application, Ser. No. 32,315, filed Apr. 27, 1970, which has now been abandoned.
Claims:
What is claimed is

1. A guide structure for acoustic surface waves comprising, in combination,

2. The guide structure specified in claim 1 wherein said lossy piezoelectric member is in the form of a substantially flat thin film.

3. The guide structure specified in claim 1 wherein said drift electron conduction channel member is a semiconductor material characterized by high electron mobility.

4. The guide structure specified in claim 2 further including a high resistivity substrate to support said lossy piezoelectric film and said drift electron conduction channel member.

5. The guide structure specified in claim 4 wherein said drift electron conduction channel member is a semiconductor layer epitaxially mounted on said substrate and said lossy piezoelectric thin film is deposited on said substrate overlying said epitaxial semiconductor layer.

6. The guide structure specified in claim 5 wherein said high resistivity substrate is formed of chromium doped gallium arsenide, said epitaxial semiconductor layer is formed of gallium arsenide and said lossy piezoelectric thin film is formed of aluminum nitride.

7. The guide structure specified in claim 1 wherein said drift electron conduction channel member forms a network of interconnected drift electron conduction channels and wherein said drift field voltage source means includes means for selectively creating a drift electron flow along a predetermined route through said network of interconnected drift electron conduction channels.

8. The guide structure specified in claim 1 further including means for reversing the polarity of energization of said drift electron conduction channel member by said drift voltage source means.

9. The guide structure specified in claim 7 further including means for reversing the polarity of energization of said drift electron conduction channel member by said drift field voltage source means.

10. A method of guiding acoustic surface waves comprising the steps of,

11. The method specified in claim 10 wherein the step of creating a drift electron flow along a predetermined channel is accomplished by selectively connecting a drift field voltage source across the ends of a selected path through a network of interconnected drift electron conduction channels.

12. The method specified in claim 10 further including the step of selectively controlling the direction of drift electron flow along said predetermined channel.

Description:
BACKGROUND OF THE INVENTION

Sophisticated radar systems have been proposed which envision real-time processing of the microwave return signals. The source of these signals is generally high power radars with bandwidths of the order of hundreds of megahertz and therefore, processing of the signals without loss of information will require that the processing systems have comparable bandwidth capability, although the frequency range employed can be shifted, for example, by mixing with the output of a suitable local oscillator.

In such a radar processing system, the storing or delaying of one signal and comparing it with the next signal received is a typical processing function performed by a delay line and the delay line, in particular, illustrates the advantages of using acoustical or microsonic techniques in conjunction with the more familiar electromagnetic spectrum devices. For example, a microsound or acoustical delay line one centimeter long can provide as much delay as 1 kilometer of coaxial cable. Consequently, if one requires a complex processing system consisting of vast number of delays, switching devices, etc., as alluded to above in the area of real-time radar signal processing, the use of microsound or acoustical devices rather than standard electromagnetic hardware would greatly reduce the structural volume of devices required.

For many years, the field of microsonics was limited to the use of volume wave devices such as ultrasonic delay lines. Moreover, the losses associated with acoustical transducers were quite large and the propagating signals were inaccessible until their emergence at the surface of the acoustic medium. More recently, however, acoustic surface waves have been propagated on crystals with very low losses and, in addition, amplification of such acoustic surface waves has been accomplished by exciting the acoustic surface waves on a minimum loss, single crystal piezoelectric medium and then imparting energy to the surface waves by means of the interaction between such surface waves and high mobility drift electrons established in an adjacent semiconductor with a drift velocity slightly in excess of the acoustic wave velocity; the critical dimension in the amplifier structure being that the spacing between the piezoelectric surface and the semiconductor must be comparable to or smaller than an acoustic wavelength. In such acoustic surface wave amplifiers, the single crystal piezoelectrics utilized to attain minimum acoustical losses have an inherent high orientation dependence in order to achieve good piezoelectric property; i.e., electromechanical conversion.

On the other hand, a need presently exists for efficient structures for guiding such acoustic surface waves, if full advantage is to be taken of the above-discussed properties of microsonics; i.e., in order to effect better utilization of the surface area of the surface wave supporting medium and perform the various processing functions desired such as, for example, signal delay and signal routing. Along these lines, it has previously been proposed to construct guide structures for acoustic surface waves by geometrically constraining the surface waves through either the shape of the surface of the wave supporting medium or by variation of materials. In either event, the surface waves are confined (guided) in the desired path due to refraction or reflection effects arising from variations in the acoustic velocities between dissimilar regions on the surface. By way of example, in one proper art guide structure a narrow guide strip is mounted on an acoustic surface wave supporting substrate material effective to produce a so-called localized "perturbed" region having elastic properties different from the substrate. In this previously proposed guide structure, the acoustic surface (or bulk) waves are guided or restricted in their propagation due to reflections at the edges of the guide strip material resulting from discontinuity in the propagation velocity between the guide material strip and the substrate. Unfortunately, such a guide structure is considered practical only where the width of the guide strip is approximately equal to an acoustic wavelength or less, inasmuch as energy losses are otherwise very large; whereas, fabrication of such a structure becomes increasingly more difficult as the frequency of the surface wave is increased.

SUMMARY OF THE INVENTION

In accordance with the present invention, the guiding of acoustic surface waves does not depend upon geometrical constraints but rather is accomplished by generating or exciting the surface waves in a piezoelectric material which is purposely made lossy (rather than having minimum loss as in surface wave amplifiers) and then maintaining the wave amplitude only in the chosen propagation path or paths by employing the above-described amplifying principle to balance the losses in the piezoelectric material. In other words, a drift electron flow is established adjacent the lossy piezoelectric, with the drift electron channel defining the desired propagation path for the acoustic waves, such that the drift electrons will impart energy to sustain the acoustic surface waves only along the desired propagation path. In all other directions in the piezoelectric, the acoustic surface waves will die out; i.e., be severely attenuated, due to the lossy nature of the piezoelectric.

More specifically, in the proposed acoustic surface wave guide structure, the piezoelectric material is polycrystalline in nature, rather than a single crystal material as used in surface wave amplifiers, and the necessary level of acoustical energy loss is thus attained by means of the well-known grain boundary scattering effect. Generally speaking, the acoustical loss or attenuation coefficient α for the polycrystalline piezoelectric is selected, in accordance with the present invention, such that the surface waves will be propagated along the desired guide path with no significant attenuation, while at the same time maintaining good isolation between adjacent guide paths. Moreover, the polycrystalline nature of the piezoelectric material utilized in the proposed guide structure imposes no orientation dependence restrictions on the direction of propagation, thus permitting the surface waves to be guided along angled or curved paths and facilitating the creation of junctions between paths; thereby providing for structures in which the acoustical waves can be made to mix or can be fanned out.

In one embodiment of the present invention, a guide structure for the acoustic surface waves is formed by epitaxially mounting a semiconductor material on a high resistivity substrate to form a drift electron conduction channel. A lossy thin film of polycrystalline piezoelectric material is then brought in close proximity to the semi-conductor drift electron path by depositing the piezoelectric film on the substrate overlying the epitaxial semiconductor. With a source of drift field voltage connected across the ends of the semiconductor drift electron channel and a suitable transducer connected to create or excite the acoustic surface waves in the lossy piezoelectric film, the surface waves will propagate along a path, determined by the path of the drift electron flow, towards a second or receiving transducer.

In view of the foregoing discussion, it should be readily apparent that, by means of the proposed structure of the present invention, acoustic surface waves may be propagated through any desired length path, to achieve varying amounts of delay, by proper configuration of the drift electron flow channel and/or selectively routed in any desired path or paths through the lossy piezoelectric film by proper selective application of drift field voltages to a network of drift electron flow channels or paths.

One object of the present invention is thus to provide a method and apparatus for guiding acoustic surface waves.

Another object of the present invention is to provide a method and apparatus for guiding acoustic surface waves in a lossy piezoelectric material due to the interaction of the acoustic waves and an adjacent drift electron flow.

Another object of the present invention is to provide a method and apparatus for selectively routing or guiding acoustic surface waves through a network of propagation paths.

Another object of the present invention is to provide a method and apparatus for guiding acoustic surface waves in a lossy piezoelectric material by selectively establishing a drift electron flow in a network of conduction channels which interacts with the acoustic surface waves and imparts energy to the surface waves to sustain propagation thereof.

A further object of the present invention is to provide a guide structure for acoustic surface waves which takes advantage of existing semiconductor technology in order to reduce the required physical dimensions of the guide structure and improve its operating characteristics.

Other objects, purposes and characteristic features of the present invention will in part be pointed out as the description of the present invention progresses and in part be obvious from the accompanying drawings wherein:

FIG. 1 is a perspective view of one embodiment of an acoustic surface wave guide structure proposed in accordance with the present invention and containing two adjacent guide or propagation paths;

FIG. 2 is a partial perspective view of one-half of the guide structure of FIG. 1 with the upper piezoelectric thin film removed to expose a semiconductor drift electron channel epitaxially mounted in the upper surface of the substrate;

FIG. 3 is a partial cross-sectional view of the guide structure of FIGS. 1 and 2;

FIG. 4 is a perspective view of a further embodiment of the proposed guide structure of the present invention with the piezoelectric thin film removed to show an alternate configuration of a drift electron channel;

FIG. 5 is a perspective view of a still further embodiment of the proposed guide structure of the present invention with the piezoelectric thin film removed to show a network configuration of drift electron channels; and

FIG. 6 is a diagrammatic illustration of circuitry for controlling the polarity of energization of a drift electron channel.

Referring now to the drawings and particularly to FIG. 1, the acoustical surface wave guide structure constituting the illustrated embodiment of the invention is shown as containing two adjacent guide or propagation paths "A" and "B," separated by a minimum distance d. The structure associated with each of these guide paths is the same and has been illustrated in more detail in FIGS. 2 and 3.

More specifically, the illustrated embodiment of the present invention comprises a substrate 10 of high resistivity (106 - 108 ohm/centimeter) material such as chromium doped single crystal gallium-arsenide. Epitaxially mounted in the upper surface of the substrate 10 is a layer or film (approximately 3 micrometers thick) of high electron mobility semiconductor material, such as n-type gallium-arsenide, for example, in the form of one or more drift electron conducting channels (two of which are assumed in the embodiment of FIG. 1 with one, designated at 11b, being shown in detail in FIG. 2) which can be formed by epitaxial chemical vapor deposition. The desired pattern or configuration of the conduction channel(s) 11 can be created either during the formation of the semiconductor film by suitable masking or subsequent to the film formation by pattern etching.

Even though the sole function of the substrate 10 might be to support the other materials comprising the proposed guide structure, it must however be compatible with those other materials. For example, in the presently preferred embodiment of the invention the semiconductor material used in the conduction channels 11 is preferably formed from a high quality single crystal in order to attain high electron mobility, to minimize electrical losses and to reduce the magnitude of the required drift fields. This, in turn, imposes the requirement that the substrate 10 upon which the semiconductor is prepared should also be a defect-free single crystal. In addition to the gallium-arsenide on gallium-arsenide combination noted above, other suitable substrate/semiconductor combinations include epitaxial silicon on sapphire and silicon on silicon.

As is well-known to those familiar with surface wave amplifiers, the interaction between the drift electrons and the electric field associated with the acoustic waves takes place entirely within the semiconductor; this electric field decaying exponentically as it penetrates the semiconductor. Since the depth of penetration decreases as the conductivity of the semiconductor increases and since conductivity is proportional to the product of carrier mobility and concentration, it is desirable to keep the number of carriers in the semiconductor to a minimum inasmuch as it is preferable to have as high a carrier mobility as possible. Subject to these constraints there is no point in having the semiconductor film any thicker than the electric field penetration depth. As noted earlier, the thickness of a typical semiconductor channel (see FIGS. 2 and 3) is approximately three micrometers.

A lossy, polycrystalline piezoelectric thin film 12 approximately one micrometer thick is then deposited on the substrate 10, overlying the epitaxial semiconductor layer 11, with uniform spacing between the surface of the piezoelectric 12 and the electron drift region, inasmuch as the necessary interaction between the surface waves and the drift electrons cannot take place unless the drift electrons occupy a region in space closer than approximately one acoustic wavelength to the surface on which the surface waves are propagating.

By way of example, the piezoelectric 12 can be formed from polycrystalline aluminum nitride deposited by reactive sputtering or similar technique. The polycrystalline nature of the piezoelectric 12 prevents the structure from being orientation dependent, as is essential for proper guiding of the surface waves; i.e., the piezoelectric 12 is substantially isotropic over a dimension comparable to the width of a guide path, typically 10-3 centimeter, which requirement can be met if the microcrystallites comprising the piezoelectric film 12 have lateral dimensions less than approximately 10-4 centimeter and are randomly oriented in the plane of the film 12. Moreover, the grain boundary scattering effect of this polycrystalline material 12 provides the mechanism to attain the necessary level of acoustical energy loss, as defined hereinabove; i.e., the attenuation coefficient α for acoustic waves propagating on the surface of piezoelectric film 12 must be sufficient to insure that a signal So which is being maintained with unity net gain along a guide path by the interaction with adjacent drift electrons, with an interaction gain coefficient β, will be reduced to a level below that of the acceptable noise N at an adjacent guide path.

In other words, if the minimum separation between guide paths "A" and "B" (see FIG. 1) is designated as d, then the value of the signal Sac at one path or channel (e.g., path "B") due to a signal So in the adjacent path or channel (path "A") is given by the expression Sac = So e-α d, where e is the base of the natural logarithm. On the other hand, if the interaction gain coefficient β for a particular guide path is equal to the attenuation α, the general expression for the signal S in the guide path (e.g., path "A") is S = So e(β - α) X = So, where x is any distance along a guide path at which S is to be determined. If, as is proposed in accordance with the present invention, the signal Sac must be less than the acceptable noise N and since Sac = So e-αd, the minimum desired acoustical attenuation or loss factor α is given by the inequality α > Lne (So /N) 1/d. Expressed in decibels per centimeter, this minimun attenuation α for the piezoelectric 12 becomes α (db/cm) >10/d (Log10 So /N). Considering the other or maximum limit, in order to prevent significant attenuation along a guide path, the acoustical loss or attenuation α must be less than the maximum or saturation interaction gain βmax. achievable along the guide path. Obviously, the exact value of acoustical loss or attenuation α will vary from one guide structure to another depending upon the requirements of practice. A typical range for the factor α might be 50 db/cm < α < 70 db/cm; where,

So /N = 10, d = 0.2 cm, and βmax = 70 db/cm.

Fabricated on the upper surface of the piezoelectric thin film 12 are two pairs of interdigital transducers 13a, b and 14a, b, of well-known design, two of which, 13a, b, are connected to receive associated RF input signals and the others of which, 14a, b, are connected to suitable RF output circuitry. The transducers 13a, b converts the RF input signals into corresponding acoustic surface waves applied to the surface of the piezoelectric thin film 12; whereas, the transducers 14a, b receive or pick up the acoustic surface waves after they have propagated along guide paths "A" and/or "B" on the piezoelectric 12 and converts each of the acoustical signals back into an RF output signal which is time delayed from the associated input RF signal, by an amount dependent upon the propagation time of the acoustic surface wave in and therefore the propagation path length of the associated guide path on the piezoelectric.

It should be understood that the transducers are shown in the drawings as being of the well-known interdigital type merely for the purposes of illustration and that where it is necessary, in order to excite an acoustic wave, that the electric field applied to the piezoelectric 12 have a large component normal to the piezoelectric surface, one electrode could be placed under the piezoelectric film 12 and a counterelectrode placed on top of the film 12. Furthermore, if it is required that the transducer impedance match the impedance of the piezoelectric surface, the counterelectrode could be formed with a proper number of parallel elements, in much the same manner as interdigital transducers are impedance matched to a surface.

The piezoelectric film 12 is also provided with apertures 15a, b and 16a, b which permit connection of drift field voltage sources 17a and b to the end terminals of the epitaxial semiconductor channels (e.g., terminal contacts 18b and 19b for channel 11b in FIG. 2) in order to establish a drift electron flow from left to right in FIGS. 1 and 2, for example, in each of the epitaxial semiconductor layers. By way of example, the apertures or windows 15a, b and 16a, b might be formed by etching and the contact terminals (e.g., 18b and 19b) might be formed by diffusing a gold-tin film into the surface of the semiconductor material. Preferably, these contacts are ohmic in nature; i.e., non-rectifying.

In accordance with the well-known and above-mentioned acoustic surface wave amplifying principle, the drift electron flow within a semiconductor channel; e.g., channel 11b in FIG. 2, imparts energy to the acoustic surface wave introduced into polycrystalline piezoelectric thin film 12, at the associated input transducer 13a, b, only along the direction of electron flow in that semiconductor channel; i.e., as represented by the dotted propagation paths "A" and "B" in FIG. 1. In all other directions, the acoustic surface wave will die out, due to the lossy nature of the piezoelectric thin film 12, as defined hereinabove. Accordingly, by properly positioning the transducers 13a, b and 14a, b relative to the drift electron flow channels, as shown in FIGS. 1 and 2, the surface waves injected at input transducers 13a and b will propagate or be guided along paths "A" and "B" and be received by the output transducers 14a and b respectively. As noted above, the acoustical loss or attenuation factor α for the piezoelectric 12 is such that good signal isolation is maintained between the adjacent guide paths "A" and "B".

It will thus be noted that the configuration of a semiconductor drift electron channel defines the shape of the associated guide or propagation path for the acoustic surface waves injected at input transducer 13a or b, provided of course that the polycrystalline piezoelectric material of film 12 is isotropic in the plane of the film so that the interaction between the surface waves and the drift electrons in the semiconductor is independent of the direction of surface wave propagation, as proposed in accordance with the present invention. Therefore, it should be obvious that any desired configuration or length of propagation path for the acoustic surface waves may be obtained by merely changing the configuration of the associated epitaxial conducting channel. For example, the epitaxial semiconductor layer may be of the serpentine configuration shown at 20 in the embodiment of FIG. 4 and thereby cause the input acoustic surface waves injected into the lossy thin film 21, at transducer 22, to follow a relatively long, winding propagation path back to the output transducer 23. In the embodiment of FIG. 4, the drift field voltage source (not shown) would be connected, via apertures 24 and 25, to the end terminals 26 and 27 of the drift electron channel 20 shown in FIG. 4.

In view of the foregoing discussion, it will be appreciated by those skilled in the art that the substrate material can support an entire network of drift electron flow channels or paths and that the acoustic surface waves can be routed along the lossy piezoelectric thin film, as desired, by selective application of drift field voltages to the appropriate semiconductor path or paths. In other words, where interaction between the drift electrons and the acoustic surface wave occurs, the surface wave propagates. Otherwise, it dies out due to the lossy nature of the piezoelectric thin film along which it is propagating. This, coupled with the facts that the acoustic surface waves will only propagate in the direction of the drift electron flow and that drift electron flow direction is controlled by the choice of polarity of the drift field voltage source, permits various manners of processing of the acoustic surface waves to be performed on the piezoelectric thin film, so that maximum utilization of the surface area of the structure is made possible, in accordance with the present invention.

One manner of selectively routing or guiding the acoustic surface waves is illustrated in FIG. 5 of the drawings. More specifically, the semiconductor drift electron material is disposed on the substrate 28 in a Y-network configuration having a single channel portion 29 which leads into a pair of branch channel portions 30 and 31. The piezoelectric thin film 32 is formed with apertures 33, 34, and 35 which permit connection of a drift field source to the ends of the semiconductor channels 29, 30 and 31 respectively, as will be described in more detail shortly, and also transducers 36, 37 and 38 which are aligned with the channels 29, 30 and 31 respectively.

In FIG. 5, one side of a drift field voltage source 39 is connected to end terminal 40 of semiconductor channel 29; whereas, the other side of the source 39 is adapted to be selectively connected to the end terminals 41 and 42 of channels 30 and 31 respectively, in accordance with the position of switch 43. For simplicity, these connections from the source 39 to the semiconductor channels 29, 30 and 31 have not been illustrated as being made through apertures 33, 34 and 35 in the piezoelectric 32.

In operation, an input RF signal could be applied to transducer 36 and thereby excite an acoustic surface wave in the piezoelectric 32. This surface wave will then be selectively routed or guided to branch channel 30 or 31, depending upon to which of these branch channels the switch 43 connects the other side of drift field source 39; e.g., if the terminal end 42 is connected to the source 39, the acoustic surface wave will propagate along channel 29 and into branch channel 31 where it will be received by transducer 38.

It should be apparent from the above that much more complex processing of an acoustic surface wave (and therefore the RF signal) can be performed, in accordance with the present invention, than was described above in connection with FIG. 5. For example, by controlling the polarity of energization of the drift electron channels or paths, in addition to which channel or path is energized, the direction of surface wave propagation can also be controlled. Moreover, the drift field source shown throughout the drawings can be either simply a battery or some other potential source involved in the over-all processing arrangement.

Simplified circuitry for controlling the polarity of energization of the drift electron conduction channels 11 (e.g., see FIG. 2) is illustrated in FIG. 6 and comprises a polarity reversing switch 44 which is connected between a drift field source 17 and the associated terminals 18 and 19 of a conduction channel 11. In one position of switch 44, drift electron flow occurs from terminal 18 towards terminal 19 and the acoustic surface waves can therefore propagate from transducer 13 towards transducer 14; whereas, with switch 44 reversed, the drift electron flow is towards terminal 18 and therefore acoustic wave propagation can occur only from transducer 14 towards transducer 13.

Various other modifications, adaptations and alterations of the present invention are of course possible in light of the above teachings. It should therefore be understood at this time that the invention defined by the appended claims may be practiced otherwise than as specifically described hereinabove.