Wave transmission lines and networks – Coupling networks – Delay lines including elastic surface wave propagation means
Reexamination Certificate
2001-07-13
2003-05-06
Summons, Barbara (Department: 2811)
Wave transmission lines and networks
Coupling networks
Delay lines including elastic surface wave propagation means
C333S153000, C333S193000, C331S154000, C257S014000
Reexamination Certificate
active
06559736
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to tunable surface acoustic wave technology and pertains more particularly to ZnO based monolithically integrated tunable surface acoustic wave (MITSAW) technology and electronic and photonic systems employing MITSAW devices.
BACKGROUND OF THE INVENTION
Basic SAW Technology
SAW devices have been widely used for signal processing since 1964, when the interdigital transducer (IDT) was introduced. The basic principle of a SAW device is to apply an input IDT and an output IDT in mutually spaced relation to a piezoelectric member, to apply an electrical signal to the input IDT, thereby causing a surface acoustic wave to propagate in the piezoelectric member, and to obtain the electrical signal generated in the output IDT by the propagated surface acoustic wave. The time for the propagated wave to travel from its generation at the input IDT to its arrival at the output IDT constitutes a time delay and the piezoelectric member constitutes a delay path.
Tunable SAW Technology Using Quantum Well
A problem in SAW technology to date has been the lack of tunability of acoustic velocity, which would allow tuning of the center frequency of the SAW filters. A conductive element near the piezoelectric surface changes the acoustic velocity by coupling with the electric fields of the acoustic wave. Ideally, tunability of the acoustic velocity is limited by the electromechanical coupling coefficient of the piezoelectric material. Early attempts include the use of a semiconductor film in close proximity to the piezoelectric surface. The variable finite conductivity of the semiconductor interacts with the electric fields associated with the acoustic wave, and slows the wave. An improved approach is to use a two dimension electron system (2DES) to tune the acoustic velocity.
More particularly, the acoustic velocity of the propagated surface acoustic wave is controlled by changing the conductivity of the 2DES through reverse biasing of the quantum well. When the quantum well is depleted, the acoustic wave propagates at the near open-circuit velocity of the piezoelectric layer and its underlying substrate system. On the other hand, when the quantum well is forward biased, the acoustic velocity approaches short-circuit velocity.
In this reported device, a GaAs substrate was used as a piezoelectric medium, and the 2DES was formed in an Al
x
Ga
1−x
As quantum well. As the piezoelectric coupling of GaAs is very small, the reported tunability range was <0.1%. An alternative hybrid GaAs—LiNbO
3
device where the 2DES was formed in a GaAs quantum well, which was epitaxially lifted off, and bonded to the LiNbO
3
substrate. The effective coupling coefficient of this structure was reported to be 3.5% and a velocity tunability of 1.5% was reported. However, the epitaxial liftoff technology is very complicated, with low. yields and poor reliability; therefore, it is unsuitable for commercial applications.
SAW Filters
A known SAW filter includes a piezoelectric layer disposed on an underlying substrate, an input IDT and an output IDT A varying electrical signal source applies signals to the input IDT and a load is connected to the output IDT
16
.
Modern communications systems are increasingly moving to higher data rates to accommodate the demand for enhanced capabilities, such as data and multimedia communications. Coupled with an expanding user base, this increasing data rate translates to larger bandwidths and higher frequencies in the engineering requirements. High frequency, low loss, low power, miniaturized and integrated filters, for example, are required for the exponentially expanding wireless communications industry. As multi-functional systems are deployed, adaptive and programmable filters are needed.
While digital filter technology is advanced, digital signalprocessors consume high power, are limited to the lower end of the frequency spectrum, being subject to circuit speed limitations and their performance is dependent on the analog preprocessing and post-processing circuits. Analog devices and circuits, which can achieve the aims of high frequency, low loss, low power, miniaturized and adaptive filtering, are needed as low cost, light weight, high performance alternatives.
SAW devices have been widely used in communications systems. They are easy to fabricate and are low cost, light weight, and very versatile devices. High performance filter specifications can be realized, using space-domain sampling, as opposed to time domain sampling in digital techniques. Very complex filter functions can be implemented, with independent design of frequency and phase response.
The major limitation of conventional SAW filters is that their frequency and phase response is set at the time of their design, and cannot be changed during operation. However, in many modem communication systems adaptive signal processing is desired for increased signal to noise performance, and security concerns. Further, tunability of the time or frequency domain response is desirable for the communications system to adapt to its operating environment.
Two distinct types of surface acoustic wave filters have emerged to meet these demands. The first type is the programmable SAW filter, which changes the filter parameters such as center frequency, band width, and pass band shape. This type of SAW device includes filter banks, multiple interdigital transducer (IDT) filters and electrode configurable devices. The second type of SAW filter was created to meet the tunability demands. Early designs were based on the voltage-controlled width of a depletion layer in a semiconductor bulk diode applied, with the input and output IDTs to the piezoelectric member. However, the current programmable SAW filters are large in size, complex and costly with relatively high insertion loss, which render them unsuitable for many applications.
SAW Voltage Controlled Oscillators
In providing SAW voltage controlled oscillators, the art has typically placed a SAW device in a feedback path, e.g., connecting one SAW IDT to the input terminal of an amplifier and connecting the output terminal of the amplifier to the other SAW IDT.
The frequency tuning range of the voltage controlled oscillator (VCO), which is built on the given piezoelectric material, is inversely proportional to the delay time of the SAW device. The shorter the delay time of the SAW, the higher is the frequency tuning range. Heretofore known SAW VCOs are seen as either having a less than desired operating frequency and frequency tuning range due to small electromechanical coupling coefficients and low acoustic velocity, such as GaAs/Al
x
Ga
1−x
As devices, or having complicated structures with low yields and poor reliability due to the hybrid processing technology, such as GaAs—LiNbO
3
.
Zero-power Remote SAW Wireless Sensors
Zero-power passive wireless sensors are important for environmental monitoring and identification applications. Their principle advantage is that they do not need a power source, as they derive energy from an interrogation signals. They are particularly attractive for hazardous environments, such as interiors of engines, chemical reaction chambers, high-voltage lines and the like.
Presently there are two known types of zero-power passive SAW wireless sensors. The first type consists of GaAs/Al
x
Ga
1−x
As quantum well and SAW structures, which are seen having very small electromechanical coupling coefficients with accompanying low acoustic velocity, resulting in a small dynamic range for wireless sensor read-out. The second type, which consists of GaAs/LiNbO
3
hybrid structure, suffers from low yields, poor reliability and high cost due to complicated bonding technology.
SAW Optical Delay Lines
The optical delay line technology is used for optical signal processing. Presently known and cost-effective SAW optical delay lines also suffer from the limitation of less than desired electromechanical coupling coefficients. They are further limited in frequency of operation, not operable in the ultraviole
Emanetoglu Nuri W.
Lu Yicheng
Hoffmann & Baron , LLP
Rutgers The State University of New Jersey
Summons Barbara
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