There is an increasing need for acoustic components to be used in microwave communications and radar systems which acquire and process large amounts of signal data. These acoustic components require a wide bandwidth capability, a large storage delay time and a built in filter or coding function. A substantial number of current requirements for signal processing systems can be met by using tapped surface acoustic wave devices. Such devices can simplify processing circuitry and represent a reliable, low cost alternative to present electronic techniques.
Surface wave devices can be used to perform matched filter functions in communications and radar systems where it is desirable to ambiguously transmit data in the presence of noise and/or jamming signals. Fixed coded devices are now in common usage where a series of phase interdigital metallic electrodes are implemented on a piezoelectric substrate for pseudo random (PN) coding and decoding. Electronic switching and logic functions in lumped circuit and hybrid form have been incorporated with the fixed coded devices to develop fixed programmable surface wave encoders and decoders. While integrated circuits can be used in such hybrid configurations, the separate manufacture and interconnection of acoustic and electronic devices creates cost inefficiencies and reliability problems due to circuit complexity. Because the development trend in signal processing subsystems is toward a higher degree of component integration in a miniaturized form factor and ultimately requires production of substantial quantites, monolithic building blocks combining the acoustic encoding and decoding functions with electronic functions such as switching, mixing and amplification on a single substrate should lead to considerable cost savings, improved reliability and size reduction.
One approach which might be pursued and which is disclosed in this application is the use of a silicon substrate with sputtered zinc oxide for surface wave generation and silicon MOSFET structures for detection and switching.
The use of a single crystal silicon substrate with MOSFET detector geometry utilizing piezoresistive surface wave detectors and a piezoelectric film transducer offers several advantages. Silicon is a low acoustic loss material and mainstay of the semiconductor microelectronics industry. The feasibility of efficiently launching surface wave using piezoelectric film transducers on silicon has been demonstrated in the art. Moreover, silicon MOSFET device technology permits the development of the required high yield, large scale integrated detectors. Further, it is possible to have programmable phase codes stored on the same silicon chip, in a read only memory circuit, which are selected therefrom by input logic. It is also possible to introduce any desired phase code by other suitable means such as a digital shift register. By using monolithic integrated circuit fabrication, a bucket brigade or other type of charge transfer device may be used to input the phase control circuit to have both phase and amplitude programmablity functions. A device of this type would be extremely useful in line-of-sight microwave communication channels where propagation effects introduce amplitude as well as phase distortions to a PN modulated signal. By having amplitude and phase control, channel distortions could be minimized to enhance signal to noise ratio.