Patterned Ferroelectric Films for Tunable Microwave Devices
- Wednesday, 02 January 2008
Microwave performance is enhanced by appropriate patterning.
Tunable microwave devices based on metal terminals connected by thin ferroelectric films (see Figure 1) can be made to perform better by patterning the films to include suitably dimensioned, positioned, and oriented constrictions. The patterns (see Figure 2) can be formed during fabrication by means of selective etching processes.
The following observations regarding prior ferroelectric-based microwave devices and circuits constitute part of the background and impetus for the present patterning concept:
- The basic principle of design and operation of a ferroelectric-based microwave device calls for a continuous film of ferroelectric material that extends from one metal terminal to another on a lowloss dielectric substrate.
- The performances of conventional ferroelectric- based devices and circuits can be degraded by excessive losses and spurious resonances.
- Designers often seek to obtain linear tuning-versus-bias-voltage profiles. In general, the tuning-versus-bias voltage profile of such a device is difficult to control in the absence of suitable patterning. The desired linear profiles (more specifically, changes in frequency or phase proportional to changes in bias voltage) have not been observed.
- Ferroelectric materials are intrinsically lossy, and losses are especially pronounced in ferroelectric-based narrowband filters, in which resonant elements must be separated by large distances to obtain the necessary isolation. In a typical prior ferroelectric-based device, the electric field is distributed uniformly across the unpatterned ferroelectric film; hence, if such a film is part of a narrow-band filter, spanning the required large distance, and the loss can be unacceptably high.
- Heretofore, the high permittivities of ferroelectric materials have given rise to large capacitances that have been detrimental to performance at microwave frequencies.
This work was done by Félix A. Miranda of Glenn Research Center and Carl H. Mueller of Analex Corp.
Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-17411.