Tech Briefs

Patterned Ferroelectric Films for Tunable Microwave Devices

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.

Figure 1. A Tunable Microwave Device is exemplified here as a one-pole microstrip filter with etchedferroelectric layer.
If the width of the ferroelectric film in such a device is reduced at one or more locations, then both the microwave field and any applied DC bias (tuning) electric field become concentrated at those locations. The magnitudes of both the permittivity and the dielectric loss of a ferroelectric material are reduced by application of a DC field. Because the concentration of the DC field in the constriction(s) magnifies the permittivity- and loss-reducing effects of the applied DC voltage, the permittivity and dielectric loss in the constriction(s) are smaller in the constriction(s) than they are in the wider parts of the ferroelectric film. Furthermore, inasmuch as displacement current must flow through either the constriction(s) or the low-loss dielectric substrate, the net effect of the constriction(s) is equivalent to that of incorporating one or more low-loss, lowpermittivity region(s) in series with the high-loss, high-permittivity regions. In a series circuit, the properties of the low-capacitance series element (in this case, the constriction) dominate the overall performance. Concomitantly, the capacitance between the metal terminals is reduced.

Figure 2. A Coupled Section of the Filter shows etched ferroelectric layer.
By making the capacitance between the metal terminals small but tunable, a constriction increases the upper limit of the frequency range amenable to ferroelectric tuning. The present patterning concept is expected to be most advantageous for devices and circuits that must operate at frequencies from about 4 to about 60 GHz. A constriction can be designed such that the magnitude of the microwave electric field and the effective width of the region occupied by the microwave electric field become functions of the applied DC electric field, so that tunability is enhanced. It should even be possible to design the constriction to obtain a specific tuning-versusvoltage profile.

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.