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| Project: |
Voltage-Controlled
Delay Lines using Interdigital BST Varactors |
| Investigators: |
Bruce Liu, Troy
Taylor, Pete Hansen, Jim Speck, Baki Acikel, Bob York |
| Funded
by: |
DARPA FAME
Program |
Description:
UCSB has pioneered the development of
monolithic VCDLs that are specifically optimized for low insertion
loss (see Nagra,
MTT Sept 99). The BST interdigital
varactor topology allows for an exceedingly simple and low-cost
implementation of the VCDL, requiring minial materials and fabrication
complexity.  The
interdigital varactor structure is a single, top-layer metalization
scheme deposited directly on the thin-film ferroelectric layer, which
in turn is directly deposited on a dielectric substrate. Since
the BST is grown directly on the substrate (rather than a refractory
base electrode), different microstructural and electrical
characteristics are expected. This topology also allows for the
use of high conductivity electrodes for minimizing series resistance
in the varactor. The tradeoff is a lower tunability as compared
with MIM (parallel-plate) structures, and high control
voltages.
Our work concentrates on low-cost glass and sapphire
substrates. To minimize the control voltage, we use our i-line
stepper for 1 micron gaps in the interdigital structure, leading to a
typical control voltage range of 0-90V for 100nm BST films (50%
tunability). An X-band VCDL (shown at right) has been
fabricated with >360 degress of differential delay and a 53o/dB
figure of merit. This circuit uses 98 individual BST varactors.
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| Viewgraphs: |
360o
X-band Interdigital VCDL |
|
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Papers:
 | Y.
Liu, A.S. Nagra, E.G. Erker, P. Periaswamy, T.R. Taylor, J.S. Speck and R.A.
York, “BaSrTiO3 Interdigitated Capacitors for Distributed Phase
Shifter Applications”, IEEE Microwave Guided Wave Lett, vol.
10, pp.448-450, Nov 2000 |
| A.S. Nagra and R.A. York, "Distributed
analog phase shifters with low insertion loss", IEEE Trans.
Microwave Theory Tech., vol. 47, no. 9, pp. 1705-1711, September 1999. |
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