Low Voltage Tunable Band Pass Filters Using Barium ...

Integrated Ferroelectrics, 112: 1–7, 2009 Copyright © Taylor & Francis Group, LLC ISSN 1058-4587 print / 1607-8489 online DOI: 10.1080/10584587.2009.484656

Low Voltage Tunable Band Pass Filters Using Barium Strontium Titanate Parallel Plate Capacitors

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T. S. Kalkur,1,∗ N. M. Sbrockey,2 G. S. Tompa,2 Pamir Alpay,3 J. E. Spanier,4 E. M. Galow,4 and M. W. Cole5 1

Microelectronics Research Laboratories, University of Colorado at Colorado Springs, Colorado Springs, CO 80933-7150 2 Structured Materials Industries, Piscataway, NJ 08854-3723 3 University of Connecticut, Storrs, CT 06269 4 Drexel University, PA 19104 5 US Army Research Lab, APG, MD 21005

ABSTRACT In this paper, we report the results of third order combline inter-digited band pass filters implemented with parallel plate barium strontium titanate (BST) capacitors on sapphire substrates with platinum electrodes. The frequency characteristics of the filters were simulated using Sonnet software package. The resonators were implemented on FR-4 substrates using a LPKF rapid prototyping machine. The resonators were loaded with BST capacitors prepared on sapphire substrates by metalorganic decomposition (MOD). The measured center frequency of the filters was 1.35 GHz with a 3 db bandwidth of 0.15 GHz for zero bias voltage to the ferroelectric capacitors. The center frequency can be tuned from 1.35 GHz to1.45 GHz for an applied tuning voltage of 3 Volts.

INTRODUCTION Frequency adaptive circuits are becoming extremely important for present and future communication systems. Filters are one of the most important circuit blocks for communications [1–3]. Tunable filters offer the opportunity of replacing a bank of filters with a single filter to cover wide frequency range. In addition, tunable filters gives us the possibility of trimming the characteristics after fabrication and packaging. A variety of tunable devices such as p-n Received September 27, 2009; in final form December 13, 2010. ∗ Corresponding author. E-mail: [email protected] 1


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junction varactors, GaAs MESFETs, and MEMS varactors have been used to implement tunable filters [4–8]. P-N junction based varactors cannot be fabricated on a variety of substrates such as sapphire and alumina, they can be tuned only in the reverse bias and they are supposed to have significant junction noise. Although MEMS based varactors offer high “Q” factors, they are slow to tune and packaging is more challenging. Varactors based on ferroelectric capacitors such as Barium Strontium Titanate (BST) offer a unique opportunity to implement tunable filters because of their tunable high dielectric constant and they can be integrated on most of the substrates such as sapphire and alumina [9, 10]. Their tuning characteristics are independent of the polarity of the applied bias. These capacitors are expected to have low noise because of the absence of p-n junction. Most of research work reported in the literature on tunable filters were based on Barium Strontium Titanate inter-digited capacitors. These capacitors need large tuning voltages in the range of 50 to 200V. Parallel Plate BST capacitors offer a great advantage of tuning the capacitors within 3 to 5 V. These voltages are compatible with CMOS technology. In this paper, we report the results of tunable band pass filters implemented with tunable BST capacitors.

FABRICATION OF BST CAPACITORS The tunable BST capacitors were fabricated on sapphire substrates. The bottom electrode 30 nm titanium/200 nm of Platinum was deposited by DC magnetron sputtering. The bottom electrode was patterned by photolithography and ion-milling. BST film was deposited by spin on metal-organic decomposition(MOD). The films were annealed in oxygen atmosphere at a temperature of 800◦ C for 30 minutes. Top electrode platinum of thickness 200 nm was deposited by DC magnetron sputtering. The top electrode was patterned by standard photolithographic technique and ion milling. The devices on the saw wafers were scribed with diamond saw. The cross-sectional view and plan view of the BST capacitor is shown in Fig. 1. The capacitance versus voltage characteristics of the capacitors were determined by HP 4275 LCR meter. Figure 2 shows the typical capacitance characteristics of a BST capacitor at a measurement frequency of 1 MHz with a small signal voltage amplitude of 100 mV. The capacitance of the capacitor changes from 0.96 pF at zero bias to 0.6 pF at 3 V bias voltage.

DESIGN OF TUNABLE FILTERS The resonators were loaded with tunable capacitors. The design parameters are: a) design frequency f0 , b) Resonator critical length (θ 0 ), c) instantaneous bandwidth and d) tuning device capacitance Cso at f0 .

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Figure 1. Structure of fabricated BST capacitor. (a) Cross-sectional view b) Plan view.

The design requirements for the filter, center frequency, f0 = 1.5 GHz, bandwidth, 10% of center frequency, 150 MHz and ripple in the pass band 1 db. To minimize the complexity of the filter, third order filter was chosen. The Chebyshev coefficients for the third order filter are: g0 = 1, g1 = 1.593, g2 = 1.0967, g3 = 1.5963, g4 = 1. The line width and separation of microstrips were calculated by US Microwaves coupled microstrip synthesis program. The width of the resonators, W = 1.87 H, and separation S = 1.17 H, where H is the thickness of the dielectric. 1.00E-12 9.00E-13

Capacitance in Farads


Low Voltage Tunable Band Pass Filters

-3

-2

8.00E-13 7.00E-13 6.00E-13 5.00E-13 4.00E-13 -1 0

1

2

3

Applied Bias in volts

Figure 2. Capacitance vs voltage characteristics of a typical capacitor.


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Figure 3. Structure of the tunable filter used for simulation in sonnet.

The full wave electromagnetic simulation was performed on this filter structure using SONNET. Figure 3 shows the two dimensional and three dimensional structure used for the simulation. In addition to tunable ferroelectric capacitors, fixed capacitors to apply the bias voltages to tunable capacitors are also included in the simulation. Figure 4 shows the simulated response of the filter obtained by SONNET. The center frequency of the simulated filter is 1.91 GHz, lower 3 db frequency is 1.835 GHz and upper 3db frequency is 2.05 GHz. Frequencies corresponding to 20% insertion loss in pass are 1.57 GHz and 2.35 GHz. The maximum return loss in pass band is greater than 25 db. The layout pattern of the filter simulated for optimized design in SONNET was down loaded to LPKF rapid prototyping machine to define patterns on FR-4 substrates. Ferroelectric varactors, bias capacitors and SMA connectors were assembled on this FR-4 board. The photograph of the tunable filter is shown in Fig. 5. The tunable filter was characterized for return loss and insertion loss using Agilent’s Network analyzer. Figure 6 shows the insertion loss of the filter at bias voltage of 0 V and 3 V. For an applied voltage of 0 V, the center frequency is 1.35 GHz and 3db frequency band width is 150 MHz. The minimum insertion loss in the pass band is about 5 db. For a bias voltage of 3 V, the center frequency has shifted to 1.45 GHz with a 3 db band width of 140 MHz. The simulated center frequency is higher than the measured center frequency and this is attributed to parasitics



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Figure 4. Simulated S11 (return loss) and S21 (insertion loss) of tunable filter. (See Color Plate I)

Figure 5. Prototype of the tunable filter.

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T. S. Kalkur et al. Frequency in Hz

0


Insert ion Loss db

5E+08

9E+08

1.3E+09

1.5E+09

-10 Bias voltage 0V

-2.0

Bias voltage 3V

-30

-4.0

Figure 6. Insertion loss of the tunable filter implemented with ferroelectric varactor. (See Color Plate II)

associated with the indium ribbons used in the assembly of discrete ferroelectric capacitors on the RF bread board. CONCLUSIONS Parallel plate BST capacitors were fabricated on sapphire substrates to fabricate tunable capacitors for tunable band pass filters. The capacitance of these capacitors can be tuned from 0.96 pF to 0.6 pF with a DC bias of 3 V. These tunable capacitors were integrated on coupled resonator band pass filters fabricated on FR-4 board. The center frequency of the filter could be tuned from 1.35 GHz to 1.45 GHz with a DC bias of 3 V. ACKNOWLEDGMENT This project was supported by STTR Phase II from Army Research Labs, W911NF-08-C-0124.

REFERENCES 1. G. Matthaei, L. Young, and E. M. T. Jones, Microwave Filters, Impdedance—Matching Networks and Coupling structures. Archtech House, 1980.



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