Narrowband Barium Strontium Titanate (BST) Tunable ... - IEEE Xplore

Narrowband Barium Strontium Titanate (BST) Tunable Bandpass Filters at X-band Zhiping Feng1, Wael M. Fathelbab2, Peter G. Lam3, Vrinda Haridasan1, Jon-Paul Maria3, Angus I. Kingon4, and Michael B. Steer1 1

Department of Electrical & Computer Engineering, North Carolina State University, Raleigh, North Carolina, 27695-7911, USA. 2 Department of Electrical and Computer Engineering, South Dakota School of Mines and Technology, Rapid City, South Dakota, 57701, USA. 3 Department of Material Science & Engineering, North Carolina State University, Raleigh, North Carolina, 27695-7914, USA. 4 Department of Engineering, Brown University, Providence, Rhode Island, 02912, USA. Abstract — A 3rd-order combline tunable filter with very narrow bandwidth at X-band is implemented utilizing ferroelectric Barium Strontium Titanate (BST) interdigitated varactors. The filter and its varactors are integrated on a ceramic substrate together with a simple resistive biasing circuit. Upon the application of a bias voltage ranging from 0 to 90 V the passband of the filter tuned from 8.127 to 9.973 GHz while maintaining a fractional bandwidth of approximately 4.8-5.9 %. Over the tuning band the passband insertion loss varied from 10.7 dB to 7.5 dB while the return loss was better than 15 dB. The tuning ratio is 23 %. Index Terms — Barium Strontium Titanate, ferroelectrics, tunable combline filter, interdigitated capacitors.

I. INTRODUCTION Modern telecommunication systems require fully reconfigurable and/or tunable RF frontends. This fact has created several challenges requiring the need for novel microwave circuits as well as the need for multifunctional materials to perform such tasks. The operation of all wireless telecommunications systems relies on the unique properties of the electromagnetic spectrum for signal propagation. However, to make the most efficient usage of this finite resource RF frontends are in urgent need of high performance microwave filters that operate at multiple bands while exhibiting low insertion loss. In recent years, the usage of frequency-agile materials based on ferroelectrics has been the focus of attention [1-6]. Ferroelectrics have the unique properties of changing their dielectric permittivity upon the application of an electric field. Furthermore, the ease of depositing such materials on microwave substrates to create ferroelectric varactors has made them the prime candidates for reconfigurable system applications. Offering very high tuning rates, low loss, and ease in manufacturing, Barium Strontium Titanate (BST) thin films with the composition of (Ba0.6Sr0.4)TiO3 has been the most commonly utilized ferroelectric for tunable microwave circuits.

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The bandwidth requirement of tunable bandpass filters in many applications is around 500 MHz to 1 GHz determined by the capabilities of available analog-to-digital converters with sufficient bit-resolution (e.g. 10 to 12) for subsequent additional frequency selectivity through digital signal processing. Most tunable BST filters have been, so far, achieving passbands with fractional bandwidths around 1214% [5, 6]. For the first time, this paper reports on the design, fabrication, and characterization of tunable BST bandpass filters realizing 5 % fractional bandwidths at X-band. It is well known that there is a trade-off between the fractional bandwidth of a microwave filter and the finite Q of its distributed resonators. The relationship between the insertion loss and bandwidth of the filters has been studied through simulation and also measurement. The filter is implemented on an alumina substrate with integrated BST-based varactors using a single-step and two step metallization process. Measured results are presented. II. DESIGN OF THE TUNABLE BANDPASS FILTER WITH BST GAP CAPACITORS Microwave bandpass filters are typically designed to have a Chebyshev response to achieve high selectivity while maintaining low in-band insertion loss. The 3rd-order prototype network utilized in this design is shown in Fig. 1. Such a prototype provides all the necessary data about the inter-resonator couplings and can easily be converted into the combline filter topology using standard techniques [7]. The lengths of the three resonators is flexible to choose but in order to achieve high passband tunability each resonator was set to be a quarter wavelength long at approximately twice the center frequency of the filter in the tuned state. This optimum choice of the electrical length leads to a high tuning range for a small capacitance variation as well as yielding a miniaturized overall circuit size [7]. The physical layout of the filter upon appropriate impedance scaling is shown in Fig. 2. Simply by altering the values of the loading capacitors upon the application of an electric field the frequency tuning of the

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passband is achieved. This leads to the additional requirement of the design of an integrated biasing circuit which the tunable filter must accommodate for the tuning of its varactors with minimal effect on its overall frequency performance. As shown in Fig. 2, three high impedance bias lines were connected through surface-mounted 3 k resistors to a bias supply. The layout of the filter shown in Fig. 2 also demonstrates that each capacitor (being implemented as a gap capacitor) was split into two to allow the connection of the bias lines.

Fig. 1. A lumped element bandpass prototype network in a 50  system with passband centered at 8.5 GHz with 5 % fractional bandwidth. (C = 7.726 pF, L1 = L2 = 0.0454 nH, and K = 52.732).

The 15 mil (381 μm) thick alumina substrate with a dielectric constant of 9.9 and loss tangent of 0.0002 has been chosen for this work because of its good RF performance and the matching of thermal expansion coefficients with BST layer. Bias line

L

S

S

Fig. 2. Layout of the fully integrated tunable filter on an alumina substrate with BST-based gap capacitor and a biasing circuit. L is the length of resonators and S is the space between resonators. The insert shows the picture of a BST gap capacitor.

The physical layout of the filter was obtained through the usage of ADS1 leading to the following dimensions: L 55 mil; Z 12 mil; Z1 13 mil; S 32 mil ; where, L is the length of the resonator, the width of center resonator is Z1 and the width of all others is Z ; and S is the space between resonators. The diameter of each via was 150 μm and the value of each tuning gap capacitors was 0.85 pF with the width of 505 μm and a gap of 4 μm. 1 2

Coorstek, Golden, Colorado Advanced Design System (ADS), Agilent Technologies, Inc., Palo Alto, CA.

III. FABRICATION OF THE TUNABLE BST FILTER Coorstek1 alumina substrates with both sides polished were used in the study. The first step in the fabrication process was the formation of the openings of the vias which were subsequently filled with a proprietary gold-based compound compatible with subsequent thermal processing. The BST thin-films were deposited on the alumina substrate using a radio frequency magnetron sputtering technique. A deposition time of 60 minutes resulted in a film thickness of 0.6 μm. Post deposition anneal was done ex-situ in air at 900 °C for 20 hours to crystallize and densify the BST films. Detailed deposition conditions are described elsewhere [8]. A hysteresis test was performed to confirm that the thin film was in the paraelectric phase. The crystalline structure of the film was investigated using a diffractometer with a CuK radiation source and showed a fully crystalline BST perovskite structure. Photolithography was used for the subsequent steps for metal patterning. Two different metals were used for the fabrication of the device including a final copper plating step. Due to the large difference in dimensions between the gaps in BST gap capacitors and the overall device, it becomes difficult to pattern the entire device in one single step, thus a two step process was used to outline the resonators and feed lines and the BST gap capacitors separately. The first layer of metal used was silver; silver provides the lowest resistivity of all known metals and it can be etched easily in a solution of methanol, ammonium hydroxide and hydrogen peroxide. The silver layer was deposited by sputtering and then annealed in air at 450 ºC for 15 mins. The annealing step was necessary to reduce the resistivity of the metal as well as to increase the adhesion of the silver to the substrate. After the annealing step, photolithography is used to pattern the whole device except the BST gap capacitors, and then the uncovered surface is etched with the solution previously mentioned. Once the silver layer was patterned, a second lithography step was used to pattern the BST gap capacitor. A 70 nm of chromium and 350 nm of gold were deposited by sputtering, and a liftoff step in acetone was used to obtain the BST gap capacitors. Finally a copper plating step was used to increase the thickness of the metal to about 3 μm, which is well beyond the recommended 3 skin depths. IV. BANDWIDTH AND INSERTION LOSS When the Q of the resonators of the bandpass filter is not infinite, there is a trade-off between the fractional bandwidth and insertion loss of a microwave filter. In this section, the relationship between insertion loss and bandwidth of filter will be discussed for the combline filter through simulation and experimentation. In the 3rd-order combline tunable filter, the coupling between the three resonators determines the bandwidth of the filter, when the coupling is reduced, the bandwidth is also reduced. The coupling is affected by the

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space and connection between the resonators. Sonnet 2 has been used for the simulation of the filters with different space between the resonators (S). Fig. 3 shows the simulation results with difference coupling spaces when keeping all other factors the same, such as Q-factor of BST capacitors, metal loss and substrate loss. While bandwidth is reduced from 0.95 GHz to 0.47 GHz by increasing the coupling spacing 15 mil (381μm), the insertion loss increased from 5.4 dB to 9.7 dB.

For 0.5 GHz bandwidth, 8-10 dB insertion loss is expected for this filter.

Fig. 5. Measured results of the test BST combline filter with/without crossbar. Red line is for the filter with the crossbar, the blue line is for the same filter with the crossbar removed.

IV. MEASUREMENT RESULTS AND DISCUSSION

Fig. 3. Sonnet simulation results of the combline filter with different coupling space between the resonators. Pink line with dots for the filter with 24.1 mil coupling space, solid blue line for 27.1 mil, red line with less dots for 34.1 mil, and dash green line for 39.1 mil.

The crossbar between the resonators also increases the signal coupling from one resonator to the other, so crossbar significantly affects the bandwidth and insertion loss. To study the effect, a test BST combline filter has been fabricated with crossbar, which can be removed easily by probe. Fig. 4 show the photo of the test figure before and after the crossbar removed.

(a)

(b)

Fig. 4. Photos of the test BST combline filter before (a) and after (b) the cross bar removed.

The measured results of the test BST filter without/with the crossbar are shown in Fig. 5. The measured insertion loss of filter increased about 4.8 dB from 6.94 dB to 11.29 dB and the bandwidth is reduced from 1.15 GHz to 0.57 GHz after the crossbar removed. The measured results and the simulation results both confirmed that the bandwidth of the filter has a big impact on the insertion loss when the filter is not lossless. 2

The Sonnet High Frequency Electromagnetic Software 8.0

HP 8510C vector network analyzer has been employed to measure S-parameter of the filters with 250 μm pitch microcoax GSG probes and SOLT calibration from GGB industries3. The measured responses of the filters from the one step fabrication are shown in Fig. 6. The measured center frequency is tuned from 7.678 GHz to 9.923 GHz for bias voltage from 0 v to 60 v. The tuning ratio is about 29.2%. The insertion loss varied from 13.77 dB at 0v to 7.22 dB at 60v. The all of return losses are better than 12 dB. The 1-dB bandwidth of the filter is 0.48 GHz close to the design requirement of a 5% fractional bandwidth. The filter shows excellent tuning ratio but the insertion loss is higher than what expected. To reduce the insertion loss further and increase the yield, the two step fabricating process is used to fabricate the second filter sample. The measured tuning response of this filter is shown in Fig. 7, where the bias of the varactors is varied from 0v to 30v and 90v. The center frequency of the filter is tuned from 8.127 GHz to 9.973 GHz for 90v bias voltage, and the tuning ratio is about 22.7%. The insertion loss is from 10.68 dB at 0v to 7.452 dB at 90 v. All of the return losses are better than 15 dB. The out-of band rejection at 1 GHz from the bandedge was more than 26 dB at frequencies below the passband and more than 40 dB above the passband. Center frequencies and insertion losses at different bias voltages are given in. 1-dB bandwidth is 0.478 GHz with zero bias voltage and the fractional bandwidth is 5.8 %, while 1-dB bandwidth is 0.49 GHz at 90 v and the fractional bandwidth is 4.9 %, matching the requirement of a 5 % fractional bandwidth.

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ACKNOWLEDGEMENT This material is based upon work supported in part by the U.S. Army Research Office as a Multi-disciplinary University Research Initiative on Multifunctional Adaptive Radio Radar and Sensors (MARRS) under grant number DAAD19-01-10496 and by the US Army Communications and Electronics Command as a DARPA Grant through Purdue University under grant number DAAB07-02-1-L430. Fig. 6. Measured tuning of the BST tunable filter with one step fabrication. The arrow shows the direction of movement of S21 when the bias voltage increasing from 0 to 60 v by 10 v step.

Fig. 7. Measured tuning of the BST tunable filter with two step fabrication. The arrow shows the direction of movement of S21 when the bias voltage increasing from 0 v to, 30 v and 90 v.

V. CONCLUSION A fully integrated BST narrowband tunable filter at X-band was designed, fabricated and tested on an alumina substrate. The filter comprised three transmission line resonators loaded by BST-based varactors that are biased by a supply voltage through a simple biasing circuit. Two demonstrations of the BST tunable filters at X-band have been characterized. The improvement of the insertion loss has been obtained by two step fabrication. The filter fabricated using two step process shows less than 0.5 GHz bandwidth over a 22.7 % tuning range with a maximum bias voltage of 90 v, when the passband insertion loss varied from 10.68 dB to 7.45 dB as the center frequency of the filter varied from 8.127 to 9.97 GHz. This is the first demonstration of a fully integrated BST tunable bandpass filter that operates at 8.13 to 9.9 GHz with 5.8 % narrow bandwidth.

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