Hybrid Ferroelectric/Superconducting Microwave Circuits. Robert R. Romanofsky NASA Glenn Research Center Cleveland, OH

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1 Hybrid Ferroelectric/Superconducting Microwave Circuits Robert R. Romanofsky NASA Glenn Research Center Cleveland, OH Recent progress in processing and deposition techniques along with better modeling and novel circuit designs have resulted in thin film ferroelectric-superconductor and ferroelectric-metal components that rival the microwave performance of state-of-theart counterparts. Not long ago a coupled microstripline phase shifter patterned on a YBa 2 Cu 3 O 7-δ /SrTiO3/LaAlO 3 multilayer structure exhibited more than 400 degrees of contiguous phase shift with less than 5 db of insertion loss at Ku-band when cooled to 77 K. A figure of merit around 100 o /db was ultimately achieved. Similar devices were constructed using Ba x Sr 1-x TiO 3 films and metal electrodes for room temperature operation. The best figure of merit thus far is about 70 o /db. It is unclear what role surface effects from different electrodes play in determining performance. Attempts to reduce the high loss tangent of these thin films (compared to single crystals) have included annealing and the use of dopants. With Ba 1-x Sr x TiO 3 films at room temperature annealing seemed to have little effect in terms of the tunability to loss ratio of K-band phase shifters. However, closer to the Curie temperature there are very definite effects at low (i.e. MHz) frequencies. Annealing appears to increase the maximum dielectric constant by about a factor of 2, and also increases the tanδ to a lesser extent. The largest values of phase shift per insertion loss have been obtained from 1% Mn doped laser ablated films grown by the Naval Research Laboratory. As opposed to DRAM applications, for microwave circuits relatively thick films are required. The socalled dead-layers at ferroelectric interfaces may explain why some researchers have reported much lower values for peak dielectric constant than we routinely see with microwave devices. Besides thickness, film crystallinity directly influences tunability. But we consistently observe degradation in film crystallinity with increasing thickness. An accurate theoretical model of the ferroelectric coupled line phase shifters based on a variational solution for line capacitance and well-known coupled line theory has been developed. The role of film thickness is examined. Besides phase shifters a Ku-band voltage controlled oscillator based on a 3λ ring resonator patterned over a 2 µm thick SrTiO 3 film demonstrated a 5 % tuning range at 40 K. Other components that are under development include tunable bandpass filters intended for cryogenic operation and tunable microstrip patch antennas. A prototype 16- element phased array radar, based on Ba x Sr 1-x TiO 3 films, for potential automotive collision warning applications will also be discussed. And, finally, a design for a new type of scanning phased array antenna called the ferroelectric reflectaray will be presented.

2 Outline Background: Applications of High Tc Superconductivity and Ferroelectric Films to Satellite & Terrestrial Communications Coupled Microstrip Phase Shifters Using Au and YBa 2 Cu 3 O 7-δ Electrodes on SrTiO 3 and Ba x Sr 1-x TiO 3 Films A Theoretical Approach Based on a Variational Formulation of Line Capacitance Tunable Resonators and Voltage Controlled Oscillators Materials Issues: Reliability, Uniformity, and Cost The Prospect of Phased Array Antennas Based on Ferroelectric Technology

3 Lewis Research Center Direct Data Distribution (D 3) A Pathfinder for Commercial Communications Insertion D 3 Approach Hitchhike Experiment on Shuttle for NASA missions risk mitigation and commercial services demonstration Features Direct distribution of space data from LEO at 622 Mbps to end user or archive facility via terrestrial networks Commercially owned and operated space and ground segments at commercial frequency JSC Potential Enterprise Benefits Off-load TDRSS for Latency tolerant data delivery from LEO S/C Communications outage restoration Commercial service providers reduce cost 19 GHz MMIC Transmit Phased Array LeRC/Raytheon Cooperative Agreement with 50/50 cost sharing 155 Mbps Multi-channel Digital Encoder- Modulator LeRC/SICOM SBIR II and Space Act Agreement Cryoreceiver-based 0.9 m Tracking Earth Terminal LeRC integration of InP PHEMT LNA, HTS filter, and cryocooler technologies Direct Data Distribution 1.ppt

4 Lewis Research Center Cryoreceiver Cryogenically cooled receiver will achieve a noise temperature of ~ 150º K, a factor of ~ 4 reduction at 19 GHz, improving receiver performance by a factor of 4 (6db) 6db is very highly significant to communication system designers - Enables a smaller space (or Earth) antenna... or increased data rate... or lower power... HTS Filter InP LNA (PHEMT) Cold Finger To Down Convertors Orthomode Coupler Cassegrain Reflector Low Thermal Loss Waveguides Low Sidelobe Feed Horn Features Exploits three key technologies: Reliable cryocooler HTS Filter InP PHEMT low noise receiver (LNA) chip Other design features: Vacuum can with low thermal loss waveguides Low sidelobe feed horn Vacuum Can 1 W Cryocooler For further information: Contact Robert Romanofsky (216) A cryoreceiver is baselined for the D 3 Earth terminal - Allows smaller diameter reflector (0.9 m instead of 1.8 m) with wider beam width, simplifying tracking

5 Lewis Research Center X-Band Cryoreceiver Space Qualified X-band Hybrid Superconductor/Semiconductor Receiver [Cooperative development by NASA Lewis Research Center and the Jet Propulsion Laboratory for High Temperature Superconductor Space Experiment (HTSSE); delivered to NRL in 1994]

6 Cryogenic S-Parameter Characterization Station

7 Lewis Research Center On-Wafer Cryogenic Probe Station

8 Cross-Sectional View of Ferroelectric Coupled Microstripline Phase Shifter y w W s Y 1 0 Odd mode E-field X h 0 1 h 1 Y 2 2 h 2 Ground plane x h 0 >>(h 1 +h 2 ), H 1 <<h 2 W>>(2w+s)

9 Ferroelectric Phase Shifter Radial Stub Bias Tee V+ Magnesium Oxide Substrate (500 micron - optically transparent) Ferroelectric Phase Shifter Element (1 of 4) GRD 180 degrees total phase shift at 400 volts bias 1 cm Barium Strontium Titanate (0.3 micron - optically transparent) CETDP Product Gold - 2 micron Photograph of Four Element Coupled Microstripline Phase Shifter

10 V S21 (db) V 100V 50V 150V Measured Insertion Loss (including SMA launchers) of an 8-element 50 Ω PLD coupled microstripline phase shifter at 290 K as a function of bias voltage. Substrate is 0.3 µm MgO with 400 nm Ba 0.60 Sr 0.40 TiO 3 film. l = 350 µm, s = 7.5 µm and w = 30 µm. Bandwidth compression from the filtering effect is evident. Marker 1, 2, 3, and 4 are at 5.75, -5.38, -6.00, and 6.49 db, respectively.

11 High Voltage Bias Tee for Ferroelectric Circuits Fail-safe bias tee allows voltages of up to 500 V to be applied to ferroelectric devices and circuits connected to expensive and delicate microwave instrumentation Measured insertion loss of packaged bias-tee

12 Insertion Phase (deg) Insertion loss (db) 8-section 50 Ohm coupled microstrip phase shifter at 40 K using YBa 2 Cu 3 O 7-δ electrodes and laser ablated SrTiO 3 films on (100) single crystal 0.25 mm LaAlO 3. Hysteresis is unremarkable. l = 470 µm, s = 7.5 µm, and w = 25µm.

13 Performance of 8-section coupled line thin ferroelectric film phase shifters from different vendors

14 Performance Comparison of Various PLD Samples. Lewis Research Center 8 element CMPS using the 305 µm thick MgO design also employing BST mesas sample Substrate BST film thick fopt GHz Tuning w/400 V Max Loss (db) Max K ( /db) UM 112 MgO 60: UM 142 MgO 60: element CMPS using the 305 µm thick MgO design, 1% Mn doped films with no mesas sample Substrate BST film thick fopt GHz Tuning w/400 V Max Loss (db) Max K ( /db) NRL MgO 60:40 am-a (160V) NRL MgO 60:40 am-a NRL MgO 60:40 am-a NRL MgO 60:40 am-a element CMPS using the 254 µm thick LaAlO 3 design sample Substrate BST film thick fopt GHz Tuning w/400 V Max Loss (db) UM NBST001 LaAlO3 50: UM NBST017 LaAlO3 50: (360 V) SCT B111497B LaAlO3 40: Max K ( /db)

15 Experimental Comparison of SrTiO 3 Film Thickness Effects for a 25 Ω Coupled Lewis Research Center Microstrip Phase Shifter on 0.25 mm LaAlO 3 Data from SrTiO 3 films showed that phase shift increased almost linearly with film thickness, actually closer to ϕ ~ t 0.67 in agreement with models If ferroelectric phase shift/loss is constant, then thicker films will yield better performance: lower overall loss since conductor loss is constant & more compact

16 A Simple Example of a Variational Solution to a Partial Differential Equation 2 φ/ x φ/ y 2 + ρ/ε = 0 (1) Given the boundary conditions: φ(a,y)=φ(-a,y)=φ(x,b)=φ(x,-b) = 0 (2) Assume a solution: φ(x,y) = (a 2 -x 2 )(b 2 -y 2 )(k 1 +k 2 x 2 +k 3 x 4 + ) (3) Implying that the x dimension is >> y δφ =(a 2 x 2 )(b 2 y 2 )δk 1 (4) where δφ is the first small variation in φ. Variational form of equation 1 for a one-term solution: Substituting (4) into this expression: k 1 = 5/8(ρ/ε)(1/(a 2 + b 2 )) The final solution is: φ(x,y) = 5/8 (1 (x/a) 2 )(1 (y/b) 2 )(1 + (b/a) 2 ) -1 [(ρ/ε)b 2 ]

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18 Return loss or insertion loss (db) S 21 S 11 ε 100 r = Frequency (GHz) Phase (degrees) 0 ε r = Frequency (GHz) Theoretical calculation for the bandpass characteristic of an 8-section coupled microstripline phase shifter using a 0.5 m Ba x Sr 1 x TiO 3 film on 0.3 mm thick MgO. The coupled length was 350 m, w = 30 m, and s = 10 m. The permittivity of the film was tuned from 3500 j0.05 (no bias) to 500 j0.005 (maximum bias).

19 Lewis Research Center Comparison of the Quasi-TEM Solution and an E-M simulator for the Case of Microstrip The model is useful for a wide variety of multilayer transmission lines. In this case, the strip spacing (s) was allowed to increase just until the even and odd mode capacitance became equal (Zoo=Zoe). Table 1. Data for a 2 micron ferroelectric layer on.25 mm thick LaAlO3, Zo 50 Ohms, s>>w,h (micro mode) Er Ferroelectric Layer Eeff (Sonnet) Eeff (Variational)

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21 3 λ 25 Ω Au/SrTiO 3 /LaAlO 3 Tunable Ring Resonator

22 Lewis Research Center Electronically Tunable K-Band Oscillator First demonstration of a cryogenic tunable oscillator using both GaAs PHEMT and thin film ferroelectric technologies that operates in the microwave frequency range.

23 Cryogenic GaAs PHEMT/Ferroelectric Ku-Band Tunable Oscillator Lewis Research Center A Side-Coupled Au/SrTiO 3 3λ Ring Resonator Provided Over 500 MHz Tuning at Ku-Band. The Laser Ablated Ferroelectric Film was 2 µm Thick. Bias was between 0 and 250 V.

24 BER Degradation Due To Phase Noise and Q Lewis Research Center (After Leeson) Phase Noise dbc/hz Un-coded QPSK

25 Comparison of the Resistivity of an Evaporated Thin Gold Film and Bulk Gold and Implications for Device Design Gold Phase Shifter Skin Effect ρ(t), Ωcm + 1 µm Au Bulk Au Frequency (GHz) T, K Measured resistivity of evaporated gold on alumina Effect of film thickness on the insertion loss of a single section coupled microstrip Ba x Sr 1-x TiO 3 phase shifter using thin film resistivity values

26 Extracting λ L (0) from Strip Resonators Circuit model of a [superconducting] transmission line near resonance coupled to a [superconducting] feed line Correction factor as a function of reduced temperature for a Tl-Ba-Ca-Cu-O thin film (800 nm) 50 Ω ring resonator on 500 µm LaAlO3. The normalized resonant frequency shifts because of the temperature dependent impedance and coupled susceptance.

27 Modeled Effect of YBa 2 Cu 3 O 7-δ Penetration Depth and Film Thickness Insertion loss and return loss of a 6-pole YBa 2 Cu 3 O 7-δ microstrip band-pass filter on 0.30 mm MgO with t=200 nm and λ o =200 nm Insertion loss and return loss of a 6-pole YBa 2 Cu 3 O 7-δ microstrip band-pass filter on 0.30 mm MgO with t=200 nm and λ o =300 nm

28 Lewis Research Center Life Cycle Testing of Ba x Sr 1-x TiO 3 Devices Fig. 2. Transmission S-parameter (S 21 ) insertion loss versus voltage cycle for a Au/BSTO(0.75 µm)/mgo (300 µm), four-element CMPS at 18 GHz and 300 K. Ba:Sr ratio is 60:40. Fig. 3. Transmission S-parameter (S 21 ) phase shift (degrees) versus voltage cycle for a Au/BSTO(0.75 µm)/mgo (300 µm), four-element CMPS at 18 GHz and 300 K. Ba:Sr ratio is 60:40.

29 Lewis Research Center Insertion Loss and Phase Shift of a 4-Section Coupled Microstrip Phase Shifter on High Resistivity Si at 298 K An ultimate goal is to integrate superconducting or metallic transmission lines and electrodes with crystalline ferroelectric films on silicon to enable more complicated microelectronic circuits and to reduce manufacturing costs. The epitaxial heterostructures described here were very tunable and exhibited low loss and may lead to exciting new devices. The fundamental structural similarities between cuprate superconductors and perovskite ferroelectrics encourages such development. The large difference in lattice constant and thermal expansion coefficient between these materials and silicon appears to be a tractable problem.

30 Lewis Research Center Cross Sectional TEM of a heterostructure consisting of 80 nm (ZrO 2 ) 0.91 (Y 2 O 3 ) 0.09 /200 nm Bi 4 Ti 3 O 12 /375 nm Ba 0.6 Sr 0.4 TiO 3 films on 100 Si A thin interfacial layer is present at the YSZ-BTO interface. The BTO-BSTO interface appears abrupt.

31 Prototype 16-Element Linear Phased Array Antenna Based on Ba 0.60 Sr 0.40 TiO 3 Thin Films

32 Measured Principal Plane Boresight Patterns of the 16 Element Phased Array at 23.7 GHz Normalized Amplitude [db] E-plane (co-polar) H-plane (co-polar) Angle from Boresight [Degrees]

33 16-Element Ferroelectric Phased Array Controller

34 Lewis Research Center Reflectarray Prototype Subarray Characteristics 2832 elements element subarrays 3 db insertion loss per phase shifter 39 dbi gain at 19 GHz Printed CP Element with integrated Ferroelectric Film Phase Shifter 19.4 diameter Patent Pending

35 Acknowledgements NASA Glenn Research Center: Sam Alterovitz Felix Miranda Carl Mueller Fred Van Keuls Joe Warner University of Maryland Chadwick Canedy Rammamoorthy Ramesh Naval Research Laboratory Jim Horwitz MicroCoating Technologies George Cui Jerry Schmitt Wen-Yi Lin