This work deals with the performance of coupled microstripline phase shifters (CMPS) fabricated using BaxSr1-xTiO3 (BST) ferroelectric thin films. The CMPS were fabricated using commercially available pulsed laser deposition BST films with Ba:Sr ratios of 30:70 and 20:80. Microwave characterization of these CMPS was performed at upper Ku-band frequencies, particularly at frequencies near 16 and 18 GHz. X-ray diffraction studies indicate that the 30:70 films exhibit almost a 1:1 ratio between the in-plane and out-of-plane lattice parameters, suggesting that their cubics create strain-free films suitable for producing CMPS devices with reduced hysteresis in the paraelectric state.
The quality of performance of the CMPS was studied based on their relative phase shift (Δφ = φnV– φ0V, where n= 0 to 400 volts DC) and insertion loss within the DC bias range of 0 to 400 V (i.e., E-field ranges within 0 to 53 V/μm). The performance of the CMPS was tested as a function of temperature to investigate their operation in the paraelectric, as well as in the ferroelectric, state (i.e., above and below the Curie temperature, respectively). The novel behavior discussed here is based on the experimental observation of the CMPS. Remarkably, these devices were hysteresis-free in the paraelectric state, and only showed Δφ hysteresis while performing in the ferroelectric state. This behavior, observed for the aforementioned cation ratio, highlights the relevance of good crystalline structure for high-quality CMPS.
Elimination of Δφ hysteresis is essential for practical microwave applications such as voltage-controlled oscillators and beam-steerable devices, particularly electronically steerable phased array antennas, which require accurate phase shift versus tuning voltage profiles for reliable operation. Accordingly, the optimization of the interplay among film microstructure, Ba content, and dielectric constant is critical for reliable CMPS devices. The origin of hysteresis is most likely related to fixed charges and ferroelectric domain phenomena in the ferroelectric state, as well as remnant ferroelectric domains in the paraelectric state. Consequently, to achieve minimum device hysteresis in the
paraelectric domain, the BST films selected for the CMPS devices should be of optimal film composition [i.e., FWHM (full width at half maximum) < 0.05°], with minimum film strain (i.e., in-plane to out-of-plane lattice parameters ratio as close as possible to 1), and moderate values of dielectric constant (≈800 at V= 0) to enable acceptable tunability at manageable insertion losses.
This work was done by Félix A. Miranda and Robert Romanofsky of Glenn Research Center, Carl H. Mueller of Qinetiq North America, and Frederick Van Keuls of Ohio Aerospace Institute. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Electronics/Computers category.
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