A Statistical Analysis of Laser Ablated Ba 0.50 Sr 0.50 TiO 3 /LaAlO 3 Films for Microwave Applications
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A Statistical Analysis of Laser Ablated Ba0.50Sr0.50TiO3 /LaAlO3 Films for Microwave Applications R.R. Romanofsky*, N.C. Varaljay*, S.A. Alterovitz*, F.A. Miranda*, C.M. Mueller**,F.W. VanKeuls***, J. Kim****, and K.S. Harshavardhan**** *NASA Glenn Research Center, Cleveland, OH 44135 **Hadron, Inc. Cleveland, OH 44135 ***Ohio Aerospace Institute, Brookpark, OH 44142 ****Neocera, Inc., Beltsville, MD 20705 ABSTRACT The NASA Glenn Research Center is constructing a 616 element scanning phased array antenna using thin film BaxSr1-xTiO3 based phase shifters. A critical milestone is the production of 616 identical phase shifters at 19 GHz with ≈4 dB insertion loss and at least 337.5o phase shift with 3 percent bandwidth. It is well known that there is a direct relationship between dielectric tuning and loss due to the Kramers-Kronig relationship and that film crystallinity and strain, affected by the substrate template, play an important role. Ba0.50Sr0.50TiO3 films, nominally 400 nm thick, were deposited on 48 0.25 mm thick, 5 cm diameter LaAlO3 wafers. Although previous results suggested that Mn-doped films on MgO were intrinsically superior in terms of phase shift per unit loss, for this application phase shift per unit length was more important. The composition was selected as a compromise between tuning and loss for room temperature operation (e.g. crystallinity progressively degrades for Ba concentrations in excess of 30 percent). As a prelude to fabricating the array, it was necessary to process, screen, and inventory a large number of samples. Variable angle ellipsometry was used to characterize refractive index and film thickness across each wafer. Microstructural properties of the thin films were characterized using high resolution X-ray diffractometry. Finally, prototype phase shifters and resonators were patterned on each wafer and RF probed to measure tuning as a function of dc bias voltage as well as peak (0 field) permittivity and unloaded Q. The relationship among film quality and uniformity and performance is analyzed. This work presents the first statistically relevant study of film quality and microwave performance and represents a milestone towards commercialization of thin ferroelectric films for microwave applications. INTRODUCTION Scanning phased array antennas could offer a highly desirable solution for futuristic near Earth and deep space science mission scenarios. For example the Laser Interferometer Space Antenna will consist of three spacecraft flying 5 million km apart in the shape of an equilateral triangle. The formation flying spacecraft will form a giant Michelson interferometer, measuring the distortion of space caused by passing gravitational waves. The antenna needs to measure the distance between proof masses separated by 5 million km with an accuracy of 20 picometers. Hence mechanically induced vibrations from gimbaled parabolic communications system antennas are unacceptable. While other solutions exist an affordable and efficient phased array would be preferred. Numerous applicatio
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