A Statistical Analysis of Laser Ablated Ba 0.50 Sr 0.50 TiO 3 /LaAlO 3 Films for Microwave Applications

Abstract : The NASA Glenn Research Center is constructing a 616 element scanning phased array antenna using thin film Ba(x)Sr(1-x)TiO3 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 337.5 deg 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. Ba(0.50)Sr(0.50)TiO3 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.