Fabrication and Optimization of Al Doped Ta2O5 Thin Films to Enable Temperature Stable Phase Shifters

• Published in: ECS transactions 2006-10, Vol.3 (5), p.35-45
• Main Authors: Cole, Melanie W., Nothwang, William, Hirsch, Sanual, Hubbard, Clifford, Ngo, Eric
• Format: Article
• Language: English
• ISSN: 1938-5862; 1938-6737
• DOI: 10.1149/1.2357194

The premier candidate active material for tunable microwave phase shifter devices is single composition, paraelectric BaSrTiO3 (BST). However, there is concern that in practical applications the device performance will be compromised due to the temperature dependence of the BST based device capacitance. We report a device design which controls the magnitude and the sign of the temperature coefficient of capacitance (TCC) via a multilayer paraelectric BST/buffer layer/ferroelectric BST coplanar device structure. To realize this multilayer device structure we have designed, fabricated, and optimized a 10 mol% Al doped Ta2O5 barrier layer with low loss (tan δ=0.004), moderate permittivity (εr=42.8), low TCC (-20 ppm/oC), and a low bias stability of capacitance (0.4%). The thin film integration of the barrier layer with the BST layers was optimized for structure, microstructure, interfacial/surface morphology, and dielectric properties as a function of Al doping concentration, annealing temperature, material growth and integration process parameters.