Reliability of Thin Film Phase Shifter Materials for Electronically Steered Antennas
ARMY RESEARCH LAB ABERDEEN PROVING GROUND MD WEAPONS AND MATERIALS RESEARCH DIRECTORATE
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Electronically steered antennas are crucial for the Army to realize the Future Force goals. Low cost phase shifter devices will enable this technology to be fielded on a wide variety of Army platforms. It is essential that any device being incorporated in such a critical technology be reliable. Residual stress, which is known to have a drastic effect on the material, electrical, and dielectric properties, becomes of particular importance in thin film materials. where the residual stress can be several orders of magnitude higher than in bulk materials. Residual stress will be a primary determiner of the long-term reliability. Barium strontium titanate BaxSr1-xTiO3 thin films are the principal materials of interest in phase shifter applications, primarily because of their low loss, high dielectric constant and large tunability. The effects of acceptor 3-10 Mg doping, added to further tune film properties, on the BST thin films were investigated in parallel with the annealing temperature 600 to 950 deg C, a strong determiner of film crystallinity. Both doped and undoped films were fabricated, and they were deposited on MgO single crystal substrates using metal organic solution deposition MOSD Cole et al., 2003. The residual stress in these films was measured in three ways. A Tencor stress analysis system was employed to measure the change in the substrate curvature due to the film stress Saha and Nix, 2002, and a nano-indentation method was used to calculate the residual stress in a system by measuring the maximum penetration, the force at maximum penetration, the measured modulus and the slope of the initial unloading curve Suresh and Giannakopoulos, 1998. These two methods were validated using the third XRD lattice calculations Cullity, 1978. Stresses as high as 2 GPa were observed under certain conditions, and it was possible to tune the stress level within the films by varying the dopant concentration and annealing temperature.
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