Search

Electronically scanned phased array antennas (ESAs) provide the means for achieving high data rate, beyond line of sight, on-the-move (OTM) communications. The phase shifter is a key component of ESAs, and the Army's Communications Engineering Research and Development Center (CERDEC) has recently acknowledged BaSrTiO3 (BST) thin film technology as the premier candidate for achieving affordable high performance phase shifter elements. Unfortunately, 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 material 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 an Al doped Ta2O5 barrier layer with low loss, moderate permittivity, low TCC, and excellent bias stability of capacitance. The 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., See also ADM002075. Presented at the Army Science Conference (25th) held in Orlando, FL on 27-30 November 2006. The original document contains color images.

Successful integration of paraelectric Ba1-xSrxTiO3 (BST) based thin films with affordable Si substrates has a potential significant commercial impact as the demand for high-frequency tunable devices intensifies. Utilizing a coplanar device design we successfully designed, fabricated, characterized, and optimized a high performance Ta2O5 thin film passive buffer layer on Si substrates, which will allow the integration of BST films with large area affordable Si substrates. This passive buffer layer thin film was fabricated via the industry standard metalorganic solution decomposition technique. The anneal optimized Ta2O5 based thin film possessed excellent material properties. Specifically, these material properties include an enhanced dielectric constant (epsilon r =45.6), low dielectric loss (tan delta=0.006), low leakage current or high film resistivity (rho=10 to the 12th Omega-cm at E=1 MV/cm), excellent temperature stability (temperature coefficient of capacitance of 52 ppm/deg C), and excellent bias stability of capacitance (^1.41% at 1 MV/cm). Additionally, the permittivity and dissipation factor exhibited minimal dielectric dispersion with frequency. The dielectric passive buffer layer film was typified by a uniform dense microstructure with minimal defects, and a smooth, nano-scale fine grain, crack/pinhole free surface morphology. This work also demonstrated that the coefficient of thermal expansion(CTE) mismatch between the Ta2O5 buffer and BST active thin films in the coplanar device design serves to enhance the dielectric tunability of the device. The development of this passive buffer layer thin film materials technology will enable the direct integration of paraelectric active thin films (BST) with silicon substrates. The impact of this materials technology is paramount, as this phase shifter materials technology design will promote broad scale implementation of affordable On The Move (OTM) phased array antenna systems across a variety of platform7, See also ADM001736, Proceedings for the Army Science Conference (24th) held in Orlando, FL on 29 Nov-2 Dec 2004. The original document contains color images. Prepared in collaboration with Oak Ridge Inst. for Science and Education, Oak Ridge, TN.

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 Ba(x)Sr(1-x)TiO3 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., See also ADM001736. Presented at Proceedings for the Army Science Conference (24th) held in Orlando, FL on 29 Nov- 2 Dec 2004. The original document contains color images.