The US Army is actively pursuing technologies to enable transformation goals of a lighter, faster, more potent force via affordable, electronically scanned phased array antennas (ESA's) that will provide the means for achieving a high data rate with beyond-line-of-sight, mobile communications. In order to transition this technology to Army applications, it is important that the cost of each device be decreased from current technology. Traditionally, paraelectric, active thin films of magnesium doped barium strontium titanate, have been deposited on expensive ceramic (MgO, LaAlO3, SrTiO3, Al2O3) substrates, and compositionally designed for tunable microwave applications. By integrating an active, thin film material with a large area, low cost, microwave friendly substrate, the cost could be significantly reduced. While Si is not a suitable substrate for microwave applications, a low cost, microwave friendly, buffer layer on silicon would be.
A high performance Ta2O5 thin film, passive, buffer layer on Si substrates has been successfully designed, fabricated, characterized, and optimized via metalorganic solution decomposition technique. The optimized Ta2O5 based thin film exhibited suitable microwave material properties, including an enhanced dielectric constant (εr = 45.6), low dielectric loss (tan δ=0.006), low leakage current, high film resistivity (ρ=1012 Ω-cm at E=1 MV/cm), excellent temperature stability (temperature coefficient of capacitance of 52 ppm/°C), and outstanding bias stability of capacitance (∼1.41% at 1 MV/cm). The permittivity and dissipation factor, also of extreme importance, 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 and pinhole free surface morphology. At elevated processing temperature, there was negligible elemental interdiffusion at the interface between the substrate and buffer layer as verified by Rutherford Backscatter Spectroscopy and Auger Spectroscopy, ensuring long-term reliability of the heterostructure.
By developing a passive, thin film material that is microwave friendly, we have demonstrated the direct integration of paraelectric active thin films with silicon substrates. This should allow phase shifter materials technology to be implemented across a wide spectrum of Army and commercial applications, specifically, affordable, mobile phased array antenna systems for a variety of DoD applications.