High frequency electrical characterization of BST capacitors

We have characterized the dielectric properties of barium strontium titanate (Ba1-xSrxTiO3, x=0.5; BST) capacitors up to 5 GHz employing a simple, single port scattering parameter measurement technique. Metal-insulator-metal type capacitors with areas ranging from 400 to 10,000 mu m(2) were defined by standard photolithographic and etching techniques BST films were rf-magnetron sputter-deposited on metallized (111) silicon. The bottom electrode was made by evaporating 100A of Ti for adhesion followed by 1500A of Pt. The top electrode consisted of 3000A of gold with a 100A titanium adhesion layer. The test capacitors were surrounded by a reference capacitor approximately 140 times in area to facilitate high frequency measurements with air coplanar (ACP) probes. The test and the reference capacitors were in series during characterization ensuring that the resulting capacitance was dominated by the smaller test structure. Single port scattering parameters (S11) were obtained using a HP8510B network analyzer. HP Microwave and RF Design System (MDS) was used for data analysis. The BST capacitor structures exhibited a roll-off near 1 GHz where the dielectric constant was reduced by 10-20% of the value at 50 MHz.