Anodization for forming thin film embedded capacitors in MCM-D and MCM-L substrates

The High Density Electronics Center (HiDEC) at the University of Arkansas has developed processes for anodizing Al and Ta metallization for the purpose of forming low defect density decoupling capacitors for applications in MCM-D and MCM-L (flex) substrates. The anodization processes were optimized in order to produce uniform, consistent thin films with high yield and low leakage. Correlations were also developed to show the relationship between the final anodization voltage and the resulting capacitance and dielectric thickness. Defect densities of less than 0.1 fatal defects per cm² were achieved for both types of capacitors. The aluminum oxide capacitors in the 50 to 70 nF/cm² range were fabricated on five-inch silicon wafers by anodizing sputtered aluminum in a solution consisting of tartaric acid and water. The capacitors exhibited capacitance values from 20-60 nF/cm² while exhibiting leakage values below 10^-7 A/cm² at 10 volts. Tantalum oxide capacitors were created similarly by anodizing sputtered tantalum over copper base plates in tartaric acid, ethylene glycol, and water. These capacitors were fabricated on both silicon wafers and flexible polyimide films. Capacitance values ranged from 50-180 nF/cm² and exhibited low impedance in the range of several GHz. Leakage values were below 10^-6 A/cm² at 10 volts and effective series resistances were less than 30 milliohms for both silicon-based and flex-film capacitors. Thermal annealing and aging effects were also investigated to determine the feasibility of incorporating these structures in single chip and multi-chip packaging