Amorphous $Ta_2O_5$ thin film capacitors were fabricated on polymer dielectric films using DC reactive magnetron sputtering for MCM-D applications, especially as decoupling or by-pass capacitors.
To obtain high quality amorphous $Ta_2O_5$ thin films, Ar/$O_2$ gas flow rate ratio of 60 to 50 sccm at the sputtering power of 200 W was selected. The $Ta_2O_5$ deposition rate was 10 nm/min. X-ray diffraction pattern showed that the deposited films were amorphous, with a refractive index around 2.10 measured by an ellipsometer. By comparing with the standard amorphous $Ta_2O_5$, these deposited films were proven to be stoichiometric $Ta_2O_5$ by RBS and XPS measurements. Dielectric constant of the films was 24.5 at 1 MHz. The leakage current density of the Al/$Ta_2O_5$/Al (MIM) capacitors fabricated on a Si-substrate was $9 \times 10^{-9}A/cm^2$ at an electric field strength of 0.5 MV/cm and its breakdown field strength was above 2 MV/cm.
Low temperature annealing at 150℃ in nitrogen, oxygen or vacuum increased capacitors yields from 60-70%(as-deposited) to more than 90%. However, the heat treatment in $O_2$ gas atmosphere was detrimental in the leakage current of the capacitors, $7 \times 10^{-8} A/cm^2$. To explain these difference in Leakage current, conduction mechanisms study in the amorphous $Ta_2O_5$ thin film capacitors was done. For the as-deposited films, Poole-Frenkel conduction mechanism at high electric field. But the $O_2$-annealed films, Only Poole-Frenkel conduction mechanism could be seen. This fact shows that the Al/$Ta_2O_5$ interface is different between the as-deposited films and $O_2$-annealed films. Hence, Large leakage current in $O_2$-annealed films is presumably due to the Oxygen-rich layer formation on the $Ta_2O_5$ surface.
The leakage current density of the capacitors fabricated on polymer films was about $7 \times 10^{-8}A/cm^2$, an order of magnitude higher than those fabricated on a Si-substrate due to the surface irregularities of the polymer films. However, these capacitor properties were still good enough to be used for MCM applications.
As a summary, amorphous $Ta_2O_5$ MIM thin film capacitors were successfully fabricated on polymer dielectrics during MCM-D multilayer processing and these capacitor characteristics are good enough to be used as decoupling capacitors.
