Ta Thin-Film Capacitors with an Al Underlay for High-Frequency Applications

The use of thin-film Ta capacitors for precision applications is limited to frequencies below 10 KHz, due to the resistance of the Ta anode. The dissipation factor, Or tan
\delta
, in a resistancecapacitance (RC) series network is given by tan
\delta
=tan
\delta
' +
\omeg
CR2, where tan
\delta
' is an intrinsic loss due tO the dielectric, independent of frequency
\omeg
, and R
2
is the series resistance of electrodes and leads. By depositing the Ta on top of an Al film, which has a much lower resistivity than the Ta, the series resistance and therefore tan
\delta
Can be reduced, and the useful frequency range can be extended into the 1-MHz region. The processing, electrical properties, and life test characteristics are reported for a thin-film RC test circuit, with an Al underlay for high-frequency applications. In the process sequence selected, 0.25-1.0 µm of Al was deposited by evaporation or magnetically enhanced sputtering. Nitrogen doped body-centered cubic (bcc) Ta was then deposited by dc diode or magnetically enhanced sputtering, and the capacitors were patterned and anodized to 190V. Tantalum nitride resistors, a Ti-Pd-Au counterelectrode, and a 350°C 1-h stabilization completed the test circuits. Processing studies focused on the identification and elimination of problems associated with Al nodules (evaporation only), the adhesion of Al to a Ta
2
05 underlay, Al hillocks, resistance at the Al-Ta interface, and etching and undercutting. Values of tan
\delta
below o.oo2 were recorded from 1-100 KHz for 700- and 3300-pF capacitors with an Al underlay. At 1 MHz the dissipation factors were 0.0026 and 0.005 for the 700 and 3300 pF capacitors, respectively, compared with a value of 0.07 for 3300-pF controls without the underlay. Calculations Showed that the series resistance was reduced by a factor of 20 with the incorporation of an Al underlay. Electrical properties other than tan
\delta
were found to be unaffected by the presence of Al.