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

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2 Author(s)
Feinstein, L. ; Bell Labs,Allentown, PA ; Pagano, R.

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 tandelta, in a resistancecapacitance (RC) series network is given by tandelta=tandelta' +omegCR2, where tandelta' is an intrinsic loss due tO the dielectric, independent of frequencyomeg, and R2is 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 tandeltaCan 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 Ta205 underlay, Al hillocks, resistance at the Al-Ta interface, and etching and undercutting. Values of tandeltabelow 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 tandeltawere found to be unaffected by the presence of Al.

Published in:

Components, Hybrids, and Manufacturing Technology, IEEE Transactions on  (Volume:4 ,  Issue: 1 )

Date of Publication:

Mar 1981

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