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New Niobium Pentoxide (Nb2O5) Integrated Capacitors for Decoupling Applications
Keywords: Niobium Oxide, Integrated Capacitors, Oxygen diffusion
Integrated capacitors are attractive to the electronics industry as they potentially consume less space and show lower parasitics compared with their surface mount counterparts. One of the major applications is their use as decoupling capacitors in printed circuit boards or chip package substrates. Tantalum oxide (Ta2O5) has been shown to be an excellent candidate for integrated decoupling because of its relatively high dielectric constant (k ~ 24) and the ability to form sub-micron thick layers. Increasing circuit density calls for even larger capacitance density than can be provided by Ta2O5. To obtain higher density, stacking of planar capacitors and geometrical variations have been done by various researchers. These solutions render high capacitance per unit area, but they involve too many process steps, making them not very cost effective. Thus, there is the need for a dielectric material with a higher dielectric constant that can deliver high capacitance density with a simple planar geometry. Niobium pentoxide (Nb2O5) with a k ~ 41 is a promising material for planar applications. Nb2O5 is a proven dielectric material for electrolytic type surface mount capacitors. Also, lower cost and better availability are driving niobium oxide as an alternative to the tantalum oxide dielectric. In this paper, we report niobium oxide used to fabricate thin film capacitors that can give a significantly higher capacitance density than the tantalum oxide dielectric. However, oxygen has a higher solubility in niobium than in tantalum, which can lead to the formation of sub-oxides along with the desired Nb2O5. These suboxides are partly conducting and can create paths for high leakage currents. Niobium oxide has been formed by reactive sputtering, followed by anodization to densify the oxide to ensure complete formation of Nb2O5. This combination technique eliminates suboxide formation in the dielectric. Another issue addressed is the oxygen diffusion from the dielectric to adjacent layers. A diffusion barrier of niobium nitride (NbN) has been sputter deposited above and below the dielectric forming a ‘sandwich’ structure. Both the dielectric formation technique and ‘sandwich’ structure were first proven with tantalum capacitors and high yield planar capacitors are now being demonstrated with niobium oxide dielectric.
Ms. Susan Jacob,
University of Arkansas
Fayetteville, AR

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