Aluminium Oxide Capacitors With a Low Dielectric Constant

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Aluminium oxide, commonly referred to as alumina, is one of the most desirable engineering materials due to its superior strength, stiffness and dielectric properties. It exists in a variety of crystalline phases which all revert to the most stable hexagonal alpha phase at elevated temperatures making it suitable for structural applications.

Alumina is also a very good dielectric material, with an extremely low permittivity and tangent loss. Combined with its high quality factor, dielectric breakdown can be achieved at very low electric fields, making it a very effective capacitor material. The ability to store electrical energy in the form of charges is a key requirement for many electronic devices, and is the primary reason that the capacity of a capacitor is inversely proportional to its thickness.

This article reports on the synthesis and characterization of hydrothermally as-grown g-phase alumina by atomic layer deposition (ALD). The as-prepared material was confirmed to be of crystalline structure through X-ray diffraction spectroscopy, with a bending vibration in the Al-O-Al band in the FTIR spectra confirming that the as-prepared material was g-Al2O3.

The electrical characteristics were determined using complex impedance measurements and spectroscopic ellipsometry. It was found that the capacitance increased with increasing film thickness, and this was attributed to Fowler-Nordheim tunneling via a thin interfacial oxide layer at the Si substrate. Leakage currents measured at high electric fields were also consistent with this interpretation. ALD films grown at 80, 100, 150 and 250 degC showed a significantly higher dielectric breakdown strength than those grown at lower temperatures. This was attributed to the decrease in carbon impurity levels and oxygen defects, as confirmed by X-ray photoelectron spectroscopy.

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