Back end of line (BEOL) metal-insulator-metal (MIM) capacitors reduce the need for discrete off-board components and have become a core passive device in integrated circuits. Applications include analog-to-digital converters, analog noise filters, DC voltage decoupling, and electrostatic discharge (ESD) protection. To enable continued scaling, capacitance density must be increased, either by introducing higher dielectric constant (κ) materials or by reducing the insulator film thickness. However decreasing insulator film thicknesses increases both leakage current density and voltage nonlinearity (characterized by the quadratic voltage coefficient of capacitance or αVCC). In addition high-κ materials typically have a large positive αVCC. Although a promising route to simultaneously meeting these competing requirements is to use a nanolaminate of insulators, which allows for combining of layers with complementary properties. Previous work has demonstrated nanolaminate MIIM devices with high capacitance density, low leakage current density, and low αVCC using PECVD SiO₂ or uncommon materials. ^1–3

In this work, MIIM capacitors using bilayers of Al₂O₃ and SiO₂ were deposited sequentially using plasma enhanced atomic layer deposition (PEALD). PEALD allows for low deposition temperatures, precise thickness control, and conformal coverage over high aspect ratio structures. Al₂O₃ and SiO₂ are attractive due to their common usage in IC fabrication, large metal-insulator barrier heights, and high dielectric breakdown strength. In addition SiO₂ is one of the few materials that exhibits a negative αVCC. Spectroscopic ellipsometry was used to characterize the growth rate and nucleation delay on TaN and Si substrates. The dielectric constants of Al₂O₃ and SiO₂ were found to be 4.6 and 8.7, respectively. αVCC values were plotted as a function of thickness and fit with a power law. Appropriate layer thicknesses were chosen to offset the negative αVCC of SiO₂ with the positive αVCC of Al₂O₃ in order to minimize the effective αVCC for a given capacitance density. The initial results for 8 nm Al₂O₃ / 3.5 nm SiO₂ MIIM devices show capacitance density of 5.4 fF/μm², 2 nA/cm² leakage at 1V, and αVCC of 70 ppm/V², simultaneously meeting the ITRS 2014 requirements for capacitance density (> 5 fF/μm²), leakage current density (< 10 nA/cm² at 1V), and voltage nonlinearity (< 100 ppm/V²). Current work is underway to optimize this nanolaminate to meet the ITRS 2017 requirements.

¹ S. Van Huylenbroeck et al, Electron Device Lett. IEEE 23, 191 (2002).

² S.J. Kim et al, Electron Device Lett. IEEE 25, 538 (2004).

³ T.H. Phung et al, Electrochem. Soc. 158, H1289 (2011).