Paper TF+EM+MI-WeM1
Electrode Modulated Electric Field Capacitance Nonlinearity in ALD Al₂O₃ and HfO₂ Metal-Insulator-Metal Capacitors

Wednesday, November 1, 2017, 8:00 am, Room 21

Back-end-of-line metal-insulator-metal capacitors (MIMCAPs) require increasing capacitance density (C_ox) while maintaining low leakage current density (J_leak). In addition, analog and mixed signal (AMS) applications are particularly sensitive to nonlinearity of capacitance-voltage (CV), empirically characterized by the quadratic voltage coefficient of capacitance, α_VCC. Scaling of MIMCAPs for AMS applications is increasingly challenging as C_ox, J_leak, and α_VCC are all inversely proportional to dielectric thickness (d_ox). Despite its technological importance, the fundamental mechanisms responsible for α_VCC are not fully understood. It is well established that the "bulk" dielectric material has a dominant effect, where α_VCC increases with increasing dielectric constant and roughly as 1/d_ox². However, the influence of the electrode interfaces is not currently understood. Of the few studies that have considered the impact of the electrodes on α_VCC, most have focused on interfacial layer oxides (ILOs).

In this work, metals with low enthalpy of oxide formation (ΔH_ox), are used to examine the influence of the top electrode interface, in the absence of a significant ILO, on the CV nonlinearity of TaN bottom electrode MIMCAPs with various thickness ALD HfO₂ and Al₂O₃. If non-linearity is purely a bulk effect then normalizing for d_ox, one would expect that the electric field coefficient of capacitance (α_ECC) should be independent of d_ox. Instead, we find that α_ECCdecreases with decreasing d_ox, indicating either an ILO or the direct influence of the interface. A plot of capacitive equivalent thickness vs. optical thickness rules out an ILO. For Au, Ag, Pd, and Ni, α_ECC increases with increasing d_ox, saturating for thick oxides. It has been proposed that for positive α_VCC materials (C_ox(V) increases with voltage), electrostriction and Maxwell stress lead to a vertical compression of the oxide under applied fields that results in increased capacitance. We further propose that the oxide must expand horizontally to maintain volume. This expansion results in compressive stress in the oxide and tensile stress in the metal, concentrated near the interface. The electrode then serves to inhibit the lateral expansion of the dielectric, reducing overall α_ECC. Indeed α_ECC of 10 nm oxides was found to increase roughly linearly with increased oxide/metal % lattice mismatch. As thinner oxides are used to achieve higher C_ox, the metal electrodes exert influence over a greater % of the oxide thickness, increasing the electrode importance and its impact on nonlinearity.