Probing the Dielectric Properties of Ultrathin Al/Al₂O₃/Al Trilayers Fabricated Usingin SituSputtering and Atomic Layer Deposition

Dielectric properties of ultrathin Al 2 O 3 (1.1–4.4 nm) in metal–insulator–metal (M–I–M) Al/Al 2 O 3 /Al trilayers fabricated in situ using an integrated sputtering and atomic layer deposition (ALD) system were investigated. An M–I interfacial layer (IL) formed during the pre-ALD sample transfer even under high vacuum has a profound effect on the dielectric properties of the Al 2 O 3 with a significantly reduced dielectric constant (ε r ) of 0.5–3.3 as compared to the bulk ε r ∼ 9.2. Moreover, the observed soft-type electric breakdown suggests defects in both the M–I interface and the Al 2 O 3 film. By controlling the pre-ALD exposure to reduce the IL to a negligible level, a high ε r up to 8.9 was obtained on the ALD Al 2 O 3 films with thicknesses from 3.3 to 4.4 nm, corresponding to an effective oxide thickness (EOT) of ∼1.4–1.9 nm, respectively, which are comparable to the EOTs found in high- K dielectrics like HfO 2 at 3–4 nm in thickness and further suggest that the ultrathin ALD Al 2 O 3 produced in optimal conditions may provide a low-cost alternative gate dielectric for CMOS. While ε r decreases at a smaller Al 2 O 3 thickness, the hard-type dielectric breakdown at 32 MV/cm and in situ scanning tunneling spectroscopy revealed band gap ∼2.63 eV comparable to that of an epitaxial Al 2 O 3 film. This suggests that the IL is unlikely a dominant reason for the reduced ε r at the Al 2 O 3 thickness of 1.1–2.2 nm but rather a consequence of the electron tunneling as confirmed in the transport measurement. This result demonstrates the critical importance in controlling the IL to achieving high-performance ultrathin dielectric in MIM structures.