Rare Earth Oxides in Microelectronics

Abstract

A feasibility study of rare earth oxides for replacing SiO₂ gate oxide for CMOS integrated circuits has been conducted. Rare earth oxides have relatively higher dielectric constants and are suitable as gate dielectrics. Two-dimensional device simulations reveal that the desirable dielectric constant, without affecting the short-channel performance, is less than 50. The dielectric constant values of rare earth oxides satisfy this condition. One of the issues in rare earth oxides that needs to be addressed is the hygroscopic properties of the film. These physical and electrical changes in the oxides caused by this moisture absorption, which are not suitable in electronics applications, can be suppressed by coating a metal film or by using a passivation layer.

Of all the rare earth oxides, it is found that La₂O₃, after a proper heat treatment, has the best electrical properties for gate insulator applications in MOSFETs, because of its higher barrier height for the conduction band electrons and valence band holes as well as its higher dielectric constant. The smallest gate leakage current demonstrated through experimental results was 3 × 10^–4/cm³ at 1 V for an EOT of 0.6 nm. The conduction mechanism through La₂O₃ has been modeled, and has been shown to be mainly by SCLC.

With post-metallization annealing (PMA) after Al gate electrode formation, La₂O₃ gated MISFET has shown high effective electron mobility of 319 cm²/Vs. This value is lower compared to SiO₂, but still one of the highest among all the high-κ MISFETs, for a gate oxide EOT value which is slightly larger than 2.3 nm. The PMA recovers the flat band shift and improves the mobility presumably by the diffusion of Al into La₂O₃ which compensates the positive charges in the film. However, the growth of an interfacial Al₂O₃ layer, which results in an increase in EOT, is still a problem. The solution could be to replace the Al gate electrode with a less reactive metal with La₂O₃, and doping La₂O₃ films with some elements which compensate the negative charge.

Interfacial layer suppression between the silicon substrate and the La₂O₃ film has been accomplished by using Y₂O₃ as a buffer layer, which is necessary to achieve an EOT less than 1 nm.

Reliability and yield of rare earth oxides still need to be investigated, but the results shown here hold promise for rare earth oxides, especially La₂O₃, as a suitable candidate for the post-Hf-based gate insulator in advanced CMOS integrated circuits.