Service de Physique de l'Etat Condensé

Chemical and electronic structure of high permittivity (high-κ) gate oxides
N. T. Barrett, B. Delomez ; Coll. O. Renault, P. Besson, Y. Le Tiec, F. Martin (CEA/LETI-Minatec)

Fig.1:Role of high-k gate oxide in a MOSFET, scaling requires capacitance and very thin EOT without leakage currents.

High-κ dielectric materials are required to replace SiO2 (κ=3.9) in sub-0.1 μm CMOS, providing better insulating properties: reduced electron tunneling with capacitance characteristics of a much thinner SiO2, thus yielding a very thin equivalent oxide thickness (EOT). [1] Hafnium oxide (HfO2) and its silicates (HfxSi(1-x)O2) have already demonstrated the best compatibility with the main electrical requirements of advanced CMOS technology favouring low leakage currents (conduction band offset ΔEc > 1 eV).

We first investigated and verified the formation of thin interface silicate between a HfO2 high-κ oxideand SiO2 bottom layer, necessary to minimize interface states. We show that the silicate layer may be restricted to two atomic layers, thus playing a negligible role in determining the electronic properties of the stack. Between HfO2 and Si, the valence- and conduction-band offset are determined as 3.2 and 1.4 eV respectively. The thermal stability of the interfacial silicate layer was also addressed. By a quantitative treatment of the Si 2p core-level intensities, we conclude that post deposition annealing under nitrogen leads to the extension of both the interface silicate and the SiO2 interface oxide.

 

Fig.2 :(left)Modification of valence band onset following nitridation of hafnium silicate.(right) Effect of variation in the local work function on the core level binding energies deduced from photoemission.

Nitriding hafnium silicate increases the boron diffusion barrier, limits the growth of an interface silicate, and improves κ. Our results show that nitridation raises the valence-band maximum (VBM) by adding N 2p states in the band gap. VB offsets of 1.6-1.9 eV are measured after nitridation. The best gate oxide would be 1.5nm Hf0.6Si1-0.6O2:N on 0.5nm SiO2 giving EOT ≈ 0.89nm, TC ~ 1100°C

Examination of the role of substrate doping on the measured binding energies show that the principal contribution to screening in the oxide is the dipole at the oxide/substrate interface which raises the local work function. Final state screening in the gate oxide and band-bending at the SiO2/Si interface must be included in order to deduce accurate band offsets.

 

Maj : 29/05/2008 (1024)

 

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