| AVS 60th International Symposium and Exhibition | |
| Electronic Materials and Processing | Tuesday Sessions |
| Session EM+PS-TuM |
| Session: | High-k Oxides for MOSFETs and Memory Devices I |
| Presenter: | C. Adelmann, Imec, Belgium |
| Authors: | C. Adelmann, Imec, Belgium K. Opsomer, Imec, Belgium S. Brizzi, BTU Cottbus, Germany M. Tallarida, BTU Cottbus, Germany D. Schmeisser, BTU Cottbus, Germany T. Schram, Imec, Belgium S.A. Chew, Imec, Belgium N. Horiguchi, Imec, Belgium S. Van Elshocht, Imec, Belgium L.-A. Ragnarsson, Imec, Belgium |
| Correspondent: | Click to Email |
HfO2 has been the standard gate dielectric for MOSFETs for several technology nodes because of its large dielectric constant (~18 for amorphous or monoclinic HfO2). To continue MOSFET scaling, replacement dielectrics for HfO2 are of interest with an even larger dielectric constant. The polymorphism of HfO2 offers the possibility to increase the dielectric constant by stabilizing the cubic phase of HfO2 with a dielectric constant of ~30. The stabilization of the cubic phase has been demonstrated by introducing dopants (typically about 10%) such as Al, Si, or rare earths.
Numerous studies have demonstrated the advantage of doped cubic HfO2 over undoped HfO2 in terms of leakage vs. equivalent oxide thickness (EOT) for films with thicknesses of ~5-10 nm. These stacks lead to EOT values >>1 nm and are thus not relevant for future CMOS technology nodes. However, no clear advantage has been shown for scaled films with EOT values <1 nm.
In this paper, we study the behavior of Gd- and Al-doped HfO2 in capacitors with EOT values below 1 nm (physical thicknesses of 2-3 nm). While Gd- and Al-doped HfO2 show similar leakage for 10 nm thick films, capacitors with 2.5 nm Gd-doped HfO2 show several orders of magnitude higher leakage than their Al-doped counterparts, indicating that the behavior for thick and thin films is not correlated. However, EOT vs. HFO2 thickness measurements show that dielectric constants of the order of 30 can be maintained even for 2.5 nm thick films.
The ultimate scaling limits were explored for Al-doped HfO2. It was found that the scaling of Al-doped HfO2 is limited by the crystallization temperature of the films, which becomes too large for acceptable temperature budgets for (gate-last) MOSFET processing for thicknesses approaching 2 nm. This was confirmed by x-ray absorption spectroscopy at the O K-edge. Thinner doped HfO2 films remain amorphous and exhibit a lower dielectric constant. In-situ XRD showed that the crystallization temperature of thick films (10 nm) was increased significantly by Al-doping. However, for 2 nm films, the comparison with undoped HfO2 led to similar crystallization behavior indicating that the effects of thin films and doping do not necessarily add up. The lowest EOT values that could be achieved for gate-last MOSFET compatible processing were of the order of 8 Å including an interfacial SiO2 contribution of about 4 Å. However, for such stacks, leakage current densities could be achieved which were about 2 orders of magnitude lower than HfO2 capacitors with identical EOT values. This indicates that doped HfO2 films offer solutions for very low gate leakage at scaled EOT values down to values as low as 8 Å.