High-k dielectrics.
Students : R. Boujamaa, V. Brizé, M. Kahn, Y. Lai, L. Libralesso, S. Margueron, E. Rauwel, O. Salicio, J. Ubrig,
Objectives : The general goals of these studies on high K films and on their interfaces are i) the control of the growth by CVD at nanometer-scale, ii) the design ofnew materials (solid solution, nanocomposites), iii) the detailed characterization of their structural and electrical properties, iv) their integration in devices.
Main results :
HfO2 thin films on Si/SiO2 : interface engineering and solid solutions : The HfO2 (or Hf silicate)gate oxide is usually nitrided after deposition in order to increase its chemical stability. We haveshown that it can be directly in situ nitrided during the growth by CVD using original precursors, Hf guanidinates,which contain nitrogen. [Patent, S. Daniele et al. FR08 01445, extension PCT/FR2009/000272 (IRCELYON, LMGP)]. Moreover some of these precursor molecules lead to a delayed crystallisation of HfO2 (up to 475°C) while the crystallisation temperature is about 370°C with the otherwise used Hf(OtBu)2(mmp)2 precursor.
The regrowth of the SiO2 interface during HfO2 deposition is a key issue for the control of the final properties
of the transistors. We investigated an original route to prevent oxide growth at the silicon/high ! dielectric
interface by inserting an ultrathin organic layer. Long alkoxy (O-(CH2)9-CH3) and functionalized alkyl
(-(CH2)9-CH3), -(CH2)10-COHH) chains were grafted by thermal hydrosilylation after ammonium fluoride
cleaning (atomically flat terrace surfaces). Full methylation was obtained by electrochemical reaction. The
grafted organic layers are dense (close to 100% Si-H substitution for methyl). Thin HfO2 films (3 to 5.5 nm)
were deposited by MOCVD at 350°C. ATR and XPS spectra show that minimal silicon oxidation is obtained for an adequate grafting. While the mean static dielectric permittivity k of the monoclinic phase of HfO2 (stable phase at room temperature and atmospheric pressure) is of ~ 14-16, ab initio calculation show that the tetragonal and cubic phases are expected to exhibit higher permitivities : k // ~ 20 and k ^ ~33 for the tetragonal phase and k ~ 26 for the cubic phase. The values for the orthorhombic phase (with Pnma space group) are intermediate of those of the cubic and tetragonal phases. The stabilization of high symmetry phases of HfO2 in thin films is thus motivated by the possibility of further decreasing the EOT values (k enhancement) as well as tuning the work function of the metal gate electrode. We investigated such stabilization in HfO2 thin films grown on p-type Si/SiO2 substrates by the addition of heterovalent cations (trivalent Y3+, Sc3+ or divalent Mg2+). For Sc and Mg, additive contents up to ~ 40 at. % were explored while
for Y3+ the whole range of composition, from few % up to 100 at. % was considered [C. Dubourdieu et al., J.
Vac. Sci. Technol. A 27(3) 503 (2009), V. Brizé et al., ECS Trans. 13, 157 (2008)]. For all studied additives,a pure cubic phase was stabilized for contents of ~ 6 to 7 at. % additive and for a growing temperature as low as 470°C.
Figure 1.14 - Left : TEM plane view image of a Hf- Mg-O film (22 at. % Mg) and electron diffraction showing
the cubic phase. Right : ATR spectra of Hf-Y-O films showing that the stabilization of the cubic phase
is achieved at 6.5 at. % (thickness 4-10 nm) and cross section TEM image of a cubic film (6.5 at. % Y).
Figure 1.15 - Relative dielectric permittivity ! as a function of Y content in Hf-Y-O polycrystalline films deposited on (001) Si/SiO2 by MOCVD.
The grain size of the films is in the nanometer range (see Fig. 1.14) and strain related to surface tension induces a shift of the frontier between the stability domains, which are very different from those of bulk ceramics. We also showed that thickness is an important parameter in the stabilization. The formation of a solid solution in the films was clearly evidenced, for the first time, from combined X-ray photoelectron spectroscopy (XPS) and X-ray diffraction analyses. The interface of the films was investigated by XPS and transmission electron microscopy (TEM). It was found to be composed of a silicate for the three investigated additives.The silicate is thicker as the film thickness increases and as the additive content increases. We then proposed some interface engineering in order to limit and accurately control the interfacial silicate layer thickness during the integration of HfO2-based solid solutions on Si/SiO2.
The electrical properties of the films were measured on MOS structures. For all additives, we found
that the dielectric permittivity k exhibits an optimum with the additive content, which is in good agreement
with other groups observation (see Fig. 1.15). We achieved k values of ~ 25 (Sc), 30 (Y) and 35 (MgO)
[Patent C. Dubourdieu et al., FR07 03089, extension PCT/FR2008/050753 (LMGP, STMicroelectronics)].
Emerging dielectrics : the perovskite SrTiO3 on Si/SiO2 : Due to its very high permittivity (er ~
300 in bulk) SrTiO3 is considered as a candidate for future integration in CMOS (EOT < 1 nm) or MIM
structures. The CVD growth of SrTiO3 on Si/SiO2 was studied in details and focused has been put on the control of the interface with silicon and on the control of the stoichiometry of the films [C. Dubourdieu et al., ECS Trans. 19, 669-684 (2009) - invited paper]. We showed that the formation of a silicate layer Ts = 700°C) at the interface with SiO2 can be suppressed by decreasing the reactive species flow rate at the substrate surface. We also investigated the use of a new Sr precursor {Sr[N(SiMe)3)2 ]2 }2 , that does not contain any oxygen atoms, in the very first steps of the growth. A significant reduction of the interface was demonstrated [Patent : N. Blasco et al., EP07 300805 (Air Liquide, IRCELYON, LMGP)]. Finally, integration of SrTiO3 in transistors was performed in collaboration with LETI-CEA and INL. The inversion zone was evidenced for both p and ntype devices, showing the success of this technology. The inversion current of a nMOSFET transistor on the gate current characteristic was 1.2 10-11 A/µm at -1V, which confirmed the good quality of the deposited oxide. A transistor effect was demonstrated on a n-MOSFET.
Figure 1.16 - Final structure of the transistors prepared at INL (on 2" wafers - base wafers were prepared
at LETI-CEA on 200 mm wafers : locos isolation,implants) - C-V curve measured on a MOSFET n-type
structure with metal gate stack SrTiO3/Al ((W=10µm, Lg=2µm, EOT = 2.4nm).
Collaborations :
- Air Liquide, ESRF,Forschungzentrum Jülic,IHP Microelectronic ,Allemagne
- IMEP-LAHC, Grenoble , INL, Ecully , IRCELYON, PMC, Palaiseau, LETI-CEA, Grenoble, LTM, Grenoble,
- Tyndall, Cork, Ireland, EMAT, Anvers, SAFC Hitech, UK, STMicroelectronics, Crolles, Univ. de Darmstadt, Germany
- MEDEA+ T207 (CMOS 65 nm) & MEDEA+ FOREMOST : funded by the French Ministry of Industry,
- NoE SINANO, Programme Thématiques Prioritaires Micro-Nano-Technologies Région Rhône-Alpes, Cifre STMicroelectronics (09 - 11)
- IHP Microelectronics, Allemagne
- LTM Grenoble.
Staff
C. Dubourdieu
Engineer and technical support
H. Roussel
I. Matko
L. Rapenne