The structural and interfacial properties of HfO2/Si by the plasma oxidation of sputtered metallic Hf thin films
Introduction
Continued scaling of device dimensions has led to greater emphasis on such issues, and indeed fundamental limits imposed by gate leakage and intrinsic reliability are expected to prevent reduction of the thickness of the SiO2-based gate dielectric in MOS below ∼1.2 nm. The trend in reducing lateral dimensions of devices brings a reduction of the capacitance of the involved MOS structures. Therefore, to keep device areas small and prevent leakage current while maintaining the same gate capacitance equivalent to a thinner (∼1.0 nm and below) SiO2 layer, a physically thicker film and a material with a significantly higher dielectric constant (high-k) is required. By far Y2O3, Ta2O5, Al2O3, TiO2 and SrTiO3 (STO) have received considerable attention as the gate oxide thickness of MOS devices are being scaled down [1], [2], [3], [4], [5].
Moderate high-k materials such as Al2O3, has limited effective oxide thickness (Teff) shrinking beyond a sub-0.1 μm gate dielectric due to medium permittivity of K<10 [6]. Meanwhile, many of the high-k materials are not thermally stable on silicon. The formation of SiO2 or metal silicates often occurs when there materials are grown on silicon or during subsequent annealing and ultrahigh-k materials such as STO have been predicted to cause poor channel effects because of the fringing field-induced-barrier lowering effect produced due to its relatively high dielectric constant [7].
Recently, HfO2 and ZrO2 have attracted great attention as a replacement for the nitrided-SiO2 gate oxide film, since they are thermodynamically stable in contact with Si. Among them, HfO2 has been recently highlighted [8], [9], [10] due to its reasonably high dielectric constant (∼25), a relatively large band gap (5.68 eV) [11], high heat of formation (271 kcal/mol) and good thermal stability on Si against reactions with the formation of SiO2-like interface. In addition, excellent metal-oxide-semiconductor capacitors with HfO2 have been demonstrated [12]. Therefore, it seem that HfO2 will be a very promising candidates gate dielectric materials.
High-quality HfO2 thin films have been deposited by PVD [13], [14], CVD [15], Metal-organic chemical-vapor-depositon (MOCVD) [16] and atomic-layer deposition [17]. Among these deposition techniques, one of the serious problems is the interfacial layer growth due to the oxidization of the Si substrate surface, which is brought about in excess oxygen ambient at elevated temperature. CVD processes are also suffering from contaminants that need to be removed by a high temperature deposition or a post deposition annealing. PVD has advantages of simple process, high purity and low cost-of-ownership. In this paper, we report on a plasma oxidation of sputtered metallic hafnium films to form the stoichiometric HfO2 at temperature as low as 400°C and focus on the effect of the postdeposition annealing on the properties of as-grown films on silicon substrate. The properties of structural and interfacial of high-k HfO2 films prepared are demonstrated.
Section snippets
Experimental details
The p-type Si (1 0 0) substrates (cut from one piece of as-purchased Si wafer used in chip manufacturing. This is to ensure that each substrate has an identical native oxide layer, surface morphology, and crystalline orientation.) with a resistivity of 2–5 Ω cm were pre-cleaned by a standard clean process (NH4OH:H2O2:H2O, 2:1:7) at 60°C for 15 min. This cleaning process removes organic matters and other impurity ions adhered to the surface of the substrates, but typically results in a chemical oxide
Results and discussion
Fig. 2 shows a typical series of XRD patterns of the as-grown and annealed HfO2 films at various temperatures. In Fig. 2(a), the as-grown HfO2 films show a featureless diffraction and demonstrate an apparent amorphous structure. Spectra (b) in Fig. 2 show only weak diffraction peaks attributed to tetragonal phase of HfO2 films; no diffraction peaks of Si substrate have been found. This result is in agreement with the report from Neumayer, which demonstrated the crystallization of HfO2 was only
Conclusions
In summary, by subjecting HfO2 films prepared by the plasma oxidation of sputtered metallic Hf film on the substrate to an oxygen/nitrogen postdeposition anneal (PDA), the structural and interfacial properties in relation to annealing temperature were investigated. Based on the X-ray diffraction and SEM analysis of the as-deposited and as-annealed HfO2 samples in O2/N2 ambient, we have shown that higher temperature annealing will bring about the transformation of amorphous structure to
Acknowledgements
We are grateful to National Major Project of Fundamental Research: Nanomaterials and Nanostructures (Grant No.19994506) and Hundred Talent Program from Chinese Academy of Science (Grant No.B20010404).
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