The structural and interfacial properties of HfO₂/Si by the plasma oxidation of sputtered metallic Hf thin films

Abstract

The thermal stability and interfacial structure characteristics for HfO₂ dielectrics formed on Si substrates by plasma oxidation of sputtered metallic Hf films were investigated. The sputtered Hf metallic films were annealed under oxygen and nitrogen atmospheres. The structural characteristics and surface morphology of the Hf/HfO₂ layers at varies of postannealing temperatures (from 600°C to 900°C) were examined by X-ray diffraction and scanning electron microscopy. The structure of the formed HfO₂ films undergo a transformation of tetragonal to monoclinic phase with increase of the annealing temperature and demonstrated a polycrystalline structure at high temperature annealing. The growth and properties of the interfacial SiO₂ layers formed at the HfO₂/Si interface were observed by using fourier transform infrared spectroscopy. It has been found that formation of the interfacial layer depends on the postdeposition annealing temperature. N₂-annealed HfO₂ films will lead to the decomposition of interfacial SiO₂ layer and bring about the slight shift of Si–O–Si bonds vibration peak position toward lower wave numbers.

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 (T_eff) 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.