**1. Introduction**

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A very great number of metal oxide thin films are produced by sol-gel methods. Metallic compounds dissolved in organic solvents are hydrolyzed and polymerized by adding H2O with an acid or a base and heating to obtain metal oxide sols. Metal oxide thin films are prepared by coating the sols on substrates followed by firing (Kozuka, 2005).

The sol-gel method produces amorphous or crystalline thin gel films of metallic solid compounds by solidifying a sol formed by hydrolyzing and polymerizing a solution containing metallic compounds. Sol-gel processes are widely employed in the field of chemistry to prepare ceramic powders and thin films of hafnium oxide (HfO2) (Nishide et al., 2000) and zirconium oxide (ZrO2) ( Liu et al., 2002) for obtaining high-quality ceramics and insulators, offering the advantages of low cost, relative simplicity, and easy control of the composition of the layers formed. This chapter describes the characterization of sol-gelderived and crystallized HfO2 and ZrO2 thin films intended for use as gate insulators with high dielectric constants in electronic devices.

In the electronic device field, the continuing miniaturization of silicon (Si) ultra-large-scaleintegration (ULSI) devices has required an ultrathin gate Si dioxide (SiO2) and oxynitride film; upon scaling down to 32-22 nm technology nodes and beyond, thinner SiO2 gate oxide films have been required. At these thicknesses, gate leakage currents due to direct tunneling become comparable to the off-currents of metal-oxide-semiconductor (MOS) field-effect transistors (FETs), increasing the off-state power consumption of the devices. In further scaled-down advanced Si complementary MOS (CMOS) devices, scaling trends have required the substitution of gate SiO2 by insulators with higher dielectric constants (high-*k*) ( Huff & Gilmer, 2004). The aim of using high-*k* materials is to increase the film thickness, thus reducing the tunneling leakage current, while scaling the capacitance of the equivalent oxide thickness (EOT) below the direct tunneling limit of SiO2 ( Huff & Gilmer, 2004). Several high-*k* material candidates, such as HfO2 (Blanchin et al., 2008), ZrO2 (Niinisto et al., 2004), Al2O3, ZrO2-Y2O3 (YANG, 1996), Y2O3 (Nishide & Shibata, 2001),La2O3 (Ng et al., 2005), and gate stack structures have been proposed and some materials have been put into practical use. All of them are either oxides or silicates of 4d or 5d transition metals or rare earth elements.

Characterization of Sol-Gel-Derived and Crystallized

HfO2, ZrO2, ZrO2-Y2O3 Thin Films on Si(001) Wafers with High Dielectric Constant 317

(b)

SiO2

Fig. 1. Cross-sectional views of HfO2 films obtained by using a high-resolution TEM: (a) a HfO2 film fired at 450 oC and (b) a HfO2 film fired at 700 oC (Shimizu et al., 2004).

**10 nm** 

Si

HfO2

Hf 4f 7/2 16.2eV

 (a) (b) Fig. 2. XPS spectra of sol-gel-derived HfO2 films. Solid lines are observed spectra and those fitted by the nonlinear least-squares algorithm. Dashed lines for O 1s spectra have two

Intensity (arb.unit)

O 1s

531.8eV

HBC

534 532 530 528 526

530.1eV

HfO2

Si

Si

LBC

Binding Energy (eV)

Gaussian peaks corresponding to Hf-OH (531.8 eV) and Hf-O (530.1 eV)

24 22 20 18 16 14 12

Binding Energy (eV)

(Shimizu et al., 2007).

Intensity (arb.unit)

SiO

(a)

Hf 4f

HfO2 and ZrO2 thin films are the most promising candidates as alternative high permittivity (high-*k*) oxides for replacing the SiO2 gate dielectric material used in CMOS devices (Gusev, 2005, Wilk et al., 2001). Because of the higher permittivity, the dielectric gate insulator thickness can be increased for a given capacitance, resulting in reduced tunneling leakage current. HfO2 has promising properties such as high permittivity (25~40) (Oniki et al. 2009, Wilk et al., 2000, 2001), a conduction band offset as high as 1.5 eV (Lucovsky, 2002), and a wide band gap (~5.68 eV) (Robertson & Chen, 1999, Robertson, 2000).

In device fabrication processes, HfO2 and ZrO2 thin film layers are deposited by chemical vapor deposition (CVD) or physical vapor deposition (PVD) or sputtered onto Si substrates (Gao et al., 2000, Wang et al., 2005) using argon (Ar) and O2 mixed gases. A sol-gel process offers various advantages for fabricating ZrO2, HfO2, ZrO2–Y2O3 (YANG, 1996) and HfO2 – Y2O3 (Nishide et al., 2000) thin films. The properties of a sol-gel-derived thin film depend on the composition of the sol solution, and residual H2O may affect the performance of the film. Investigations of the basic structural and optical properties of sol-gel-derived HfO2 films have shown that HfO2 films formed on quartz substrates begin to crystallize at a firing temperature of 550 oC as determined from X-ray diffraction (XRD) patterns (Nishide et al., 2000). From the interplanar spacing they derived from the XRD patterns and a comparison of their data with data from a Joint Committee on Powder Diffraction Standards (JCPDS) card, they determined the crystalline phase of the sol-gel-derived HfO2 film to be monoclinic. Recently, on the basis of high-resolution transmission electron microscopy (HRTEM) measurements in combination with results of electron beam nanodiffraction analyses, sol-gel-derived HfO2 thin films on Si(001) wafers were found to crystallize in a monoclinic face-centered cubic (fcc) structure (Shimizu et al., 2004). Sol-gel-derived ZrO2 thin films fired in air at 350 and 450 °C on Si(001) wafers are reported to be amorphous and around 9-10 nm in thickness. Crystallization occurs first at 550 °C as amorphous/tetragonal (011), and finally at 700°C, the ZrO2 film crystallizes into tetragonal (011)/monoclinic (111) and (111) structures (Shimizu et al., 2009). Electrical characteristics have been evaluated using capacitors with an Al/ZrO2 and/or HfO2/Si sandwich structure. The leakage current and dielectric constant of the films have been examined using current-voltage (*I-V*) and capacitance-voltage (*C-V*) methods. On the basis of *C-V* characteristics, the dielectric constant (relative permittivity: *ε*ZrO2 and *ε*HfO2) of sol-gel derived ZrO2 and HfO2 thin films fired in air were shown to be far higher than that of silicon dioxide (SiO2: 3.9) (Shimizu et al., 2009, 2010). This chapter summarizes the characterizations of sol-gel-derived HfO2, ZrO2 and ZrO2-Y2O3 thin films on Si(001) wafers with the aim of showing their suitability as alternative gate insulator materials in advanced CMOS devices.
