Zhiqiang Chen

Application of Metastable Phase Diagrams to Silicate Thin Films for Alternative Gate Dielectrics

Using the concept of metastable phase diagrams, we discuss the microstructure evolution during annealing of amorphous ZrO2–SiO2 and HfO2–SiO2 thin films for gate dielectric applications. These systems are characterized by a low solid solubility, a liquid miscibility gap and a kinetic barrier to the formation of the complex, crystalline silicate. We show that phase partitioning is expected for most compositions. Compositions within the metastable extensions of the spinodal are unstable and will spontaneously unmix in the amorphous phase upon heating. Hafnia- or zirconia-rich phases will subsequently crystallize to form HfO2 or ZrO2. Most compositions outside the metastable extensions of the liquid phase miscibility gap must phase separate above the crystallization temperature by nucleation of crystalline HfO2 or ZrO2 out of an amorphous silica-rich matrix. We present calculations of the metastable extensions of the miscibility gap and spinodal. The calculations predict that SiO2-rich compositions, investigated for gate dielectric applications, will show spinodal decomposition if they contain less than ~90 mol% SiO2 at the typical device processing temperature of 1000°C. Experimental studies of Hf-silicate films with three different compositions, between ~40 and 80 mol% HfO2 that lie inside and outside the miscibility gap, respectively, are presented. All three compositions show phase separation. Despite the different pathways of microstructure evolution, the final phase separated microstructures are similar. Experimental verification of the pathways that lead to these microstructures requires further studies.

Stability of ZrO2 layers on Si(001) during high-temperature anneals under reduced oxygen partial pressures

Electron energy-loss spectroscopy and high-resolution transmission electron microscopy were used to investigate ZrO2 layers grown by electron-beam evaporation in a molecular-beam epitaxy system. ZrO2/Si layers were investigated before and after uncapped annealing at 1000u200a°C under different oxygen partial pressures. The thickness of a SiO2-like, low-dielectric constant layer at the silicon interface was found to depend on the oxygen partial pressure during annealing. At oxygen partial pressures of about 10−4u2009torr the interfacial silicon oxide thickness increased through oxygen diffusion through the ZrO2 layer and silicon consumption at the interface. At oxygen partial pressures in the range of approximately 10−5u2009torr, only a thin (1 nm) interfacial silicon oxide layer was present, as required for low-equivalent oxide thicknesses of gate stacks incorporating alternative oxides. Further reduction of the oxygen partial pressures (about 10−7u2009torr) during annealing resulted in zirconium silicide formation at th...

Composition control and dielectric properties of bismuth zinc niobate thin films synthesized by radio-frequency magnetron sputtering

Radio-frequency sputtering was used to deposit near-stoichiometric Bi1.5Zn1.0Nb1.5O7 (BZN) films. Composition, crystallinity, and phase purity of the films were investigated by Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy (TEM). X-ray diffraction detected cubic pyrochlore in films that were annealed above 400u200a°C. Films were fully crystallized at 750u200a°C. TEM and electron diffraction confirmed the cubic pyrochlore structure of the grains. Dielectric constant and loss were measured using planar Si/SiO2/Pt/BZN/Pt and Al2O3/Pt/BZN/Pt capacitor structures, respectively. After annealing at 750u200a°C, films on Pt coated silicon wafers showed a permittivity of 170, low dielectric losses, and a large electric field tunability of the dielectric constant at a measurement frequency of 1 MHz. Dielectric loss tangents improved when substrates were moderately heated during deposition.Radio-frequency sputtering was used to deposit near-stoichiometric Bi1.5Zn1.0Nb1.5O7 (BZN) films. Composition, crystallinity, and phase purity of the films were investigated by Rutherford backscattering spectrometry, x-ray diffraction, and transmission electron microscopy (TEM). X-ray diffraction detected cubic pyrochlore in films that were annealed above 400u200a°C. Films were fully crystallized at 750u200a°C. TEM and electron diffraction confirmed the cubic pyrochlore structure of the grains. Dielectric constant and loss were measured using planar Si/SiO2/Pt/BZN/Pt and Al2O3/Pt/BZN/Pt capacitor structures, respectively. After annealing at 750u200a°C, films on Pt coated silicon wafers showed a permittivity of 170, low dielectric losses, and a large electric field tunability of the dielectric constant at a measurement frequency of 1 MHz. Dielectric loss tangents improved when substrates were moderately heated during deposition.

Phase Separation in Hafnium Silicates for Alternative Gate Dielectrics Influence on the Unoccupied States

We have used X-ray absorption near-edge fine-structure (XANES) analysis of oxygen K-edges in combination with high-resolution transmission electron microscopy to investigate phase separation in Hf-silicate thin films subjected to high temperature anneals. We show that the oxygen K-edge fine structures can, in a first approximation, be interpreted as an overlap of features from amorphous silica and crystalline hafnia phases in the phase separated microstructures. Increasing amounts of silica in the samples resulted in an increase of bulk silica-like features in the oxygen K-edges. The lowest conduction band levels are determined by the Hf d states of hafnia for all compositions investigated. XANES was not able to detect silicate-type bonding that is expected to be present in some of the films.

Chemical, Physical, and Electrical Characterizations of Oxygen Plasma Assisted Chemical Vapor Deposited Yttrium Oxide on Silicon

Understanding and controlling interface and bulk chemical stability of chemical vapor deposited high-k dielectrics is an important research issue. We report thin Y 2 O 3 films deposited by oxygen plasma assisted chemical vapor deposition using two yttrium diketonate precursors. Unacceptable large hysteresis in capacitance-voltage data, presumably due to the incorporation of fluorine. is observed for the films from the F-containing precursor. For films deposited with the hydrogenated precursor and exposed to air after deposition, transmission electron microscopy shows a triple layer structure after annealing, and electron energy loss spectroscopy and X-ray photoelectron spectroscopy show the film to be stoichiometric Y 2 O 3 on top and yttrium silicate/SiO 2 at dielectric/Si interface. This structure is also confirmed by Fourier transform infrared spectroscopy, X-ray diffraction, and atomic force microscopy. Prenitridation of the silicon surface prior to dielectric deposition impedes the reaction with the substrate. promoting the Y 2 O 3 structure. A substantial consumption of silicon substrate is directly demonstrated by a carefully designed etching experiment. Possible mechanisms consistent with the observed results, including Si diffusion, crystallization of Y 2 O 3 , and reaction with absorbed OH, are discussed.

Reactions of Y2O3 films with (001) Si substrates and with polycrystalline Si capping layers

We use electron energy-loss spectroscopy in scanning transmission electron microscopy to investigate interfacial reactions of chemical vapor deposited Y2O3 films with the Si substrate and with in situ polycrystalline Si (“poly-Si”) capping layers after postdeposition annealing. We find that in situ capping layers significantly reduce the formation of SiO2 at the interface with the substrate, but silicates form at the substrate and the capping layer interfaces. Predeposition nitridation of the Si surface can impede the reaction at the substrate interface, resulting in crystallization of Y2O3 in the film interior. Possible mechanisms of the silicate formation are discussed.

Electron energy-loss spectroscopy analysis of interface structure of yttrium oxide gate dielectrics on silicon

Interface stability of high dielectric constant gate insulators on silicon is an important issue for advanced gate stack engineering. In this article, we analyze the silicon/dielectric interface structure for thin Y2O3 and Y silicate films deposited by chemical vapor deposition on clean and prenitrided Si(100) using high-resolution transmission electron microscopy, electron energy-loss spectroscopy, and x-ray photoelectron spectroscopy. The analysis shows the films to be stoichiometric Y2O3 on top and Y-silicate/SiO2 at the dielectric/Si interface. Prenitridation of the silicon surface impedes the reaction between the depositing film and the substrate, promoting a Si-free Y2O3 structure. Possible mechanisms leading to the observed Y2O3 and Y silicate structures are discussed.

Electron energy-loss spectroscopy study of thin film hafnium aluminates for novel gate dielectrics

We have used conventional high‐resolution transmission electron microscopy and electron energy‐loss spectroscopy (EELS) in scanning transmission electron microscopy to investigate the microstructure and electronic structure of hafnia‐based thin films doped with small amounts (6.8 at.%) of Al grown on (001) Si. The as‐deposited film is amorphous with a very thin (∼0.5 nm) interfacial SiOx layer. The film partially crystallizes after annealing at 700 °C and the interfacial SiO2‐like layer increases in thickness by oxygen diffusion through the Hf‐aluminate layer and oxidation of the silicon substrate. Oxygen K‐edge EELS fine‐structures are analysed for both films and interpreted in the context of the films’ microstructure. We also discuss valence electron energy‐loss spectra of these ultrathin films.