Hyun Chul Sohn

Investigation of phase transition of Ge2Sb2Te5 and N-incorporated Ge2Sb2Te5 films using x-ray absorption spectroscopy

For this study, the phase-change materials Ge2Sb2Te5 and N-doped Ge2Sb2Te5 films were investigated using x-ray absorption near-edge structure and extended x-ray absorption fine structure. During the phase transition, change in electronic structure is observed by the shift of the absorption edge energy, i.e., structural coordination of Ge–Te changes from tetrahedral to octahedral coordination, of which the interatomic distances are 3.12 and 2.83A, respectively. In addition, nitrogen incorporation into the film led to a p-p orbital hybridization and a different crystallization behavior. The hybridization caused by the formation of a Ge–N bond was related to suppression of the phase transition.

Effect of In incorporated into SbTe on phase change characteristics resulting from changes in electronic structure

The effects of the In content of InSbTe films with various stoichiometries (Sb2Te2.7, In0.5Sb2Te2.9, and In2.6Sb2Te2.9) on phase change characteristics were investigated. With increasing incorporation of In atoms into Sb2Te3, various crystalline phases, i.e., In2Te3, Sb, and In3SbTe2, were observed due to the bond energy between the constituent atoms, while only Sb2Te3 and the Sb2Te2 phases were observed in the case of Sb2Te2.7 and In0.5Sb2Te2.9 films. In addition, the shifts in binding energy of the Sb 3d and In 3d peaks in x-ray photoelectron spectra after the annealing treatment were directly related to the amount of incorporated In. The observed changes in electronic structure suggest that the changes in electrical conductivity and crystalline phase are directly related to the extent of In incorporation.

Effects of ceric ammonium nitrate (CAN) additive in HNO3 solution on the electrochemical behaviour of ruthenium for CMP processes

In order to develop a new chemical mechanical polishing process for ruthenium (Ru), the present work deals with the effect of a ceric ammonium nitrate (CAN) additive on the electrochemical behaviour of physical vapour deposited Ru films in a 1 M HNO3 solution employing electrochemical methods and surface analytical techniques. By adding CAN to HNO3 solution, the polarisation curves showed an increase in the corrosion potential and current, suggesting that Ru is anodically polarised by CAN as an oxidising additive. To characterise the influence of CAN, open-circuit potential (OCP) and potentiostatic anodic current transient curves were examined in CAN-containing HNO3 solution and the resulting surfaces were then characterised by scanning electron microscopy and X-ray photoelectron spectroscopy. It is proposed that Ru is oxidised to heterogeneous Ru2O3 and RuO2 films on the Ru surface in CAN-containing HNO3 solution and galvanic corrosion occurs at grain boundaries, caused by the difference in OCP between the grain interiors and boundaries. The grain boundaries are oxidised to RuO4, a volatile species, resulting in a roughened and porous structure.

(Bi, La)4Ti3O12 (BLT) thin films grown from nanocrystalline perovskite nuclei for ferroelectric memory devices

Using nanocrystalline perovskite nuclei, (117) oriented-(Bi,La)4Ti3O12(BLT) thin films were grown using a noble bake process for nonvolatile ferroelectric memory devices. The c-axis oriented BLT thin films have a remanent polarization (2Pr) of 8.0μC∕cm2 at a 3V driving voltage, and the (117) oriented films have a 2Pr value of about 25μC∕cm2. The BLT capacitors, grown on a platinum electrode via nanocrystalline perovskite nuclei, had fatigue and imprint free characteristics after applying 1×1011 switching cycles and for ten years at a 125°C stress. The average sensing margin of the (117) oriented BLT thin films was approximately 700mV for a 0.65μm2 cell size and a sufficient signal margin for ten years was indicated, based on the extrapolation of the measured data for high density ferreoelectric random access memory applications.

Interfacial reaction induced phase separation in LaxHfyO films

Amorphous LaxHfyO films containing La at concentrations (x) of 50 and 20% were prepared by atomic layer deposition on ultrathin SiO2 films (1 nm). We examined the electronic structures and microstructures of the LaxHfyO films by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Phase separation into La2O3 and HfO2 was observed in the LaxHfyO films subjected to annealing temperatures over 900 °C, although the mixture of La2O3 and HfO2 is thermodynamically stable. The structural changes that occurred as the result of phase separation were dependent on the concentrations of La and Hf in the films. During the annealing treatment, silicate was produced due to interfacial reactions and the interfacial reactions were found to be dependent on the La2O3 content in the LaxHfyO films, which has a significant influence on the phase separation process and resulting film structure.

Physical and electrical characteristics of physical vapor-deposited tungsten for bit line process

We attempted to explain the phenomenon that the electric resistivity of tungsten film increases as the thickness decreases and that physical vapor-deposited (PVD) tungsten shows a much lower resistivity than chemical vapor-deposited (CVD) tungsten. The crystallinity and electric conductivity of an under-layer does not affect the electric resistivity of tungsten film. The low resistivity of PVD tungsten originates from a large grain size. PVD tungsten with large grains is free from grain boundary scattering, while CVD tungsten with small grains exhibits grain boundary scattering. As film thickness decreases down to the mean free path of tungsten, the surface scattering effect surpasses the grain boundary scattering effect. Consequently, the resistivity of PVD and CVD tungsten becomes equal because surface scattering increases the resistivity of both large- and small-grained films. The same rule is adaptable for the resistance change of a narrow line structure. CVD tungsten shows a high resistance because of grain boundary scattering originating from a small grain size. However, if the line-width is reduced to the mean free path, grain boundary scattering disappears, making surface scattering only the factor that increases electric resistivity. Thus, CVD tungsten shows the same resistance as PVD tungsten in a very narrow line structure.

Characteristics of the nanoscale titanium film deposited by plasma enhanced chemical vapor deposition and comparison of the film properties with the film by physical vapor deposition

TiSi2 film was grown by plasma enhanced chemical vapor deposition (PECVD) method for lower contact resistance. We obtained a uniform TiSi2 film on Si substrate. The growth rate of TiSi2 on Si substrate is four times higher than that of Ti on silicon dioxide. The thickness ratio of TiSi2∕Si is very high at the early stage of growth. The high ratio, 8.8, results from a long incubation time before the formation of Ti film on silicon dioxide and ignoring the incubation time, thickness ratio is almost constant in the given thickness range. The average of thickness ratio is determined as 4.25. C49 (060) phase was formed on Si substrate by PECVD method and C49 TiSi2 phase was not changed into C54 one by rapid thermal annealing (RTA) at 900°C. We could not observe boron segregation at the TiSi2∕Si interface. On the contrary, we observed inward diffusion of boron into Si substrate during the silicide formation. We obtained 1.21×103Ω of contact resistance by PECVD method. When Ti thickness is less than 1nm, the con...

Boron pile-up at the interface between plasma enhanced chemical vapor deposited TiSi2 film and BF2-doped Si

We have investigated the redistribution of boron in BF2-doped Si substrate during plasma enhanced chemical vapor deposition (PECVD) of TiSi2 with secondary ion mass spectrometry. Boron concentration was observed to be increased at the TiSi2/Si interface after deposition of PECVD TiSi2. Contact resistance with PECVD TiSi2 was measured to be constant in the range of −30–125 °C, implying that the accumulated boron was electrically active and charge transport was field emission in nature.