Christos G. Takoudis

ALD and Characterization of Aluminum Oxide Deposited on Si ( 100 ) using Tris(diethylamino) Aluminum and Water Vapor

© The Electrochemical Society, Inc. 2006. All rights reserved. Except as provided under U.S. copyright law, this work may not be reproduced, resold, distributed, or modified without the express permission of The Electrochemical Society (ECS). The archival version of this work was published in Katamreddy, R., R. Inman, G. Jursich, A. Soulet, and C. Takoudis, 2006, ALD and characterization of aluminum oxide deposited on Si (100) using tris(diethylamino) aluminum and water vapor: Journal of the Electrochemical Society, v. 153, no. 10, p. C701-C706.

Metalorganic chemical vapor deposition of aluminum oxide on Si: Evidence of interface SiO2 formation

Thin films of aluminum oxide were deposited on H-passivated Si(100) substrate using trimethylaluminum and oxygen at 0.5 Torr and 300 °C. Fourier transform infrared (FTIR) and x-ray photoelectron spectroscopic analyses of these films showed no aluminum silicate phase at the film–substrate interface. The O/Al ratio in the deposited film was found to be higher than that in stoichiometric Al2O3. On annealing the as-deposited samples in Ar at 900 °C, an absorption peak due to the transverse optical phonon for the Si–O–Si stretching mode appeared in the FTIR spectra. A combination of Z-contrast imaging and electron energy-loss spectroscopy in the scanning transmission electron microscope confirmed that the annealed samples developed a layer of silicon dioxide at the aluminum oxide–Si interface. Our results suggest that excess oxygen present in the deposited film reacts with the underlying Si substrate and forms silicon oxide.

Chemical vapor deposition of silicon under reduced pressure in hot-wall reactors

Abstract A detailed mathematical model is discussed for low pressure, hot wall, chemical vapor deposition reactors. Although this model originates from previous studies on low pressure chemical vapor deposition (LPCVD) (Jensen and Graves, 1983), it includes two-dimensional diffusional mass transport between successive wafers, convective and diffusive mass transport in the annulus formed by the edges of the wafers and the reactor wall, and new overall reaction kinetics for the silicon depositing from silane. The system of the resulting coupled reactor equations is solved by finite difference methods. The chemical vapor deposition (CVD) of polysilicon from SiH 4 is studied in detail within the context of the model presented. Experimental data obtained in a LPCVD reactor operating in the electrical engineering department at Purdue University and other data reported in the literature compare very well with the predictions of the model discussed.

Isothermal sustained oscillations in a very simple surface reaction

Abstract A simple model is used to illustrate that a bimolecular Langmuir—Hinshelwood surface reaction with two empty sites in its reaction step, non-equilibrium in the adsorption steps, and coverage independent parameters may lead to sustained oscillatory reaction rates. The two empty sites in the reaction step play an essential role in the establishment of these oscillations. Numerical simulation is used to demonstrate the periodic behavior predicted by the model. Several similar surface reaction models with coverage independent parameters can also yield oscillations. A mechanism with one vacant site in the adsorption steps, two vacant sites in the reaction step and only two dimensionless non-zero parameters may lead to sustained oscillations.

Rapid thermal oxidation of silicon in ozone

Rapid thermal oxidation (RTO) of Si in ozone gas is studied at temperatures between 200 and 550 °C, and the properties of the resulting ultrathin oxides are characterized using in situ mirror-enhanced reflection Fourier transform infrared (IR) spectroscopy. Thus, the frequency and intensity of the longitudinal optical vibrational mode of the Si–O–Si asymmetric stretching from ultrathin oxide films (<30 A) are probed in different processing environments and related to the oxidation kinetics and interfacial layer properties. The oxidation rate in ozone is found to be comparable to the one in pure oxygen at approximately 200 °C higher temperature. Analyses of the oxidation in ozone show a fast oxidation regime followed by a slow one with activation energies of 0.13±0.01 and 0.19±0.04 eV, respectively. Two regions are also observed for the oxidation in pure O2 with activation energies of 0.20±0.03 eV for the fast oxidation regime and 0.36±0.04 eV for the slow one. X-ray photoelectron spectroscopy results and IR spectral feature frequency shifts suggest that the RTO of silicon in ozone ambient results in a thinner, less-stressed interfacial layer than the one obtained in pure O2. Preliminary electrical characterization using surface charge analyses indicates that the oxides formed in ozone are of superior quality.

Investigation on the diffusion barrier properties of sputtered Mo∕W–N thin films in Cu interconnects

Mo∕W–N bilayer thin film structures deposited on Si using sputtering have been studied as a copper diffusion barrier. The thermal stability of the barrier structure after annealing Cu∕Mo∕W–N∕⟨Si⟩ samples in N2 for 5min is studied using x-ray diffraction (XRD), scanning electron microscopy/energy dispersive spectroscopy, and four point probe measurements. The failure of the barrier structure is indicated by the abrupt increase in sheet resistance value and the formation of Cu3Si phase as probed by XRD. Our results suggest that the Mo (5nm)∕W–N (5nm) barrier is stable and can prevent the formation of Cu3Si at least up to 775°C.

Surface-enhanced Raman spectroscopy at transition metal-gas interfaces: Adsorption and reactions of sulfur dioxide on platinum-, rhodium-, and ruthenium-coated gold

Abstract Surface-enhanced Raman (SER) spectra obtained using 647-nm excitation are reported for the adsorption and oxidation of sulfur dioxide in flowing argon-based streams at ambient pressures and at 300 K on electroelectrochemically roughened gold, and on gold surfaces modified by thin (2–3 monolayer) electrodeposited films of platinum, rhodium, and ruthenium. The spectra were recorded by using a spectrograph/charge-coupled device (CCD) detector system, enabling real-time spectral sequences to be obtained within 1–2 s. Dosing with sulfur dioxide yielded a band at 1135 cm −1 on platinum as well as on unmodified gold, assigned to a symmetric S-O stretch of molecularly adsorbed SO 2 . This band is weak on ruthenium, and absent on rhodium. A feature at 300–320 cm −1 is also obtained on gold, platinum, rhodium, and ruthenium, and is identified as the metal-adsorbate stretch for sulfur formed by dissociative SO 2 adsorption. On platinum and gold, oxidation of the adsorbed SO 2 requires the presence of water and CO in addition to O 2 in the feed stream. Under these conditions, the 1135-cm −1 feature is replaced by bands at 1010 and 1020 cm −1 (Pt) or 1010 cm −1 (Au), ascribed to adsorbed sulfate. On ruthenium, including only water and O 2 in the feed yields SO 2 oxidation to sulfate, characterized by a band at 1030 cm −1 . The stability of each species to temperature excursions up to 475 K is noted. Comparison is also made with potential-dependent SER spectra obtained for SO 2 at the corresponding metal-aqueous electrochemical interfaces. Similarly to the gas-phase systems, the surface oxidation as well as molecular adsorption of SO 2 can be monitored at these electrochemical interfaces with SERS. Dissociative chemisorption to yield adsorbed sulfur is strongly potential-dependent, as expected since the process is reductive. Some virtues of such parallel gas-phase and electrochemical surface spectroscopic studies are pointed out.

Annealing behavior of atomic layer deposited hafnium oxide on silicon : Changes at the interface

Thin films of hafnium oxide are deposited on Si(100) substrates by means of atomic layer deposition using tetrakis(diethylamino)hafnium and water on Si(100) at 300°C. Detailed studies of temperature induced annealing effects on the HfO2∕Si interface are done using angle resolved x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy, and time of flight secondary ion mass spectroscopy (ToF-SIMS). As-deposited films show mostly native silicon oxide at the interface. Crystallization of HfO2 film initiates at about 600°C. As the annealing temperature is increased, the hafnium silicate content in the film is found to increase and the mostly silicon oxide interlayer is found to grow thicker under Ar atmosphere. Also, the formation of hafnium silicide is found to take place at temperatures ⩾800°C. The XPS data shows decomposition of the interfacial hafnium silicate layer into hafnium oxide and silicon oxide at 1000°C along with increasing formation of hafnium silicide. The ToF-SIMS data ...

Understanding the Effect of a Fluorinated Ether on the Performance of Lithium–Sulfur Batteries

A high performance Li-S battery with novel fluoroether-based electrolyte was reported. The fluorinated electrolyte prevents the polysulfide shuttling effect and improves the Coulombic efficiency and capacity retention of the Li-S battery. Reversible redox reaction of the sulfur electrode in the presence of fluoroether TTE was systematically investigated. Electrochemical tests and post-test analysis using HPLC, XPS, and SEM/EDS were performed to examine the electrode and the electrolyte after cycling. The results demonstrate that TTE as a cosolvent mitigates polysulfide dissolution and promotes conversion kinetics from polysulfides to Li2S/Li2S2. Furthermore, TTE participates in a redox reaction on both electrodes, forming a solid electrolyte interphase (SEI) which further prevents parasitic reactions and thus improves the utilization of the active material.

Fabrication of Anti-Aging TiO2 Nanotubes on Biomedical Ti Alloys

The primary objective of this study was to fabricate a TiO2 nanotubular surface, which could maintain hydrophilicity over time (resist aging). In order to achieve non-aging hydrophilic surfaces, anodization and annealing conditions were optimized. This is the first study to show that anodization and annealing condition affect the stability of surface hydrophilicity. Our results indicate that maintenance of hydrophilicity of the obtained TiO2 nanotubes was affected by anodization voltage and annealing temperature. Annealing sharply decreased the water contact angle (WCA) of the as-synthesized TiO2 nanotubular surface, which was correlated to improved hydrophilicity. TiO2 nanotubular surfaces are transformed to hydrophilic surfaces after annealing, regardless of annealing and anodization conditions; however, WCA measurements during aging demonstrate that surface hydrophilicity of non-anodized and 20 V anodized samples decreased after only 11 days of aging, while the 60 V anodized samples maintained their hydrophilicity over the same time period. The nanotubes obtained by 60 V anodization followed by 600 °C annealing maintained their hydrophilicity significantly longer than nanotubes which were obtained by 60 V anodization followed by 300 °C annealing.