Oliver Chyan

Electrodeposition of Copper Thin Film on Ruthenium A Potential Diffusion Barrier for Cu Interconnects

The electrochemical deposition of copper (Cu) thin film on polycrystalline ruthenium (Ru) electrode surface was investigated in a sulfuric acid plating bath. Scanning electron microscopic characterization indicated that a continuous thin Cu film (150 A and above) could be conformally coated on Ru with good control of thickness. The nucleation and growth of Cu on Ru was studied using the potentiostatic current-transient method. The results support a predominantly progressive nucleation of Cu on the Ru surface. In addition, X-ray diffraction patterns indicated (i) a principally (111) texture of the electrochemically grown Cu on Ru and (ii) the absence of any new phase or compound formation between the two metals, even after annealing up to 800°C. Scotch tape peel tests showed that Cu films adhered strongly to Ru, both before and after the annealing treatments. The lack of metallurgical interaction and strong adhesion between Cu and Ru at elevated temperatures underscore the potential application of Ru as a new Cu diffusion barrier.

5nm ruthenium thin film as a directly plateable copper diffusion barrier

Interfacial stability of electroplated copper on a 5nm ruthenium film supported by silicon, Cu∕(5nmRu)∕Si, was investigated using Rutherford backscattering and high-resolution analytical electron microscopy. Transmission electron microscopy (TEM) imaging shows that a 5nm Ru film is amorphous in contrast to the columnar microstructures of thicker films (20nm). Direct Cu plating on a 5nm Ru film yielded a homogeneous Cu film with over 90% plating efficiency. It is demonstrated that 5nm Ru can function as a directly plateable Cu diffusion barrier up to at least 300°C vacuum anneal. TEM reveals an interlayer between Ru∕Si, which expands at the expense of Ru upon annealing. Electron energy loss spectroscopy analyses show no oxygen (O) across the Cu∕(5nmRu)∕Si interfaces, thereby indicating that the interlayer is ruthenium silicide (RuxSiy). This silicidation is mainly attributed to the failure of the ultrathin Ru barrier at the higher annealing temperature.

Diffusion studies of copper on ruthenium thin film a plateable copper diffusion barrier

Diffusion studies were carried out on physical vapor deposited Cu/Ru(∼20 nm)/Si samples using secondary ion mass spectroscopy (SIMS) and transmission electron microscopy (TEM). Back side SIMS depth profiling revealed well-defined interfaces and showed that Cu interdiffusion was impeded by Ru thin film up to 450°C vacuum annealing. TEM showed a 20-22 nm Ru barrier layer with a columnar microstructure oriented vertically with respect to Si substrate. TEM results corroborate with SIMS data to indicate stability of the Ru film barrier for annealing temperatures up to 450°C. Direct Cu electroplating on ultrathin Ru barrier layers (<20 nm) was investigated in sulfuric acid. The electroplated Cu film is shiny, smooth, and without agglomeration under scanning electron microscopy. Excellent adhesion between interfacial layers was confirmed by the scribe-peel test. The interfacial characterization results indicate that Ru thin film is a promising candidate as a directly plateable Cu diffusion barrier.

Copper Deposition on HF Etched Silicon Surfaces: Morphological and Kinetic Studies

The kinetics and morphologies of Cu deposition on HF-treated silicon surfaces were investigated by atomic force microscopy (AFM), inductively coupled plasma mass spectroscopy (ICP/MS), and graphite furnace atomic absorption spectroscopy (GFAAS). The early stage (<60 s) of Cu deposition, as characterized by AFM, was found to be dominated by the nucleation of nanometer-sized Cu nuclei on HF-treated silicon surfaces. After 60 s of Cu deposition, the total grain number of Cu deposits was leveled to a constant plateau. However, a significant grain size increase of deposition Cu nuclei was noticed. We employed an AFM volume-integration technique in conjunction with the ICP/MS and GFAAS measurements to demonstrate that the Cu deposition rate was limited by the diffusion of Cu 2+ ions across the stationary solution layer toward the silicon surface.

Response of Ceriodaphnia dubia to ionic silver: discrepancies among model predictions, measured concentrations and mortality

Silver thiosulfate, often a waste product of photoprocessing, is less bioavailable or toxic to aquatic organisms than is ionic silver. We conducted duplicate 48-h Ceriodaphnia dubia tests in reconstituted laboratory water using treatments of 92.7 nM Ag+ with various concentrations of thiosulfate. Expected Ag+ concentrations were generated for thiosulfate treatment levels using MINEQL + chemical equilibrium modeling. Ag+ concentrations in treatments were determined using a novel silicon-based sensor. Based on predicted Ag+ and published 48-h LC50 values for C. dubia, we did not expect to observe adverse effects. Yet, 100% mortality was observed at low thiosulfate treatments, whereas > 85% and > 95% survival was observed at higher thiosulfate treatment levels, respectively. Our results indicate that biotic responses match the sensor-based Ag+ concentrations. However, there is a discrepancy between these empirical results and responses expected to occur with Ag+ concentrations as predicted by MINEQL + chemical modeling. By correlating silicon sensor data with toxicity results obtained from our laboratory, our work clearly relates a specific chemical form (Ag+) to toxicity results.

Interfacial Diffusion Studies of Cu ∕ ( 5 nm Ru ) ∕ Si Structures Physical Vapor Deposited vs Electrochemically Deposited Cu

In contrast to physical vapor deposition (PVD), the electrochemical deposition (ECD) process is dependent upon substrate resistivity. ECD of Cu on ultrathin Ru diffusion barriers remains a technological challenge due to large resistivity increase over a wide plating area. Results are presented from the comparative investigation of interfacial stability and Cu diffusion processes in PVD and ECD Cu/(5 nm Ru)/Si structures. Cu can be conformally electroplated onto (5 nm Ru)/Si surfaces (ca. 1 cm 2 ) with over 94% efficiency. However, lesser uniformity and conformality of ECD Cu are observed on (5 nm Ru)/Si samples with larger surface areas. The transmission electron microscopy (TEM) reveals that ECD Cu film is less densely packed (ca. 70 nm) than PVD Cu. HRTEM studies in conjunction with surface analyses using optical microscopy and four-point probe resistivity measurements show that 5 nm Ru can successfully impede Cu diffusion up to 300°C for 10 min, but fails at 450°C. Interfacial profiling data obtained from back side secondary-ion mass spectrometry (SIMS) analysis agree with TEM results. X-ray photoelectron spectroscopy (XPS) investigation on nitric acid-etched PVD and ECD Cu/(5 and 20 nm Ru)/Si samples shows the presence of residual ECD Cu after annealing, suggesting that ECD Cu diffuses further into Ru than PVD Cu.

Underpotential Deposition of Copper on Electrochemically Prepared Conductive Ruthenium Oxide Surface

Copper underpotential deposition (UPD) on a conductive hydrous ruthenium oxide (RuO x H y ) surface was studied by progressive cyclic voltammetry and X-ray photoelectron spectroscopy (XPS). Cu UPD on an electrochemically prepared RuO x H y surface started at 0.15 V in a 2 mM CuSO 4 solution and reached a coverage plateau of ca. 0.9 monolayer (ML) beyond -0.05 V (vs. Ag/AgCl). XPS data confirmed that Cu deposits underpotentially on RuO x H y surface. The anodic polarization potential determines the chemical states and affects the Cu UPD/bulk deposition of RuO x H y electrode. We observed close to a 55% increase of underpotential shift for Cu UPD on RuO x H y (ca. 170 mV) in comparison to Ru surface (ca. 110 mV). The results suggest that interfacial binding of the first ML of Cu on RuO x H y is stronger than Ru by 12 kJ mol - 1 . Possible applications of Cu UPD to Ru-based plateable seedless Cu diffusion barrier are discussed.

Enhancement of the energy photoconversion efficiency through crystallographic etching of a c-plane GaN thin film

Using a simple and inexpensive crystallographic etching technique on a GaN thin film, the energy photoconversion efficiency was increased by 100%. Prior to etching, the thin films solar-to-hydrogen conversion efficiency at the applied bias of 0.5 V versus the counter electrode in 1.0 M HCl solution was 0.37%. After etching, the efficiency doubled to 0.75%. After five hours of continuous gas collection, the unetched GaN thin film yielded a stable photocurrent of 0.41 mA cm−2 which produced 0.10 mL of H2 gas. The etched sample, on the other hand, resulted in an improved stable photocurrent of 0.83 mA cm−2 and yielded a greater volume of 0.70 mL of H2 gas, with the presence of H2 confirmed through gas chromatography. Further investigations have shown that the increased hydrogen generation capacity was possibly caused by three factors: one, increase in surface area caused by the etching process; two, decrease in surface donor concentration caused by the etching as probed through Mott-Schottky plots; and three, the appearance of stepped edges and etched facets that show greater photocatalytic activity than the original c-plane when probed through the photodeposition of Ag particles.

Comparative Studies of Hydrogen Termination on Single-Crystal Silicon Surfaces by FT-IR and Contact-Angle Measurements

The hydrogen termination process on a Si(100) surface has been studied by multiple internal reflection infrared spectroscopy (MIR-IS) and contact-angle measurements. Three main silicon hydride absorption peaks at 2087, 2104, and 2114 cm−1 were found to gradually increase with the hydrofluoric (HF) acid etching. Eventually, a constant peak height was reached as an indication of complete hydrogen termination. Integration of all the surface hydrides absorption peaks (2000 to 2200 cm−1) provides direct quantitative evaluation of the hydrogen termination process. On the other hand, water contact-angle data were shown to consistently lag behind the IR measurement in determining the extent of hydrogen termination on the silicon surface. Analysis of the surface free energy of HF-etched silicon surfaces indicates that the degree of the hydrogen termination determined by water contact-angle measurements is subjected to inaccuracies due to the preferential hydrogen-bonding interaction between the water and silicon surface oxide.

Enhanced Electrochemical Properties of Arrayed CN x Nanotubes Directly Grown on Ti-Buffered Silicon Substrates

The effects of a Ti buffer layer on structural and electrochemical properties of arrayed nitrogen-containing carbon nanotubes (CNx NTs) directly grown on Si substrates have been investigated. Cyclic voltammograms using Fe(CN)(6)(3-)/Fe(CN)(6)(4-) as redox couple were measured to study the electrochemical activities of CNx NTs. The highest peak current density was achieved at an optimal Ti thickness of 20 nm owing to the good conductivity of TiSi2 and high number density of NTs. Therefore, we have demonstrated the direct growth of aligned NTs on Ti- buffered Si with improved electrochemical activity that is believed to be suitable for advanced microsystem applications. (c) 2006 The Electrochemical Society.