Guoshun Pan

Atomic scale deformation in the solid surface induced by nanoparticle impacts

Nanoparticle impacts on an ultra-smooth surface always occur in nano-machining processes, such as polishing of a monocrystalline silicon wafer, which is an important process in the manufacture of semiconductors. A fundamental understanding of nanoparticle impacts on a solid surface is important to control and prevent the deformation of the surface. In this study, a cylindrical liquid jet containing de-ionized water and SiO2 nanoparticles impacts obliquely on a single crystal silicon surface at a speed of 50?m?s?1. The microstructure of the impacted surface was examined using a high resolution transmission electron microscope, an atomic force microscope, etc. Some crystal defects, lattice distortion, grain refinement and rotation of grains in the surface layer of the silicon wafer after exposure for 30?s have been observed. However, when the exposure time is extended to 10?min, an amorphous layer containing crystal grains is exhibited in the subsurface, and many craters, scratches and atom pileups can be found in the surface.

Progress in material removal mechanisms of surface polishing with ultra precision

Chemical mechanical polishing (CMP) process is commonly regarded as the best method for achieving global planarization in the field of surface finishing with ultra-precision. The development of investigation on material removal mechanisms for different materials used in computer hard disk and ultra-large scale integration fabrication are reviewed here. The mechanisms underlying the interaction between the abrasive particles and polished surfaces during CMP are addressed, and some ways to investigate the polishing mechanisms are presented.

Probing Particle Movement in CMP with Fluorescence Technique

Chemical mechanical polishing (CMP) is now widely used in semiconductor manufacturing. Characterizing the particle movement in CMP is essential to understanding of the complicated material removal mechanisms for CMP. In this study, an experimental system based on fluorescence technique has been designed and developed. The particle displacement and velocity in the interface between a transparent wafer and a polishing pad were observed in-situ with the system. It can be found that some particles were embedded on the polishing pad and rotated with the pad. And the others were able to move free in slurry flow. All the particles in the experiments could be classified into four groups. The counting procedure suggested that the percentage of free particles in the field of view was between 10 and 50%. By analyzing the velocity of free particles at various rotation speeds and applied pressure, it was found that some of the free particles were able to travel at a speed 12―18 times the linear speed. The uncertainty in the velocity measurements was less than 8%. The objective of this study was to offer deep insights into the material removal mechanisms by measuring the real-time particle movement in CMP.

Nitrogen/sulfur co-doped non-noble metal material as an efficient electrocatalyst for the oxygen reduction reaction in alkaline media

This work demonstrates the feasibility of nitrogen/sulfur co-doped non-noble metal materials (Fe–N/C–TsOH) as platinum-free catalysts for the oxygen reduction reaction (ORR) in alkaline media. Electrochemical techniques such as cyclic voltammetry (CV), rotating disk electrodes (RDEs) and rotating ring-disk electrodes (RRDEs) are employed with the Koutecky–Levich theory to investigate the ORR kinetic constants and the reaction mechanism. It is found that the catalysts doped with TsOH (p-toluenesulfonic acid) show significantly improved ORR activity relative to a TsOH-free catalyst. The overall electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098, respectively, for the catalysts with and without TsOH-doping. Catalysts heat treated at 600 °C exhibit relatively higher activity. In addition, the catalyst doped with TsOH (Fe–N/C–TsOH-600) not only exhibits exceptional stability in 0.1 mol L−1 KOH solution but also has higher methanol tolerance compared to commercial Pt/C catalyst in 0.1 mol L−1 KOH. To some extent, increasing the Fe–N/C–TsOH-600 loading on the electrode favors a faster reduction of H2O2 to intermediate to H2O. X-ray photoelectron spectroscopy analysis indicates that pyrrolic N groups are the most active sites, and that sulfur species are structurally bound to carbon in the forms of C–S(n)–C and oxidized –SO(n)– bonds, an additional beneficial factor for the ORR.

Atomically smooth gallium nitride surface prepared by chemical-mechanical polishing with different abrasives:

For chemical-mechanical polishing of epitaxial gallium nitride (GaN), a two-step experiment method with two kinds of abrasives, aluminum oxide (Al2O3) and colloidal silica (SiO2), was put forward. The average material removal rates of GaN by the slurry with Al2O3 and SiO2 abrasives were 594.79 and 66.88 nm/h, respectively. An atomically flat surface with roughness (Ra) of 0.056 nm was obtained after the second chemical-mechanical polishing process with SiO2-based slurry, which presented an atomic step-terrace structure. The material removal characteristics of GaN surfaces were investigated in detail. A model was proposed to describe the different behaviors of the two kinds of abrasive during chemical-mechanical polishing process.

Atomic Step Formation on Sapphire Surface in Ultra-precision Manufacturing

Surfaces with controlled atomic step structures as substrates are highly relevant to desirable performances of materials grown on them, such as light emitting diode (LED) epitaxial layers, nanotubes and nanoribbons. However, very limited attention has been paid to the step formation in manufacturing process. In the present work, investigations have been conducted into this step formation mechanism on the sapphire c (0001) surface by using both experiments and simulations. The step evolutions at different stages in the polishing process were investigated with atomic force microscopy (AFM) and high resolution transmission electron microscopy (HRTEM). The simulation of idealized steps was constructed theoretically on the basis of experimental results. It was found that (1) the subtle atomic structures (e.g., steps with different sawteeth, as well as steps with straight and zigzag edges), (2) the periodicity and (3) the degree of order of the steps were all dependent on surface composition and miscut direction (step edge direction). A comparison between experimental results and idealized step models of different surface compositions has been made. It has been found that the structure on the polished surface was in accordance with some surface compositions (the model of single-atom steps: Al steps or O steps).

Direct observation of oil displacement by water flowing toward an oil nanogap

A fluorescence microscope and a light microscope were employed to observe the phenomenon of water flowing toward an oil nanogap between two solid surfaces. It was found that water was able to displace hexadecane in the nanogap confinement, which contradicted previous viewpoints. An increase in water flow speed contributed to entrainment of water into the contact region, due to inadequate oil supply. Surface energy was found to be another factor that influenced the displacement phenomenon. It was easier for water to enter the contact region on the surface with a greater surface energy, since less energy is required to separate the contact of hexadecane and solid surface and to form water’s own contact.

Chemical Mechanical Planarization of Copper Using Ethylenediamine and Hydrogen Peroxide Based Slurry

Chemical-mechanical planarization (CMP) of copper is a committed step hi the IC manufacturing. In this work, the slimy including ethylenediamine and hydrogen peroxide was studied. Result showed that the material removal rate of copper increased with the concentration of ethylenediamine, and the effect of the concentration of hydrogen peroxide was also studied. When the concentration of ethylenediamine was 100mM/L, with the concentration of 0.6% hydrogen peroxide, we got a high material removal rate, that is 1899mn/min. Some corrosion inhibitors like benzotriazole and potassium sorb ate were added to the slurry to improve the surface after CMP.

Effects of mixed inhibitors in copper chemical mechanical polishing at a low down pressure

The individual and synergistic effects of benzotriazole and sodium dodecyl sulfonate on copper disc are investigated at a down pressure of 3.4 kpa. The corrosion rate is reduced by using benzotriazole, but the surface quality is poor owing to the slow formation rate of copper–benzotriazole protection film. For sodium dodecyl sulfonate, the corrosion rate is further reduced, but slight corrosions can be still observed due to the incomplete coverage of sodium dodecyl sulfonate molecules. The combination of sodium dodecyl sulfonate and benzotriazole leads to the maximum decrease of corrosion rate. Under the benzotriazole/sodium dodecyl sulfonate ratio of 1:3, the average roughness achieves as low as 1.025 nm owing to the soft and dense coverage of benzotriazole/sodium dodecyl sulfonate molecules. Besides, the trends of removal rate, roughness and friction coefficient are proved to be similar for copper discs and copper wafers in slurries with different inhibitors. Therefore, the results presented here are relevant for further developments in the area of low-pressure chemical mechanical planarization of copper lines overlying fragile low-k dielectrics in the new interconnect structures.

Synthesis of dual-doped non-precious metal electrocatalysts and their electrocatalytic activity for oxygen reduction reaction

Abstract The pyrolyzed carbon supported ferrum polypyrrole (Fe-N/C) catalysts are synthesized with or without selected dopants, p -toluenesulfonic acid (TsOH), by a facile thermal annealing approach at desired temperature for optimizing their activity for the oxygen reduction reaction (ORR) in O 2 -saturated 0.1 mol/L KOH solution. The electrochemical techniques such as cyclic voltammetry (CV) and rotating disk electrode (RDE) are employed with the Koutecky-Levich theory to quantitatively obtain the ORR kinetic constants and the reaction mechanisms. It is found that catalysts doped with TsOH show significantly improved ORR activity relative to the TsOH-free one. The average electron transfer numbers for the catalyzed ORR are determined to be 3.899 and 3.098, respectively, for the catalysts with and without TsOH-doping. The heat-treatment is found to be a necessary step for catalyst activity improvement, and the catalyst pyrolyzed at 600 °C gives the best ORR activity. An onset potential and the potential at the current density of – 1.5 mA/cm 2 for TsOH-doped catalyst after pyrolysis are 30 mV and 170 mV, which are more positive than those without pyrolized. Furthermore, the catalyst doped with TsOH shows higher tolerance to methanol compared with commercial Pt/C catalyst in 0.1 mol/L KOH. To understand this TsOH doping and pyrolyzed effect, X-ray diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) are used to characterize these catalysts in terms of their structure and composition. XPS results indicate that the pyrrolic-N groups are the most active sites, a finding that is supported by the correspondence between changes in pyridinic-N content and ORR activity that occur with changing temperature. Sulfur species are also structurally bound to carbon in the forms of C–S n –C, an additional beneficial factor for the ORR.