Xinchun Lu

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.

Tribochemistry and Superlubricity Induced by Hydrogen Ions

Friction behavior of aqueous solution at macroscale is quite different from that at nanoscale. At macroscale, tribochemistry usually occurs between lubricant and friction surfaces in the running-in process due to a high contact pressure, and most such processes can lead to friction reduction. In the present work, we reported that the hydrogen ions in aqueous solution played an important role in tribochemistry in running-in process (friction reducing process), which could result in the friction coefficient reducing from 0.4 to 0.04 between Si(3)N(4) and glass surfaces at macroscale. It is found that the running-in process and low friction state are closely dependent on the concentration of hydrogen ions in the contact region between the two friction surfaces. The lubrication mechanism is attributed to tribochemical reaction occurring between hydrogen ions and surfaces in the running-in process, which forms an electrical double layer and hydration layer to lower friction force. Finally, the running-in process of H(3)PO(4) (pH = 1.5) was investigated, which could realize superlubricity with an ultralow friction coefficient of about 0.004.

Monoatomic layer removal mechanism in chemical mechanical polishing process: A molecular dynamics study

Molecular dynamics simulations of nanoscratching processes were used to study the atomic-scale removal mechanism of single crystalline silicon in chemical mechanical polishing (CMP) process and particular attention was paid to the effect of scratching depth. The simulation results under a scratching depth of 1 nm showed that a thick layer of silicon material was removed by chip formation and an amorphous layer was formed on the silicon surface after nanoscratching. By contrast, the simulation results with a depth of 0.1 nm indicated that just one monoatomic layer of workpiece was removed and a well ordered crystalline surface was obtained, which is quite consistent with previous CMP experimental results. Therefore, monoatomic layer removal mechanism was presented, by which it is considered that during CMP process the material was removed by one monoatomic layer after another, and the mechanism could provide a reasonable understanding on how the high precision surface was obtained. Also, the effects of the...

Abrasive rolling effects on material removal and surface finish in chemical mechanical polishing analyzed by molecular dynamics simulation

In an abrasive chemical mechanical polishing (CMP) process, materials were considered to be removed by abrasive sliding and rolling. Abrasive sliding has been investigated by many molecular dynamics (MD) studies; while abrasive rolling was usually considered to be negligible and therefore was rarely investigated. In this paper, an MD simulation was used to study the effects of abrasive rolling on material removal and surface finish in the CMP process. As the silica particle rolled across the silicon substrate, some atoms of the substrate were dragged out from their original positions and adhered to the silica particle, leaving some atomic vacancies on the substrate surface. Meanwhile, a high quality surface could be obtained. During the abrasive rolling process, the influencing factors of material removal, e.g., external down force and driving force, were also discussed. Finally, MD simulations were carried out to examine the effects of abrasive sliding on material removal under the same external down force as abrasive rolling. The results showed that the ability of abrasive rolling to remove material on the atomic scale was not notably inferior to that of abrasive sliding. Therefore, it can be proposed that both abrasive sliding and rolling play important roles in material removal in the abrasive CMP of the silicon substrate.

Effects of surface deformation on corrosive wear of stainless steel in sulfuric acid solution

Corrosive wear behaviors of austenitic, ferritic and duplex stainless steel in sulfuric acid solution were investigated. Transmission electron microscopy with energy dispersive X-ray, scanning electron microscopy, microhardness tester, etc, were used to study the mechanism of surface deformation of stainless steel after corrosive wear tests. The result indicated that the corrosive wear rate of austenitic stainless steel (ASS) was the highest and that of duplex stainless steel (DSS) was the lowest if the applied loads were higher than 25 N. Because martensitic transformation induced brittleness and increased corrosion rate of ASS surface, the corrosive wear rate of ASS increased largely though the surface microhardness of ASS was largely increased during corrosive wear under high loads. The corrosive wear fate of ferritic stainless steel (FSS) also increased largely under high loads though FSS had the best corrosive wear resistance under low loads. The corrosive wear rate of DSS had a linear relationship with loads. The abilities of surface deformation strengthening of two phases were very different. The investigation indicated that surface deformation strengthening of DSS with proper ratio of gamma phase was one of the important methods to improve corrosive wear resistance of stainless steel

Extrusion formation mechanism on silicon surface under the silica cluster impact studied by molecular dynamics simulation

Molecular dynamic simulation is applied in analyzing the deformation of silicon surface under the impact of large silica cluster. The mechanism of such a deformation is largely different from the cases of ion bombardment and indentation. With the impact of large silica cluster, the silicon surface is extruded due to the combinational effects of thermal spread, phase transformation, and crystallographic slip. It is found that thermal spread is the most significant one among these three effects. The extrusions on silicon surface will be in embryo during the impact unloading stage and will grow up during the cluster rebounding stage. Furthermore, the critical impact velocity to induce the formation of extrusions on silicon surface is associated with the incidence angle of the cluster, while it is independent from the size of the cluster. The findings are instructive in optimizing the process parameters for ultraprecision machining of silicon wafer.

Nanoscale Effect on Ultrathin Gas Film Lubrication in Hard Disk Drive

Since the current thickness of the gas film between the slider and the disk in Hard Disk Drive is already only one order of magnitude larger than the diameter of gas molecules, the nanoscale effect cannot be neglected any longer In this paper a nanoscale effect function, N p is proposed by investigating the unidirectional flow of the rarefied gas between two parallel plates, and four kinds of formerly and currently employed lubrication models are modified. The calculated results using the modified Reynolds equations indicate that the nanoscale effect weaken the rarefaction effect to some extent for ultra-thin gas film lubrication.

Film Forming Characteristics of Oil-in-Water Emulsion with Super-Low Oil Concentration

The oil in water emulsion has been widely used in many fields such as rolling operations. The mechanism and characteristics of film forming have been widely investigated before. However, the mechanism is still dubious and film forming characteristics are seldom discussed under an oil concentration of 0.05%. In this paper, a lubricating film testing apparatus is used to investigate the film forming characteristics and tribological behaviors under different speed of oil-in-water emulsion between a steel ball and a glass disc. By carrying out experiments under an extremely low concentration of oil, some new phenomena are found in our experiments. Oil concentration is even low to 0.0005%. The results indicate that the speed-thickness curves are changed as the condition changes. The effect of droplet size and the stability of emulsion are both considered to be important. The frictional behaviors are investigated under different conditions. The film forming mechanism of oil-in-water emulsion is also discussed by direct observations of emulsion in the contact area. A new viewpoint on the lubrication of emulsion is put forward in this paper.

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.

Ultra-thin Al2O3 films grown by atomic layer deposition for corrosion protection of copper

Ultra-thin Al2O3 films with thickness in the range of 4.5–29.4 nm were prepared on a copper substrate by atomic layer deposition (ALD) at the temperature of 150 °C to protect the substrate from corrosion. Auger electron spectroscopy (AES) was employed to analyze the elemental components of the film surface and to detect elemental distribution in a depth direction of the film, and atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed to measure the surface morphology before and after the corrosion experiment. Electrochemical impedance spectroscopy (EIS) was used to measure anti-corrosion properties of the film in a 0.1 M NaCl solution. The results demonstrate that high quality ultra-thin Al2O3 films with a uniform in-depth stoichiometry are achieved on the copper substrate and the films can efficiently decrease the corrosion of copper. A thicker Al2O3 film can provide better corrosion resistance because of its lower porosity. When the film thickness is 7.8 nm or above, the copper surface can be well protected, which is embodied by the fact that the AFM and SEM images of the surface do not show a great difference before and after corrosion.