Molecular dynamics investigations on polishing of a silicon wafer with a diamond abrasive

During the final stages of polishing silicon wafers, much of the interactions between silicon and diamond abrasive takes place at the silicon asperities. These interactions, leading to material removal, were investigated in a MD simulation of polishing of a silicon wafer with a diamond abrasive under dry conditions. Simulations were conducted with silicon asperities of different geometries, different abrasive configurations, and polishing speeds. Under the conditions of polishing, the silicon atoms from the asperities were found to bond chemically to the surface of the diamond abrasive. Continued transverse motion of the diamond abrasive (relative to the silicon asperity) leads to tensile pulling, necking, and ultimate separation of the silicon asperity material instead of conventional material removal in polishing (chip formation) involving cutting/ploughing, which takes place in the absence of chemical bonding between the abrasive and the asperity material. This phenomenon has not been reported previously in the literature. The thrust and cutting forces initially increase due to the increase in the number of asperity atoms affected finally reaching a maximum. This is followed by a decrease of these forces due to tensile pulling and formation of individual strings followed by ultimate separation or breakage of the final string. The ratio of thrust force (F _z ) to the cutting force (F _x ), i.e. |(F _z /F _x )| was found to increase continuously to a maximum of ～0.8 followed by continuous decrease to ～0.25. This is in contrast to a more or less constant value of ～2 in the case of tools with rounded radii or tools with large negative rake angles, where material is removed in the form of chips ahead of the tool. Three regions of the asperity have been identified that are useful in the development of a phenomenological model for polishing that enables computation of material removal rates: (1) the region directly in front of the abrasive for which the probability of the removal of an asperity atom is close to unity, (2) the distant region where this probability is nearly zero, and (3) an intermediate region from which the probability of removal is close to half.     

Possibilities of C 1s XPS and N(E) C KVV Auger spectroscopy for identification of inherent peculiarities of diamond growth

The interaction of C-atoms and CH_n-radicals with uncleaned and argon cleaned silicon substrate and with diamond surface after H-treatment have been studied in situ by XPS and Auger spectroscopy. It was found the formation of a new chemical surface state of carbon atoms in the case of carbon atoms and radicals interaction with cleaned silicon. The same chemical state was revealed on the H-treated diamond surface. Graphite-like structure of carbon atoms was observed on the surface of unlearned silicon and H-treated diamond after interaction with carbon atoms and radicals. N(E) C KVV Auger spectrum for the new chemical state of carbon atoms significantly differs from typical spectra for sp²- and sp³-bonded carbon materials. The high energy part of this spectrum was interpreted under the hypothesis of sp³-bonded carbon atoms but with shifted fermi level position.     

A novel evaluation method for thin films mechanical characterizations using cutting system

In this study, we suggest a nano-cutting system to determine the shear strength of the thin films using fracture mechanics analysis of the diamond blade. Based on Merchant's cutting model, we analyze the thin films cutting process with regard to shear angle and resistant forces as initiation of the yield in the chip to establish a direct correlation between the cutting forces and the shear strength. Validating the proposed method was conducted using homogenous polycarbonate disk showing similar shear strengths between different cutting directions. Next, we examined a thin copper electroplated film used in traces of printed circuit board. A thin copper film was examined and found the intrinsic shear strength (307.5 MPa) and adhesion force (44 N/m) between the film and substrate. The result was comparable with tensile strength values reported in the literatures. Finally, we used SEM to visually verify the feasibility of nano-cutting technique to determine thin film properties.     

Electrons diffusion study on the nitrogen-doped nanocrystalline diamond film grown by MPECVD method

Nitrogen-doped nanocrystalline diamond (NNCD) films were deposited onto p-type silicon substrates with three different layer structures: (i) directly onto the silicon substrate (NNCD/Si), (ii) silicon with undoped nanocrystalline diamond layer which was deposited in the same way as the above mentioned NNCD by the recipe Ar/CH₄/H₂ with a ratio of 98%/1%/1% (NNCD/NCD/Si), and (iii) silicon wafer with 100 nm thickness SiO₂ layer (NNCD/SiO₂/Si). Atomic force microscopy (AFM), X-ray diffraction (XRD) and Raman spectroscopy were employed to characterize the morphology and microstructure of the as-grown nitrogen-doped diamond films. Silver colloid/silver contacts were made at to measure the current-voltage (I-V) characteristics for the three different structures. Electrons from a CVD reactor hydrogen plasma diffuse toward the p-type silicon substrate during a deposition process under the high temperature (∼800 °C). The study concluded that the SiO₂ layer could effectively prevents the diffusion of electrons.     

Molecular dynamics study of particle emission by reactive cluster ion impact

Molecular dynamics (MD) simulations of sputtering process with fluorine cluster impact onto silicon targets were performed. By iterating collisional simulations on a same target, accumulation of incident atoms and evolution of surface morphology were examined as well as emission process of precursors. When (F₂)₃₀₀ clusters were sequentially irradiated on Si(1 0 0) target at 6 keV of total incident energy, column-like surface structure covered with F atoms was formed. As the number of incident clusters increased, sputtering yield of Si atoms also increased because the target surface was well fluoridised to provide SiF_x precursors. Size distribution of emitted particles showed that SiF₂ was the major sputtered particle, but various types of silicon-fluoride compounds such like Si₂F_x, Si₃F_x and very large molecules consists of 100 atoms were also observed. This size distribution and kinetic energy distribution of desorbed materials were studied, which showed that the sputtering mechanism with reactive cluster ions is similar to that under thermal equilibrium condition at high-temperature.     

Second virial coefficient and mechanical moduli of metallic glasses

The relationship between the bulk, shear moduli and second virial coefficient of amorphous materials is derived according to their dependences with the radial distribution function. Lennard-Jones–Gaussian potential is used to investigate the relationship between second virial coefficient and temperature, where Lennard-Jones potential represents interactions with the nearest neighbor atoms, and Gaussian potential is responsible for the multi-atom interactions including the next nearest neighbor atoms and heterogeneous structures for a metallic glass. The results show that deep potential well formed by Gaussian potential causes a large second virial coefficient at low temperatures, which is very obvious for the larger fragility glasses. The quadratic form relationship of shear modulus and compositions is proposed, and confirmed by the experimental results of Pd_xNi_100−x−20P₂₀ alloy.     

Study of the atom-phonon coupling model for (SC) partition function: first order phase transition for an infinite linear chain

In spin-conversion (SC) compounds containing molecules organized around an iron (II) ion the fundamental level of the ion is low spin (LS), S = 0, and its first excited one is high spin (HS), S = 2. This energy diagram is due to the ligands field interaction on 3d electrons and to the spin pairing energy. Heating the compound increases the magnetic susceptibility which corresponds to a change of populations of both levels and consequently a change of spin value of the molecules. This mechanism, called spin conversion (SC), can be accompagnied by thermal hysteresis observed by studying magnetic susceptibility or high spin fraction. In that case one considers that the (SC) takes place through a first-order phase transition due to intermolecular interactions. In the atom-phonon coupling model the molecules are considered as two-level systems, or two-level atoms, and it is assumed that the elastic force constant value of the spring which links two atoms first neighbours is depending on the electronic states of both atoms. In this study we calculate the partition function of a linear chain of N atoms (N ≤ 16) and we describe the role of phonons and that of the parameter Δ which corresponds to the distance in energy between both levels. The chain free-energy function is F _atph . We introduce for the chain a free-energy function defined by the set (F _HS , F _LS , F _barr ) and we show that F _atph tends towards the previous set when N → ∞. The previous set allows to describe a first order phase transition between a (LS) phase and a (HS) one. At the crossing point between the function F _LS and F _HS , and around this point, there is an intermediate free-energy barrier which prevents the chain to change phase which can lead to thermal hysteresis. The energy gap between the free-energy function F _atph and that defined by the set (F _HS , F _LS , F _barr ) is small. So we can expect that a nanoparticule takes for free-energy function that defined by the set and then displays a thermal hysteresis.     

Nucleation of diamond on silicon wafers using C60 in the hot filament chemical vapour deposition system

Scanning electron microscopy and Raman shifts were used to study the process of diamond nucleation and growth using C60 in the hot filament chemical vapour deposition (HFCVD) system.The process of nucleation and growth of diamond films on silicon wafer using C60 as intermediate layer in HFCVD system is described.In order to increase the density of diamond nuclei on the wafers,it is not necessary to use negative bias.The UV-light pre-treatment is not beneficial for improving the diamond nucleation.The multi-layers of C60 molecules,but not a monolayer,can increase the density of diamond nuclei in the presence of H atoms.     

氧化硅团簇切削单晶硅粗糙峰的分子动力学模拟研究

应用分子动力学模拟方法研究了氧化硅团簇在不同的切削 深度下切削单晶硅粗糙峰的过程, 考察了切削过程中粗糙峰和氧化硅团簇形态变化、团簇的受力状况、粗糙峰原子配位数和温度分布等. 模拟结果表明: 切削深度小于0.5 nm时, 被去除的材料以原子或者原子簇形式存在, 并黏附在颗粒表面被带走; 当切削深度增大至1 nm时, 材料的去除率增大, 并形成大的切屑. 在切削过程中, 由于压力和温度的升高, 粗糙峰切削区域的单晶硅转变为类似Si-Ⅱ相和Bct5-Si相的过渡结构, 在切削过程后的卸载阶段, 过渡结构由于压力和温度的下降转变为非晶态结构.     

单晶硅纳米切削中C–C键断裂对金刚石刀具磨损的影响

本文基于分子动力学方法模拟金刚石刀具纳米切削单晶硅, 从刀具的弹塑性变形、C–C键断裂对碳原子结构的影响以及金刚石刀具的石墨化磨损等方面对金刚石刀具的磨损进行分析, 采用配位数法和6元环法表征刀具上的磨损碳原子. 模拟结果表明: 在纳米切削过程中, 金刚石刀具表层C–C键的断裂使其两端碳原子由sp³杂化转变为sp²杂化, 同时, 表面上的杂化结构发生变化的碳原子与其第一近邻的sp²杂化碳原子所构成的区域发生平整, 由金刚石的立体网状结构转变为石墨的平面结构, 导致金刚石刀具发生磨损; 刀具表面低配位数碳原子的重构使其近邻区域产生扭曲变形, C–C键键能随之减弱, 在高温和高剪切应力的作用下, 极易发生断裂; 在切削刃的棱边上, 由于表面碳原子的配位严重不足, 断开较少的C–C键就可以使表面6 元环中碳原子的配位数都小于4, 导致金刚石刀具发生石墨化磨损.     

Role of the substrate thickness for the structural properties of organic-organic heterostructures

F₁₆CuPc deposited on pentacene is characterized by the coexistence of two different configurations: F₁₆CuPc is found in the standing up phase (“s-configuration”) on top of pentacene terraces and in a lying down phase (“l-configuration”) at pentacene step edges. By combining AFM and grazing incidence X-ray diffraction we show that the ratio between F₁₆CuPc in l- and s-configurations increases with thickness of the pentacene substrate film, demonstrating the role of the pentacene steps as nucleation centers for the F₁₆CuPc l-configuration. Experiments performed with ultra-thin pentacene thicknesses disclose that the F₁₆CuPc l-configuration does not grow on top of the first and second pentacene layers, pointing to the action of long-range interactions with the substrate.     

Effects of hydrogen and oxygen on the electrochemical corrosion and wear-corrosion behavior of diamond films deposited by hot filament chemical vapor deposition

A diamond film was deposited on silicon substrate using hot filament chemical vapor deposition (HFCVD), and H₂ and O₂ gases were added to the deposition process for comparison. This work evaluates how adding H₂ and O₂ affects the corrosion and wear-corrosion resistance characteristics of diamond films deposited on silicon substrate. The type of atomic bonding, structure, and surface morphologies of various diamond films were analyzed by Raman spectrometry, X-ray diffraction (XRD) and atomic force microscopy (AFM). Additionally, the mechanical characteristics of diamond films were studied using a precision nano-indentation test instrument. The corrosion and wear-corrosion resistance of diamond films were studied in 1 M H₂SO₄ + 1 M NaCl solution by electrochemical polarization. The experimental results show that the diamond film with added H₂ had a denser surface and a more obvious diamond phase with sp³ bonding than the as-deposited HFCVD diamond film, effectively increasing the hardness, improving the surface structure and thereby improving corrosion and wear-corrosion resistance properties. However, the diamond film with added O₂ had more sp² and fewer sp³ bonds than the as-deposited HFCVD diamond film, corresponding to reduced corrosion and wear-corrosion resistance.     

Dislocation nucleation at amorphous intergrain boundaries in deformed nanoceramics

A theoretical model is proposed for lattice dislocation nucleation in deformed nanocrystalline ceramics with amorphous intergrain boundaries. According to the model, a lattice dislocation dipole nucleates at an amorphous intergrain boundary through a local plastic shear along the boundary cross section. The energy parameters of this nucleation process are calculated. It is demonstrated that the dislocation nucleation at amorphous intergrain boundaries is energetically favorable and can occur as an athermic process (without energy barrier) in the nanocrystalline phase of cubic silicon carbide 3C-SiC and in the TiN/a-Si₃N₄ nanocomposite over wide ranges of structural parameters and mechanical loads.     

Electronic band structure and effective masses of electrons and holes in the α and β phases of silicon nitride

The electronic band structure of the two main crystallographic modifications of silicon nitride, namely, the α-Si₃N₄ and β-Si₃N₄ phases, is calculated from the first principles. The estimates obtained for the effective charges of silicon and nitrogen atoms and those for the effective masses of electrons and holes in the α-Si₃N₄ phase are in good agreement with the available experimental data for amorphous silicon nitride a-Si₃N₄. The calculations performed demonstrate that the effective mass tensor determined for the β-Si₃N₄ phase differs substantially from the effective mass tensor obtained for the α-Si₃N₄ phase.     

纳米粒子碰撞下的单晶硅表面非晶相变

应用分子动力学模拟研究了在纳米粒子的碰撞作用下，单晶硅表面局部区域的物相转变和结构演变. 模拟表明在碰撞过程中，基体表面碰撞区域从初始的单晶体转变为熔融态，经历过冷液体状态之后凝固成为了非晶态. 模拟揭示的凝固转变温度与硅玻璃化温度很接近. 在颗粒反弹阶段，与发生的冷却过程和压力去除过程相一致，碰撞区域从瞬态的、高度无序、高度致密的过冷状态开始，经历了结构有序度的增加和向相对疏松状态的转变. 碰撞之后所得非晶硅的平均配位数为5.27，其中配位数5，6原子构成了碰撞区域原子总数的61.5%.     

X-ray photoelectron spectroscopic studies of black silicon for solar cell

The black silicon has been produced by plasma immersion ion implantation (PIII) process. The microstructure and optical reflectance are characterized by field emission scanning electron microscope and spectrophotometer. Results show that the black silicon appears porous or needle-like microstructure with the average reflectance of 4.87% and 2.12%, respectively. The surface state is investigated by X-ray photoelectron spectroscopy (XPS) technique. The surface of the black silicon is composed of silicon, carbon, oxygen and fluorine element. The formation of Si_xO_yF_z in the surface of black silicon can be proved clearly by the O 1s, F 1s and Si 2p XPS spectra. The formation mechanism of the black silicon produced by PIII process can be obtained from XPS results. The porous or needle-like structure of the black silicon will be formed under the competition of SF_x⁺ (x ≤ 5) and F⁺ ions etching effect, Si_xO_yF_z passivation and ion bombardment.     

Amorphous silicon hexaboride at high pressure

ABSTRACT

We investigate the pressure-induced structural phase transformation of amorphous silicon hexaboride (a-SiB₆) using a constant pressure first principles approach. a-SiB₆ is found to undergo a gradual phase transformation to a high-density amorphous phase (HDA) in which the average coordination number of both B and Si atoms is about 6. The HDA phase consists of differently coordinated motifs ranging from 4 to 8. B₁₂ icosahedra are found to persist during compression of a-SiB₆ and the structural modifications primarily occur around Si atoms and in the regions linking pentagonal pyramid-like configurations to each other. Upon pressure release, an amorphous structure, similar to the uncompressed one, is recovered, indicating a reversible amorphous-to-amorphous phase change in a-SiB_6. When the electronic structure is considered, the HDA phase is perceived to have a wider forbidden band gap than the uncompressed one.     

Mechanical, structural, and spectroscopic properties of C70 fullerite phases produced under high pressure and shear

X-ray diffraction patterns, Raman spectra, and the hardness of C₇₀ fullerite subjected to a high pressure with shear are investigated. It is shown that these conditions favor the phase transformation of molecular fullerite into the hard amorphous phase. The hardness of a specimen removed from a diamond anvil cell loaded up to 26 GPa under shear deformation applied is found to be equal to 30 GPa.     

Mössbauer effect of119Sn in amorphous superconducting metals

Mössbauer spectra for¹¹⁹Sn in crystalline and disordered Sn, as well as in crystalline and liquid-like amorphous Sn_1-x Cu _x (X=0.10?0.18), have been measured at 2.6 K≦T≦108 K. The Debye-Waller-Factor (DWF) obtained from the spectra is identical for the crystalline and for the disordered phase. The DWF of the amorphous phase is smaller than the DWF of the crystalline phase athigh temperatures, but it shows a stronger temperature dependence than the DWF of the crystalline phase and reaches the latter one at about 4 K. From this low-temperature result we conclude that the differences of the Eliashberg functionα ²(ω)F(ω) and of the superconducting transition temperatureT _c in these two phases cannot be related to changes in the phonon spectrumF(ω), but must result from changes of the interaction parameterα ² (ω). A comparison between DWF,α ² F, and specific heat data is performed. From the values for the isomeric shift of the Mössbauer line we can show that the hybridisation and covalency of the electronic bonds present in the crystalline and in the disordered phases are destroyed in the amorphous phase. Both, the DWF and the isomer shift demonstrate that the electronic properties of crystalline and amorphous Sn(Cu) differ appreciably. The electronic and superconducting properties of amorphous Sn(Cu) are similar to the properties of the high pressure phase of tin.     

机械-化学方法在硅(100)表面制备芳香烃重氮盐单层膜

利用机械-化学方法同时实现硅表面的图形化和功能化. 在芳香烃重氮盐(C₆H₅N₂BF₄)中用金刚石刀具刻划单晶硅(100),使单晶硅表面的Si-O键断裂,形成硅的自由基,进而它们与溶液中含有的有机分子共价结合以形成自组装单层膜. 用原子力显微镜对自组装前后的表面形貌进行表征,用飞行时间二次离子质谱和红外光谱对自组装单层膜进行检测和分析,通过确认C₆H₅离子的存在证明自组装单层