Evaluation of repeated single-point diamond turning on the deformation behavior of monocrystalline silicon via molecular dynamic simulations

A three-dimensional molecular dynamics simulation study is conducted to investigate repeated single-point turnings of a monocrystalline silicon specimen with diamond tools at nanometric scale. Morse potential energy function and Tersoff potential energy function are applied to model the silicon/diamond and silicon/silicon interactions, respectively. As repeated nano-cutting process on surfaces often involve the interactions between the consequent machining processes, repeated single-point diamond turnings are employed to investigate the phase transformation in the successive nano-cutting processes. The simulation results show that a layer of the damaged residual amorphous silicon remained beneath the surface after the first-time nano-cutting process. The amorphous phase silicon deforms and removes differently in the second nano-cutting process. By considering the coordination number (CN) of silicon atoms in the specimen, it is observed that there is an increase of atoms with six nearest neighbors during the second nano-cutting process. It suggests that the recovery of the crystalline phase from the amorphous phase occurred. Moreover, the instantaneous temperature distributions in the specimen are analyzed. Although the tangential force (F _X ) and the thrust force (F _Y ) become much smaller in the second cutting process, the material resistance rate is larger than the first cutting process. The larger resistance also induces the increase of local temperature between the cutting tool and the amorphous layer in the second cutting process.     

Nanopolishing of silicon wafers using ultrafine-dispersed diamonds

In the present study, two new methods are proposed for the polishing of silicon wafers using ultrafine-dispersed diamonds (UDDs). The first proposed polishing method uses a polishing tool with an ultrafine abrasive material made through the electrophoretic deposition of UDDs onto a brass rod. Dry polishing tests showed that the surface roughness of the silicon wafer was reduced from R_a=107 to 4 nm after polishing for 30 min. The second method uses a new polishing pad with self-generating porosity. By polishing with the new pad in combination with the polycrystalline UDD in a water suspension, it is possible to achieve the specified surface roughness of the silicon wafer much faster than when using a conventional pad made of foamed polyurethane. The tests showed that the surface roughness of the silicon wafer was reduced from R_a=107 to 2 nm after polishing for 90 min.     

Planarization process of single crystalline silicon asperity under abrasive rolling effect studied by molecular dynamics simulation

In the chemical mechanical polishing (CMP) process, the complex behaviors of abrasive particles play important roles in the planarization of wafer surface. Particles embedded in the pad remove materials by ploughing, while particles immersed in the slurry by rolling across the wafer surface. In this paper, processes of the particle rolling across a silicon surface with an asperity under various down forces and external driving forces were studied using molecular dynamics (MD) simulation method. The simulations clarified the asperity shape evolution during the rolling process and analyzed the energy changes of the simulation system and the interaction forces acted on the silica particle. It was shown that both the down force and the driving force had important influences on the amount of the material removed. With relatively small down forces and driving forces applied on the particle, the material removal occurred mainly in the front end of the asperity; when the down forces and driving forces were large enough, e.g., 100?nN, the material removal could take place at the whole top part of the asperity. The analysis of energy changes and interaction forces provided favorable explanations to the simulation results.     

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.     

Inhibitors for organic phosphonic acid system abrasive free polishing of Cu

Organic phosphonic acid system abrasive free slurry for copper polishing is developed in our earlier work. Since material removal rate is too high to be applied as precision polishing slurry for copper, inhibitors are needed. Experiment results also show us that the most commonly used inhibitor benzotriazole is unsuitable for this abrasive free slurry, and then another kind of compound inhibitors for this organic phosphonic acid system abrasive free slurry are developed. The compound inhibitors, consisting of ascorbic acid and ethylene thiourea, can control the material removal rate and also reduce surface roughness. XPS results show that, in the compound inhibitors, ascorbic acid participates in the surface chemical reaction, forms passivating layer on copper surface and helps to control the material removal rate. Corrosion current calculated from polarization curve is consistent with material removal rate. Ethylene thiourea contributes to the reduction of surface roughness, which can be indicated by the peak shape change of S_2p in XPS results.     

Investigation of material removal mechanism of silicon wafer in the chemical mechanical polishing process using molecular dynamics simulation method

Chemical mechanical polishing (CMP) technology, being the mainstream technique of acquiring global planarization and nanometer level surface, has already become an attractive research item. In the case of CMP process, the indentation depth lies in the range of nanometer or sub-nanometer, huge hydrostatic pressure induced in the local deformation area which makes the material removal and surface generation process different from traditional manufacturing process. In order to investigate the physical essence of CMP technique, the authors carry out molecular dynamics (MD) analysis of chemical mechanical polishing of a silicon wafer. The simulation result shows that huge hydrostatic pressure is induced in the local area and leads to the silicon atom transform from the classical diamond structure (α silicon) to metal structure (β silicon). This important factor results in the ductile fracture of silicon and then in the acquisition of a super-smooth surface.     

Investigating the mechanisms of diamond polishing using Raman spectroscopy

Recent research has shown that a phase transformation of diamond to a different form of carbon is involved when diamonds are polished in the traditional fashion. The question as to how this phase transformation is activated and maintained to produce high wear rates is of great technological interest since it may radically change the way we view the processing of diamond. This paper describes the use of Raman spectroscopy to examine debris produced on the diamond polishing wheel, both during its preparation and during polishing. In addition, polished diamond surfaces were examined for the possible existence of non-diamond surface layers in an attempt to identify material removal mechanisms. Raman spectroscopy proves ideal for these analyses because its relatively high spatial resolution is well suited to the analysis of small wear features and debris particles, and because of the wealth of information it reveals about chemical structure. This level of structural information has been lacking in previous analyses of diamond polishing debris. In addition to the non-diamond carbon found in the wear debris, significant quantities of two iron oxides, magnetite (Fe₃O₄) and haematite (α-Fe₂O₃), were also found. An interesting observation was that a transformation from magnetite to haematite could be induced either by using high power laser excitation or by frictional heating during polishing. It is suggested that some of the Raman peaks previously attributed to lonsdaleite might better be explained by the presence of these oxides.     

抛光对光纤连接器回波损耗的影响

分别选用氧化铝和氧化硅材料的抛光砂纸,在施加不同抛光压力、抛光时间,以及抛光助剂等工艺条件下,实验研究了抛光对连接器回波损耗的影响规律。通过实验发现:氧化铝砂纸干式抛光使光纤连接器的回波损耗仅保持在32～38dB之间;氧化硅砂纸干式抛光会造成光纤端面污损,使得连接器的回波损耗降低到20dB以下;氧化铝与氧化硅砂纸湿式抛光均可使光纤连接器的回波损耗提高到45～50dB,但氧化铝砂纸湿式抛光会造成80nm以上的光纤凹陷。因此,制作高回波损耗的光纤连接器应优先选用氧化硅砂纸湿式抛光工艺,抛光时间应控制在20～30s。     

Static dissolution rate of tungsten film versus chemical adjustments of a reused slurry for chemical mechanical polishing

Tungsten is widely used as deposited layer for the multi-level interconnection structures of wafers. The chemical composition of abrasive slurry plays an important role in chemical mechanical polishing (CMP) process. Removal of tungsten is driven by complex oxidation mechanisms between slurry components. The slurry for tungsten CMP generally contains oxidizer, iron catalyst, complexing agents and stabilizers in a pH adjusted solution of abrasive particles. Interaction between iron complex and H₂O₂ in the slurry is the main factor governing the chemical mode of material removal, oxidation potencies and kinetics.In this study, we investigate the effects of chemical additives in silica (SiO₂)-based slurry on the removal rate of the tungsten film. Experiments were carried out in static batch as a preliminary study to understand and optimize chemical mechanisms in CMP-Tungsten process. Experiment designs were conducted to understand the influence of the chemical additives on the main performances of W-CMP. Used slurry, concentrated and retreated with chemical adjustments, is compared to the original slurry as a reference.     

Effect of scratching speed on deformation of soda–lime–silica glass

The grinding and polishing of a fundamentally brittle material like glass to an utmost precision level for ultra-sophisticated applications ranging from mobile devices to aerospace as well as space shuttle components to biomedical appliances pose a big challenge today. Looking simplistically, the grinding and polishing processes are basically material removal by multiple scratching at a given speed. Unfortunately however, the role of the scratching speed in affecting the material removal mechanism in soda–lime–silica (SLS) glass is yet to be comprehensively understood. Therefore, the present work explores the surface and subsurface deformation mechanisms of SLS glass scratched under a normal load of 5 N at various speeds in the range of 100–1000 μm?s^?1 with a diamond indenter of ～200 μm tip radius. The results show important roles of the time of contact, the tensile stress behind the indenter and the shear stress just beneath the indenter in governing the material removal mechanisms of the SLS glass.     

Electron microscopy analysis of debris produced during diamond polishing

This paper deals with an analysis of debris produced during the polishing of diamond. The debris is carefully collected 'as ejected' to shorten the history of the freshly removed material. Using high-resolution electron microscopy as well as electron-energy-loss spectroscopy, the structure of the material is revealed and analysed in terms of density, percentage of sp 2 hybridized carbon, and oxygen content. Debris from polishing in the so-called hard and soft directions were involved in this investigation. Overall the structure of all debris is amorphous carbon. The material appears to be composed of small clusters, some nanometres in diameter, in which the graphite basal planes can be recognized. Very few and very small nanometre-sized diamond particles were found in the debris from polishing in the hard direction. The results support a polishing mechanism based on a mechanically induced transformation of diamond to graphite, after which material removal easily occurs. The well-known anisotropy observed in polishing can be explained satisfactorily on the basis of this model. Finally, in appendices, the art of polishing and the role of the black powder during preparation of the scaife are discussed.     

Acoustic emissioin monitoring of strength in brittle materials

It has been established that for porcelain, ferrite and cement-porcelain mortar the tensile strength, σ_F, during subsequent loading is closely related to the stress σ_O at which during the unloading period of the proof test the acoustic emission (AE) response changes from a continuous activity to a discrete one. The coefficients in the equation σ_F = A + Bσ_O in bending were found to be the same as those for radial compression. For a very homogeneous material with no continuous AE during unloading, σ_F correlates well with the relative change, κ, in pulse amplitude when two proof tests are carried out. The relationships σ_F(σ_O) and σ_F(κ) are independent of the mode and direction of abrasive machining.     

Mechanism of small variations in energy of ultracold neutrons interacting with a surface

The cause of the small heating of ultracold neutrons (UCNs) by ～10^?7 eV with a probability of 10^?8–10^?5 per collision with a surface was investigated. Neutrons heated in this way will be called vaporized UCNs (VUCNs). It was established that a preliminary heating of a sample in vacuum up to a temperature of 500–600 K can increase small-heating probability P _VUCN by a factor of at least ～100 and 10 on a stainless steel and a copper surface, respectively. For the first time, an extremely vigorous small heating of UCNs was observed on a powder of diamond nanoparticles. In this case, both the VUCN spectrum and the temperature dependence of probability P _VUCN were similar to those previously obtained for stainless steel, beryllium, and copper samples. On the surface of single crystal sapphire, neither the small heating of UCNs nor nanoparticles were found. All these facts indicate that VUCNs are likely produced by inelastic scattering of UCNs on weakly bound surface nanoparticles being in permanent thermal motion.     

熔融石英玻璃无水环境固结磨粒抛光特性

为了克服游离磨粒抛光的随机性、磨料浪费以及产生的水合层等问题,提出了一种无水环境下熔融石英玻璃固结磨粒抛光技术。研究实现了稳定的抛光轮烧结工艺,并应用于熔融石英玻璃抛光加工,通过对加工产物和抛光轮粉末进行EDS能谱分析和XRD衍射分析,从微观上初步阐述了固结磨粒抛光的去除机理;从宏观上探索压力和转速对去除效率和表面粗糙度的影响。实验结果表明:加工过程中,在法向力和剪切力作用下,CeO₂磨粒和熔融石英发生化学反应,CeO₂将SiO₂带出玻璃,实现材料去除;同时,压力和转速对加工效率影响并不遵循Preston公式,温升和排屑成为决定去除效率的关键。     

Hysteresis and elastic interactions of microasperities in dry friction

Velocity independent dry friction of a slider upon a base is due to an hysteretic response of relative displacement to a tangential driving force F. We show that the purely elastic model for multistability considered in a previous publication is in no way essential: multistability arises just as well from adhesion. We emphasize the physical consequences of multistability for dynamic/static, a.c./d.c. friction. When the slider is moved from rest by an amount the transition from the zero force static configuration to dynamic behaviour is progressive, spreading on a range equal to the width of the hysteresis cycle. When is small, an elastic restoring force ensues, in agreement with observations. The competition of that elastic pinning with bulk elasticity generates a screening length which we believe is the natural size of Burridge Knopoff blocks. We then study the effect of elastic interactions between asperities: it is weak for dilute asperities, but its long range makes it important. In lowest order the interaction mediated displacement of a given asperity has logarithmically divergent fluctuations: they become comparable to the asperity radius when the slider size reaches another characteristic “Larkin length”, which for dilute micronic asperities is exponentially large. We give arguments suggesting that individually monostable asperities display collective multistability on scales larger than . For individually multistable sites we show that elastic interactions give rise to cascade processes in which the spinodal jump of a given asperity triggers the jump of others. We estimate the size of these cascades that should show up in the noise spectrum. Received: 3 February 1998 / Accepted: 19 March 1998     

Subsurface damage in alumina and alumina–silicon carbide nanocomposites

The subsurface plastic deformation below alumina (Al₂O₃) and Al₂O₃–silicon carbide (SiC) nanocomposite surfaces subjected to grinding, polishing and annealing has been measured by high-resolution grazing-incidence parallel-beam X-ray powder diffraction and transmission electron microscopy. The variation with angle in the full width at half-height maximum (FWHM) of the X-ray Bragg peaks was successfully modelled by a FWHM distribution that fell exponentially with increasing depth. Consistent parameters were extracted from data taken using both prism and pyramidal reflecting planes. Correlation was found between the depth at which the FWHM fell to 1/e of the surface value and the depth of damage observed by transmission electron microscopy. The associated surface strain in the nanocomposite was found to increase linearly with increasing diameter of the diamond polishing particles. In ground 5?vol.%?SiC nanocomposite, these random surface strains fell by a factor of 7 and the depth of damage increased by a factor of 3 after annealing at 1250°C for 2?h. No differences were observed in the Bragg peak FWHM as a function of angle for material polished with 1?µm diamond grit before and after annealing.     

A Pressure Sensor Using Fiber Bragg Grating

In the present investigation, a Q-switched Nd:YAG laser is used to study the various aspects of diamond processing for fabricating integrated optic and UV optoelectronic devices. Diamond is a better choice of substrate compared to silicon and gallium arsenide for the fabrication of waveguides to perform operations such as modulation, switching, multiplexing, and filtering, particularly in the ultraviolet spectrum. The experimental setup of the present investigation consists of two Q-Switched Nd:YAG lasers capable of operating at wavelengths of 1064 nm and 532 nm. The diamond cutting is performed using these two wavelengths by making the “V”-shaped groove with various opening angle. The variation of material loss of diamond during cutting is noted for the two wavelengths. The cut surface morphology and elemental and structural analysis of graphite formed during processing in both cases are compared using scanning electron microscopy (SEM) and laser Raman spectroscopy. Both the Q-Switched Nd:YAG laser systems (at 1064 nm and 532 nm) show very good performance in terms of peak-to-peak output stability, minimal spot diameter, smaller divergence angle, higher peak power in Q-switched mode, and good fundamental TEM ₀₀ mode quality for processing natural diamond stones. Less material loss and minimal micro cracks are achieved with wavelength 532 nm whereas a better diamond cut surface is achieved with processing at 1064 nm with minimum roughness.     

大口径碳化硅反射镜数控抛光工艺北大核心CSCD

研究了针对600mm口径方形轻质碳化硅元件的数控抛光工艺过程,采用国产OP1000数控研磨抛光机床对一块600mm×480mm的方形碳化硅元件进行数控抛光加工。在经过两周的加工时间,碳化硅光学元件的通光口径均方根(RMS)值收敛到了35nm(大约为λ/18,λ=632.8nm)。在加工过程中针对大口径椭圆形碳化硅反射镜采用了合适的加工参数优化,例如在加工过程中的不同阶段选择了不同颗粒度的金刚石微粉作为特定阶段的抛光辅料以保证光学元件的表面粗糙度。对计算机控制数控加工技术的快速收敛过程也进行了阐释。     

Modeling effects of abrasive particle size and concentration on material removal at molecular scale in chemical mechanical polishing

A novel material removal model as a function of abrasive particle size and concentration was established in chemical mechanical polishing (CMP) based on molecular scale mechanism, micro-contact mechanics and probability statistics. A close-form equation was firstly developed to calculate the number of effective particles. It found nonlinear dependences of removal rate on the particle size and concentration, being qualitatively agreement with the published experimental data. The nonlinear relation results from the couple relationship among abrasive number, slurry concentration and surface atoms’ binding energy with the particle size. Finally, the system parameters such as the operational conditions and materials properties were incorporated into the model as well.     

Analysis of the current-transport mechanism across a CVD diamond/silicon interface

This work presents a study on the mechanism of injection and charge transport through a CVD diamond/n⁺-Si interface. The current-voltage-temperature characteristics of CVD diamond/silicon heterojunctions measured in the temperature range 119-400 K have been interpreted according to thermionic theory and thermionic field-emission theory. This junction shows deviations from the ideal thermionic theory current model, suggesting the presence of surface states, thin-layer depletion and/or non-homogeneity in the diamond/silicon interface. The T₀ anomaly has been used to explain the behaviour of the ideality factor with temperature. At very low temperatures tunnelling may occur because the E₀₀ values for these junctions are close to the value expected by thermionic field-emission theory. The usual activation-energy plot deviates from linearity at low temperatures. This deviation has been corrected supposing a ln(J_S/T²) versus 10³/nT plot. Under these conditions the Richardson constant is found to be 0.819 A cm⁻² K⁻², which is close to the theoretical value of 1.2 A cm⁻² K⁻². Field-emission device is a promising application for diamond/silicon structure.