Ultra-precision Grinding Process Research Articles

Dislocation slip mechanism and prediction method during the ultra-precision grinding process of monocrystalline silicon

Prediction of dislocation depth during ultra-precision grinding of monocrystalline silicon is a major challenge. To provide atomistic perspective, a single grain scratching molecular dynamics (MD) model of monocrystalline silicon is established to analyze the initiation and slip mechanism of dislocation. The simulation results show that the initiation of dislocations depends on the normal scratching force, and the dislocation length decreases with the decrease of scratching force and the increase of scratching velocity. Besides, the active slip system was identified by calculating Schmid factor, and the slip stress τ(111)[110],p (i.e. Peierls stress) of 2 GPa for ±1/2[110] dislocation on (1 1‾ 1) plane was obtained. Furthermore, a prediction model for dislocation depth was established based on the analysis of shear stress τ(111)[110]. Finally, a set of nano-scratch experiments were carried out, and the depth and slip direction of dislocation that close to the predicted were obtained by TEM method. This study provides a new method for predicting and controlling dislocation damage of monocrystalline silicon during ultra-precision grinding.

Ductile deformation and subsurface damage evolution mechanism of silicon wafer induced by ultra-precision grinding process

In the field of advanced packaging, large-size silicon wafers are usually thinned by ultra-precision grinding technology relying on the process of workpiece self-rotation. However, the damage caused by mechanical material removal deteriorates the wafer surface flatness and die strength, while its atomic-scale formation principle has yet to be revealed. This work explores the ductile deformation and subsurface damage evolution mechanism of silicon wafer during the ultra-precision grinding process. Some grinding tests are first conducted using different diamond wheels. The surface topography and energy dispersive spectroscopy (EDS) mapping of ground wafers are measured by scanning electron microscope (SEM). The phase transition properties of the ground silicon wafer are confirmed via Raman spectrum. Next, the specimen of groove cross-section is fabricated and its morphology is examined via transmission electron microscopy (TEM) to analyze the subsurface damage characteristics. The subsurface defects at the atomic scale, involving amorphous layer, dislocations, stacking faults and lattice distortions, are observed. Due to its controllable load and similarity of the cutting motion, nanoscratch is utilized for investigating the influence of abrasive interactions on subsurface deformation during the grinding process. Thus, a series of multiple nanoscratch tests are performed in both varied and constant force modes. The material removal behaviors under different scratching conditions are illustrated. This work provides a fundamental understanding of developing a high-efficiency and low-damage thinning method for 12-inch silicon wafers.

A Method of Restraining the Adverse Effects of Grinding Marks on Small Aperture Aspheric Mirrors.

The grinding method is used as the preliminary processing procedure for small aperture aspheric mirrors. Regular grinding marks produced in the grinding process significantly affect the mid-spatial frequency error; however, because of their small radius of surface curvatures and high steepness, they are difficult to polish using traditional methods. Therefore, in this study, the ultra-precision grinding and polishing process of fused quartz material was investigated, and the influence of grinding marks was analyzed, which achieved the purpose of restraining the grinding marks in the grinding process. The generation mechanisms of horizontal and vertical grinding marks were analyzed by means of simulation and experiment, and the relationship between different grinding process parameters and surface quality was explored. A magnetorheological finishing (MRF) spot method was used to explore the effects of grinding marks on subsurface damage (SSD). The elastic adaptive polishing method was used to polish an aspheric lens with high steepness and small caliber. Based on the principle of an elastic adaptive polishing mathematical model, the grinding marks were suppressed, and the mid-spatial frequency error of the lens was reduced by optimizing the polishing path and composition of the polishing fluid. The final roughness reached 10 nm Ra. In this paper, the source of wear marks and their influence on the mid-spatial frequency error of small aperture aspheric mirrors are analyzed, and the grinding marks were suppressed by elastic adaptive polishing.

Effect of grinding damage on cutting force and ductile machining during single grain scratching of monocrystalline silicon

Understanding the initiating and suppression mechanisms of existing defects are the key to optimizing the ultra-precision grinding process of hard and brittle materials. This paper discusses the effects of grinding damage on material removal mechanism from the perspectives of residual scratch depth, scratch morphology and normal scratching force, through conducting the contrast variable-depth nano-scratch experiment on polished and ground monocrystalline silicon and establishing the single grain scratching Smoothed Particle Hydrodynamics (SPH) model. The experimental and simulation results show that radial crack on ground silicon surface is almost invisible due to the existence of grinding marks, which indicates that it is difficult to judge the ductile brittle transition (DBT) point by the traditional method of observing cracks. With the increase of penetration depth, pop-in will occur in the residual scratch depth of polished and ground silicon, indicating that the material removal mode has transformed, which provides an important basis for judging the DBT point. The DBT critical normal scratching force (38.93 mN) and residual scratch depth (−0.14 μm) of ground silicon are much higher than those of polished silicon, implying ground silicon is easier to achieve ductile processing. Meanwhile, when the penetration depth is the same, compared to polished silicon, the normal scratching force of ground silicon is generally larger. Finally, a single grain scratching SPH model of pre-stressed silicon is established. And the simulated phenomenon and conclusions are consistent with the experiment, which proves that the surface compressive stress is an important factor that causes the removal characteristics of ground silicon to be different from polished silicon. This study provides a theoretical basis for understanding the ultra-precision grinding removal mechanism of hard and brittle materials.

Study of deterministic surface micro-texture generation in ultra-precision grinding considering wheel oscillation.

Ultra-precision grinding is crucial for manufacturing high-end optics and molds, while the unbalanced wheel vibration is inevitable and becomes even more critical in surface generation, which resulted in undesired waviness and micro-texture on the ground surface. In this paper, to understand and control the micro-texture generation, a theoretical model has been developed to predict the deterministic surface micro-texture generation resulted from unbalanced tool vibration in ultra-precision grinding, in which the overlap trajectories of grinding wheel with an arc cutting edge were analyzed and calculated. The simulation work was performed and a double phase mechanism involved in deterministic textural pattern and structure has been revealed. Both theoretical and experimental results proved that phase shift is an important factor to determine micro-texture evolution in the ultra-precision grinding process. On this basis, a novel tool path strategy has been proposed to fabricate deterministic micro-structure by coordinating oscillation motion of the grinding wheel and phase shift control, in which a rhombus-shaped micro-structure array can be generated. A small adjustment for the phase shift was conducted and it was found that the more complex micro-texture with different textural patterns and micro-structure can be machined. The results indicated that the phase control for the tool path planning is an effective method to fabricate flexible and tunable micro-texture surfaces in ultra-precision grinding.

The material removal and the nanometric surface characteristics formation mechanism of TiC/Ni cermet in ultra-precision grinding

In this paper, the material removal mechanism of TiC/Ni cermet is firstly investigated based on the analysis of the nano-indentation and the diamond scratching test, and the grinding induced surface damage mechanics is then explored according to the surface topography, the surface morphology and the material microstructure analysis. The results show that the material removal experienced plastic deformation, plowing and fracture under the dynamic scratching process, where the material microstructure plays a determinant role on the obtained surface characteristics of the TiC/Ni cermet. To achieve a smooth surface and ductile material removal by the ultra-precision grinding process, a group of the machining parameters is selected to reveal the formed nanometric surface characteristics, where the dislodgement of the hard TiC particles and the surface relief formation were induced by the varied material removal rate between the binding phases and the TiC hard particle under the scratching of the diamond grits. In addition, the preferred TiC (200) crystalline plane in the ground surface layer appears for the interface fracture between the TiC grains and the rim structure, while the Ni (111) crystalline plane shows a preferred growth for the extrusion in the subsurface deformed layer.

The Effect of Diamond Wheel Wear on Surface and Sub-Surface Quality in Fused Silica Optics Grinding

During the ultra-precision grinding process of fused silica optics, the micro-removal processes of brittle and hard fused silica material by diamond grits in different wheel wear states are inconsistent, which leads to the instability of the grinding quality of the components. First, the wear forms of diamond wheel in the process of grinding brittle and hard fused silica material were analysed. The main wear form of diamond grinding wheel was diamond abrasive wear. Then taking the cumulative removal volume of material as the evaluation parameter of wheel wear state, the evolution laws of surface roughness and sub-surface defects of components under different cumulative removal volume of material were studied. With the aggravation of grinding wheel wear, the surface roughness and depth of sub-surface defects layer decreased first, and then fluctuated and increased slowly. In the rapid wear stage, the falling diamond particles scratched deep grooves on the surface of components, and the roughness and sub-surface depth increased sharply. According to the results, it’s reliable to monitor the wheel wear state and predict the life of wheel.

Nanomechanical characterization of RB-SiC ceramics based on nanoindentation and modelling of the ground surface roughness

Reaction bonded silicon carbide (RB-SiC) ceramics are the primary structure and mold materials for the optical industry and mostly are machined by means of ultra-precision grinding to achieve a satisfactory surface quality. However, it is not easy to attain the theoretical prediction of surface quality, particularly surface roughness, because of different mechanical characterization of Si/SiC phases inside the RB-SiC ceramics. In this work, the nanoindentation tests were performed to investigate the nanomechanical characterization of individual phase inside the RB-SiC ceramics. On the basis of the nanoindentation results of RB-SiC, a theoretical model was established to predict surface roughness in the ultra-precision grinding process, which considered the different removal mechanisms of Si matrix and SiC particles. The comparison of the prediction results of existing and novel models and single-factor experimental results shows that the novel model was well consistent with the experiment.

Machinability improvement of silicon carbide via femtosecond laser surface modification method

In this paper, the laser-induced periodic surface structures (LIPSS) of single-crystal SiC and the machining properties of the modified layer are studied. The distribution of surface features as a function of laser repetition frequency and the phase of micro/nanostructures of the surface modification layer are investigated. Furthermore, wear and machinability of the modified SiC layer are analyzed using nano-scratching. The results indicate that surface modification with the femtosecond laser can effectively reduce the pushing and extrusion of material as well as facilitate removal of materials. The hybrid-grinding method can also reduce the influence of the size effect during processing, which effectively improves machinability of SiC during ultra-precision grinding process.

Analytical Elastic–Plastic Cutting Model for Predicting Grain Depth-of-Cut in Ultrafine Grinding of Silicon Wafer

Grain depth-of-cut, which is the predominant factor determining the surface morphology, grinding force, and subsurface damage, has a significant impact on the surface quality of the finished part made of hard and brittle materials. When the existing analytical models are used to predict the gain depth-of-cut in ultra-precision grinding process of silicon wafer, the results obtained become unreasonable due to an extremely shallow grain depth-of-cut, which is inconsistent with the theory of the contact mechanics. In this study, an improved model for analyzing the grain depth-of-cut in ultra-fine rotational grinding is proposed, in which the minimum grain depth-of-cut for chip formation, the equivalent grain cutting tip radius, elastic recovery deformation in cutting process, and the actual number of effective grains are considered in the prediction of the ultrafine rotational grinding of brittle materials. The improved model is validated experimentally and shows higher accuracy than the existing model. Furthermore, the sensitivity of the grain depth-of-cut to three introduced factors is analyzed, presenting the necessity of the consideration of these factors during the prediction of grain depth-of-cut in ultrafine grinding.

Simulation Research on Micro Contact Based on Force in Silicon Wafer Rotation Grinding

Silicon wafer rotation grinding with cup type diamond wheel is a typical ultra precision grinding process. In this paper, a simulation model based on force for micro contact between wheel micro unit and silicon wafer is established from the stable ductile grinding process. Micro contact process in grinding is simulated using the nonlinear explicit finite element analysis software LS-DYNA. The stress-strain results on silicon wafer and wheel micro unit are analyzed by finite element method. The results show that the critical displacement and load corresponding elastic to plastic - plastic to brittle exist on silicon wafer. In silicon plastic zone tangential sliding can produce plastic groove and uplift. Wear of wheel micro unit can be based on the simulation data to judge. The research provides support for wafer grinding and wheel wear mechanism.

Study on the technological method of ultra-precision grinding for parabolic mirror of monocrystalline silicon

Curved optics of monocrystalline silicon plays an important role in high power laser system and its form accuracy and subsurface damage (SSD) have a significant impact on the performance of the laser system. In order to improve grinding accuracy and control the depth of SSD, the ultra-precision grinding (UPG) process of Si is studied. Firstly, the grinding theory is introduced and the relationships among the depth of cutting, the depth of subsurface cracks and the load of single particle are analysed. Then the influence of different process parameters on the surface quality of Si was investigated by some process experiments. Based on the result of process experiments, the processing of a 195 mm-diameter parabolic mirror of Si was carried out. The results show that the form accuracy of peak-valley value is promoted up to 3.77 μm from 21.4 μm and Rms value of the roughness is improved up to 0.11 μm from 0.625 μm.

Study on the technological method of ultra-precision grinding for parabolic mirror of monocrystalline silicon

Curved optics of monocrystalline silicon plays an important role in high power laser system and its form accuracy and subsurface damage (SSD) have a significant impact on the performance of the laser system. In order to improve grinding accuracy and control the depth of SSD, the ultra-precision grinding (UPG) process of Si is studied. Firstly, the grinding theory is introduced and the relationships among the depth of cutting, the depth of subsurface cracks and the load of single particle are analysed. Then the influence of different process parameters on the surface quality of Si was investigated by some process experiments. Based on the result of process experiments, the processing of a 195 mm-diameter parabolic mirror of Si was carried out. The results show that the form accuracy of peak-valley value is promoted up to 3.77 μm from 21.4 μm and Rms value of the roughness is improved up to 0.11 μm from 0.625 μm.

ALON高陡度薄壁保形非球面的超精密磨削工艺

ALON高陡度薄壁保形非球面的超精密磨削工艺

Grinding Performance of Diamond Grinding Tools for Sapphire Crystal

In order to improve the efficiency and reduce the cost of sapphire crystal machined by loose abrasive lapping, the fixed-abrasive diamond grinding tools for sapphire grinding were investigated in this paper. Four vitrified-resin composite bond diamond grinding tools with different grain sizes (40μm, 20μm, 7μm, 2.5μm) were developed. The grinding experiments were performed with the developed diamond grinding tools and the grinding performance of four grinding tools were evaluated by comparing the surface roughness and the material removal rate (MRR) of sapphire. The experiment results show that with the increase in grain size, both the MRR and the surface roughness increase. A high-efficiency and high-quality ultra-precision grinding process using diamond grinding tools with different grain sizes was proposed.

렌즈 성형용 유리탄소 금형의 초정밀연삭

In this study, glassy carbon was ground for lens core of glass mold press. Ultraprecision grinding process was applied for machining of core surfaces. During the process, brittle crack occurred because of hard-brittleness of glassy carbon. Author investigated optimized grinding conditions from the viewpoint of ductile mode grinding. Geometrical undeformed chip thickness was adopted for critical chip thickness that enables crack free surface. Machined cores are utilized for biaspheric glass lens fabrication and surfaces of lens were compared for verification of ground surface.

Study on Material Removal Mechanism of Silicon Nitride during ELID Ultraprecision Grinding

Based on the analyzing ultraprecision grinding process of hard and brittle materials, taking ELID grinding of silicon nitride ceramic as an example, active control technology of passivating films state was introduced in this paper. ELID ultraprecision grinding process respectively at adaptive dynamic balance mode, discontinuous electrolyzing mode and discontinuous grinding mode had been comparatively studied. By means of AFM used for analyzing surface topography of parts, studies show that material removal method for ELID grinding is always a combination of micro brittle fracture, plastic shearing, lapping and polishing action, and which is the main material removal mode depends on the actual grinding contact state. Finally, finishing surface generating mechanism for silicon nitride in ELID ultraprecision grinding was summed up.

Development of Ultra-Precision Grinding Process for Aspheric Surface Based on Feed Rate Controlling and Error Compensation Methods

In this paper, a new method of improving uniformity of surface roughness through controlling feed rate was proposed. A CNC grinding software based on feed rate controlling and error compensation for aspheric surface was developed. The structure and function of the software system was described. An experimental test for grinding aspheric surface of WC mould was conducted for evaluating the performance of the compensation process by using this developed software. After compensation grinding for several times, form accuracy of PV 126 nm, roughness of Ra 3.4 nm can be obtained.

Rhenium-iridium coating effect of tungsten carbide mold for aspheric glass lens

Recently, with increasing lightness and miniaturization of high resolution camera phones, the demand for aspheric glass lens has increased because plastic and spherical lenses are unable to satisfy the required performance. An aspheric glass lens is fabricated by high temperature and pressure molding using a tungsten carbide molding core; so precision grinding and coating technology for the molding core surface is required. In this paper, Aspheric glass lens for 5 megapixel 4 time zoom camera phone modules was developed. The grinding condition of the tungsten carbide molding core was found after applying design of experiments (DOE). Also, the ultra-precision grinding process was investigated under this condition by experiment. Rhenium-iridium (Re-Ir) coating was applied on the ground surface of the tungsten carbide molding core. The influence of Re-Ir coating was compared and evaluated. The surface roughness of the molding core were improved by application of Re-Ir coating on the surface of the tungsten carbide molding core. Aspheric lenses were also molded with the non-coated molding core and the Re-Ir coated molding core. The surface roughness of the aspheric glass lens improved by about 0.4 nm (aspheric surface) and 0.5 nm (plane surface).

A Study on the Grinding to Improve Profile Accuracy of Aspheric Lens

This study presents the development of an ultra-precision grinding system based on a new grinding technique called the “In-Process Grinding Method (IPGM)”. IPGM which is used for grinding aspheric lens increases both the production and grinding performance, and significantly decreases total production costs. To enhance the precision grinding productivity of ultra-precision aspheric lens, we present here an ultra-precision grinding system and process for the aspheric micro-lens. The tool path was calculated and CNC program generation and tool path compensation were performed for aspheric lens. Using this ultra-precision grinding system, aspheric lens, 4mm in diameter, were successfully performed. The profile error after the first grinding without any compensation was less than 0.6μm, and surface roughness Ra was 0.01μm. In-process grinding was performed with compensation. Results of the profile accuracy P-V 0.3μm and surface roughness Ra 0.006 μm were obtained.