Ultra-precision Process Research Articles

Unveiling the synergistic interaction: Investigating the enhanced mechanism of 4H–SiC chemical mechanical polishing with the addition of sodium silicate and manganese dioxide

Silicon carbide (SiC), the quintessential semiconductor material, poses an acute challenge to the realization of highly efficient ultra-precision processing. This study has investigated the effect of the combined ratio of sodium silicate and manganese dioxide on the polishing rate and surface roughness of 4H–SiC in chemical mechanical polishing (CMP). The synergistic mechanism of sodium silicate and manganese dioxide on slurry spreading performance and particle dispersion stability is characterized by surface wettability, surface tension, scanning electron microscopy, zeta potential analysis and X-ray photoelectron spectroscopic. Based on the above studies, the synergistic mechanism of sodium silicate and manganese dioxide on the CMP of 4H–SiC is attributed to the improvement of the suspension stability of the abrasive as well as the oxidizing ability of the slurry.

A novel slurry for atomic-scale polishing of KDP crystals

Abstract Atomic-scale surfaces and structures have been playing a significant role in the next generation of devices and products. Potassium dihydrogen phosphate (KDP) crystals are crucial in energy sectors but challenging for ultra-precision processing due to deliquescence, brittleness, and low hardness. This paper introduces a novel chemo-mechanical slurry designed for achieving atomic-scale polishing of KDP crystals. The slurry employs a combination of polyethylene glycol (PEG) and anhydrous ethanol (AE) to counter deliquescence. In addition, graphite oxide (GO) with KOH is incorporated to prevent the embedding of SiO2 abrasives and the dissolution of KDP in deionized water (DW). The mechanism underlying the formation of an ultra-smooth surface is elucidated based on the analysis of the X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) test. Response surface method (RSM) is used to optimize the slurry parameters and finally obtaining an atomic-scale surface with Sa 0.3 nm.

Multi-objective optimization design and experiment for a magnetic micro-vibration isolator considering stiffness trade-offs.

Vibration substantially degrades machining quality and measurement accuracy in ultra-precision processes, necessitating the implementation of vibration isolators to mitigate these effects. Magnetic vibration isolators represent a promising alternative due to their wide isolation frequency range, low energy consumption, and compatibility with vacuum environments. In designing the quasi-zero stiffness magnetic vibration isolator, the particle swarm optimization algorithm is employed to balance multiple performance indicators. To address the limitations of conventional optimization methods, which excessively prioritize stiffness and fail to encompass the entire performance spectrum of magnetic vibration isolators, this study presents an innovative optimization approach. This novel strategy directly enhances vibration isolation by integrating transmissibility and compliance considerations. Structural parameters for two types of magnetic vibration isolators were established through optimizations utilizing both conventional and innovative techniques. Simulations and experiments demonstrate that isolators developed using the proposed optimization method outperform those designed with conventional strategies, thereby validating the effectiveness of this advancement. The insights presented in this paper illuminate a novel and effective pathway for the conceptualization and refinement of magnetic vibration isolators.

Exploring the mechanism of gallium nitride surface quality enhancement by green organic additives in chemical mechanical polishing

This study presents a novel approach using colloidal silica and organic salt additives for ultra-precision processing of GaN films. The chemical mechanical polishing (CMP) process significantly reduced the surface roughness (Sq) to a minimum of 0.63 nm, while improving the material removal rate by 60 % (from 70 nm/h to 112 nm/h). Contact angle experiments confirmed enhanced slurry wettability due to the organic salts, reducing the contact angle between the polishing slurry and GaN surface from 20° to less than 10°. Additionally, X-ray photoelectron spectroscopy (XPS) and quantum chemical calculations explained the inhibition of the oxide layer through adsorption energy. These insights contribute to optimizing GaN CMP processes for fabricating high-performance GaN devices.

Interfacial Heterogeneous Microstructure and Subsequent Machinability of 316L/CuSn10 Bimetallic Structure Manufactured by Laser Powder Bed Fusion

Additively manufactured bimetallic structures combine the advantages of dissimilar materials and can achieve localized properties through a customized composition distribution. However, additively manufactured parts may still lack the dimensional accuracy and surface integrity essential for precision mechanical assemblies that the post-machining process can address. Therefore, this study aims to systematically investigate the microstructure and machinability of 316L/CuSn10 bimetallic structures fabricated using laser powder bed fusion. The results show that the fusion zone of the bimetallic structure had refined grains of microscale size owing to the mixture of the primary elements of the bimetals, which resulted in the highest microhardness of 3.4 GPa. The difference in microstructure and microhardness between the single-material and fusion zones also causes significant differences in the cutting response during the ultraprecision process. The 316L stainless steel side exhibited the highest cutting force and more severe material accumulation in the chips. The cutting force drops when cutting through the fusion zone, with an observable fracture in the chips and separation of dissimilar materials on the machined grooves, indicating that the heterogeneous properties of additively manufactured 316L/CuSn10 bimetallic structures pose challenges to the improvement of surface quality. The simulation results also showed that stress accumulation occurred in the tool path through the fusion zone owing to the higher yield strength and hardness of stainless steel, indicating that lower cutting speeds and depths of cut are favorable for reducing cutting force and improving surface quality. This study provides deep insight into the microstructure evolution mechanism and a theoretical basis for improving the surface quality of additively manufactured bimetallic structures using an ultraprecision machining process.

Machining mechanism and residual stress of AlCuCrFeNi alloy

This investigation utilizes molecular dynamics simulations to explore the mechanical response of the AlCuCrFeNi high-entropy alloy to nano-scraping employing an abrasive tip traversing the workpiece surface. This study investigates the influence of various parameters, such as grain size, pressing depth, alloy composition, temperature, and sliding distance, on plastic deformation, frictional behavior, dislocation density, wear mechanisms, and von Mises stress. The results reveal that increasing grain size leads to augmented force and hardness, indicative of a reverse Hall-Petch relationship. Moreover, scraping and indentation forces escalate with decreasing aluminum concentration and temperature. The analysis underscores the pivotal role of grain boundaries in impeding stress and strain propagation. Stress and strain concentrations are particularly evident at the interface between the scraping tool and the substrate and near the grain boundaries. Grain boundary slipping, bending, and grain merging are pivotal mechanisms contributing to the distortion of polycrystalline materials, culminating in solid dislocations within grain boundaries. Furthermore, heightened compression depth and sliding distance exacerbate plastic deformation and subsurface damage. Notably, the presence of copper atoms enhances the HEA's resistance to deformation. This research enhances comprehension of the nano-scraping behavior and deformation mechanisms in AlCuCrFeNi HEA during ultra-precision processing.

Fabrication and Application of Grinding Wheels with Soft and Hard Composite Structures for Silicon Carbide Substrate Precision Processing.

In silicon carbide processing, the surface and subsurface damage caused by fixed abrasive grinding significantly affects the allowance of the next polishing process. A novel grinding wheel with a soft and hard composite structure was fabricated for the ultra-precision processing of SiC substrates, and the grinding performance of the grinding wheel was assessed in this study. Different types of gels, heating temperatures, and composition ratios were used to fabricate the grinding wheel. The grinding performance of the grinding wheel was investigated based on the surface integrity and subsurface damage of SiC substrates. The results showed that the grinding wheel with a soft and hard composite structure was successfully fabricated using freeze-dried gel with a heating temperature of 110 °C, and the component ratio of resin to gel was 4:6. A smooth SiC substrate surface with almost no cracks was obtained after processing with the grinding wheel. The abrasive exposure height was controlled by manipulating the type and ratio of the gel. Furthermore, the cutting depth in nanoscale could be achieved by controlling the abrasive exposure height. Therefore, the fabrication and application of the grinding wheels with soft and hard composite structures is important for the ultra-precision processing of large-size SiC substrates.

Ultra-precision machining process of inner surface considering shear-thickening polishing method

Shear-thickening polishing (STP) is a non-traditional flexible ultra-precision processing method that exploits the polishing fluid’s shear-thickening properties to create a shear rate between the workpiece surface and the potential flow. This increases viscosity and achieves material removal. In the past, the shear-thickening polishing method was mostly employed for the outer surface of rotating bodies. Nevertheless, to apply the STP method to ultra-precision processing of the workpiece’s inner surface, this paper proposes an active formed shear-thickening layer polishing method based on a numerical calculation model. Additionally, the main parameters affecting the machining efficiency were identified, orthogonal experiments were performed on the process parameter levels, and optimal machining conditions were derived. The optimized process was adopted to machine the inner surface of the large-size bearing ring, and the surface roughness in terms of Ra was reduced from the original 131.27 to 27.14 nm in 30 min and further to 19.52 nm in 60 min. The validation experiments highlight that the numerical model can predict the experimental results accurately, demonstrating the proposed method’s feasibility for ultra-precision machining on the inner surface with the STP method.

Wear behavior of copper material removal during fluid jet polishing: A comparative study between experiment and simulation

As a crucial part in micro-electromechanical manufacture, local ultra-precision processing of highly ductile copper is expected to be realized by fluid jet polishing (FJP), which widely utilized in optical elements. Since copper exhibits different wear behavior from stiff and brittle material, there is currently no abrasive wear prediction model applicable for copper to investigate the polishing mechanism. This research reveals that the copper material removal is dominated by deformation wear rather than cutting wear through abrasive jet impact experiments and localized wear scars analysis. A three-dimensional gas-liquid-particle triphasic wear model for copper in FJP is developed by considering impact energy and wear mechanism simultaneously. Ultimately, validation assessments at various working pressures and impingement angles achieve the goodness-of-fit up to 0.92–0.97 in quantitative comparison between simulations and experimental measurements, which demonstrate the wear prediction ability of the proposed model. This investigation facilitates a better understanding of copper wear mechanism and provides theoretical guidance for FJP process optimization.

Online chatter detection for single-point diamond turning based on multidimensional cutting force fusion

Single-point diamond turning (SPDT) is an advanced ultra-precision processing technology that is widely used in machining optical surfaces and microstructures. However, due to insufficient stiffness or damping of the cutting system and the high strength of the workpiece material, chatter inevitably occurs in the machining process, resulting in inferior workpieces and short tool lives. Thus, chatter detection at the early stage is vital to avoid its adverse effects, and a multidimensional cutting force fusion-based online chatter detection method for SPDT is proposed in this work. According to the characteristics of each component of the cutting force in the chatter SPDT process, the stable frequency component is filtered out, and then an artificial constant offset is added. On this basis, the cutting force-based multidimensional information fusion signal and its derivative are set as the detection variables. The chatter detection indicator is developed based on the sum of squares of the detection variable’s frequency spectrum amplitude, with the chatter warning threshold being adaptively determined by the upper limit of the 95% confidence interval of the same indicator obtained from the measurement noises under air cuts. Experiments conducted under various cutting conditions prove that the proposed method can accurately and timely detect weak chatter, thereby preventing further severe chatter and its detrimental effects on the SPDT system.

Concentration characterization of underlying intrinsic defects accompany with surface structural defects and their effect on laser damage resistance

During ultra-precision processing of potassium dihydrogen phosphate (KDP) crystals, various macro-structural surface defects are inevitably introduced, but there is no monotonic relationship between different macro-defects and laser-induced damage threshold (LIDT). Therefore, a single macro-perspective cannot fundamentally reveal the causation for low LIDT. In this work, energy, photoelectron spectra characterization and photoluminescence characteristics of surface were adopted to determine the type and concentration of intrinsic defects accompany with macro-structural defects. First-principles calculation was applied to explore the influence of oxygen vacancy (VO) defect concentration on performance in terms of crystal structure, electronic structure, and optical properties. The results show that the causation for low LIDT of cracks is VO defects. With the increase of VO defect concentration, PO43- group would collapse, leading to a decrease in the stability of crystal structure. High concentration of VO defects would introduce defective-energy level, causing irreversible alteration in energy level. Free electrons are susceptible to absorb photons and transition due to defective-energy level, severely reducing the laser damage resistance of components. This work reveals the influence of surface defects on laser damage from intrinsic defects at atomic-level, which is significance for the suppression of surface defects during ultra-precision processing and effective improvement of laser damage resistance of KDP.

Novel designed mechanical-mechanochemical synergistic micro-grinding technology and compounded abrasive micro-grinding tools

The mechanical and mechanochemical synergistic micro-grinding technology was proposed for grinding hard and brittle materials, especially for fused silica micro parts in this work. Compounded abrasive micro-grinding tools with diamond abrasives on the outer layer and cerium oxide abrasives on the inner layer were designed and fabricated. Efficient grinding was initially achieved by mechanical processing of super hard abrasives, and then ultra-precision processing was obtained by mechanochemical process to remove surface or subsurface damage. This study firstly analyzed the contact model of single diamond abrasive particle and theoretically evaluated ground surface roughness. The mechanism of abrasive layer wear was investigated for accurate error compensation. The formulation of cerium oxide abrasive layer and its processing efficiency were further analyzed. The material removal mechanisms of fused silica during mechanochemical grinding were also studied. Results showed that the material removal rate was 1.924 μm/min and the surface roughness Ra was 0.248 μm after mechanical processing. Subsequently, the surface roughness Ra was improved to 38 nm after mechanochemical processing. The study indicates that this technique is a promising approach for high-efficiency and high-quality grinding of fused silica.

Phase Measurement Method Based on Digital Dual Frequency Comb for High-Precision High-Speed Heterodyne Interferometry

The heterodyne interferometer is widely used as a precise displacement sensor in ultraprecision processing. However, there is a trade-off between measurement speed and measurement precision in the existing interference signal processing methods. In this work, a novel phase measurement method based on digital dual frequency comb is proposed. The proposed method uses the intrinsic frequency comb signal and a narrowband low-pass filter to form a band-pass filter bank, which divides the broadband brought by high-speed movement into multiple narrow bands. The signal-to-noise ratio (SNR) is improved for phase calculation. In addition, two groups of intrinsic frequency comb signals are used simultaneously for phase measurement and the rotation of the output, preventing the inaccurate phase measurement of the input signal whose frequency is the average of two frequencies in the frequency comb signal. The heterodyne laser interferometer can maintain high precision at high measurement speed. The simulation results show that the precision of the proposed method is more than 50% higher than that of the phase-locked loop method. The experiment with a circuit board combined with 100 Msps sampling 16-bit A/D converter shows that the phase measurement precision of the input signal frequency in the bandwidth range of 2.5–37.5 MHz can reach 0.77 mrad, equivalent to the displacement measurement precision of the double-pass interferometer (19 pm) at a measurement speed of less than 2.77 m/s.

Toward controllable material removal of glass–ceramic surface for low‐deformation machining

AbstractDue to its extremely high optical uniformity and excellent hot stability, glass–ceramic serves as a key material for ultraprecision imaging of lithography lens, and low‐deformation machining is of significance for achieving high‐quality surface. Aiming at controllable processing, the annealing of different nanoscratches on glass–ceramic was investigated for revealing the mechanism of material removal and damage repair. The volume change of the single‐pass nanoscratch under various normal loads and sliding velocities before and after the annealing was calculated for quantifying the contribution of shear flow, densification, and residual stress to the material removal, respectively. It is found that ductile removal under high normal load or low sliding velocity is dominated by shear flow, thereby improving removal efficiency and reducing machining deformation and defects caused by densification and residual stress. The changes of microstructures beneath the scratches before and after annealing further reveal that the excess processing energy will be absorbed in glass–crystal interface and form micro‐cracks on crystal surface. For comparison, brittle removal under variable cycles was simulated by multi‐pass nanoscratches, and it reveals that the shear flow ratio raises gradually with the increase of cycle number. These findings provide theoretical guidance for ultraprecision processing of glass–ceramic surfaces.

Wireless photoelectrochemical mechanical polishing for inert compound semiconductor wafers

Chemical mechanical polishing (CMP) remains the necessary polishing technology in chip manufacturing, but its applications on inert n-type gallium nitride (GaN) and carborundum (SiC) semiconductors are inefficient. Based on the mechanism of bipolar photoelectrochemistry, an efficient wireless photoelectrochemical mechanical polishing (WPECMP) method was proposed. The method applies a wireless electric field to the ultraviolet (UV) light-irradiated semiconductor wafer to separate photogenerated electron-hole pairs during polishing, rather than the traditional wire-connecting wafer to a power source. WPECMP is thus a universal polishing method for freestanding and semiconductor-on-insulator substrate wafers. The material removal rate (MRR) may reach ~μm h−1 level because the unpaired holes oxidize the wafer to accelerate the material removal in the mechanical polishing process. An apparatus was devised to allow the wafer to alternate between oxidation and mechanical polishing, and the method was validated by finishing GaN wafers that are inert in CMP processing. The best MRR achieved 1.2 μm h−1, one order of magnetite higher than that of the CMP technology. The WPECMP method enables wafers to achieve a damage-free surface/subsurface, a surface roughness (Sa) of less than 0.082 nm in 5 × 5 μm2, and a surface flatness of less than 3.2 nm over 45 mm. The study opens a way to develop ultra-precision processing technologies with wireless electric field assistance.

Investigation of nano-tribological behaviors and deformation mechanisms of Cu-Ni alloy by molecular dynamics simulation

With the rapid development of micro/nanodevices, it is critical to investigate and disclose the nano-sized tribological properties and deformation mechanisms. This paper aimed to understand nano-tribological behaviours and deformation process of Cu-Ni alloy. The effects of the pressed depth, alloy composition, and sliding distance on friction state, dislocation density, and von-Mises strain stress (VMSS) are comprehensively investigated using molecular dynamics simulation. The results indicate that a larger pressed depth brings about more lattice defects in the pressed region and a higher transverse force, the stacking fault and dislocation density increased rapidly as the Cu content increases, and a longer sliding distance results in more plastic deformation and the number of wear atoms generated. As the pressed depth and sliding distance rise, the plastic deformation and sub-surface damage are aggravated, however, Cu atoms improve the Cu-Ni alloy resistance of deformation. This work can enrich the understanding on the nano-tribological behaviour and deformation mechanism of Cu-Ni alloy during the ultraprecision process.

Surface quality prediction and lapping process optimisation on the fixed-abrasive lapping plate of sapphire wafers

PurposeLapping is a vital flattening process to improve the quality of processed semiconductor wafers such as single-crystal sapphire wafers. This study aims to optimise the lapping process of the fixed-abrasive lapping plate of sapphire wafers with good overall performance [i.e. high material removal rate (MRR), small surface roughness (Ra) of the wafers after lapping and small lapping plate wear ratio (η)].Design/methodology/approachThe influence of process parameters such as lapping time, abrasive size, abrasive concentration, lapping pressure and lapping speed on MRR, Ra and η of lapping-processed sapphire wafers was studied, and the results were combined with experimental data to establish a regression model. The multi-evaluation index optimisation problem was transformed into a single-index optimisation problem via an entropy method and the grey relational analysis (GRA) to comprehensively evaluate the performance of each parameter.FindingsThe results revealed that lapping time, abrasive size, abrasive concentration, lapping pressure and lapping speed had different influence degrees on MRR, Ra and η. Among these parameters, lapping time, lapping speed and abrasive size had the most significant effects on MRR, Ra and η, and the established regression equations predicted the response values of MRR, Ra and η to be 99.56%, 99.51% and 93.88% and the relative errors between the predicted and actual measured values were <12%, respectively. With increased lapping time, MRR, Ra and η gradually decreased. With increased abrasive size, MRR increased nearly linearly, whereas Ra and η initially decreased but subsequently increased. With an increase in abrasive concentration, MRR, Ra and η initially increased but subsequently decreased. With increased lapping pressure, MRR and η increased nearly linearly and continuously, whereas Ra decreased nearly linearly and continuously. With increased lapping speed, Ra initially decreased sharply but subsequently increased gradually, whereas η initially increased sharply but subsequently decreased gradually; however, the change in MRR was not significant. Comparing the optimised results obtained via the analysis of influence law, the parameters optimised via the entropy method and GRA were used to obtain sapphire wafers lapping with an MRR of 4.26 µm/min, Ra of 0.141 µm and η of 25.08, and the lapping effect was significantly improved.Originality/valueTherefore, GRA can provide new ideas for ultra-precision processing and process optimisation of semiconductor materials such as sapphire wafers.

Investigation of dynamic mechanical properties and constitutive model of (010) plane of gallium oxide crystals under shock loading

In this study, ultraprecision processing performance of new-generation optoelectronic gallium oxide crystals is investigated. Based on material mechanics, Hopkinson rod test is carried out on (010) crystal plane of gallium oxide crystal. The destruction process of gallium oxide crystals was observed by high-speed photography and three-dimensional topography. Through this experiment, the dynamic mechanical properties of the (010) crystal plane of the gallium oxide crystal were tested, and the real stress-strain curves at different rates were obtained.The formation mechanism of fracture after crystal crushing was analyzed. It was found that the compressive strength and yield strength of gallium oxide crystals showed a significant plastic increase with the increase of the impact speed, while the plastic failure strain decreased with the increase of the impact speed. Furthermore, dynamic mechanical properties of (010) crystal plane of gallium oxide crystal are tested, and actual stress-strain curves are obtained. The fracture process of gallium oxide crystals is observed via high-speed photography. Formation mechanism of fractures is analyzed using three-dimensional topography. AlongsideCombined with the quasistatic indentation test, a set of Holmquist–Johnson–Cook constitutive models suitable for gallium oxide crystals is established. Based on obtained parameters, finite element software ABAQUS is used to numerically simulate nanoindentation test of gallium oxide crystals. By comparing numerically and experimentally obtained load-displacement curve data, validity of parameters of constitutive model is verified. It provides a certain guiding significance for the subsequent ultra-precision machining of gallium oxide crystals.

Surface Form Accuracy Evaluation in Abrasive Lapping of Single-Crystal Silicon Wafers

Silicon-based materials still dominate the current semiconductor industry for the foreseeable years such that it is needed in continuously developing the related advanced manufacturing technologies. For the abrasive precision lapping of single-crystal silicon wafers, the surface form accuracy is very important which can significantly improve its efficiency and reduce the cost in the following ultra-precision polishing process. In this study, a novel driving system is proposed in the single-side planetary lapping process that could realize the irrational rotation speed ratio of the lapping plate to the workpiece, and it is found from the numerical qualitative and quantitative analysis that the uniformity of the particle trajectories moving on the target surface has been significantly improved using the irrational rotation speed ratio and hence resulting in the higher surface form accuracy than that driven by the rational rotation speed ratio. Moreover, an in-house developed irrational rotation speed ratio driving system has been designed for the experimental study, and it is found that the effect of the rational and irrational rotation speed ratios on surface roughness is not significant, while all the five essential values related to the surface form accuracy are better under the rotation speed ratio of i = 1.0772… than that under the rotation speed ratio of i = 1, which demonstrates that the irrational rotation speed ratio driving system has the advantage of being able to obtain a good surface form accuracy and agrees well with the numerical simulation results.

Investigation of ultra-precision planing process to fabricate high luminance retroreflector based on cutting force and tool vibration analysis

In ultra-precision planing process, the analysis of the critical depth of cut (DOC) is required to reduce the edge blunt and micro burrs produced by size effect which decreases of the effective area for high luminance retroreflector. However, since the machining characteristics are different according to cutting tool shape, machining material, and cutting condition, determine of the critical DOC is difficult without a comparison of machined surfaces under various DOC measured by ultra-high resolution measuring instrument. In this study, the critical DOC was analyzed using cutting force and tool vibration signals. The specific cutting energy was calculated by cutting force and cross-sectional area to analyze the stress variation according to DOC. Also, acceleration signals were converted to frequency spectrum that analyze dominant vibrating direction of the cutting tool by variation of cutting characteristic. It was confirmed that the method of using tool vibration more effective and accurate than specific cutting energy through validation of the comparison between results from analyze of the vibration signals and direction measuring surfaces. The master mold with area of 250 mm2 was manufactured by applying analyzed critical DOC. In addition, the high luminance characteristic of a retroreflection film press formed by the master mold was confirmed.