Polishing Head Research Articles

Proposal of Magnetic-Force-Assisted Polishing Method

To reduce the size of the polisher, especially the polishing head, a novel method for applying the polishing load using magnetic force is proposed. As the fabrication of next-generation power device substrates advances, such as diamond, ultra-precise planarization via chemical mechanical polishing (CMP) becomes crucial for transforming these substrates into functional devices. Achieving CMP necessitates the application of an optimal polishing load to the substrate. Deadweight and air pressure methods are the traditional mechanisms for delivering this load. However, they tend to increase the size and complexity of the polishing head mechanism, hindering its miniaturization. This study proposes leveraging the magnetic force for the application of polishing load. Such an approach not only promises the miniaturization of the polishing head but also paves the way for smaller polishers. We constructed a prototype polisher with a straightforward mechanism and conducted several tests. The removal rate measurements from these tests, when compared with those of the traditional deadweight method in prior research, validated our approach. Additionally, by adjusting the magnet spacing (which adjusts magnetic force) and the rotational speed, we found that the removal rate adheres to Preston’s law even when employing the magnetic force for polishing.

Design optimization of 3D-manufactured monolithic stage for biomedical polishing applications by neural artificial network and teaching learning studying based optimization algorithm

Ultra-precision polishing engineering is a current trend in ensuring the good surface quality for biomedical applications. In precision engineering, compliant mechanism, so-called monolithic mechanism, has an increasing trend in alternating conventional rigid counterparts which can be utilized for biomedical polishing application. Therefore, this article proposes an optimal design for the monolithic stage. The stage is employed in driving the polishing head. The stage is operated within from several millimeters to hundreds of millimeters. In this article, the stage was tended to guide the head to polish the biomedical samples. Hypercube sampling design is combined with finite element method to collect the data. The teaching learning-based optimization algorithm is firstly coupled with the artificial intelligence in modeling the behaviors of the stage (resonant frequency, displacement, strain energy, and stress). Next, the teaching learning studying based optimization algorithm is extended to search the optimal parameters of the stage. Three single objective optimization scenarios and two multiobjective optimization scenarios of the monolithic stage are conducted to show the effectiveness of the offered method. The predictors were well-formed by using the teaching learning-based optimization-coupled artificial neural network. The results found that the performance indicators (R and R2) are almost equal to one with the mean square error (MSE) and root MSE values are very small. The optimal results that the stage can work with a highest frequency of 760.7903 Hz, a largest stroke of 0.7325 mm (about 732.5 μm), and a strain energy of 2.5999 mJ. Meanwhile, the von Mises stress is almost much lower than the yield strength of AL 7075-T73. The designed method can effectively use in designing the stage. The stage can be fabricated by using selective laser melting additive manufacturing technology, and it can be employed to guide the polishing head for biomedical samples.

Development analysis of magnetorheological precession finishing (MRPF) technology.

Magnetorheological polishing (MRF) has emerged as a critical non-contact sub-aperture polishing technology due to its notable attributes of high precision and minimal damage. However, MRF's inherent D-shaped removal function leads to reduced convergence efficiency of surface form error and introduces mid-spatial-frequency (MSF) waviness. To address these challenges, we propose magnetorheological precession finishing (MRPF) technology, which ingeniously combines MRF with bonnet precession polishing to generate a Gaussian-like removal function. A pivotal component of what we believe to be a novel approach is the design and fabrication of a specialized hemispherical magnetorheological precession polishing head. The design process incorporates magnetostatic simulations and magnetic force analysis to determine the optimal generating conditions for magnetorheological ribbons. Spot polishing experiments confirm the suitability of a 30° precession angle. Experimental results demonstrate that 8-step polishing achieves a Gaussian-like removal function. Additionally, uniform polishing of fused quartz surfaces significantly reduces Ra from 0.7 µm to 2.14 nm. This research showcases the feasibility of MRPF as a new technical route to achieve Gaussian-like removal functions and nanometer-scaled surface roughness.

The role of formamidine acetate as a complexing agent in the chemical mechanical polishing process of Ta-based barrier layers for through-silicon vias wafers

The complex agent can form a soluble complex with tantalum to improve the removal rate of tantalum (Ta), but the removal rate of tantalum is less than 1000 Å/min when the traditional complex agents such as citric acid, acetic acid, phosphoric acid, ammonium salt and oxalic acid are added to the slurry, which cannot meet the requirements of through-silicon via (TSV) process. Due to the various bonding forms of amidine and the easy modification of the substituent group on the terminal nitrogen atom, the metal complexes of amidine have different stereo effects and electronic properties, and are called "universal ligands". Formamidine acetate (FA) is a new complexing agent with great potential, which is suitable for the removal of TSV tantalum materials. In order to improve the rate selection ratio of Ta and copper (Cu) in the chemical mechanical polishing (CMP) of TSV wafer barrier materials, the effect of FA as a complexing agent on the removal rate of Ta and Cu and the rate selection ratio was investigated under the conditions of working pressure of 3 psi, polishing head speed of 87 r/min, polishing disk speed of 93 r/min, and flow rate of 300 mL/min. The complexation mechanism of FA on Ta and Cu was investigated by electrochemistry, X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and density functional theory (DFT), and the surface quality before and after polishing was compared by using atomic force microscope (AFM). The results showed that with an FA concentration of 1.5 wt%, the removal rates of Ta and Cu were 1503 Å/min and 516 Å/min respectively, which verified the obvious effect of FA on improving the rate selection ratio of Ta and Cu. Surface roughness for both metals decreased from pre-polishing values of 0.793 nm to post-polishing values of 0.210 nm for Ta; from pre-polishing values of 4.32 nm to post-polishing values of 0.381 nm for Cu indicating significant improvement in surface quality after treatment with FA.

Prediction of Pad Wear Profile and Simulation of Its Influence on Wafer Polishing.

As feature sizes decrease, an investigation of pad unevenness caused by pad conditioning and its influence on chemical mechanical polishing is necessary. We set up a kinematic model to predict the pad wear profile caused by only diamond disk conditioning and verify it. This model shows the influences of different kinematic parameters. To keep the pad surface planar during polishing or only conditioning, we can change the sweep mode and range of the conditioner arm. The kinematic model is suitable for the prediction of the pad wear profile without considering the influence of mechanical parameters. Furthermore, based on the pad wear profile obtained from a real industrial process, we set up a static model to preliminarily investigate the influence of pad unevenness on the pad-wafer contact stress. The pad-wafer contact status in this static model can be approximated as an instantaneous state in a dynamic model. The model shows that the existence of a retaining ring helps to improve the wafer edge profile, and that pad unevenness can cause stress concentration and increase the difficulty in multi-zone pressure control of the polishing head.

A novel specialized material removal rate model considering the synergistic effect of dynamic pressure and shear stress for the permanent-magnet small ball-end magnetorheological polishing

Permanent-magnet small ball-end magnetorheological polishing is often used to process small glass workpiece. During polishing, the pressure and shear stress are generated by magnetorheological fluid on the workpiece surface. When conducting fixed-point polishing experiment on fused silica plane workpiece, it was found that the polished pits were mainly distributed on one side of the polishing head's rotation axis. However, the available material removal model considering only a single force cannot explain the cause of polished pit offset phenomenon. It is speculated that the synergistic effect of the two forces on material removal may lead to the offset phenomenon. In order to accurately describe the material removal mechanism of this polishing method, a novel material removal rate model is established in this research. First, a group of actual polished pits are obtained through fixed-point polishing experiment. Then, a novel pressure-shear stress synergistic material removal rate model with unknown coefficients is established. Based on this model, the fixed-point polishing process is simulated by finite element method. The simulated polished pits under different model coefficients are obtained, and the influence rules of model coefficients on the polished pit contour is analyzed. Finally, by seeking the minimal fitting error between the simulated and actual contours, a group of optimal model coefficients are determined, namely, the shear stress coefficient is 1, the dynamic pressure coefficient is 1.2, and the weight coefficient of polishing velocity is 1.9. Through verification, the relative error between the simulated and actual contours can be as low as −9.12 % to 7.13 %. The novel model reveals that the material removal is caused by both shear stress and dynamic pressure. The dynamic pressure is distributed on one side of the polishing head axis and plays a leading role in material removal, which can well explain the cause of offset phenomenon. This research fills the blank of material removal mechanism of this polishing method, and the novel model provides a theoretical basis for process optimization research.

Profile prediction for ultrasonic vibration polishing of alumina ceramics

Ultrasonic-assisted polishing methods are widely used for the precision processing of hard and brittle materials such as glass and ceramics due to their excellent polishing performance. Most research has been focused on basic removal mechanisms of ultrasonic-assisted polished materials. Some investigators have studied the profile of surfaces after conventional mechanical polishing, but less research has been done on the profile of faces after axial ultrasonically assisted polishing. This paper has established a novel force prediction model for the ultrasonically assisted polishing, which is critical for geometrical accuracy and surface finish. Also a material removal model has been developed for this process using a spherical polishing head at different tilt angles. Pressure and velocity distributions have been calculated for multi-position contact areas. The experimental study of multiple trajectory polishing of alumina ceramics showed a good correlation between the experimental and simulated surface profiles. The ultrasonic polishing process parameters were optimized by applying the gray correlation analysis. The obtained results can be used not only select ultrasonic assisted polishing process parameters but also provide theoretical underpinning for surface polishing trajectory planning.

Monitoring and meaning of vibrations in robot polishing

Robot polishing is increasingly used in the production of high-end glass work pieces such as astronomy mirrors, lithography lenses, laser gyroscopes or high-precision coordinate measuring machines. The quality of optical components such as lenses or mirrors can be described by shape errors and surface roughness. Whilst the trend towards sub nanometre level surfaces finishes and features progresses, matching both form and finish coherently in complex parts remains a major challenge. With larger or more precise optics, the influence of process instabilities on the quality of the optics to be polished has a greater impact. Vibrations at a polishing head have a negative influence on the polishing result. These vibrations are caused by bearing damage, motors and other excitations (e.g. gears, belts). The aim of this work is the determination of vibrations at a polishing head and their avoidance strategies. Different bearing conditions are considered: new and perfect bearing, a bearing that has been in contact with polish (rust) and a bearing with repeatable damage (groove milled on the running surface). It can be shown that the frequencies of bearings affect the polishing tool. Furthermore, reasons for and against vibrations in the process are discussed. For the case of vibrationless machining, avoidance strategies were presented.

Construction process of a novel multi-degree of freedom parallel passive compliant constant-force mechanism

Based on the configuration of the Stewart parallel mechanism (SPM), after analyzing the kinetostatic characteristic of a planar four-bar linkage with attached springs which works around the kinematic limb-singularity, a process of constructing a novel multi-degree of freedom (DOF) parallel passive compliant constant-force mechanism (CCFM) is presented. The kinematic limb-singularity of a rigid four-bar linkage with attached springs is obtained and then is applied to construct a constant-force compliant mechanism (CM) based on the pseudo-rigid-body-model by designing appropriate structure parameters to obtain appropriate equivalent spring stiffness of flexures. The CCFM is further used as the branches of the SPM and then a passive multi-DOF parallel mechanism (PM) is constructed. Here the method of replacing the original rigid branch of the PM with the CCFM is called the “Expanded Rigid-body Replacement Synthesis” (ERBRS). The multi-DOF parallel passive CCFM can generate a constant-force zone when makes a small pose transformation around a certain position, where each compliant branch is located at the constant-force zone. The proposed CCFM can be applied in practice such as the polishing machine which needs the constant contact-force between the polishing head and the workpiece. The proposed method of constructing a CCFM with one- or multi-degree of freedom mechanism has some important reference value on designing other types of CCFMs.

Reduction in the Volumetric Wear of a Ball Polishing Tool Using Ultrasonic-Vibration-Assisted Polishing Process

Ultraprecision freeform polishing using a bonnet or a felt ball mounted on a polishing head plays an important role in the mold and lens production industries. The volumetric wear of a bonnet or a felt polishing ball is still a problem to be solved. The objective of this study was to develop an ultrasonic-vibration-assisted ball polishing process on a CNC machining center to improve the surface roughness of a STAVAX mold steel and to reduce the volumetric wear of the polishing ball. The optimal combination of the ultrasonic-vibration-assisted ball polishing parameters for a plane surface was determined by conducting the Taguchi L18 matrix experiments, ANOVA analysis, and verification experiments. The surface roughness of the polished specimens was improved from the burnished surface roughness of Ra 0.122 μm to Ra 0.022 μm. In applying the optimal plane surface ball burnishing and vibration-assisted spherical polishing parameters sequentially to a fine-milled and burnished aspherical lens surface carrier on a five-axis machining center, the surface roughness of Ra 0.014 μm was obtainable. The improvement in the volumetric wear of the polishing ball was about 62% using the vibration-assisted polishing process compared with the nonvibrated polishing process.

Investigating the Effect of Operating Parameters on the Wear of Abrasive Tools in the Polishing Stage of Granitic Building Stones

Operating parameters affect the wear of abrasive tools during the polishing stage in building stone processing plants. This study investigates the effects of essential operating parameters including polishing head pressure, head rotation speed and water flow rate on the wear of the abrasive tools. For this purpose, a building stone abrasivity test was used to determine the weight loss of the abrasive tools during laboratory polishing of fifteen different types of Iranian granitic building stones. The standard operating parameters of the test were a polishing head pressure of 5 bar, a head rotation speed of 300 revolutions per minute (rpm), and a water flow rate of 4 L/min. The values of the operating parameters were changed to values within the range from ±25% and ±50% of the standard conditions in order to investigate the effect of variations in these parameters on the wear of the abrasive tools during the polishing stage. The results of different tests showed that the wear of the abrasive tools was directly proportional to the pressure up until a critical value of around 6.25 bar, after which it gradually decreased. This nonlinear wear behavior does not conform to Archard’s well-known classical wear law. The FESEM images of the worn surfaces showed that due to excessive load, debonded abrasive particles could not be pulled out from the pin surface and led to an interlocking phenomenon between the pin and stone surface. It was also found that the wear of the abrasive tools increased with increasing head rotation speed, while it decreased with the water flow rate. Moreover, the main wear mechanism of tests was abrasive wear and in some cases with a mixture mode of adhesion and delamination.

Gloss and Modelling Studies of Stone Polishing Using Linear Polishing Machines with Rotating Heads

The ornamental stone industry has always played an important role in the world economy. Polishing the slabs to increase their gloss is important to enhance the beauty and richness of these natural materials. Many industrial polishing machines rely on a rotating head’s movement along zigzag trajectories (controlled by belt, transverse and rotational speeds), to erode the surface as stochastically as possible (to avoid scratches and other visual defects caused by paths that are too symmetrical). Optimizing (and automatizing) these three speeds together therefore represents a significant gain for the industry, in time, energy, and quality of product. In this work we show that this optimization can be accomplished by fulfilling these conditions: (1) the displacement of the polishing head after a single zigzag movement should be smaller than its diameter; and (2) the displacement of the polishing head after a single rotation should be smaller than its radius. To assess the validity of these two conditions, we studied the polishing activity using experiments based on gloss measurements of the polished stone and computer simulations based on the geometric contacts between the tool and the stone. We concluded that: (i) a clear correlation can be established between experimental and simulation data; (ii) the two displacement parameters represent an effective way to control the quality and efficiency of the polishing process; and (iii) there is a limit to the gloss acquired through polishing processes, thus polishing above a given threshold decreases the efficiency without increasing the quality. The correspondence between experimental and modelling results opens the door to further optimizations of these polishing processes in the future.

Highest Quality and Repeatability for Single Wafer 150mm SiC CMP Designed for High Volume Manufacturing

Silicon Carbide (SiC) provides excellent characteristics such as superior thermal conductivity, high carrier mobility and extreme chemical stability in comparison with those of Silicon (Si). SiC is already showing significant device performance benefits in power devices, high performance communication, and LED lighting. However, SiC presents many challenges for wafer surface treatment because of its high hardness and remarkable chemical inertness. Today, mechanical polishing techniques on industrial batch CMP tools are the predominant methods for SiC wafer surface treatment, but material removal rate (MRR), surface defects and wafer flatness control are reaching fundamental limits with increasing wafer diameter. Batch processing typically results in a higher amount of surface scratches and defects, higher wafer to wafer variability, and higher wafer breakage rates. A unique single wafer chemical mechanical polishing (CMP) technique on 150mm n-doped, 4° off-axis, single crystal, 4H-SiC wafers was developed to create a virtually defect-free surface. A polishing head has been designed to manipulate polishing pressures at various zones of the wafer. This capability can modulate the removal thickness at each region on the wafer surface, resulting in a highly uniform wafer profile. Additionally, a CMP slurry has been formulated to maximize MRR from 2μm/hr to over 8.5μm/hr. Potassium permanganate has been selected as an oxidant and aluminum oxide particles as the abrasive. The oxidant concentration and abrasive content along with slurry pH level have also been optimized for ideal chemical and mechanical activity. Scratch-free wafer surfaces are observed with atomic force microscopy (AFM) and bright field (BF) and dark field (DF) inspection techniques. Roughness on the Si face is reduced to below 0.08nm. Total length of surface scratches was reduced to 10mm or less. Industrial metrics of wafer flatness, including total thickness variation (TTV) and local thickness variation (LTV) are modulated and improved. A test run completed on 25-wafers shows an overall 31% improvement of TTV post CMP process.

Research on robot bonnet polishing silicon carbide optical element machining method based on improved traveling salesman problem pseudo-random polishing path planning

Silicon carbide (SiC) has been widely used in optical components due to its excellent physical and electronic properties. However, it is very difficult to process the SiC owing to hardness and chemical stability. The robot bonnet polishing method was proposed to solve the problems existing in current polishing SiC optical elements methods, which have a low material removal rate and are expensive. First, according to Preston equation, Hertz contact theory and velocity analysis, the removal function model was established, the bonnet polishing head device was designed and put through simulation analysis. Next, aiming at the problem that the workpiece surface is prone to ripple errors during the traditional periodic polishing path trajectory processing, a pseudo-random polishing path based on the traveling salesman problem (TSP) is proposed. The path has the characteristics of multi-direction, high randomness, flatness and continuity, and compared with the raster path and random disturbance path, it is verified that the improved TSP polishing path has the ability to suppress the spatial intermediate frequency error. Then, single-point and multi-point polishing experiments are implemented to verify the accuracy and stability of the removal function. Finally, the application verification polishing processing of the SiC optical element is implemented by rough and fine polishing. The experimental results show that the bonnet polishing method and the improved TSP polishing path can effectively achieve the polishing of SiC optical elements and better suppress the intermediate frequency error of the optical processing surface. After three cycles of rough polishing and four cycles of fine polishing, the surface profile convergence rate is 72.3% and 50.8%, respectively, and the convergence rate is faster and the machining accuracy is higher.

Material removal predictions in the robot glass polishing process using machine learning

Robot polishing is increasingly being used in the production of high-end glass workpieces such as astronomy mirrors, lithography lenses, laser gyroscopes or high-precision coordinate measuring machines. The quality of optical components such as lenses or mirrors can be described by shape errors and surface roughness. Whilst the trend towards sub nanometre level surfaces finishes and features progresses, matching both form and finish coherently in complex parts remains a major challenge. With increasing optic sizes, the stability of the polishing process becomes more and more important. If not empirically known, the optical surface must be measured after each polishing step. One approach is to mount sensors on the polishing head in order to measure process-relevant quantities. On the basis of these data, machine learning algorithms can be applied for surface value prediction. Due to the modification of the polishing head by the installation of sensors and the resulting process influences, the first machine learning model could only make removal predictions with insufficient accuracy. The aim of this work is to show a polishing head optimised for the sensors, which is coupled with a machine learning model in order to predict the material removal and failure of the polishing head during robot polishing. The artificial neural network is developed in the Python programming language using the Keras deep learning library. It starts with a simple network architecture and common training parameters. The model will then be optimised step-by-step using different methods and optimised in different steps. The data collected by a design of experiments with the sensor-integrated glass polishing head are used to train the machine learning model and to validate the results. The neural network achieves a prediction accuracy of the material removal of 99.22%.Article highlightsFirst machine learning model application for robot polishing of optical glass ceramicsThe polishing process is influenced by a large number of different process parameters. Machine learning can be used to adjust any process parameter and predict the change in material removal with a certain probability. For a trained model,empirical experiments are no longer necessaryEquipping a polishing head with sensors, which provides the possibility for 100% control

Beyond Beauty. Byzantine steatite icon from Chełm. Archaeology, Petrography and Traceology

Abstract A fragmentarily preserved Byzantine icon made of steatite was discovered in 2015 during regular excavations in Chełm, eastern Poland. Identified as the left wing of a diptych illustrating the Twelve Great Feasts and created at the close of the 12th century, the find is one of the most important and beautiful Byzantine artefacts to have been found in Poland. The icon was uncovered within the confines of the palace complex which was created by Daniel (Danylo) Romanovych († 1264) in Chełm in the second quarter of 13th century. The icon, even though it was found within the borders of what is now Poland, is material evidence of contact between Byzantium and the social elite of the Galicia-Volhynia lands, rather than with the Polish Piasts. In this paper we concentrated on the presentation of the archaeological context of the find, which made it possible to establish that the icon arrived Chełm before the middle of the 13th century (terminus ante quem 1253), and especially on petrographic and traceological analyses of the icon. Assuming that greenish plaques were indeed the most characteristic steatite icon type, a decision was made to examine, apart from the Chełm artefact made from white rock, a greenish icon from the National Museum in Krakow as well. Petrographic analyses were based on optical microscopy, scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and X-ray powder diffraction (XRPD). Both icons were carved in steatite i. e. talc rich rock but their chemical compositions indicate the presence of other components. Artifact from Chełm is white. Porous, enriched in potassium (K) and locally blistering outer rim of the icon from Chełm was formed probably during the fire event. Presence of forsterite and subordinate amount of leucite also indicate high temperature influence. Local enrichment in calcium (Ca) is related to exchange reactions with ground compounds. Accumulation of different components on the surface of the icon’s surface was noted. The icon from the National Museum in Krakow is greenish probably because of the presence of chlorite. The results of the traceological analysis (icon from National Museum in Krakow was not analysed) indicate that the icon found in Chełm was created most likely by a skilled and experienced carver with access to the high-quality magnifying glass and specialist tools required for rendering minuscule objects and their details. The production of the icon also involved the use of a “mechanical” tool, probably a kind of a miller with a rotating polishing head, which also seems to point to a specialist workshop. The use-wear traces observed on artefact are limited to polish resulting from prolonged contact with human hands or storing the icon in a leather case. Most of the extant Byzantine icons are unprovenanced objects held in museum collections or church treasuries. Therefore, as the icon presented in this paper was discovered during archaeological excavations, it ranks among the few Byzantine artefacts to have been found outside of this realm. The petrographic and traceological analyses conducted are the first published natural science contributions to the study of Byzantine steatite icons and we hope they will provide the impetus for undertaking such research on other Byzantine finds, helping to develop Byzantine archaeology further.

Material removal characteristics of magnetic-field enhanced shear thickening polishing technology

To achieve the efficient and high-quality machining of hard and brittle ceramics used in various industries, in addition to other materials that are difficult to process, a magnetic-field enhanced shear thickening polishing technology was developed in this study. Based on the coupling effect of the magnetization enhancement and shear thickening, the proposed processing technology can improve the rheological properties of the shear thickening fluid, thus realizing an adaptive and efficient removal of microscopic materials. Herein, the microscopic material removal mechanism of this technology was analysed and the rheological properties of a magnetic shear thickening polishing fluid were studied. Based on the Preston's equation, the dynamic pressure of non-Newtonian fluids and the magnetorheological polishing theory, a material removal rate prediction model was established and it had a maximum relative error of 7.56%. The influences of the polishing head rotational speed, abrasive particle concentration and carbonyl iron powder particle concentration on the material removal function were investigated and the stability of the prediction model was verified. After polishing a zirconia workpiece for 20 min, a low-damage machining surface was obtained and the surface roughness was 8.3 nm. Therefore, the effectiveness of the material removal prediction model was verified and the findings of this study provide a theoretical basis for the realization of deterministic polishing. Moreover, the feasibility of the magnetic-field enhanced shear thickening polishing method for the ultra-precision machining of hard-to-machine materials, such as hard and brittle ceramics, was verified.

Single-Wafer Chemical Mechanical Planarization (CMP) on 150mm Silicon Carbide Substrate with Superior Surface Finish and Removal Efficiency

Silicon carbide (SiC) has excellent performances such as superior thermal conductivity, good carrier mobility and high chemical stability in comparison with those of silicon (Si). SiC has a great potential in the area of the next generation power controlling devices, high performance communication, and LED lighting. Chemical mechanical polishing (CMP) techniques combine mechanical polishing with a chemical etching action and can achieve truly defect-free surface with wafer flatness control capability, which is important for obtaining good epitaxial layer. However, Silicon carbide (SiC) presents many challenges for wafer surface treatment because of its high hardness and remarkable chemical inertness. Currently, low material removal rate (MRR) of CMP technique is a fundamental limit, it extends the CMP process time and effects the yield in mass production of substrate.In this study, we proposed an ultra-high MRR CMP process on 150mm n-doped, 4° off-axis, single crystal, 4H-SiC wafers. The process was developed at Applied Materials Mirra Durum® CMP configured with the membrane polishing head using an acidic slurry with strong potassium permanganate oxidant and thermoplastic polyurethane (TPU) pad. The effect of polishing process parameters (speed of the head and platen rotating, head sweeping distance and applied pressure) on MRR are analyzed in JMP® (SAS institute) while a design of experiments (DOE) is employed. The chemical composition of slurry, which is also critical to MRR, has also been evaluated via DOE. Potassium permanganate with certain concentration has been selected as oxidant and aluminum oxide micron particle has been used as abrasive. We have also conduct studies on the SiC wafer flatness control, which is proven to depend on structural design of polishing head. The head has been optimized to offer a flexible zone pressure control, therefore the impact on wafer flatness during the CMP process has also been minimized. In addition, accepted industry measurements of surface roughness, surface defect and wafer deformation are also taken into consideration.The successful development of CMP process provided an optimized MRR result, the reported MRR is 22.5 um/hr from C face and 6.4um/hr from Si face. The MRR have achieved highest level within literature and industry. The MRR results are analyzed by least–square modelling, which reveals that Prob >|t| is less than 0.05 for platen rotation speed and membrane pressure. Prediction profile demonstrates that MRR typically follows Preston’s law, which is a linear function of the platen speed and the pneumatic pressure applied to substrate. MRR is highly stable throughout a 2000-wafer repeat run with wafer-to-wafer uniformity of 2.4-3.7%. Scratch-free wafer surfaces are observed for the ultra-smooth wafers via atomic force microscopy (AFM) and defect inspection (Candela) tool. Roughness on Si face has been reduced from 2.0-2.5nm to less than 0.15nm with acceptable repeatability. Surface defects including scratches, pits have been greatly reduced. Total scratch length of 10mm has been achieved. Total thickness variation (TTV) is measured to characterize wafer flatness by Tropel tool, which indicates that the total amount of removed thickness on faces has a profound influence on TTV. With focused process control, post-CMP TTV can be improved by 0.6 ~2.1 um from pre-CMP TTV.

Investigations on flexible pad polishing for nano-finishing of freeform optics mold

The need for freeform surfaces are widely recognized in the optical, aerospace, biomedical, and automotive industries. However, the fabrication of freeform surfaces is very difficult and expensive because of the involvement of advanced, multi-axis, dedicated machining processes. In many applications, the machining surface needs further polishing to reduce the surface roughness. The main aim of this work is to investigate the flexible pad polishing process for the finishing of the freeform surface. To achieve this, a flexible pad polishing setup is developed in two-axis configuration that can be integrated with a diamond turning machine or any other computer numerical control (CNC) machine and capable to polish the conventional rotationally symmetric surfaces as well as freeform surfaces. The polishing parameters for the developed polishing head are optimized for the nano-finishing of the freeform mold surface of Stavax ESR steel. The effectiveness of the current polishing setup is demonstrated by measuring the improvement in surface roughness height and profile. Test results reveal that the improvement on the freeform surface is significant, that is, surface roughness ( Ra) reduces from 220 nm to 25 nm while keeping the profile within the desired tolerance.

The effect of pressure gradient and abrasive tool wear when polishing ceramic tiles

A series of experimental investigations were conducted to develop and describe the distribution of the pressure gradient below and across the topological surface of laboratory grinding tools on a ceramic tile during the gloss gaining process. The possibility of controlling the pressure required for the industrial polishing ceramic tiles would prevent excessive wear, directly reducing energy and water consumption. The present research aims to determine the effect of incremental surface pressure distribution in the polishing of ceramic tiles with line contact. For this purpose, two grinding devices were mounted together as a polishing head and built from the machine's base coordinate system on a CNC tribometer with a deflection angle of 2.2°. The grinding pressure was distributed gradually underneath and across the polishing tools by implementing a new polishing tool composed of rotating shaft with pivot joint, helical springs and two abrasive blocks attached on it. Due to the configuration of polishing tools with preferred process parameters, the impact of the material removal and gloss production was found to be minimized or maximized according to the chosen kinematics. Results implied that the process outcome in terms of gloss level and fine surface finish is influenced by the surface pressure (maximum at approximately more than 15 MPa) as well as the abrasive tool surface wear. Furthermore, in the manufacturing line, higher wear rate is posed by the coarser abrasives but less wear rate on the finer abrasives with high gloss gaining.