Polishing Pad Research Articles

Investigation of Electrolytic Condition on Abrasive-Free Electrochemical Mechanical Polishing of 4H-SiC Using Ce Thin Film

Silicon carbide (SiC) is a promising material for power electronic devices; however, the fabrication of low-cost and high-quality single-crystal SiC wafers with a smooth surface remains difficult, due to its high mechanical hardness and chemical stability. We report an abrasive-free electrochemical mechanical polishing (AF-ECMP) for a SiC surface using Ce film deposited on a polishing pad. The SiC surface is oxidized by electrolysis between the SiC (anode) and the polishing plate (cathode), the resulting oxide is then removed by interaction with the Ce film. The excess oxide produces a rough surface during continuous electrolysis on the SiC surface. The periodic electrolysis improves the surface quality, and electrolysis conditions such as duty ratio, duty cycle, and applied voltage are optimized. The abrasive ECMP, using CeO2 particles, broadens the initial scratches on SiC, whereas no scratch broadening is observed by the AF-ECMP. Despite a lower material removal rate (MRR) than the abrasive ECMP, the surface roughness was improved by the AF-ECMP with less polishing time. AF-ECMP produces an atomically smooth SiC surface with a roughness of ∼0.5 nm Ra, which is promising for low-cost and environment-friendly finishing process for SiC.

Hybrid Effect Study of Pad-Conditioning/Slurry-Supply by High-Pressure-Micro-Jet (HPMJ) Method during CMP Process

Surface clogging of polishing pad is inevitable during CMP (Chemical Mechanical Polishing) process of semiconductor wafers. In this paper, a novel slurry-feeding system utilizing HPMJ (High Pressure Micro Jet) apparatus was presented as in situ conditioning for CMP process. Clogging of pad surface can be greatly removed; the surface damage of polishing pad can also be extremely reduced by this new method. The experimental results exhibited that the material remove rate (MRR) of HPMJ method was almost constant value 100 μm h−1. However, the MRR value of conventional slurry-dripping method was about 50 μm h−1. On the other hand, surface roughness Ra changed both in hybrid HPMJ method and conventional slurry-dripping method after long-time continuous CMP process, the average Ra of hybrid HPMJ method was about 30% smaller than that of conventional slurry-dripping method, indicating that better surface quality could be obtained by HPMJ method polishing. It was demonstrated that HPMJ method showed higher MRR and lower surface roughness Ra for polishing process. HPMJ hybrid system mentioned in this study can also be applied to the polishing process of hard-to-process wide bandgap semiconductor substrate materials, such as SiC or GaN.

Mathematical modeling based on contact mechanism due to elastic and plastic deformation of pad asperities during CMP

Technologies in semiconductor industry have been developed into a three-dimensional multilayer wiring for high integration of devices. Chemical mechanical planarization (CMP) process is one of the key technologies for achieving multilayer wiring, which enables global planarization. In addition, highly integrated devices can be realized by increasing the depth of focus in the photolithography process. However, in the inter-layer dielectric (ILD) CMP of the transistor, the uppermost oxide layer has the step due to the arrangement of the devices. The ideal material removal mechanism is to gradually remove materials from the top of the step height which allows for global planarization. However, in the CMP of the patterned wafers, simultaneous polishing of the upper and lower layers occurs when the step height reaches a certain height. This means that the polishing is strongly dependent on the structural characteristics of the pattern. Especially, the difference in the material removal rate depending on the pattern density acts as a constraint in terms of device layout. Therefore, it is essential to develop an accurate prediction model of material removal rate as a function of pattern density, size and arrangement. This study aims to define the mathematical planarization model according to contact mode between a polishing pad and patterned wafer. Considering that the real contact area between the actual polishing pad and the wafer is about 1 %, the mathematical model is derived based on the microscopic deformation of the pad asperities, not the macroscopic deformation of the bulk pad. Finally, we describe the verification between the theoretical material removal rate model and step height reduction and the actual CMP results. The root mean square error of the upper layer material rate, the lower material removal rate, and the step height reduction were 24.59 nm/min, 22.03 nm/min and 22.6 nm, respectively. Compared with the previous studies, the new model of this study improved the error by up to 50.9 %.

Non-Newtonian fluid based contactless sub-aperture polishing

Compliant polishing (e.g. bonnet polishing) is widely used to finish freeform surfaces. However, direct pad/surface contact often causes scratches or embedding of abrasives, especially on metal moulds. Here, the use of non-Newtonian fluid (polymer and starch) to enable contactless polishing without need for a polishing pad is reported. Modelling and analysis are conducted to reveal the influence of fluid rheology on the material removal mechanism. Using #6000 grit alumina, Gaussian shaped removal footprints can be obtained, with a completely scratch-free roughness of 3.9 nm Ra on Nickel, in contrast with 70.3 nm Ra obtained by general water based bonnet polishing.

A Gaussian Process Model-Guided Surface Polishing Process in Additive Manufacturing

Abstract Polishing of additively manufactured products is a multi-stage process, and a different combination of polishing pad and process parameters is employed at each stage. Pad change decisions and endpoint determination currently rely on practitioners’ experience and subjective visual inspection of surface quality. An automated and objective decision process is more desired for delivering consistency and reducing variability. Toward that objective, a model-guided decision-making scheme is developed in this article for the polishing process of a titanium alloy workpiece. The model used is a series of Gaussian process models, each established for a polishing stage at which surface data are gathered. The series of Gaussian process models appear capable of capturing surface changes and variation over the polishing process, resulting in a decision protocol informed by the correlation characteristics over the sample surface. It is found that low correlations reveal the existence of extreme roughness that may be deemed surface defects. Making judicious use of the change pattern in surface correlation provides insights enabling timely actions. Physical polishing of titanium alloy samples and a simulation of this process are used together to demonstrate the merit of the proposed method.

Trajectory uniformity of the double-sided mechanical polishing of SiC single crystal substrate

SiC single crystal substrate has a useful range of physical, mechanical and electronic properties that makes it a promising material for semiconductor devices. To obtain the relationship between a certain abrasive on the polishing pad and SiC single crystal substrate, the model of double-sided mechanical polishing has been established and the kinematics equations also been derived. As a key to the process, the best optimized polishing trajectory parameters have been carried out. The specific detailed distribution ranges of abrasive distribution radius RA, speed ratio of ring gear to sun gear m and speed ratio of lower polishing disk to sun gear n have been proposed. To acquire the trajectory uniformity of the double-sided mechanical polishing, the coefficient of variation firstly based on statistical analysis has been calculated. Surface quality of the double-sided mechanical polishing of SiC single crystal substrate has been characterized by flatness, micro-morphology and surface roughness (C-face and Si-face). All these results greatly provide a key guarantee for the next step of chemical mechanical polishing to improve efficiency and ensure ultra-precision polishing quality.

Development of a novel chemical mechanical polishing slurry and its polishing mechanisms on a nickel alloy

Conventional chemical mechanical polishing (CMP) slurries of pure nickel (Ni) and its alloys usually consist of toxic and corrosive acids, which is dangerous and contaminative to the operators and environment. It is a big challenge to develop a novel environment friendly CMP slurry for Ni alloys. In this study, a novel environment friendly CMP slurry was developed, containing of silica, hydrogen peroxide (H2O2), malic acid and deionized water. The surface roughness Ra, and peak-to-valley (PV) values are 0.44, and 4.49 nm, respectively with an area of 71 × 53 μm2. To the best of our knowledge, surface roughness in this work is the lowest for pure Ni and its alloys at a scan area of 71 × 53 μm2. The CMP mechanisms are elucidated by electrochemical, X-ray photoelectron spectroscopy, and infrared measurements. Firstly, H2O2 dominated the oxidation process in CMP, forming oxides of nickel (Ni), chromium (Cr), and molybdenum (Mo) on the surface of Ni alloy. Then, the Ni oxides were dissolved by hydrogen (H) ions. The oxides of Cr and Mo were stable in malic acid. Chelating formulas are proposed between malic acid and Ni ions. Finally, the passivated film was removed by the polishing pad.

Viscoelastic behavior of polishing pad: Effects on edge roll-off during silicon wafer polishing

In semiconductor device fabrication, surface flatness of silicon wafers has a significant impact on the chip yield. Hence, there is a strong demand to prevent the deterioration in surface flatness near the wafer edge due to edge roll-off during polishing. In the present study, we investigate the viscoelastic behavior of polishing pads and its effects on the uniformity of material removal distribution near the wafer edge. On the basis of the findings, we propose polishing conditions required to improve surface flatness near the wafer edge. The double-sided polishing experiments performed using silicon wafers reveal that the proposed polishing conditions effectively reduce edge roll-off.

Simple and rapid voltammetric determination of cephalosporin drug cefixime on boron-doped diamond electrode

A boron-doped diamond electrode was used for the first time in sensitive and selective analysis of cefixime, a third-generation cephalosporin drug, using the square-wave voltammetry technique. The effect of the pretreatment procedure on the current response at the electrode was investigated. The best signals were obtained by polishing the electrode with a polishing pad after applying anodic and cathodic electrochemical pretreatment to the electrode. Cefixime gave a well-defined, irreversible and diffusion-controlled oxidation peak in 0.1 mol dm−3 H2SO4 support electrolyte at + 1.25 V (vs. Ag/AgCl 3 mol dm−3 NaCl). Under the best instrumental conditions and with the pretreatment procedure, a calibration curve was obtained with linearity between 2.2 × 10−6 mol dm−3 and 1.3 × 10−4 mol dm−3 at 5.9 × 10−7 mol dm−3 detection limit. The effect of possible interference substances on the current response for cefixime was investigated and the method was found to have sufficient selectivity. The developed method was successfully applied for the analysis of cefixime from pharmaceutical formulations. The method can be an alternative to the other analytical approaches due to its speed, simplicity, low cost, wide calibration range, and repeatable responses.

A curious observation of phenomena occurring during lapping/polishing processes.

We demonstrate that the kinematics of the polishing process is more intriguing than an idealized planetary movement as all the previous studies reported. In reality, the workpiece is pseudo-constrained by the planetary carrier and because of this its relative motion to the polishing pad also incurs a 'parasitic' movement that previously has not been observed. Here, we report and model this parasitic movement and quantify its effect upon the workpiece surface roughness. Using a motion capture system, the principal and 'parasitic' movements between the sample and polishing tool have been tracked and our models validated. It is proved that considering this parasitic movement the prediction of workpiece surface morphology can be significantly improved when compared with the idealized approach (i.e. planetary). Our observations and modelling framework open the avenue to carefully consider the compliance between the tools and workpiece in other manufacturing processes for accurate predictions of the process outcomes.

Effect of graphene additions on polishing of silicon carbide wafer with functional PU/silica particles in CMP slurry

In this paper, a functional ternary slurry consisting of polyurethane (PU) microspheres, graphene oxide (GO) nano platelets and silicon oxide (SiO2) abrasives was used to carry out the polishing process on Si face of 4H-SiC wafers. The processing parameters of the slurry include graphene weight fraction in slurry GO1–GO7 (0.1–0.7[Formula: see text]wt.%), pH value (3–5), and sonication time T5–T15 (5–15[Formula: see text]min). Polishing process is conducted with two kinds of polishing pads A and B, PU and PC (polycarbonate). Results show that material removal rate (MRR) increases with increasing GO weight fraction up to GO5; besides, MRR also increases with increasing sonication time up to T10, and with increasing pH value. Using PU pad, the GO5-T10-pH5-A slurry leads to highest MRR 102.220[Formula: see text]nm/h of the polished SiC wafer. On the other hand, surface roughness improvement rate (SRIR) increases with increasing GO weight fraction up to GO5, and increases with increasing sonication time up to T15. But SRIR is not affected by pH value. Regarding effect of pad type, on average the PU pad results in higher MRR and better SRIR compared with the PC pad. Using PC pad, GO5-T10-pH5-B leads to lower MRR of 87.627[Formula: see text]nm/h. The addition of GO as the ternary slurry demonstrates its better effect on polishing SiC wafers by comparing with the counterpart binary slurry without GO. For example, MRR by the counterpart slurry SiO212-pH5-A is 58.411[Formula: see text]nm/h, which is lower than 102.220[Formula: see text]nm/h by the ternary slurry GO5-T10-pH5-A. Both XPS and Raman spectra demonstrate that the wafer polished by the functional ternary slurry can effectively produce the softer SiO2 reactant layer on SiC wafer, and result in better polishing performance.

Design and mechanical properties of particle-reinforced polymer-matrix functionally graded materials applied on elastic polishing pad

Aiming at the functionally graded materials (FGMs) formed by mixing the abrasive particles and rubber with different kinds and mass ratios, the numerical analysis and experimental verification of its mechanical properties were carried out. The FGMs was applied on the elastic polishing pad for obtaining ultra-smooth surfaces of hard and brittle materials, such as glass ceramics and silicon wafers. Its structure and properties changed in the radial direction quasi-continuously. Regarding to the design of the FGMs, the maximum standard deviation of Young's modulus was 1.09 after 6 repeated tests, when the SiC abrasive particles and chloroprene rubber (CR) were mixed with mass ratio less than 50 phr. Furthermore, there are three types of mechanical properties, named edge stress, boundary stress and central stress, when the workpiece was applied to the elastic polishing pad mainly made of the FGMs mentioned above. If using matching module method, the abrupt phenomenon of edge stress was basically eliminated. Increasing the gradient rings could reduce the abrupt change of boundary stress, and the contact stress in the single gradient ring was basically kept constant when the FGMs was up to 8 gradient distribution. Meanwhile, due to the viscoelastic property of rubber, there was a difference of 25% between the dynamic and static stress values of the elastic polishing pad with acrylate rubber (ACM) as the matrix, and the maximum difference was less than 4.7% when using CR as the matrix. Besides, the dynamic mechanical analysis (DMA) showed that the loss factor tanδ was less than 0.1 in the processing temperature range of 20 °C–50 °C. Therefore, the FGMs with SiC or Al2O3 as particle-reinforcement, and CR as polymer-matrix could largely solve the inconsistent problem of central stress under dynamic and static conditions.

Relationships among Carrier, Friction Coefficient and Removal Rate of Double-Sided Polishing

Double-sided polishing (DSP) method is widely used in manufacturing process of sapphire substrate for LEDs. Since, several studies on the DSP method have been actively reported, however, it is not enough from the practical viewpoint such as the effect of carrier, polishing pad surface asperities and kinematic friction characteristics on the removal rate. This study focuses on the carrier used in the DSP as well as aims to elucidate the carrier characteristics and the kinematic friction coefficient for obtaining the high removal rate. In this paper, the effects of slurry hole area ratio and surface properties of carrier on the removal rate in upper/ lower surfaces of substrate were examined. Moreover, in the kinematic friction characteristics between the polishing pad and the substrate, the friction characteristics in the DSP were observed. As a result, the removal rate and kinematic friction coefficient of the upper/ lower surfaces of substrate change depending on the slurry hole area ratio and surface properties of carrier, and there is a strong relationship between them.

Study on Organic Fibers in The Polishing Pad for Sapphire Chemical Mechanical Polishing

Study on Organic Fibers in The Polishing Pad for Sapphire Chemical Mechanical Polishing

Precision Polishing of Single Crystal Diamond (111) Substrates Using a Sol-Gel (SG) Polishing Pad

In recent years, the application prospect of single crystal diamond (SCD) in the field of electronics is remarkable, and material processing technologies are the basis for the application of semiconductor materials. In this paper, a novel polishing method for SCD substrates was developed utilizing a sol-gel (SG) polishing pad, which is a semi-fixed abrasive polishing tool fabricated by SG technique. The surface morphology of SCD substrates was observed with a 3-D optical profiler and scanning electron microscopy (SEM). The results showed that the surface roughness was 1.32 nm Ra and the material removal rate of SCD (111) was about 120 nm/h. The polished diamond surfaces and wear debris were investigated using Raman spectroscopy and transmission electron microscopy (TEM), respectively. The results showed the removal mechanism of SCD using an SG polishing pad was mainly dominated by the mechanical removal and phase transformation, including the mechanical cleavage, amorphization, and graphitization.

A Global Correction Process for Flat Optics With Patterned Polishing Pad

To improve the efficiency of flat optics fabrication, a global correction method with the patterned polishing pad is developed in this paper. Through creating grooves on a polishing pad, the contact pressure distribution on the optics surface can be adjusted to change the material removal rate (MRR) distribution during polishing. Using the patterned pad, the selective removal ability of the polishing process is greatly enhanced. The predictability and stability of the MRR distribution are the preconditions to efficiently implement the proposed global correction method. Relying on the MRR distribution prediction method proposed and validated in this paper, the pad pattern can be designed based on the original surface figure of the workpieces. The designed groove pattern is created on the polishing pad using the custom-developed equipment. Then, the optical glass is polished on the designed pad with the optimized polishing time. A flat optical glass sample (Φ 100 mm) is polished with the global correction method to show its feasibility and advantage. The correction instance shows that the peak-to-valley (PV) value of the surface profile (with 3 mm edge exclusion) dropped from 1.17 µm to 0.2 µm in 14 min using a polyurethane pad with two ring grooves. Comparing with the conventional polishing process, which usually takes hours or days, the global correction method proposed in this paper can improve the efficiency of the optics manufacturing significantly.

Pollution-Free Approaches for Highly Efficient Sapphire Substrate Processing by Mechanical Chemical Polishing

In this study, two pollution-free approaches were used to improve the processing efficiency of mechanical chemical polishing (MCP) for sapphire substrates. The first was dedicated to polishing using synthetic silica abrasives with high reactivity as soft abrasives, and the second was dedicated to polishing using organic acids and bases as catalysts on the polishing pad and coolant, including citric acid, theophylline, green tea, and coffee. The results showed that the rate of material removal of MCP with highly reactive silica abrasives was more than 187.5% larger than that of pristine silica abrasives with low reactivity, and surface roughness was 21% smaller than that of pristine silica abrasives. Meanwhile, in case of citric acid, the removal mass of sapphire was more that 29.2% larger than that of an original polishing method, and surface roughness was about 15.2% smaller than that of original polishing method. However, theophylline, green tea, and coffee affected processing efficiency negligibly or negatively, due to a limited effect on increasing the pH level of the coolant, and the powder residue in the coolant obstructing the contact between the nano-silica abrasive particles and sapphire surface. These results indicate that highly reactive nano-silica abrasives and citric acid show good catalytic activity towards the sapphire processing efficiency of MCP. The promoting mechanism of highly active silica abrasives and citric acid for the processing efficiency of MCP is also discussed in detail in this study.

Design of surface grooves on a polishing pad based on slurry uniform flow

Slurry uniform flow can ensure uniform slurry distribution, temperature distribution, mechanical and chemical reactions, and timely residue removal, which is conducive to the stability of the machining process. Surface pattern of a polishing pad is one of the main factors to control the flow of slurry. To achieve the slurry uniform flow, the hydrodynamics of the slurry were calculated to design the surface grooves on polishing pad. Finite element technology was adopted to simulate the flow field distribution of the designed polishing pad. Particle image velocimetry experiments were conducted to verify the uniformity of slurry flow on the designed pad surface. The polishing experiments with designed and groove-diffused pads were conducted to obtain the high surface quality in the same polishing conditions. The results show that the spiral surface pattern established by a curve equation with 2-mm-width and 1.5-mm-depth grooves was obtained for the slurry uniform flow. The experiments reveal that the new surface pattern retards the slurry flow, causing the radial velocity of slurry to be uniform. Surface roughness Sa of CaF2 crystal polished by the designed pad is 0.251 nm, which is much better than that by the groove-diffused pad. Therefore, the slurry uniform flow is conducive to obtain a better surface quality.

Study on the surface quality of marble tiles polished with Sol-Gel derived pads

Sol–gel (SG) pads are used to polish different types of marble tiles and obtain high-gloss, low roughness, and scratch-free surfaces. The main features of such tools are semiconsolidated abrasives and flexible substrates. Through experiments with three types of polishing methods and two different types of tools used to polish marble tiles, the polishing mechanism of SG pads was confirmed. An abrasive yield phenomenon was observed for the semiconsolidated abrasive polishing tool with a flexible matrix, which resulted in a mirrored marble surface with high gloss, and the polishing steps were simplified. For SG polishing pads, a larger abrasive can be used to obtain high-gloss marble surfaces compared to the fine abrasives used in traditional resin polishing tools. Therefore, the agglomeration problem associated with fine particle size abrasives is avoided. In addition, after SG pad polishing, the surface of the marble tiles exhibited hydrophobic properties, and the oleophobic properties were simultaneously improved.

Model Friction Studies of Chemical Mechanical Planarization Using a Pin-on-Disk Tribometer

A pin-on-disk attachment for a commercial rheometer was used as a model system for chemical–mechanical planarization. The system enables real-time measurements of friction and the pin height changes as a function of velocity and load. We found the pin-on-disk tribology geometry was able to achieve a range of down pressures relevant to CMP processes (24.5 kPa at Fn = 1.5 N). It was possible to measure changes in the pin height, or gap, on the order of 1 µm during the test, which was useful to determine the amount of wear on the polishing pad and whether the system moves into hydrodynamic lubrication. The pad conditioning, or break-in process with water using a stainless steel disk with embedded diamond teeth, was conducted right before the tribology test, as in CMP processes, to give a consistent texture and remove surface impurities. Control studies of pure water at various pH’s showed the friction coefficient data and gap changes between the pad and the silicon wafer vary with the pH and ionic strength of the solution. CMP polishing was also studied with commercial slurries, pads and silicon wafers, showing that COF and pad wear can be measured reliably in the tribometer and compares well to measurements made on a 200 mm CMP commercial polisher for both water and the slurry. Overall, the pin-on-disk attachment was found to provide good insights of the tribology of a full CMP system.