Polishing Pad Research Articles

Experimental In-Situ Observatory on Brownian Motion Behavior of 105 nm Sized Silica Particles During Chemical Mechanical Polishing of 4H-SiC by an Evanescent Field

The experimentally observing optical systems for on-machine measurement have been developed to study on nano-polishing phenomena during the chemical mechanical polishing process, which is a wet process in semiconductor manufacturing. The developed optical system employs an evanescent field to selectively enhance exclusively the observatory of phenomena occurring on the surface being polished, offering a lateral resolving power of approximately 400 nm, in the slurry concentration of up to 5 wt% based on the numerical aperture of the objective lens. In addition, there is also the observability of 105 nm and down to 55 nm-sized silica particles without requiring additive fluorescence agents in or around the nano-particles, even when these particles are moving on surfaces such as silica glass or hard materials (silicon carbide: 4H-SiC). Consequently, the motion behavior of nano-particles disjoining with polishing pad asperity was explored and discussed, in this paper. Experimental results revealed that the polishing pad spatially constrains the movement of particles between the pad and the substrate surface, guiding them toward the surface being polished. During pad sliding, fluidically dragged nano-particles exhibit slower movement than the polishing pad sliding speed while retaining the Brownian motion. Furthermore, 105 nm-sized silica particles did not continuously approach to attach onto the SiC surface; the nano-particles approached in steps with reduced Brownian motion in all directions before attaching. This behavior can be attributed to the effects of van der Waals attraction and electrostatic repulsion forces between the particle and the substrate surfaces.

Additive manufactured cerium oxide foils, used as pads for polishing processes of brittle-hard materials

A new concept for polishing pads for flat and spherical surfaces is introduced which comprises additive manufactured polishing pads made of cerium oxide. By using additive manufacturing technologies, polishing processes with polishing slurry can be substituted with tools containing bonded grain. The bonded polishing pads can be fabricated using rolling processes. The pad geometries can be adjusted by using laser cutting. Furthermore, surface modifications of the pad can be applied with laser processes to favour quality and economic factors of the polishing procedure. First results from the experimental setup are showing, that lapped surfaces with a roughness Rq of ~ 500 nm can be improved to approx. Rq = 30 nm roughness by polishing with bonded grain cerium oxide foils. Further approaches for future investigations and applications are proposed.

Atomic surface achieved through a novel cross-scale model from macroscale to nanoscale.

Chemical mechanical polishing (CMP) is widely used to achieve an atomic surface globally, yet its cross-scale polishing mechanisms are elusive. Moreover, traditional CMP normally employs toxic and corrosive slurries, resulting in potential pollution to the environment. To overcome these challenges, a novel cross-scale model from the millimeter to nanometer scale is proposed, which was confirmed by a newly developed green CMP process. The developed CMP slurry consisted of hydrogen peroxide, sodium carbonate, sodium hydroxycellulose, and silica. Prior to CMP, fused silica was polished by a ceria slurry. After CMP, the surface roughness (Sa) was 0.126 nm, the material-removal rate was 88.3 nm min-1, and the thickness of the damaged layer was 8.8 nm. The proposed model was built by fibers, through integrating Eulerian and Lagrangian models and reactive force field-molecular dynamics. The results predicted by the model were in good agreement with those of CMP experimentally. A model for large-sized fibers revealed that a direct contact area of 11.12% was obtained for a non-woven polishing pad during the CMP experiments. Another model constructed via combining Eulerian and Lagrangian functions showed that the stress at the intersections of the fibers varied mainly from 0.1 to 0.01 MPa and was higher than the stress at other parts. An increase in viscosity led to a decrease in the areas with low stress, demonstrating that viscosity enhanced the stress and facilitated the removal of material. At the microscale and nanoscale, the stress of the abrasive surface exposed to the workpiece changed from 2.21 to 6.43 GPa. Stress at the interface contributed to the formation of bridging bonds, further promoting the removal of material. With increasing the compressive stress, the material-removal form was transformed from a single atom to molecular chains. The proposed model and developed green CMP offer new insights to understand the cross-scale polishing mechanism, as well as for designing and manufacturing novel polishing slurries, pads, and setups.

Changes in edge shape during silicon wafer polishing: Roll-off and roll-up formation

The edge shape of a silicon wafer is crucial for optimal device manufacturing. In polishing processes, it is necessary to form wafers into either a flat or roll-up shape, depending on the specific requirements of the process. However, the conventional process typically results in a roll-off shape. This study identifies the factors that influence a change in the edge shape during polishing. Polishing experiments were conducted to examine the effects of the initial edge shape and contact state between the wafer and polishing pad on the removal distribution. An approach for adjusting the resulting edge shapes was proposed.

Polishing performance of 3D-printed multi-material photopolymer tools on glass

Due to the increasing demands and complexity of optical glass components to be processed in the future, advanced technologies and precision machining methods will be required in order to produce high-quality optics economically. The polyjet process enables the production of 3D printed gradient index (GRIN) polishing tools with hardness gradients by combining different polymer materials for e.g. synchrospeed polishing. In terms of their geometric design, these tools are significantly more flexible than conventional ones, where the polishing cloth must be glued to metal tool holders. The composition of the polishing pad material directly influences the process efficiency and quality at which the glass is polished. In a laboratory test the influence of the mechanical properties of the multi-material pads on process efficiency, i.e. their long-term stability and the polishing rate of the glass workpiece is demonstrated.

Dispersion stabilization of carbonyl iron particles and its applications in chemical mechanical planarization

A novel aqueous magnetorheological (MR) slurry is proposed for potential application in the planarization of dielectric substrates, eliminating the need for traditional polishing pads. The preparation of a stable MR slurry hinges on the dispersion properties of abrasives and magnetic responsive particles, particularly high-density carbonyl iron particles (CIP). This study focuses on investigating the dispersion stabilities of CIP and its modified form, SiO2-coated CIP (SiO2@CIP), with the addition of ammonium polyacrylic acid (PAA-NH4) as a dispersant. Experimental measurements, including adsorption isotherms and zeta potential, combined with numerical simulations using density-functional theory, explore the adsorption behavior of PAA-NH4 on CIP and SiO2@CIP. Rheological and sedimentation analyses confirm PAA-NH4’s effectiveness in dispersing both particles. We elucidate the dispersion stabilization mechanism through calculations based on the Derjaguin−Landau−Verwey−Overbeek theory. In MR polishing, SiO2 coating on CIP improves efficiency, reducing surface roughness from over 8 nm (using CIP) to less than 8 Å (using SiO2@CIP). The addition of PAA-NH4 augments dispersion properties, leading to further enhancements in polishing efficiency, resulting in a surface roughness reduction to 2 Å and a simultaneous increase in material removal rate. We thoroughly examine the underlying factors driving these improvements.

ЭКСПЕРИМЕНТАЛЬНЫЕ ИССЛЕДОВАНИЯ КОМБИНИРОВАННОГО ПРОЦЕССА ЭЛЕКТРОХИМИКОМЕХАНИЧЕСКОГО ПОЛИРОВАНИЯ

This article presents the results of experimental studies of the electrochemical mechanical polishing process, which combines anodic dissolution, possessing high localizing properties, with the dynamic effects of abrasive grains. The essence of the combined process is described, when, under the influence of operating voltage, a continuous electrochemical dissolution of the treated surface occurs with the formation of oxide films on it, followed by their removal together with the treated metal by means of abrasive grains quasi-fixed in the polishing pad. The description and conditions of the conducted experiments are given. The influence of the weight content of the abrasive in the composition of the electrolyte-abrasive mixture on the removal of metal and the roughness of the treated surface is shown. The impact on the course of the combined process of the frequency of rotation of the polishing pad and the pressure of the workpiece on it is noted. At optimal values of the studied parameters, electrochemical-mechanical polishing makes it possible to ensure high geometric quality of the treated surface and high process productivity.

Tribological glazing evolution of fixed abrasive pad under different polishing solution conditions

PurposeThis study aims to clarify the influence mechanism of polishing solution type on the glazing evolution of fixed abrasive pad under different interfacial pressure conditions.Design/methodology/approachThe tribological experiments were carried out on the friction and wear machinery with W3-5 diamond fixed abrasive pad and quartz glass workpiece under three polishing solution types of five pressure conditions. The changes of surface morphology, porosity and hardness of fixed abrasive pad were detected by white light interferometer, optical microscope and shore hardness tester.FindingsThe results showed that the glazed phenomenon of fixed abrasive pad is occurred after a certain time, which is more obvious with the increasing of interfacial pressures. The polishing solution type has a significant effect on the glazing time, although the glazed phenomenon is inevitable. The mechanism of it is that the micro-convex peaks on the surface of the fixed abrasive pad are easily wear, and the pores are blocked by the accumulation of waste debris generated during the experiment process. Thus, a smooth and high-density hard layer is formed on the surface of the fixed abrasive pad which induces the decreasing of the friction coefficient and surface roughness value. For selected polishing solution types, the wear rate of micro-convex peaks is different due to the corrosion action difference with polishing pad surface.Originality/valueThe main contribution of this work is to provide a new investigating method for further understanding the glazing evolution mechanism of fixed abrasive pad.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2023-0257/

Metal Thickness Measurement System Based on a Double-Coil Eddy-Current Method With Characteristic Ratio Detection

Eddy-current sensor systems have been employed to determine the real-time thickness distribution of metal films and the thickness at endpoint detection in metal chemical mechanical polishing (CMP) processes, to achieve global planarization and avoid over-polishing or under-polishing. However, the large lift-off distance of 3.5 mm between the sensor coil and metal film gradually decreases with continuous wear of the polishing pad, which has a strong negative impact on the sensitivity and accuracy of the thickness measurement. In this study a double-coil eddy-current sensor system, with novel characteristic ratio detection capability, is proposed for metal film thickness measurement and compared with a traditional amplitude-based method. The linear relationship between the characteristic ratio and metal film thickness that is immune to lift-off distance is demonstrated both theoretically and experimentally. The experimental results show that the proposed method performs better than the amplitude-based method, reducing the effect of lift-off distance variation and obtaining an excellent linear measurement range from 24 to 2095 nm. The measurement accuracy is as high as 2.1 nm with an excitation frequency of 1.39 MHz and a large lift-off distance of 3.5 mm. The relative measurement error is less than 1.2% and the endpoint thickness ranges from 100 to 180 nm in the Cu-CMP process, when the lift-off distance varies from 3.6 to 3.3 mm.

Automated Vickers hardness system with closed-loop control for polishing pads for semiconductor production

Chemical mechanical polishing plays a pivotal role in enhancing the topography of wafers during semiconductor fabrication. The hardness of polishing pads holds great significance as it determines the material removal rate during the planarization process. Hence, precise determination of polishing pad hardness at both micro and macro scales is essential for optimizing the semiconductor polishing process, ensuring the prevention of wafer erosion or dishing. In this study, an automated system was developed for determining Vickers hardness of microporous polishing pads employing a closed-loop control method. The system was calibrated based on the relationship between load-unloading force and displacement observed in Vickers indentation experiments. The designed apparatus is not only successfully verified with standard values for Vickers reference blocks with less than 2% error, but it also demonstrated a smaller average relative error percentage than the commercial system using traditional open-loop measurement method. The closed-loop Vickers hardness apparatus proves to be a promising alternative for measuring hardness accurately, particularly in samples with noisy surfaces, rough surfaces, and porous microstructures.

A collocation grids method for determining pad conditioning coverage and density under long time conditioning process

In the Chemical Mechanical Planarization (CMP) process for semiconductor wafer fabrication, the polishing pads need to be continuously conditioned to ensure appropriate surface characteristics, which causes wear to the pads and subsequently affects the quality of CMP. Tuning the conditioning parameters is typically determined through expensive experimental trials. An effective pad conditioning simulation platform to model the conditioning process can significantly reduce the time and cost of the process development cycle. However, to mimic the real-life pad conditioning process, the simulation needs to track hundreds or even thousands of micro-sized diamond tips to check the contact points on the pads continuously during the conditioning process that spans many hours. This makes simulating pad conditioning over the entire pad for the duration of the process very challenging. In this work, we propose a low-cost numerical approach for simulating the pad conditioning process based on a collocation grid method and parametric mapping. Important numerical aspects such as time step size characterization, jump-over ratio, simulation efficiency, and error control are systematically analysed and discussed. Several benchmark examples are performed to evaluate the effectiveness of the proposed method.

Modeling of material removal in copper blanket wafer polishing based on the hard polishing pad microstructure

Modeling of material removal in copper blanket wafer polishing based on the hard polishing pad microstructure

Chemical mechanical polishing for potassium dihydrogen phosphate using four kinds of green developed slurries

Due to the soft-brittle, deliquescent, and temperature-sensitive nature, potassium dihydrogen phosphate (KDP) has become one of the most difficult-to-process materials. Nonetheless, high-performance devices strictly require the surface roughness of KDP less than 1 nm, which is a great challenge for the ultraprecision machining field. To overcome this challenge, four kinds of green slurries were developed. For instance, ceria and graphene oxide were selected as abrasives, sodium carboxymethylcellulose was used as thickener, fatty alcohol-polyoxyethylene ether was employed as emulgator, sodium carbonate was proposed as a pH regulator, and tiny water was applied to dissolving agent. Four kinds of various base solutions, such as corn oil, glycol, polyethylene glycol and glycerol, form four varieties of green slurries for the chemical mechanical polishing (CMP) of KDP. After CMP, the surface roughness Sa is 0.562 nm with a scanning area of 50 × 50 μm2, using the glycol slurry. Until now, to the best of our knowledge, this is the lowest surface roughness at a such large measurement area. X-ray photoelectron spectroscopy and infrared Fourier transformation confirm that CP bonds were generated between graphene oxide and KDP, resulting in the breaking away of H2PO4− from KDP. A soft layer was produced by the tiny water. CO32− reacts with H2PO4−, forming CO2 gas and protecting the concave area on the surface of KDP. The soft layer was removed by the synergistic effect among abrasives, thickener and a polishing pad. The developed green slurries suggest new insights to achieve sub-nanometer surfaces on KDP with uniquely physical nature, for the potential use in laser systems.

Characterization of Pad–Wafer Contact Area and Distance in Chemical-Mechanical Polishing

Chemical-mechanical polishing (CMP) has been an important process step in microelectronic device manufacturing for more than three decades. Physical CMP modeling utilizes the interactions between the polishing pad and the wafer. This requires data on important physical properties such as the real contact area and the effective pad-to-wafer distance under near-process conditions, as these are directly related to the removal rate, defects, etc. In the present study, a FTIR spectrometer and a custom ATR equipment with micro-structured single reflection elements (mSRE) were used to quantify the volume fraction and contact area of a IC1000 polishing pad in dry and wet state under different static loads. No stable contact over time was observed even at very high pressures of up to 350 kPa. Viscoelastic behavior of air-dried and water-soaked pad was proven on a microscale by using load response, creep-recovery and stress relaxation tests. This leads to a memory effect of the pad that should be taken into account during dynamic loading as the wafer-pad contact has a direct impact on the material removal. The presented method can therefore provide valuable information about dynamic effects of the pad behavior in near-process environment and thus help to improve the CMP modelling.

A statistical characterization model of the microscale contact state of polishing pads

Chemical mechanical polishing (CMP) greatly benefits from the wafer and pad’s microscale contact condition. With so many varying pressures and wafer layouts, it is incredibly difficult to measure all the microscale contact states. Therefore, there is an urgent need to realistically characterize the morphology and contact state of polishing pads. A statistical characterization model of the microscale contact state of polishing pads was developed in this investigation. The microscale contact states under different pressures were measured to make sure that the model was accurate. This proposed model can extract the pad morphology parameters from the microscale contact state to predict the pad surface morphology and, thus, the microscale contact state in arbitrary conditions. This study can provide some recommendations for stability in CMP and the production and use of pads.

Effect of elastohydrodynamic characteristics on surface roughness in cylindrical shear thickening polishing process

The shear thickening fluid exhibits a non-Newtonian characteristic, i.e. the viscosity follows a power law with the increasing shear rate. The shear thickening polishing technology has been proposed and developed to realize the precision finishing, utilizing the slurry with free abrasives prepared by the shear thickening fluid. In this paper, the formation mechanism and evolution of surface roughness in the cylindrical shear thickening polishing process is investigated in the elastohydrodynamics theoretically and experimentally. The elastohydrodynamic properties and the surface roughness are modeled by a novel solution method, based on the integration of polishing pad’s elastic deformation and slurry’s hydrodynamics. The Taguchi orthogonal tests of polishing the cylindrical AISI 52100 steel with the foam polyurethane pad were conducted, and the experimental surface roughness Ra of 9.689 nm was achieved under conditions of loading force 10 N, α-Al2O3 powder with grain size 1 μm and concentration 20 wt%, starch powder with concentration 25 wt%. According to the comprehensive analysis of variance and level average response, it is revealed that the slurry’s viscosity has a great influence on the surface roughness. The effecting mechanisms of processing factors on the surface roughness are interpreted from the perspective of effects transition in aspects of viscosity, pressure distribution and single grain’s cutting depth, interestingly exhibiting significant differences from that of traditional polishing methods.

Study on Development of Composite PAM/PI Gel Abrasive Tools and Finishing Performance on the Sapphire Substrates

A sol-gel method was proposed to prepare the abrasive tool to solve the problem of abrasive agglomeration in the traditional hot-pressing abrasive tool. In this paper, the effect of AM acrylamide (AM) monomer concentration on the polyacrylamide (PAM) gel performance and the effect of abrasive mass fraction on viscosity were investigated. Experiments about the effect of polyimide (PI) resin mass fraction on the mechanical properties, surface morphologies and friction wear performance of the abrasive tool were conducted. The polishing experiments were conducted to compare the machining effect of the gel abrasive tool, hot-pressing abrasive tool and the polyurethane polishing pad. Results show that the PAM gel has the best uniformity when the concentration of AM monomer reaches 7 wt%, when the mass fraction of PI resin in the abrasive tool reaches 12 wt%, the mechanical properties is the best. When the mass fraction of PI resin is 8 wt%, the gel abrasive tool has the most stable friction coefficient. The average surface roughness reaches 2.706 nm, and the flatness reaches 0.631 μm when the sapphire was processed by the gel abrasive tool. After Chemical mechanical polishing (CMP), a scratch-free surface was obtained and the average surface roughness reached 0.724 nm.

Effects of grinding-induced surface topography on the material removal mechanism of silicon chemical mechanical polishing

Ultra-precision thinning technology using workpiece self-rotational grinding followed by chemical mechanical polishing (CMP) is extensively applied in the integrated circuit manufacturing process, which enables to obtain large size ultra-thin silicon wafers with high material removal rate and low damage layer thickness. However, an in-depth understanding of the influence caused by ground surface topography on material removal mechanism in silicon CMP process has not been revealed yet. This work systematically investigates the contact characteristics of the wafer-pad interface and corrosion behaviors of the ground silicon wafer immersed in polishing solution. Firstly, some silicon wafers are ground using different wheels and subsequently polished. The material removal depth during the whole CMP process is measured. Then, the intrinsic model of polishing pad is determined via mechanical property tests. A finite element method is adopted to evaluate the contact status, displacement distortion and stress distribution at the contact region between pad and ground wafer surface. Moreover, the chemical reaction mechanism between ground silicon wafer and polishing solution is revealed by utilizing X-ray photoelectron spectroscopy and electrochemical analysis. The influence of wafer surface topography on corrosion resistance in CMP process is illustrated. Finally, corresponding experimental results are explained from an atomic scale by a ReaxFF reactive molecular dynamics simulation model. This presented study demonstrates the corrosion promotion principle of ground silicon wafer in CMP process.

An investigation of the Vickers hardness of polishing pads using simulations based on microtomography model

Porous pads used during the chemical mechanical polishing are critical in IC fabrication for achieving consistent throughput. The hardness of thermoplastic polyurethane pads is identified as a crucial parameter in calibrating the amount of material removed during the polishing process. Through micro-CT scanning technology, a three-dimensional micro-topography model of polyurethane IC1000 pad was developed. Nanoindentation measurements were conducted to evaluate the physical properties of IC1000 pad in the manufacture process. In order to investigate the Vickers hardness of the polyurethane IC1000 pad, which widely use in the fabrication of semiconductor manufacturing, the finite element method was employed to simulate the indentation between Vickers indenter diamond tip and the model of IC1000 pad. This study is not only successfully calibrating the Vickers hardness of polyurethane IC1000 pad with a 3 percent error compared to experimental verification, but also developed a novel methodology for investigating the micro-topography structure of polymer foam materials.

Chemically-induced active micro-nano bubbles assisting chemical mechanical polishing: Modeling and experiments

The material loss caused by bubble collapse during the micro-nano bubbles auxiliary chemical mechanical polishing (CMP) process cannot be ignored. In this study, the material removal mechanism of cavitation in the polishing process was investigated in detail. Based on the mixed lubrication or thin film lubrication, bubble-wafer plastic deformation, spherical indentation theory, Johnson-Cook (J-C) constitutive model, and the assumption of periodic distribution of pad asperities, a new model suitable for micro-nano bubble auxiliary material removal in CMP was developed. The model integrates many parameters, including the reactant concentration, wafer hardness, polishing pad roughness, strain hardening, strain rate, micro-jet radius, and bubble radius. The model reflects the influence of active bubbles on material removal. A new and simple chemical reaction method was used to form a controllable number of micro-nano bubbles during the polishing process to assist in polishing silicon oxide wafers. The experimental results show that micro-nano bubbles can greatly increase the material removal rate (MRR) by about 400% and result in a lower surface roughness of 0.17 nm. The experimental results are consistent with the established model. In the process of verifying the model, a better understanding of the material removal mechanism involved in micro-nano bubbles in CMP was obtained.