Conventional Chemical Mechanical Polishing Research Articles

Electrolyte Effect on Photoetching of Gallium Nitride

The limited material removal rate of conventional chemical mechanical polishing (CMP) significantly hinders the fabrication efficiency and surface quality, thereby preventing the development of gallium nitride (GaN)-based devices. Moreover, the incorporation of photoelectrochemistry in CMP has garnered increasing attention because of its potential to enhance the quality and efficiency of the GaN process. However, a considerable gap still exists in the comprehensive understanding of the specific photoelectrochemical (PEC) behavior of GaN. Here, we report the influence of the electrolyte on the PEC etching of GaN. Various acids and bases were tested, with their pH being carefully adjusted. The concentrations of the cations and anions were also examined. The results showed that photocorrosion/photoetching was more pronounced in sulfuric acid, phosphoric acid, and nitric acid environments than in alkaline environments, but it was less pronounced in hydrochloric acid. Furthermore, the effects of pH and anion concentration on photoetching were investigated, and the results revealed that photoetching in acidic environments weakened with increasing pH levels and diminished with increasing sulfate concentration. The underlying reasons contributing to this observation were explored. These findings provide ideas for improving the photoetching efficiency of GaN, thereby enriching the photoelectrochemical mechanical polishing (PECMP) technology of GaN.

Novel green chemical mechanical polishing for an aluminum alloy and mechanisms interpreted by molecular dynamics simulations and measurements

Aluminum (Al) alloy is a kind of soft-plastic metal, and traditional chemical mechanical polishing (CMP) is easy to produce scratches on the polished surface and embed of abrasives, resulting in the degradation of devices. In addition, conventional CMP generally employs toxic and corrosive slurries, leading to the pollution of environment. To address these challenges, a novel green CMP was developed for Al alloy, consisting of ceria, silica, sodium carbonate, hydrogen peroxide, glycine and deionized water. After CMP, surface roughness Sa is 0.258 nm under an area of 100 × 100 μm2, which is the lowest surface roughness reported previously in such a big measurement area for Al alloy. Transmission electron microscopy confirms that the thickness of damaged layer is merely 4.6 nm. X-ray photoelectron microscopy and Fourier transform infrared spectroscopy reveal that composite abrasives are conducive for the generation of Al-O, Al-O-Si and Ce-Al bonds on the polished surface. Molecular dynamics simulations elucidate that the subsurface damage is mainly caused by exerted loading of abrasives on amorphous layer, as well as vacancy induced through the annihilation of dislocations profoundly. These outcomes provide a new avenue to manufacture atomic surfaces of soft-plastic metals for latent applications in devices.

Polymer Nanoparticles Applied in the CMP (Chemical Mechanical Polishing) Process of Chip Wafers for Defect Improvement and Polishing Removal Rate Response.

Chemical mechanical planarization (CMP) is a wafer-surface-polishing planarization technique based on a wet procedure that combines chemical and mechanical forces to fully flatten materials for semiconductors to be mounted on the wafer surface. The achievement of devices of a small nano-size with few defects and good wafer yields is essential in enabling IC chip manufacturers to enhance their profits and become more competitive. The CMP process is applied to produce many IC generations of nanometer node, or those of even narrower line widths, for a better performance and manufacturing feasibility. Slurry is a necessary supply for CMP. The most critical component in slurry is an abrasive particle which affects the removal rates, uniformity, defects, and removal selectivity for the materials on the wafer surface. The polishing abrasive is the source of mechanical force. Conventional CMP abrasives consist of colloidal silica, fume silica or other inorganic polishing particles in the slurries. We were the first to systematically study nanoparticles of the polymer type applied in CMP, and to compare traditional inorganic and polymer nanoparticles in terms of polishing performance. In particular, the polymer nanoparticle size, shape, solid content dosing ratio, and molecular types were examined. The polishing performance was measured for the polishing removal rates, total defect counts, and uniformity. We found that the polymer nanoparticles significantly improved the total defect counts and uniformity, although the removal rates were lower than the rates obtained using inorganic nanoparticles. However, the lower removal rates of the polymer nanoparticles are acceptable due to the thinner film materials used for smaller IC device nodes, which may be below 10 nm. We also found that the physical properties of polymer nanoparticles, in terms of their size, shape, and different types of copolymer molecules, cause differences in the polishing performance. Meanwhile, we used statistical analysis software to analyze the data on the polishing removal rates and defect counts. This method helps to determine the most suitable polymer nanoparticle for use as a slurry abrasive, and improves the reliability trends for defect counts.

Influences of Nonaqueous Slurry Components on Polishing 4H-SiC Substrate with a Fixed Abrasive Pad

4H-SiC wafers are more likely to sustain a lower material removal rate (MRR) and severe subsurface damage in conventional chemical mechanical polishing (CMP) methods. To overcome the material removal bottleneck imposed by aqueous chemistry, a high-efficiency polishing of 4H-SiC wafers method by applying reactive nonaqueous fluids to self-sharpening fixed abrasive pads has been proposed in our former research works. Furthermore, to improve the material removal rate and reduce the surface roughness Sa value of 4H-SiC substrates of the Si face, the effect of organic acid, H2O2, and Triton X-100 in nonaqueous slurry on 4H-SiC polishing was investigated. The MRR of 12.83 μm/h and the Sa of 1.45 nm can be obtained by the orthogonally optimized slurry consisting of 3 wt% H2O2, 0.5 wt% Triton X-100 at pH = 3. It is also found that the addition of different levels of oxidant H2O2 and surfactant Triton X-100 components not only increased the MRR of the 4H-SiC substrates of the Si face but also achieved a lower Sa value; in that, the polishing efficiency of the Si side of the 4H-SiC wafers and the surface quality of the 4H-SiC wafers could be effectively improved by the optimization of the polishing slurry.

Highly efficient and atomic scale polishing of GaN via plasma-based atom-selective etching

Plasma-based atom-selective etching (PASE) of GaN is conducted to realize the highly efficient planarization of the GaN surface. Inductively coupled plasma with high temperature and high concentration of radicals is used as the source of PASE. The non-toxic carbon tetrafluoride is chosen over chlorine as the reaction gas, and the volatility of the etching products will be improved at high temperatures in the PASE of GaN. After 2 min of PASE, the GaN surface roughness is reduced from Sa 135.8 nm to Sa 0.527 nm. The material removal rate of PASE of GaN is measured to be 93.01 μm/min, thousands of times higher than that of the conventional chemical mechanical polishing method. The crystal structure of the GaN subsurface is well-ordered without any damage or defects. PASE is thus proven to be a non-destructive etching method. In this study, the effects of radio frequency power and reaction gas flow rate on PASE are also investigated. Surface temperature and concentration of radicals are found to be the critical factors in the PASE of GaN.

Recent Advances In Silicon Carbide Chemical Mechanical Polishing Technologies.

Chemical mechanical polishing (CMP) is a well-known technology that can produce surfaces with outstanding global planarization without subsurface damage. A good CMP process for Silicon Carbide (SiC) requires a balanced interaction between SiC surface oxidation and the oxide layer removal. The oxidants in the CMP slurry control the surface oxidation efficiency, while the polishing mechanical force comes from the abrasive particles in the CMP slurry and the pad asperity, which is attributed to the unique pad structure and diamond conditioning. To date, to obtain a high-quality as-CMP SiC wafer, the material removal rate (MRR) of SiC is only a few micrometers per hour, which leads to significantly high operation costs. In comparison, conventional Si CMP has the MRR of a few micrometers per minute. To increase the MRR, improving the oxidation efficiency of SiC is essential. The higher oxidation efficiency enables the higher mechanical forces, leading to a higher MRR with better surface quality. However, the disparity on the Si-face and C-face surfaces of 4H- or 6H-SiC wafers greatly increases the CMP design complexity. On the other hand, integrating hybrid energies into the CMP system has proven to be an effective approach to enhance oxidation efficiency. In this review paper, the SiC wafering steps and their purposes are discussed. A comparison among the three configurations of SiC CMP currently used in the industry is made. Moreover, recent advances in CMP and hybrid CMP technologies, such as Tribo-CMP, electro-CMP (ECMP), Fenton-ECMP, ultrasonic-ECMP, photocatalytic CMP (PCMP), sulfate-PCMP, gas-PCMP and Fenton-PCMP are reviewed, with emphasis on their oxidation behaviors and polishing performance. Finally, we raise the importance of post-CMP cleaning and make a summary of the various SiC CMP technologies discussed in this work.

Revealing the mechanisms for inactive rolling and wear behaviour on chemical mechanical planarization

The paper examines the influences of abrasive conditions, silica thickness, and motion parameters on the polishing process of a silicon substrate covered by a thin silica film. The silica film minimizes the direct collision of the abrasive on the silicon substrate. During the polishing process, the adhesion phenomenon plays the main role in atomic removal. Notably, the abrasive sliding motion can generate an inactive rolling motion. The inactive rolling motion can be used to explain the stick-slip motion by showing the system of periodic high-strain marks. In rolling, the deformation behaviors of the workpiece rely on the correspondence between the rolling and the moving speeds. In the fixed diamond abrasive (FDA) cases, the atomic sticking rate is lower but the workpiece deformation is higher than the loose diamond abrasive (LDA) cases. Besides, the FDA cannot create the inactive rolling motion and the stick-slip motion as the LDA. In general, the paper points out the existence of the inactive rolling motion and its role in the atomic removal process through the stick-slip motion phenomenon at the nanoscale, the advantage of this type of motion in a conventional chemical mechanical polishing (CMP) process, and the padding role of the silica layer.

Novel SiC wafer manufacturing process employing three-step slurryless electrochemical mechanical polishing

In this work, a three-step silicon carbide (SiC) wafer manufacturing process using slurryless electrochemical mechanical polishing (ECMP) is proposed. In the first step, ECMP using fixed hard abrasives is applied to an as-sliced SiC wafer to rapidly remove the subsurface damage (SSD) and waviness induced during slicing. Then, the SiC wafer is polished using the second-step ECMP by employing a ceria grinding stone to remove the residual SSD and decrease the surface roughness. Finally, a finishing ECMP step employing a low potential is used to further decrease the surface roughness. Before applying this proposed three-step process to an as-sliced SiC wafer, a suitable grinding stone was selected for the first-step ECMP by evaluating the uniformity of the induced SSD layer and the obtained surface roughness. By applying the first step, i.e., ECMP using fixed-diamond abrasives, the root-mean-square ( S q) surface roughness decreased from 163.33 to 25.45 nm within 20 min with a material removal rate (MRR) of 62 μm/h. During the 30-min second step, the S q and the maximum height ( S z) surface roughness values further decreased to 0.82 and 6.96 nm, respectively, with an MRR of 11 μm/h. Finally, the 60-min finishing step allowed the S q and S z surface roughness values to further decrease to 0.11 and 1.46 nm, respectively; furthermore, a surface with a step-terrace structure was obtained, which is comparable to the surface obtained by conventional chemical mechanical polishing. Overall, the proposed three-step slurryless ECMP method allowed for the rapid transformation of an unprocessed SiC wafer to an atomically smooth surface and is thus expected to reduce the cost and manpower required during SiC wafer manufacturing. • Novel SiC wafer manufacturing process employing three-step slurryless ECMP is proposed. • Three-step slurryless ECMP process requires less than 2 h to obtain an atomically smooth surface from an as-sliced SiC wafer. • Three-step slurryless ECMP process uses only one machine tool and one chuck to process an as-sliced SiC wafer. • Current SiC wafer manufacturing process can be significantly simplified by the three-step slurryless ECMP process.

Electrolytically Ionized Abrasive-Free CMP (EAF-CMP) for Copper

Chemical–mechanical polishing (CMP) is a planarization process that utilizes chemical reactions and mechanical material removal using abrasive particles. With the increasing integration of semiconductor devices, the CMP process is gaining increasing importance in semiconductor manufacturing. Abrasive-free CMP (AF-CMP) uses chemical solutions that do not contain abrasive particles to reduce scratches and improve planarization capabilities. However, because AF-CMP does not use abrasive particles for mechanical material removal, the material removal rate (MRR) is lower than that of conventional CMP methods. In this study, we attempted to improve the material removal efficiency of AF-CMP using electrolytic ionization of a chemical solution (electrolytically ionized abrasive-free CMP; EAF-CMP). EAF-CMP had a higher MRR than AF-CMP, possibly due to the high chemical reactivity and mechanical material removal of the former. In EAF-CMP, the addition of hydrogen peroxide (H2O2) and citric acid increased the MRR, while the addition of benzotriazole (BTA) lowered this rate. The results highlight the need for studies on diverse chemical solutions and material removal mechanisms in the future.

Polishing tool with phyllotactic distributed through-holes for photochemically combined mechanical polishing of N-type gallium nitride wafers

In this work, we further developed the photochemically combined mechanical polishing (PCMP) method for finishing N-type gallium nitride (GaN) wafers. A core improvement is to design a novel polishing tool with phyllotactic distributed through-holes, through which the wafer surface underneath through-holes can receive ultraviolet (UV)-light for the photochemical oxidation, while the rest parts undergo mechanical polishing. During PCMP, the co-rotation of the wafer and polishing tool allows the wafer surface to undergo the uniform and high-frequency conversion of oxidation and polishing. Based on the designed PCMP system and apparatus, the fundamental issues arising from such an alternate processing mode, which is different from the parallel mode of conventional chemical mechanical polishing (CMP), were investigated. Results show that the technical features of PCMP depend on the nature of the photochemical oxidation of wafers themselves if the mechanical polishing procedure can sufficiently remove oxides in time. The material removal rate (MRR) is inversely proportional to the dislocation density of wafers. Under acidic conditions, the oxidation proceeds by the GaN monocrystal step orientation, allowing PCMP to clear surface/subsurface damages (SSDs) and to prepare step-terrace structures on the wafer surface. When the polishing solution (pH = 1.5) includes 0.1 M K2S2O8 oxidants and 10 wt% SiO2 abrasives, the surface roughness Sa attains 0.21 nm in 10 × 10 μm2, and the MRR reaches 275.3 nm/h. The present study shows that the phyllotactic distributed through-hole array structure designed for polishing tools offers rich possibilities for the innovation of polishing technologies combining with various oxidation approaches.

Fabrication of silicon-on-insulator with high uniform top Si for silicon photonics applications

Silicon-on-insulator (SOI) is the most suitable platform for silicon photonics applications owing to its unique structure and excellent compatibility. To satisfy the requirement of uniformity, HCl etching process is developed to smooth the surface of top silicon layer in this paper. The top silicon mean thickness is thinned down to 220 nm; the corresponding root mean square roughness is as low as 0.165 nm after the etching process, which is comparable to chemical mechanical polishing (CMP) process. Furthermore, HCl etching process shows great repeatability of thickness range and removal amount. The within wafer thickness range can be controlled below 30 Å and the removal amount can be controlled within ±4.5 Å, which is far better than conventional CMP process.

Development of an Ultra-Precision Annular Polishing Machine Tool with Full Gas Static Pressure

In order to improve chemical mechanical polishing (CMP) efficiency and accuracy in the fabrication of planar optics, CMP models and machine tools have been developed. A three-dimensional contour map of the surface of the polishing plate was established by measuring the runout error of several circles on the polishing plate. Based on the Preston equation and the three-dimensional contour map, a CMP model that simulates material removal at any point on the work piece is proposed. This model shows that higher motion accuracy can improve efficiency and accuracy. Then, based on this point of view, a new CMP machine tool was designed, and the ultra-precision gas static pressure guide rail and turntable and Siemens 840Dsl numerical control system were applied to the new CMP machine tool. In order to validate the new machine, a series of testing and processing experiments were carried out. The straightness error of the gas static pressure guide rail can be less than 1.1 μm. The axial runout error of the gas turntable can be less than ±0.4 μm. The surface profile of the experimental workpiece can be less than 0.01λ, and the processing efficiency of the new CMP machine can reach 4 times of the processing efficiency of the conventional CMP machine. In addition, the repeatability and stability of the CMP process is improved on new machines.

Development of super-smooth flat silicon mirror substrates using bowl-feed chemical-mechanical polishing

Silicon is one of the important optical substrate materials used for making high reflectivity, high damage threshold mirrors for high power/energy IR lasers and grazing incidence synchrotron X-ray mirrors. These applications require ultra-low roughness or super-smooth substrates that ensure extremely low scattering losses. The present work is an attempt to fabricate super-smooth, flat silicon substrates using the conventional Chemical-Mechanical Polishing (CMP) method with bowl-feed technique. The roughness parameters of the ultra-smooth flat silicon surfaces fabricated using bowl-feed CMP have been measured in Atomic Force Microscope (AFM) and by hard X-Ray Reflectivity (XRR) and are found to be of the order of 5 Å. The surface morphologies of the polished silicon surfaces were also investigated using a Scanning Electron Microscope (SEM). We briefly discuss the bowl-feed CMP process of producing super-smooth, flat silicon substrates and presented the results of surface roughness measurements using various techniques.

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.

Polishing single-crystal silicon carbide with porous structure diamond and graphene-TiO2 slurries

In this study, a self-modified diamond (SMD) slurry for mechanical polishing is presented as well as a novel TiO2-graphene slurry used with the aid of UV light for a chemical mechanical polishing (CMP) technique that improves the rate of polishing. The surface characteristics and removal rate of silicon carbide (SiC) samples polished with the slurry were compared with the results obtained by polishing with both a conventional diamond slurry and CMP slurry. The experimental results indicated that the material removal rate (MRR) of SiC from the wafer surface with the SMD slurry was higher, and surface finish and lapping efficiency was better than that achievable with other slurries. The experimental data also showed that a TiO2-graphene slurry had a higher removal rate compared to the TiO2 slurry using the same intensity of UV light. The TiO2-graphene slurry showed a 3-fold increase in removal rate over that of a traditional CMP slurry. An experiment was also carried out where nanodiamond (50 nm and 500 nm) abrasive was added to the graphene-TiO2. As expected, the removal rate increased but the average surface roughness was poor because the nanodiamond particles caused small scratches. The graphene-TiO2 with nanodiamond abrasives has 5 times the removal rate achieved with a traditional CMP slurry. This novel method can facilitate the extensive production of well-polished single-crystal SiC wafers.

Fujimi’s New SiC CMP Slurry Development

Introduction Since silicon carbide (SiC) is a very hard substrate and is stable both chemically and physically, wafer processing is costly and takes a lot of time. Particularly, in the chemical mechanical polishing (CMP) process, which is the final wafering process, significant improvement in performance such as surface quality and productivity is required for high volume commercial production. At Fujimi, we have developed a high removal rate slurry of over 1.5 μm per hour on the Si-face under neutral pH and conventional CMP processing conditions. We have also proven that the slurry had a nice behavior in removal rate performance based on Prenston equation, that is, removal rate was proportional to the pressure and velocity of substrate. Furthermore, our testing result has shown great surface morphology such as no scratches and no latent scratches beneath the surface. Low surface roughness was achieved under 0.1nm of RMS at 10x10μm area. Fujimi has introduced this new SiC slurry at the Advanced Industrial Science and Technology (AIST) as a suitable stock polishing slurry to remove surface damage coming from previous grinding or lapping steps. We have demonstrated the benefit of a two-step CMP process which applies a stock step using the higher removal rate slurry followed by a fine step using our Colloidal silica type slurry. As a result, the combination process could realize to produce the SiC epi-ready wafer with fine quality surface in shorter process time than conventional process [1]. Results and DiscussionIn this study, we will discuss our SiC slurry development technologies. At Fujimi, we can control the size and shape of the abrasive grains. Utilizing our expertise in chemical and additive formulation, we can modify our slurries to meet our customers’ various performance criteria. In Figure 1, it shows a test result example in which the removal rate of the Si-Face was improved. These improved performances depended on the type of additive used in the slurry. Test was performed using substrates of 4-inch 4H-N SiC (4°off) Si-face substrates with exactly the same polishing process and pad condition for all slurries. Slurry A was a result of high speed conventional slurry used by AIST [1] as a reference slurry. Slurry B was a newly developed slurry with improved abrasive grains dispersion. Slurry C was also a newly developed slurry focusing on the improvement of oxidation efficiency. As a result, it was possible to achieve a polishing removal rate over 2 um / hour as well as maintain low surface roughness at the same time. [1] S. Hirano et al., Journal of the Japan Society for Abrasive Technology, 60, (2016)353 Figure 1

Photo-Assisted Chemical Mechanical Polishing of Si Wafer Using Phosphorescent Particles as a Luminescent Agent

Chemical mechanical polishing (CMP) is used for fabricating semiconductor wafer surfaces because it produces smooth and damage-free surfaces. Polishing efficiency is a key factor to decrease fabrication costs. Photo-assisted CMP—a promising method to improve polishing efficiency—requires a special setup allowing the material surface to be directly illuminated by the excitation light, which increases initial setup costs. Thus, a photo-assisted method that improves polishing efficiency without a special setup is strongly desired. We report a photo-assisted CMP method that employs phosphorescent particles (PhPs). X-ray analysis demonstrates that the Si surface is oxidized by photo-chemical treatment using PhPs. Further, photo-assisted CMP using PhPs added into a polishing slurry improves the material removal rate for Si under UV irradiation on the polishing pad; however, large and blocky PhPs generate defects on the polished surface. A double-layered polishing pad (abrasive layer partly stacked on the phosphorescent layer) is proposed to obtain smooth and defect-free surfaces. Compared with conventional CMP, photo-assisted CMP using the double-layered pad achieves higher polishing efficiency and smoother (0.19 nm Ra) surfaces without generating defects under optimal polishing conditions; thus, this method has potential applications to achieve semiconductor wafer fabrication with a simple polishing setup.

Photochemically combined mechanical polishing of N-type gallium nitride wafer in high efficiency

The application of conventional chemical mechanical polishing (CMP) to super inert N-type gallium nitride (GaN) wafer suffers from low material removal rate (MRR). In this work, we described and tested a photochemically combined mechanical polishing (PCMP) strategy to promote the MRR following a photo-assisted (PA) oxidation mechanism. The utilization of ultraviolet (UV)-light to irradiate GaN generates electron-hole pairs, while the use of proper oxidants to extract the conduction band (CB) electrons leads the valence band (VB) holes to be able to oxidize GaN. By means of the PCMP prototype we designed, the features of proper oxidants were investigated and the key issues on MRR and surface roughness (Ra) were clarified. Results show that PCMP generates much higher MRR than CMP, but requires certain special oxidants because common redox oxidants, such as H2O2, are preferentially oxidized by the VB holes. When polishing solution (pH = 1.5–13.5) includes 0.1 M K2S2O8 oxidants and 2 wt% SiO2 abrasives, the MRR reaches 180–254.7 nm/h and the lowest Ra attains 0.76 nm (5 × 5 μm2). Although the photo-corrosion of GaN wafer itself may cause the surface roughening, the enhancement of the mechanical polishing process to MRR can decrease Ra, revealing a key factor in the further optimization of the PCMP system and equipment. In conclusion, the present study confirms that PCMP is a promising approach in polishing GaN wafer in high efficiency.

Study on high efficient sapphire wafer processing by coupling SG-mechanical polishing and GLA-CMP

Abstract A novel surface finish process sequence has been developed with a sequential process employing sol-gel (SG) semi-fixed abrasive polishing tool and gas-liquid assisted chemical mechanical polishing (GLA-CMP) instead of general copper-resin lap or plate (CP) and chemical mechanical polishing (CMP) to enhance the surface finishing efficiency for sapphire wafer. The surface and subsurface quality of sapphire wafer after the SG and GLA-CMP process have been compared with those by CP and CMP, respectively. Results show that the SG polishing can offer a better surface quality in terms of both surface roughness and sub-surface damage for the next fine polishing step. Based on the contact mechanics models, yielding effects of the SG polishing pad are found to be effective to scratch mitigation. This trend is consistent with experimental results. Furthermore, oxygen is applied as catalyst to improve the removal material rate (MRR) of sapphire by GLA-CMP. It is found that oxygen concentration can improve MRR of sapphire polishing. Among four combinations of CP, SG, CMP, and GLA-CMP, the machining by coupling SG and GLA-CMP shows the maximum MRR approximately as high as 77.6 nm/min, more than 10.23% larger than that of CP-machined wafer and traditional CMP. Ultra-smooth wafer surface with damage-free can be achieved by this new process sequence and the sapphire wafer with qualified surface can be obtained within 50 min by GLA-CMP, which is 90 min shorter than the original 140 min polished by the conventional CMP method. Compared to the conventional surface finish process sequence, the proposed sequence shows superior processing efficiency and quality for sapphire wafers.

Material removal model of ultrasonic elliptical vibration-assisted chemical mechanical polishing for hard and brittle materials

The chemical mechanical polishing (CMP) is widely used to polish hard and brittle materials. However, it is difficult for conventional CMP to achieve high material removal rate (MRR) and high surface quality while polishing of hard and brittle materials such as monocrystalline silicon. Therefore, ultrasonic elliptical vibration (UEV) is employed to aid conventional CMP in our research, which combines the functions of conventional CMP and ultrasonic machining. In the ultrasonic elliptical vibration-aided chemical mechanical polishing (UEV-CMP) experimental setup developed by us, the workpiece attached on the ultrasonic vibrator can vibrate simultaneously in both horizontal and vertical directions during CMP. It is found that the ultrasonic elliptical vibration can effectively increase the MRR while maintaining surface quality in conventional CMP. The possible mechanism in UEV-CMP is firstly analyzed chemically to establish a reasonable material removal rate model. The effects of the ultrasonic elliptical vibration on the interaction among the abrasive particles, polishing pad, and workpiece are investigated to explain why the MRR of UEV-CMP is higher than that of the conventional CMP. A mathematic model, which includes polishing variables such as morphology and material properties of polishing pad; abrasive size; and material properties of particles, frequency, and amplitude of the ultrasonic vibration as well as polishing process parameters, is set up to interpret the increase in MRR for UEV-CMP. The results from the MRR model show that the ultrasonic elliptical vibration can improve material removal by increasing both the chemical reaction efficiency of polishing solution and mechanical impact efficiency of the abrasive particles on the workpiece surface and also by increasing vibration amplitude in vertical direction as the horizontal vibration contributes less towards the increment of MRR. Experiments are conducted for model verification, which should be that the experimental results agree well with model predictions.