Polishing Pressure Research Articles

Investigation on Material Removal Uniformity in Electrochemical Mechanical Polishing by Polishing Pad with Holes

Owing to the low polishing pressure and high removal rate, electrochemical mechanical polishing (ECMP) is a good candidate to process thin copper substrates. However, ECMP pads require punching holes to ensure electrochemical effects on substrate surface during polishing, and difference in the arrangement of holes on polishing pad leads to uneven distribution of electrochemical action on substrate, which results in different material removal rates. This paper presents a new model to investigate non-uniformity of material removal in ECMP by using track point density distribution. The coefficient of variation of track point density and density distribution of two polishing pads at different speed ratios were simulated to represent the uniformity of electrochemical action across substrate, and experiments were designed to verify the results of simulation. The simulation and experimental results show that substrate after polished by the concentric circular arrangement pad has an annular corrugation and its flatness is deteriorated. The phyllotactic arrangement pad has a more uniform polishing effect, and the flatness error of substrate which has a diameter of 100 mm is reduced from 9.7 μm PV to 4.5 μm PV after polished by this pad.

Design and test result of a superconducting double-spoke cavity

Abstract Superconducting multi-spoke cavities are outstanding alternative choice for acceleration of heavy ions in medium velocity regimes. Based on the scheme of China ADS, several researches on the superconducting double-spoke cavities were done and two prototype cavities have been developed. In this paper, the RF design, the mechanical design and fabrication considerations of the bare cavity will be described in detail. After Buffered Chemical Polishing and High Pressure Rinsing, one of the prototype cavities was installed into the Vertical Test Stand for high gradient RF testing at 4.2 K. The measurement results of the quality factor as a function of the accelerating field and the maximum surface field will be presented. An accelerating gradient of more than 15 MV/m is achieved during the test, with maximum surface electric field of 58 MV/m, and maximum surface magnetic field of 117 mT.

Lightweight Design of Multi-Objective Topology for a Large-Aperture Space Mirror

For a large-aperture space telescope, one of the key techniques is the method for designing the lightweight primary mirror assembly (PMA). In order to minimize the mirror surface error under axial gravity, lateral gravity, and polishing pressure at the same time, a method for topology optimization with multi-objective function combined with parametric optimization is introduced in this paper. The weighted compliance minimum is selected as the objective function to maximum the mirror structural stiffness. Then sensitivity analysis method and size optimization are used to determine the mirror structure parameters. Compared with two types of commonly used lightweight configurations, the new configuration design shows obvious superiority. In addition, the surface figure root mean square (RMS) of the mirror mounted by given bipod flexure (BF) under 1 g lateral gravity is minimized only with a value of 3.58 nm, which proves the effectiveness of the design method proposed in this paper.

Research on material removal model and processing parameters of cluster magnetorheological finishing with dynamic magnetic fields

Based on fluid mechanics and Preston equation, the influences of various technological parameters (including rotational speeds of workpieces, magnetic poles, and polishing disk and the machining gap) on polishing pressure on workpiece surfaces were investigated. On this basis, a material removal rate (MRR) model of cluster magnetorheological finishing (CMRF) with dynamic magnetic fields was established. Moreover, three pieces of single-crystal silicon substrates were subjected to synchronous polishing on the CMRF devices with dynamic magnetic fields in order to analyze the influence of different technological parameters on the MRR. It is showed that the theoretical simulation results favorably agree with the experimental results. The single-crystal silicon substrates with the initial surface roughness (Ra) of 0.48 μm were polished for 5 h on the optimal technological conditions: the machining gap and the eccentricity of magnetic poles are 0.9 mm and 6 mm, respectively; the workpieces are oscillated in Y direction for 40 mm at the oscillation speed of 600 mm/min; the rotational speeds of the polishing disk, magnetic poles, and workpieces are 50 r/min, 90 r/min, and 350 r/min, respectively. In this way, the super-smooth uniform surfaces with the roughness of Ra 2.4 nm were acquired.

Abrasive-free surface finishing of glass using a Ce film

A novel form of abrasive-free polishing is described, using Ce film deposited on a polishing pad by vacuum evaporation. Polishing using metallic films other than Ce, such as Al and Cu, generates defects on the glass surface with almost negligible material removal rates (MRRs), whereas Ce film with deionized (DI) water achieves smooth, scratch-free surfaces with noticeable MRR. If DI water is not supplied to the pad, there is significant deterioration in polishing performance. MRR increases with greater thickness of Ce film, and extremely smooth surface with sub-nanometer roughness is achieved using Ce film of thickness 2.5 μm. Polishing performance is dependent on conditions such as pad rotation rate and polishing pressure. By adding potassium hydroxide (KOH) to the polishing solution, the MRR almost matches that obtained with conventional abrasive polishing, but achieves ultra-smooth surface finish (Ra: 0.388 nm). The polishing method presented here uses approximately 94% less Ce compared with conventional abrasive polishing, thereby dramatically reducing consumption of this valuable rare earth element.

Research and application on force control of industrial robot polishing concave curved surfaces

In order to improve the quality of the industrial robot automatic polishing on curved surfaces and ensure the constant polishing pressure during polishing process, a method for polishing complex concave cavity surfaces with industrial robot is proposed in this article. The method can achieve stable force control and precise position control and is easy to be realized online. In order to ensure the removal rate uniformity of surface material at different normal vectors, a method for adjusting the speed of motorized spindle in real time according to the surface normal vector is proposed. After planning the trajectory and normal vectors, combined with the feedback force signal from the sensor and the proportional–integral controller in the direction of the normal vector, the robot terminal tool corrects the trajectory in the direction of the surface normal vector, indirectly realizing force control between the tool and the surface. The robot polishing system with different polishing tools has different system stiffness. In order to ensure the polishing system with different stiffness to have a better tracking performance of the contact force, an adaptive proportional–integral control algorithm proposed in this article can be used to evaluate the stiffness of polishing system and to adjust proportional–integral parameters. The simulation and experimental results indicate that the method can realize the polishing of concave cavity surface commendably.

Atomistic mechanisms of Cu CMP in aqueous H2O2: Molecular dynamics simulations using ReaxFF reactive force field

Molecular dynamics simulations using ReaxFF reactive force field are employed to study the process of the silica abrasive particle sliding on the Cu(1 0 0) substrate in the aqueous H2O2 in order to clarify the atomistic mechanisms of the Cu chemical mechanical polishing process. Results reveal that there are mainly CuH2O, OHCuOH, CuOHCu, CuOHH2O, OCuOH etc on the substrate, among them, the number of CuH2O is the most. And a part of H2O and OH in the above product come from H2O, some from H2O2, and the other are from the recombination of these two types molecule. Under the mechanical sliding effects, Cu atoms are mainly removed in the form of clusters by fracturing of CuCu bonds and CuO bonds on the Cu substrate. When the polishing pressure is different, the more Cu atoms are removed, the greater the friction force between the abrasive particles and the substrate and the more H2O molecules are decomposed as the polishing pressure increases. Besides, H2O plays a dual effect by oxidizing the Cu substrate surface as well as by the effect of lubrication. Our results may shed light on the removal mechanism of Cu atoms in the CMP process at the atomic level and help to further understand the CMP of Cu.

Effects of Process Parameters on Material Removal in Vibration-Assisted Polishing of Micro-Optic Mold.

Process parameter conditions such as vibrating motion, abrasives, pressure and tool wear play an important role in vibration-assisted polishing of micro-optic molds as they strongly affect material removal efficiency and stability. This paper presents an analytical and experimental investigation on the effects of process parameters, aimed at clarifying interrelations between material removal and process parameters which affect polishing quantitatively. The material removal rate (MRR) and surface roughness which represent the polishing characteristics were examined under different vibrating motions, grain sizes of abrasives and polishing pressure. The effects of pressure and tool wear conditions on tool influence function were analyzed. The results showed that 2D vibrating motion generated better surface roughness with higher material removal efficiency while a smaller grain size of abrasives created better surface roughness but lower material removal efficiency. MRR gradually decreases with the increase of polishing pressure when it exceeds 345 kPa, and it was greatly affected by the wear of polisher when wear diameter on the polisher’s head exceeds 300 μm.

Study on the variable pressure CCOS polishing technology

In grinding and polishing of the aspherical and freeform surface, the CCOS technology is widely used. It commonly uses constant pressure during polishing, and thus the desired amount of material to be remove depends on the dwell time. This paper focuses on the variable pressure CCOS polishing technology. It adds one more degree of freedom to the polishing process, in which the desired amount of material to be removed is controlled by both the polishing pressure and the dwell time. Firstly, a mathematical model was established for the variable pressure polishing process. Then, the stability and response speed of the output force of the polishing tool, and the stability of removal function was measured and analyzed. Finally, a material removal experiment that applied sinusoidal force was carried out on a K9 material mirror. Results show that frequency of the measured force is the same as that of the ideal sinusoidal polishing force, with a standard deviation of the force error being about 0.35 N. Its effect on PV and RMS of the finish surface is less than 9%. The spatial period of the measured surface profile is the same as that of surface profile obtained by simulation of the sinusoidal polishing process. The surface profile error is within 17%. In this paper, variable pressure polishing was achieved, and its effectiveness for optical processing was verified.

Research on ice fixed-abrasive polishing mechanism and technology for high-definition display panel glass

This study introduces an ice fixed-abrasive polishing (IFAP) technology. Using silica solution IFAP pad and Al2O3 IFAP pad, orthogonal tests were performed on polishing high-definition display panel glass, respectively. The results show that the polishing efficiency and effect polished with silica solution IFAP pad are better than those polished with Al2O3 IFAP pad. The optimised silica solution IFAP parameters are: polishing pressure 0.1 MPa, polishing time 40 min, table velocity 80 r/min and the ratio of accelerator and slurry 1:10. Finally, the IFAP mechanism was studied and it is suggested that IFAP is a comprehensive effect of mechanical removal and microchemical reaction, combined with fixed abrasive polishing and free abrasive polishing.

Simulation Model of Polishing Pressure in Chemical-Mechanical Polishing

Simulation Model of Polishing Pressure in Chemical-Mechanical Polishing

Damage-free highly efficient polishing of single-crystal diamond wafer by plasma-assisted polishing

Single-crystal diamond (SCD) is considered to be an ideal material for next-generation power devices. Plasma-assisted polishing (PAP) without using an abrasive was applied to polish SCD fabricated by chemical vapor deposition. Argon-based plasma containing water vapour was used in the PAP to modify the surface of polishing plate and SCD (100), and SCD was polished under a polishing pressure ranging from 10 to 52.6kPa. Raman spectroscopy measurement showed that there was no residual stress on the polished SCD surface, and a polishing rate of 2.1μm/h and a surface roughness of 0.13nm Sq were obtained.

Cobalt “Buff Step” Chemical Mechanical Planarization

In this study, silicon dioxide wafers were polished as part of a cobalt “buff step” CMP using a novel and state-of-the-art application-specific slurry and pad combination to study the thermal, tribological, and kinetic attributes of the process. Removal rate and mean pad temperature showed increases with higher polishing pressures and sliding velocities, while coefficient of friction stayed relatively constant across all investigated conditions. Under all conditions, the process was tribologically robust as it remained in “boundary lubrication”. As the removal rate exhibited a non-Prestonian behavior, a well-established Langmuir-Hinshelwood model was used to simulate, for the first time for this process and this set of consumables, removal rate data such that chemical and mechanical rate constants could be extracted and understood in light of the process parameters chosen. Results indicated that the process was mechanically limited at nearly all combinations of pressure and velocity except at the highest values investigated, indicating that small deviations from set polishing power can greatly affect material removal rate.

Effect of CVD-Coated Diamond Discs on Pad Surface Micro-Texture and Polish Performance in Copper CMP

The effect of pad surface characteristics on the thermal, tribological and kinetic attributes of copper CMP was investigated. Three CMC D100 pads with very different surface micro-textures were generated using three very different CVD-coated diamond conditioning discs. Pad samples were collected after polishing and analyzed for their surface contact area and topography using confocal microscopy. The contact area, contact density, and asperity height increased with increasing conditioner aggressiveness. Copper removal rates and pad surface temperatures increased with increasing polishing pressure and sliding velocity for all pads albeit in a non-Prestonian manner. The pad generated by the most aggressive disc caused the highest removal rates yet it showed the lowest overall coefficient of friction (COF). The tall asperities, open pores, and adequate contact of this pad produced a greater removal rate than pad surfaces with shorter asperities and glazed pores produced by the less aggressive discs. Trends in COF, temperature and removal rate were successfully simulated using a two-step modified Langmuir-Hinshelwood model which also yielded values for the chemical and mechanical rate constants. The simulation results indicated that the process was chemically limited for all polishing conditions, and that the process became even more chemically limited as P × v increased.

Role of the Adhesion Force during Copper Chemical Mechanical Planarization

The adhesion force between slurry particles and the wafer surface has a significant effect on the material removal and ultra-smooth surface formation during the chemical mechanical polishing/planarization (CMP) process. Based on zeta potentials measurements, we calculate the value of adhesion force, which cannot be ignored compared with the Hertz contact force for small sized abrasive particles at low polishing pressures. More over, the impacts of slurry (particle size and pH value) and wafer surface roughness on the adhesion force are evaluated. Theoretical analyses manifest that the adhesion force mainly originates from the van der Waals force and electrical double-layer force, which increases with the increase of the particle size and slurry pH, while decreases with the increase of the wafer surface roughness. The relationships between the adhesion force and scratch depth indicate that scratching would be avoided while enhancing the adhesion force during CMP. These findings would provide some inspirations to understand the CMP technology in depth, and they are also beneficial to engineering super-smooth and super-lubricant surfaces with nano-scaled roughness.

Numerical and experimental study on low-pressure abrasive flow polishing of rectangular microgroove

A low-pressure abrasive flow polishing (LAFP) technology is presented to address the issue of fabricating rectangular microgroove of Cu and SUS304 materials. The effects of abrasive flow on the polishing efficiency and uniformity are analyzed numerically and experimentally. The rectangular structure of polishing microchannel is designed and optimized by simulating the trajectory and erosion action of particles using computational fluid dynamics (CFD) model. The distributions of turbulence intensity and shear force in the polishing microchannel are analyzed. Polishing experiments are conducted to fabricate microgrooves on Cu and SUS304 materials. The experimental results reveal the increase of material removal with the increase in particle size, polishing pressure, and particle concentration. Surface roughness of Ra 4.8nm and Ra 12.7nm are respectively obtained when polishing the rectangular microgroove of Cu and SUS304 materials. Simulation and experimental results show that LAFP using polishing microchannel can fabricate microstructures with high-quality surface.

Chemical kinetics mechanism for chemical mechanical polishing diamond and its related hard-inert materials

Hard-inert materials such as diamond, silicon carbide, gallium nitride, and sapphire are difficult to obtain from the smooth and damage-free surfaces efficiently required by semiconductor field. Therefore, this study proposed a chemical kinetics model to evaluate the material removal rate of diamond in chemical mechanical polishing process and to investigate the material removal mechanism by examining the surface information with optical microscopy, surface profilometry, and atomic force microscopy as well as X-ray photoelectron spectroscopy. The theoretical and experimental results show that chemical and mechanical synergic effect may promote the diamond oxidation reaction in chemical kinetics. The material removal rate is acceptable when the mechanical activation coefficient is smaller than 0.48. The 2.5 μm B4C abrasives, the polishing temperature of 50 °C, and the polishing pressure of 266.7 MPa are optimal parameters for diamond polishing with potassium ferrate slurry. It provides the highest material removal rate of 0.055 mg/h, the best surface finish (about Ra 0.5 nm) and surface quality (no surface scratches or pits). It then discusses how mechanical stress may promote the chemical oxidation of oxidant and diamond by forming “C-O,” “C=O,” and “O=C-OH” on diamond surface. The study concludes that chemical kinetics mechanism is effective for the investigation of the synergic effect in chemical mechanical polishing hard-inert materials.

Cobalt CMP Development for 7nm Logic Device

This study presented the advancement of cobalt CMP development for 7nm logic device at GLOBALFOUNDRIES. Two-step CMP process was developed. Overburden cobalt was removed and polishing stopped on the liner via endpoint detection during the bulk CMP. Bulk CMP was followed by buff CMP, where the liner and a certain amount of ILD films were removed. At first, the effect of polish pressure on blanket cobalt removal rate was investigated during bulk CMP. Results showed the Prestonial relationship. Results also showed that bulk CMP had wide over polish time window due to good stop ability of the bulk slurry on the liner. During buff CMP, removal rate selectivity between cobalt, liner, and ILD films were studied and compared for different types of slurries. Higher liner removal rate and lower cobalt/ILD rate could be achieved by slurry screening. Finally, the cleaning solution was also evaluated. Defects post cobalt CMP are comparable to tungsten CMP.

Cobalt CMP Development for 7nm Logic Device

Cobalt metallization for CA (contacts) and TS (trench silicide) of 7nm logic device may dramatically reduce resistance and improve 5-7% performance compared to its counterpart tungsten. This new integration scheme requires novel cobalt deposition, CMP, cleaning and etching processes. This study presents the advancement of cobalt CMP development in GLOBALFOUNDRIES Inc.. Two-step CMP process was developed. Overburden cobalt was removed and polishing was stopped on the liner material via endpoint detection during the bulk CMP. Bulk CMP was followed by buff CMP, where the liner material and a certain amount of ILD films were removed. At first, the effect of polish pressure and sliding velocity on blanket cobalt removal rate was investigated. Results showed the prestonial relationship. Subsequently, process parameters (polish pressure, sliding velocity, over polish time, etc.) were optimized for the bulk CMP. End point was captured successfully and was stable at the optimized process conditions. In addition, process had wide over polish time window due to good stop ability of the bulk slurry on the liner material. During buff CMP, removal rate selectivity between cobalt, liner, and ILD films were studied and compared for different types of slurries with different components concentration. Higher liner removal rate and lower cobalt/ILD rate were achieved at an optimized slurry components concentration. Finally, cleaning solutions and process parameters were optimized. Surface topography (dishing, erosion, etc.) and defects post cobalt CMP were extensively characterized. Results demonstrated comparable CMP performance between cobalt and tungsten. Figure 1

Analysis of magnetorheological fluid behavior in chemo-mechanical magnetorheological finishing (CMMRF) process

Advanced nanofinishing is an important process in manufacturing technologies due to its direct influence on optical quality, bearing performance, corrosion resistivity, bio-medical compatibility and micro-fluidics attributes. Chemo-mechanical magnetorheological finishing (CMMRF) process, one of the advanced nanofinishing process, was developed by combining essential aspects of chemo-mechanical polishing (CMP) process and magnetorheological finishing (MRF) process for surface finishing of engineering materials. The CMMRF process was experimentally analyzed on silicon and copper alloy to generate surface roughness of the order of few angstroms and few nanometers respectively. However, the process needs theoretical exploration towards better understanding, process optimization and result prediction. Hence, an attempt has been made for theoretical study of CMMRF process to analyze the effects of MR fluid under various process parameters. The present theoretical work is split as per following two sub-activities to simplify intricacy of the work.1) FEA-CFD simulation to analyze magnetism, polishing pad formation and polishing pressure during the CMMRF process. The simulation results are used to conduct experiments on aluminium alloy.2) A mathematical model has been developed to predict material removal as well as surface roughness during the CMMRF process. Model validation is conducted by comparing finite element simulation results with the experiments on aluminium alloy.The theoretical results show good agreement with the experimental data and the same has been discussed in this paper.