Polishing Pressure Research Articles

구리 ECMP에서 전류밀도가 재료제거에 미치는 영향

Dept. of Mechanical Engineering, Northeasten University(Received November 30, 2014; Revised March 25, 2015; Accepted April 10, 2015)Abstract − RC delay is a critical issue for achieving high performance of ULSI devices. In order to minimizethe RC delay time, we uses the CMP process to introduce high-conductivity Cu and low-k materials on the dam-ascene. The low-k materials are generally soft and fragile, resulting in structure collapse during the conventionalhigh-pressure CMP process. One troubleshooting method is electrochemical mechanical polishing (ECMP)which has the advantages of high removal rate, and low polishing pressure, resulting in a well-polished surfacebecause of high removal rate, low polishing pressure, and well-polished surface, due to the electrochemical accel-eration of the copper dissolution. This study analyzes an electrochemical state (active, passive, transpassive state)on a potentiodynamic curve using a three-electrode cell consisting of a working electrode (WE), counter elec-trode (CE), and reference electrode (RE) in a potentiostat to verify an electrochemical removal mechanism. Thisstudy also tries to find optimum conditions for ECMP through experimentation. Furthermore, during the low-pressure ECMP process, we investigate the effect of current density on surface roughness and removal ratethrough anodic oxidation, dissolution, and reaction with a chelating agent. In addition, according to the Faraday’slaw, as the current density increases, the amount of oxidized and dissolved copper increases. Finally, we confirmthat the surface roughness improves with polishing time, and the current decreases in this process.Keywords −electrochemical mechanical polishing(전기화학 기계적 연마), potentiodynamic curve(변전위 곡선),passivation layer(부동태 층), current density(전류밀도), material removal rate(재료제거율)

Wear process of single crystal diamond affected by sliding velocity and contact pressure in mechanical polishing

In this work, the influences of sliding velocity and contact pressure on the wear rate of diamond substrate in mechanical polishing are investigated experimentally and theoretically. The experimental observations indicate that the wearing process only consists of the wear-in and stable wear periods. The removal thickness of diamond crystal first increases nonlinearly in the wear-in stage, but then linearly in the subsequent stable wear stage. A diamond carbon amorphization-dependent model is established to calculate the linear variations of removal thickness, which gives a satisfactory prediction accuracy as compared to the experimental data. Although the experiment results demonstrate that the higher sliding velocity or contact pressure will cause a higher wear rate of diamond substrate, the action laws are thoroughly different according to the theoretical prediction. A greater sliding velocity increases the amorphization rate of diamond carbons and the scratching frequency of diamond grits. However, a higher contact pressure produces a larger contact area, which forces more diamond grits to scratch on the surface of diamond substrate.

Material removal mechanism of cluster magnetorheological effect in plane polishing

A cluster magnetorheological (MR) plane polishing based on a combination of magnetorheological finishing (MRF) and cluster mechanism (arrange in a regular array) has been developed for large planar optical workpiece. In this paper, the polishing pressure of an individual MR micro-grinding head on workpieces was investigated, and the mathematical model for cluster MR plane polishing was established based on the Preston equation and the properties of the workpiece material. An arc-shaped polishing belt was processed on monocrystalline 6H-SiC and Si wafer by the cluster MR polishing apparatus which polishing disk inlaid with cylindrical flat bottom magnetic pole arrange in the same direction regularly and monocyclically. The profile of arc polishing belt’s cross section and the surface morphology were observed and analysed to verify the obtained model. The abrasives with different size were found to have an equal effect on the processed surface because of the accommodated-sinking effect of the polishing pad constituted by the cluster MR micro-grinding head. It was found that the monocrystalline 6H-SiC and Si wafer were polished in a plastic mode with a smooth, damage-free surface.

Studies of an LL-type 500 MHz 5-cell superconducting cavity at SINAP**Supported by National Natural Science Foundation of China (11175237)

A low loss- (LL) type 500 MHz 5-cell superconducting niobium prototype cavity with a large beam aperture has been developed successfully including the optimization, the deep drawing and electron beam welding, the surface treatment and the vertical testing. The performance of the fundamental mode was optimized and the higher order modes were damped by adopting an enlarged beam pipe for propagation. Surface preparation or treatment including mechanical polishing, buffered chemical polishing and high pressure rinsing with ultra-pure water and so on was carried out carefully to ensure a perfect inner surface condition. The vertical testing results show that the accelerating voltage higher than 7.5 MV was obtained while the quality factor was better than 1×109 at 4.2 K. No obvious multipacting or field emission was found during the test. However, a quench happened while increasing the field a little higher than 7.5 MV that at present limited the cavity performance.

A molecular-scale analytic model to evaluate material removal rate in chemical mechanical planarization considering the abrasive shape

This paper presents a theoretical model of material removal rate (MRR) in the process of chemical mechanical planarization (CMP), in which the morphology effect of abrasive particles is taken into account using a shape factor. In contrast to previous models based on the continuum material removal mechanism, the physical basis of the current model is that the wafer material is primarily removed by de-bonding the chemistry-weakened surface molecules. It is shown that a non-linear relationship between the shape factor and the MRR is given rise to by two opposite effects. On one hand, the increase in the shape factor results in the decrease in the number of the effective particles participating in the material removal, and subsequently contributing to the decrease in the MRR. On the other hand, the particle/wafer contact diameter is enhanced with the increase in shape factor, leading to the increase in the MRR. The simulation results match well with the published experimental data in the range of low polishing pressure conditions. With further increase in the pressure, the effect of the shape factor on the MRR becomes significantly pronounced, which might be ascribed to the influence of the abrasive shape on the micro-contact state of the pad/abrasive/wafer. In addition to the shape factor, most variables involved in the CMP process, such as processing conditions (velocity, pressure), pad properties (modulus, hardness and asperity size) abrasive characteristics (size, concentration and distribution) and wafer hardness, were also addressed in the present model.

Failure Mechanisms of CVD Diamond Wafers and Thin Films During Polishing

This article investigates the failure mechanisms of CVD diamond wafers and thin films during a fast dynamic friction polishing process. To explore the evolution of temperature and stress fields, a comprehensive finite element analysis was systematically carried out, with the aid of experimental examination. It was found that the discontinuity and sharp change of the stresses across the film-substrate interface causes debonding failure of a CVD diamond thin film specimen. In the case of a CVD diamond wafer, however, the high surface tensile stress and bulk bending is responsible for the cracking. It was concluded that specimen cracking is sensitive to the polishing pressure, and that the polishing window for the CVD thin films is smaller. Polishing time is a critical factor, because a longer time corresponds to a higher thermal stress. This article points out that using the combination of a smaller polishing load and a greater sliding speed is a good option in selecting polishing parameters. To minimize cracking, a stepwise polishing process can be used. With the proper parameters obtained in this study, very smooth, high-quality surfaces of CVD diamond wafers and thin films can be produced in a short polishing duration of minutes.

Influence of Polishing Parameters on Abrasive Free Chemical Mechanical Planarization (AFCMP) of Non-Polar (11-20) and Semi-Polar (11-22) GaN Surfaces

In this study, an Abrasive free Chemical Mechanical Planarization (AFCMP) of non-polar (11-20) and semi-polar (11-22) GaN surfaces has been demonstrated. Effect of processing parameters, such as types and concentration of oxidizers, polishing pressure, platen velocity, and pH of the slurries, on the material removal rate (MRR) and surface quality (roughness) have been studied in details. The maximum MRR has been found to be 1.14 μm/hr and 1.85 μm/hr for non-polar (11-20) and semi-polar (11-22) GaN surfaces respectively, under optimized condition of 38 kPa polishing pressure, 90 rpm (100 for non polar surface) platen velocity, 30 rpm carrier velocity, slurry pH 2 and 0.4 M Oxidizer concentration. Root mean square (RMS) surface roughness of ∼1.9 nm and ∼0.8 nm, over large scanning area of 0.70 × 0.96 mm2, has been achieved on AFCMP processed non-polar (11-20) and semi-polar (11-22) GaN surfaces, respectively, using optimized slurry chemistry and processing parameters.

BTA Free Alkaline Slurries Developed for Copper and Barrier CMP

With the technology node moving down to below 32 nm, ultra low k materials are routinely used currently. Integration of low k materials in the backend presents challenges in metal CMP processes. This paper reports the development of the alkaline slurries containing a novel additive that can be used for primary and secondary Cu CMP as well as barrier CMP. The slurries exhibited a relatively high removal rate in Cu CMP, especially at low polish pressure, because of the strong affinity between copper and functional groups in the additive. The smooth Cu surface with sub-nanometer roughness can be obtained with the Cu slurries plus the nonionic surfactant, BRIJ30. The Cu CMP slurries bring about satisfactory results of dishing and oxide erosion in the experiments. The k value of the low k material does not show significant difference after CMP with the barrier slurry. Since the slurries are free of BTA, it will be beneficial for post CMP cleaning of wafers. The low leakage current in the electric test could be attributed to the low level of metal contamination because of unique complexing action of the chelator.

Topology optimization-based lightweight primary mirror design of a large-aperture space telescope.

For the large-aperture space telescope, the lightweight primary mirror design with a high-quality optical surface is a critical and challenging issue. This work presents a topology optimization-based design procedure for a lightweight primary mirror and a new mirror configuration of a large-aperture space telescope is obtained through the presented design procedure. Inspired by the topology optimization method considering cast constraints, an optimization model for the configuration design of the mirror back is proposed, through which the distribution and the heights of the stiffeners on the mirror back can be optimized simultaneously. For the purpose of minimizing the optical surface deviation due to self-weight and polishing pressure loadings, the objective function is selected as to maximize the mirror structural stiffness, which can be achieved by minimizing the structural compliance. The total mass of the primary mirror is assigned as the constraint. In the application example, results of the optimized design topology for two kinds of mass constraints are presented. Executing the design procedure for specific requirements and postprocessing the topology obtained of the structure, a new mirror configuration with tree-like stiffeners and a multiple-arch back in double directions is proposed. A verification model is constructed to evaluate the design results and the finite element method is used to calculate the displacement of the mirror surface. Then the RMS deviation can be obtained after fitting the deformed surface by Zernike polynomials. The proposed mirror is compared with two classical mirrors in the optical performance, and the comparison results demonstrate the superiority of the new mirror configuration.

Path planning of mechanical polishing process for freeform surface with a small polishing tool

Products with freeform surface are widely applied in industries, and the surface quality plays an important role in order to fulfill the targeted functions. As polishing path of small polishing tool affects the polishing removal function considerably, it is highly necessary to study the polishing path of freeform surface for obtaining good polishing efficiency and well-proportioned surface quality. By combining the Preston polishing removal function, the material removal model of small polishing tool under the control of constant polishing force and pressure is established. Based on this model, the material removal functions of scan line, Archimedean spiral, and Hilbert fractal polishing path are derived. The simulation results show that the Hilbert fractal polishing path has the best comprehensive performance. By using the projection relation of differential geometry, the optimal path generation algorithm of the Bezier surface based on Hilbert fractal polishing path is established. The polishing experiments are conducted on a self-developed polishing machine which is based on a parallel manipulator. The experimental results demonstrate that the surface roughness is improved from level 9 to level 11.

Reducing the mechanical action of polishing pressure and abrasive during copper chemical mechanical planarization

Chemical mechanical planarization (CMP) is a critical process in deep sub-micron integrated circuit manufacturing. This study aims to improve the planarization capability of slurry, while minimizing the mechanical action of the pressure and silica abrasive. Through conducting a series of single-factor experiments, the appropriate pressure and the optimum abrasive concentration for the alkaline slurry were confirmed. However, the reduced mechanical action may bring about a decline of the polishing rate, and further resulting in the decrease of throughput. Therefore, we take an approach to compensating for the loss of mechanical action by optimizing the composition of the slurry to enhance the chemical action in the CMP process. So 0.5 wt% abrasive concentration of alkaline slurry for copper polishing was developed, it can achieve planarization efficiently and obtain a wafer surface with no corrosion defect at a reduced pressure of 1.0 psi. The results presented here will contribute to the development of a “softer gentler polishing” technique in the future.

Ceria-Based Slurries for Non-Prestonian Removal of Silicon Dioxide Films

A slurry with a non-Prestonian dependence on the polishing pressure can help in minimizing dishing and erosion during shallow trench isolation chemical mechanical planarization. Here, we show that ceria-based slurries containing diallyldimethylammonium chloride (DADMAC) yield a non-Prestonian blanket film polish rate with a low threshold pressure (1–2 psi) when polishing plasma-enhanced chemical vapor (PECVD) tetraethylorthosilicate (TEOS) deposited oxide as well as thermal oxide films. The polishing mechanism of this non-Prestonian slurry was investigated by a series of experiments involving zeta potential measurements, thermogravimetric analysis (TGA) and UV-vis spectroscopy and it was shown that more DADMAC molecules are adsorbed on silica particles (as oxide film representatives) than on ceria particles and the binding strength between DADMAC and silica is much higher than that with ceria surface.

Effect of three kinds of guanidinium salt on the properties of a novel low-abrasive alkaline slurry for barrier CMP

The influence of three kinds of guanidinium salt on the removal rate selectivity of different materials was studied during the barrier chemical mechanical polishing (CMP) process at first. The three kinds of guanidine saltguanidine hydrochloride, guanidine nitrate and guanidine carbonate. Then we compared the effect of the three kinds of guanidine salt on the dishing, erosion and surface roughness value. In the end, the reaction mechanism was studied through electrochemical analysis. All the results indicate that there is a better performance of the slurry with guanidine hydrochloride than the slurries with the other two kinds of guanidine salt. It effectively improved the removal rate selectivity and the surface roughness under the premise of low abrasive concentration and low polishing pressure, which is good for the optimization of the alkaline slurry for the barrier CMP process.

Study on Process Parameters in CMP Ultra-Thin Stainless Steel Sheet

Chemical mechanical polishing (CMP) is the most effective technology to achieve ultra-smooth without damage surface in ultra-precision machining of stainless steel substrate. In this paper, according to the slurry ingredients obtained by former research, the influences of the CMP process parameters, such as the rotational velocity of the platen and the carrier, the polishing pressure and the abrasive size on the material removal rate (MRR), have been studied in CMP stainless steel substrate based on the alumina (Al2O3) abrasive. The research results show that the material removal rate increases with the increase of the abrasive size, the rotational velocity of the platen and the polishing pressure significantly and the surface roughness increases with the increase of the abrasive size. This study results will provide the reference for optimizing the process parameters and researching the material removal mechanism in CMP stainless steel sheet.

The Research of Extruded Pressure Auto-Adjustment Method in Abrasive Flow Polishing

Aiming at the quality problems of ‘over polishing’ and ‘insufficient polishing’ in the process of traditional extrusion abrasive flow machining,An abrasive flow polishing method at the constantly small pressure difference is developed, the hydraulic proportional control system is designed, and the mathematical modeling of the hydraulic proportional control system for small pressure difference constant extrusion polishing abrasive flow machining is set up. The simulation results show that the polishing pressure can be well controlled and the stability of polishing velocity is improved, which will improve the extrusion abrasive flow polishing quality.

Research on a processing model of CMP 6H-SiC (0001) single crystal wafer

Firstly, this paper presents an orthogonal test of six factors and five levels, called the chemical mechanical polishing (CMP) process parameters experiment, for determining the best process parameters and ranking the influencing factors from primary to secondary. The three most important factors are the polishing pressure, the polishing liquid concentration and the relative velocity ratio of polishing disk to polishing carrier. Then, based on this analysis, the three factors and three levels of the quadratic orthogonal regression test are put forward. A mathematical model impacting the surface roughness has also been set up. Finally, this work has achieved a polished wafer, whose material removal rate (MRR) is in the range of 70–90 nm/hand the surface roughness (Ra) is between 0.3 nm and 0.5 nm.

Design of Double-Sided Polishing Machine for Functional Crystal Substrate

Responding to the demand on the ultra-precision equipment for machining monocrystalline silicon, sapphire, zirconia ceramics and other large-size functional crystal substrates in the microelectronics and optoelectronics manufacturing, this study analyzes the current state-of-the-art of polishing technologies and the technical challenges to achieving high surface quality substrates. In this study, a planetary double-sided polishing machine is designed to solve the problem between achieving high polishing quality and efficiency. The machine is characterized by process stability, large polishing pressure and high polishing speed, etc. which is verified by polishing zirconia substrates with satisfactory results: material removal rate of 5 μm/h and the surface roughness Ra of 1 nm.

Identifying the Microtopographic Features Generated by Different Ultra-Precision Machining Techniques by Power Spectral Density

An atomic force microscope (AFM) was used to image the ultra-precision machined surfaces of K9 glasses, potassium dihydrogen phosphate (KDP) crystals, and silicon wafers. Each surface was generated by two machining techniques: (1) coarse-grain-sized diamond wheel grinding and regular polishing were used on K9 glasses; (2) single-point diamond turning and deliquescent polishing were performed on KDP crystals; (3) standard chemical mechanical polishing and atmospheric pressure low temperature plasma polishing were employed on silicon wafers. One-dimensional and two-dimensional power spectral densities (PSDs) of each sample surface were calculated from the measurement data. The influence of each machining process on the sample surface texture was analyzed based on the PSDs. The experiment results indicate that the power spectral density is a great guidance for the selection and improvement of ultra-precision machining techniques.

Effects of pressure and shear stress on material removal rate in ultra-fine polishing of optical glass with magnetic compound fluid slurry

Optical glasses used in a range of industrially important optoelectronic devices must be polished to nano-level roughness for proper device operation. Polishing process with magnetic compound fluid slurry (MCF polishing) under a rotary magnetic field is an influential candidate for the method to precisely polish optical glass. MCF slurry has been successfully utilized to polish a variety of materials, ranging from soft optical polymers to hard optical glasses. MCF was developed by mixing a magnetic fluid and a magnetorheological fluid with the same base solvent, and hence includes not only μm-sized iron particles but also nm-sized magnetite particles. To elucidate the behaviour of material removal in MCF polishing, this study measured the normal and shear forces generated in the polishing zone during polishing. From these measurements, the distributions of pressure P and shear stress τ were obtained. The distribution of material removal rate (MRR) was investigated through spot polishing of borosilicate glass. The effects of three process parameters, namely magnet revolution speed, MCF carrier rotational speed and working gap, on pressure P, shear stress τ and the MRR were also investigated. The results revealed that P is higher near the centre of the interacting area (i.e. the polishing spot centre) and the point of maximum shear stress τ appears at about 5mm from the polishing spot centre. All of P, τ and MRR are sensitive to MCF carrier rotational speed and working gap but insensitive to magnet revolution speed. Shear stress is more sensitive to these process parameters than the pressure. Cross-sectional profiles of the polishing spots exhibit a characteristic symmetric W-shape; material removals are minimal at the spot centre and maximal at approximately 8.2–10.2mm from the spot centre depending on the process parameters. MRR is proportional to the MCF carrier rotational speed and is negatively correlated with working gap. An MRR model involving both the pressure and shear stress in MCF polishing is proposed. In the model, MRR is more dominated by shear stress than by pressure.

Development of a Self-Propelled Multi-Jet Polishing Tool for Ultra Precision Polishing

As the needs of optical glasses are on the rise, the precision on shape, form, surface qualities and the scaling down of sizes are rising, too. The standards and surface finish of reference mirrors used in measuring appliances are crucial; hence, enhancement of the surface finish is indispensable in manufacturing industries. This paper proposes a self-propelled multi jet polishing technique for ultra precision polishing process in which bladeless Tesla turbine was used as a prime mover. The turbine is characterized by high swirling velocity at the outlet; therefore, high kinetic energy in the course of away from the turbine was used as polishing energy. Simulation of the flow of the field of turbine blades using computational fluid dynamics software (CFD) has also been presented. With a newly designed and manufactured polishing tool, this paper investigates the optimal polishing parameters for surface roughness improvement of crown optical glasses using Taguchis experimental approach; signal-to-nose (S/N) ratio and ANOVA analysis was also carried out to determine the effect of main factors on the surface roughness. Consequently, a 2.5μm size of Al2O3 abrasive, 10wt% abrasive concentration, 80rpm of polishing head, 6 numbers of nozzles, 6 kg/cm2 of pressure, and 45min. of polishing time have been found to be the optimal parameters. It was observed that about 94.44% improvements on surface roughness; Ra, from 0.360μm to 0.020μm has been achieved using the optimal parameters. In addition to this; angular speed of polishing head, pressure and polishing time were found to have significant effect on surface roughness improvement.