Hydrodynamic analysis of chemical mechanical polishing process

Chemical Mechanical Polishing (CMP) refers to a material removal process done by rubbing a work piece against a polishing pad under load in the presence of chemically active abrasive containing slurry. The CMP process is a combination of chemical dissolution and mechanical action. The mechanical action of CMP involves hydrodynamic lubrication. The liquid slurry is trapped between the work piece (wafer) and pad (tooling) forming a lubricating film. For the first step to understand the mechanism of the CMP process, hydrodynamic analysis is done with a semiconductor wafer. Slurry pressure distribution, resultant forces and moments acting on the wafer are calculated in typical conditions of the wafer polishing, and then nominal clearance of the slurry film, roll and pitch angles at the steady state are obtained.     

Optimization of carrier morphology in terms of Fourier sine series for chemical mechanical polishing process

To establish a 2D axisymmetric quasi–static finite element model during the chemical mechanical polishing process, revolutions of the wafer and the pad were assumed to be the same, the axisymmetric uniformly distributed pressure form was given, and both the wafer–pad interface and wafer–film interface were considered as contact boundaries. Next, the height of the contact interface between the carrier and the film near the zone of nonuniform von Mises stress distribution of the wafer surface was changed to the form of a Fourier sine series. Finally, a quadratic programming method was utilized to solve the coefficients of the Fourier sine series and then an optimal morphology in terms of Fourier sine series under the condition of minimum nonuniformity of the wafer surface was achieved. The result found that the nonuniformity was reduced remarkably and its improved rate reaches 88.45% under the optimal surface of the carrier.     

Modal analysis of multi-layer structure for chemical mechanical polishing process

In this study, a three-dimensional finite element model was established to perform modal analysis of the chemical mechanical polishing process. The contact boundary conditions were considered in the wafer and pad, and the influence of the static load exerted on the carrier was considered in order to investigate dynamic behaviour of the wafer. The analysis was in two steps. Firstly, a given pressure was exerted on the carrier and the geometric nonlinear effect and large deformation theory were used to carry out static analysis. Secondly, the results of the analysis were used to perform modal analysis of the wafer. The results gave way to four conclusions. (1) Due to the offset configuration of the wafer and pad, the von Mises stress distribution was asymmetric. Therefore, the stress on the wafer appeared to be almost uniform near its centre, goes through a maximum near the edge, and decreased as the edge is approached. This tendency is similar to that of the removal rate profile experiment, which proved that the proposed finite element model for CMP is acceptable. (2) Due to the extremely thin thickness of the film, wafer and pad, most mode shapes are predominant in out-plane deformation. Furthermore, since the y-axis is symmetric in the three-dimensional model, there were double roots in some modes. (3) When the load was larger, the tangent stiffness and the natural frequency would also be reduced. The pressure changes did not have much affect on mode shape. (4) Since the soft materials of the pad and film have significantly different Young’s modulus’ than hard materials, the natural frequencies of harder materials for the pad and film increase.     

In-situ neural network process controller for copper chemical mechanical polishing

Process control is one of the key methods to improve manufacturing quality. This research proposes a neural network based run-to-run process control scheme that is adaptive to the time-varying environment. Two multilayer feedforward neural networks are implemented to conduct the process control and system identification duties. The controller neural network equips the control system with more capability in handling complicated nonlinear processes. With the system information provided by this neural network, batch polishing time (T) an additional control variable, can be implemented along with the commonly used down force (p) and relative speed between the plashing pad and the plashed wafer (v). Computer simulations and experiments on copper chemical mechanical polishing processes illustrate that in drafting suppression and environmental changing adaptation that the proposed neural network based run-to-run controller (NNRTRC) performs better than the double exponentially weighted moving average (d-EWMA) approach. It is also suggested that the proposed approach can be further implemented as both an end-point detector and a pad-conditioning sensor.     

A study of a finite element model for the chemical mechanical polishing process

In this paper, relative velocity at a given point on the wafer was first derived. The revolutions of wafer and pad are assumed the same and the axisymmetric uniformly distributed pressure form is given. Thus, a 2D axisymmetric quasic-static model for chemical-mechanical polishing process (CMP) was established. Based on the principle of minimum total potential energy and axisymmetric elastic stress-strain relations, a 2D axisymmetric quasic-static finite element model for CMP was thus established. In this model, the four-layer structures including wafer carrier, carrier film, wafer and pad are involved. The von Mises stress distributions on the wafer surface were analysed, the effects of axial, hoop, radial and shear stresses to von Mises stress and the effects of axial, hoop, radial and shear strains to deformation of the wafer were investigated. The findings indicate that near the wafer centre, von Mises stress distribution on the wafer surface was almost uniform, then increased gradually with a small amount. However, near the wafer edge, it would decrease in a large range. Finally, it would increase dramatically and peak significantly at the edge. Besides, the axial stress and strain are the dominant factors to the von Mises stress distributions on the wafer surface and the wafer deformation, respectively.     

Improvement of rounding effect in chemical mechanical polishing process for nano-scale manufacturing

In the present work, the rounding effect in the CMP process was examined in the process conditions such as the head pressure, platen and head speed, and deposition thickness. The rounding effect according to each process condition is measured from SEM and compared with each other. From the experimental results, CMP process condition to reduce the rounding effect is determined and the rounding effect has been improved from 55nm to 29nm, which is about 47% reduction.     

化学机械抛光的研究进展

化学机械抛光简称CMP技术是迄今唯一的可以提供整体平面化的表面精加工技术,可广泛用于集成电路芯片、计算机硬磁盘、微型机械系统等表面的平坦化。介绍了半导体加工领域CMP技术的特点,重点叙述了CMP技术的发展历程、设备特性、研磨抛光耗材和应用领域的技术现状,指出CMP急待解决的技术和理论问题,并对其发展方向进行展望。     

An integrated material removal model for silicon dioxide layers in chemical mechanical polishing processes

The material removal rate in CMP processes obeys Preston's equation, which can be expressed as a linear function of the applied areal power density under usual operating conditions. However, some experimental results have shown a nonlinear relationship between the CMP material removal rate and the applied areal power density, suggesting non-Prestonian behavior under certain operating regimes. Although the material removal rate is caused by the coupled effect of both mechanical and chemical actions in actual CMP processes, the treatment of the chemical action as a mere supplementary means of softening the surface makes it difficult to explain this non-Prestonian behavior. In this work, we propose an integrated material removal model for silicon dioxide during CMP based on a multiscale mechanical abrasion model coupled with the slurry chemical diffusion effects. The synergetic effects on the material removal mechanism due to both mechanical and chemical actions are incorporated in the model, and the total material removal rate is predicted by accounting for both effects. Consequently, the non-Prestonian behavior often shown in silicon dioxide CMP may be explained using the proposed model. The validity of the model is supported by comparing the predicted material removal rates with experimental values available in the literature.     

Highly efficient chemical mechanical polishing method for SiC substrates using enhanced slurry containing bubbles of ozone gas

In this study, a highly efficient method for chemical mechanical polishing (CMP) of silicon carbide (SiC) substrates using enhanced slurry was proposed and developed. The enhanced slurry contains bubbles of ozone gas generated by ozone gas generator in pure water mixed with a conventional commercially available slurry. Therefore, the enhanced slurry has an oxidizing effect on the Si-face of SiC substrates. To confirm the effectiveness of bubbles enclosing ozone gas, both nano-indentation test and X-ray photoelectron spectroscopy (XPS) analysis were conducted. As a result, the hardness decrease of the Si-face of the SiC substrate was confirmed through the nano-indentation test, and the generation of reaction products was confirmed on Si-face of SiC substrate in the XPS analysis. According to a series of experimental results of our proposed highly efficient CMP method for SiC substrates, the removal rate can be increased when the enhanced slurry was applied, comparing with that for the not only conventional commercially available slurry but also commercially available dedicated slurry.     

Effects of wafer curvature caused by film stress on the chemical mechanical polishing process

A theoretical model based on two-body contact theory is established to simulate the contact pressure distribution arising from wafer curvature which is caused by film stress during CMP process. Both wafer and pad deformations during the contact process are considered. The profiles of the contact pressure distribution for wafers with different curvature radius are simulated. The influences of wafer curvature on mean removal rate and within wafer removal rate nonuniformity (WIWNU) are simulated and compared with the experimental data. According to the two-body contact model, when the pad is in contact completely with the wafer, the profile of the contact pressure has almost the same trend whether the wafer has an upward or a downward curvature. The mean value of the contact pressure will increase with increasing of radius of downward curvature. WIWNU will decrease with increasing pre-polish wafer bow from concave (upward curvature) to convex (downward curvature). The results from the simulation correlated with the experimental data and demonstrated the validity of the model. The results are helpful for controlling and reducing the wafer to wafer removal rate nonuniformity and within wafer removal rate nonuniformity in CMP.     

Effect of conditioner load on the polishing pad surface during chemical mechanical planarization process

During the Chemical mechanical planarization (CMP), the pad conditioning process can affect the pad surface characteristics. Among many CMP process parameters, the improper applied load on the conditioner arm may have adverse effects on the polyurethane pad. In this work, we evaluated the pad surface properties under the various conditioner arm applied during pad conditioning process. The conditioning pads were evaluated for surface topography, surface roughness parameters such as Rt and Rvk and Material removal rate (MRR) and within-wafer non-uniformity after wafer polishing. We observed that, the pad asperities were collapsed in the direction of conditioner rotation and blocks the pad pores applied conditioner load. The Rvk value and MRR were founded to be in relation with 4 > 1 > 7 kgF conditioner load. Hence, this study shows that, 4 kgF applied load by conditioner is most suitable for the pad conditioning during CMP.

     

Experimental investigation of process parameters for roll-type linear chemical mechanical polishing (Roll-CMP) system

The fabrication processes for electronic components are now demanding a higher degree of planarity for integration and multistacking, with chemical mechanical polishing (CMP) processes replacing conventional etching or mechanical polishing owing to their ability to attain global planarization. As CMP has been applied to more and more fields, new types of CMP machines have been developed. This study introduces a novel roll-type linear CMP (Roll-CMP) process that uses a line-contact material removal mechanism to for the polish flexible substrates, and examines the effect of the process parameters on the material removal rate (MRR) and its nonuniformity (NU). The parameters affecting the Roll-CMP process include down force, roll speed, table feed rate, slurry flow rate, slurry temperature, and the table oscillation length. Increasing the down force, roll speed, slurry flow rate, and slurry temperature resulted in a high average MRR (MRR_avg). Further, the MRR_avg was found to decrease with an increase in the oscillation length because of the effect of the polishing area. A large down force, high roll speed, high table feed rate, and high slurry flow rate were effective for reducing the NU. These results will be helpful for understanding the newly developed Roll-CMP process.     

The crystallographic change in sub-surface layer of the silicon single crystal polished by chemical mechanical polishing

The effect of the contact nominal pressure on the surface roughness and sub-surface deformation in chemical mechanical polishing (CMP) process has been investigated. The experimental results show that a better surface quality can be obtained at the lower pressure, and the thickness of sub-surface deformation layer increases with the increase of the pressure. In CMP process, polishing not only introduces amorphous transformation but also brings a silicon oxide layer with a thickness of 2–3 nm on the top surface. The atomic structure of the material inside the damage layer changes with the normal pressure. Under a higher pressure (125 kPa), there are a few crystal grain packets surrounded by the amorphous region in which the lattice is distorted, and a narrow heavy amorphous deformation band appears on the deformation region side of the interface. Under a lower pressure, however, an amorphous layer can only be observed.     

Analysis of retaining ring using finite element simulation in chemical mechanical polishing process

During the chemical mechanical polishing process (i.e., CMP for short), it is expected to attain the requirement of global planarization. However, the stress concentration, which occurred when approaching the wafer edge, has resulted in over-grinding. The increasing material removal rate has also contributed to the wafer’s nonuniformity. In this paper, a retaining ring surrounding the wafer carrier was added to the conventional CMP mechanism in order to improve the over-grinding phenomenon and avoid the wafer sliding from the carrier as much as possible. The revolutions of the wafer and the pad were assumed to be the same, and the force forms of the carrier and the retaining ring were axisymmetric uniformly-distributed. In addition, when the principle of minimum potential energy was applied, a two-dimensional axisymmetric quasi-static finite element model for CMP including the carrier, the retaining ring, the film, the wafer and the pad could be established. Following this model, the von Mises stress distribution of the wafer surface without a retaining ring was analyzed to verify the model. The effects of the gap between the ring and the pad and the ratio of the ring load and the carrier load on the stress and the wafer’s nonuniformity were investigated. The results indicated that the von Mises stress distribution of the wafer surface was almost uniform near the wafer center, and the maximum value appeared near the edge. The value decreased as the edge was approached, but it rose again very close to the edge. Besides, the wafer’s nonuniformity would be prohibited while the gap and the load were designed within the certain range to supply the retaining ring.     

化学机械抛光机理研究

虽然CMP过程材料去除机理得到了广泛深入研究,取得了许多理论成果,但是每一种理论都存在着或多或少的缺陷,没有一种理论完全正确的解释了CMP的机理。在众多CMP材料去除机理里,比较得到认可的是抛光垫、颗粒和工件表面三体磨损理论和化学腐蚀理论[1~4]。但是表面质量较好的工件在CMP之后没有观察到划痕,三体磨损理论受到了极大的挑战。而化学腐蚀理论很难解释腐蚀速度和腐蚀后表面质量关系[5,6]。本文结合CMP过程的实际环境,实验得到了化学机械抛光的抛光机理。     

化学机械抛光的理论模型研究综述

化学机械抛光(Chemical Mechanical Polishing,CMP)是当今唯一能够提供全局平面化的技术,其抛光机理的研究是当前的热点.综述了考虑抛光液和抛光垫特性的抛光机理模型,分析了各模型的相关特点,最后对CMP模型的发展和研究方向提出展望.     

利用复合磨粒抛光液的硅片化学机械抛光

为了提高硅片的抛光速率,利用复合磨粒抛光液对硅片进行化学机械抛光.分析了SiO2磨粒与聚苯乙烯粒子在溶液中的ζ电位及粒子间的相互作用机制,观察到SiO2磨粒吸附在聚苯乙烯及某种氨基树脂粒子表面的现象.通过向单一磨粒抛光液中加入聚合物粒子的方法获得了复合磨粒抛光液.对硅片传统化学机械抛光与利用复合磨粒抛光液的化学机械抛光进行了抛光性能研究,提出了利用复合磨粒抛光液的化学机械抛光技术的材料去除机理,并分析了抛光工艺参数对抛光速率的影响.实验结果显示,利用单一SiO2磨料抛光液对硅片进行抛光的抛光速率为180 nm/min;利用SiO2磨料与聚苯乙烯粒子或某氨基树脂粒子形成的复合磨粒抛光液对硅片进行抛光的抛光速率分别为273 nm/min和324 nm/min.结果表明,利用复合磨粒抛光液对硅片进行抛光提高了抛光速率,并可获得Ra为0.2 nm的光滑表面.     

Preparation of La-doped colloidal SiO2 composite abrasives and their chemical mechanical polishing behavior on sapphire substrates

Chemical mechanical polishing (CMP) has become a widely accepted global planarization technology. Abrasive is one of the key elements in CMP process. In order to enhance removal rate and improve surface quality of sapphire substrate, a series of novel La-doped colloidal SiO₂ composite abrasives were prepared by seed-induced growth method. The CMP performance of the La-doped colloidal SiO₂ composite abrasives on sapphire substrate were investigated using UNIPOL-1502 polishing equipment. The analyses on the surface of polished sapphire substrate indicate that slurries containing the La-doped colloidal SiO₂ composite abrasives achieve lower surface roughness, higher material removal rate than that of pure colloidal SiO₂ abrasive under the same testing conditions. Furthermore, the acting mechanism of the La-doped colloidal silica in sapphire CMP was investigated. X-ray photoelectron spectroscopy analysis shows that solid-state chemical reactions between La-doped colloidal SiO₂ abrasive and sapphire surface occur during CMP process, which can promote the chemical effect in CMP and lead to the improvement of material removal rate.     

抛光垫在蓝宝石衬底化学机械抛光中的应用研究

在分析化学机械抛光中常用抛光垫的材质、性能、表面结构基础上,研究了抛光垫对蓝宝石衬底抛光质量的影响规律:材质硬的抛光垫可提高衬底的平面度;材质软的抛光垫可改善衬底的表面粗糙度;表面开槽的抛光垫可提高抛光效率;表面粗糙的抛光垫可提高抛光效率;对抛光垫进行适当的修整可使抛光垫表面粗糙;用聚氨酯类抛光垫能够使得蓝宝石衬底的抛光面小于0.3nm的表面粗糙度.     

Estimating chemical mechanical polishing pad wear with compressibility

A polishing pad is an important component in a chemical mechanical polishing (CMP) system. Few investigations have specialized in the variation of the characteristics of the pad as it undergoes wear. All of the information concerning a pad, such as compressibility and pad life, comes from the manufacturer. No acknowledged standard or instrument exists for determining a pad’s quality. This study obtained the variation of the compressibility (K) of major types of pad (single-layer pad and composite pad) with polishing time (pad wearing) by theoretical modeling and real experiments. The K of single-layer or composite pads changes due to wear, and the trends in the K values of the two types of pads are exactly opposite. A finite element method (FEM) is used here to show that the variation in K strongly affects the polishing process. Theoretically, the compressibility of a pad can be used to judge whether the pad is good for polishing or not.