Effect of mechanical process parameters on friction behavior and material removal during sapphire chemical mechanical polishing

The effect of mechanical process parameters such as down force and rotation speed on friction behavior and material removal rate (MRR) was investigated during chemical mechanical polishing (CMP) of sapphire substrate. It was found that the increase in both rotation speed and down force can enhance the MRR and friction force almost linearly depends on the down force and rotation speed. The coefficient of friction (COF) decreases with increasing rotation speed under a fixed down force but keeps constant regardless of variation in down force under a fixed rotation speed. Moreover, the relationship between friction force and MRR was obtained. MRR was proportional to friction force with increasing down force whereas converse proportional to that with increasing the rotation speed. In addition, MRR data are fitted to the Preston equation in the sapphire CMP.     

SiLK dielectric planarization by chemical mechanical polishing

As the feature size of integrated circuits is driven to smaller dimensions the importance of the inter- and intralayer isolator capacitance in future metallization schemes becomes more pronounced. Organic polymers with low dielectric constants are one class of material choice for the replacement of SiO₂. However, their successful integration into functional circuits requires new fabrication procedures. The embedded dielectric scheme offers an evolutionary path for their successful integration into a subtractive etched, aluminum-based integrated circuit. This scheme can effectively lower the total capacitance while minimally changing the rest of the metallization fabrication process. However, the non-conformal deposition of spin-on polymers requires an effective planarization process. Therefore, this paper focuses on the planarization capability of a chemical mechanical polishing process (CMP) using SiLK resin as the interlayer dielectric material. The experimental results demonstrate the high planarization capability of the CMP process using a commercially available slurry. The post-CMP degree of planarization is greater than 95% for all feature dimensions and this planarity can be achieved rapidly. SiLK dielectric coatings are therefore considered as a promising candidate to replace SiO₂ in existing Al/W-based technologies.     

Effects of process parameter variations on the removal rate in chemical mechanical polishing of 4H-SiC

The effects of temperature, slurry pH, applied pressure, and polishing rotation rate on the material removal rate during chemical mechanical polishing (CMP) of 4H-silicon carbide wafers using colloidal silica slurry and polyurethane/polyester fiber polishing pads have been studied. Measured removal rates varied from around 100 Å/hr to nearly 2500 Å/hr depending on the values of the various parameters. The amount of material removed was determined by measuring the wafer mass before and after polishing. Variations in temperature and slurry pH did not produce significant changes in the measured removal rates. Higher polishing pressures resulted in increased material removal rates from 200 to 500 Å/hr but also produced excessive polishing pad damage. Variations in pad rotational speeds produced the largest changes in material removal rates, from around 200 to around 2000 Å/hr for rotational speeds between 60 and 180 rpm, but the variations were non-linear and somewhat inconsistent. This CMP formula is shown to consistently produce damage free surfaces but the optimum removal rate is slow.     

A model for wafer scale variation of removal rate in chemical mechanical polishing based on elastic pad deformation

It is well known that within wafer non-uniformity (WIWNU), due to the variation in material, removal rate (MRR) in the whole wafer plays an important role in determining the quality of a wafer planarized by CMP. Various material removal models also suggest that the MRR is strongly influenced by the interface pressure. In the present work, an analytical expression for pressure distribution at the wafer and pad interface is developed. It is observed that depending on the wafer curvature and polishing conditions, the interface pressure may exhibit significant variation. The analytical model predictions are first verified against finite element method (FEM) simulations. The predicted analytical pressure profiles are then utilized in Preston's equation to estimate the MRR, and these MRR predictions are also compared to experimental observations. The analytical results suggest, that for a specified wafer curvature there exists a certain polishing condition (and vice versa) that will enable holding the WIWNU within a specified tolerance band. The proposed model facilitates the design space exploration for such optimal polishing conditions.     

化学机械抛光中的硅片夹持技术

目前半导体制造技术已经进入0.1 3μm时代,化学机械抛光(CMP)已经成为IC制造中不可缺少的技术.本文描述了下一代IC对大尺寸硅片(≥300mm)局部和全局平坦化精度的要求,介绍了目前工业发达国家在化学机械抛光加工中硅片夹持技术方面的研究现状,分析了当前硅片夹持技术中存在的问题,并指出了未来大尺寸硅片超精密平坦化加工中夹持与定位技术的发展趋势.     

液体喷射抛光技术研究

液体喷射抛光(FJP)技术是近几年提出的一种新型光学抛光工艺,本文简介了这种抛光技术的原理,介绍了我们的研究结果,其中包括在抛光机理方面提出抛光液中磨料粒子的径向流动对工件产生的径向磨削剪切作用是材料去除的关键;建立了较完善的描述FJP过程的数学模型;提出了获得较理想工作函数的方法;得到了求解时间驻留函数的一种算法;另外还研究了不同FJP溶液及一些工艺参数对样品表面粗糙度的影响、被抛光材料特性对材料去除率及表面粗糙度的影响等.结合这些研究建立了一套非球面数控液体喷射抛光没备和相关工艺,并利用其对实际非球面做了数控抛光实验,最后得到其截面的误差优于0.15靘PV,表面粗糙度Ra达到2.25nm,实验结果表明此技术可用于非球面的数控抛光.     

Experimental investigation on mechanisms of silicon chemical mechanical polishing

In this research, we conducted a series of experiments to investigate the mechanisms of chemical mechanical polishing (CMP) of silicon. Experimental approaches include tribological tests of frictional and lubricating behavior, chemical analysis, and surface characterization. Specifically, the effects of pH in slurry, surface roughness of wafers, and nano-particle size on removal rate were studied. A transmission electron microscope (TEM), a scanning electron microscope (SEM), and x-ray characterization tools were used to study the change of surface structure and chemistry. Experimental results indicate that the removal rate and planarization are dominated by the surface chemistry.     

Effect of organic acids on copper chemical mechanical polishing

In chemical mechanical polishing (CMP) of Cu, organic acids are often used as additives of slurries. This paper studied the effects of citric acid, oxalic acid, glycolic acid and glycine on Cu CMP performance. Our experiments explored the difference of these organic acids in surface reactions with Cu. The results showed that organic acids could chelate the passive film of Cu, and oxalic acid would further form precipitates with copper ions to change the chemical and mechanical action during CMP. Potential-pH diagrams, electrochemical polarization and impedance analyses were used to examine the behaviors of Cu in various organic acid slurries. The results indicated that the proposed equivalent circuits from impedance analysis for Cu CMP system could provide a good index to surface roughness. Furthermore, we also discussed the effects of used organic acids on reducing particle contamination after Cu CMP by measuring the difference of isoelectric points between Cu and α-Al₂O₃. The result showed that the addition of organic acid could efficiently decrease particle contamination.     

A model for wafer scale variation of material removal rate in chemical mechanical polishing based on viscoelastic pad deformation

It is well known that within-wafer nonuniformity (WIWNU) due to the variation in material removal rate (MRR) in chemical mechanical polishing (CMP) significantly affects the yield of good dies. The process control for a batch CMP operation is further complicated by wafer-to-wafer nonuniformity (WTWNU) caused by MRR decay when a number of wafers are polished with the same unconditioned pad. Accordingly, the present work focuses on modeling the WIWNU and WTWNU in CMP processes. Various material removal models suggest that the MRR is strongly influenced by the interface pressure. It is also well known that the viscoelastic properties of the pad play an important role in CMP. In the present work, an analytical expression for pressure distribution (and its associated MRR) at the wafer-pad interface for a viscoelastic pad is developed. It is observed that under constant load, which is typical during main polishing in CMP, the spatial distribution of the interface pressure profile may change with time from edge-slow to edge-fast, depending on the combination of wafer curvature, down pressure, and pad properties. For constant displacement operations, the pressure profile retains its edge-slow or edge-fast characteristics over time. The analytical model predictions of MRR based on viscoelastic pad properties also correlate very well to existing experimental observations of MRR decay when an unconditioned pad is used to polish a number of wafers. Based on these observations, it may be conjectured that the viscoelastic material properties of the pad play a primary role in causing the observed MRR decay. The analytical results obtained in the present work can also provide an estimation of evolution of thickness removal distribution over the entire wafer. This may be used for determining the optimum thickness of the overburden material and its polishing time, and for effective control of CMP processes.     

Size effect of nanoparticles in chemical mechanical polishing - a transient model

When a workpiece to be polished is placed on the carrier of a polishing machine, it is pressed down to the polishing pad. Large abrasives make contact between the pad and the workpiece before the smaller ones. The larger abrasives are pressed into the pad and indented into the workpiece. These particles are the active abrasives and participate in material removal. The abrasives with a size less than the gap between the pad and the workpiece move freely in the valleys/voids of the pad and are inactive. As the gap decreases during the polishing process, smaller abrasives trapped between the pad and the workpiece become active in polishing. Thus, the process of chemical-mechanical polishing is dynamic, while all previous modeling is static. This paper establishes a dynamic model for the abrasives. The modeling considers the transient motion of the workpiece/particle/pad in the vertical direction and the change of the roughness of the workpiece. A study of the transient motion shows an increasing number of active particles and a changing polishing rate in the first 2 min. It also demonstrates that the viscoelastic properties of the pad and the workpiece surface roughness are important factors in determining the polishing rate. This paper also shows that when the average particle size is smaller than an optimum size, the polishing rate increases with increasing particle size for the same particle density or same wt% abrasives. Yet, if the average particle size is larger than the optimum size, the polishing rate decreases with increasing particle size.     

超大规模集成电路制造中硅片化学机械抛光技术分析

目前半导体制造技术已经跨入0.13μm 和300mm时代,化学机械抛光(CMP)技术在ULSI制造中得到了快速发展,已经成为特征尺寸0.35μm以下IC制造不可缺少的技术。CMP是唯一能够实现硅片局部和全局平坦化的方法,但CMP的材料去除机理至今还没有完全理解、CMP系统过程变量和技术等方面的许多问题还没有完全弄清楚。本文着重介绍了化学机械抛光材料去除机理以及影响硅片表面材料去除率和抛光质量的因素。     

Damageless Cu chemical mechanical polishing for porous SiOC/Cu interconnects

Both chemical and mechanical damages to porous SiOC film should be minimized in the Cu-CMP (chemical mechanical polishing) process for the 32-45 nm node Cu interconnect process. This paper first discusses chemical damage that occurs during direct CMP on a porous SiOC film. We found that the k-value increase after direct CMP was caused by the surfactants added to the cleaning chemicals to suppress watermark generation on the hydrophobic SiOC film surface. The surfactants assisted water molecule diffusion into the pores by improving the wettability of the film surface. N₂ annealing after direct CMP removed moisture inside the pores and restored the k-value increase. Second, the paper discusses low-pressure electro-CMP (e-CMP) technology that we developed to reduce mechanical stress on the porous SiOC film. A high removal rate and good planarization performance were obtained by optimizing the cathode area of the electro-cell and carbon material of the e-CMP pad.     

Aluminum-germanium-copper multilevel damascene process usinglow-temperature reflow sputtering and chemical mechanical polishing

A low-temperature multilevel aluminum-germanium-copper (Al-Ge-Cu) damascene technology was developed using reflow sputtering and chemical mechanical polishing (CMP). The maximum processing temperature for the fabrication of multilevel interconnections could be reduced to 420°C using Al-1%Ge-0.5%Cu, whereas the conventional reflow temperature was not less than 500°C. No degradation due to reflow heat cycles was observed in terms of Al-Ge-Cu wiring resistance. Electromigration test results indicated that the mean time to failure (MTTF) of Al-1%Ge-0.5%Cu was longer than 10 years at the operating condition, which was equivalent to that of Al-1%Si-0.5%Cu. The Al-1%Ge-0.5%Cu triple-level interconnection was fabricated using reflow sputtering to fill vias and wiring trenches and subsequent CMP for Al-Ge-Cu films     

Effect of abrasive particle concentration on preliminary chemical mechanical polishing of glass substrate

Effect of abrasive particle concentration on material removal rate (MRR), MRR per particle and the surface quality in the preliminary chemical mechanical polishing (CMP) of rough glass substrate was investigated. Experimental results showed that the MRR increases linearly with the increase of abrasive concentration and reaches to the maximum when the abrasive concentration is 20 wt.%, and then tends to be stable. When the abrasive concentration increases from 2 to 5 wt.%, the MRR per particle increases greatly and reaches a peak. Then the MRR per particle decreases almost linearly with the increase of the abrasive concentration. The root mean squares (RMS) roughness almost decreases with increasing particle concentration. In addition, in situ coefficient of friction (COF) was also conducted during the polishing process and the zeta potentials of abrasive particles in slurry with different solid concentration were also characterized. Results show that COF value is not related to zeta potential but be sensitive to glass surface conditions in terms of rough peaks in preliminary polishing of glass substrate.     

Effects of abrasive size distribution in chemical mechanical planarization: modeling and verification

Recently, a comprehensive model has been developed by Luo and Dornfeld ("Material removal mechanism in chemical mechanical polishing: theory and modeling", IEEE Trans. Semiconduct. Manufact., vol. 14, pp. 112-133, May 2001) to explain the material removal mechanism in chemical mechanical planarization (CMP). Based on the model, the abrasive size distribution influences the material removal from two aspects, one, the number of active abrasives, and the other, the size of the active abrasives. In this paper, experimental evidence supporting this view of abrasive size effects is discussed. A detailed model of wafer-abrasive-pad contact is developed to explain how and where abrasive size distributions come into the comprehensive material removal model. A material removal rate formulation as a function of abrasive size distribution is proposed and verified. In the future, the application of the model to the CMP process optimization, for example, improving the nonuniformity, or obtaining minimum surface scratching and preferred material removal rate by changing abrasive size distribution, may be attempted.     

Ultra-smooth BaTiO3 surface morphology using chemical mechanical polishing technique for high-k metal-insulator-metal capacitors

The surface roughness of barium titanate (BTO) following its implication by aerosol deposition method (ADM), is a very important characteristic affecting its potential for use in high-k metal-insulator-metal capacitors. The ADM is the best candidate to deposit ceramic films but has two major problems: macroscopic defects and rough interface effects on the BTO surface. In this work, a chemical mechanical polishing (CMP) technique is applied to obtain an ultra-smooth BTO surface morphology by the optimization of several factors including the slurry type, the head rotational speed, and the down pressure. Statistically, we were able to achieve a root mean square (RMS) value of the BTO surface of 1.746 nm by utilizing a two-step polishing process, applied at a head rotational speed of 70 rpm under 5 kg/cm² of down pressure; this RMS value is improved at least 8 times over previous studies. This analysis is based on representative pattern images, three-dimensional images, line profiles, histograms, and power spectra of selected BTO surface areas, further verified with data from both energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy.     

Electrochemical corrosion effects and chemical mechanical polishing characteristics of tungsten film using mixed oxidizers

In this paper, the effects of mixed oxidizers on tungsten-chemical mechanical polishing (W-CMP) process were studied using three different kinds of oxidizers such as Fe(NO₃)₃, KIO₃ and H₂O₂. Moreover, the interaction between the tungsten film and the oxidizer was discussed by potentiodynamic polarization test, in order to compare the CMP performances and electrochemical behavior of the tungsten film as a function of mixed oxidizers. The potentiodynamic polarization results indicated that the corrosion current densities of the 5 wt% H₂O₂ and 5 wt% H₂O₂ + 5 wt% Fe(NO₃)₃ were higher than the other mixed oxidizers. Such an electrochemical corrosion effect implies that slurries with the highest removal rate have a high dissolution rate at lower pH. Therefore, we conclude that W-CMP performances are strongly dependent on the kind of oxidizers and the amounts of oxidizer additive.     

Platinum chemical mechanical polishing (CMP) characteristics for high density ferroelectric memory applications

The key component of ferroelectric random access memory (FeRAM) is a capacitor including a ferroelectric thin film and electrode materials. Platinum is one of the suitable metals which meet requirements such as low resistivity, high thermal stability, and good oxygen resistance. Generally, the ferroelectric and the electrode materials were patterned by a plasma etching process. The application possibility of chemical mechanical polishing (CMP) processes to the patterning of ferroelectric thin film instead of plasma etching was investigated in our previous study for improvement of an angled sidewall which prevents the densification of FeRAM. In this study, the characteristics of platinum CMP for FeRAM applications were also investigated by an approach as bottom electrode materials of ferroelectric material in CMP patterning. The removal rate was increased from 24.81 nm/min by the only alumina slurry (0.0 wt% of H₂O₂ oxidizer) to 113.59 nm/min at 10.0 wt% of H₂O₂ oxidizer. Electrochemical study of platinum and alumina slurry with various concentrations of H₂O₂ was performed in order to investigate the change of the removal rate. The decreased particle size in the alumina slurry with an addition of 10.0 wt% H₂O₂ oxidizer made the improved surface roughness of the platinum thin films. Micro-scratches were observed in all polished samples.     

大直径硅晶片化学机械抛光及其终点检测技术的研究与应用

化学机械抛光是硅片全局平坦化的核心技术,然而在实用阶段上,这项技术还受限于一些制造系统整合上的问题,其中有效的终点检测系统是影响抛光成效的重要关键.若未能有效地监测抛光运作,便无法避免硅片产生抛光过度或不足的缺陷.本文在介绍CMP机制与应用的基础上,系统分析了CMP终点检测技术的研究现状及存在的问题.     

High-quality conformal silicon oxide films prepared by multi-step sputtering PECVD and chemical mechanical polishing

High-quality conformal oxide films were obtained by using multi-step sputtering (MSSP) plasma enhanced chemical vapor deposition (PECVD) process with argon ion sputtering and chemical mechanical polishing (CMP). The repeated deposition by plasma enhanced chemical vapor deposition (PECVD) and anisotropic etching of oxide films by multi-step sputtering PECVD improve the step coverage and gap filling capability significantly. The argon plasma treatment enhances the binding energy of Si-O in the SiO₂ network, and the temperature dependence of stress for MSSP oxide film showed no hysteresis after the heating cycle up to 440 °C. The stress-temperature slope of MSSP oxide film was found to be much less than that of conventional PECVD oxide film. The slope for 1.1 μm thick film is about 5.8×10⁵ dynes/cm²/°C which is smaller than that of thermally grown oxide film. It seems that MSSP oxide film reduces stress-temperature hysteresis and becomes more dense and void-free in the narrow gaps with inter-metal spacing of 0.5 μm. After filling of the narrow gap, we adopted the CMP process for global planarization and obtained good planarization performance. The uniformity of the film thickness was about 4% and the degree of the planarization was over 95% after CMP process.