Metal Chemical Mechanical Polishing Research Articles

(Invited) Novel Brush Design for Post CMP Cleans

Chemical Mechanical Polishing (CMP) process is one of the key processes in semiconductor wafer processing. A sub 10nm wafer fabrication process may require more than 40 dielectric and metal CMP steps. As CMP process requirements get more stringent, CMP slurries are formulated with smaller and smaller metal oxide abrasive particles. The abrasive particles may also be surface modified or doped with other metal oxides. Post CMP cleaning chemistry formulations are designed to remove polish byproducts, pad residues and abrasive particles from wafer surface. Chemical composition and PH are tuned for modulating wafer surface-particle interactions so brushes can mechanically “sweep” them off the surface. PVA brushes, used of post CMP cleans, are a highly porous network of crosslinked polyvinyl acetate with pore size in the range of 30 – 100 um and 80% - 90% porosity, depending on the type of brush being used. While PVA brushes have largely met the process capability requirements, they have limitations for advanced cleans such as nano-particle removal. As CMP process continues to expand to new dielectric and metal films for different process steps in wafer fabrication, there is a need for customized brush material and surface design for optimal cleaning efficiency.We discuss a new engineered post CMP brush design that shows high efficiency in nano-particle cleans and allows high level of material tunability. New brush design is an engineered composite with an open cell microporous core, overlaid with engineered fibers in form of loops. Brush design is compatible with existing post CMP cleans. Figure 1(a) shows the new design, where in DIW enters the brush through the center of microporous core and flows outwards to the surface. As water exits the surface, it flows over and across the loops and contacts the wafer surface. Flow resistance through core and loops can be independently tuned, which allows better fluid distribution and more effective cleans. Figure 1(b) show a close-up schematic of interaction between loop and particle. Flexing of the loops on brush surface provides improved contact as well as cleaning ability Brush design can be optimized for chemical and mechanical properties with choice of fiber material, fiber diameter and loop height. Figure 2 shows the Zeta potential of PVA and different loop materials. The fiber material maybe selected such that zeta potential of the loops can enhance mechanical action in removing particle. Mechanical action of the Loops can be modeled as a cantilever and force on the particle is exerted by flexing of the loops.Deflection of loops can be approximated with flexure stiffness of rods with point load at free end. For initial purposed loop is approximated as single element. The equation can be rewritten to calculate cleaning force P at the free end for given loop deflection (Y), P = 3EIY/L3 (1)Where E is elastic modulus, I = moment, L = length of fiber and for rod shapes; I = Pi*(r4/4)The adhesion of particles on the surface is due to the effect of van der Waals and electrostatic forces. The total adhesion force on the surface can be written as: F=F vdw + F edl (2) where Ft is total adhesion force, Fvdw is the van der Waals force, and Fedl is the electrostatic force.Fvdw = AR/6r2 (3)Where A is the Hamaker constant, R is particle radius, r is the separation distanceFor effective removal of particles, P >/= F (4)Flex force and adhesion force were modeled based on equation (1) and (3) and are shown in Figure 3. New brush design was used for post CMP cleaning of silicon dioxide wafers polished with ceria slurry. Two different brush designs were tested for cleaning efficiency. Modeling data and comparison with experimental results is presented. Figure 1

Role of alumina particles in chemical-mechanical synergies in ruthenium polishing

Abrasive particles are usually considered to only play a "mechanical wear" role in the metal chemical mechanical polishing (CMP) process, neglecting their influence on chemical reactions. This study reveals the chemical role of Al2O3 particles in ruthenium (Ru) CMP by comparing the properties and polishing performance of Al2O3-SiO2 mixed slurry and pure SiO2 slurry. Several testing methods are used to investigate the mechanical and chemical actions of Al2O3 particles, including particle size distribution, Zeta potential, electrochemical reaction kinetics, oxide layer property and hydroxyl radical detection test. The results indicate that Al2O3 particles enhance the mechanical action by reducing the Zeta potential, and promote the chemical action by catalyzing hydrogen peroxide to produce hydroxyl radical (Fenton-like reaction) on Ru surface. These increase the material removal rate of Ru CMP with mixed slurry by nearly 5 times, while the surface roughness decreases by 60 %. The mechanism of chemical reaction promoted by increasing the hydroxyl radical concentration on Ru surface is confirmed from the aspects of increased corrosion current, oxide layer thickness and proportion of higher-valence Ru oxide (RuO2) in the oxide layer. Among them, the thickened oxide layer is a significant factor contributing to the substantial improvement in the material removal rate of Ru and the reduction in surface roughness. This study enhances the understanding of the chemical-mechanical synergistic mechanism of abrasive particles in metal polishing and elucidates the principle of this mechanism on promoting material removal and surface quality. It also provides new research ideas and theoretical guidance for efficient and high-quality metal polishing.

Metal Thickness Measurement System Based on a Double-Coil Eddy-Current Method With Characteristic Ratio Detection

Eddy-current sensor systems have been employed to determine the real-time thickness distribution of metal films and the thickness at endpoint detection in metal chemical mechanical polishing (CMP) processes, to achieve global planarization and avoid over-polishing or under-polishing. However, the large lift-off distance of 3.5 mm between the sensor coil and metal film gradually decreases with continuous wear of the polishing pad, which has a strong negative impact on the sensitivity and accuracy of the thickness measurement. In this study a double-coil eddy-current sensor system, with novel characteristic ratio detection capability, is proposed for metal film thickness measurement and compared with a traditional amplitude-based method. The linear relationship between the characteristic ratio and metal film thickness that is immune to lift-off distance is demonstrated both theoretically and experimentally. The experimental results show that the proposed method performs better than the amplitude-based method, reducing the effect of lift-off distance variation and obtaining an excellent linear measurement range from 24 to 2095 nm. The measurement accuracy is as high as 2.1 nm with an excitation frequency of 1.39 MHz and a large lift-off distance of 3.5 mm. The relative measurement error is less than 1.2% and the endpoint thickness ranges from 100 to 180 nm in the Cu-CMP process, when the lift-off distance varies from 3.6 to 3.3 mm.

Endpoint Detection Based on Optical Method in Chemical Mechanical Polishing.

Endpoint detection is an important technology in chemical mechanical polishing (CMP), which is used to capture the material interface and compensate the variations of consumables and incoming wafer thickness. This paper aimed to apply optical detection in metal CMP. An in situ optical measurement system was developed for a 12-inch CMP tool. Kinematic analysis of the scanning trajectory of the laser device indicated the relative position relationship between the device and the wafer. Average optical data within the wafer described the material removal of metal CMP. Furthermore, optical data and location described the non-uniformity of the entire wafer surface. In this research, the polishing condition and the residual of the wafer edge are characterized by optical trace. Pauta Criterion is used to discriminate the inflexion point of the material interface. The results reveal that the interface capture is accurate and effective.

Towards a deep understanding of oxidation in the material removal of GCr15 bearing steel during chemical mechanical polishing

Oxidation is critical to metal chemical mechanical polishing (CMP). In this work, the role of oxidation in the material removal of GCr15 bearing steel during CMP was thoroughly investigated. Six representative oxidizers were selected to acquire the material removal rate (MRR) data. There are three distinct MRR trends. The oxidizers can be divided into three categories based on the MRR and surface composition results. For the first, with the concentration of H2O2 or KIO3 increasing, the MRR first sharply increases, then decreases and stabilizes; For the second, as the concentration of KClO3 or NaClO increases, the MRR first increases and then levels off; For the third, with the concentration of K2S2O8 or Fe(NO3)3 increasing, the MRR almost linearly increases. According to the reaction kinetics and steady state, a fundamental MRR model considering iron oxidation and removal processes was built, and then applied to the three categories of oxidizers combined with the specific oxidation paths. The fitting results agree with the MRR data. Moreover, the key to realizing efficient removal of GCr15 steel is Fe2+ for H2O2, KIO3, KClO3 and NaClO, while Fe3O4 for K2S2O8 and Fe(NO3)3. The findings can deepen the understanding of oxidation in metal CMP.

Monitoring the Physicochemical Degradation of Polishing Pad Soaked in Hydrogen Peroxide during Chemical Mechanical Polishing

The porous polyurethane polishing pads are extensively used in the chemical mechanical polishing. The pad property is very important and influences the polishing effectiveness and the wafer surface quality. Hence, in the present work, the physicochemical degradation of unused polyurethane polishing pads during metal chemical mechanical polishing (CMP) in the presence of hydrogen peroxide (H2O2) was studied. It was found that the polishing pads were reacted chemically with H2O2 resulting in degradation of the polymer chain network of polishing pad and reduction in the pad elastic modulus. Pad polymer network chain scission was observed during FT-IR evaluation confirming the degradation of the pad. CMP experiments with the H2O2 soaked pads resulted in change in material removal rate (MRR) and the within-wafer non-uniformity (WIWNU). Further Pad elastic modulus estimated by colorimetric evaluation and CIE Lab color coordinates.

Defect reduction of replacement metal gate aluminum chemical mechanical planarization at 28 nm technology node

The replacement metal gate (RMG) defectivity control is very challenging for high-k metal gate (HKMG) RMG chemical mechanical polishing (CMP) processes. In this study, three major defect types, including fall on particle, micro-scratch and corrosion, post Al metal CMP process have been investigated to provide the total solutions for preventing the formation of these defects. The fall on particles were fully removed as a repulsive force created between the surfaces of the Al metal gate and polishing abrasives by introducing appropriate acid based slurries and chemical solutions. Furthermore, the micro-scratches could be significantly eliminated as implementing a reliable soft pad at the final polishing step. Severe integranular corrosion and pitting issues on the surface of the RMG structures were also found to be effectively prevented by creating a capable passivation layer with minimized micro-galvanic corrosion level on the top surface of the Ti-doped Al metal gate structures.

Process development of high-k metal gate aluminum CMP at 28nm technology node

The replacement metal gate (RMG) height range and defectivity (fall on particle, Al residue, corrosion and micro-scratch) performance controls are very challenging for high-k metal gate (HKMG) RMG chemical mechanical polishing (CMP) processes. In this study, a robust aluminum metal CMP (Al-CMP) process development was investigated to meet the criteria of RMG at 28nm technology node. The Al metal gate height loss post-Al-CMP was found to increase with increasing the total area and pattern density of metal gates. Adding at least one extra ghost metal gate dummy structure on both sides of each metal gate and optimizing the metal gate dummy structure can effectively eliminate the metal gate height loss. Furthermore, implementing a real-time profile control (RTPC) end-point detecting system with lower down force polishing condition can further reduce the Al metal gate height loss and range levels. The defectivity issues of the microscratches and fall on particles can be fixed by using a soft pad with chemical wafer buff process on the final Al-CMP polishing step.

Metal CMP Optimization Based on Chemically Formed Thin Film Analysis

The conventional demands for development in semiconductor industry are changing as the Moore's Law is approaching to its limits. This paper demonstrates a theoretical optimization approach for the planarization of metal films by Chemical Mechanical Polishing (CMP) process. Optimal removal rate and a smooth surface finish post CMP can be achieved by the combined effect of the chemical and mechanical components of the process. Metal CMP necessitates a protective oxide film formation in the presence of surface active agents, corrosives, pH regulators etc' to achieve global planarization. Formation and mechanical properties of the chemically modified films determine the stresses develop in the film structure delineating the stability of the chemically altered films on the surface of the metal wafer. The balance between the stresses built in the film structure versus the mechanical actions provided during the process can be used to optimize the process variables and furthermore help define new planarization techniques for the next generation microelectronic device manufacturing which is expected to deal with atomic level structures.

Study of Copper Residue Generated During Cu CMP Process

Copper (Cu) chemical-mechanical polishing (CMP) has been a critical technology for copper damascene manufacturing process for metal interconnects. This paper investigated the mechanism of the copper residue generated during Cu-CMP process. Both electrical testing and various physical measurement techniques have been extensively used for the characterization. For bulk Cu CMP process, the wear surface of the retaining ring caused reduction of the effective down pressure on the wafer, resulting in occurrence of residual copper. For barrier metal CMP process, the protrusion of the copper lines (negative oxide erosion) on the fine pitch line and space array was the main contributor to copper residue on the upper interconnection layer. This paper also examines the effects of slurry selectivity on topography correction and dielectric erosion, in multi-step Cu CMP processes. We proposed an optimal selectivity of Cu/Ta(tantalum)/SiO (0.6/1/0.8) of barrier metal polishing slurry to reduce copper residue between metal patterns and to ensure electrical isolation between neighboring circuit.

Tribological Behavior of Metal CMP and Detection of Process Abnormality

We examined a real-time coefficient of friction (COF) monitoring to evaluate the correlation between the pad surface response during polishing and material removal rate for metal chemical-mechanical polishing (CMP) applications. The results showed that a correlation of the removal rate and COF in tungsten (W) CMP was contrary to that in copper (Cu) CMP. Polishing by-products generated during CMP affects the friction force between the probing tip and the pad surface. The balance between the chemical and mechanical factors indicates the different frictional behavior between W CMP and Cu CMP. Furthermore, we detected the failures in the CMP process, which extracts an out-of-range deviation of the continuously monitored COF in product wafers, and reveal them in tantalum CMP. These results demonstrated the usefulness of an in situ CMP process monitor.

Cu CMP에서 Citric Acid가 재료 제거에 미치는 영향

Chemical mechanical polishing (CMP) achieves surface planrity through combined mechanical and chemical means. The role of slurry is very important in metal CMP. Slurry used in metal CMP normally consists of oxidizers, complexing agents, corrosion inhibitors and abrasives. This paper investigates the effects of citric acid as a complexing agent for Cu CMP with . In order to study chemical effects of citric acid, X-ray photoelectron spectroscopy (XPS) was peformed on Cu sample after etching test. XPS results reveal that CuO, layer decrease but layer increase on Cu sample surface. To investigate nanomechanical properties of Cu sample surface, nanoindentation was performed on Cu sample. Results of nanoindentation indicate wear resistance of Cu surface decrease. According to decrease of wear resistance on Cu surface removal rate increases from in Cu CMP.

텅스텐 CMP에서 산화제 영향에 관한 연구

Chemical mechanical polishing(CMP) has become the process of choice for modem semiconductor devices to achieve both local and global planarization. CMP is a complex process which depends on numerous variables such as macro, micro and nano-geometry of pad, relative velocity between pad and wafer stiffness and dampening characteristics of pad, slurry, pH, chemical components of slurry, abrasive concentration, abrasive size, abrasive shape, etc. Especially, an oxidizer of chemical components is very important remove a target material in metal CMP process. This paper introduces the effect of oxidizer such as <TEX>$H_2O_2,\;Fe(NO_3)_3\;and\;KIO_3$</TEX> in slurry for tungsten which is used in via or/and plug. Finally the duplex reacting mechanism of <TEX>$oxidizer(H_2O_2)$</TEX> through adding the <TEX>$catalyst(Fe(NO_3)_3)$</TEX> could acquire the sufficient removal rate in tungsten CMP.

CVD Rhenium and PVD Tantalum Gate MOSFETs Fabricated With a Replacement Technique

This letter reports the first replacement metal gate MOSFETs with chemical vapor deposition (CVD) Rhenium (Re), and physical vapor deposition (PVD) Tantalum (Ta) as the stacked gate electrode. Transistors with PVD Ta electrode are fabricated with a replacement and a nonself-aligned method for comparison. Our data show that CVD Re can be implemented as a gate electrode material for MOS transistors. The CVD Re process has the advantage of reducing the plasma and radiation damages to the gate oxide. A thick layer of PVD Ta covering a thin layer of CVD Re forms the stacked gate structure and makes the metal chemical-mechanical polishing feasible.

Chemical mechanical planarization for microelectronics applications

The progressively decreasing feature size of the circuit components has tremendously increased the need for the global surface planarization of the various thin film layers that constitute the integrated circuit (IC). Global planarization, being one of the major solutions to meet the demands of the industry, needs to be achieved following the most efficient polishing procedure. Chemical mechanical polishing (CMP) is the planarization method that has been selected by the semiconductor industry today. CMP, an ancient process used for glass polishing, was adopted first as a microelectronic fabrication process by IBM in the 80 s for SiO 2 polishing. To achieve efficient planarization at miniaturized device dimensions, there is a need for a better understanding of the physics, chemistry and the complex interplay of tribo-mechanical phenomena occurring at the interface of the pad and wafer in presence of the fluid slurry medium. In spite of the fact that CMP research has grown by leaps and bounds, there are some teething problems associated with CMP process such as delamination, microscratches, dishing, erosion, corrosion, inefficient post-CMP clean, etc.; research on which is still developing. The fundamental understanding of the CMP is highly necessary to characterize, optimize and model the process. The CMP process is ready to make a positive impact on 30% of the US$ 135 billion global semiconductor market. This paper presents an overview of CMP process in general, the science and mechanism of polishing, different metal and dielectric CMP processes. The impact of consumables on the CMP process, post-CMP cleaning, modeling of different CMP processes as well as the future trends are also discussed.

A chemical mechanical polishing model incorporating both the chemical and mechanical effects

A comprehensive model for the material removal in a chemical mechanical polishing (CMP) process is presented in which both chemical and mechanical effects are taken into consideration. The chemical effects come into play through the formation of chemically modified surface layer on the wafer surface that, in turn, is removed mechanically by the plastic deformation induced by slurry particles. This model describes the influence of most, if not all, variables involved in the CMP process including slurry characteristics (solid loading, particle size and distribution, modulus), pad properties (modulus, hardness, asperity sizes and distribution) and processing conditions (down-pressure, velocity). Although more elaborate experimental verification of the model is yet to follow, this model appears to be capable of explaining many experimental observations on both oxide and metal CMP that, otherwise, could not be explained properly.

A chemical mechanical polishing model incorporating both the chemical and mechanical effects

A comprehensive model for the material removal in a chemical mechanical polishing (CMP) process is presented in which both chemical and mechanical effects are taken into consideration. The chemical effects come into play through the formation of chemically modified surface layer on the wafer surface that, in turn, is removed mechanically by the plastic deformation induced by slurry particles. This model describes the influence of most, if not all, variables involved in the CMP process including slurry characteristics (solid loading, particle size and distribution, modulus), pad properties (modulus, hardness, asperity sizes and distribution) and processing conditions (down-pressure, velocity). Although more elaborate experimental verification of the model is yet to follow, this model appears to be capable of explaining many experimental observations on both oxide and metal CMP that, otherwise, could not be explained properly.

CMP of ultra low- k material porous-polysilazane (PPSZ) for interconnect applications

In this article, we investigated the impact of chemical mechanical polishing (CMP) on an ultra low dielectric constant (ultra low- k) material Porous-Polysilazane (PPSZ) with slurries of metal polishing during interconnect manufacture process. Since the CMP processing of metals such as TaN and Cu are inevitable steps for interconnect fabrication, we have utilized two types of slurries (marked as TaN and Cu slurries) to evaluate their effects on the dielectric properties of PPSZ films. Electrical and material analyses have shown surface planarity and dielectric properties of PPSZ films will not be degraded during these metal CMP processes. This indicates that the ultra low- k PPSZ films are promising for inter-level dielectric (ILD) applications in ultra large-scale integrated circuits (ULSI) technology.

A Physical Model for Dishing during Metal CMP

A physically based model for dishing during metal chemical mechanical polishing (CMP) is presented. The control of dishing is important for advanced damascene metal interconnect technologies. However, the modeling of dishing is still in an initial phase. Material removal mainly takes place at the contact areas between the rough polish pad and the substrate. Therefore, a numerical model of the contact area of the pad and wafer, and the number of contacts and the size distribution of the contact areas as function of polish parameters and pad properties, was developed. The number of contacts appeared directly proportional to the applied pressure. In addition, the contact size distribution was strongly dependent on pad morphology. It was weakly dependent on the applied pressure and the Young’s modulus of the pad. Thereafter the dishing phenomenon was modeled using these results. For an arbitrary linewidth, the metal was removed by two groups of pad asperities, one group with contact size smaller than the linewidth (group S) and one with contact size larger than the metal linewidth (group B). By considering the material removal rate of these two groups of asperities separately and using a modified form of Preston’s law, a model was developed that describes the dishing during (over)polishing for an arbitrary metal line. The model contains one free parameter. The model was compared with experimental results obtained from state-of-the-art copper CMP. Excellent agreement was observed between data and the model. © 2003 The Electrochemical Society. All rights reserved.

The application of chemical–mechanical polishing for planarizing a SU-8/permalloy combination used in MEMS devices

Abstract Chemical–mechanical polishing (CMP) allows the planarization of wafers with two or more materials at its surface. Subject of this investigation was to apply CMP for planarizing electroplated permalloy (NiFe 81/19) in micromolds formed by photosensitive epoxy (SU-8). This material remained in the structure as an insulation and planarization layer. This material combination was intended for fabricating magnetic Micro Electro-mechanical Systems (MEMS). For planarizing a SU-8/permalloy combination, metal CMP slurry was used in conjunction with standard CMP polishing pads. SU-8 effectively served as a polishing stop. For this reason it was necessary to optimize the SU-8 properties to exhibit an optimal hardness and wear resistance. The mechanical properties of the SU-8 depended on the thermal treatment during hard bake after development. The resulting SU-8 layer not only had a sufficient hardness to stop the removal of permalloy, but also could be planarized quite well.