Abstract This paper proposes a model that integrates the microscale asperity contact and macroscale elastohydrodynamic lubrication (EHL) to simulate the pressure distribution in the chemical mechanical planarization (CMP). This model involves modified Reynolds equation used to describe the status of fluid field, the equation of the average asperity contact pressure by using statistics for solid contact pressure due to asperity contact, and the equation of the elastic pad deformation in bulk. Results show that with increasing relative velocity or load, the magnitude of the sub-ambient pressure decreases, the greater asperity contact pressure is formed to support the load, and the friction force also increases to cause the greater rotation angles. The magnitude of the fluid pressure is of the same order of magnitude as the applied normal load. Therefore, the addition of this fluid pressure may significantly change the distribution of the contact stress. The reason of the sub-ambient pressure existed is the deformation of the pad. In the material removal rate model, the elastic deformation of asperities is assumed, and the contact pressure is determined by Hooke¡¦s law. The indentation depth can be obtained from the force balance imposed on the particles by the wafer and the pad. Results show that the material removal rate decreases with increasing abrasive size, due to the increasing contact area between the abrasive and wafer. Keywords¡GElastohydrodynamic Lubrication, Chemical Mechanical Polishing

The Chemical-mechanical polishing (CMP) process is now widely employed in the ultralarge scale integration chip fabrication. Due to the continuous advances in semiconductor fabrication technology and decreasing sub-micron feature size, the characterization of erosion, which affects circuit performance and manufacturing throughput, has been an important issue in Cu CMP. In this paper, the erosion in Cu CMP is divided into two levels. The wafer-level and die-level erosion models were developed based on the material removal rates and the geometry of incoming wafers to the Cu CMP process, including the Cu interconnect area fraction, linewidth and Cu deposition thickness. Experiments were conducted to obtain the selectivity values between the Cu, barrier layer and dielectric, and the values of within-wafer material removal rate ratio, β, for the validation of the new erosion model. It was compared with the existing models and was found to agree better with the experimental data. / Singapore-MIT Alliance (SMA)

Long the dominant method of wafer planarization in the integrated circuit (IC) industry, chemical-mechanical polishing is starting to play an important role in microelectromechnical systems (MEMS). We present an experiment to characterize a polysilicon CMP process with the specific goal of examining MEMS sized test structures. We utilize previously discussed models and examine whether the same assumptions from IC CMP can be made for MEMS CMP. We find that CMP at the MEMS scale is not just pattern density dependent, but also partly dependent on feature size. Also, we find that new layout designs relevant to MEMS can negatively impact how well existing CMP models simulate polishing, motivating the need for further model development. / Singapore-MIT Alliance (SMA)

A formidable challenge in the present multi-step Cu CMP process, employed in the ultra-large-scale integration (ULSI) technology, is the control of wafer surface non-uniformity, which primarily is due to dielectric erosion and Cu dishing. In contrast with the earlier experimental and semi-theoretical investigations, a systematic way of characterizing and modeling dielectric erosion in both single- and multi-step Cu CMP processes is presented in this paper. Wafer- and die-level erosion are defined, and the plausible causes of erosion at each level are identified in terms of several geometric and physical parameters. Experimental and analytical means of determining the model parameters are also outlined. The local pressure distribution is estimated at each polishing stage based on the evolving pattern geometry and pad deformation. The single-step model is adapted for the multi-step polishing process, with multiple sets of slurry selectivities, applied pressure, and relative velocity in each step. Finally, the effect of slurry-switching point on erosion was investigated for minimizing dielectric erosion in the multi-step Cu CMP. Based on the developed multi-step erosion model, the physical significance of each model parameter on dielectric erosion is determined, and the optimal polishing practices for minimizing erosion in both multi-step and single-step polishing are suggested. / Singapore-MIT Alliance (SMA)

The present success in the manufacture of multi-layer interconnects in ultra-large-scale integration is largely due to the acceptable planarization capabilities of the chemical-mechanical polishing (CMP) process. In the past decade, copper has emerged as the preferred interconnect material. The greatest challenge in Cu CMP at present is the control of wafer surface non-uniformity at various scales. As the size of a wafer has increased to 300 mm, the wafer-level non-uniformity has assumed critical importance. Moreover, the pattern geometry in each die has become quite complex due to a wide range of feature sizes and multi-level structures. Therefore, it is important to develop a non-uniformity model that integrates wafer-, die- and feature-level variations into a unified, multi-scale dielectric erosion and Cu dishing model. In this paper, a systematic way of characterizing and modeling dishing in the single-step Cu CMP process is presented. The possible causes of dishing at each scale are identified in terms of several geometric and process parameters. The feature-scale pressure calculation based on the step-height at each polishing stage is introduced. The dishing model is based on pad elastic deformation and the evolving pattern geometry, and is integrated with the wafer- and die-level variations. Experimental and analytical means of determining the model parameters are outlined and the model is validated by polishing experiments on patterned wafers. Finally, practical approaches for minimizing Cu dishing are suggested. / Singapore-MIT Alliance (SMA)

Electrochemical Mechanical Polishing (ECMP) has become increasingly important due to the continuous decrease of the device size in integrated circuit (IC) fabrication. Tantalum (Ta) is a promising material as a substitute for copper in ICs. This dissertation studies the tribology and electrochemistry of Ta ECMP. The present research uses experimental combined analysis approaches. A specially designed experimental setup assembling a tribometer and a potentiostat was used to carry out Ta ECMP. The friction force and electrochemical reactions were measured simultaneously. Using this setup, we found the factors which affected the frictional behaviors of Ta during ECMP. The technique of single frequency electrochemical impedance spectroscopy (EIS) was employed to investigate the material removal mechanisms in Ta ECMP. The results presented the competing mechanisms of removal and formation of a surface oxide layer of Ta. In order to further the investigation in a nanoscale, the atomic force microscope (AFM) was used to measure the material removal rate. The Preston equation for the Ta ECMP was established. A new methodology was developed to study the oxidation state and process of Ta during ECMP. Through comparing the material removal rate measured by using the AFM and the calculated one via the Faraday’s law, the distribution of the Ta suboxides and pentoxide, as well as the oxidation process, was revealed. The oxidation process was strongly dependent of the applied anodic potential, thickness of the oxide layer, mechanical forces, and surface orientation. A polymer environmental cell was designed and produced. Using this cell and AFM, it was found that the material removal in the nanometer scale was a function of the surface orientations. This research is beneficial for optimization of the Ta ECMP process. This dissertation includes six chapters. After Introduction and Motivation and Objectives, the material, setup, and testing conditions are discussed in Chapter III. Chapter IV discusses the tribology and material removal mechanisms in Ta ECMP, while Chapter V the oxidation of Ta during ECMP, followed by Conclusions and Future Work.

ABSTRACT The polishing stocks used in various ultra-precision polishing machines consist of abrasives, polishing disk (pad), and polishing fluids. They are expendable goods. To ensure the machining ability and the repeat accuracy of machining characteristics, the polishing disc (pad) must use the dressing mechanism to produce sharp new grains. As a result, the grinding surface on the abrasive wheel becomes thinner gradually, then losses it¡¦s machining ability, and finally must be changed. Hence, in this project, an idea of an ultra-precision abrasive machining is proposed by using the continuous composite electroplating on the polishing disc. In this idea, the machining ability of Cu polishing disc can be ensured due to the use of the continuous Sn-Al2O3 composite electroplating. Hence, it can save the cost of the ultra-precision machining using in the semiconductor wafer. In this study, after 60 minutes continuous composite electroplated polishing, the thickness of the composite coating on the surface of Cu polishing disc can increase 6.13£gm. It means the surface of disc can be grew and renewed at every moment. The removal amount of the wafer is 10.8£gm. The surface of wafer was Ra=0.5453£gm and Rmax=5.464£gm at the start ,but came to Ra=0.0019£gm and Planess=2.649£gm/36mm after 60 minutes polishing.

The Chemical Mechanical polishing (CMP) process is now widely employed in the Integrated Circuit Fabrication. However, due to the complexity of process parameters on the material removal rate (MRR), mechanism of material removal and pattern effect are not well understood. In this paper, three contact regimes between the wafer surface and the polishing pad were proposed: direct contact, mixed or partial contact, and hydroplaning. The interfacial friction force has been employed to characterize these contact conditions. Several polishing models are reviewed with emphasis on the mechanical aspects of CMP. Experiments have been conducted to verify the mechanical polishing models and to identify the dominant mechanism of material removal under typical CMP conditions. / Singapore-MIT Alliance (SMA)

In this work, membrane set(s) had been incorporated into different crossflow electrofiltration (CEF) /electrodialysis (ED) treatment modules for treating various CMP wastewaters and simultaneously generating two streams of electrolytic ionized water (EIW). In general, CMP wastewaters have high alkalinity, turbidity, total solids content and silica content. In this investigation, CMP wastewaters were obtained from two wafer fabs in Taiwan and characterized by various standard methods. Then they were treated by the aforementioned treatment modules. Experiments were carried out based on the fractional factorial design and the L8 orthogonal arrays of the Taguchi method. Experimental factors such as electric field strength, transmembrane pressure for CEF, etc. were used to investigate their effects on the permeate qualities (i.e., oxidation-reduction potential, pH, etc.). According to the results of analysis of normal probability plots, analysis of variance (ANOVA) and regular analysis, the electric field strength was presumed to be a very significant parameter. Experimental results showed that filtrate flux increased with the increasing applied electric field strength. The permeate has a turbidity of below 1 NTU, TOC of below 3 mg/L, and TDS of below 250 mg/L under various operating conditions. Other permeate qualities were 15~22 mg/L of K, 53~68 mg/L of silica, 2~4 mg/L of NH4+ and 134~680 £gS/cm of electrical conductivity. But the values of electrical conductivity, pH, and oxidation-reduction potential (ORP) varied substantially for the anolyte EIW and catholyte EIW. Using these novel treatment modules, the optimal ORP and pH values of the anolyte EIW were 211.8 mV, 4.52 and 214.1 mV, 4.83, respectively, for single- and multi-membrane sets. The optimal ORP and pH values of the catholyte EIW were -165.0 mV, 11.21 and -172.0 mV, 10.81, respectively, for single- and multi-membrane sets. It is clear that permeate obtained in this study is suitable for high-level recycling. To further upgrade the water quality of permeate obtained above, a reverse osmosis (RO) unit was added to the treatment system. The water quality of silica for post-RO permeate were decreased from 53.7 to 0.98 mg/L for the anolyte EIW and from 68.05 to 1.32 mg/L for the catholyte EIW. The removal rates of Na and K by the RO unit were not significant. In addition, other unique properties of EIW (e.g., pH, ORP, and cluster size of water molecules) remained almost the same in post-RO permeate. The total recovery rate of the treated water could be above 85%. Therefore, the treated water at this stage could be reused as the cleaning media for the wafer surfaces or reused for the DI water production apparatus.

In this study, two types of chemical mechanical polishing wastewaters (designated Cu-CMP wastewater and mixed-CMP wastewater, respectively) from a wafer fabrication plant was treated by a simultaneous electrocoagulation/electrofiltration (EC/EF) process using laboratory-prepared TiO2/Al2O3 composite membranes. First, tubular membrane supports of Al2O3 were prepared by the extrusion method. Then the slip composed of nanoscale TiO2 (prepared by sol-gel process) and 1 wt% of corn starch was applied on the aforementioned tubular membrane supports by the dip-coating method, followed by sintering to obtain tubular TiO2/Al2O3 composite membranes. These tubular inorganic composite membranes then were incorporated into an EC/EF treatment module for the treatment of CMP wastewaters. The permeate qualities were evaluated. In addition, the effects of different operating modes (i.e., the flow-through mode and recirculation mode) on membrane flux and permeate quality were conducted. Finally, the effects of changing the backwash time and backwash cycle on the membrane flux were also investigated. Experimental results have shown that the slip containing 75 v/v% of TiO2 sol and 25 v/v% of corn starch solution would yield a membrane layer with a thickness of 13 £gm and a pore size of 15 nm. On the CMP wastewater treatment, the removal efficiencies of copper ion and total organic carbon (TOC) were found to increase with the increasing electric field strength. This relationship, however, did not apply to other water quality items. Under the optimal operating conditions of using the recirculation mode, the removal efficiencies for turbidity and TOC for Cu-CMP wastewater were determined to be 98% and 90%, respectively. Similarly, a turbidity of < 1 NTU (a removal efficiency of 99%) was obtained for mixed-CMP wastewater. By using the same optimal operating conditions for the recirculation mode to treat Cu-CMP wastewater, initial fluxes of 300 L/h¡Em2 and 280 L/h¡Em2 were obtained for the flow-through mode and recirculation mode, respectively. The corresponding initial fluxes for mixed-CMP wastewater were 370 L/h¡Em2 and 360 L/h¡Em2, respectively. For the case of the recirculation mode, the removal efficiencies of total solids content, silicon, copper ion, TOC, and turbidity for Cu-CMP wastewater were 71%, 85%, 72%, 90% and 99%, respectively. The corresponding removal efficiencies of 68%, 88%, 78%, 90% and 99%, respectively were determined for the case of the flow-through mode. On the other hand, the removal efficiencies of total solids content, silicon, TOC, and turbidity for mixed-CMP wastewater using the recirculation mode were 76%, 84%, 78% and 99%, respectively; whereas 78%, 86%, 72% and 99%, respectively for the flow-through mode. Based on the above findings, the operating mode is not a significant parameter in influencing the membrane flux and quality. Permeate obtained in this work was found to be recyclable for the use in irrigation and make-up water for cooling towers. Backwashing was found to be important to the membrane flux in this study.