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Along with the remarkable growth in the complexity of semiconductor fabrication technology, chemical mechanical planarization (CMP) has evolved and become progressively more sophisticated over the years, enabling the implementation of novel integration schemes. This paper discusses current research and development trends in one specific aspect of the CMP technology, namely, ceria particle usage for advanced technology nodes and provides some perspectives on how to improve CMP performance metrics of the current ceria abrasives and ceria-based CMP slurries and move forward to the next phase.

Temperature is one of the parameters that needs to be continuously monitored and controlled during handling, storage, and transportation to achieve repeatable and consistent CMP performances. Here, we investigated the effect of three different storage temperatures (15, 25, and 45oC) on several slurry health parameters (particle size, zeta potential, pH, total dissolved solids, conductivity, and dissolved oxygen (DO) concentration) of a ceria-based slurry and also on removal rates of SiO2 films during polishing. The changes in all the parameters of slurries stored at 15oC and 45oC for three weeks, except for DO concentration, were reversible and returned to their values at 25oC when the slurries were brought back to 25oC. DO concentration increased by ~14 % and decreased by ~18 % in the slurries stored for three weeks at 15oC and 45oC, respectively, compared to that of ceria slurry stored at 25oC. They did not return to the original values even after keeping them at 25oC for 6 hours. The increase in storage temperature causes an increase in the adsorption of dissolved oxygen onto the ceria surface, which can transform Ce3+ species into less reactive Ce4+-superoxo or a Ce3+-peroxo species, resulting in lower SiO2 removal rates.

Brush scrubbing is commonly employed for cleaning contaminated polished wafers, especially after chemical mechanical polishing. Here we report the results from real-time video imaging of the brush cleaning of ∼90 nm ceria particles from thin oxide films on transparent glass substrates using evanescent wave microscopy to identify the interactions among the particles, brush, film and cleaning liquid. Two cleaning liquids, DI water (pH ∼ 6) and 0.1 M NH4OH solution at pH ∼ 11, were used. It was found that purely hydrodynamic forces are unable to remove the ceria particles from the oxide film surface and direct contact between brush asperities and film is crucial for particle detachment to occur. However, such a direct contact also causes significant particle loading of the brush as well as redeposition of some of the already dislodged particles. Preventing such redeposition is crucial for efficient surface cleaning. The dislodged particles that are transferred into the fluid flow on the film continued to move without redepositing. Since the 0.1 M NH4OH solution at high alkaline pH created an environment for charge repulsion among the ceria particles, PVA brush and oxide film, particle removal from the film is more rapid and somewhat more efficient compared to DI water.

We demonstrated that the movement of the single ceria particles located at the glass interface could be imaged in situ using evanescent wave (EW) microscopy. (1,2) Here, the flat glass surface of the lens became our model for the SiO2 film surface of interest. 3D trajectories, diffusion coefficients, and interactions of ceria particles with a glass surface at pH 3, 5, and 7 were investigated using EW microscopy. We also defined the types and characteristics of particle motions by analyzing the mean-square displacement (MSD) versus time curves. (1) The removal of single ceria particles from the glass films in the cleaning solution was monitored in situ using EW microscopy coupled with a brush set-up. In situ measurement data collected during cleaning are likely to provide more important insights into the underlying mechanisms of various complex reactions that occur at the particles/brush–wafer interface. (3) This talk will also introduce lateral force microscopy (LFM) mode in the atomic force microscopy (AFM) to obtain the removal force of single ceria particles from the SiO2 film. LFM is a powerful tool to monitor the torsion bending (or twisting) of the cantilever with contact mode, which is corresponding to the removal force. Types of particle removal mechanism depending on the characterization of single particles (size, shape, and surface) will be discussed. Our results will provide a better understanding of how the abrasive particles, the additives, and the film surfaces interact together during cleaning. (1) Seo, J., Gowda, A., Khajornrungruang, P., Hamada, S., Song, T., & Babu, S. (2020). Trajectories, diffusion, and interactions of single ceria particles on a glass surface observed by evanescent wave microscopy. Journal of Materials Research, 35(3), 321-331. (2) Seo, J., Gowda, A., Khajornrungruang, P., Hamada, S., & Babu, S. (2020). 3D trajectories and diffusion of single ceria particles near a glass surface and their removal. Journal of Materials Research, 1-10. (3) Seo, J. (2020). A review on chemical and mechanical phenomena at the wafer interface during chemical mechanical planarization. Journal of Materials Research, 1-23.

It has been very difficult to remove smaller ceria particles from oxide surfaces after polishing. As the particle size decreases from 90 to 10 nm, cleaning efficiency of SC1 solution decreases from ~94 % to ~19 %, presumably because smaller ceria particles with a higher surface concentration of Ce3+ are more strongly coupled with silicon dioxide surfaces via strong Ce-O-Si bonding. (1) As was reported earlier, where the cleaning of oxide surfaces of ceria particles from slurries containing no additives, we reported that equimolar solutions containing H2O2 and NH4OH (4.2 mol l−1 each of H2O2 and NH4OH), aided by ultrasonic cleaning, and at high pH could almost completely remove even 10 nm sized ceria particles from silicon dioxide film surfaces. (1) However, a cleaning solution containing H2O2 and NH4OH at equal molar (4.2 mol l−1) was not effective in cleaning oxide and nitride surfaces contaminated with ceria particles in the presence of additives such as proline and citric acid. (2) Recently, we reported a cleaning solution comprising 1 wt% ascorbic acid, 1 wt% ammonium carbonate and 50 ppm Triton X-100 at pH 12, aided by ultransonic energy, to clean ~30 nm ceria particles from both oxide and nitride films with ~ 99% cleaning efficiencies. The binding between the additive-covered ceria particle particles and the oxide/nitride surfaces could be broken by the nucleophilic attack of hydroxyl ions. Ascorbic acid and ammonium carbonate prevent ceria particle redeposition by adsorbing on the surface of the removed particles and blocking the active Ce3+ species. Triton X-100 reduces adhesion between the particle and the film surfaces during cleaning. (1) Seo, J., Gowda, A., & Babu, S. V. (2018). Almost complete removal of ceria particles down to 10 nm size from silicon dioxide surfaces. ECS Journal of Solid State Science and Technology, 7(5), P243. (2) Gowda, A., Seo, J., Ranaweera, C. K., & Babu, S. V. (2020). Cleaning Solutions for Removal of∼ 30 nm Ceria Particles from Proline and Citric Acid Containing Slurries Deposited on Silicon Dioxide and Silicon Nitride Surfaces. ECS Journal of Solid State Science and Technology, 9(4), 044013.

We investigated the effect of friction force between ceria abrasive and SiO2 film on dishing in STI CMP. The control of adsorption amount of poly acrylic acid (PAA) on ceria surface led to the reduction of the friction force during CMP. The reduced friction force by a thick surface layer on ceria resulted in the decrease of the dishing in STI structure during over-polishing process. In the patterned wafer, the dishing decreased from 976 to 594 A/min at 37.5% pattern density (Si3N4/SiO2 = 30/50 μm) as a maximum adsorption amount increased from 0.49 to 0.64 mg/m2.

A previously developed aqueous cleaning solution (4.2 mol l−1 each of H2O2 and NH4OH) was found to be ineffective in cleaning oxide/nitride surfaces after contamination with ceria particles from slurries containing proline or citric acid. However, a cleaning solution consisting of 1 wt% ascorbic acid, 1 wt% ammonium carbonate and 50 ppm triton X-100 at pH 12, aided by ultrasonic cleaning, removed these ceria particles, even those as small as ∼30 nm, from both oxide and nitride surfaces with efficiencies >99% as determined by AFM imaging. Fourier transform infrared (FTIR) spectroscopy results indicated that ceria particles treated with these additives can also bind with oxide/nitride surfaces through Si–O–C and Si–O–H bonds, in addition to any Ce–O–Si, where the C and H atoms are from the additives adsorbed on the ceria particles. All these bonds are broken effectively by the nucleophilic attack of hydroxyl anions in the cleaning solution while triton X-100 in the cleaning solution reduces adhesion between the particles and the film surface and facilitates cleaning via a wetting mechanism. More importantly, ascorbic acid and ammonium carbonate prevent particle redeposition by complexing with the removed particles and blocking the active Ce3+ species on their surface.

We have investigated the synergistic effect with mixing of three different-sized SiO2 abrasives (30 nm-SiO2, 70 nm-SiO2 and 200 nm-SiO2) and the corresponding W chemical mechanical planarization (CMP) performances. W removal rate significantly increased when the different-sized SiO2 abrasives were mixed, which is attributed to the increase in the total contact area between the abrasives and the W film. Based on the statistical model, we obtained the optimal mixing ratio (30 nm, 70 nm, 200 nm) = (0.49, 0.23, 0.28) for the highest W removal rate. These results, investigated in this study, show that the removal rate of W film can be improved via simple mixing process.

Localized galvanic corrosion at the tungsten (W) and titanium nitride (TiN) interface causes fatal defects (e.g., corrosion and pitting issues) in chemical mechanical planarization (CMP) process. In this study, we control the corrosion potential difference between W and TiN via mixing of two corrosion inhibitors in order to decrease those defects. The roles of corrosion inhibitors in CMP slurry are analyzed by open circuit potential (OCP) measurements, linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Through these electrochemical approaches, we found that two mixed inhibitor system can decrease the galvanic potential and current density. CMP result with W/TiN patterned structure wafer shows that mixed inhibitor system prevents galvanic corrosion at the W/TiN interface during CMP, leading to decreasing the defects. We believe that the results investigated in this study will provide researchers with the importance of electrochemical approaches in W CMP.

Ceria has been widely used as an abrasive for polishing SiO2 film due to the high removal rate of SiO2 during Shallow Trench Isolation (STI) CMP.1 As the scale of transistor has been reduced, the number of maximum acceptable defects was decreased. Recently, numerous researchers have studied prevention of defect formation on the SiO2 film by using solution-grown ceria with spherical shape. Although solution-grown ceria is spherical shape and smaller particle size, it leads to unexpected results such as removal rate of SiO2 and higher defects compared to solid-state ceria with irregular shape. Recently, the study of surface chemistry appears to be more important than morphology, as ceria reacts with the SiO2film to remove it. Here, the effect of the surface chemistry of solution-grown ceria on the CMP performance was investigated. Nitrate- and hydroxyl-functionalized ceria nanoparticles were prepared by ion-exchange method. Figure 1(a) shows the adsorption behavior of silicate ions on nitrate- and hydroxyl-ceria particles at pH 7.0. The presence of surface nitrate group induces high affinity between ceria particles and silicate ions at pH 7. Figure 1 (b) shows the effect of nitrate group on the surface on the CMP performance. Nitrate-ceria particles increase the removal rate of SiO2film, which correlates well with the adsorption behavior of silicate ions on the ceria particles. To interpret the adsorption behavior of nitrate- and hydroxyl-functionalized ceria particles, the adsorption reaction at the molecular level through quantum mechanical calculation was investigated.2 As the second step, the hydroxyl-ceria adsorption on the H-terminated SiO2 surface is water generating reaction (Figure 2e). The binding energies of the both cases are -2.865 and -3.813 eV, respectively. On the other hand, only one step adsorption was considered for the nitrate-ceria without generating water molecule (Figure 2f). The binding energy of this nitrate ceria on the SiO2surface is much higher than that of the hydroxyl-ceria as -4.383 eV. In this study, we revealed that surface functional groups such as hydroxyl and nitrate groups have a significant influence on the characteristics of CMP. The adsorption behavior based on surface chemistry is important for understanding of reaction between ceria and SiO2. Acknowledgement This work was supported by the Global Research Laboratory (GRL) Program (K20704000003TA050000310) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (Information and Communication Technologies) and Future Planning, and the International Cooperation program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government of Ministry of Trade, Industry & Energy (2011T100100369) Reference [1] Cook, L. M.“Chemical Processes in Glass Polishing.” Journal of Non-Crystalline Solids, 120, (1-3), 152-171, (1990) [2] Perdew, J. P. et al. “Accurate and Simple Analytic Representation of the Electron-Gas Correlation-Energy” Physical Review B, 45, (23), 13244-13249, (1992)

Shallow trench isolation (STI) process has been a crucial technology to isolate the transistors in ultra large scale integration (ULSI) device [1]. In STI process, step height of dielectric material is inevitably formed after gap-filling process due to the different pattern density. Chemical mechanical planarization (CMP) process has been employed to remove the step height. Remaining step height would cause defects such as dishing and erosion, therefore the reduction of step height can be considered as one of the critical issues in STI-CMP process. Several groups have studied about the step height reduction using ceria due to its high removal rate of SiO2 [2]. Lim et al. reported that the effect of particle size on step height reduction. They showed that the large particle size could improve the step height reduction by its high removal rate. Recently, the needs of small particle have been increased because the defect control has been more important as the design rule is getting narrow. Therefore, it is inevitable to develop new methodology to enhance the step height reduction. Here, we used different amino acid to modify the surface of ceria and enhance the interaction between the ceria slurry and SiO2. We investigated the effect of ceria slurries prepared with L-serine, proline, glutamine and amino butyric on step height reduction. We calculated the interaction forces between ceria with different amino acids and SiO2 film by Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. The result shows that ceria slurry with L-serine or proline has higher attractive interaction forces with SiO2 than the slurry with glutamine or amino butyric acid (Fig. 1). The results of SiO2 pattern as different pattern density and line width show that the ceria slurry with L-serine or proline produces higher step height reduction than the slurry with glutamine or amino butyric (Fig. 2). As a result, we found that the high interaction forces between ceria slurry and SiO2 contribute to high step height reduction and it is stronger in case of the slurry with L-serine or proline than the slurry with glutamine and amino butyric acid.

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