Ceria CMP Slurry for the Construction of Floating Gates in MLC NAND Flash Memory below 51 nm

Abstract

A ceria slurry, which is capable of high removal selectivity between silicon oxide and poly-Si, in the chemical mechanical planarization ͑CMP͒ process was investigated for the construction of poly-Si gates of a multilevel cell ͑MLC͒ NAND flash memory below 51 nm. Anionic phosphate fluorosurfactant was incorporated into the ceria slurry for use as a passivation agent to suppress poly-Si removal. The ceria slurry with fluorosurfactant showed high silicon oxide-to-poly-Si removal selectivity ͑295:1͒, resulting from the passivation layers on the poly-Si due to the selective adsorption of the fluorosurfactant. The selective adsorption behavior of the fluorosurfactant can be explained by the difference in surface characteristics between the oxide and the poly-Si, which was supported by the experimental results.The NAND flash memory has shown promise for high density cell integration and has received interest as a mass storage device. [1][2][3][4][5] Presently, the cell integration density and processing speed of the NAND flash memory is increasing very rapidly due to its simple structure that is suitable for high resolution lithography and poly-Si as a floating gate. 6,7 The integration density of the NAND flash memory is doubled by multilevel cell ͑MLC͒ operation. 8 However, below 51 nm, capacitive coupling between floating gates results in interference between adjacent memory cells and a subsequent threshold voltage shift. 8,9 This interference is strongly affected by misalignment between the floating gate and the active region of the device. To resolve these serious problems, self-aligned poly-Si floating gates are constructed using the chemical mechanical planarization ͑CMP͒ process. 7,10 In a conventional, shallow trench isolation ͑STI͒ process, silicon oxide is polished by STI CMP, and then the exposed silicon nitride films are removed to form the gates. However, in the STI process for the MLC NAND flash memory, the poly-Si film is directly deposited as a floating gate without silicon nitride deposition, leading to the formation of self-aligned poly-Si floating gates. Therefore, silicon oxide-to-poly-Si removal selectivity is the most important factor in the STI CMP process ͑herein called "poly-Si stop CMP"͒.To achieve a high silicon oxide-to-poly-Si removal selectivity, selective adsorption of polymeric molecules to poly-Si films is necessary. In the conventional STI CMP process, the difference in the surface potentials between the silicon oxide and the silicon nitride films enables the selective adsorption of anionic polymeric molecules. 11,12 This selective adsorption prevents the silicon nitride films from being polished, resulting in an improved removal selectivity. However, for silicon oxide-to-poly-Si removal selectivity, the difference in surface energies between silicon oxide and poly-Si film is a key factor for achieving high removal selectivity in the poly-Si stop CMP process. As the poly-Si film has a lower surface energy than the oxide film, hydrophobic polymeric molecules are preferred for adsorptio...

Hydrogenated ceria nanoparticles for high-efficiency silicate adsorption

“…Given that their versatile characteristics based on high oxygen storage capacity (OSC) and low redox potential, ceria (CeO 2 ) nanoparticles (NPs) have garnered considerable interest in various fields for applications including solid oxide fuel cells, [1][2][3] UV shielding, 4 gas sensors, 5,6 biomedicine [7][8][9] and chemical mechanical planarization (CMP). [10][11][12][13] The unique properties of CeO 2 are attributed to their low vacancy defect formation energy, which causes oxygen vacancies on the surface of CeO 2 NPs. [14][15][16] Deviations from stoichiometry in the CeO 2 lattice result from the oxygen vacancy, which reduces the oxidation state of cerium ions from Ce 4+ to Ce 3+ .…”

Frictional Characteristic of Polymeric Additive for the Slurry of Chemical Mechanical Planarization Process

“…2 Other applications than STI CMP, such as reverse oxide to nitride selectivity or SiO 2 to poly-Si selectivity with chemical additives, were also reported. [3][4][5] Removal selectivity of STI CMP is mostly achieved by surface reaction between chemical additives and CeO 2 (ceria) abrasives. It is reported that ceria abrasives have higher selectivity than SiO 2 (silica) abrasives.…”