Controlled Synthesis Of SiO₂/CeO₂, PS/CeO₂ Composite Microspheres With Core/Shell Structure And Their Chemical Mechanical Polishing Performances

Chemical mechanical polishing/Planarization (CMP) is one of the most efficient planarization technologies in the manufacturing of ultra-large scale integration (ULSI) and precision optical devices. In CMP processes, abrasives in slurry play an important role in improving CMP performances, such as material removal rate (MRR), roughness, number of defects, and surface flatness. CMP performances depend on the choice of abrasives including type, morphology, size distribution and mechanical properties. In this dissertation, the silica/ceria (SiO2/CeO2) and polystyrene/ceria (PS/CeO2) composite microspheres with core-shell structure were synthesized by liquid phase method, and the SiO2-CMP performance were investigated. The obtained results are as follows.SiO2/CeO2 composite microspheres with core-shell structure were prepared by immersion method and In-situ chemical precipitation method with tetraethyl orthosilicate (TEOS) as silica source and cerium nitrate hexahydrate as the cerium source, respectively. TEM and FESEM results showed the surfaces of SiO2 core were coated uniformly by CeO2 nanoparticles. XPS results showed the CeO2 (shell) was chemically bound with SiO2 (core), and as a result of Si—O—Ce was formed. The CeO2 shell thickness of composite microspheres could be controlled by adjusting the cerium ion concentration in the sol and the concentration of Ce(NO3)3·6H2O in the reaction solution, respectively.A simple method was proposed to prepare PS/CeO2 composite microspheres without surface modification or addition of surfactant (stabilizer). Firstly, negative-charged polystyrene microspheres were prepared via soap-free emulsion polymerization by potassium peroxydisulfate (KPS, anionic) as initiator. Then, Ce3+ cations could be absorbed onto the surfaces of the negatively charged PS microspheres. Under slow hydrolyzing of HMT, the opposite-charged OH- was released in the solution. Ce3+ combined with OH- slowly hydrolyzed from HMT driven by electrostatic attraction. By using this method, PS/CeO2 composite microspheres with different core size and/or shell thickness could be obtained. The results indicated that the as-prepared core-shell structured composite microspheres (100-300 nm in diameter) possessed thin shell (5-20 nm) composed of CeO2 nanoparticles (particle diameter of 5-10 nm), and the final CeO2 contents of the composite microspheres ranged from 30 to 70 wt%.Atomic force microscopy (AFM) was employed to probe the mechanical properties of PS microspheres and PS/CeO2 composite microspheres. On the basis of Hertz's theory of contact mechanics, elastic moduli were measured by the analysis of force-displacement curves captured on the microsphere samples. The average elastic modulus value of the PS microspheres was found to be approximately 2.8 GPa. The compressive modulus is slightly less than the moduli of polystyrene bulk materials. The PS core size and/or the CeO2 shell thickness affected the mechanical properties of composite microspheres. For a fixed PS core size or CeO2 shell thickness, the elastic modulus of composites increased with an increase of the CeO2 shell thickness or the PS core size, respectively. The elastic moduli of the composites displayed a significant shift from the value of pure CeO2 toward that of PS core. Compared with traditional inorganic particles, the as-prepared PS/CeO2 composite microspheres exhibited the especial non-rigid mechanical properties. This approach would provide fundamental insights into the actual role of organic/inorganic core/shell composite abrasives in CMP.Compared with pure SiO2 and CeO2 abrasives, SiO2/CeO2 composite abrasives led to lower topographical variations as well as few scratches. The surface roughness (RMS) values of polished GaAs wafer, silica glass, and silicon oxide dielectric layer was 1.09, 0.346 and 0.428 nm, respectively. The as-prepared SiO2/CeO2 composite abrasives exhibited the core-shell structure, which could provide the direct contact between CeO2 nanoparticles and wafer. Such character could partially avoid the mechanical damage induced by hard agglomerates, which could improve the CMP performance.The CMP results showed that the PS/CeO2 composite abrasives were beneficial to elimination of scratches and decrease surface roughness (RMS) in the process of SiO2-CMP. In addition, there was an obvious effect of core size and/or shell thickness of the composite abrasives on oxide CMP performance. At a fixed CeO2 shell thickness, the surface roughness values increased with an increase of PS core size. And for a given PS core, the surface roughness value decreased and then increased with the increase of CeO2 shell thickness. The improvement of CMP performance might be attributed to a synergistic effect of the core-shell structure. The spring-like effect coming from the elastic polymer core increased the contact area between wafer and abrasives and decreased the contact stress during CMP, which was in favor of reducing roughness and mechanical damage. Furthermore, assisted by the mechanical compliance coming from the elastic PS core, the planarity of wafer after CMP could also be enhanced. In addition, inorganic shell could improve the surface hardness of the polymer core and possessed an enhanced chemical activity of composite abrasives.