Synthesis Of CeO₂ Nanoparticles Via Alcohol-water Method And Its Polishing Performance On GaAs Wafer

Nano-CeO₂ particles were prepared by homogenous precipitation in alcohol-water solvent using cerium and hexamethylenetetramine(HMT) as starting materials, and were characterized by TEM, HRTEM, SAD, SEM, XRD, BET, UV-vis and FT-IR. The process parameters which influenced the size and dispersion of nanometer CeO₂ particles were investigated. The prepared nano-CeO₂ particles were collocated into polishing slurry using to polish GaAs wafer, the material removal mechanism and the effect of abrasive size on the surface performance after polished were also studied.Experiment results indicated that Ce(NO₃)₃ concentration, mole ratio of HMT to Ce(NO-3)₃, volume ratio of alcohol to water had obvious influence on the size of resultant CeO₂ precursors, of which the optimal process parameters were obtained by taking advantage of the comparison of light transmission rate measured by the spectrophotometer, and revealed as follows: Ce³⁺ concentration was 0.02mol/l, mole ratio of HMT to Ce(NO₃)₃ was 20:1, and volume ratio of alcohol to water was 3:1, particles prepared by the above method were of smaller size and better dispersion than those synthesized in water solution. The grain sizes of CeO₂ nanocrystallites grew up slowly when calcined below 400°C, but increased rapidly when the calcination was higher than 400°C, the diffusion growth mechanism was taken into consideration, and the driving force of grain growth was determined by the diffusion energy and growth activity energy.Nano-CeO₂ particles can be synthesised in any of four alcohol-water composite solvents, it is found that the particle size become bigger in case of the solvent altering from methanol to isopropanol. However, the particle size decreased when the solvent changed into butanol. It was found that the result was due to the competition between two contradictory mechanism of electrostatic repulsion and grain refinement, of which the dominant one determined the final size of nano-particles.As prepared different size CeO₂ nano-particles were used to polish GaAs wafer, it was found that the 8nm nano-particles exhibited the best polishing efficacy, those smaller or bigger than such size got the higher surface roughness. It was concluded that the chemical polishing manner played a dominant role for the difficulty of abrasives penetrating into the soft layer on condition that the particles were too small. On the reverse, when the particles were bigger than certain size, the loading pressure on each particle increased resulting the rise of surface roughness for the depth which particles embeded into martrix increased. It can be concluded that the best polishing efficacy can be obtained on conditon that the depth of particles indented into wafer surface was equal to or near to the thickness of soft layer.