Experimental And Theoretical Research On Nano-Particle Removal In Post Chemical Mechanical Polishing Cleaning

The preparation of ultraclean wafer surface and hard-disk substrate is important in the fabrication of semiconductor devices and hard-disk manufacturing. The criticality of post chemical mechanical polishing (post-CMP) cleaning is increasing as critical particle size decreasing to nano scale. In this thesis, the mechanisms for nano particle contaminations adhesion and removal are studied by experiments and theories. Potential cleaning methods are given and tested.The hard disk samples are grabbed randomly from real product line. A scatter spot method and scanning electron microscope (SEM) are used to analyze the particle contaminations after each process of post-CMP cleaning. The results indicate that brush scrub together with megasonic is a refined method for cleaning nano-particles. However, contaminations like metallic particles and bacteria from the cleaning equipments and dryer will cause pollution. Pits originated from plating process are smaller than 100nm and will hold particle contaminations which can not be removed by physical methods.An aqueous solution of nonionic surfactant for cleaning the NiP substrate was investigated. The macromolecule surfactant changes the particle sizes in the solution and the anionic surfactant and high pH value maintain a repulsion of the particles from the surface and dispersion of partilces. Increasing the ethoxy group will enlarge the steric stabilization and increase the water solubility, yet the magnitude of adsorption diminishes and the surface tension of the solution increases. AEO9 with concentration of 5~10 times of CMC at pH value of 10~12 is the optimization formula which can balance these aspects.A 3-D lubrication model between the soft porous brush and rigid flat wafer surface shows that the fluid pressure has negative regions in inlet area. The porous structure leads to the decrease of the hydrodynamic drag force but results in a normal fluid velocity away from the wafer surface. The hydrodynamic force is determined by gradient of fluid pressure when the pore is small. When the size of pore is larger than the height of brush surface asperity, the hydrodynamic drag force is determined by the relative velocity of the two surfaces. Increase of brush velocity and deflection of brush nodule is effective for cleaning. A low wafer rotation speed is recommended to keep the cleaning uniformity. The hydrodynamic drag force is asymmetry over the whole wafer and the removal moment is increasing during the cleaning process.The forces on the particles are calculated and effect of particle size is discussed. The adhesion forces are the van der waals force, the electrostatic force, the capillary force and the static friction between particle and wafer and the brush load. The van der waals force between particle and wafer is the most important adhesion force.The removal forces are van der waals force, electrostatic force and friction between the brush and particle, contact elastic force and hydrodynamic drag force.A new model for nano particle removal is presented. There are four situations of the particles relative to the brush roughness asperities for which the forces on the particle and removal mechanisms are different. When the particle is in contact with a brush asperity or on the wafer surface and in a semi-infinite fluid flow field, the particle may be removed by hydrodynamic force and elastic force in the presence of surfactant. When the particle is embedded in the brush asperity, the remove will realized when the friction caused by adhesion between the brush and particle overcome the adhesion force between particle and wafer surface. The removed particles will be in the flow field or adhered on the brush surface and may redeposit on the wafer surface.