Understanding the Reasons behind Defect Levels in Post-Copper-CMP Cleaning Processes with Different Chemistries and PVA Brushes
Results from a series of post-CMP PVA scrubbing marathon runs performed in a high-volume manufacturing fab are scientifically explained via a series of controlled laboratory tests. The major differences in the ingredients within the cleaning solutions, and some of the key physical properties of the brushes are identified and their effects on various critical factors are studied. These include the magnitude of shear forces present in the brush-solution-wafer interface, the water uptake and porosity of the brushes, the diffusivity of a given cleaning solution through the micro-pores and macro-pores of each type of brush, the open-circuit potential in a dynamic process, and the availability of the cleaning fluid between the brush nodules and the wafer surface. Results show a strong inverse correlation between wafer-level defects and shear force. The latter is shown to decrease with solution availability at the brush-wafer interface which in turn is shown to depend on brush porosity and the diffusion rate of the solution through the pores. Our understanding is further strengthened by dynamic electrochemical analysis data where we see a greater interfacial chemical activity (i.e., an increase in corrosion current) as solution availability is increased.