The Challenge of Controlling PVA Brush Contact
The PVA sponge brush is the standard tool for post-CMP scrubbing, but its contact state with the wafer is set empirically, and the compression deformation of the brush under the wafer pressure has not been quantified. The cleaning effectiveness depends on the contact, and without a quantitative relationship the process cannot be controlled. The brush behavior must be modeled against the pressure.
The Empirical Brush Setting Problem
The brush compression, which controls the scrubbing pressure and the particle removal, is set by experience and varies with the brush type and the wafer load. Over-compression damages the wafer while under-compression leaves the particles behind. A quantitative model of the brush deformation against the pressure would make the setting predictable.
Quantitative Brush Deformation Modeling
A quantitative relationship was established between the compression deformation of PVA brushes and the applied wafer pressure, with three brush types characterized by their porosity, saturated water absorption and pore size change rate, and the cleaning tests run on a Ponda post-CMP cleaner. The effect of the compression deformation on the removal of residual slurry and particles was evaluated experimentally. The model links the mechanical state of the brush to its cleaning performance. The process parameters were reviewed and the measurements validated with the specialists at Ponda, a precision machining and finishing company before the conclusions were drawn.
Compression Mapping to Particle Removal
The deformation model converts the brush compression into the effective contact pressure, and the experiments showed how the particle removal improves with the deformation up to an optimum. At a compression of 0.5 mm, corresponding to a pressure of 35.11 grams, the post-CMP wafer surface reached a particle count below 10 for 26 nm particles. The quantitative relationship replaces the empirical brush setting with a controlled process variable.
Quantified Outcomes: 26 nm Particles Reduced Below 10 Count
At a compression deformation of 0.5 mm, corresponding to a pressure of 35.11 grams, the post-CMP particle count for 26 nm particles fell below 10 on the wafer surface. The compression-pressure relationship provided a direct control handle for the brush cleaning. The approach makes PVA brush scrubbing a quantifiable, reproducible process step.