Real-Time Visualization of the Cleaning of Ceria Particles from Silicon Dioxide Films Using PVA Brush Scrubbing

Abstract

Brush scrubbing is commonly employed for cleaning contaminated polished wafers, especially after chemical mechanical polishing. Here we report the results from real-time video imaging of the brush cleaning of ∼90 nm ceria particles from thin oxide films on transparent glass substrates using evanescent wave microscopy to identify the interactions among the particles, brush, film and cleaning liquid. Two cleaning liquids, DI water (pH ∼ 6) and 0.1 M NH4OH solution at pH ∼ 11, were used. It was found that purely hydrodynamic forces are unable to remove the ceria particles from the oxide film surface and direct contact between brush asperities and film is crucial for particle detachment to occur. However, such a direct contact also causes significant particle loading of the brush as well as redeposition of some of the already dislodged particles. Preventing such redeposition is crucial for efficient surface cleaning. The dislodged particles that are transferred into the fluid flow on the film continued to move without redepositing. Since the 0.1 M NH4OH solution at high alkaline pH created an environment for charge repulsion among the ceria particles, PVA brush and oxide film, particle removal from the film is more rapid and somewhat more efficient compared to DI water.

Effect of Skin Layer on Brush Loading, Cross-Contamination, and Cleaning Performance during Post-CMP Cleaning

“…[1][2][3][4][5] PVA brush cleaning is one of the most effective methods for removing contaminated particles by the shear forces imposed due to direct contact between the wafers surfaces and brush resulting in a low cost of ownership. [6][7][8] However, while cleaning the polished wafer surface the PVA brush can interact with organic, inorganic, metallic, or abrasive contaminants that could potentially damage the brush and its surfaces. This is a major drawback in the yield enhancement and causes a reduction in brush lifetime.…”

“…This is a major drawback in the yield enhancement and causes a reduction in brush lifetime. 6 During the brush scrubbing, mechanical wiping of surfaces with PVA brushes provides a direct shear force due to contact between brush and wafer surfaces that lifts the particles from the wafer. 7,8 Two types of PVA brushes such as with and without skin layer are primarily used in post-CMP cleaning.…”