It has been very difficult to remove smaller ceria particles from silicon dioxide surfaces. One of the likely reasons is that the smaller ceria particles with a higher surface concentration of Ce 3+ are more strongly coupled with silicon dioxide surfaces via strong Ce-O-Si bonding. Here, we show that some equimolar solutions containing H 2 O 2 and either NH 4 OH or KOH, aided by ultrasonic cleaning, and at high pH can almost completely remove even 10 nm sized ceria particles from silicon dioxide film surfaces. The particles on the oxide surface before and after cleaning were imaged using atomic force microscopy and counted. The optimal composition of H 2 O 2 -NH 4 OH solutions was identified by investigating ten solutions with different compositions using an augmented simplex-centroid method along with four H 2 O 2 -KOH solutions. Equimolar mixtures of H 2 O 2 and NH 4 OH or KOH with the highest possible perhydroxyl ion concentration [HO 2 − ] showed ∼99% cleaning efficiency for 10 nm ceria particles. We propose that the strong Ce-O-Si bonding can be ruptured effectively by the nucleophile HO 2 − , formed by the decomposition of H 2 O 2 in the high pH environment, and show that [HO 2 − ] is highest in equimolar solutions. The cleaning efficiencies for 10 nm, 30 nm and 90 nm particles correlate very nicely with [HO 2 − ].

Abstract

A previously developed aqueous cleaning solution (4.2 mol l−1 each of H2O2 and NH4OH) was found to be ineffective in cleaning oxide/nitride surfaces after contamination with ceria particles from slurries containing proline or citric acid. However, a cleaning solution consisting of 1 wt% ascorbic acid, 1 wt% ammonium carbonate and 50 ppm triton X-100 at pH 12, aided by ultrasonic cleaning, removed these ceria particles, even those as small as ∼30 nm, from both oxide and nitride surfaces with efficiencies >99% as determined by AFM imaging. Fourier transform infrared (FTIR) spectroscopy results indicated that ceria particles treated with these additives can also bind with oxide/nitride surfaces through Si–O–C and Si–O–H bonds, in addition to any Ce–O–Si, where the C and H atoms are from the additives adsorbed on the ceria particles. All these bonds are broken effectively by the nucleophilic attack of hydroxyl anions in the cleaning solution while triton X-100 in the cleaning solution reduces adhesion between the particles and the film surface and facilitates cleaning via a wetting mechanism. More importantly, ascorbic acid and ammonium carbonate prevent particle redeposition by complexing with the removed particles and blocking the active Ce3+ species on their surface.

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.