MICRO:Surface Chemistries, by Joseph Zahka, p.85 (March '99)

Joseph Zahka and Roger Blum, ash residue, and bulk photoresist are removed from wafer surfaces using various chemicals. While these chemicals are generally classed as mixtures of solvents, amines, corrosion inhibitors, and suspending agents, the specific formulations are often proprietary and can vary depending on the application. These chemicals are among the most difficult to filter in recirculated baths and spray tools. Since they are designed to remove polymers from the wafer at temperatures as high as 120°C, they have the potential to compromise the polymers and O-rings used in particle filters. Even at elevated temperatures the viscosity of the organic solutions can remain high, reducing flow in the system. Because of the relatively large amount of material being removed, stripping of bulk resist is one of the few cleaning steps in the fab during which the removal of contamination from the wafer surface may be visible to the naked eye. The chemical filters must have high particle-holding capacity, good particle removal capability, and a high flow rate to aid in the dissolution process and prevent the redeposition of residue when the wafer is removed from the bath. This article describes the application of organic photoresist strippers to remove bulk resist and heavy postetch residues in a current wafer processing scheme. The physical properties for specific formulations, such as viscosity and surface tension, are listed and the effect they have on filter choice and operation are explained. A model is used to predict flow and retention in a bulk strip recirculated bath, and the selection process for an optimum filtration system is described. Several examples show the effect of filter selection on surface particles and on the lifetime of the chemical filtration system. Photoresist films and other polymer residues vary in composition and, therefore, in how difficult they are to remove.