Wafer cleaning

Microchips are manufactured in cleanrooms where the concentration of dust and other contaminants is kept as low as possible. The air is continuously filtered and directed through the production area in such a way that released particles are efficiently removed from the working environment. Personnel are also a major source of contamination. For this reason, special cleanroom garments are worn that release as few particles as possible and, depending on the required cleanliness class, cover large parts of the body. This helps prevent skin flakes, hair, sweat and other substances released by humans from entering the manufacturing environment. Contamination can also be introduced through ambient air, process chemicals and gases, as well as by the manufacturing equipment itself. Common types of contamination include:

• microscopic contamination: for example dust and material particles originating from ambient air, process gases or equipment components
• molecular contamination: for example hydrocarbons from oils, lubricants or pumping systems
• ionic contamination: for example sodium or chloride ions introduced by sweat or process chemicals
• atomic contamination: for example metal atoms, particularly heavy metals, originating from process solutions or abrasion of equipment and tool components

Microscopic contamination consists of particles that attach to the wafer surface. Sources of this contamination include the ambient air, personnel clothing, abrasion from moving parts in process equipment, inadequately filtered liquids such as etch and cleaning solutions or developer, or etch residues remaining after dry etch processes.

Microscopic contamination causes shadowing effects when particles on the wafer surface prevent exposure of the photoresist; in contact exposure, poor resolution results when relatively large particles are located between the mask and the wafer. In addition, they can shield accelerated ions from the wafer surface during implantation processes or dry etching.

Particles can also become enclosed by layers, creating unevenness. Subsequent layers can crack open at these locations, or resist can accumulate there, resulting in areas that are not fully exposed due to the excessive resist thickness.