The Piranha solution is used to clean organic contamination off of silicon wafers prior to processing. This is usually the first of many chemical cleans and is often followed by the RCA clean. Piranha and Acetone cleans are great for removing fingerprints and other organic residue that is hard to remove otherwise and ends up collecting on your wafers throughout processing and can make a hydrophobic surface. Typically this is 3 parts of concentrated sulfuric acid and 1 part of 30% hydrogen peroxide solution, although it can vary.
Take great care when preparing the solution, and anytime you are diluting concentrated Sulfuric acid the proper procedure must be followed.