Preparation and Use
Many different mixture ratios are commonly used, and all are called piranha. A typical mixture is 3:1 concentrated sulfuric acid to 30% hydrogen peroxide solution; other protocols may use a 4:1 or even 7:1 mixture. A closely related mixture, sometimes called "base piranha", is a 3:1 mixture of ammonium hydroxide (NH4OH) with hydrogen peroxide.
Piranha solution must be prepared with great care. It is highly corrosive and an extremely powerful oxidiser. Surfaces must be reasonably clean, and completely free of organic solvents from previous wash steps, before coming into contact with piranha solution. Piranha solution cleans by dissolving organic contaminants, and a large amount of contaminant will cause violent bubbling and a release of gas that can cause an explosion.
Piranha solution may be prepared by adding the peroxide to the acid. Mixing the solution is exothermic. The resultant heat can bring solution temperatures up to 120 °C. It must be allowed to cool reasonably before applying any heat. The sudden increase in temperature can also lead to violent boiling, or even splashing of the extremely acidic solution. Also, explosions may occur if the peroxide solution concentration is more than 50%. Once the mixture has stabilized, it can be further heated to sustain its reactivity. The hot (often bubbling) solution will clean organic compounds off substrates, and oxidize or hydroxylate most metal surfaces. Cleaning usually requires about 10 to 40 minutes, after which time the substrates can be removed from the solution.
The solution may be mixed before application or directly applied to the material, applying the sulfuric acid first, followed by the peroxide. Due to the self-decomposition of hydrogen peroxide, piranha solution should be used freshly prepared. Piranha solution should not be stored.
Immersing the substrate (such as a wafer) into the solution should be done slowly to prevent thermal shock that may crack the substrate material.
Read more about this topic: Piranha Solution
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