Piranha Etch The Removal of Organics Using Sulfuric Peroxide by Scott Clark, MSCE   Overview The origins of organic material on wafers include residual photo resist, equipment oils, and human beings--skin oils left behind in the form of fingerprints, skin flakes, and particles from a persons breath. The removal of organic material is of the essence since they act as a mask when attempting to use an HF solution to remove the oxide layer. That is, organic material will prevent the removal of oxides which may contain contaminants thereby inhibiting the deposition of the selected dopant to that area of the wafer. This results in an uneven dopant distribution which will affect the operating characteristics of the integrated circuit located in that area. Chemistry and Application Typically, mixtures of 98% H2SO4 (sulfuric acid) and 30% H2O2 (hydrogen peroxide) in volume ratios of 2-4:1 are used at temperatures of 100°C and higher. This mixture is often referred to as “Piranha etch” (because of its voracious ability to remove organics) or, in some cases, “Caro’s acid”, however, the latter term is not strictly accurate. Strictly speaking, Caro’s acid is composed of 98% H2SO4, 30% H2O2, and DI water in volume ratios of 380:17:1. A variation of the Piranha solution is to use (NH4)S2O8 (ammonium persulfate) in place of hydrogen peroxide. When ammonuim persulfate is added to hot sulfuric acid HO-O-(SO2)-(SO2)-O-OH (H2S2O8--peroxydisulfuric acid) is formed. This reaction is given as: The reaction of peroxydisulfuric acid with organics forms CO2, H2O, and H2SO4. This reaction is represented by: One of the advantages of the system is that one of the byproducts is sulfuric acid. When H2O2 is used one of the by products is water, thus if too much is added the solution becomes dilute degrading the effectiveness of the cleaning solution. Since sulfuric acid is a by product when ammonium persulfate is used the addition of excess ammonium persulfate does not degrade the effectiveness of the solution. Since the Piranha etch solution is widely used it will be discussed in greater detail. It is reported that a treatment of 10 to 15 minutes at 130°C is most effective.1 In this process sulfuric acid is used to convert the organic materials to carbon. The carbon reacts with the atomic oxygen present--due to the dissociation of hydrogen peroxide--to form CO2. A gas phase product which readily escapes the process tank. The remaining liquid is very viscous and vigorous DI water rinsing is essential for its removal from the wafer surface. Piranha etch chemistry is highly effective at removing organic contaminants, however, it does not remove inorganic contaminants such as metals. The primary limitation to the removal of organic contaminants from wafers is the conversion of organic material to carbon. Therefore, it is important to consider the affects of adding too much H2O2 to the process tank. Too much hydrogen peroxide added to the system will rapidly dilute the sulfuric acid which, in turn, will result in less clean product. Hydrogen peroxide dissociates into atomic oxygen and water; it is the water formed from this dissociation that dilutes the sulfuric acid and thereby reduces the cleaning effect of the chemistry. Thus, the typical operating procedure is to first pour up the sulfuric acid then heat it to the desired temperature. Hydrogen peroxide is spiked (added) into the process tank just prior to the introduction of the wafers. Atomic oxygen begins to evolve immediately and stops with in ca. 10 minutes. Hence, the introduction of hydrogen peroxide just prior to the introduction of the wafers ensures that there will be a relative abundance of atomic oxygen to facilitate the complete removal of carbon in the form CO2; it also reduces the dilution effects caused by the addition of water to the sulfuric acid. It is noteworthy point out that Piranha solutions and its varients are very hazardous and extreme care must be taken when using this chemistry. It is recommended that operators wear goggles, face shields, and gloves when working near these mixtures. References 1. Kern, W., in “Handbook of Semiconductor Wafer Cleaning Technology” (Kern, W. ed.), p. 19. Noyes Publications, New Jersey, 1993.

 

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