Editor’s Note: This article was originally published in December 2016 and has been updated with additional information and reposted in March 2023.

Silicon wafers are fabricated with repeated etching and cleaning steps to produce the micro-structures required for the final silicon semiconductor products. Piranha or SPM (sulfuric peroxide mix) solutions can clean organic material from wafers and oxidize most metals. The powerful chemical action that makes it a favorite for resist strip and for cleaning wafers with organic residue also makes it difficult to use. High-quality silicon wafer cleaning equipment designed to handle the corrosive chemicals safely is required for carrying out piranha etch safely and effectively.

What is Piranha Etch and How is it Used

Piranha etch is a highly corrosive mixture of concentrated sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) commonly used in the microelectronics industry to clean and etch silicon wafers. It is called “piranha” due to its aggressive nature, similar to the ferocious piranha fish found in the Amazon River.

The most common ratio is approximately three parts acid to one part peroxide, but solutions of up to seven parts acid to one part peroxide are sometimes used. The solution is highly exothermic and is prepared by slowly adding the peroxide to the acid. The mixture heats up rapidly and is often used at temperatures of around 130 degrees C. Once the operating temperature and the desired concentration are reached, the wet bench equipment must heat the solution to maintain the temperature and keep the etch rate constant.

Piranha etch is primarily used to remove organic and inorganic contaminants from silicon wafers, which can negatively affect the performance of electronic devices. The highly oxidizing nature of hydrogen peroxide and the dehydrating property of sulfuric acid make piranha etch highly effective in removing photoresist residues, heavy metals, and other contaminants that may be present on the surface of silicon wafers. The underlying surfaces are hydroxylated, making them hydrophilic or attractive to water, a characteristic that can be used in subsequent silicon semiconductor manufacturing process steps.

Piranha Etch Process Steps

The process of using piranha etch involves several steps. First, the silicon wafer is rinsed with deionized water to remove loose particles or dust. Then, the wafer is immersed in a piranha etch bath for a short period, typically 10-30 minutes, depending on the level of contamination. During this time, the piranha etch reacts with the contaminants on the wafer, breaking them down and removing them from the surface.

After the piranha etch treatment, the wafer is rinsed several times with deionized water to remove any remaining piranha etch solution. It is important to note that piranha etch is a very dangerous substance and should be handled with extreme caution. The mixture is highly reactive, exothermic and produces toxic fumes that can cause severe burns and respiratory problems. Proper safety equipment, such as gloves, goggles, and fume hoods, should always be used when working with piranha etch.

Spiking the Piranha Solution to Maintain Concentration

When a facility wants to reuse a piranha solution for an extended period, the solution must be spiked with extra hydrogen peroxide. Hydrogen peroxide is unstable in the solution and decomposes, reducing the etching power of the bath. Spiking with additional hydrogen peroxide lets operators use a piranha solution for up to eight hours rather than replacing it every two hours. Spiking saves money by conserving sulfuric acid but requires wet bench equipment to handle the process and the spiking.

Modutek’s Silicon Wafer Cleaning Solutions

Modutek’s wet bench and silicon wafer cleaning equipment provide wafer cleaning solutions. The company can supply standard equipment or customize wet bench solutions to meet specific customer requirements. Priorities are high-quality materials, excellent designs, and low cost of ownership to achieve optimized process results. Piranha wafer cleaning is supported with Modutek’s QFa high-temperature recirculating Quartz Tanks and the QA constant temperature baths. Both can be installed in a wet bench station, and the process can be controlled automatically, semi-automatically, or manually.

The QFa series high-temperature re-circulating Quarz Tanks provide fast even heating over a temperature range of 30 to 180 degrees C. The heat-up rate can be up to 2 degrees C per minute, and the temperature control is up to plus/minus 1 degree C. The quartz bath is made of flame-polished semiconductor-grade quartz insulated with silica fiber rated up to 1260 degrees C.

The QA series constant temperature quartz baths are made from the same materials with the same control characteristics as the QFa series, but they feature a magnetic stirrer, an aspirator valve system, a gravity drain, and a quartz bubbler. Both baths are available in standard sizes or can be custom-made to fit customer requirements.

Modutek’s Equipment Provides Safe and Reliable Operation

Modutek can provide a complete range of silicon wafer cleaning equipment that includes baths suitable for piranha cleaning applications. The equipment is designed with safe and reliable operation in mind, and the cleaning equipment eliminates contaminants and impurities to the greatest extent possible. Modutek’s wet bench equipment allows operators to use piranha cleaning methods safely to increase facility throughput while maintaining or improving output quality. Contact Modutek for a free quote or recommendations on using the right equipment for your wafer cleaning application.