Using Modutek’s Teflon Tanks with KOH and TMAH Etching Processes

Using Modutek’s Teflon® Tanks with KOH and TMAH Etching ProcessesEditor’s Note: This article was originally published in June 2014 and has been updated with additional information and reposted in March 2023.

KOH (potassium hydroxide) and TMAH (tetramethylammonium hydroxide) are two commonly used chemicals for etching silicon, glass, and other materials. The etching process involves dissolving the surface of the material, leaving behind a desired pattern or shape. When using these chemicals, it is important to choose a tank material that is resistant to their corrosive properties, such as Teflon®.

Modutek provides Teflon® tanks that are well-suited for use with both KOH etching and TMAH etching processes. These tanks are highly resistant to chemical corrosion, making them a safe and reliable option for handling these caustic substances. However, there are several additional considerations that should be kept in mind when using KOH and TMAH for etching.

Etching Rates and Material Compatibility

Another important consideration when using KOH and TMAH for etching is the etching rate. The rate at which these chemicals etch materials can be affected by various factors, including the concentration of the solution, the temperature, and the surface orientation of the material being etched. Proper optimization of these factors is necessary to achieve the desired etching rate and profile.

It is also important to consider the compatibility of the material being etched with the chosen etchant. While KOH and TMAH can etch a wide range of materials, not all materials are compatible with these etchants, and some may require specialized etching solutions. Prior to using these chemicals, it is important to check the compatibility of the material being etched with the chosen etchant.

After etching, residues may be left on the surface of the material being etched. These residues can be removed using a variety of techniques, such as rinsing with deionized water or using a plasma cleaner. The choice of technique will depend on the nature of the residue and the material being etched.

Tank Maintenance and Safety Considerations

Proper tank maintenance is essential for ensuring the longevity of Teflon® tanks used in KOH and TMAH etching processes. The surface of the tank should be kept free from scratches and cleaned regularly with appropriate cleaning agents. Additionally, the tanks should be checked for leaks and other damage periodically to ensure that they remain in good condition.

It is important to take appropriate safety precautions when handling and using these chemicals. Both KOH and TMAH can be dangerous if not handled properly, so it is essential to wear protective gear such as gloves, safety goggles, and lab coats. Proper ventilation should also be used which can be provided with chemical fume hoods. Additionally, proper chemical waste disposal procedures should be followed to ensure the safe disposal of these substances.

Modutek’s Equipment Support for KOH and TMAH Etching

Modutek’s TFa and TI series PFA Teflon® Tanks, and other wet bench equipment, fully support KOH etching and TMAH etching processes.  These Teflon® Tanks are designed together with other wet bench equipment to decrease impurities and unwanted byproducts due to advanced welding techniques with PFA sheet material. Continual adjustments to the Si etching process are not necessary after the initial process has been established. Modutek constructs all Teflon® Tanks and related equipment per individual customer specifications, and routinely design products that are compatible with a customer’s existing wet bench equipment.

Modutek’s Teflon® Tanks have an operating temperature range from 30 – 100º C, with a process temperature control of ± 0.5º C and a heat-up rate from 2-3º C per minute (depending on the size of the system).  The modular design allows for new installation or upgrades into any wet etching station configuration.

Teflon® Tank Configurations that Support KOH and TMAH Etching:

  • Temperature Controlled Re-circulating Baths (TFa Series)
  • Temperature Controlled Static Baths (TI Series)

Teflon® Tank design features:

  • Manual cover with overlapping seal
  • Minimizes water lost
  • No concentration deficiency over a long etch time

Heat source options include: 

  • Teflon® inline heating
  • Immersion heating in overflow weir for TFa Series and main tank for TI Series

Custom Teflon® Tanks and Additional Options: Static KOH Etching bath with condensing coils

  • Custom size Teflon® tanks can also be built to match any size (no limitations)
  • Bottom drain features
  • Magnetic stirrer for agitation (TI and TT series)
  • Water condensing refluxor system available on all baths
  • Auto lid feature
  • DI water or IPA spiking system available

Benefits of Modutek’s Teflon® Tanks:

  • Modular design
  • Two available heat sources
  • All Teflon® fluid path
  • Process temperature control of ± 0.5º C
  • Process etch uniformity wafer to wafer <2%
  • In-house heater maintenance and repair
  • 360-degree overflow filtration
  • Uniform heating throughout the bath

KOH etching tank with condensing coils and semi auto robotModutek provides world-class service, installation, and support for all Teflon® Tanks and related wet bench equipment. In addition, Modutek provides quality products that focus on reliability, precision, throughput, usability, and up-time. For more information contact Modutek for a free quote or consultation.

How Piranha Etch is Used in Silicon Wafer Cleaning

How Piranha Etch is Used in Silicon Wafer CleaningEditor’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.