Monthly Archives: July 2022

Why Stainless-Steel Stations Are Needed for Solvent Processing

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During silicon wafer fabrication, BEOL (Back End of Line) processes may require solvent-based stations to ensure compatibility with semiconductor manufacturing steps. FEOL (Front End of Line) acid-based chemistry may not be suitable for BEOL finishing. Instead, stainless-steel stations with solvents such as acetone, IPA (Isopropyl Alcohol) or EDP (Ethylenediamine Pyrocatechol) complete wafer etching, stripping, and cleaning. Stainless steel is used for these stations because it is impervious to solvents and because it reduces the risk of fire when solvents are highly inflammable.

Why BEOL Wafer Fabrication Steps May Require Solvent-Based Stations

A wet bench FEOL fabrication starts by repeatedly cleaning, etching, and stripping wafers to build microscopic structures in the silicon. The common acid-based processes used include RCA clean, with chemicals such as hydrogen peroxide and hydrochloric acid; KOH etch, with potassium hydroxide; and Piranha clean, with sulfuric acid. As well as etching the silicon wafers, these chemicals remove both organic and metallic impurities from the surfaces of the wafers, leaving them completely clean and pure for the next process steps. The structures etched during these FEOL steps form the physical parts of the semiconductor components such as transistors.

In the BEOL process steps, the transistors and other components are interconnected and connected to plugs or sockets that interface with outside signals. The connecting paths are metallic, usually made of copper or aluminum, so that they can conduct electrical charges efficiently. Once the metallic paths are deposited on the silicon wafers, acid-based processes can no longer be used because the acidic chemicals damage the metallic paths. Instead, a solvent manufacturing process is used with solvents that are compatible with metals but can complete the silicon wafer processing and produce the final semiconductor components.

Solvent-Based Wet Benches Need Special Safeguards and Protection

The solvents used in wet bench stations are highly inflammable and operators have to be protected against being exposed to fumes or contact. Stainless steel stations using inflammable solvents have to be explosion-proof and satisfy the Class 1, Division 2, Group D explosion-proof standards. They should be designed with fire suppression and wired to the NFPA 70 and 79 standards. An emergency power off mushroom button with safety interlocks helps protect operators if something goes wrong and PVC safety shields help prevent accidental contact with the chemicals. An experienced wet bench manufacturer can ensure that these safety regulations are followed and that operating the solvent stations is as safe as possible.

Modutek’s Stainless Steel Solvent Stations Operate Safely and Meet Customer Requirements

With over 30 years of experience in wet process technology, Modutek designs and builds stainless steel solvent stations to operate safely and meet customer requirements. These stations are made of 304 stainless steel and are wired to the NFPA 70& 79 standards. Fire suppression is included and extensive customization is available.

Stations can be fully automated, semi-automatic, or manual. Fully automated stations reduce operator error and feature excellent repeatability, both for batch processing and for keeping variables constant over an extended period of continuous operation. Semi-automatic stations provide many of the benefits of fully automated operation but at a lower cost. Manual operation is ideal for testing or for prototypes. Once the process has been optimized with manual adjustments, automatic operation can reduce defects and improve facility performance. SolidWorks Simulation software is available to analyze the process and calculate chemical flows. The software can improve etching performance, improve yields and increase throughput.

On the performance side, Modutek stainless steel stations can operate with temperatures from thirty to one hundred degrees centigrade, heat up the bath at a rate of 2 degrees centigrade per minute, and maintain the temperature with an accuracy of plus/minus 1 degree centigrade. Heating elements are four-sided and a process temperature controller is available as an option.

Modutek’s Expertise in Wet Process Equipment

 Modutek designs and builds their own wet process equipment in-house using their own expertise and experience. Their wet bench equipment is designed to meet high quality standards, and are customized to meet specific customer requirements. Solvent-based stainless-steel stations deliver top performance for BEOL processing and Modutek can ensure the equipment meets specific customer needs. Contact Modutek for a free consultation to discuss your specific process requirements.

How the Piranha Etch Process Improves Silicon Wafer Cleaning

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While the Piranha solution quickly removes organic residue from silicon wafers, the process is difficult to control and may produce explosive gas mixtures. The Piranha solution is made up of one part hydrogen peroxide and three parts sulfuric acid, although the ratio may vary for specific applications.

The mixture is exothermic and the heat released when the hydrogen peroxide is added to the sulfuric acid drives up the temperature of the solution towards the normal process operating temperature of 130 to 180 degrees centigrade. Both the temperature and the concentration may vary, reducing the useful lifespan of the solution. When the challenge to control temperature and maintain the concentration is met, the Piranha solution can deliver superior silicon wafer cleaning performance.

Spiking the Piranha Solution Can help Maintain Concentration

The hydrogen peroxide in the Piranha solution is not stable and decomposes to produce water. The rate of decomposition is higher the higher the temperature, and the water dilutes the solution. Operators can compensate for the lower concentration by periodically spiking the solution with additional hydrogen peroxide. As hydrogen peroxide is added, the temperature rises, and more hydrogen peroxide decomposes to form water. This interdependence of the concentration with the temperature complicates keeping a tight control on the process, but spiking the Piranha solution lengthens the solution’s lifespan from a few hours to about one day.

Modutek’s “Bleed and Feed” Method Increases the Piranha Solution Lifespan

Modutek has developed an innovative solution to the control and lifespan issues of the Piranha process. The company uses quartz tanks with a “clean” and a “dirty” tank to provide pre-mixed Piranha solution to the active process. Silicon wafer cleaning takes place in the “dirty” tank while the clean tank has a Piranha solution with a programmable concentration. When the concentration of hydrogen peroxide drops in the “dirty” tank, a small amount of the low-concentration solution is drained from the tank and an equal amount is added from the “clean” tank. Instead of the destabilizing spike of hydrogen peroxide, the small but frequent addition of pre-mixed fresh Piranha solution keeps both the concentration and temperature stable and makes possible a tight control of the process, significantly prolonging lifespan.

The “Bleed and Feed” Method Delivers Important Benefits

Modutek’s “Bleed and Feed” method is fully programmable, allowing operators to adapt it to any application to deliver improved silicon wafer cleaning. The concentrations of the “clean” and “dirty” tanks, the amount of the “bleed” and the amount of the “feed” are all independently adjustable so the desired concentrations can be maintained for an extended period. Specific benefits of the “bleed and feed” method include the following:

  • Predictable strip rate because both the temperature and the concentration are tightly controlled.
  • Programmable settings allow a flexible operation.
  • Risk of an explosion from spiking with too much hydrogen peroxide is reduced.
  • Longer solution lifespan results in reduced use of chemicals.
  • Reduced costs for purchase, storage, and disposal of chemicals.
  • Reliable process parameters result in excellent repeatability between batches and accurate maintenance of process variables over extended operation.
  • Reduced downtime because the Piranha solution has to be replaced less often.
  • Safer operation because the process is maintained in a stable equilibrium.

With a more stable Piranha process, silicon wafer cleaning is improved with less contamination and fewer particles. Product defect rates are lower and customers will see higher yields. Overall wet process line performance is improved, with lower costs and better output quality.

Modutek Provides Innovative Solutions for Wet Processing Requirements

Modutek continues to build on its experience in wet process technology and has in house expertise to develop innovative solutions for their customers. The Piranha solution “bleed and feed” method lets customers achieve their production goals more easily. Modutek provides solutions that meet customer needs and ensures that the delivered equipment performs as required. Contact Modutek for a free consultation to discuss the equipment needed to support your specific process requirements.