How the Piranha Etch Process Improves Silicon Wafer Cleaning

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.

Improving Piranha Etch Process Results in Silicon Wafer Cleaning

Improving Piranha Etch Process Results in Silicon Wafer CleaningPiranha etch is a popular process for silicon wafer cleaning, but it has to be tightly controlled to be effective. The mixture of about one part hydrogen peroxide and three parts sulfuric acid rapidly removes organic matter from silicon wafers. The etch process takes place in heated quartz tanks where both temperature and chemical concentration affect the etch rate.

Precise control is difficult because mixing the original solution is exothermic, and the hot mixture is then maintained at between 130 and 180 degrees centigrade. The hydrogen peroxide decays to form water, diluting the mixture. The rate of decay depends on the temperature, but adding extra hydrogen peroxide to keep the concentration steady increases the temperature.

With several key variables interdependent, accurate control with traditional control systems is not easy and results can be varied. The mixture, with periodic addition or spiking with hydrogen peroxide, has a useful life of only about one day. Modutek has developed a control strategy that improves process results while reducing chemical use.

The Modutek “Bleed and Feed” Process Control Strategy Delivers Improved Results

Modutek’s “Bleed and Feed” process control strategy keeps the concentration of the Piranha mixture at the desired level while increasing the lifespan of the mixture. The company uses a two-tank “clean” and “dirty” tank design. When the concentration of hydrogen peroxide in the dirty tank goes down, a small amount of mixture is drained from the tank and is replaced by an equal amount from the clean tank. The clean tank then receives a fresh amount of mixture. The clean tank and dirty tank mixtures and the amounts of the bleed and the feed are all programmable so that the desired concentrations can be maintained for an extended period. Using the “Bleed and Feed” method, the Piranha mixture has a much longer life span.

“Bleed and Feed” Control Strategy Advantages

The control strategy of frequently adding small amounts of a programmable hydrogen peroxide/sulfuric acid mixture to the main “dirty” tank mixture to maintain its concentration provides several advantages to the silicon wafer cleaning process. Instead of spiking the mixture with hydrogen peroxide, the Modutek method adds an already mixed dosage to the main mixture, reducing temperature instability and allowing better control of the process. When the main mixture is relatively stable, it lasts longer and doesn’t have to be replaced as often. The temperatures and concentrations are more stable and the control of the etch or strip rate is more precise.

The Modutek “Bleed and Feed” Process Control Delivers Substantial Benefits

With more precise control of the process and a more stable Piranha mixture, Modutek’s “Bleed and Feed” process control improves Piranha etching results. Specific benefits include the following:

• Better control of the process increases reliability of strip results

• Precise temperature and concentration levels result in a constant strip rate

• Programmable dosage levels add control flexibility for predictable results

• Reproducing process parameters creates excellent repeatability

• Longer mixture life reduces chemical use

• Replacing the mixture less often reduces down time

• Chemical purchase and disposal costs are lower

• Risk of an accident from adding too much hydrogen peroxide is eliminated

Overall process results and wet etching performance are improved with the better outcomes and reduced costs of a Piranha etch process using Modutek’s “Bleed and Feed” process control. Customers incorporating Modutek’s new control strategy can expect reduced product failure rates, better output quality and overall improved wet process station performance.

Modutek continues to improve their silicon wafer cleaning equipment to provide customers with better process results using the “Bleed and Feed” process control in new wet bench stations. The company offers free consulting and can help customers choose an appropriate wet bench configuration or custom-build one to meet specific customer needs.