How the SPM Clean Process is Supported in a Wet Bench Process

How the SPM Clean Process Is Supported in a Web BenchEditor’s Note: This article was originally published in Oct 2016 and has been updated with additional information and reposted in May 2023.

Semiconductor manufacturing involves multiple processing steps which include cleaning silicon wafers. The sulfuric peroxide mix (SPM) cleaning process is widely utilized in semiconductor manufacturing. The process utilizes a mixture of approximately 3 parts sulfuric acid to 1 part hydrogen peroxide which is highly effective at eliminating organic and inorganic contaminants from semiconductor wafers. The SPM process is typically performed in wet benches specifically designed for this cleaning process.

The SPM Clean Process

Sulfuric acid in the SPM solution reacts with organic contaminants like photoresist and residue, while hydrogen peroxide oxidizes and removes inorganic contaminants like metal ions and particles from semiconductor wafers. This reaction generates heat which helps to accelerate cleaning. Process engineers using the SPM process in semiconductor manufacturing need to make sure that the chemical ratio and temperature are maintained within safe limits and that the solution and wafers are contained safely in impervious baths. During the process concentration levels may vary from 3 to 1 to a maximum of 7 to 1 and the temperature used may be as high as 140 degrees centigrade. When the solution operating values are chosen, the baths should be maintained at those concentration and temperature values to keep the strip rate uniform.

Advantages of SPM Clean

SPM Clean offers several advantages over other cleaning methods. Its key advantage lies in its effectiveness at removing organic and inorganic contaminants simultaneously. The process is relatively quick usually taking just a few minutes. In addition, the SPM clean process does not damage or etch the substrate beneath, making it ideal for sensitive materials like silicon dioxide and silicon nitride.

Equipment for the SPM Process

SPM cleaning solutions are typically applied in a wet bench designed specifically to facilitate this cleaning method, typically composed of materials compatible with SPM solutions like polypropylene, Teflon, and quartz. Wet benches come equipped with safety features such as fume hoods, exhaust systems, and emergency shutoff switches to help minimize chemical exposure. For consistent and reliable results, SPM clean process equipment typically features process control and automation features to facilitate consistent operations. SPM solution concentration, temperature, and flow rate can all be carefully managed in order to ensure an efficient cleaning process. In addition, automated wafer handling systems may also help minimize contamination risks while improving process efficiencies.

Safety Considerations

To ensure the safe and effective implementation of the SPM Clean process in wet benches, certain practices should be observed. These include adhering to safety procedures, performing regular equipment and tool maintenance checks, optimizing cleaning process performance, and complying with local regulations for waste management and disposal.

Using the SPM Clean Process in Modutek’s Wet Benches

Modutek provides manual, semi-automatic, and fully automatic wet bench systems that support the SPM clean process. Wet Bench systems are available in a wide variety of configurations as well as in custom designs. The fully automated stations use Modutek’s in-house software for automatic process execution with high accuracy, reliability, and repeatability. The semi-automated stations can achieve similar results with robotic controls at a lower cost. The manual stations have the same safety and process features without the cost of automation.

Quartz Recirculating Baths

To work well, the SPM solution must be heated rapidly in a bath that can withstand high temperatures and that will not react with aggressive chemicals. Heating must be even and controlled within a narrow range. At high temperatures, hydrogen peroxide decomposes and the solution must be spiked with more peroxide to maintain its concentration.

Quartz recirculating baths fulfill all these requirements. The quartz can be manufactured in a pure enough form to withstand the temperatures and corrosion while the recirculation allows tight control of concentration with the addition of chemicals as required.

This spiking allows operators to use the solution for a longer period of time rather than discarding the solution when the concentration falls below acceptable levels. Re-using sulfuric acid for as long as possible reduces costs and is desirable from an environmental point of view as well.

Modutek Series QFa Series Quartz Baths

Modutek’s quartz baths support the SPM Clean process and satisfy key conditions for high safety and reliability as well as low cost of ownership. The tanks are made of semiconductor-grade flame-polished quartz and are insulated with high-density alumina-silica fiber rated to 1260 degrees centigrade. The four-sided heating element promotes fast, even heating and the seamless sloped flange and the dual safety snap switch help ensure safe and convenient operation.

The quartz baths have an operating temperature range of 30 to 180 degrees centigrade and they feature a standard heat-up rate of 2 degrees centigrade per minute. The operating temperature can be controlled to within plus/minus 1-degree centigrade and a liquid level sensor is available as an option.

Standard tanks are available in dimensions ranging from 7.75 to 21.5 inches inner side length and in square and rectangular formats. Depths range from 6.75 to 14.5 inches and available heaters are rated 2 to 6 KW. Modutek can design systems with custom vessel sizes to satisfy specific requirements.

The Bottom Line

Modutek’s wet benches and quartz baths are designed with low cost of ownership in mind while emphasizing features that reduce errors and improve reliability. Semiconductor manufacturers using the SPM process can achieve higher throughput and better output quality using Modutek’s equipment. Contact Modutek for a free consultation or quote on wet bench equipment to support your wafer cleaning processes.

How Precise Control of the SPM Process Improves Processing Results

How Precise Control of the SPM Process Improves Processing ResultsWhile the SPM (Sulfuric acid Peroxide Mix) or Piranha process quickly removes organic contaminants such as photoresist, it is difficult to control. The cleaning action depends on both the temperature and the concentration of the mixture. Both vary if the process is left to carry on without intervention. Better controls can improve cleaning performance, reliability, and repeatability while increasing the lifespan of the mixture. Modutek has developed advanced controls that maintain temperature and concentration precisely at their desired levels to improve cleaning results.

The SPM Process Suffers from Decreasing Concentration and Slower Cleaning

The SPM mixture is typically about three parts sulfuric acid to one part hydrogen peroxide. The preparation of the mixture is highly exothermic. Once the mixture stabilizes, the wafer cleaning process occurs in a heated tank at 130 to 180 degrees centigrade. The hydrogen peroxide is unstable and decomposes to form water and oxygen. As the amount of water in the mixture increases and the concentration of hydrogen peroxide goes down, the cleaning effectiveness of the mixture deteriorates.

Small amounts of hydrogen peroxide are periodically added to the mixture to correct the problem of a decreasing hydrogen peroxide concentration. This spiking with hydrogen peroxide causes a sudden temperature increase due to the exothermic nature of the mixing process. While spiking solves the problem of a decreasing hydrogen peroxide concentration, the temperature can’t be kept constant. An SPM mixture that is not spiked is useful for about two hours, but spiking with hydrogen peroxide increases the lifespan of the mixture to about one day.

Modutek Has Developed Advanced Controls to Improve Process Results

Modutek’s “Bleed and Feed” process control strategy keeps the concentration of hydrogen peroxide and the mixture temperature within narrow limits. The temperature variations due to spiking and the need for spiking are eliminated. With concentration and temperature controlled accurately, the strip rate of the mixture remains constant and predictable. Reliable timing and repeatability of the process are excellent.

Modutek achieves these results by using a two-tank control system. The process tank is the “dirty” tank, while the second tank is the “clean” tank. At periodic intervals, a small amount of the mixture is drained from the dirty tank. It is replaced with a fresh mixture from the clean tank. The clean tank mixture is then replenished with new chemicals. A programmable controller controls the process, so the dosage intervals and the amount of mixture to be drained are adjustable and recorded by the programmable controller. As a result, once the process controls are optimized, the SPM wafer cleaning process can be run exactly the same way each time.

Modutek’s “Bleed and Feed” Reduces Costs and Provides Benefits

Customers who have switched to Modutek’s “Bleed and Feed” controls find that their costs are reduced, and the process line performance is better. Modutek’s control strategy reduces chemical use, and the mixture lasts longer when the temperature variations caused by spiking are eliminated. Downtime due to having to replace the bath chemicals is reduced as well. The risk of accidents from operators adding too much hydrogen peroxide or from spills is eliminated. In addition to savings from lower chemical purchases, reduced costs include lower chemical handling, storage, and disposal expenses.

Support for the “Bleed and Feed” Process

Modutek offers the “Bleed and Feed” process controls for the SPM process on new wet process stations. As one of the leading semiconductor equipment manufacturers, Modutek continues to work on improving its silicon wafer cleaning equipment to meet customer needs. Since Modutek designs and builds its equipment in-house, innovative features can be developed to support unique customer requirements. Wet bench processes are often specialized, and Modutek can utilize in-house expertise to customize equipment as needed. Innovation and customization make Modutek a valuable partner for wet bench technology.

How Innovative Changes to the SPM Process Improves Results

How Innovative Changes to the SPM Process Improves ResultsWhen the SPM process requires frequent spiking with hydrogen peroxide, precise control is difficult and the useful lifetime of the mixture is reduced. The SPM process, using a mixture of sulfuric acid and hydrogen peroxide, is a popular wafer cleaning process because it quickly removes large amounts of organic material from the surface of silicon wafers. It is ideally suited for stripping photoresist and it hydroxylates surfaces making them hydrophilic (having an affinity for water) in preparation for subsequent wafer processing steps. Spiking with hydrogen peroxide keeps the process going but it means the mixture has to be replaced about once a day. Modutek has developed an innovative way to control the process, resulting in significant cost savings and improved results.

How the SPM Process Cleans Wafers

The mixture of sulfuric acid and hydrogen peroxide removes organic material very quickly because it is highly corrosive, but it is also unstable. The hydrogen peroxide decays to form water that reduces the mixture concentration and slows down cleaning. To keep the process going, the mixture is periodically spiked with hydrogen peroxide, bringing the concentration back up. At the same time, adding hydrogen peroxide to sulfuric acid is exothermic, raising the temperature of the mixture. Cleaning takes place more quickly at higher temperatures, but the hydrogen peroxide decays more quickly as well, influencing control accuracy. Repeated spiking limits the useful life of the mixture to about one day. Daily replacement of the chemicals is expensive and results in delays for the processing line.

“Bleed and Feed” Process Change Saves Money and Improves Output

Modutek has developed an innovative SPM control strategy that prolongs the life of the SPM mixture while precisely controlling mixture temperature and concentration. In the “bleed and feed” method, Modutek uses a two tank system with a clean tank and a dirty tank. Small programmable amounts are regularly drained from the dirty tank and replaced with mixture from the clean tank. To restore the mixture in both tanks, sulfuric acid is added to the clean tank to replace the amount drained and small amounts of hydrogen peroxide are added to both tanks to maintain the required concentration. This semi-continuous process keeps the mixture concentration within narrow limits without spiking and without the temperature changes caused by spiking.

The “bleed and feed” method is PLC-controlled and the amounts to be drained, the replacement amounts and the frequency of operation can all be adjusted for optimal process results. The concentration and temperature of the mixture remain within tight limits and the process control is more accurate. The useful life of the mixture can be extended to as much as a week and the more precise process control gives improved results.

Benefits from “Bleed and Feed” Process Change

Modutek’s “bleed and feed” method means that chemical use and disposal is reduced substantially. When the wafer cleaning process performance is precise and predictable, better repeatability and consistent cleaning times result in better process line performance. When mixture life is stretched from one day to a week, the cost of chemicals is greatly reduced while the process equipment can stay in service longer since it doesn’t have to be shut down as often for changing the mixture. Downtime is lower and throughput increases.

As a leading semiconductor equipment manufacturer, Modutek continues to work closely with customers to develop innovations that can improve process results. The “bleed and feed” control method increases productivity and reduces costs while the decreased use of chemicals is environmentally friendly. The new process control method could be of interest to manufacturing and research facility managers who can use it to lower production costs, improve output quality and decrease the plant’s environmental footprint. Modutek offers free consultation can discuss details on how the new SPM control method can improve the process for your application.

New SPM Process Provides 75% Improvement in Chemical Savings

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Earlier this year Modutek started implementing its new SPM process with automatic adjustment of the sulfuric peroxide mix concentration and the company now has benchmark results from real process applications. Normally the SPM mixture deteriorates continuously as the hydrogen peroxide turns to water and dilutes the concentration. The SPM mixture has to be completely replaced every eight hours or so at the outside. Costs are substantial for downtime during new mixture pour and for the replacement of the chemicals as well as for neutralizing and disposing of the corrosive mixture in an environmentally safe way. Trying to lengthen the period during which an SPM mixture can be used has been an ongoing process for many companies but without results that provide a safe and reliable alternative.

In the Modutek “Bleed and Feed” system, a “dirty” tank and a “clean” tank are used to continuously compensate for changes in the mixture concentration caused by the degradation of hydrogen peroxide. Periodically when needed, a programmable amount of mixture is drained from the dirty tank and another programmable amount of mixture is automatically transferred from the clean tank to the dirty tank. To compensate, an appropriate amount of sulfuric acid is added to the clean tank and both tanks receive enough hydrogen peroxide to restore the concentration. The “Bleed and Feed” process is designed to maintain the SPM mixture concentration automatically over extended periods.

Benchmark Results from a Real “Bleed and Feed” Application

One of Modutek’s customers that use their semiconductor manufacturing equipment now has real-life results from the use of the company’s “Bleed and Feed” SPM process improvements. The results show significant reductions in chemical use, downtime for re-pour and disposal volumes. Detailed results are based on a 2 6-inch carrier bath size with an 8-hour shift and are as follows:

  • Volume of sulfuric acid used reduced by 77%
  • Chemical disposal volume reduced by 75%
  • Acid neutralizer use reduced
  • Acid re-agent use and cost reduced
  • No downtime to drain and re-pour
  • System drains reduced from three times a day to once a week
  • Increased throughput
  • No original process result changes
  • Increased safety due to less operator interaction with dangerous chemicals

While other companies have tried to develop other SPM clean process improvements involving less chemical use, Modutek’s system is now proven to provide excellent results in real life applications and operate reliably.

Improved SPM Process in Custom Designed Systems

While Modutek has developed a greatly improved method for maintaining SPM process mixture concentrations and reducing chemical use, the new method can be applied in any custom-designed SPM process wet bench. Modutek designs and assembles wet bench products and semiconductor manufacturing equipment at their facility in San Jose, California and can supply systems that exactly meet its customers’ requirements.

Process engineers and facility managers concerned about the environmental impact of high toxic chemical use and the increasing costs of chemicals such as sulfuric acid can contact Modutek for a free quote or consultation to discuss solutions to these issues. Modutek can suggest wet bench systems and follow up with proposals from their complete line of wet bench products. If the SPM process is needed by a customer, Modutek now has the benchmark results on reduced chemical use and improved facility operation.

 

Changes to the SPM Process Improves Efficiency and Results

The sulfuric acid hydrogen peroxide mixture (SPM) process effectively strips and cleans wafers but suffers from instability due to the decomposition of the hydrogen peroxide. A heated bath improves performance but increases the rate at which hydrogen peroxide changes into water. Periodically spiking the mixture with extra hydrogen peroxide can restore the required concentration but affects the strip rate. As a result, the SPM mixture has to be changed frequently, leading to high costs for chemicals and frequent downtime on wet bench stations. Working closely with a local semiconductor manufacturer, Modutek has developed a “bleed and feed” process upgrade that addresses these problems and results in substantially longer mixture life and improved process results.

The SPM Process

A typical mixture for the SPM strip and clean process is one part hydrogen peroxide to three parts sulfuric acid. The sulfuric acid parts can go as high as 7 parts but once the process has started, if the concentration changes, the strip rate can change as well. The mixture is heated to 130 to 140 degrees centigrade to increase the strip rate. The higher the temperature, the faster the hydrogen peroxide decomposes to water, diluting the mixture.

To keep hydrogen peroxide in the mixture, more of the chemical is periodically added to the process, spiking the concentration back to its original level. Despite this, the mixture is normally usable for only a few hours at a time and typically has to be replaced several times a day. Replacing the mixture is costly and requires repeated downtime when processing has to wait for a new pour. In addition to the costs of the chemicals, there are treatment and disposal costs that are required to comply with environmental regulations.

Modutek’s “Bleed and Feed” Process Change

Modutek has developed a hydrogen peroxide spiking method that keeps the chemical mixture at a predictable concentration and avoids the requirement for frequent replacement. The method reduces consumption of the chemicals and lets the system run for extended periods without down- time.

Using a two-tank dirty and clean tank process, the “bleed and feed” method involves draining a programmable amount of mixture from the dirty tank prior to each process step. Another programmable amount is fed from the clean tank to the dirty tank. Then programmable amount of sulfuric acid is added to the clean tank and both tanks receive programmable amount of hydrogen peroxide.

When the entire mixture has to be replaced, normally about once every week, the system has a quick drain feature that reduces down time and gets the process up and running again quickly. The whole “bleed and feed” spiking method is PLC controlled and the periodic adding and spiking of chemicals allows the mixture to be used continuously for days at a time.

Benefits of the Modutek Process Change

In addition to savings related to the reduced use of chemicals, the new update improves overall efficiency and achieves better results. Instead of having to shut down the wet bench station every shift to pour new acid, the process can run for days and only requires a complete chemical change once per week. In addition, the process is cleaner and can deliver better cleaning and stripping performance.

Modutek is offering the SPM process upgrade on their new wet bench stations and can provide details regarding chemical use and performance. The company has the experience and expertise to help customers more effectively use the SPM and other processes to ensure they obtain the results they need. Call Modutek at 866-803-1533 for any questions you have with using either the updated SPM process or other wet processes needed for your application.