Improving Silicon Wafer Cleaning with the Piranha Etch Process

The Piranha etch process removes organic material from silicon wafers rapidly and completely. Semiconductor manufacturing involves the repeated etching and cleaning of the silicon wafers and the Piranha mixture is a favorite method for the resist strip of wafers to prepare them for further processing. Modutek can provide high temperature re-circulating and constant temperature quartz baths and the company has developed a new “bleed and feed” control method to improve the silicon wafer cleaning process.

Modutek Quartz Baths for Piranha Etch

Modutek quartz baths are based on extensive experience and the use of the highest quality materials. The baths themselves are made of virgin boron-free fused quartz in a flame-retardant polypropylene housing. The QFa series is a high temperature re-circulating bath with a temperature range of 30 to 180 degrees centigrade while the Qa series is a constant temperature bath with the same temperature range.

Both bath series are temperature controlled to plus/minus 1 degree centigrade with a heat-up rate of 2 degrees centigrade per minute. Process control can be fully automated, semi-automatic or manual. The baths are available in a variety of sizes and Modutek will construct custom units as required. The units are ideal for Piranha etch applications because of their quick and even heating, accurate temperature control and extended vessel life.

Improving Piranha Process Control

The Piranha process uses an aggressive mixture of sulfuric acid and hydrogen peroxide to dissolve organic residue on the silicon wafers. The mixture is heated to about 130 degrees centigrade to improve the strip speed. Control of the cleaning is difficult because mixing sulfuric acid and hydrogen peroxide is exothermic and heats up the solution when the mixture is first prepared. As the mixture cools, it has to be heated to maintain its temperature and the strip rate.

The hydrogen peroxide in the mixture is unstable and decomposes to form water, diluting the mixture and slowing the strip rate. Heating the mixture increases the rate of decomposition of the hydrogen peroxide. To keep the concentration and the strip rate constant, the sulfuric acid and hydrogen peroxide mixture is periodically spiked with extra hydrogen peroxide. This addition keeps the strip rate elevated but the overall process is hard to control and the mixture has to be replaced completely about once per day. To improve the Piranha process and silicon wafer cleaning, the concentration and temperature variation issues have to be addressed.

The Modutek “Bleed and Feed” Process Control Method

Modutek has developed a method of improving control of the Piranha process by using a two tank system with a clean and a dirty tank. When the concentration of hydrogen peroxide goes down, a small amount of mixture from the dirty tank is drained and discarded. The drained amount is replaced from the clean tank. The stripping process can continue and the concentration is maintained at the desired level. The clean tank has its sulfuric acid and hydrogen peroxide replenished. All “feed and bleed” amounts are programmable to match specific process variables.

Benefits of the Process Change

The “bleed and feed” control method can be fully automated and the frequent addition of small amounts of sulfuric acid and hydrogen peroxide mixture keeps the strip rate constant and allows for continuous use of the mixture over an extended period of time. The benefits include:

  • Savings of chemicals can reach 75 percent while chemical purchase and disposal costs are correspondingly lower.
  • Process efficiency is increased due to less downtime for replacement of the chemicals.
  • Process results are improved due to a more constant strip rate.

As a leading semiconductor equipment manufacturer, Modutek provides customers with high quality equipment that offers the highest degree of process control. Modutek supports the new “bleed and feed” process change for Piranha strip in the company’s new wet bench stations. Call for a free consultation to discuss your specific process requirements.

Reviewed and Approved by Douglas Wagner
President & CEO, Modutek Corporation

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.

 

Why Particle Removal is Essential in Silicon Wafer Cleaning

During silicon wafer processing, impurities and particles are deposited on wafer surfaces or are left over from previous process steps. Such particles can cause defects in the final semiconductor product. With the reduced size of today’s silicon wafer microstructures, even the tiniest particles can block etching and affect the diffusion processes. The result appears in the final semiconductor circuit as either as a defect or reduce quality and life expectancy of the product. As a result, the focus of many wafer cleaning operations is to leave the silicon wafer surface intact but free of contaminating particles.

How Particles are Removed from Silicon Wafer Surfaces

The removal of particles can be difficult because they often have a chemical or electrostatic affinity for the silicon surface. They are attracted to the silicon wafer because of electrostatic charges and specific mechanisms have to be used to dislodge and remove them. The smaller the particle, the more such attraction may play a role and the harder it is to remove every particle from the wafer.

Mechanisms to remove particles include silicon wafer cleaning with a chemical that reacts with the particles, cleaning with a solution that dissolves the particles or washing the particles from the wafer surface. In each case, a specific type of equipment is required and traditional standard processes can be used together with new technologies aimed at removing even the smallest particles.

Cleaning Processes

Many chemical processes used to clean silicon wafers have remained unchanged since they were first used 30 years ago. These methods use aggressive chemicals to remove contamination from the wafers, which are then rinsed with de-ionized water and dried. These methods remove most of the contaminants but are less effective in removing the smallest particles. Refined older methods and new technologies such as megasonic cleaning are now often used to complete the cleaning process.

Cleaning methods used at different stages of the silicon wafer fabrication process include the following:

  • The RCA clean process, often carried out in two steps called SC1 and SC2, prepares a wafer for further processing. SC1 cleans wafers with a mixture of ammonium hydroxide and hydrogen peroxide to remove organic residue. SC2 uses hydrochloric acid and hydrogen peroxide to remove metallic residues and particles.
  • The Piranha cleaning process removes large amounts of organic residue such as photoresist. It uses sulfuric acid and hydrogen peroxide in a particularly corrosive mixture that acts quickly but must be handled with care.
  • Megasonic cleaning dislodges particles and other contaminants using microscopic cavitation bubbles generated by a megasonic cleaning system. The bubbles form and collapse in time with the MHz sound waves, delivering a scrubbing action that overcomes particle attraction to the silicon wafer surface.
  • The Ozone cleaning process uses ozone to convert organic particles and contaminants to carbon dioxide. All organic traces on a wafer surface are completely removed, leaving the silicon wafer free from particles.

One of the most critical processes for silicon wafer cleaning is the pre-diffusion clean process that takes place just before the wafers are placed in the diffusion oven. Any of the above methods or a combination of cleaning methods can be used to ensure that wafers are free of particles and the diffusion will be even and consistent.

Equipment Used for Wafer Cleaning

Modutek’s wet bench technology supports all the above cleaning methods and can be provided within their manual, semi-automated or fully automatic systems. The company can offer equipment for traditional cleaning and for the new megasonic and ozone methods as well. All cleaning equipment is available in standard configurations but Modutek can also design custom products to meet the needs of any of their customers’ silicon wafer cleaning requirements. If you need highly reliable equipment to support your semiconductor manufacturing processes call Modutek for a free consultation or quote at 866-803-1533 or email [email protected].

 

How Pre-Diffusion Cleans Are Used in the Wafer Cleaning Process

The silicon wafers processed in semiconductor manufacturing facilities and research centers are repeatedly cleaned in aggressive chemical cleaning baths used to remove surface contaminants from the wafers. The cleaning is important because surface impurities affect the diffusion of dopants in the diffusion ovens. Depending on the next diffusion step, wafers have to be completely clean and several types of material may have to be removed. The wafer cleaning process can include multiple cleaning methods to prepare the wafers for the subsequent diffusion step.

Organic Material and Photoresist

When the previous manufacturing step included masking, wafers may still have photoresist adhering to their surfaces. Any such materials have to be removed because they will interfere with the doping process. If contamination is heavy, Piranha etch, a mixture of sulfuric acid and hydrogen peroxide is often used. The sulfuric acid dissolves the carbon-based compounds and the peroxide oxidizes the carbon to carbon dioxide, which is given off as a reaction product.

General Cleaning and Metallic Particles

For lighter organic contamination the two-step RCA clean process (originally developed by RCA Corporation) is often used. The first or SC1 step cleans organic contamination from the wafers but not metallic particles. The second or SC2 step removes the metallic particles and creates a protective layer on the silicon surface.

SC1 clean involves immersing the wafers in an aqueous mixture of ammonium hydroxide and hydrogen peroxide. This removes traces of organic matter and non-metallic contaminants but it leaves metallic particles on the wafer surfaces. These particles have to be removed before diffusion takes place because they may otherwise be embedded into the wafer in addition to the desired doping and cause defects in the semiconductor material.

In SC2, a hydrochloric acid and hydrogen peroxide solution in deionized water dissolves the metallic particles and leaves a clean wafer surface. In addition, a passivation layer is created that protects the silicon from further contamination. The wafers are now ready for further processing.

Native Oxide Layer Removal

The SC1 clean results in an oxide layer forming on the silicon wafers, and even if SC1 is not used, silicon oxide forms naturally on the clean, bare wafers. For some semiconductor fabrication steps, the native silicon oxide layer has to be removed. The silicon oxide layer is stable and very few chemicals can attack and remove it. Hydrofluoric acid is often used but it is extremely dangerous to handle and a safe set-up is important. Usually the wafers are dipped briefly into the acid, which acts very quickly. After a rinse with deionized water and drying, the wafers are ready for the diffusion oven.

Other Cleaning Methods

For some semiconductor fabrication processes, Modutek has pioneered wafer cleaning methods that avoid the use of harsh chemicals. Megasonic cleaning uses megasonic sound waves in a cleaning solution to dislodge particles and clean wafers. Vibrations and the cavitation bubbles of the high-frequency waves dislodge submicron particles and remove contaminating films. Modutek’s ozone treatment is a fast and cost-effective alternative to chemical cleaning for some applications. The ozone converts organic residue to carbon dioxide and does not leave metallic particles behind.

With its extensive experience in wet processing applications and its expertise in providing high-quality wafer cleaning process equipment, Modutek can advise customers on solutions for their semiconductor fabrication needs. The company can evaluate specific fabrication processes and suggest equipment from its complete line, ensuring that the proposed stations fulfill requirements safely and effectively. Modutek’s wet benches support the pre-diffusion cleaning processes as well as general cleaning, stripping and etching and they are available in manual, semi-automated and fully automated stations. Contact Modutek at 866-803-1533 for a free quote or consultation to discuss which process equipment will work best to support your application.

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.

 

Why Quartz Baths Are Used in the Wafer Cleaning Process

Why Quartz Baths Are Used in the Wafer Cleaning ProcessThe cleaning of silicon wafers during semiconductor manufacturing uses strong, corrosive chemicals to remove deposits from the surface of the silicon. Critical factors for effective cleaning are an absence of contamination in the chemical bath, the impervious nature of the container and a tight control of the bath temperature. Baths made from high purity quartz don’t react with chemicals used in semiconductor manufacturing and are inert as a source of contamination. Quartz conducts heat well so fast temperature rise and accurate control are possible. A high-quality quartz bath is an ideal container for the semiconductor wafer cleaning process.

How Quartz Baths Are Used

The semiconductor manufacturing process involves multiple steps, each of which may require cleaning of the silicon wafer before or after the production process segment. Highly corrosive chemicals such as sulfuric acid, hydrogen chloride or hydrogen peroxide are used to remove materials such as organic residue from the surface of the wafers in preparation for manufacturing steps such as etching or diffusion. Effective cleaning is a critical factor in the success of semiconductor manufacturing because impurities or contaminating particles left on a wafer can lead to defective products or products of inferior quality.

During the cleaning process, the silicon wafers are immersed in chemicals within the quartz bath. Depending on the cleaning process, the baths may be heated, kept at a given temperature for a defined time period and chemicals may be re-circulated. For example, in RCA clean, wafers are first immersed in a mixture of ammonium hydroxide and hydrogen peroxide. These chemicals remove organic contaminants and particles from the surface of the silicon wafers. After rinsing and drying, the wafers are immersed in a mixture of hydrochloric acid and hydrogen peroxide. In this step metallic contaminants and particles are removed. The wafers are then cleaned for subsequent processing steps.

If particles remain on the wafers or if impurities are introduced during the cleaning process, manufacturing steps such as diffusion will be affected. With microscopic semiconductor structures, even tiny particles can obstruct an electrical connection or affect electrical characteristics. The semiconductor product may then fail prematurely or not work properly. When the quartz container is eliminated as a source of contamination, the focus is on the wafer cleaning process itself and its effectiveness in removing contaminants and particles.

How Quartz Baths Facilitate Reliable Cleaning

Quartz baths can meet all the criteria for effective cleaning of silicon wafers but they have to be designed well and constructed with top quality materials. Modutek series QFa re-circulating baths are made with semiconductor grade flame polished quartz to reduce the possibility of contamination. They feature quick, even heating and are available in a variety of standard as well as custom sizes. Operation is reliable and safe with excellent wafer cleaning performance and high quality output.

Operating characteristics include a temperature range of 30 to 180 degrees centigrade and the tanks can heat the bath at a rate of 2 degrees centigrade per minute. Temperature control is accurate within plus/minus 1 degree centigrade and heating is even through the bath. Standard tank sizes range from 7.75 inches to 21.5 inches a side with a depth of up to 14.5 inches. Custom sizes are available.

Modutek uses its experience of over 30 years designing quartz baths to deliver the highest quality baths with excellent cleaning performance. Modutek baths feature safe operation, reliable output and a low cost of ownership. The company can advise customers how best to meet their semiconductor wafer cleaning needs and can make specific proposals for solutions based on the company’s complete range of wet bench processing equipment.

How Megasonic Cleaning Improves the Silicon Wafer Cleaning Process

How Megasonic Cleaning Improves the Silicon Wafer Cleaning ProcessAs silicon microscopic circuits and structures shrink in size, the elimination of contaminants from becomes increasingly important. When silicon wafer cleaning is effective, it removes particles as small as 0.1 µm to prevent them from affecting the silicon fabrication process. Traditional wafer cleaning with chemicals may leave some of the smallest particles in place and production line output quality can suffer. The semiconductor components produced may be of inferior quality or fail completely. Megasonic cleaning with sound waves in the MHz range generated in a water cleaning solution can remove particles down to 0.1 µm in size and improve cleaning performance.

How the Megasonic Process Cleans

The Megasonic Cleaning System consists of a high-frequency generator, transducers that convert the electric signal from the generator to sound waves in the water, and a cleaning tank to hold the cleaning solution and the silicon wafers. Sound waves in the MHz frequency range travel through the cleaning liquid and generate microscopic cavitation bubbles in the low-pressure wave troughs. When the bubbles collapse in the high-pressure wave peaks, they produce tiny jets of water.

When the bubbles collapse near a wafer, the resulting jets hit the silicon and dislodge any particles adhering to the surface. The particles are carried away by the water currents and the microscopic bubbles are so numerous that all surfaces are cleaned. The bubbles and the cleaning effect are present throughout the liquid and they penetrate into holes, crevices and microscopic structures, cleaning completely.

Megasonic Cleaning Benefits

In addition to cleaning silicon wafers and removing microscopic particles more effectively than traditional cleaning methods, Megasonic Cleaning provides several other benefits over the use of chemicals. The rise in output quality is accompanied by lower costs, a safer process environment and shorter process times.

When a semiconductor fabrication facility uses fewer chemicals for cleaning wafers, costs decrease. The facility has to purchase smaller amounts of chemicals, storage costs are lower and costs for disposal are less. Depending on the process, Megasonic Cleaning may allow a facility to eliminate certain chemicals completely, resulting in even higher savings.

The Megasonic Cleaning process is safe and environmentally friendly. The megasonic waves and the cleaning solution do not present any danger to equipment or operators who can set a timer and come back when the cleaning process is finished. The water-based solution is not toxic and does not require special disposal. Compared to chemical cleaning, the storage and handling of Megasonic cleaning materials is safe and easy, power consumption is lower and less water is required.

Modutek’s Megasonic Cleaning System

The Modutek Megasonic Cleaning System was developed together with ultrasonic technology leader Kaijo Corporation to reflect the specific demands of silicon wafer cleaning. The high operating frequency in the MHz range ensures gentle but effective cleaning of delicate materials without pitting the silicon surface or damaging the silicon wafer structures.

Modutek’s partnership with Kaijo lets the company offer an integrated system that features the Quava Megasonic generator and transducer within Modutek’s cleaning baths. Megasonic’s baths are available in the indirect heating MSI series that can heat the cleaning solution up to 140 degrees centigrade while the direct heating MSD series can heat the cleaning solution up to 70 degrees centigrade. Both baths are ideal for submicron particle removal with a high power density and high efficiency.

The Megasonic System is available with 600 W, 900 W and 1200 W power ratings at the standard 950 kHz frequency. Frequencies of 200 kHz, 430 kHz, 750 kHz and 2 MHz are also available for applications that require the more robust cleaning action from the lower frequencies or for very delicate components at the higher frequency. In any case, Modutek can advise customers to make sure they select the ideal Megasonic Cleaning configuration for their specific silicon wafer cleaning applications.

Why Pre-Diffusion Cleans Are Critical in Wafer Cleaning Processing

Why Pre-Diffusion Cleans Are Critical in Wafer Cleaning ProcessingBefore silicon wafers are placed in a diffusion furnace, they must be cleaned to remove impurities and particles from their surfaces. Such pre-diffusion cleaning takes place several times during semiconductor manufacturing as microscopic structures are fabricated in the silicon. Various wafer cleaning process methods include the use of different chemical baths, ozone treatment or megasonic cleaning for ensuring that wafers are clean and free from contamination. Modutek offers a complete range of equipment that supports the various cleaning methods and helps ensure the highest quality output from semiconductor manufacturing facilities.

Cleaning Methods

Standard cleaning methods involve immersing the silicon wafers in baths of powerful chemicals that remove contaminants and particles from the surface of the wafers. Silicon is relatively inert to reactions with the chemicals used but organic and metallic impurities on the wafer surfaces are oxidized and dissolved.

In the RCA wafer cleaning process silicon wafers are placed into a solution containing ammonium hydroxide and hydrogen peroxide. This bath removes organic contaminants but may leave metallic traces. As a result, the RCA clean process is often carried out in two parts called SC1 (standard clean 1) and SC2. After the ammonium hydroxide/hydrogen peroxide (APM) bath, the SC2 step consists of a bath containing hydrochloric acid and hydrogen peroxide (HPM). SC2 removes metal ions and leaves the wafer clean and ready for further processing.

Piranha etch clean is used to remove large amounts of organic residue from silicon wafers. It consists of a bath containing sulfuric acid and hydrogen peroxide and can dissolve hard to remove organic material such as photoresist. The solution’s strong oxidizing properties hydroxilate the surfaces of the silicon wafers, making them hydrophilic or attractive to water. This property may be an advantage for subsequent cleaning steps.

Megasonic cleaning is another method for obtaining clean, particle-free silicon wafers for diffusion. The method uses cavitation bubbles produced by megasonic waves in a cleaning solution to remove particles and contaminants from wafers. The bubbles produce a scrubbing action that cleans wafer surfaces without the use of expensive and corrosive chemicals.

In some cleaning applications, ozone may be used to obtain clean, particle-free silicon wafers. The wafers are first rinsed with deionized water to remove water-miscible contaminants and are then exposed to ozone in an ozone chamber.  All organic contaminants and particles are converted to carbon dioxide leaving the wafer surface clean and particle-free.

The Importance of Effective Cleaning

The presence of particles on the silicon wafer surface affects the subsequent diffusion steps and impacts the quality of the final product. Particles may be diffused into the silicon along with the doping substances and produce unknown electrical anomalies. They may remain on the surface and block subsequent etching processes to change microscopic structures. They may interfere with the creation of tiny conductor paths on the surface of the silicon and they can create chemical patches that don’t react as expected.

With the trend to smaller size geometries in silicon wafer structures, the potential damage that a single particle can cause has increased. Particles may affect the performance of a semiconductor product or its reliability. The product lifetime may be reduced or its functionality impaired. Defective products reduce plant performance while low quality products hurt the facility’s reputation. Ideally semiconductor manufacturers need equipment that will help them produce high quality semiconductors with low reject rates by emphasizing effective wafer cleaning methods with a maximum reduction in the presence of particle contamination.

Modutek Wafer Cleaning Solutions

Modutek offers a complete line of chemical baths and wet processing equipment that supports RCA clean with SC1 and SC2, piranha clean, megasonic cleaning and ozone clean. Each wafer cleaning process is designed to provide effective pre-diffusion cleaning to produce silicon wafers with a minimum of particle contamination.

In addition to traditional chemical cleaning, Modutek has pioneered advanced ozone cleaning and is using megasonic cleaning for additional particle removal for smaller device geometries. The company can help semiconductor manufacturing facilities increase their throughput, improve their performance and reduce their costs by drawing on the extensive experience they have developed in the various silicon wafer cleaning applications.

How the Advanced Ozone Cleaning Process Improves Wafer Yields and Reduces Costs

How-the-Advanced-Ozone-Cleaning-Process-Improves-Wafer-Yields-and-Reduces-CostsModutek’s advanced ozone cleaning process offers two methods that can improve silicon wafer cleaning and reduce chemical use. The main part of the process uses ozone to convert organic residue on wafers into carbon dioxide through oxidation. The wafers coming out of this process have reduced levels of particle contamination when compared to traditional cleaning methods using chemical baths. The improved cleaning performance of the patented advanced ozone cleaning process can increase facility throughput and improve semiconductor quality while reducing chemical use.

 

How Advanced Ozone Cleaning Works

With ozone cleaning in Modutek’s DryZone gradient dryer, wafers are first rinsed with deionized water to remove inorganic impurities before being exposed to ozone in an ozone chamber. The Coldstrip ozone cleaning method operates at sub ambient temperatures while Organostrip cleans at room temperature.

The ozone’s powerful oxidizing action converts the carbon of organic residue to carbon dioxide and leaves wafers clean and particle-free. The ozone is dissolved in either plain water or in a mild solvent solution and substrates receive a stable hydrophilic surface.. No harsh chemicals are required and the exhausts and waste products of the process are harmless.

 

Advantages and Benefits

The advantages of Modutek’s advanced ozone cleaning process in comparison to traditional chemical cleaning methods derive from the more compact process equipment, the absence of harsh chemicals and better cleaning performance. Benefits include lower costs, higher quality products due to lower particle counts and an environmentally-friendly cleaning process that doesn’t require compliance with onerous regulations.

Where Piranha cleaning requires as many as six processing tanks and traditional solvent cleaning five tanks, the Organostrip ozone cleaning method requires only three tanks while the Coldstrip method requires two. As a result, equipment costs and space required will be less.

One of the main issues with chemical bath cleaning is the short lifetime of the cleaning solution. The Piranha clean mixture deteriorates immediately and cleans effectively for only a few hours. Other solvents have a solution life span measured in days, after which the solution has to be replaced. Because the ozone cleaning methods do not rely on solution components that react chemically with each other, the lifetime of an ozone solution is at minimum greater than a week.

Other advantages of the ozone cleaning method are its compatibility with metal films, the absence of water stains and the low particle count. Where a typical particle count for ozone cleaning is less than 15, cleaning with Piranha solution or solvents produces particle counts in the thousands.

The resulting benefits for a semiconductor manufacturing facility can be substantial. The process costs for Coldstrip are estimated at $21,000 per year while Organostrip runs to $104,000. These amounts compare to approximately $300,000 for Piranha strip and $200,000 for other solvent methods.

In addition to saving money on process costs, the costs of chemical storage, handling and disposal must be factored in. All of these have a component for regulatory compliance that can be very costly, depending on the jurisdiction in which the facility is operating. The ozone cleaning process has no such costs.

Finally there is a key benefit to the reduced particle counts obtained with the ozone process. Subsequent diffusion and etching will result in fewer defective components and a higher quality final product. Overall, ozone cleaning allows a semiconductor manufacturing facility to improve production line performance and substantially reduce costs.

 

Modutek Can Help

Modutek offers the equipment for applying the patented ozone cleaning process to semiconductor manufacturing and can help in determining the ideal configuration for a particular application. The company also delivers equipment for traditional chemical bath cleaning methods and is therefore ideally placed to advise how the new ozone cleaning methods can improve production. Semiconductor manufacturers can take advantage of this capability to cut costs and increase productivity in their plant.

How Piranha Etch is Used in Silicon Wafer Cleaning

How Piranha Etch is Used in Silicon Wafer CleaningThe fabrication of silicon wafers is carried out 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 the cleaning of 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

A piranha solution is made up of a mixture of sulfuric acid and hydrogen peroxide. 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 operating temperature and the desired concentration are reached, the wet bench equipment has to heat the solution to maintain the temperature and keep the etch rate constant.

When a facility wants to re-use a piranha solution for an extended period of time, 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 a facility use a piranha solution for up to eight hours rather than having to replace it every two hours. Spiking saves money by conserving sulfuric acid but it requires wet bench equipment that can handle the process and the spiking.

Semiconductor fabrication facilities and research labs use piranha etching to clean residue from silicon wafers and glass. The solution removes trace organic contaminants and strip residue while oxidizing metals. The underlying surfaces are hydroxilated making them hydrophilic or attractive to water, a characteristic that can be used in subsequent silicon semiconductor manufacturing process steps.

Modutek’s Silicon Wafer Cleaning Solutions

Modutek specializes in semiconductor manufacturing equipment and wet bench technology that includes solutions for wafer cleaning. The company can supply standard equipment or can customize wet bench solutions to fit specific customer requirements. Priorities are high quality materials, excellent designs, low cost of ownership and high quality results. Piranha wafer cleaning is supported by Modutek’s QFa quartz recirculating 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 baths 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, gravity drain and a quartz bubbler. Both baths are available in standard sizes or can be custom made to fit customer requirements.

Modutek can provide a complete range of wet bench and 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 wet bench equipment allows operators to use piranha cleaning methods safely to increase facility throughput while maintaining or improving output quality. Call or email Modutek for a free quote or recommendations on using the right equipment for your wafer cleaning application.