How Specialized Wet Bench Equipment Improves the RCA Clean Process

How Specialized Wet Bench Equipment Improves the RCA Clean ProcessThe RCA clean process removes contaminants from the surface of silicon wafers so that additional wet process semiconductor manufacturing steps can take place. The process consists of two steps, with the SC1 step removing organic compounds and the SC2 step dislodging any remaining metallic residues or particles. Specialized wet benches ensure that contaminant and particle removal is as complete as possible and the silicon wafers remain clean. To accomplish this, the equipment carefully rinses away contaminants and minimizes the handling of the wafers. Automation can be used to replicate process parameters reliably and consistently. Modutek can supply both standard and customized specialized equipment to carry out the RCA clean process.

RCA Clean SC1 Removes Most of the Wafer Surface Contamination

The SC1 cleaning step uses chemicals to dissolve impurities while leaving the underlying silicon surface unaffected. The wafers are placed in a solution containing equal parts of NH4OH (ammonium hydroxide) and H2O2 (hydrogen peroxide) in five parts of de-ionized water. The solution is heated to about 75 degrees centigrade and the wafers are left in the solution for ten to fifteen minutes. Organic residues are dissolved and particles are removed. A thin layer of silicon oxide forms on the wafer and there is some contamination with metallic ions and particles.

RCA Clean SC2 Removes Metallic Impurities

For SC2, the newly cleaned wafers are placed in a bath containing equal parts of hydrochloric acid and hydrogen peroxide in five parts of de-ionized water. The exact ratio may vary depending on the application. The bath is heated to about 75 degrees centigrade with the wafers soaking for about ten minutes. The solution specifically eliminates alkali residues, metal hydroxides and other metallic particles. The wafers are now completely clean and free of all types of particles.

Wafer Cleaning Equipment Has to Fulfill Specialized Functions

Specialized equipment is needed to carry out RCA clean process steps effectively. The required chemicals have to be delivered in the right quantities to the cleaning baths and then, when the chemicals are no longer needed, they have to be neutralized and disposed of safely. The concentration of the chemicals, the bath temperature and the timing are all important for being able to subsequently reproduce the desired cleaning performance. Contaminants and particles have to be rinsed away and filtered out. Key features of effective cleaning are a low particle count on the wafer surface and a resulting low rejection rate for fabricated wafers.

Modutek’s Wet Process Equipment Provides Specialized Features

Modutek’s wet benches and chemical stations support both of the RCA cleaning steps. Chemical delivery systems ensure that the right amounts of chemicals are supplied to the process safely and that waste chemicals are neutralized before disposal. The FM4910 material of construction keeps particle contamination low and is safe to use with the acid and base processes of RCA clean. All baths include a continuous flow de-ionized water chamber with chemical circulation and filtration to rinse away and remove contaminants. The SolidWorks flow simulation software lets operators calibrate dosages precisely and store settings for future use.

As well as fulfilling customer needs from its extensive line of wet process equipment, Modutek can build custom systems for specific applications. Customers who wish to incorporate drying, etching or stripping functions as well as RCA clean in their process lines can rely on Modutek to design and build exactly what they need.

With 40 years of experience in the wet process and technology sector and one of the leading semiconductor equipment manufacturers, Modutek has the expertise in house to provide specialized equipment for the RCA clean process and other semiconductor manufacturing applications. Customers can count on Modutek to help them find effective solutions and supply the corresponding equipment. Contact Modutek for a free quote or consultation to discuss your process equipment requirements.

How Are Isotropic and Anisotropic Processes Used to Improve Silicon Wet Etching?

How Are Isotropic and Anisotropic Processes Used to Improve Silicon Wet EtchingThe microscopic structures produced by silicon wet etching can be created with a high degree of precision by using both isotropic and anisotropic processes. Isotropic etching is faster but may etch under masks to create rounded shapes. Anisotropic etching can be controlled more precisely and can produce straight sides with exact dimensions. In each case, controlling the etch bath temperature and the etchant concentration is critical for successful micro-structure creation and for repeatability for subsequent batches.

How Isotropic and Anisotropic Etching Differ

Silicon wafers have a mono-crystalline lattice structure that repeats in all directions but is not equally dense in all directions. Vertical planes contain a different number of silicon atoms than diagonal planes. This means that etching with certain etchants is slower in the directions with more atoms while it progresses faster in the directions with fewer atoms.

Etchants used for isotropic etching, such as hydrofluoric acid, etch at the same speed in all directions, independently of silicon atom density. For etchants used for anisotropic etching, such as potassium hydroxide (KOH), the etching speed depends on the number of silicon atoms in a crystal lattice plane and therefore depends on the direction of the different planes.

The difference in anisotropic etching speeds allows a better control of shapes etched into the silicon wafers. With a corresponding orientation of the silicon wafer, etching can be timed to produce straight or angled sides and sharp corners. Etching under masks can be reduced.

How Isotropic and Anisotropic Etching Are Used in Semiconductor Manufacturing

Isotropic etching is harder to control than anisotropic etching but it is faster. In the initial stages of silicon wafer fabrication, large features are etched into the silicon. At this stage of manufacturing, etching speed is important for facility throughput. Isotropic etching is used to quickly create these large shapes with rounded sides and corners. Although process engineers and operators have less control over the shape of the feature being etched, accurate temperature and concentration control are still important to ensure that the rounded shapes being created are the same on wafers processed in different batches.

After the large shapes are etched with an isotropic process, the micro-structures and metal paths require better control of the details. Anisotropic etching provides this control as long as the lattice structure of the silicon wafer is oriented correctly. Anisotropic KOH etching is reliable and easily controlled. It can be used to create the precise, straight-sided shapes that are required in the final semiconductor product. Accurate control of the temperature and etchant concentration is even more important for anisotropic etching because these process parameters strongly influence the etching speeds in the various directions and therefore influence the final shapes that are etched.

Modutek Teflon Tanks Support Both Isotropic and Anisotropic Etching

For silicon wet etching processes in which the etch speed is temperature dependent, Modutek’s heated Teflon tanks provide rapid heating and tight temperature control. The tanks are either recirculating or static and they can be built into any new wet bench configuration. The tanks feature 360-degree overflow filtration and uniform heating throughout the bath. The heat-up rate is 2 to 3 degrees centigrade per minute and the temperature control accuracy is plus/minus 0.5 degree centigrade. The temperature controls of these tanks are ideally suited for both isotropic and anisotropic etching.

In terms of control of etchant concentration, Modutek can provide for the injection of de-ionized water into the tanks. Because etchant concentration affects the etch speed, accurate concentration control is important for final product quality and repeatability. Modutek can analyze customer requirements, find and build custom solutions and ensure that process control is precise enough to deliver the required results, both for isotropic and for anisotropic etching.

How Megasonic Cleaning Reduces Costs and Improves Silicon Wafer Yields

How Megasonic Cleaning Reduces Costs and Improves Silicon Wafer YieldsBetween process steps that etch silicon wafers and deposit circuit paths, semiconductor manufacturing relies on wafer cleaning to remove material from previous process steps and microscopic contaminating particles. As structures and circuits on wafers decrease in size, even the tiniest particles can interfere with etching and the creation of circuits and micro-structures.

Such interference can cause defects in the final semiconductor product, lower product performance or cause a reduced product lifespan. The use of Megasonic Cleaning results in a gentle cleaning action that dislodges particles from wafer surfaces while leaving the wafer, its circuits and structures undamaged. Megasonic Cleaning reduces chemical use and final product rejection rates using a process that is completely safe and environmentally friendly.

Megasonic Cleaning Delivers Quick Results with Safe, Gentle Cleaning Action

Megasonic cleaning systems generate microscopic cavitation bubbles in the cleaning solution. When a bubble bursts, it produces a tiny jet that dislodges contaminants from the surfaces of the parts to be cleaned. Lower ultrasonic frequencies generate comparatively large bubbles and powerful jets while high frequencies clean with smaller bubbles and less energetic jets.

The tiny structures etched into the silicon and the microscopic metal filaments deposited on the wafers are easily damaged. Products such as microprocessors, micro electro mechanical systems (MEMS), and controllers may not work properly if precise cleaning procedures are not followed. Using frequencies above 950 kHz in the megahertz range for cleaning the wafers ensures that the Megasonic Cleaning action takes place with the tiniest bubbles and the least energetic jets. As a result, Megasonic Cleaning is gentle enough to remove contaminating particles while leaving silicon surfaces, micro-structures and metallic deposits intact.

Reduced Use of Chemicals Saves Money

The traditional wafer cleaning process uses strong chemicals to strip contaminants and particles from wafer surfaces. In addition to the cost of purchasing the chemicals, there are costs for storage, handling and disposal. As environmental regulations become more strict, hazardous chemicals have to be stored in special containment facilities. Chemical delivery systems have to include special measures that guard against spills. Once the chemicals are used, they have to be neutralized and disposed of in a way that doesn’t harm the environment. All these measures are expensive and will become more costly as regulations tighten.

Workplace safety is another costly aspect of the use of hazardous chemicals. Operators have to be protected against inadvertent contact with corrosive materials and many of the chemicals emit dangerous vapors that require expensive ventilation equipment. Operators that work with hazardous chemicals need protective clothing that reduces productivity and work accidents can lead to serious injury.

Megasonic Cleaning removes contaminants from wafers and dislodges particles more effectively than chemical methods. The reduced use of chemicals saves money and leads to a safer, more productive workplace environment.

Modutek’s Megasonic Cleaning Improves Sub-Micron Particle Removal

Modutek has partnered with Kaijo Corporation, a world leader in megasonic and ultrasonic cleaning technology, bringing Megasonic Cleaning to wet bench semiconductor manufacturing. Modutek has integrated Megasonic Cleaners into the company’s wet bench equipment to produce unparalleled low particle counts. The bubbles of the Megasonic Cleaning system agitate the cleaning bath to disrupt boundary layers, allowing effective cleaning action around complex microscopic structures and inside holes. The tiny jets produced by the bursting bubbles break the bonds between the wafer surfaces and surface particles and allow the particles to be removed by cleaning solution currents.

The low particle counts achieved by Modutek’s wet bench stations incorporating Megasonic Cleaning reduce defects and improve yields for semiconductor manufacturing lines. Product quality rises while costs go down. Using the latest Modutek Megasonic Cleaning equipment improves overall facility performance for semiconductor manufacturing plants and research labs. Contact Modutek for a free consultation on selecting the right equipment to meet your manufacturing 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.

Improvements to the SPM Process Provides Significant Results

Wet bench semiconductor manufacturing relies on the SPM wafer cleaning process to quickly strip photoresist and other residue from silicon wafers. Because the hydrogen peroxide in the sulfuric acid peroxide mixture is unstable, hydrogen peroxide continuously degrades into water and the degradation is accelerated if the mixture is heated to increase the strip rate. To keep the SPM process concentration stable, hydrogen peroxide has to be added, spiking the concentration back to its original level. After several hours of repeated spiking, the mixture has to be replaced.

While other semiconductor equipment manufacturers provide less efficient solutions, Modutek has developed a “bleed and feed” method that keeps the SPM hydrogen peroxide concentration steady. The company has now collected operational results from customers using the new method and a comparison with the traditional SPM process shows substantial savings from reduced use of acid and better operational results.

How Modutek’s “Bleed and Feed” Method Works

The Modutek “bleed and feed” method uses a two-tank configuration with a clean tank and a dirty tank. Periodically an amount of mixture is drained from the dirty tank and replaced with mixture from the clean tank. The clean tank receives an amount of sulfuric acid to replace the drained amount and both tanks receive enough hydrogen peroxide to bring the concentration up to the set level.

The method is PLC-controlled with the initial drain amount, the replacement amount, the hydrogen peroxide amounts and the “bleed and feed” frequency all programmable. The SPM process concentration is maintained and the strip rate remains unchanged, but the SPM mixture has a much longer life. The cost savings result from the SPM mixture not having to be changed frequently.

The “Bleed and Feed” Method Reduces the Consumption of Chemicals

Operational data from customers shows that the Modutek “bleed and feed” method substantially reduces the use of the SPM process chemicals. Data was collected from a semiconductor manufacturing facility using a 2 6-inch carrier bath size and operating on an eight-hour shift.

The use of sulfuric acid was reduced by 77% while the volume of chemicals disposed at the end of the process went down 75%. Acid neutralizer and acid re-agent use declined as well. The reduced cost of chemicals and the savings from lower disposal volumes were substantial.

“Bleed and Feed” Can Increase Throughput

The spiking with hydrogen peroxide of the traditional SPM wafer cleaning process means that mixture replacement causes frequent down times for drain and re-pour. Use of the Modutek “bleed and feed” method reduced system drains from three times per day to once per week on average. The once-per-week change is supported by a quick-drain feature that lets the change-over to a new mixture take place quickly. Overall throughput was increased.

Operational Advantages of “Bleed and Feed”

In addition to cost savings from reduced chemical use and higher product volumes from increased throughput, customers saw operational advantages from using Modutek’s “bleed and feed” method. The reduction in chemical handling lowered the possibility of spills and increased operator and employee safety. At the same time, strip rates and other SPM process characteristics were at least maintained at previous levels or improved.

Customers Can Immediately Take Advantage of “Bleed and Feed”

The Modutek “bleed and feed” method is especially attractive to facility managers, process engineers and researchers concerned about the environmental impact of high volumes of toxic chemicals. Modutek can deliver the new method as part of its standard wet bench stations or prepare a custom design for a specific application. Since Modutek designs and builds its own equipment, the company has the expertise to propose custom solutions for any SPM process applications and allow all their customers to benefit from the new “bleed and feed” method. Contact Modutek for a free consultation or quote on equipment that is designed to meet your specific manufacturing requirements.

Specialized Wafer Etching for Critical Wet Processing Applications

Improving the performance of critical wet process applications depends on selecting wet bench processes designed for the tasks at hand. Specialized equipment can support different etching and cleaning processes and can be used to make sure that the process selected can fulfill specific application requirements. Modutek’s wet process stations are specialized for the different processes but also guarantee safe and reliable operation.

Specific Tasks Require Specialized Processes

Depending on the etching or cleaning task to be carried out, specific wafer etching process characteristics are needed. The fabrication of solar cells has different requirements than the creation of microscopic structures for a processor. The major process characteristics that can impact wafer manufacturing include the following:

  • Low particle count
  • Etching speed
  • Selectivity
  • Precise control
  • Undercutting

Low particle counts are always desirable but can be critical for the smallest structures. A high particle count can result in high rates of rejection during final testing because the particles cause defects in tiny structures.

Etching speed is related to throughput. If the goal is to remove a substance as quickly as possible, high etching speed can be prioritized at the expense of other characteristics such as precision. Ideally the process speed is optimized so the process step doesn’t become a bottleneck.

For highly selective processes, the substrate material is not affected by the etchant while the material to be removed is etched rapidly. While selectivity is a basic characteristic of wet bench processing and is always important, some applications require especially high selectivity and the process has to be chosen accordingly.

Precise control becomes more and more critical as the dimensions of the micro-structures decrease and the packing of components becomes more dense. Since this tendency is a current trend in wafer manufacturing, control precision is becoming more important.

Undercutting compromises control precision but can be limited or eliminated with anisotropic etching by choosing the appropriate process and the orientation of the silicon crystal planes. When undercutting under the mask is not acceptable, anisotropic etching results in clean and sharp corners.

Choosing the right process to deliver the etching characteristics needed can have a high impact on wafer etching process line performance. Key performance indicators such as product rejection rates, throughput, down times and operating costs can vary greatly depending on the best match of process to application. An experienced equipment manufacturer can help customers choose the best solutions.

Modutek Provides Wet Process Solutions for Different Application Requirements

Modutek offers a complete line of wet process stations and can deliver solutions for general etching and cleaning as well as for specialized applications. Among the processes supported by Modutek equipment are the following:

  • Piranha etch
  • KOH process
  • Buffered Oxide Etch (BOE)
  • Ozone resist strip

Piranha etch cleans organic material from substrates and is often used to remove photoresist from silicon wafers. It cleans rapidly and does not etch the underlying silicon. Modutek can evaluate whether piranha etch is a good fit for an application.

KOH etching is characterized by good control and the possibility of use for anisotropic etching. The etch rate is controlled by changing the temperature of the KOH solution and Modutek offers baths with tight temperature control.

BOE is used primarily to etch silicon dioxide and silicon nitride. It features good control and does not undercut masks. Modutek supports BOE with its sub ambient filtered baths.

Ozone resist strip cleans organic residue while reducing the particle count. The powerful oxidizing action of the ozone combines with carbon in the organic compounds to leave wafers clean and free of particles. Modutek offers the ozone strip process as a chemical-free alternative to traditional strip methods.

With extensive experience in all areas of wet process wafer fabrication, Modutek can help choose the specialized process required for an application and deliver equipment of the highest quality. In supporting special applications as well as general etching and cleaning, Modutek ensures that it can meet all the wet process needs of its customers.

 

Wet Processing Application Capabilities Provided by Modutek

Modutek provides a complete line of wet processing equipment to semiconductor manufacturing plants and research facilities. The company’s wet process stations support all common cleaning and etching applications. Modutek works with customers to continuously improve wet processing performance and reduce contamination and particle counts to a minimum. Wet process applications supported by Modutek equipment include the following:

Standard Clean 1 (SC1 Clean)

The SC1 Clean process uses a mixture of ammonium hydroxide and hydrogen peroxide in water to remove organic material and other non-metallic contaminants from the surface of the silicon wafer. Additional cleaning steps are required if metallic particles have to be removed.

RCA Clean

RCA Clean is a two-step process in which the first step, immersing the silicon wafer in a mixture of ammonium hydroxide and hydrogen peroxide, is similar to SC1 Clean. The second step bathes the silicon wafer in a mixture of hydrochloric acid and hydrogen peroxide. This step removes metallic contamination and particles from the wafer surface, leaving it ready for further process steps.

SPM Process Clean

SPM Clean stands for cleaning with a sulfuric peroxide mixture and the method bathes silicon wafers in a solution of sulfuric acid and hydrogen peroxide. This is also known as Piranha Clean and it removes heavy organic material contamination such as photoresist from the surface of the silicon wafer.

KOH Etch

KOH etching uses a potassium hydroxide solution for anisotropic etching of the silicon wafer. The process is safe and reliable and produces precise etching that can be controlled by varying the KOH concentration and the temperature. KOH etch is one of the most common etching applications.

Nitride Etch

Silicon nitride masks are etched with phosphoric acid to create highly selective etch masks where silicon dioxide masks can’t be used. While silicon dioxide masks resist etching with KOH, for example, sometimes long etching times mean that the etching selectivity versus silicon may not be enough and silicon nitride masks are used instead.

Aluminum Etch

Aluminum layers are deposited on silicon wafers to create conducting paths between the semiconductor components on the wafer. The aluminum layers have to be etched so that only the desired conducting paths are left. Etching is performed under vacuum using a variety of etchants, including phosphoric and nitric acids. The vacuum eliminates the hydrogen bubbles created in a chemical reaction with the etchants.

How Modutek Supports These Wet Processing Applications

A complete line of wet process equipment has to include chemical handling systems, stations that can support the different cleaning and processing methods and automation where needed. Modutek has chemical delivery, pumping and neutralization systems; different types of baths, tanks and rinsers; as well as fume hoods, scrubbers and dryers. The company can advise customers on the different options for a wet process line and deliver suitable equipment.

For chemical handling, Modutek can offer chemical delivery systems, pumps carts, lift stations and chemical collection systems. The systems include bulk storage or local storage container options and, once collected, the chemicals can be neutralized.

Processing and cleaning station options include quartz baths and Teflon tanks. Rotary wafer etching systems are available as are vacuum metal etchers and stainless steel solvent stations. Baths can be sub-ambient, ambient or temperature controlled. Stations can be manual, semi-automatic or fully automated. In each case, controls and automation deliver a precise and reliable setting of variables and excellent repeatability of process steps.

Modutek’s extensive experience in developing wet processing equipment and on-going efforts to develop advanced process systems provide customers with superior performance with lower cost of ownership. For a free quote or consultation on selecting the right equipment for your manufacturing process contact Modutek at 866-803-1533.

 

Safely Controlling the Silicon Nitride Etching Process

Safely controlling the Silicon Nitride Etching ProcessSilicon nitride etch removes silicon nitride from silicon wafers during the fabrication process of semiconductor components. A solution of phosphoric acid in water etches silicon nitride rapidly and consistently as long as the temperature of the solution and the concentration of phosphoric acid are kept constant. Maintaining consistent process conditions during the silicon nitride wet etching process is difficult because adding water to phosphoric acid can result in an energetic explosive reaction. The accurate monitoring of the solution is extremely important for safe control of the process.

The Silicon Nitride Wet Etching Process

Silicon nitride is used as a mask to produce micro-structures and connections in semiconductor manufacturing. In most etching applications, the etch rate can be varied by changing the temperature or chemical concentration, but silicon nitride etch is best controlled at its boiling point and at a concentration of 85 percent phosphoric acid in a de-ionized water solution.

The phosphoric acid etching solution is a viscous liquid that is heated until it boils at about 160 degrees centigrade. The high temperature means some of the water will boil off and be lost as steam, increasing the acid concentration of the remaining liquid. As the concentration increases, the boiling point of the solution rises and water has to be added to keep the process variables constant.

The addition of water to the solution is dangerous because, if too much water is added at once, the solution stops boiling and the added water collects as a film above the viscous acid. As the temperature rises again and the acid starts boiling, the large quantity of water from the film mixes with the acid and may cause an explosive reaction.

Instead, the control system has to ensure that only small amounts of water are added at a time and these small amounts are immediately mixed with the remaining acid solution. Such a control strategy results in constant process characteristics, a safe operation and a high quality output.

How Modutek’s Nb Series Wet Etching Baths Ensure Safe Operation

Modutek has developed a bath control system that combines consistency with safe operation. For the Nb series etching baths, the phosphoric acid solution is kept boiling with a constantly-on heater that maintains the solution at its boiling point. As water evaporates and the acid concentration rises, the boiling point increases and the solution temperature goes up. The temperature rise is detected by a thermocouple and a small amount of water is added to the solution to bring the concentration back down.

Because the solution is constantly boiling, the small amount of water is immediately mixed in with the rest of the acid. The amount of water is too little to stop the solution from boiling and the heater is powerful enough to always maintain a vigorous boiling condition. To ensure that water is added only when the solution is boiling, a second thermocouple senses the presence of steam above the bath liquid and blocks the addition of water when no steam is present. A third thermocouple monitors the bath temperature to switch off the heater if the liquid overheats.

The Benefits of the Modutek Silicon Nitride Etch Bath

The advanced control system of Modutek’s Nb series baths allows semiconductor manufacturers and research labs to safely implement the silicon nitride wet etching process to achieve optimum consistency characteristics. The temperature and concentration of the etching bath remain within tight limits due to the two-level control, monitoring the temperature to correct the acid concentration.

Modutek offers the Nb series baths in their fully automatic, semi-automatic and manual wet bench stations as part of its complete line of wet process equipment. The company constantly works with customers to continuously develop improvements in wet process technology. Contact Modutek for a free consultation to discuss your specific process requirements.

Why High Temperature Quartz Baths Are Required for Silicon Wafer Cleaning

Silicon wafers undergo many process steps during the manufacture of semiconductor components and cleaning the wafers properly is an important factor in successful fabrication. Process steps include etching and diffusion, both of which involve coating the wafer with masking material to guide the etching chemicals or diffusion targets. Once a step is completed, the masking chemicals have to be completely removed. If traces remain or if other impurities are introduced, the quality of the final semiconductor product will be affected.

Silicon wafers are cleaned with aggressive chemicals to remove organic masking material and other possible contaminants. As a result, the cleaning solution container has to be able to resist the action of corrosive chemicals while remaining inert without becoming a source of contamination itself. Baths made of quartz are highly stable, resist corrosion and can be designed and built to avoid contamination. Quartz baths are an ideal solution to the challenges of silicon wafer cleaning and the maintaining of high levels of wafer cleanliness.

How Quartz Baths Clean Silicon Wafers

Quartz baths provide a clean and safe container for the chemical reactions that are used in silicon wafer cleaning. Depending on the materials used, the quartz bath may simply hold the wafers and the cleaning chemicals, but some processes require heating, circulating the solution or the addition of chemicals. In each case, the control actions must be accurate, reliable and easily duplicated.

For example, materials such as hydrochloric acid, sulfuric acid and hydrogen peroxide may be used for cleaning the silicon wafers. Quartz is impervious to these strong chemicals and the quartz tanks remain unaffected while the acids clean the silicon wafers. With SPM clean, sulfuric acid and hydrogen peroxide are mixed and maintained at a constant temperature of about 130 degrees centigrade. Periodic spiking with hydrogen peroxide is necessary because it decomposes in the solution. The mixture cleans wafers rapidly but can’t attack the quartz bath container.

Using bath containers that are inert and don’t contaminate is important because the electrical and physical structures making up modern semiconductor components are extremely sensitive to contaminating particles. As these structures become smaller and more tightly packed on a wafer, even a single particle can interfere with the etching or the diffusion process. Such interference from particles reduces the yield of semiconductor components and affects the productivity of the semiconductor manufacturing facility.

The Benefits of Using Modutek’s Quartz Baths

Modutek has over 30 years experience in quartz bath design and manufacture so that the company’s baths fulfill all the basic requirements for semiconductor processing and deliver additional benefits. Modutek can provide standard units where they satisfy customer requirements but can also design and build custom systems for special applications.

Modutek’s QFa series high temperature re-circulating quartz baths are safe and reliable with a low total cost of ownership. The semiconductor grade quartz of the baths is flame polished to reduce contamination and the vessels are designed for an especially long service life. Particle addition from re-circulating flow is kept to a minimum and remote control is available. Tank sizes range from an inner measurement of 7.75 x 7.75 inches and 9 inches high to 21.5 x 11.5 inches and 10.5 inches high. Custom sizes are available as well.

The QFa baths have an integrated heater that provides an operating range of 30 to 180 degrees centigrade. The heaters produce a temperature rise of 2 degrees per minute and control accuracy is plus/minus 1 degree centigrade, these variables depending on the operating conditions. The wide temperature range, fast thermal response and accurate temperature control make the Modutek quartz baths ideal for silicon wafer cleaning. The high precision guarantees excellent repeatability of the process between batches and the fast heating rate reduces process times. Overall, Modutek’s quartz baths can help improve the semiconductor production performance of manufacturing facilities and research labs. For a free quote or consultation on selecting the right equipment for your manufacturing process contact Modutek at 866-803-1533.

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

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