How Stainless-Steel Stations Provide Safe Solvent Processing

How Stainless Steel Stations Provide Safe Solvent ProcessingSolvents used in semiconductor manufacturing can remove oils and organic contaminants from silicon wafers in preparation for further processing steps. Acetone, isopropyl alcohol (IPA) and ethylene diproxitol (EDP) are solvents commonly used to clean wafers, remove photoresists, and in pattern transfers for the creation of microscopic structures on the wafer. The solvents are inflammable and require special safety measures to reduce the risk of fire and explosion, and they have to be disposed of safely. Stainless steel wet benches incorporating stainless steel processing tanks are an ideal solution for safe processing with solvents.

Safety Requirements for Stainless Steel Wet Bench Stations

The safe operation of stainless-steel solvent stations has to include the following safety features:

  • fire suppression
  • fire fighting
  • protection against exposure
  • protection against leaks
  • safe disposal

Fire is one of the principal safety hazards because solvents or their vapors can catch fire and explode. Fire suppression measures can include electrical design limiting the potential energy of sparks so they can’t ignite solvents and designing equipment to Class 1, Division 2 specifications.

Firefighting equipment has to be able to extinguish fires, should they occur. For inflammable liquids such as solvents, gas-type fire systems such as those using carbon dioxide are needed. It’s critical to have corresponding safety protocols and procedures to ensure workers are evacuated before triggering the extinguishing gases because the firefighting gas displaces oxygen and could cause suffocation.

Many solvents are harmful when workers are exposed to them over extended periods, either through direct contact or through breathing the vapors. A complete enclosure of the solvent processing tanks along with effective exhausts and leak detection limits the amount of solvent to which workers can be exposed.

Disposal of used solvents has to be carried out safely and according to environmental regulations. The disposal can be on-site if the facility has the corresponding capability, or the waste can be stored in a carboy for pick-up by a specialized disposal company.

Modutek Stainless Steel Wet Bench Stations are Safe and Customizable

Modutek stainless steel stations are made from 304 stainless steel and are available in fully automatic, semi-automatic, or manual versions. Dry to dry and fume hood designs are possible, and the stations feature casters and leg levelers. SolidWorks simulation software calculates the process flow characteristics and all design, assembly, and testing are carried out in-house. The stations can be built to the size and length required and extensive customization is available to exactly match customer requirements.

On the safety side, all stations are wired to the NFPA 70 & 79 standards and designed with fire suppression. Safety interlocks with emergency power off buttons are included. The station tanks have a one-inch lip exhaust and a continuous flow deionized water manifold. PVC safety shields are included and the stations have a nitrogen head case purge with Photohelic pressure interlocks. A Teflon nitrogen gun and deionized water hand spray are also included.

As a leading wet bench manufacturer, Modutek works closely with customers to determine how best to help them meet their objectives. Careful wet bench design can increase productivity and output while reducing costs. Modutek is focused on delivering wet bench equipment such as stainless-steel solvent stations using the best available materials and safe, high-quality designs.

With over 40 years of experience in the manufacture of wet benches and with the ability to carry out all design, assembly, and testing work in the company’s San Jose facility, Modutek is the ideal partner for semiconductor fabrication facilities and research labs. Modutek’s in-house experience allows for easy customization and safe execution of wet process semiconductor manufacturing projects of all kinds. Contact Modutek for a free consultation to discuss your specific solvent processing requirements.

How Teflon Tanks Improve the KOH Etching Process

How Teflon Tanks Improve the KOH Ethcing ProcessEtching silicon wafers with potassium hydroxide (KOH) is a popular process for semiconductor manufacturing because it is relatively safe compared to other etching methods and because it features good control of the etch rate. When carried out in Teflon tanks, contamination is reduced and the etch rate can be controlled.

A key factor for successful etching is to determine the required etch rate. If the rate is too fast, the KOH may etch too far into the silicon, while if the rate is too slow, the etched holes might be too shallow. Improving the KOH etching process means applying several control methods to the etch rate to ensure the resulting etched shapes are exactly correct.

Process Factors that Affect the KOH Etch Rate

The KOH etch rate in silicon wafers is influenced by the following factors:

  • Process temperature. The higher the temperature of the KOH solution, the faster KOH will etch the silicon.
  • Solution concentration. A higher concentration increases the etch rate. Normally the concentration of the KOH solution is about 30 percent, but it can vary from 10 to 50 percent, with a corresponding effect on the etch rate.
  • Doping. Doping means adding impurities to the silicon crystal. When boron is placed into the silicon crystal lattice at a specific location, etching stops in that direction. Boron doping can influence the shapes to be etched in this way.
  • Crystal lattice orientation. The silicon crystal atoms are arranged in a cubic lattice that has a greater atom density in some directions than in others. Etching is slower in directions with a higher atom density.

All four factors have to be taken into account when designing the mask to obtain the microscopic structures in the silicon. The silicon wafer has to be oriented correctly to give the different etch rates along with the different lattice directions. Doping has to be in a place where etching is required to stop and the correct KOH solution concentration has to be mixed. These are initial conditions that are established before the process starts. A target temperature can be set as well but the temperature can be varied to adjust the etch rate during processing. The ability to vary the etch rate by changing the temperature results in excellent control of the KOH etching process.

Modutek Teflon Tanks Feature Rapid Heating and Precise Temperature Control

Modutek’s Teflon tanks for KOH processing are available in a circulating or a static design. The heat source is either inline or immersed in the overflow weir. An all-Teflon liquid path reduces the possibility of contamination. The heated tanks can improve the KOH etching process with short heat-up times and precise temperature control. As a result, Modutek Teflon tanks can keep the etch rate steady by maintaining an accurate temperature set point or can allow the etch rate to be adjusted with fast and reliable controlled temperature changes.

Both models of Teflon tanks feature uniform heating throughout the baths, level and temperature limit settings, and a drain interlock. Heat up rates are 2 to 3 degrees centigrade per minute and the temperature is controlled with a precision of plus/minus 0.5 degrees centigrade. The operating temperature is from 30 to 100 degrees centigrade and a cooling refluxor with Teflon cooling coils is included.

Modutek’s Teflon tanks are available in standard sizes but can work with customers to design and build systems with custom sizes and special requirements. With their high-quality materials, precise temperature controls, and customization capabilities, Modutek’s Teflon tanks can improve the KOH etching process to deliver better semiconductor manufacturing results. Contact Modutek for a free consultation to discuss your specific process requirements.

How On-Site Acid Neutralization Systems Provide Safe Chemical Disposal

When the right acid neutralization system is chosen, it can automatically add the required neutralizing chemicals to acid waste left over after a semiconductor manufacturing process step. Ideally such a system should use the least amount of chemicals while ensuring adequate neutralization of waste to comply with environmental standards. The system has to have enough capacity to handle the waste chemical flow and it must operate reliably, protecting against spills and documenting chemical use and discharge. With overall environmental standards tightening and local regulations becoming more stringent, semiconductor manufacturers need acid neutralization systems that provide safe and reliable disposal of chemical waste.

How to Choose Continuous Flow or Batch Acid Neutralization Systems

Acid neutralization systems can operate on a continuous basis or can neutralize individual batches of waste chemicals. For mass production of semiconductor parts, fabrication lines operate over extended periods, using fixed amounts of chemicals and producing uniform quantities of waste.

For these large-scale operations, a continuous flow acid neutralization system is the best choice. Because the nature of the waste chemicals is known and remains consistent over time, the neutralization process can add fixed amounts of low-pH chemicals to continuously neutralize the waste flow. Neutralization can be fine-tuned because the overall process doesn’t change.

Batch neutralization systems are better suited for the production of small quantities of semiconductor products. Typical applications are prototype production, production of small quantities for testing or the manufacture of special components. For these operations, the waste chemicals have to be neutralized in small volumes and each process is different. The batch acid neutralization system has to measure the pH of the waste chemicals, and determine what low-pH additions are appropriate. The neutralization is different for each batch as is each outflow.

Automation Can Play a Key Role in Effective Acid Neutralization Systems

Automated acid neutralization systems can help reduce chemical use, ensure regulatory compliance and reduce spills. The system automatically adds the required chemicals to the waste solution, tracks chemical use, measures outflow pH and records the results. Workplace safety is increased and human error is reduced. The records of chemical use and outflow pH can demonstrate that environmental standards are met.

Automating continuous flow systems is comparatively easy because the process parameters are known and they don’t change. The complete automation of batch neutralization systems can be challenging because key variables as well as the chemicals involved can vary from batch to batch.

For a continuous flow chemical acid neutralization system, the acid to be neutralized and the low-pH chemical to be used are known. The automation only has to add the required amount of neutralizing chemical on a continuous basis and record the resultant pH. For slight variations, a control loop can adjust the amount of neutralizing chemical to achieve the required output pH.

A fully automated batch neutralization system first has to measure the batch pH and add an amount of low-pH neutralizer. Depending on the chemicals involved, the automated system may have to add different neutralizing chemicals in several stages to achieve the required output. The automated system is complicated to program and difficult to set up. For batch neutralization systems, operator involvement to select the type of neutralization required can simplify the operation of the automated system and make it more effective.

Modutek Can Help Select the Right Acid Neutralization System

Modutek has a full line of chemical handling equipment, including acid neutralization systems. The company works closely with customers to determine their needs and can customize systems to meet special requirements. Once delivered, Modutek makes sure that the acid neutralization systems operate as specified and customers are completely satisfied. Contact Modutek for a free consultation to discuss the chemical disposal requirements needed at your facility.

Using Acid Fume Scrubbers to Maintain a Clean Work Environment

When semiconductor manufacturing process steps include etching or cleaning with acids, the process station exhaust can be harmful. Acid droplets in the discharge air can create health risks for workers and can corrode equipment. Worker safety standards and more stringent environmental regulations may set limits on the permissible acid droplet concentration.

To address these issues, semiconductor manufacturers need acid fume scrubbers to remove and neutralize acid droplets in the process discharge air. Once the acidic mist is passed through the scrubbers, the output air is clean enough to be discharged into the workspace or outside. Employees can work in a clean and safe work space and the outside environment is not contaminated with acid.

Acid Fume Scrubbers Can Remove up to 99 Percent of Acid Mist

Acid fume scrubbers work by passing the acid mist over a wet bed made up of special, large surface packing material. The scrubber has to be able to handle the flow volume from the process exhaust. Modutek’s scrubbers can remove up to 99% of acid mist and are available in sizes ranging from 500 to 25,000 CFM. This capacity range will satisfy the flow requirements from the smallest to the largest process volumes.

Modutek’s acid fume scrubbers use a high surface area packing material made by industry leader Lantec. The packing material is kept wet with a solution containing acid neutralizing chemicals. As the acid mist passes over the surface of the wet packing material, the acid droplets dissolve in the neutralizing solution. The acid droplets are almost completely removed from the process discharge air, resulting in a clean and non-corrosive air output. The acid in the neutralizing solution is rendered harmless and the solution can be discharged safely.

When Modutek’s high-efficiency acid fume scrubbers are used with the right flow capacity, the corrosive air output from wet process manufacturing stations can made safe for release into the work environment. This is especially important for semiconductor manufacturing operations that rely on a clean, particle-free process space.

Advantages of Modutek’s Fume Scrubbers

Modutek has built its fume scrubbers for a low total cost of ownership combined with design flexibility and ease of operation. The high efficiency resulting from careful design and the use of the Lantec packing material means that a higher percentage of acid fumes are removed and the resulting output air is cleaner and safer. The neutralizing solution from the packing material is recycled, reducing the use of water. A bottom drain makes maintenance easier and the scrubbers have a double containment system for extra safety.

The operator interface gives operators all the information they need for effective operation of the scrubbers. Pressure gauges and flow meters allow operators to get a complete picture of scrubber operation while water level indicators with discharge metering give operators control of neutralizing solution discharge. In addition to wet process station applications, acid fume scrubbers can be used with other chemical handling equipment Modutek offers to provide a safe working environment.

Benefits from Using Modutek Acid Fume Scrubbers

Modutek’s acid fume scrubbers allow facility managers to maintain a clean and safe work environment and neutralize acidic droplets that are discharged from wet process semiconductor manufacturing stations. Even when the use of fume scrubbers is not mandated by worker safety standards or environmental regulations, it often makes sense to install the scrubbers for the many benefits they provide. Benefits can include the following:

  • The high acid mist removal rate improves workplace air quality and results in higher employee job satisfaction.
  • Ease of use means operators don’t need extensive training and can reliably achieve operational targets.
  • Low cost of operation and maintenance allows for a quick return on investment.
  • The flexible design and wide range of capacities ensure that there is a system suitable for most applications.

Based on the needs of your facility Modutek can recommend specific acid fume scrubbers from its complete product line to ensure the installed unit meets application requirements.

How Process Controls Are Improved with Automated Wet Bench Equipment

how-process-controls-are-improved-with-automated-wet-bench-equipmentWhen semiconductor manufacturing is automated, tight process control can improve overall facility performance. An optimized manufacturing process can reduce consumption of consumables and production results are more uniform. Once a batch has run successfully, an automated process ensures that it will run the same way for subsequent batches. With good design, automation reduces silicon wafer handling and particle contamination. A wet process equipment provider has to be able to customize the automated functions to meet the needs of the application, but once it is set up, the process can run with a minimum of amount of monitoring from an operator.

Automation Can Reduce Chemical Use

An optimized automated process uses the least amount of chemicals required to carry out the etching or cleaning function and it continues to do so reliably. Spills and human error are eliminated while chemical use is tracked. Chemical treatment prior to disposal is closely monitored and neutralizing chemical use is reduced to a minimum while still ensuring compliance with environmental regulations. When overall chemical use is reduced, the cost savings can be substantial.

Output Quality is Improved with Automation

As semiconductor components and micro-structures become more tightly packed, they are more sensitive to dosage variations and process parameter inaccuracies. Processes such as etching are often dependent on temperature and chemical concentration. The timing of the process steps is critical in obtaining exactly the required etched pattern. Fully automated wafer fabrication equipment has these variables programmed in and runs the process accurately and the same way each time. Semiconductor component output is consistent with fewer defects and rejections.

Automated Neutralization Reduces Environmental Impact

Using an automated system to treat, neutralize and discharge chemical waste has several advantages. Waste from wet process manufacturing can be toxic and hazardous so human error during neutralization can have severe consequences for the environment and surrounding communities. Automation can ensure that acid and solvent chemical waste is neutralized and an automated system can block discharge if the output pH is not within predefined limits. Because all chemical use and discharge is tracked, compliance with environmental regulations can easily be demonstrated.

An Experienced Provider Can Customize Both Equipment and Automation

Depending on what kind of semiconductor components are being produced, a manufacturing or research facility may require highly customized equipment to carry out the process steps required. The automation has to flexible enough to accommodate custom process steps and the equipment provider has to be able to program the automation accordingly. Ideally, an experienced provider will work with customers to exactly define what is needed and then build the wafer fabrication equipment with the required automation. Such cooperation can result in equipment and controls designed and built for the specific purpose and delivering exceptional performance for the customer.

Modutek Designs and Builds Its Own Equipment with Automation

Modutek designs and builds its own equipment in-house based on extensive experience in wet process technology. This leaves Modutek in an ideal position to customize wafer processing equipment as needed to meet customer requirements. Because the expertise is in-house, Modutek engineers and technicians can adjust designs to reflect customer needs. This ability extends to the wet bench stations as well as to the automation that runs them.

Modutek’s SolidWorks Simulation software calculates precise dosages and process parameters and is also developed and supported in-house. Modutek can offer assistance for custom design of process stations, supporting equipment, and software to ensure customers get the systems they need. After the process line is built, installed, and tested, Modutek can provide full support with their own in-house specialists to make sure the automated semiconductor manufacturing process continues to operate as expected.

How Chemical Carts Provide Safe Handling and Disposal of Chemicals

How Chemical Carts Provide Safe Handling and Disposal of ChemicalsChemical handling equipment is an important part of a semiconductor manufacturing operation because the aggressive chemicals used in etching and cleaning semiconductor wafers have to be stored, delivered to the process, and disposed of safely. Chemical carts, also referred to as chemical pump carts or chemical collection carts, are a convenient way to remove waste chemicals from process tanks and ready them for disposal. These carts are compact units that can be rolled up to the processing line to pump out processing tank contents. Carts have to be designed for safe pumping and containment of the chemicals and for moving them to a safe location for further treatment.

Chemical Cart Design Features

Modutek’s chemical carts allow operators to quickly remove used chemicals from a process line and let the process continue to the next step with a minimum of delay. The carts are easily rolled up to the process tank that contains the used chemicals and operators can safely transfer even toxic or corrosive chemicals. Modutek’s chemical carts have the following features:

  • Compact profile at 36 inches long, 18 inches wide, and 42 inches high
  • Made of white polypropylene resistant against acids; also available in stainless steel for solvent applications
  • Mounted on swivel casters that can be locked
  • Suction wand for emptying process tanks
  • Independent discharge tube for transfer of waste chemicals to a neutralization system
  • Pneumatic pump, three-eighths inch
  • On-cart five-gallon chemical tank with double containment that is Department of Transport approved
  • High-level sensor interlocked with pump for additional safety
  • All-Teflon piping

Once operators have pumped waste chemicals into the cart tank, the cart can be rolled out to a neutralization area or the waste can be shipped off-site for neutralization and disposal by a specialized company. In either case, the amount of chemicals and the treatment are recorded to ensure compliance with local environmental regulations.

Chemical Handling Equipment Increases Workplace Safety

Chemical handling equipment such as chemical carts allows operators to receive, store, deliver and discharge dangerous chemicals safely. Well-designed chemical handling equipment can reduce waste, deliver accurate dosages to the process and track chemical use. Containment of spills is an important safety and environmental issue that can be addressed by good design practices. Modutek’s extensive experience with wet process technology and the acids used to ensure that the company’s chemical handling systems operate in a completely safe and reliable manner.

Chemical Handling Functions Require Different Types of Equipment

While chemical carts used for the disposal of used chemicals fulfill one particular role, similar units can take on other functions. Chemical delivery systems handle chemical reception, storage, and delivery to the process. They can include remote bulk storage such as in large tote chemical containers that can supply high volumes of chemicals or 55-gallon drums located at the process. Chemical delivery systems can include software for dosage control and software can control the mixing of chemicals as well.

Chemical lift stations can fulfill a role similar to chemical carts but the units are permanently installed. They are sometimes located behind the wet process station they are serving, but Modutek has developed a space-saving design that allows an integration of the lift station in the process line. When chemical waste or waste water enters the lift station tank, the liquids are pumped to a neutralization area for further treatment.

Modutek’s product line of chemical handling equipment allows manufacturers to make their workplaces safer and save money. Chemical carts and other chemical handling equipment can be customized in terms of flow, capacity, and functionality to respond exactly to customer needs. Modutek can leverage its experience in semiconductor manufacturing equipment to help customers choose the right systems and ensure that the chosen systems perform as needed.

How Specialized Equipment Improves Silicon Wet Etching Processes

Results from common wet etching processes such as KOH and Piranha etch can be improved with the use of specialized equipment. Depending on the process, key parameters can be controlled especially well and innovative procedures can reduce particle counts. When specialized equipment is used in silicon wet etching processes, wafer output can increase, defects can be reduced and output quality can improve. Equipment such as temperature controls, recirculating baths, and special control strategies can optimize specific processes and positively impact overall facility performance.

KOH Etching is Reliable and Easy to Control

Etching with potassium hydroxide (KOH) is a favorite process for applications requiring tight control of the etch rate. The KOH etch speed depends on the concentration of the bath and the temperature. A base etch rate can be attained by using an appropriate concentration and the rate can then be varied by controlling the temperature.

Modutek Teflon tanks are specially suited for KOH etching because they minimize contamination and feature tight temperature control. The TI static tanks and the TFa recirculating tanks are temperature-controlled models while the TT series of tanks operate at ambient temperature. The TI and TFa models operate over a temperature range of 30 to 180 degrees centigrade and temperature control is within plus/minus 0.5 degrees centigrade. Modutek Teflon tanks with KOH etching feature excellent output quality with low defects and high reproducibility.

Piranha Etch Works Quickly

Piranha etch uses a mixture of sulfuric acid and hydrogen peroxide to quickly remove organic residue such as photoresist from silicon wafer surfaces. The mixture has to be handled carefully because it is highly exothermic when first prepared. Normal operation is often at around 130 degrees centigrade so the process requires tanks that can withstand high temperatures and can be heated.

Modutek quartz baths are ideal tanks for Piranha etching. The QFa series high temperature circulating baths can withstand the initial temperature spike from mixture preparation and the subsequent heating to operating temperature. The quartz tanks heat at a rate of two degrees centigrade per minute and temperature accuracy is plus/minus one-degree centigrade. Modutek quartz baths with Piranha etch represent an inexpensive, safe and reliable etching process.

Buffered Oxide Etch (BOE) Etches Silicon Dioxide

Buffered oxide etch uses buffered hydrofluoric acid to etch thin films of silicon dioxide or silicon nitride. The Modutek F-series sub-ambient circulating baths reduce consumption of acid and filter out contaminating particles. Temperatures of 10 to 60 degrees centigrade are maintained within plus/minus 1-degree centigrade and the sub-micron particle filter reduces the particle count and improves yields. Modutek has designed the F-series baths especially for BOE applications and delivers a safe and reliable process with low-cost operation.

Silicon Nitride Etching Requires Special Control Measures

A phosphoric acid and de-ionized water mixture etches silicon nitride in a process that requires special stabilizing controls. The mixture usually operates at its boiling point of about 160 degrees centigrade. As the etching proceeds, de-ionized water evaporates, leaving an increasing concentration of acid behind. The higher concentration increases the boiling point and the etch rate. To keep the process working consistently, the concentration has to be restored to the lower level.

Adding enough water to restore the concentration risks an explosive reaction when the water mixes with the acid. Modutek has designed a control system that frequently adds small quantities of water to the silicon nitride wet etching baths whenever the temperature rises. The small amount of water is mixed in immediately and the concentration remains steady. Modutek’s specialized control system ensures improved stability and safe operation.

Modutek’s extensive experience in wet process fabrication allows the company to design and build specialized equipment to meet the needs of its silicon wet etching process customers. Such special equipment can improve output quality, reduce costs, and increase yields.

Improving Standard Clean Particle Removal in a Wet Bench Process

When silicon wafer yields are not improved with the Standard Clean process, improved particle removal can be achieved with advanced processing equipment. Standard Clean or RCA clean was developed by the RCA company in 1965 and it has changed little since then. It has always been successful in cleaning wafers and removing most particles. However, with silicon circuits and structures decreasing in size and product density on wafers increasing, even the tiniest particles can cause device defects.

RCA clean works by first dissolving and removing organic materials from the surface of the wafers using powerful chemicals. In a second step, metallic impurities are removed and the wafer is rinsed clean. The two-step process and the rinsing remove most particles but the smallest particles often remain attached to the wafer with surface tension and electrostatic charges. Such particles can affect diffusion in wafers or block the etching of electronic structures or circuit paths.

The submicron particles are hard to remove completely with chemicals or by rinsing. The use of megasonic cleaning technology to generate very high-frequency sound waves in the wafer bath provides the capability to dislodge all remaining particles, allowing them to be rinsed away. Megasonic cleaning improves Standard Clean particle removal process and helps ensure defect-free production of microscopic structures on the silicon wafer.

How Megasonic Cleaning Works to Improve Standard Clean Particle Removal

The two steps making up the RCA wafer cleaning process are SC1, using a mixture of ammonium hydroxide and hydrogen peroxide, and SC2, using a mixture of hydrochloric acid and hydrogen peroxide. The SC1 process removes most of the contamination from the silicon wafer surface, but introduces metallic ion contaminants that can influence subsequent wafer processing steps. The SC2 process dissolves the remaining contaminants, allowing impurities to be rinsed away as they float to the surface of the cleaning mixture.

Standard Clean takes place in heated inert tanks such as quartz baths that are carefully designed to limit additional contamination. The chemical mixtures are heated to about 80 degrees centigrade and processing takes about 10 minutes for each step. After rinsing, the silicon wafers are clean but may still have an unacceptable number of submicron particles adhering to the wafer surfaces.

Megasonic cleaning can remove the majority of these particles while also dislodging the few larger particles that may still be present. The process involves adding a megasonic transducer to the tank holding the wafers. The transducer converts very high-frequency signals from a megasonic generator into sound waves that travel through the rinsing water to strike the wafer surfaces. Sound wave frequencies are typically at or above 1 MHz but may range as low as 200 kHz depending on the specific cleaning application.

As the sound waves travel through water, they create microscopic cavitation bubbles that form and burst in tune with the sound wave frequency. When the bubbles burst, they produce tiny but intense jets of water. The jets from the bubbles near the wafer surfaces dislodge any remaining particles, leaving them to be rinsed away at the surface of the bath, for example via an overflow weir. The additional Megasonic Cleaning step is very effective in particle removal because the tiny jets break any bonds that make the particles adhere to the wafer. The result is extremely low particle counts on wafers that undergo both the RCA wafer cleaning process and Megasonic Cleaning.

Modutek Designs and Builds Specialized Equipment to Improve Wet Bench Processes

Megasonic Cleaning is one example of Modutek’s implementation of designs and technologies that improve wet process performance for their customers. For over forty years Modutek has been providing specialized semiconductor manufacturing equipment to meet unique industry needs. As a leading semiconductor equipment manufacturer, the company offers free consulting and can provide recommendations on equipment from its complete product lines. When Modutek supplies wet bench equipment to customers, the company follows a well-defined process to ensure their equipment performance meets customer requirements.

The Importance of Pre-Diffusion Cleans in Silicon Wafer Cleaning

The Importance of Pre-Diffusion Cleans in Silicon Wafer CleaningWhen wafer cleaning immediately prior to diffusion is effective, semiconductor manufacturing output is of high quality and the defect rate is reduced. A major pre-occupation of pre-diffusion cleaning is the removal of microscopic particles from the surface of the silicon wafer. Particles can prevent even diffusion and may themselves be diffused into the silicon, causing defects.

Silicon wafer cleaning can be carried out in chemical baths or with megasonic cleaning systems. Several cleaning steps using different processes are sometimes required to get a specific level of cleanliness. The goal is to obtain wafers free from metallic or organic contamination and with as few surface particles as possible. As silicon microstructures are designed with smaller elements and with a higher component density, adequate wafer cleaning becomes more and more critical.

RCA Clean Uses Two Steps to Remove Organic and Metallic Contamination

RCA clean uses ammonium hydroxide, hydrogen peroxide and hydrochloric acid to remove surface contaminants from the silicon wafer. In the Standard Clean 1 (SC1) process, the wafers are placed in a mixture of ammonium hydroxide and hydrogen peroxide. The corrosive mixture removes organic matter but may leave metal ions behind.

In the Standard Clean 2 (SC2) process, the remaining metallic traces are removed by immersing the wafers in a mixture of hydrochloric acid and hydrogen peroxide. At the end of the RCA wafer cleaning process, organic and metallic contamination has been removed while as many remaining particles as possible are rinsed away as well.

Piranha Clean Removes Heavy Organic Contamination

When wafers are heavily contaminated with organic materials such as photo resist, the Piranha process cleans more quickly than RCA clean. The wafers are immersed in a mixture of sulfuric acid and hydrogen peroxide and the mixture may be heated to speed up the reaction. The piranha clean process hydroxilates the wafer surface, making it hydrophilic. This affinity for water is sometimes a useful feature for additional cleaning measures.

Megasonic Cleaning Removes Contaminants and Particles

Megasonic cleaning avoids the use of corrosive chemicals and is especially effective for dislodging microscopic particles from the wafer surface. The Megasonic process works by generating high-frequency sound waves in the cleaning bath. An ultrasonic generator produces the high-frequency electronic signal and a transducer converts the signal into sound waves that travel through the cleaning solution. The sound waves create tiny cavitation bubbles that produce a scrubbing action against the wafer surface. The action of the bubbles cleans the wafer.

Microscopic contaminating particles adhere to the wafer and are difficult to remove. With integrated circuits featuring increasingly smaller geometries, even the tiniest particles can cause defects. Megasonic silicon wafer cleaning operates in the frequency range near or above 1 MHz and the cavitation bubbles agitate the cleaning solution at the operating frequency. Such agitation breaks the bond holding the particle on the wafer surface and the floating particles can be rinsed away.

Modutek Wet Bench Equipment Supports Pre-Diffusion Cleaning Methods

RCA, Piranha and Megasonic cleans are commonly used in pre-diffusion clean, but each fabrication facility optimizes for its own sequence of semiconductor manufacturing processes. A production line may use one or several cleaning methods, and may have special requirements for production parameters such as size, temperature or timing.

Modutek designs and builds its own complete line of wet bench equipment and can advise customers on choosing the best processing stations for their applications. Ideal solutions often require extensive customization to optimize yield and reduce costs to a minimum. Modutek can provide custom solutions including pre-diffusion cleans, and can design new equipment for integration into the customer’s manufacturing line to meet specific wafer processing requirements.

How Process Controls Improve KOH Etching Results

How Process Controls Improve KOH Etching ResultsWhile potassium hydroxide (KOH) etching is a versatile process for creating silicon microstructures, precise and responsive controls are required to get superior results. The KOH wet bench process is popular because KOH etches quickly and it is less hazardous than some other processes. Combined with accurate chemical delivery and reliable process controls, KOH etching can deliver exact etching dimensions and reproducible results for batch processing.

KOH Etching Can Create Complex Shapes

The KOH process is used widely because, in addition to etching quickly, it can be set up to etch at different speeds in different directions. For example, technicians may want to etch downward, into the silicon wafer, more quickly than etching in a horizontal direction. They may want to create a rectangular shape, where the long side has a different etch rate than the short side.

The etching direction is influenced by the crystal lattice orientation of the silicon wafer and possible doping of the wafer with boron. The etching speed depends on the KOH mixture concentration and temperature. To obtain improved KOH etching results, the etching process has to be set up to include all these factors and produce the desired result every time the process runs.

When the microstructures to be etched into the wafer require different etching speeds, the process uses the fact that the crystal lattice can be denser in one direction than another. Because there are more atoms to etch away in the denser direction, etching progresses more slowly. Boron doping is a way to stop etching in a particular direction. KOH will not etch areas with boron impurities, so etching stops at the doped silicon.

Once the wafer with the correct crystal orientation and doping is ready for etching, a basic etch speed can be determined with the KOH concentration. The etch speed is controlled by the KOH mixture temperature but the concentration has to be high enough for the desired etch speed.

Typical KOH concentrations can vary from 10 percent to 50 percent with 30 percent KOH by weight representing a common value. The mixture is heated to between 60 and 80 degrees centigrade and held steady at the temperature that gives the desired etch speed. With right wafer crystal orientation, doping in the right places and the correct etch speed, the microstructures will be completed in a given time.

Teflon Tanks with Precise Controls Produce Improved Output

The Teflon tanks designed and manufactured by Modutek feature the precise temperature controls and accurate support equipment that an effective KOH process requires. To etch the silicon wafer with a high degree of accuracy, operators have to make sure the concentration of KOH is at exactly the right value and have to be able to control the temperature within narrow limits.

Modutek’s wet bench equipment can deliver the KOH mixture to the Teflon tanks at the right concentration. The tank temperature controls can produce a heating rate of 2 to 3 degrees centigrade per minute, leading to rapid correction of temperature deviations. The controller accuracy is plus/minus 0.5 degrees centigrade, precise enough for excellent etching speed control.

The precise controls mean that the etch rate remains exactly at the desired level. The dimensions of the silicon microstructures are etched exactly as planned and the KOH process delivers high-quality output. Equally important is that the next time a batch is run with identical requirements and settings, the output is reproducible and the silicon microstructures are the same.

Modutek offers a complete line of wet bench processing equipment and can adapt the KOH etching tanks as well as other processing stations to meet the needs of its customers. All equipment is designed and built-in house, allowing for extensive customization and unparalleled customer support and service.