Why Pre-Diffusion Cleans Are Essential for Silicon Wafer Processing

Why Pre-Diffusion Cleans Are Essential for Silicon Wafer ProcessingSilicon wafers must be completely clean before they go through the diffusion process. If contaminating particles are present on the wafer surfaces during the diffusion process, they will cause defects in the final semiconductor product. Pre-diffusion cleaning can be carried out with several methods. RCA clean and Piranha etch use chemicals to strip away wafer contamination. Megasonic cleaning uses high-frequency sound waves to dislodge surface contaminants and particles. No matter which cleaning method is chosen, cleaning must be done to reduce contaminating particle counts to a minimum. An experienced manufacturer of wet process stations can integrate the required cleaning methods into wet benches. They can then ensure that the silicon wafers are cleaned thoroughly.

RCA Clean is a Common Silicon Wafer Cleaning Method

RCA clean was originally developed at the RCA corporation and remains a popular all-round silicon wafer cleaning method. It consists of two parts: Standard Clean 1 and 2 (SC1 and SC2). SC1 removes organic material but leaves metallic contamination behind. SC2 cleans the remaining metallic particles and produces a completely clean wafer.

The SC1 cleaning bath contains a solution of ammonium hydroxide and hydrogen peroxide. The cleaning bath is heated to about 75 degrees centigrade, and the wafers are immersed for 10 to 15 minutes. All organic matter and many insoluble contaminants are removed, but some metallic ions stay attached to the wafer surface.

The metallic ions are removed during the SC2 cleaning step. The wafers are placed into a solution of hydrochloric acid and hydrogen peroxide. The solution is heated to about 75 degrees centigrade, and the wafers are immersed for about 10 minutes. Once the wafers are rinsed with deionized water and dried, they are ready for the diffusion processing steps.

Piranha Etch Quickly Cleans Heavy Contamination

When silicon wafers are heavily contaminated or need to be stripped of photoresist from previous process steps, a Piranha mixture is often used to begin the wafer cleaning process. The mix of sulfuric acid and hydrogen peroxide quickly removes large amounts of mainly organic contaminants. While it works more rapidly than RCA clean, it operates at an elevated temperature of 130 to 180 degrees centigrade and is hard to control precisely. Modutek’s proprietary “bleed and feed” process control improves process stability. The advanced controls allow for more precise temperature settings and better cleaning performance while maintaining the rapid removal of contaminants.

Megasonic Cleaning Provides Improved Removal of Contaminating Particles

Megasonic cleaning uses high-frequency sound waves in the cleaning bath to dislodge light contamination from wafer surfaces. The cleaning method features reduced use of toxic and expensive chemicals while reducing particle counts to a minimum. Even the smallest sub-micron particles can distort diffusion and cause defects in the final semiconductor product. These tiny particles are especially difficult to remove because they tightly adhere to wafer surfaces due to static charge and surface tension. Megasonic cleaning generates microscopic bubbles in the cleaning solution. When these bubbles collapse, the resulting scrubbing action removes the particles.

Modutek’s Wet Benches Support Pre-Diffusion Cleans for Specific Wafer Processing Requirements

Modutek wet process stations support all standard silicon wafer cleaning methods. Pre-diffusion cleans can be integrated into a wet bench to satisfy specific customer requirements. Since Modutek designs and builds equipment in-house, wet bench stations can be customized to meet specific customer needs. Based on its in-house expertise, Modutek can recommend solutions for wafer processing and propose equipment from its complete line of wet process stations. Once the equipment is built and delivered, Modutek can provide continuous customer support for the supplied stations. Contact Modutek for a free consultation to discuss your specific process requirements.

How Precise Control of the SPM Process Improves Processing Results

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

The SPM Process Suffers from Decreasing Concentration and Slower Cleaning

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

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

Modutek Has Developed Advanced Controls to Improve Process Results

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

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

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

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

Support for the “Bleed and Feed” Process

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

How Advanced Ozone Cleaning Reduces Costs and Improves Wafer Yields

How Advanced Ozone Cleaning Reduces Costs and Improves Wafer YieldsWhen ozone is used to clean silicon wafers, it reduces the use of aggressive chemicals and it can decrease the wafer particle count. Modutek’s advanced ozone cleaning process can clean more quickly than many chemical-based processes and it delivers other benefits. Ozone is used to remove organic contaminants from wafer surfaces in either the Coldstrip sub ambient process or the Organostrip process. Either process can reduce overall wafer fabrication costs and improve manufacturing facility performance.

Advanced Ozone Cleaning Outperforms Chemical Stripping

With increased regulation of the use of dangerous chemicals and pressure on businesses to make their operations more environmentally friendly, Modutek developed the advanced ozone cleaning processes to help semiconductor manufacturers and research labs reduce chemical use. Ozone is introduced in Modutek’s DryZone system in compact units using the Coldstrip or Organo strip processes. Both cleaning methods also feature a reduced particle count compared to chemical cleaning methods.

In the Coldstrip process, wafers are first rinsed with deionized water to remove soluble non-organic contaminants. The process operates at four to ten degrees centigrade. After the rinsing, ozone is introduced into the chamber and combines with the carbon of the organic compounds on the wafer surfaces. The ozone-carbon reaction forms carbon dioxide, leaving wafers clean and almost free of particles.

The Organostrip process also uses ozone but operates at ambient temperature. Ozone is introduced into the process dissolved in acetic acid, a mild solvent in which ozone has a very high solubility. The wafers are rinsed with the ozone-acetic acid solution and the high level of ozone rapidly oxidizes the organic contaminants. The waste products of the process do not require special treatment. Both processes work quickly and feature excellent cleaning performance with low particle counts.

Changing to the Advanced Ozone Cleaning Process Delivers Substantial Benefits

The major benefits of changing from chemical cleaning to an advanced ozone cleaning process is a reduction in the use of toxic chemicals and improved cleaning performance with a lower particle count. These two factors are at the root of additional benefits resulting in cost savings, better yields and improved safety for employees.

Reduced Use of Chemicals

When the use of toxic chemicals is reduced, there are cost reductions in addition to savings resulting from fewer chemical purchases. Other savings include reduced costs for chemical storage, handling and disposal. Employees benefit from lower exposure to harmful chemicals and from increased workplace safety because there is less danger of spills or accidents.

Regulatory compliance costs are also reduced. As environmental regulations become more onerous, costs rise and compliance becomes more difficult. Reduced chemical use results in lower compliance costs, and a more environmentally friendly operation helps improve community relations and the environmental reputation of the business.

Lower Particle Contamination

Lower particle counts are the result of better cleaning performance. Particle counts play ang increasingly important role in wafer structures that are smaller and more tightly packed. Even a single particle can block a conducting path or the etching of a tiny detail. Fewer particles means a lower rate of defective products and higher yields. Product quality and longevity may also be improved.

Improved Cleaning Performance and Wafer Yields with Less Equipment

In addition to a lower particle count, the improved cleaning performance of the advanced ozone cleaning process results in shorter cleaning times within a smaller footprint. Modutek’s DryZone units used for the ozone cleaning process are compact and take up less space than the corresponding chemical cleaning stations. Cleaning is more rapid resulting in higher throughput. As a result of these benefits, the overall performance of the silicon wafer fabrication facility can improve substantially.

Modutek Provides Advanced Equipment for Manufacturers

As one of the leading semiconductor equipment manufacturers, Modutek can advise customers on how to implement a change to ozone cleaning. Modutek offers a free consultation and quote on their equipment, and ensures their equipment will support processes that meet customer requirements.

How the Piranha Etch Process Improves Silicon Wafer Cleaning

While the Piranha solution quickly removes organic residue from silicon wafers, the process is difficult to control and may produce explosive gas mixtures. The Piranha solution is made up of one part hydrogen peroxide and three parts sulfuric acid, although the ratio may vary for specific applications.

The mixture is exothermic and the heat released when the hydrogen peroxide is added to the sulfuric acid drives up the temperature of the solution towards the normal process operating temperature of 130 to 180 degrees centigrade. Both the temperature and the concentration may vary, reducing the useful lifespan of the solution. When the challenge to control temperature and maintain the concentration is met, the Piranha solution can deliver superior silicon wafer cleaning performance.

Spiking the Piranha Solution Can help Maintain Concentration

The hydrogen peroxide in the Piranha solution is not stable and decomposes to produce water. The rate of decomposition is higher the higher the temperature, and the water dilutes the solution. Operators can compensate for the lower concentration by periodically spiking the solution with additional hydrogen peroxide. As hydrogen peroxide is added, the temperature rises, and more hydrogen peroxide decomposes to form water. This interdependence of the concentration with the temperature complicates keeping a tight control on the process, but spiking the Piranha solution lengthens the solution’s lifespan from a few hours to about one day.

Modutek’s “Bleed and Feed” Method Increases the Piranha Solution Lifespan

Modutek has developed an innovative solution to the control and lifespan issues of the Piranha process. The company uses quartz tanks with a “clean” and a “dirty” tank to provide pre-mixed Piranha solution to the active process. Silicon wafer cleaning takes place in the “dirty” tank while the clean tank has a Piranha solution with a programmable concentration. When the concentration of hydrogen peroxide drops in the “dirty” tank, a small amount of the low-concentration solution is drained from the tank and an equal amount is added from the “clean” tank. Instead of the destabilizing spike of hydrogen peroxide, the small but frequent addition of pre-mixed fresh Piranha solution keeps both the concentration and temperature stable and makes possible a tight control of the process, significantly prolonging lifespan.

The “Bleed and Feed” Method Delivers Important Benefits

Modutek’s “Bleed and Feed” method is fully programmable, allowing operators to adapt it to any application to deliver improved silicon wafer cleaning. The concentrations of the “clean” and “dirty” tanks, the amount of the “bleed” and the amount of the “feed” are all independently adjustable so the desired concentrations can be maintained for an extended period. Specific benefits of the “bleed and feed” method include the following:

  • Predictable strip rate because both the temperature and the concentration are tightly controlled.
  • Programmable settings allow a flexible operation.
  • Risk of an explosion from spiking with too much hydrogen peroxide is reduced.
  • Longer solution lifespan results in reduced use of chemicals.
  • Reduced costs for purchase, storage, and disposal of chemicals.
  • Reliable process parameters result in excellent repeatability between batches and accurate maintenance of process variables over extended operation.
  • Reduced downtime because the Piranha solution has to be replaced less often.
  • Safer operation because the process is maintained in a stable equilibrium.

With a more stable Piranha process, silicon wafer cleaning is improved with less contamination and fewer particles. Product defect rates are lower and customers will see higher yields. Overall wet process line performance is improved, with lower costs and better output quality.

Modutek Provides Innovative Solutions for Wet Processing Requirements

Modutek continues to build on its experience in wet process technology and has in house expertise to develop innovative solutions for their customers. The Piranha solution “bleed and feed” method lets customers achieve their production goals more easily. Modutek provides solutions that meet customer needs and ensures that the delivered equipment performs as required. Contact Modutek for a free consultation to discuss the equipment needed to support your specific process requirements.

How Advanced Ozone Cleaning Reduces Chemical and Solvent Usage

As environmental concerns mount and regulatory compliance becomes more difficult, companies that reduce the use of hazardous chemicals and solvents will have a competitive advantage. In addition to reducing costs, the use of fewer corrosive chemicals improves workplace safety and can lead to better semiconductor manufacturing facility performance. Modutek’s patented advanced ozone cleaning process saves money and reduces the contaminating particle count on wafer surfaces. Depending on process requirements, ozone cleaning can out-perform traditional wafer cleaning with harsh chemicals. Modutek’s ozone cleaning process is compact, fast, and safe.

Advanced Ozone Cleaning Converts Organic Impurities to Carbon Dioxide

Modutek has developed the Cold Strip sub ambient process that operates with deionized water and the Organostrip process that uses a mild acid at room temperature for photo resist strip. This also eliminates the use of NMP solvent base strippers. Both use ozone to clean organic contaminants from wafer surfaces and both leave harmless residues that allow for disposal without special neutralizing procedures.

The Cold Strip Advanced Ozone Cleaning Process operates at four to ten degrees centigrade and uses deionized water to strip silicon wafers of water-soluble inorganic impurities. After the wafers are rinsed with deionized water, ozone is introduced into the chamber. The ozone reacts with the remaining organic impurities and converts them to carbon dioxide. Particle contamination is reduced as well because the ozone reacts with organic particles. Wafers are more effectively cleaned with a reduced particle count.

In the Organostrip process, wafers are rinsed with a solution of a mild acid and ozone at room temperature. The acid has a high ozone solubility that lets the ozonated solution remove organic compounds from the wafer surfaces quickly and completely. The process is compatible with metals such as gold, copper, and aluminum, and the waste products of the Organostrip process are not hazardous.

Modutek uses its DryZone solvent gradient dryer for the Advanced Ozone Cleaning Process that the company developed to address customer concerns. Customers experienced high particle counts and issues with the use of hazardous chemicals, including their effect on the environment. The developed process successfully addresses these issues and delivers effective wafer cleaning.

Modutek’s Advanced Ozone Cleaning Process Provides Substantial Benefits

Modutek developed the Advanced Ozone Cleaning Process to address specific customer concerns, but the Cold Strip and Organostrip processes also improve wafer cleaning and facility operations. The benefits of using ozone cleaning include the following:

  • Reduced cost for process chemicals. The acids used in a traditional wafer cleaning process, such as sulfuric or hydrochloric acids, are expensive, and the cleaning processes such as Piranha clean use substantial volumes of chemicals. Ozone and the mild acids or solvents used in ozone clean are less expensive and the solutions last longer.
  • Reduced cost for chemical handling. The storage, delivery, and disposal of hazardous chemicals used for traditional wafer cleaning is expensive. The handling of ozone and the mild chemicals used for ozone cleaning requires no special measures.
  • Improved workplace safety. The chemical baths used for traditional wafer cleaning often operate at high temperatures and they can be dangerous. The ozone cleaning process chemicals are safe and the operation is done at sub ambient or at room temperature.
  • Higher throughput. Advanced ozone cleaning takes less time than traditional cleaning methods.
  • Improved yields. Ozone cleaning reduces particle counts and results in higher output quality with fewer product defects.

Modutek’s Continued Process Improvement Delivers Innovative Solutions for Customers

Modutek works closely with customers to identify new concerns and solve existing processing problems. Using extensive in-house experience and expertise in wet process technology allows Modutek to provide innovative solutions to meet customer needs. New wet process steps such as Advanced Ozone Cleaning can help customers improve semiconductor manufacturing facility performance and increase profitability.

How Megasonic Cleaning Improves Silicon Wafer Manufacturing

When silicon wafers are cleaned between manufacturing steps, it is critical to remove all contamination from the wafer surfaces. The remaining traces of process chemicals or microscopic particles can disrupt the etching process and result in defective or low-quality semiconductor devices.

Megasonic cleaning uses high-frequency sound waves in the cleaning tank to remove contaminants and particles from the silicon wafers. The technology can save time and money because it works quickly and does not require expensive chemicals. Silicon wafers cleaned with Megasonic cleaning are completely clean with a reduced particle count. As a result, the technology can improve the operation of semiconductor manufacturing lines for semiconductor fabricators and research labs.

Megasonic Cleaning Reduces the Use of Toxic Chemicals

The cleaning of silicon wafers after the completion of each semiconductor manufacturing step is accomplished by soaking the wafers in mixtures of chemicals including hydrochloric acid or sulfuric acid. In addition to the cost of the chemicals themselves, there are ongoing costs related to the storage, delivery, and disposal of these chemicals. The possibility of leaks and the disposal of the waste chemicals represent environmental hazards that are continuously being mitigated with tighter regulations. Reducing the use of aggressive chemicals can save money beyond their direct cost and can improve the environmental footprint of the semiconductor manufacturing facility.

With Megasonic cleaning, a frequency generator produces an electric signal in the MHz range that is transmitted to a transducer. The transducer that is immersed in the cleaning solution converts the signal to sound waves in the cleaning tank. The sound waves create microscopic cavitation bubbles that deliver a gentle scrubbing action against the surface of the silicon wafer. The cleaning intensity is strong enough to dislodge impurities and contaminants but will not damage the wafer surface or the microscopic structures that have been etched into it.

When Megasonic cleaning is used to replace some of the traditional cleaning steps, the use of chemicals is reduced. Megasonic cleaning uses plain water or water with the addition of mild detergents. The cost benefits and reduced environmental impact can be substantial, and the areas where Megasonic cleaning is used will have increased worker safety and reduced chemical exposure.

Megasonic Cleaning Can Deliver Improved Cleaning Performance

While acid baths work well for cleaning general contaminants from silicon wafer surfaces, ensuring low particle counts can be challenging. Contaminating particles can block etching and cause defects in the final semiconductor devices. As functions become more and more closely packed on the wafer and microscopic structures become smaller, a single particle can affect the etched shapes and current paths. A key factor for improving output quality and reducing defects is reducing particle counts to a minimum.

Microscopic particles can be difficult to remove from the surface of silicon wafers because they often develop a static charge that allows them to cling to the wafer. Chemicals can dissolve the substances that make the particle adhere to the wafer surface but the static charge often remains as an additional bond. With Megasonic cleaning, very small cavitation bubbles form and collapse in tune with the sound wave frequency in the MHz range. When a bubble collapses near the wafer surface, it emits a powerful jet that dislodges any particle still clinging to the surface. Wafers that have been cleaned with Megasonic cleaning systems have a lower particle count as well as a high degree of cleanliness.

Modutek Can Help with Megasonic Cleaning Integration

As a leading manufacturer of wet process semiconductor manufacturing equipment, Modutek can help customers integrate Megasonic cleaning into their wafer cleaning process. Modutek offers free consultation and can show customers how to realize the benefits of lower costs, better output quality, and improved yields.

How Specialized Equipment Improves Wafer Cleaning Results

When equipment is specially designed for a specific cleaning process, residue and particle removal from wafers can be improved. A better wafer cleaning process reduces semiconductor defects and can result in improved output quality. Specialized equipment can clean more quickly, allowing higher facility throughput. Specialized and customized equipment can be adapted for batch processing, continuous manufacturing or prototyping, improving production line performance, and reducing costs.

Specialized Equipment Delivers Specific Advantages

Modutek has developed specialized equipment that addresses issues with specific semiconductor manufacturing processes. Resulting advantages include better control of process variables, more efficient cleaning, reduced space requirements, and more flexible automation. While standard solutions are also available and are sometimes sufficient, the specialized equipment will deliver superior results in specific cases.

Piranha Etch Clean is often used for post-etch clean and for the removal of organic matter. Modutek has implemented a specialized control strategy that gives more reliable cleaning results while cutting chemical costs. The “bleed and feed” method provides tight control of temperature and chemical concentration to deliver superior cleaning performance.

A wafer cleaning process used in semiconductor manufacturing such as Piranha Clean has to take place in special tanks that are impervious to the process chemicals. Modutek’s heated quartz tanks minimize particle contamination while delivering precise temperature control. They are available in recirculating and constant temperature models are made from boron-free virgin fused quartz.

For many process steps, wafers must be carefully rinsed after cleaning. Modutek’s specialized quick dump rinsers minimize rinsing time while reducing the consumption of deionized water. Modutek’s specialized design has eliminated particle entrapment by reducing particle contamination at the rinsing stage.

After rinsing, wafers have to be dried while limiting additional particle contamination. Modutek offers IPA vapor drying stations but has developed a specialized single-chamber unit that rinses and dries the wafers without moving them to a new station. Moving the wafers exposes them to additional particle contamination and the single chamber IPA vapor dryer features extremely low particle counts.

As microscopic structures on silicon wafers become smaller and more tightly packed, a reduced particle count becomes critical for some wafer processes. Modutek’s use of Megasonic cleaning uses high-frequency sound waves in a cleaning solution to remove almost all remaining particles on wafer surfaces. The cavitation bubbles created by the sound waves can dislodge even the smallest particles, delivering an almost completely particle-free wafer.

Specialized Software Control for Automation

To control the wafer cleaning process, specialized software and automation can help deliver exactly the cleaning chemicals required while eliminating human error. Modutek uses its own SolidWorks Simulation Professional and Flow Simulation software and can adapt the programs to any process. Modutek’s wet bench stations are available in manual, semi-automatic, and fully automated versions with the automation designed to support the latest wet process technology. The specialized automation can be customized for specific applications to satisfy customer process requirements.

How Modutek’s Specialized Equipment Improves Performance

Specialized equipment can reduce particle counts, improve cleaning, reduce costs and increase production capacity. Modutek works closely with customers in a variety of industries and develops specialized wafer cleaning equipment to meet their unique needs. When additional customization is needed, Modutek can adapt equipment and software as required. Because the company designs and builds its products in-house, it can make the changes needed itself without relying on third parties.

With Modutek’s specialized equipment, customers ranging from large semiconductor manufacturers to small research labs can achieve excellent production results, especially after the additional customization that Modutek can provide. Superior wafer cleaning performance impacts all aspects of semiconductor manufacturing, from initial prototyping to full production line fabrication. Modutek can help with the choice of specialized equipment, suggest customizations, and ensure that the delivered equipment meets expectations.

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.

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.