Category Archives: Wafer Etching Process

Using Modutek’s Teflon Tanks with KOH and TMAH Etching Processes

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Using Modutek’s Teflon® Tanks with KOH and TMAH Etching ProcessesEditor’s Note: This article was originally published in June 2014 and has been updated with additional information and reposted in March 2023.

KOH (potassium hydroxide) and TMAH (tetramethylammonium hydroxide) are two commonly used chemicals for etching silicon, glass, and other materials. The etching process involves dissolving the surface of the material, leaving behind a desired pattern or shape. When using these chemicals, it is important to choose a tank material that is resistant to their corrosive properties, such as Teflon®.

Modutek provides Teflon® tanks that are well-suited for use with both KOH etching and TMAH etching processes. These tanks are highly resistant to chemical corrosion, making them a safe and reliable option for handling these caustic substances. However, there are several additional considerations that should be kept in mind when using KOH and TMAH for etching.

Etching Rates and Material Compatibility

Another important consideration when using KOH and TMAH for etching is the etching rate. The rate at which these chemicals etch materials can be affected by various factors, including the concentration of the solution, the temperature, and the surface orientation of the material being etched. Proper optimization of these factors is necessary to achieve the desired etching rate and profile.

It is also important to consider the compatibility of the material being etched with the chosen etchant. While KOH and TMAH can etch a wide range of materials, not all materials are compatible with these etchants, and some may require specialized etching solutions. Prior to using these chemicals, it is important to check the compatibility of the material being etched with the chosen etchant.

After etching, residues may be left on the surface of the material being etched. These residues can be removed using a variety of techniques, such as rinsing with deionized water or using a plasma cleaner. The choice of technique will depend on the nature of the residue and the material being etched.

Tank Maintenance and Safety Considerations

Proper tank maintenance is essential for ensuring the longevity of Teflon® tanks used in KOH and TMAH etching processes. The surface of the tank should be kept free from scratches and cleaned regularly with appropriate cleaning agents. Additionally, the tanks should be checked for leaks and other damage periodically to ensure that they remain in good condition.

It is important to take appropriate safety precautions when handling and using these chemicals. Both KOH and TMAH can be dangerous if not handled properly, so it is essential to wear protective gear such as gloves, safety goggles, and lab coats. Proper ventilation should also be used which can be provided with chemical fume hoods. Additionally, proper chemical waste disposal procedures should be followed to ensure the safe disposal of these substances.

Modutek’s Equipment Support for KOH and TMAH Etching

Modutek’s TFa and TI series PFA Teflon® Tanks, and other wet bench equipment, fully support KOH etching and TMAH etching processes.  These Teflon® Tanks are designed together with other wet bench equipment to decrease impurities and unwanted byproducts due to advanced welding techniques with PFA sheet material. Continual adjustments to the Si etching process are not necessary after the initial process has been established. Modutek constructs all Teflon® Tanks and related equipment per individual customer specifications, and routinely design products that are compatible with a customer’s existing wet bench equipment.

Modutek’s Teflon® Tanks have an operating temperature range from 30 – 100º C, with a process temperature control of ± 0.5º C and a heat-up rate from 2-3º C per minute (depending on the size of the system).  The modular design allows for new installation or upgrades into any wet etching station configuration.

Teflon® Tank Configurations that Support KOH and TMAH Etching:

  • Temperature Controlled Re-circulating Baths (TFa Series)
  • Temperature Controlled Static Baths (TI Series)

Teflon® Tank design features:

  • Manual cover with overlapping seal
  • Minimizes water lost
  • No concentration deficiency over a long etch time

Heat source options include: 

  • Teflon® inline heating
  • Immersion heating in overflow weir for TFa Series and main tank for TI Series

Custom Teflon® Tanks and Additional Options: Static KOH Etching bath with condensing coils

  • Custom size Teflon® tanks can also be built to match any size (no limitations)
  • Bottom drain features
  • Magnetic stirrer for agitation (TI and TT series)
  • Water condensing refluxor system available on all baths
  • Auto lid feature
  • DI water or IPA spiking system available

Benefits of Modutek’s Teflon® Tanks:

  • Modular design
  • Two available heat sources
  • All Teflon® fluid path
  • Process temperature control of ± 0.5º C
  • Process etch uniformity wafer to wafer <2%
  • In-house heater maintenance and repair
  • 360-degree overflow filtration
  • Uniform heating throughout the bath

KOH etching tank with condensing coils and semi auto robotModutek provides world-class service, installation, and support for all Teflon® Tanks and related wet bench equipment. In addition, Modutek provides quality products that focus on reliability, precision, throughput, usability, and up-time. For more information contact Modutek for a free quote or consultation.

How Teflon Tanks Improve the KOH Etching Process

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

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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.

Specialized Wafer Etching for Critical Wet Processing Applications

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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.

 

Safely Controlling the Silicon Nitride Etching Process

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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.

How Silicon Wet Etching Processes Are Improved with Specialized Equipment

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how-silicon-wet-etching-processes-are-improved-with-specialized-equipmentWhile Modutek’s wet bench equipment supports all common wet etching processes, the continuous development of new wet bench technologies allows the company to offer specialized equipment that can improve performance for specific applications. Key factors for high output quality, fast processing, high yields and reliable outcomes are cleanliness, repeatability, tight control and precise dosages. Modutek’s standard line of silicon wet etching equipment already scores highly on these factors but the company’s specialized equipment can further improve performance for certain processes.

Buffered Oxide Etch (BOE)

The BOE process requires tight and precise temperature control while filtering lowers the particulate count and reduced acid consumption results in lower costs. Modutek’s F-series sub-ambient circulation bath is specially designed for BOE applications and addresses these factors. The temperature controller is accurate to less than plus/minus one degree centigrade in a range from 10 to 60 degrees centigrade with sub-ambient capability. The circulating bath includes 10.0 to 0.2 micron filtration and is designed for low acid consumption. Using a wet bench system specialized for the BOE process can reduce costs and defects in products while providing safe and reliable operation.

Piranha Etching

The sulfuric acid/hydrogen peroxide mixture of the Piranha solution is extremely corrosive, and the mixture is exothermic when first prepared. The Modutek QFa Series of Quartz High Temperature Recirculating Baths are ideal for withstanding the high temperatures, first of the initial heating after mixing and then during the process that is heated to preserve mixture reactivity. The baths have heaters that can raise the temperature of the mixture by two degrees Centigrade per minute and the temperature controller can maintain the temperature at plus/minus one degree centigrade over a range of 30 to 180 degrees centigrade. These Modutek quartz baths are characterized by the high degree of safety, reliable operation and low cost of ownership.

Potassium Hydroxide (KOH) Etching

Modutek Teflon tanks are ideal for use in KOH etching because they can be custom fitted to any wet bench configuration and come in three models to satisfy common heating and recirculating requirements. The TFa and TI series of Teflon tanks are temperature controlled to an accuracy of plus/minus 0.5 degrees centigrade over a range of 30 to 100 degrees centigrade. The TFa series are overflow re-circulating models while the TI series are static. The TT series are static ambient temperature models. All Teflon tanks are made from PFA Teflon with special welding techniques to minimize the presence of undesirable by-products in the process and to reduce contamination. Modutek’s Teflon tanks improve KOH process control, increasing reliability and reproducibility of process conditions.

Silicon Nitride Etching

Etching silicon nitride with a phosphoric acid deionized water mixture is challenging because the concentration changes as the deionized water evaporates and because adding water to the process can cause an explosion. Modutek’s silicon nitride wet etching bath is specially designed to address these issues and provide reliable and safe process control. The mixture is kept at the boiling point, which changes as the concentration increases. Depending on the temperature of the mixture, small amounts of deionized water are added to the boiling mixture to maintain the concentration at the required level. Using the boiling point temperature as an indication of the concentration and allowing the boiling mixture to immediately distribute small amounts of water lets Modutek control the process while maintaining safe operation.

With its extensive experience in silicon wet etching process technology, Modutek can deliver specialized equipment suited to a particular process and help customers select the right equipment for their applications. With equipment specially designed for the process used, customers obtain superior results while often saving money as well. Contact Modutek for a free consultation or quote on using the right silicon wet etching equipment for your application.

 

Why Teflon Tanks Are Used with the KOH Etching Process

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KOH etching is a preferred silicon wafer fabrication method because it works rapidly and reliably while handling of the chemicals is relatively safe. The process relies on a contamination-free environment and uses the temperature and KOH concentration to control the etching speed. Teflon tanks are used in this process because the Teflon is stable and doesn’t degrade. The KOH etching process controls allow accurate and repeatable temperature control for reliable fabrication results and high-quality production. KOH etching in Teflon tanks with temperature control is an excellent process for silicon wafer nanostructure fabrication in both manufacturing facilities and research environments.

How KOH Etching Works

KOH or potassium hydroxide etches silicon quickly and at a constant rate that depends on the concentration of the solution and the temperature of the liquid. In addition to these controllable variables, silicon etching is influenced by the crystal planes of the silicon and the doping concentration. These factors can be used to direct etching in certain directions and to stop etching at given points. The result is that KOH etching can produce the precise nanostructures needed as long as the crystal planes and doping are used correctly.

The controllable variables have to be kept tightly to set values and variations have to be minimized. Important factors are the maintenance of the KOH solution at the desired concentration and temperature. The Teflon tanks have to be designed to support the consistent and accurate etching necessary for defect-free wafer fabrication.

Modutek Teflon Tank Features

Modutek Teflon tanks are designed to support KOH etching leading to wafer fabrication of the highest quality. The temperature-controlled tanks are available in re-circulating and static versions and come in standard sizes for single or double carrier capacities as well as in custom sizes when required. The re-circulating tanks have an all-Teflon fluid path, and in the available custom configurations, are easy to integrate into existing fabrication lines.

Modutek Teflon tanks eliminate contamination from the tanks or fluid paths by ensuring the KOH solution only comes into contact with Teflon, which in unaffected by the corrosive chemical. In addition, Teflon welding for the tanks is carried out with advanced PFA sheet welding techniques that reduce impurities and by-products. The result is that the KOH solution stays pure.

A key element of the KOH etching process control is the initial concentration of the KOH solution and keeping it constant throughout the process step. Modutek Teflon tanks have sealed lids that minimize water loss, even during lengthy etching. Water condensing systems and de-ionized water spiking systems are available if the application requires it.

The other variable affecting the etch rate is the temperature of the KOH solution. Modutek tanks have either inline heating or immersion heating in the overflow weir. Temperature can be controlled between 30 and 100 degrees Centigrade with an accuracy of plus/minus 0.5 degrees Centigrade. Heating is rapid with an average heat-up rate of 2 to 3 degrees Centigrade per minute depending on the details of the installation. Rapid heating and tight control result in superior etching performance and consistent output.

To make operation of the KOH etching flexible and convenient, the Modutek Teflon tanks systems have standard drain interlocks and low level, high limit, and high-temperature alarms. Temperature process controllers and remote operation timers are also available.

Modutek Teflon KOH tanks are designed specifically for applications such as KOH etching and Modutek can supply custom systems specially adapted to customer requirements. Tank sizes and controls can be designed to match customer applications. Modutek can rely on its experience and expertise to supply KOH etching systems that meet the highest quality standards and deliver the best output possible.

 

How the KOH Etching Process is Improved Using Modutek’s Teflon Tanks

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Among the different approaches available to foundries that create intricate integrated circuitry on semiconductor chips, etching with potassium hydroxide (KOH) is frequently preferred for the error-free mass-production that it allows.

The improvement in precision that the KOH etching process brings to semiconductor fabrication is attributable in large part to the use of deionized water. When employed with high alkalinity in excess of pH 12, this process can be thermally adjusted for precise degrees of etching.

How does the use of potassium hydroxide improve the semiconductor etching process?

While dry semiconductor etching processes do exist, they tend to present challenges in process control. Semiconductor wafers etched in this way tend to suffer from quality control issues. Dry etching processes can be difficult to build and run, as well — toxic and explosive chemicals often seen as byproducts, after all.

Greater precision

Etching with potassium hydroxide offers greater precision, and lends itself to improved control, as well.

The fluids employed in the KOH etching process are stored in tanks built into the equipment. One of the most cost-effective and meaningful choices to make in KOH etching processes: to place Teflon tanks within the etching equipment in order to hold the fluids employed. Teflon tanks can make the KOH etching process safer than other options available.

It is possible to structure the KOH etching process in a way to easily create repeat manufacturing projects, as well. When such projects come in from different clients, fabrication units can focus their strengths on setting up the equipment, worrying as little as possible about safety or reliability.

Opting for fewer contaminants and greater cleanliness

Customization is one of the greatest most significant improvements that Modutek brings to the KOH etching equipment business — one where generic, mass-produced Teflon tanks are the norm. Modutek’s Teflon tanks are especially capable of bringing impurity quantities to levels that are impressively low. The result is greater reliability and consistency over an extended period of time.

Greater control

Modutek’s Teflon tanks come with highly customizable temperature settings. Temperature control is achieved either through in-line heating and cooling equipment installed, or through immersion in liquid tanks. It is precisely varied anywhere between 30°C to 100°C. Temperature changes dialed up our achieved at a fast rate of 2°C a minute, on average (although tank size does affect response rates).

Modutek brings customization to the table

Modutek designs Teflon tanks for the specific needs of each client installation. Whether a fabrication plant requires temperature controlled recirculating baths for their KOH etching processes or temperature controlled static baths, Modutek’s Teflon tanks deliver both freedom from water loss and freedom from concentration deficiency.

From dual heating Teflon tanks for fast and consistent water-based etching, to the installation of drains and valves that enable rapid cleanup, high-tech remote data interfaces, controllers and timers, Modutek’s clients have their choice of every advance in KOH etching.

 

Etching Silicon Wafers Without Using Hydrofluoric Acid

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Etching Silicon Wafers Without Hydrofluoric AcidHydrogen fluoride is an excellent etching chemical for silicon wafer fabrication, etching rapidly to remove silicon oxide, for example after an initial RCA clean. While highly effective, hydrofluoric acid is extremely dangerous. Its vapor can cause death and it is toxic enough that even a small area of skin exposed to the chemical can cause cardiac arrest. As a result, storage and handling is difficult and disposal problematic. Safety and environmental considerations have resulted in a search for alternatives. Depending on the silicon wet etching application, other chemicals may be suitable for replacing hydrofluoric acid and Modutek can help customers make the change by recommending equipment that supports alternative chemical processes.

Why Hydrofluoric Acid is Especially Dangerous

Hydrofluoric acid is extremely toxic and penetrates the skin to cause secondary effects that require medical attention. Its vapor can cause lungs to fill up with liquid and exposure to even small amounts can cause death. Neutralization and disposal is difficult and any mistakes run the risk of severe environmental contamination. Given the dangers, silicon fabrication facility owners, managers and process engineers are looking for alternative process chemicals.

While hydrofluoric acid is dangerous as a corrosive chemical, its added danger comes from its absorption into the body. It affects the nerves at sites where the skin has been exposed to the acid and victims may initially feel little pain as a result, often delaying treatment. At the same time, hydrofluoric acid can penetrate deep into the body and attack underlying tissue and bones, causing lasting damage. It also disrupts the calcium chemistry of the blood, eventually leading to cardiac arrest. Exposure of as little as 25 square inches of skin can result in deep burns that are slow to heal and in death if medical treatment is delayed.

Bare skin and the eyes are the most common areas of exposure. Treatment starts with extensive rinsing, for about 15 minutes, of the exposure site. Repeated application of calcium gluconate gel helps mitigate the effect on blood calcium. Anyone with more than four square inches of skin exposed should be admitted to hospital for monitoring of blood chemistry. Normal safety measures to avoid contact with hydrofluoric acid are to cover all skin and wear eye protection.

Hydrofluoric acid is hazardous waste and its discharge is tightly regulated. The acid must first be neutralized and then discharge limits on fluoride and metals must be observed. A complete neutralization process often starts with adding a basic solution to the acid and then precipitating out other materials until they fall within discharge limits. After discharge, the resulting sludge represents an additional waste problem. While other chemicals may be subject to similar discharge procedures, hydrofluoric acid represents an additional risk if neutralization is not carried out properly.

Possible Alternative Etching Solutions

Depending on the particular silicon wet etching application, other etchants may be substituted for hydrofluoric acid. Potassium hydroxide (KOH) is a safer chemical and can be used for etching in many applications.  Tetramethylammonium hydroxide (TMAH) and nitride etch are other possible alternatives that may be used.

Modutek can help facilitate a switch from hydrofluoric acid where possible. The company has over 35 years experience with silicon wafer fabrication equipment and has the in house expertise to advise customers about possible alternatives.

In addition to analyzing customer process needs and making recommendations, Modutek can supply the equipment required from their complete line of wet bench and chemical delivery stations. Modutek’s equipment, while able to handle hydrofluoric acid, is also designed to work with other chemical processes that don’t use it. Modutek can offer standard or customized components such as Teflon tanks or quartz baths to help customers wishing to use other chemicals where they can. Contact Modutek for a free consultation and quote on selecting the right equipment to support the silicon wet etching process for your application.

Selecting Equipment for the KOH Wafer Etching Process

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Selecting Equipment for the KOH Wafer Etching ProcessKOH etching is increasingly used for creating microscopic structures in silicon. The wafer etching process uses a 20 to 30 percent solution of potassium hydroxide to create cavities in the unmasked parts of a silicon wafer. KOH etching is comparatively safe and etching can be tightly controlled through regulation of the bath temperature and doping of the silicon. Semiconductor fabrication facilities favor this etching method because of its high precision, good repeatability and cost effectiveness.

How It Works

The silicon wafer is masked with a material impervious to KOH, usually silicon dioxide or silicon nitride. The mask is based on the layout of the structures and circuits of the final semiconductor product and determines where cavities will be etched. The silicon wafers are immersed in the KOH solution and the temperature is controlled to give the desired etch rate.

KOH etching is extremely sensitive to temperature variations at temperatures above 60 degrees centigrade, with the etch rate in microns per hour approximately doubling for every ten degree rise in temperature. A typical rate is 1 micron per minute for a bath at 80 degrees centigrade. Accurate control of the temperature is a key factor in precise etching results.

While varying the temperature can control the etch rate, the orientation of the crystal planes in the silicon and doping of the silicon with boron also influence the rate and direction of the etch. The anisotropic nature of the silicon crystal can be used to create slopes and shapes along the crystal planes and the etch can be stopped in specific locations with boron doping. This way KOH etching can create complex shapes and circuit paths for semiconductor products.

Modutek KOH Bath Equipment

Modutek supports KOH etching as one of its wafer etching process options in the company’s line of wet bench process equipment. The TFa and TT series Teflon heated tanks are ideal for the requirements of KOH etching. The tanks are available for standard carrier sizes for single or double capacity and Modutek can also build custom sizes if needed. The modular nature of the tanks means they can easily be integrated in any new or existing wet etching station.

The TFa series high temperature overflow tanks and the TT series static tanks are both PFA Teflon tanks with an all-Teflon fluid path. Heating can be inline or immersion heating in the overflow weir. The operating temperature range is from 30 to 100 degrees centigrade and the tanks can heat up at the rate of 2 to 3 degrees centigrade per minute. Temperature control accuracy is plus/minus 0.5 degrees centigrade and liquid levels and high temperatures are monitored. The high precision temperature control and the rapid heating rate ensure that heating is uniform throughout the bath.

Additional features and specifications of the Modutek KOH etching tanks include 360 degree overflow filtration through serration overflow, an optional condensing Teflon refluxor with Teflon cooling coils, a pneumatically actuated auto cover, an aspirator valve system and a Teflon gravity drain. An RS232 interface, temperature controllers and remote operation timer switches complete the remote operation capabilities.

Modutek can help customers considering KOH etching in their selection of the appropriate systems. The company has a complete range of wet bench process equipment, chemical handling systems and equipment repair capabilities. In addition to their standard lines, Modutek can customize components to meet the specific needs of their customers. Experienced company personnel can work with customers to design optimal configurations and layouts for their clean room facilities. The KOH etching tanks are the ideal solution for semiconductor fabricators that need to etch microstructures in silicon wafers.