Category Archives: KOH Etching

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.

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.

 

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.

Tips on Improving Your Silicon Wafer Etching Process

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Tips on Improving Your Silicon Wafer Etching ProcessSemiconductor manufacturing facilities and research labs need silicon wafer etching equipment that delivers accurate and repeatable results in a safe and reliable environment. The structures on semiconductor wafers and for micro-electromechanical systems (MEMS) have to be produced to meet tight tolerances and with no contamination by impurities or stray particles. An improved silicon wafer etching process depends on selecting the right etching process and using high-quality equipment, which are key factors for consistent output with a low rate of defects.

Silicon Wafer Etching Processes

Wafer etching is a multi-stage fabrication process and different chemical baths are used to achieve specific purposes. Process steps include cleaning, masking and etching of the wafers themselves or deposited metal. These steps result in the production of microscopic structures in the silicon wafer and in electrical connections where required. Key factors for improved etching performance are control of etching speed and the cleanliness of the fabrication environment.

Silicon nitride is used as a masking material and is etched with hot phosphoric acid. Key factors for improved results are the accurate control of the acid concentration and temperature through the addition of de-ionized water.

Piranha etching is carried out with a mixture of sulfuric acid and hydrogen peroxide. The etching solution cleans organic residue from silicon wafers and is often used to remove photoresist. Cleaning with piranha etching can be appropriate at various stages of the semiconductor fabrication process, and its effective application depends on the correct mixture and accurate control of the process.

The KOH etching process creates microscopic structures in the silicon using a solution of potassium hydroxide. Variables controlling the etch rate include bath temperature, KOH concentration, silicon crystal orientation and impurities in the silicon. Improved etching performance means tightly controlling the etch rate to get the precise structures needed.

Buffered oxide etch (BOE) uses hydrofluoric acid and a buffering agent to etch thin masking films of silicon nitride or silicon dioxide. If properly controlled, it results in a highly consistent and repeatable process that is compatible with photoresist.

The key to improving silicon etch performance is to use wet process equipment from a supplier that has the required experience and who can deliver high-quality equipment designed for excellent etching performance. Modutek has over 35 years experience in the field, and can advise customers on which equipment best supports their particular wet processing requirements and can offer standard or customized equipment as needed.

Etching Equipment Design Features

Choosing the right wafer etching process for each stage of semiconductor fabrication is important, but it is the design characteristics of the equipment that ensure improve etching results. Critical design features of Modutek’s etching equipment include the following:

  • Filtration and flow control. Filtered etch baths feature continuous filtration in the 10- to 0.2-micron range and reduced acid consumption.
  • Temperature control. Temperature controlled circulators can heat or cool an etching bath and keep the temperature at the required levels. Operating range is 10 to 60 degrees centigrade with an accuracy of plus/minus 1 degree.
  • Tank design. Teflon tanks for KOH etching are available in many configurations and in standard carrier sizes for single or double capacity. Quartz baths for a variety of etching applications including piranha clean are available in a range of sizes.

Modutek helps customers improve their silicon etching performance by advising them on the right equipment for their specific etching applications and by ensuring the equipment delivers accurate performance in a consistent way. When silicon etching processes are carried out reliably with repeatable results, facility productivity increases and output quality improves.

Commercial Market Opportunities for MEMS Technology

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Using KOH Etching for Batch ProcessingMicro-electro-mechanical systems (MEMS) is a technology that incorporates the use of microscopic devices ranging in size from several millimeters to less than one micron. They usually have both electrical and mechanical functions and often include parts that can move in response to process conditions, commands from outside the structure or commands from integrated electronics. Typical miniaturized MEMS devices include sensors, actuators and other microscopic structures manufactured on silicon wafers using similar techniques as for semiconductors. They are popular because they are inexpensive to fabricate and are easily integrated into systems containing electronic controls, often on the same silicon wafer.

MEMS Applications

MEMS were first used in the automotive industry and then found increased application in the medical and optical fields. Typical macroscopic functions that were taken over by MEMS counterparts were pressure sensors, inertial sensors and chemical sensors. The MEMS sensors were less expensive to manufacture and performed better than the corresponding large sensors. In addition, different sensors could be placed on one silicon wafer along with signal processing electronics. For example, one unit could measure chemical pressure, detect impurities and generate alarm signals.

As MEMS fabrication methods became more sophisticated, the systems were able to take on additional functions and include different structures. For example, today’s MEMS can include mirrors, gears, pumps, nozzles and valves. Associated electronics can evaluate the signals, perform complex analysis and send results to external receivers via radio frequency elements. Such devices are found in the automotive industry in car airbags, tires and fuel systems. Technical products such as ink jet printers, digital projectors, bar code readers and display screens use MEMS to reduce costs and miniaturize components. Medical systems and devices use the miniature MEMS for diabetic evaluation, insulin delivery, blood analysis and needle less injectors. As the number of applications increases, the market for MEMS continues to grow as does the interest in inexpensive, reliable and rapid fabrication methods.

How MEMS are Produced

To create the microscopic structures that make up MEMS, manufacturers use technology similar to that used to produce microelectronics. For silicon wafer based MEMS, the wafers are masked, etched and cleaned in multiple steps. Silicon wet etching is one method that offers a number of advantages.

Wet etching with potassium hydroxide, or KOH etching, is inexpensive and reliable. KOH is safe and etches quickly. Repeatability is good and the process can be controlled by changing the temperature, the concentration of KOH, the crystal plane orientation of the silicon and the doping concentration. The KOH process allows for very precise etching and it can be automated. Other methods of creating microscopic structures on silicon wafers are more expensive, more risky or not as reliable.

Modutek’s Support for the KOH Etching Process

Modutek offers a variety of wet bench processing options including KOH etching. The company’s KOH silicon etching tank is designed and manufactured in house and can be built to customer specifications to satisfy specific wet process requirements. Its design emphasizes cleanliness with an all-PFA construction and consistency in the process to deliver accurate etching results.

The tanks are manufactured to reduce impurities and contaminants to a minimum with the PFA sheet material welded together using advanced welding techniques. The resulting process tanks deliver excellent performance and high quality output. Custom installations to work with existing wet bench equipment are available. Modutek uses its experience in the wet bench process to ensure that customer needs are met and the equipment functions as expected. If you have challenging wet processing applications contact Modutek for a free quote or consultation on solutions that will address your requirements.

Using KOH Etching for Batch Processing

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Using KOH Etching for Batch ProcessingEtching baths with varying concentrations of potassium hydroxide (KOH) and controlled temperatures are used in creating microscopic structures on silicon wafers. Modutek can supply equipment such as PFA Teflon tanks, circulation and filtering systems and cassette handling equipment that supports KOH etching. The company designs and builds individualized systems for customers who want to use KOH etching and the resulting systems deliver accurate control, high repeatability, consistent performance and superior output quality.

The KOH Etching Process
The KOH etching process is well understood and is a preferred method for creating silicon wafer nano structures for semiconductor manufacturing and in research facilities. The process delivers controlled and uniform results with high precision and it is relatively safe.

The silicon surface is masked to allow etching and cavity creation in specific areas according to the layout of the final semiconductor product. The etch rate can be controlled by varying the concentration of KOH and the bath temperature. KOH etching is also impacted by the following:

  • The lattice planes of the crystalline silicon
  • Boron doping of the silicon
  • The presence of atomic defects in the silicon crystal
  • Natural impurities

Orienting the lattice planes in a particular direction can influence and control the direction of the etching and the shape of the cavities created in the silicon. Similarly, boron doping can be used to stop etching in a particular direction. Together with the mask, these factors permit the creation of cavities of the required shapes and orientations. Defects in the silicon crystal or natural impurities may reduce the quality of the final product and can result in defective items.

Advantages of KOH Etching
KOH etching provides excellent results and is used extensively because of the following advantages:

  • Safe and easy to handle
  • Rapid etching
  • Excellent reproducibility
  • Low cost
  • High precision when process variables are tightly controlled

Modutek can supply equipment that builds on the listed advantages to deliver corresponding benefits to the customer.

Modutek’s KOH Etching Process Equipment
Modutek’s wet bench technology supports KOH processing, and Modutek examines each request in detail before building the equipment for a particular application. This includes the KOH processing tank, the recirculating system and the silicon wafer handling. Sizes, capacities and control characteristics are adjusted for the customer’s requirements.

The systems supplied by Modutek use the latest technology to provide high accuracy and excellent output quality. Etch rates can be controlled precisely and etch uniformity is guaranteed. Even long etch times, such as up to 3 days, can be handled reliably and safely.

Modutek’s KOH processing tanks are PFA Teflon with all-Teflon fluid paths. The tanks are manufactured onsite using advanced PFA sheet welding techniques designed to minimize impurities and unwanted by-products. The tanks are available as static or as recirculating baths.

The static baths can be ambient temperature baths or temperature controlled from 30 to 100 degrees centigrade. The recirculating temperature controlled baths feature heat-up rates of 2 to 3 degrees centigrade per minute and a temperature control accuracy of plus/minus 0.5 degrees centigrade. The available semi automatic cassette handling reduces costs and increases reliability.

In each case Modutek makes sure that the equipment supplied fits into the existing or a new installation and is compatible with any other wet processing systems. Modutek’s specialists work to ensure the supplied equipment satisfies the customer’s requirements and meets or exceeds performance expectations.