Isotropic and Anisotropic Silicon Wet Etching Processes

The silicon wet etching of monocrystalline wafers produces microscopic structures that are used in micromechanical devices and semiconductor components. Areas of the silicon wafer not to be etched are protected by masks made of materials such as silicon dioxide or silicon nitride. The exposed areas of the silicon are etched when the wafer is immersed in a chemical bath.

In isotropic etching, an isotropic etchant such as hydrofluoric acid etches the silicon equally in all directions. This means that the wafer is etched directly downwards and also sideways under the mask. The resulting cavity has rounded corners and edges and is larger than the opening in the mask.

For anisotropic etching, the anisotropic etchant such as potassium hydroxide (KOH) etches with different speeds in different directions. This means that the etch rate in the downward direction can be faster than in the sideways directions under the mask. When properly designed, the anisotropic etch can produce cavities with straight sides and less undercutting of the mask.

More details on isotropic and anisotropic silicon wet etching can be found in the following documentation:

How Anisotropic Etching Works

In monocrystalline silicon wafers, the silicon atoms are arranged in a crystal lattice. The crystal has planes which have different atom densities. For example, the 111 plane in silicon is a diagonal plane that has a different atom density than the vertical 100 or 110 planes. As a result, the etching speed for certain etchants differs according to the plane of the silicon crystal through which etching takes place.

To design a successful anisotropic etching process, the etching speed of the etchant and the orientation of the crystal planes have to be coordinated. For example, if the aim is to etch a deep cavity with straight walls, the crystal planes with a slow etching speed have to be oriented along the cavity walls while the plane with a high etching speed should form the bottom. Rather than producing etched cavities with rounded walls and edges, anisotropic etching can create clearly defined straight-walled cavities along the silicon crystal planes.

Controlling the Etching Speed to Obtain Specific Structures

The silicon wafer masks define where etching can take place but the depth of the etched cavity and its shape can be determined by choosing the right etchant and controlling the etching rate. Isotropic etching is often used to create larger features in the initial stages of silicon wafer processing while anisotropic etching can produce straight-edged microstructures in the finishing stages.

Beyond the choice of the type of etching used, the etch rate depends on the concentration of the etchant and the etching solution temperature. Once the etch rate is determined for an etchant concentration at a specific temperature, the masked wafer is immersed in the etchant just long enough to produce the size of cavity required. In each case the precise concentration has to be mixed and the temperature has to be maintained at the target level. Subsequent wafers can be processed exactly the same way for reproducible results.

Differences between Isotropic and Anisotropic Silicon Wet Etching

While isotropic etching is harder to control precisely and hydrofluoric acid is difficult to handle, isotropic etching is faster than anisotropic etching. As a result it is often used for large geometries for which etching speed is an important factor. Anisotropic KOH etching is more precise but it depends on the orientation of the silicon crystal planes. If the wafer crystal orientation doesn’t match the requirements of the process, the resulting cavity will not have the desired characteristics.

Silicon wet etching equipment such as Modutek’s Teflon tanks support both types of processes and they come in temperature controlled and ambient versions. For processes dependent on temperature for controlling etch rate, the tank temperature controllers provide rapid and accurate heating. Concentration can be maintained through supplementation of de-ionized water and custom tank sizes are available if needed. For a free consultation or quote on selecting the right silicon wet etching equipment for your etching process contact Modutek at 866-803-1533.

 

Advantages of Using the KOH Etching Process

Advantages of Using the KOH Etching Process for Silicon EtchingThe KOH etching process uses a potassium hydroxide solution to etch silicon wafers and produce microscopic structures in the silicon. In subsequent semiconductor fabrication steps, the micro structures are used in the manufacture of integrated circuits, processors and other electronic devices.

Compared to other etching processes KOH etching is comparatively safe, etches silicon rapidly and can be tightly controlled. These characteristics are especially important for batch processing when a process step has to be reproduced precisely many times. While other chemical processes are required for specific semiconductor cleaning and etching steps, industrial plants and research facilities prefer the KOH process for general silicon etching.

How the KOH Etching Process Works

The KOH solution is prepared by adding KOH to water in an etching tank made of material impervious to aggressive chemicals. Silicon wafers are masked with silicon nitride or silicon dioxide, substances that the KOH solution does not etch. When the wafers are immersed in the KOH solution, silicon is removed from the areas that are not masked by the chemical action of the KOH etching solution.

The etch rate can be controlled by changing the concentration of the solution and by changing the temperature. The concentration is fixed once the process is established and is usually around a 30% solution by weight, but may be as low as 10% and as high as 50%. Typically the solution temperature is about 60 to 80 degrees centigrade and the etch rate increase is very sensitive to an increase in temperature.

Other factors influencing the etch rate are the crystal lattice planes of the silicon and the presence of boron doping. Different crystal lattice orientations are etched at different rates so that the crystal lattice planes influence the design of the masks and their placement. Boron doping can be used to stop the etching in a specific direction. Taken together, all the ways the etch rate can be controlled allow the creation of complex shapes in the silicon.

Controlling KOH Etching

Obtaining the desired etching results from the KOH process can be divided into two groups of control measures. The concentration, doping and lattice orientation are determined initially before the process starts and establishes itself. At that point, temperature control can still change the etching rate. The initial control measures can be put in place with the required precision, concentration and other characteristics but the temperature of the etching solution must be controlled accurately on a continuing basis.

Because the KOH etching process is very temperature sensitive, maintaining the temperature at the exact set point is important. The temperature controller must be accurate during the process and also from one batch to the next. Tight control during the etching process ensures that the etch rate remains constant while precisely keeping the same temperature for a given set point from one batch to the next ensures accurate reproducibility of process conditions and identical output across different batches.

Modutek’s Teflon Silicon Etch Tanks

Using Modutek’s Teflon tanks allows operators to fully benefit from the advantages of KOH etching. The tanks are designed with KOH etching in mind and feature a wide temperature range, tight temperature control and rapid heating. Custom sizes are available and custom installations can be designed to fit any new or existing wet bench application.

Heating in Modutek’s Teflon tanks is either in line or through an immersion heater in the overflow weir. Temperatures can be controlled to plus/minus 0.5 degrees centigrade and the temperature range is 30 to 100 degrees centigrade. The heating rate is 2 to 3 degrees per minute, depending on the size of the system, and heating is even throughout the bath. Since temperature control is a key requirement for successful KOH etching, these system characteristics allow for excellent reproducibility between batches and for the precise control needed for high quality output.

Apart from excellent temperature controls, Modutek’s Teflon tanks feature all TFA Teflon with advanced manufacturing techniques designed to minimize contamination. Options such as an auto lid feature or a condensing refluxor are available if needed. Modutek Teflon tanks are an ideal solution for KOH etching applications and the company can help select the model and options that best satisfy specific customer needs. Contact Modutek for a free consultation on selecting the right equipment for specific process requirements.

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

 

Why Teflon Tanks Are Used with the KOH Etching Process

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

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.

 

How Teflon Tanks Are Used in Wet Processing Applications

How Teflon Tanks Are Used in Wet Processing ApplicationsModutek’s Teflon tanks are custom designed for use in new and existing wet processing stations. Their modular configuration allows easy integration in customer clean room facilities. Advanced PFA Teflon welding techniques reduce impurities and unwanted by-products in the process, and Modutek can supply heated models for use in processes that require temperature control. Standard carrier sizes are available for single or double capacity, and Modutek can design and build custom tanks for specific customer requirements. Additional options and features make Modutek’s Teflon tanks suitable for a variety of wet process applications.

Teflon Tank Configurations

Modutek offers three types of Teflon tanks:

  • The TT series are ambient temperature baths
  • The TI series are temperature controlled static baths
  • The TFa series are temperature controlled overflow recirculating baths

Modutek can work with customers to analyze their process requirements and recommend the best tank configuration. All tanks are manufactured from the highest quality material and come with first-class support from in house technicians at Modutek.

Temperature-Controlled Teflon Tanks

Modutek offers static and recirculating temperature-controlled Teflon tanks for KOH (potassium hydroxide) and TMAH (tetramethyl ammonium hydroxide) anisotropic etching of silicon. The heat sources can be in line and an immersion heater in the overflow weir. The tanks feature all-Teflon fluid paths and low level, high limit and high-temperature signaling. A drain interlock is standard.

The temperature control delivers fast heating of two to three degrees centigrade per minute within a range of 30 to 100 degrees centigrade. Process temperature control is accurate to within plus/minus 0.5 degrees and the tanks feature an optional Teflon refluxor with Teflon cooling coils. The fast and accurate temperature control system lets operators achieve excellent consistency and repeatability for temperature-sensitive processes such as TMAH and KOH etching.

Teflon Tank Features

Modutek designs their Teflon tanks for ease of use, convenient installation, quick repair and excellent compatibility with new or existing customer facilities. Specific features include:

  • 360-degree serration overflow
  • Manual cover with overlapping seal
  • Modular design
  • Minimal water loss
  • Consistent chemical concentration over time
  • One year warranty

The design of Modutek’s Teflon tanks results in fewer defects in output and a higher product quality overall.

Teflon Tank Options

Modutek tanks can be ordered with the following options:

  • A pneumatically operated automatic cover
  • Teflon gravity drain system
  • Aspirator valve system
  • An RS232 Interface
  • Remote operation timer switches
  • Temperature process controller/timer
  • Teflon refluxor with Teflon cooling coils
  • Magnetic stirrer for agitation
  • DI water or IPA spiking system

Benefits of the Modutek Teflon Tanks

Modutek’s design and configuration results in the following specific benefits:

  • Uniform heating throughout the bath
  • Accurate and rapid temperature control
  • Process etch variability wafer to wafer less than 2 percent
  • Modutek in house maintenance and repair support
  • Extensive customization to meet specific requirements

Based on the standard Teflon tank design, Modutek can use its design expertise to supply Teflon tanks of the highest quality, sized to customer specifications and with the features and options required for the customer’s application.

Modutek Support

With over 35 years experience in wet bench stations, Modutek can offer effective support for customer wet process applications. The company manufactures a full line of semiconductor fabrication equipment and can help customers analyze their needs. With its wet process expertise and its experienced technical personnel, Modutek can offer help with selecting the best possible wet bench equipment configuration. The company offers free consulting to ensure that customers select the right equipment. Whether it is Teflon tanks or a complete wet process semiconductor manufacturing line, Modutek can help determine what the requirements will be, suggest optimal solutions and deliver the appropriate equipment.

Improvements to the Silicon Wet Etching Process

Improvements to the Silicon Wet Etching ProcessAs a result of improvements in design, and materials, the silicon wet etching process can produce better wafer yields with reduced defects. Modutek provides equipment engineered to incorporate such improvements that are designed for reliability, safety and low cost of ownership. The company’s wet etching products include features that offer specific advantages and deliver improved results. Process engineers and fabrication facility managers can order equipment customized to meet their needs and to ensure they receive the process improvements they expect.

Modutek’s QFa Series High Temperature Recirculating Quartz Baths and the Qa Series Constant Temperature Quartz Baths are designed to set the quality standard in quartz baths. Used for etching, stripping and heating, the baths feature fast, even heating to improve process yield and minimize damage from runaway temperatures. The high quality internally flame polished virgin fused quartz process vessels reduce downtime and increase reliability due to longer vessel life.

Although Modutek’s Teflon tanks come in standard carrier sizes, the company will build custom sizes to match any requirement. Tank configurations are Temperature Controlled Recirculating Baths, Temperature Controlled Static Baths and Teflon Ambient Baths. Inline heating or immersion heating are available. Modutek can help design the optimal configuration for both new as well as existing systems you may already have.

Modutek’s silicon nitride etch baths deliver excellent process control while retaining flexibility in installation and operation. The control system monitors the temperature of the bath to determine when adding de-ionized water is required to maintain the acid to water ratio. As water is lost to evaporation, the boiling point of the bath increases and the control system adds small amounts of water to the boiling liquid, ensuring that the water mixes in immediately. This method tightly controls the boiling of the chemicals and avoids “bumping.” The availability of remote metering means that the product is easy to install and Modutek can help find the best solution for any process station.

Modutek’s line of Quick Dump Rinsers can increase process efficiency by reducing rinse times and DI water consumption. The Rinsers feature state-of-the-art engineering to eliminate particle entrapment and minimize rinsing times. Dual overhead spray manifolds, a contoured vessel design and a large machined dump door ensure efficient and rapid operation.

For facilities that could benefit from using KOH etching, Modutek offers customized solutions manufactured according to customer specifications. KOH etching is a repeatable process that delivers fast etching at a low cost. Modutek’s solutions use the company’s Teflon tanks with advanced welding techniques and all-PFA material. The products can be designed to particular specifications or to be compatible with your existing wet bench equipment. Once the wet etching process is established, continual adjustments are not necessary. Modutek can integrate the new products into an existing line to deliver process improvements and increased efficiency.

Making use of improved designs or higher quality materials can generate substantial benefits from reduced costs and better throughput. The increased ability to manufacture customized products that are optimized for a particular application can further improve silicon wet etching results. For any questions about silicon wet etching products or for help selecting the right manufacturing equipment for your requirements call Modutek at 866-803-1533.

Using the KOH Etching Process with Modutek’s Teflon Tanks

Potassium hydroxide-KOH etching is an efficient chemical process that allows for precise impressions to be made within a body of silicon. The process uses DI (deionized) water in combination with heavy alkalinity (pH levels greater than 12); the solution is then thermally adjusted for the appropriate degree of etching needed. This process allows for greater precision than other forms of etching when it comes to manufacturing silicon wafers.

What Are the Benefits of Using KOH Etching?

The precision KOH etching grants is very beneficial to the product’s outcome. Due to its chemical composure and procedural approach, dry etching is more apt to create physical flaws; it also can make for a more hazardous environment, as it risks the exposure of toxic — even explosive — chemicals.

KOH etching, on the other hand, is almost entirely aquatic. The liquids for the etching process are contained within Modutek’s own Teflon Tanks, meaning that exposure to hazardous chemicals is drastically reduced to create a safe environment for manufacturing. The structuring behind the process also means that it can easily be set up for repeat manufacturing; the resources required for the etching are more stable and affordable, meaning that clients can duplicate their etchings at a much more feasible rate.

How Do Teflon Tanks Bolster the KOH Etching Process?

Modutek’s Teflon Tanks are designed with the foresight to emphasize the maximum potential KOH etching can provide. Furthermore, each Teflon Tank is fashioned to cater to the specific needs of the client, regardless of unorthodoxy. In unison with wet bench equipment, these Teflon Tanks greatly reduce the impurities other methods of etching may establish, providing you with a more reliable and consistent output every time.

The Teflon Tanks can be thermally adjusted to operate between 30 — 100º C (86 — 212º F), and on average can heat up at a rate of 2 — 3º C per minute, though smaller or larger tanks may have varying thermal increase rates. Thermal output can be mustered either from inline Teflon heating, or from submersion within an overflow bank for each type of Teflon Tank. Each tank is also fitted with cooling coils that can quickly reduce the solution’s temperature.

There are two types of Teflon Tanks that support KOH etching: Temperature Controlled Re-circulating Baths (TFa Series), and Temperature Controlled Static Baths (TI Series). These two series of Teflon Tanks are designed with overlapping covers to minimize the loss of water during the etching process; they also boast the ability to eliminate concentration deficiency.

Because Modutek designs their Teflon Tanks with the client’s specifications in mind, the customization each tank receives is advantageous to the desired end result. Dual heating methods produce a quick, balanced aid to water-based etching. Drains and valves can be installed into the tanks for safe and easy clean-up. Remote timers, controllers, or even a data interface can be equipped to make a simple process even simpler.

When compared to other forms of etching (whether wet or dry), Modutek’s support of KOH etching produces more precise and affordable results in a safer and more expeditious work environment. Teflon Tanks can be produced individually or en masse depending on the output the client is in need of. Each tank can also be fitted with a number of practical options that can improve the etching process dramatically.

No client should work with an environment or product that diminishes the quality of their manufacturing process; customization leads the way for an innovative and effective product. Modutek’s support of KOH etching in their catalytic Teflon Tanks will provide exceptional results for clients that use this process.