How Process Controls Improve KOH Etching Results

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

KOH Etching Can Create Complex Shapes

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

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

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

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

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

Teflon Tanks with Precise Controls Produce Improved Output

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

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

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

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

Improving the KOH Etching Process Using Teflon Tanks

Etching with potassium hydroxide (KOH) is commonly used for general purpose etching of silicon wafers because the process can be tightly controlled and because it is relatively safe. During the process, the etch rate is controlled through the temperature. When KOH etching is done using Modutek’s Teflon tanks, performance can be improved through tank features such as tight temperature control and high control accuracy.

Controlling the KOH Etching Process

The KOH etch rate is affected by solution concentration, lattice orientation, doping and solution temperature. The silicon wafer is masked with a substance such as silicon nitride, which is impervious to KOH. The solution etches the parts of the silicon wafer that is not masked and a combination of the measures that affect the etch rate allows operators to etch complex shapes that are used in electronic circuits and microstructures.

A higher concentration results in an increased etch rate. The KOH concentration can range from about 10 percent to 50 percent but is usually around 30 percent by weight. KOH etching is designed to work with a fixed concentration that is determined at the start of the process and is not changed once etching is under way.

The silicon wafer crystal lattice has different atom densities in different directions, with atoms closer together in some directions than in others. As a result, KOH etches at different speeds in the different directions or lattice orientations. When designing the shapes to be etched, the silicon crystal can be placed in an orientation that produces deeper or longer etching in certain directions, giving designers control of the etched shapes.

Designers can also stop KOH etching in specific directions by doping the silicon crystal with boron. Used with the other measures influencing KOH etching, boron doping completes the initial conditions used to create shapes in the silicon wafers. Masking, concentration, lattice orientation and doping are set up to produce the desired shapes before the process begins.

Temperature Control is Key to KOH Etching

Once KOH etching is under way, temperature control lets operators affect the etch rate during the process. KOH etching is sensitive to temperature changes and the solution temperature can be used to control etching and influence the dimensions of the etched shapes. Keeping the temperature constant keeps the different etch rates in different directions constant as well. Changing the temperature changes the different etch rates and can influence etched shapes. For example, if an etching direction has been blocked by boron doping, different etch rates in the remaining directions will result in different etched shapes.

As a result, temperature control becomes critically important for precise KOH etching. Ideally, the solution heats up quickly to a set temperature and stays exactly at that temperature until the set point is changed. The solution then quickly heats up to the new temperature and again maintains it accurately. A rapid and precise temperature control system can improve KOH etching performance.

How Modutek’s Teflon Tanks Improve KOH Etching

Modutek’s Teflon tanks are designed to eliminate contamination by using an all-Teflon liquid path design. The heated tanks are available in a circulating design or with an immersion heater in the overflow weir. Standard sizes are available but Modutek can build custom sizes for special applications as required and can fit new tanks into existing wet bench equipment.

Modutek’s tanks feature an advanced temperature control system ideal for improved KOH etching. The tank heating system has a 30 to 100 degree centigrade operating range and the heating rate is 2 to 3 degrees centigrade per minute, depending on the size of the tank. Process temperature control is accurate within plus/minus 0.5 degrees centigrade and the temperature controls are accurate from batch to batch. With Modutek’s Teflon tanks, silicon wafers etched with the KOH process are uniform, resulting in high output quality. Overall facility performance can be improved and component defects reduced. Contact Modutek for a free consultation or quote on Teflon tanks or other equipment designed to meet specialized manufacturing requirements.

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