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.