Hydrogen 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.
An increasing number of silicon wafer fabrication applications, such as those with large size substrates or densely packed components, require tight process control. Accurate chemical dosage and precise process timing become critical for a high-quality output. Manual and semi-automatic controls depend on operator training and reliable operator execution of instructions, leaving open the possibility of human error and a variation in how process steps are carried out. Fully automated wet benches and chemical stations can improve process control accuracy and help with consistent execution. Software can help calculate process variables and ensure correct dosages. Full automation of the wafer fabrication process with the use of software for process control and robotics for wafer handling can result in better repeatability, reduced waste and improve production line performance.