The silicon nitride wet etching process uses a phosphoric acid solution in water to quickly and effective etch mask material. However, this process is usually hard to control. The etch rate depends on the concentration of the solution and the bath temperature. Ideally, the solution has to be about 85% phosphoric acid and 15% de-ionized water at an optimized temperature of 165 degrees Centigrade. Adding water to the solution to control the concentration will change the temperature, and can be dangerous. Managing such an unstable solution with two interdependent variables is difficult, but using standard controls cannot accurately maintain the etching rate and remove the silicon nitride.
To maintain proper control, small amounts of water are added to the solution continuously, and the solution is maintained at its boiling point. To optimize the process Modutek introduced the silicon nitride wet etching process control solution that improves etch control and results. It features precise control of the temperature, concentration of the solution, and etch rate while ensuring the operation is safely maintained.
Modutek’s control concept uses an always-on heater to keep the 85% phosphoric solution at its boiling point. As water is lost to steam, the solution’s concentration starts to increase, and the temperature begins to rise above the original value. A thermocouple senses the temperature rise and signals the system to add water. A small amount of water is added and immediately mixed into the solution by the boiling action. The concentration is maintained at the original 85% phosphoric acid and 15% water; the temperature remains at the boiling point of 165 degrees Centigrade at this concentration. The etch remains constant and the operation is safe.
An additional thermocouple is located above the hot acid senses the presence of hot steam. Another thermocouple detects the temperature of the acid. If there’s no steam or the acid becomes too hot, the process shuts down. The emergency shutdowns don’t take effect during regular operation, but they are additional safety features in case of equipment malfunction or operator error.
For additional details on how Modutek’s control strategy improves etching rates, read the complete article, “How Process Controls Improve Silicon Nitride Wet Etching Result.” If you have any questions, or would like to set up a free consultation, contact Modutek 866-803-1533 or email Modutek@modutek.com.