When silicon nitride wet etching is used to etch masks on silicon wafers, precise control of the etch rate is important. If the etch rate varies, the mask may not be correct and the microscopic structures to be created in the silicon may not have the right shape or depth. In the silicon nitride wet etching process, the etch rate depends on both the temperature and the concentration of the etching solution. Both may vary as the process progresses and precise control becomes difficult. An ideal control strategy maintains both the concentration and the temperature at known levels. The etch rate then remains constant and the mask is etched accurately according to the applicable specification.
Silicon Nitride Mask Etching Is Effective but Hard to Handle Safely
Semiconductor manufacturers and research labs use silicon nitride wet etching because the process etches the mask material quickly and reliably. A mixture of 85 percent phosphoric acid and 15 percent deionized water is heated to the boiling point. As the mixture boils, it releases steam that reduces the amount of water in the etching solution. As the concentration of phosphoric acid rises, so does the boiling point, the temperature of the mixture and the etch rate. To keep the process under control, deionized water has to be added to bring the concentration back down.
Adding water to phosphoric acid is exothermic. To prevent a sudden increase in temperature, water has to be added in small amounts, but if too many small amounts are added to the mixture, it will stop boiling. The water then forms a thin film on top of the mixture and eventually mixes with the phosphoric acid all at once, leading to a sudden temperature spike and a possible explosion. A complex control system with variable set points can ensure safe operation but control of the etch rate is poor.
Modutek’s Silicon Nitride Etch Controls Are Simple and Precise
Modutek addresses the control issues of silicon nitride wet etching by heating the etch bath continuously. An always-on heater heats the original 85/15 percent phosphoric acid/deionized water mixture to its boiling point of 165 degrees centigrade. As the mixture loses steam and the concentration and temperature both start to rise, a thermocouple detects the slightly increased temperature and triggers the addition of a small amount of deionized water. The added water is immediately mixed into the etching solution by the boiling action. The amount added restores the concentration and keeps the temperature at the boiling point.
Modutek has refined this control concept to accurately maintain the etching solution exactly at its original concentration and at its boiling point. As a result, with both the concentration and the temperature remaining constant, the etch rate remains predictable and constant as well. Mask etching is reliable and the final mask has precisely the desired characteristics.
Because the Modutek control strategy relies on maintaining the etching solution at the boiling point, the control system has additional safety features to identify dangerous conditions that could result if the mixture stops boiling. Two thermocouples detect the presence of steam over the hot acid and acid overtemperature. If there is no steam, the mixture is no longer boiling. If the acid temperature is too high, the solution is no longer being maintained at the original boiling point. In either case, the process is shut down because equipment failure is likely and a dangerous condition may be present.
Modutek Provides a Complete line Equipment Solutions
Modutek offers a complete line of silicon wet etching equipment and works with customers to develop innovative solutions to their problems. The silicon nitride etching control system improves etching performance and results remain consistent over different batches. The number of defective products is reduced and product quality increases. When customers purchase etching systems and other equipment, Modutek will ensure they meet their requirements. Contact Modutek for a free consultation to discuss your specific process requirements.