The silicon nitride wet etching process uses a phosphoric acid solution in water to etch mask material quickly and reliably but the process is hard to control. Etching speed depends on the concentration of the solution and the bath temperature. Adding water to the solution to control the concentration is dangerous and, at the same time, it can change the temperature. Controlling such an unstable solution with two interdependent variables is difficult, but in the absence of reliable controls, the etching rate is not accurately maintained and the removal of silicon nitride is not precise.
How Silicon Nitride Etching With Phosphoric Acid Works
In the silicon nitride wet etching process, the silicon wafer is masked with a silicon nitride coating that has to be selectively etched. A solution of about 85 percent phosphoric acid and 15 percent de-ionized water is used to etch the silicon nitride at an optimized temperature of 165 degrees centigrade. To speed up etching, the phosphoric acid solution is heated to its boiling point. As the solution boils, water from the solution is lost as steam and the concentration of the solution rises. The boiling point of the solution rises as the concentration rises and the etch rate increases as both the temperature and the concentration increase.
To correct the etch rate, water has to be added to the bath to bring the concentration and the temperature back down. Adding water to boiling phosphoric acid is dangerous because it can reduce the temperature of the solution to below boiling. When the phosphoric acid solution is no longer boiling, the added water can form a thin film on top of the acid instead of mixing in continuously. Eventually, the film will mix with the acid all at once, and a “bump” can result from the exothermic reaction.
To make sure small amounts of water mix into the solution continuously, the solution has to be kept at its boiling point. Because the boiling point is variable, standard controls with a fixed temperature set point don’t work. Modutek has developed an innovative silicon nitride etching process control solution that features precise control of the temperature, concentration and etch rate while ensuring safe operation.
The Modutek Control Strategy Improves Etching Results
Modutek’s control concept uses an always-on heater to keep the 85 percent phosphoric acid solution at its boiling point. As water is lost to steam, the concentration of the solution starts to increase and the temperature starts 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 is immediately mixed into the solution by the boiling action. The concentration is maintained at the original 85 percent phosphoric acid and 15 percent water and the temperature stays at the boiling point of 165 degrees centigrade with this concentration. The etch rate remains constant and the operation is safe.
Because the safety of the Modutek control strategy relies on the constant boiling of the phosphoric acid solution, an additional thermocouple is located above the hot acid to sense the presence of hot steam. Another thermocouple senses the temperature of the acid. If no steam is present or if the acid becomes too hot, the process is shut down. These emergency shut-downs don’t take effect during normal operation, but are an additional safety feature in case of equipment malfunction or operator error.
Modutek’s Control Strategy Delivers Improved Etching Performance
Modutek has tested and fine-tuned its control strategy with their Nb Series silicon nitride etching baths for customers using manual, semi-automated, and fully automated wet bench stations. Customers that have been using the new equipment have achieved an average etch rate of 65 angstroms per minute while limiting oxide etch as the controls ensure safe operation at a consistent and optimum operating point.
A reliably-controlled and constant etch rate yields predictable etching and silicon nitride removal within product tolerances. When etching is not the same from batch to batch, product quality suffers and product lifespan can be reduced. Modutek manufactures a complete range of silicon wet etching equipment, and with the company’s innovative control system, rejection rates are reduced and facility output is improved.