Silicon nitride is used as a mask when etching silicon wafers during the semiconductor manufacturing process. Before the silicon wafer can be processed further, the silicon nitride has to be removed. A solution of phosphoric acid and de-ionized water etches the silicon nitride while leaving the wafer unaffected. As a result, the silicon nitride is stripped from the wafer surface and a clean wafer is left for further fabrication steps.
To optimize the silicon nitride wet etching process, the phosphoric acid solution is kept at a high temperature. This means some of the de-ionized water boils off as steam and has to be replaced. Adding water to phosphoric acid can be extremely dangerous and can result in an explosion. In addition, keeping the temperature of the solution constant is important for accurate control of the stripping process. Modutek has improved both the safety and the control of the nitride wet etching process with its Nb series silicon nitride wet etching baths.
How the Process Works
A solution of 85 percent phosphoric acid and 15 percent de-ionized water is heated to a boil at about 160 degrees centigrade. Some of the water boils off as steam and part of the steam is condensed and placed back in the solution. Some steam escapes and de-ionized water has to be added periodically. The solution is kept at the boiling point by a heater and the process temperature can be tightly controlled. With the Modutek system, nitride etch is uniform and there are no “bumps” in the chemical concentration or temperature.
How Modutek Controls the Etching Process Safely and Accurately
The two challenges faced when operating a boiling phosphoric acid bath is the safe replacement of the de-ionized water lost to steam and an accurate control of the process. If water is added to the solution too quickly, the solution may stop boiling and a film of water can accumulate on top of the viscous acid. If the water of the film then suddenly mixes with the acid, a strong reaction can occur, and an explosion is possible. Adding water in this way also makes accurate control of the process difficult.
In the Modutek system, the heater that boils the solution is always switched on. The solution is always heated up to the boiling point and further heating only increases the boiling rate but not the temperature. This control strategy results in a clearly defined temperature for the boiling solution, which will always be at its boiling point.
The boiling point of a phosphoric acid and water solution varies with the concentration, increasing as the solution loses water to steam. A thermocouple in the bath senses the increasing temperature and gives the signal to add water. De-ionized water is slowly added to the boiling solution. Because the acid is boiling rapidly, the small amounts of water are immediately mixed in and don’t form a surface film. Adding water in this way is safe and controlling the process by monitoring the concentration through the temperature rise is accurate.
Additional Safety Features
Since the silicon nitride wet etching process depends on keeping the solution at the boiling point, additional safety interlocks in Modutek’s etching baths are in place to insure no water is added if the solution is not boiling. A thermocouple above the solution senses the presence of hot steam and shuts off the water valve if steam is not present. To ensure the solution does not overheat, another thermocouple switches off the heaters when the temperature of the solution reaches 170 degrees centigrade. With its advanced control system and the additional safety interlocks, Modutek has improved the silicon nitride wet etching process with better control, accuracy and increased safety.