Improved Process Control with Modutek’s Silicon Nitride Wet Etching Bath

Improved Process Control with Modutek's Silicon Nitride Wet Etching BathWith over 30 years of experience in the design and manufacturing of silicon wet etching equipment, Modutek has recently introduced their series Nb silicon nitride etch baths, an upgrade to their previously available series Na. The new series features improved process control resulting in greater flexibility and better safety. The Nb series benefits include uniform nitride etching, smooth chemical boiling control, and remote DI water metering.

Silicon nitride etching takes place in a fluid that is 85% phosphoric acid and 15% DI water. The acidic solution boils at 160 degrees C, resulting in some water boiling off as steam. The Modutek Nb series etch bath includes a condenser that returns some of the steam back to the bath, but water has to be added to the bath to replace the steam that escapes.

Adding water to acid can be hazardous, but Modutek has developed a safe procedure to add the required water to the acid bath. When water is added to a viscous liquid such as phosphoric acid, the water stays on top of the acid. As more water is added, a sudden reaction can occur when the water mixes with the acid. Instead, the Modutek bath adds small amounts of water to the acid solution when the liquid is boiling. The boiling action ensures that the water to replace losses through steam is immediately mixed into the solution and avoids sudden large reactions.

To ensure that the acid solution is always boiling, the bath heater is on when the bath is operating. The bath solution remains at the boiling point, and the operation of the heater does not have to be controlled. Instead, the temperature controller monitors the temperature to determine when to add water to the bath.

The boiling point of the bath solution varies with the concentration of the acid. As the solution loses water through the escape of steam, the boiling point increases. The temperature controller detects the increase in temperature and adds a small amount of water to restore the concentration of the acid solution. In this way, the temperature controller uses the temperature of the boiling solution to maintain the bath concentration at its set point.

Because keeping the solution boiling is critical to the process and for safety, the Modutek silicon wet etching bath includes additional monitoring to detect boiling. A thermocouple located above the surface of the liquid activates an interlock if there is no hot steam over the solution. The interlock cuts power to the water valve and prevents water from being added when the bath solution is not boiling. The same interlock activates when the lid of the bath is off because the loss of steam and heat can cause the solution to stop boiling.

Additional safety interlocks are provided by thermocouples that detect bath overtemperature and tank overtemperature. The former activates an interlock to shut off the heaters when the bath temperature exceeds 170 degrees C, indicating there is a problem with the water supply, the set points, or the acid solution itself. The latter shuts off the heaters when the tank overheats, for example, if an empty tank is switched on.

Modutek’s silicon nitride etching baths which use etched foil resistance heaters, PVDF or Teflon condensers with safety covers, a heat-up rate of 2 to 3 degrees C per minute, and process temperature control of plus/minus 0.5 degrees C. The benefits of this design include accurate nitride etching control with good nitride etching uniformity, chemical boiling control without bumping, easy installation with remote metering, and a high level of safety.

To learn more about the Silicon Wet Etching products sold by Modutek, or to learn how our products can be redesigned to work with your Nitride Etch Baths or Quartz Baths and Sub Ambient Systems (BOE), contact us at 866-803-1533 or email to discuss your requirements and get a free quote.