Nitride Etch process

For the demanding Nitride Etch process, Imtec offers a high-tec solution grounded in both a thorough understanding of the chemistry involved and the specific requirements of the Silicon Nitride etching process.

To obtain the constant high temperature required to etch Silicon Nitride at the optimum rate is not merely a matter of heating the phosphoric acid to the highest possible temperature (the boiling point), and allowing the process to run. Rather, when the phosphoric acid is heated, the water in the solution begins boiling off and evaporating, increasing the acid concentration and, consequently, increasing the boiling point. The combination of higher temperatures and higher acid ¬concentrations can lead to undesirable etch rates on the SiO2 and the quartz vessel.

Manufacturers often solve this problem by injecting di water into the system to control the boiling point of the phosphoric acid and consequently the temperature of the bath. However, introducing large amounts of water to the system often results in uneven temperatures in the unit and it creates difficulties maintaining the boiling point.

Imtec solves this problem by creating a closed “reflux” system above the bath using a cooling collar and a lid. As the water vapor rises, it condenses onto the water-cooled coils and drips back down into the bath. A di water drip is still necessary to precisely control the boiling point, but the closed system means that much less water is required. And the water that drips down from the cooling collar enters the bath evenly from all sides, protecting against uneven temperatures within the bath.

Using this closed system, Imtec’s Nitride Etch Accubaths are able to precisely control the temperature and acid concentration. Our 952 Control Panel uses an Imtec proprietary algorithm that controls heater output and di drip. Using the customer-set parameters, the control panel injects small amounts of D.I. water when the temperature rises above the set value, thus decreasing the phosphoric acid concentration and lowering the boiling point. Using 100-ohm RTD sensors to monitor the temperature, the 952 control panel can maintain the set point with a variation of only +/- 0.2° C.

According to research performed at Imtec, etch rates for Silicon Nitride increased by a factor of 6 in a closed reflux system as opposed to an open system at the same temperature. In addition, the etch rates for Silicon Dioxide, the masking material, decreased by a factor of 2.2. While the optimum temperature set point will depend on the application, it is clear that the closed reflux system provides a faster, more selective etching mechanism. The Imtec system also saves expense on resources- less DI water is necessary to control the boiling point, and heaters don’t waste energy maintaining higher temperatures when the phosphoric acid concentration increases.

Imtec’s Quartz Nitride Reflux System was designed to address the unique needs of the Silicon Nitride Etch process. Each bath is backed by Imtec’s standard safety and reliability features, such as redundant over-temperature sensors, Alumina-silica insulation, Inconel heaters, and high-purity quartz fabricated and molded in-house to spec.

Imtec’s experienced engineers can assist you from the design and specifications of your bath to choosing an optimum temperature setpoint for your process, and back you up with our excellent customer support.