Silicon Nitride Etch process

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

Research has shown that the greatest etch rates and best selectivity are achieved when the phosphoric acid has the highest ratio of water at a given temperature, i.e. the boiling point. As long as the boil point is maintained, the etch rate of both Si3N4 and SiO2 can be controlled precisely. Maintaining a boiling solution however, is not easy to do using standard baths and controllers. When the phosphoric acid is heated, the water in the solution begins boiling off, increasing the acid concentration and, increasing the boiling point. A standard temperature controller will continue to maintain the temperature, but the water concentration will decrease and the etch rates will change.

In order to keep enough water in the solution some method of water addition must be provided. The amount of water and how it’s added can have a dramatic effect on how precisely the temperature (and boil point) can be maintained. Through careful study and working with customers IMTEC came up with a unique system that set the standard for Silicon Nitride etching.

IMTEC found that the best control could be achieved when a minimum amount of water was added slowly and only when needed. In order to minimize water addition, a closed “reflux” system is created above the bath using a condensing 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. This reduces overall fluctuations in temperature as well as improving uniformity throughout the bath.

Using this closed system, IMTEC’s Silicon Nitride Etch Accubaths are able to precisely control the temperature and acid concentration. Unlike a standard temperature controller, Our 952 Dedicated Nitride Etch Controller uses an IMTEC proprietary algorithm that controls both the heater output and DI drip. By adding DI water slowly as needed, the 952 controller can maintain the set point with a variation of only +/- 0.2° C.

The combination of the closed refluxing system and IMTEC’s proprietary control algorithm assures that uniform conditions are created throughout the bath which provides a precisely controlled etch rate and the best possible selectivity.

IMTEC has also been on the forefront of improving tank lifetimes. The chemical fumes and high temperatures that Nitride Etch tanks are subjected to are known to reduce bath life substantially by attacking the sealant that prevents liquid and fumes from entering the heater area. IMTEC recognized this problem years ago and created the Aquaseal©.

The Aquaseal creates a water barrier between the sealant and the harsh environment and completely eliminates the most common failure mode seen on heated quartz tanks. Tanks with the Aquaseal option have an industry-leading 30 month warranty and typically last 5 times as long as competitive products

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 specification. UL Recognition and ISO 9001 certification assures consistent products and quality.

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


Design Concept

The customer-selected process nitride etch quartz tanktemperature for the desired nitride etch rate and selectivity is best attained by a balance of three factors: optimum (high) process temperature, acid concentration and the highest water fraction. (Highest water fraction for a selected process temperature is coincident with the chemistry boiling point.) In nitride-etching, a specific acid concentration is picked for selectivity - and semiconductor process is then conducted for fastest etch rate for that acid concentration at the highest attainable temperature (at standard pressures): its boiling point. We have developed a special nitride controller that senses and maintains the chemistry boilingpoint (rather than the process bath temperature). IMTEC’s advanced 952 controller provides the manipulation of the significant nitride-etch control parameters: the phosphoric acid concentration, boil rate and boiling point temperature.

Customer Benefits

The unique new quartz-nitride control system from IMTEC provides the semiconductor process user with the following twin benefits:

  • Process uniformity
  • Lot-to-lot repeatability

Prevents Stratification

IMTEC’s well-tested and proven control technique virtually eliminates the all-too common problem of semiconductor bath stagnation which results from the loss of boil. The previous safety risks of water stratification become insignificant when using our control system.

Nitride etch system

Learn more about the Accubath product line and Silicon Nitride Etch System with the links below:

Accubath Product Line

Silicon Nitride Etch System Brochure

Silicon Nitride Etch Process