Silicon Nitride Etch Process

For the demanding Silicon Nitride Etch process, we offer a Quartz Silicon Nitride Etch Systems 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 Deionized water (DIW) 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 DIW 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.

Accubath™ Quartz Silicon Nitride Etch Systems

The customer-selected process temperature 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 boiling point (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.

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

Nitride Etch Trend – Single Wafer Processing

Single wafer processing in semiconductor manufacturing was first introduced almost 15 years ago. Dry etching equipment initially adopted the technology for various processing advantages including lowering the cost of consumables and high repeatability and reliability of the process.

In some instances, wet etch has been slower to adapt single wafer processing because throughput is impacted. But several key processes in wet etching are now trending toward single wafer. The Nitride Etch process is a prime example for the following reasons:

  1. The elevated temperatures used in the etch process
  2. The need to keep etch temperatures constant
  3. The need to keep the process repeatable

Operating at elevated temperatures, like in the silicon nitride etch bath, is complicated by the requirement that the acid concentration must be strictly maintained to regulate the temperature of the bath. In-line heaters are one way to maintain the needed temperature of the etch process and ensure repeatability. Different types of in-line heaters for different applications. Different types of in-line heaters designed for different materials. For high purity fluids heating and gas heating inline heaters, quartz is the ideal material. But for heating HF (hydrofluoric acid), KOH (potassium hydroxide), and other high PH chemistries quartz has limitations, we found a better solution by using high purity PFA as a heat transfer material. Accuheat™ in-line heaters are specifically designed for the silicon nitride etch application. The traditional application uses the in-line heater as part of the recirculation loop, either as the sole heat source or in combination with a heated quartz tank for faster heat-up times and better control.

Accuheat In-line Heaters – Key Benefits

  • Uniform heating
  • No purge or cooling required
  • Easy to install, less expensive to operate
  • Minimal heat-up time – no energy wasted
  • Redundant safety features for a safe environment
  • Zero maintenance

In-line Heaters – Zero Maintenance

  • No lamp replacement
  • No calibration
  • No reflectors to clean
  • No leaks – Hot Grip Nuts stay tight
  • Self-draining