Why Are PTFE Heaters Specified for Hot DI Water Rinse Tanks in Semiconductor Fabs?

Metal contamination at parts-per-trillion levels can destroy yields of devices in semiconductor production. Hot deionised water rinses are common and any heater in touch with this water must be free of leachable metals. PTFE heaters can meet this rigorous criteria.

Hot DI Water Semiconductor Fabrication
Hot deionised water is utilised in semiconductor fabs for a variety of important cleaning procedures, including wafer cleaning, resist stripping and general rinsing. The water is constantly re-circulated through the system and has a high resistance of 18 megaohm-centimeters (MΩ-cm) at 25°C. This guarantees the water is ultrapure, critical to avoiding contamination throughout the production process for semiconductors.

The DI water is usually kept at the temperature of 70-90°C for effective removal of pollutants and to optimise the cleaning process. This temperature range is important for the best processing of the wafers and is obtained by means of specialised heaters.

Metal-sheathed heaters: risk of contamination
Many industrial heating applications utilise metal-sheathed heaters such as stainless steel or Incoloy. These are a serious hazard in high-purity applications such as semiconductor production. Trace ions (e.g. iron, nickel, chromium) are leached into the DI water from metal heaters over time. These metal ions can damage the quality of water and contaminate it at very low amounts. This contamination can significantly affect wafer quality, lowering device yields and requiring more frequent replacement of the resin bed to recover water purity.

To avoid these problems, the materials used to create the heaters in touch with ultrapure DI water should not introduce any pollutants into the water.

Reasons for Specifying PTFE Heaters
Hot DI water rinse tanks in semiconductor fabs are specified with PTFE (polytetrafluoroethylene) heaters because of their excellent chemical resistance and inertness. PTFE has no metal content and hence will not leach any metallic ions into the water guaranteeing that the water remains ultrapure. This is important to sustain the high water resistivity needed for semiconductor production.

PTFE heaters are tolerant to 70-90°C operating temperature ranges, making them a good fit for this application. PFA ( perfluoroalkoxy ) is sometimes used instead of PTFE when even better purity or temperature tolerance is needed . PFA can handle higher temperatures while still maintaining the material integrity .

One of the primary advantages of PTFE in DI water rinse tanks is its long-term stability. PTFE has no contamination problems, thus it does not require the periodic maintenance or replacement as metal heaters do. This provides more constant and reliable performance in high purity conditions, saving operating costs over time.

Technical Considerations for PTFE Heaters 
For semiconductor manufacturing, the resistivity of DI water must be kept at 18 MΩ-cm at 25°C in order to be really ultrapure. PTFE heaters are the only heater sheath materials allowed to be in direct contact with ultrapure water for these demanding applications. The watt density of the heater must be conservative enough to avoid any localised boiling or particle production. This helps to avoid hot spots which could lead to undesirable contamination or physical damage of the water system.

Purity Specification Limits for Metal Contamination
Limits for metal contamination are very low in high purity semiconductor applications. For example, the contamination level should be less than 1 ppb (parts per billion) of iron, nickel or other metal ions in DI water. The PTFE heaters are constructed of a 100% non-metallic material and are ideally suited to satisfy these rigorous purity requirements.

Summary
In semiconductor manufacturing, the ultrapure water quality is kept using PTFE heaters so that no metal ions are leached into the water during the cleaning process. The selection of materials for high purity applications such as semiconductor fabs is motivated by the need to control contamination. PTFE heaters are specifically designed for these demanding requirements and provide a reliable option to heating DI water without affecting water quality or device yields.