The Effects of Higher Temperatures in Piranha Etch processes

A sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) mixture (SPM) is used for various wet cleaning process steps in semiconductor manufacturing. It’s often called Piranha Etch because it’s so aggressive and is typically used for photoresist stripping, cleaning, and other residue removal steps.

The most common process for the photoresist removal process is typically a combination of plasma resist strip to remove the majority of the resist followed by a SPM wet cleaning process to remove the remainingorganic residue. The plasma stripping process does have an effect on the subsequent residue remaining after theplasma exposure. With lower doses of implanted photoresist, the SPM chemistry can stand alone for removal. Higher implant doses require the plasma stripping step prior to Piranha.

Historically,semiconductor processes have been limited to approximately 180°C due to limitations with quartz heated tanks, as well as plumbing connections, and sensors used for temperature and level monitoring.

With the increasing use of single wafer processing and in several batch processing applications, there is a need to run the Piranha etch at higher temperatures than the standard 180° C. (To keep throughput high, either parallel single wafer processing stations are needed, running up the cost of capital equipment, or the etch needs to happen faster. Hence the increased temperatures demanded). Increased temperatures may allow for new applications due to the dramatic increase in chemical reactivity that occurs at higher temperatures.

Two batch processes where a fab may be able to eliminate a plasma asher with a high temp Piranha etch are:

  1. Wafer reclaim
  2. Baked on resist removal

Note that the chemical reaction of the SPM logarithmically increases as the temperature goes up. So the reliability of the etch tank and the wet bench and how these separate tools work together is paramount to success. Processes that were previously thought to be too slow due to temperature limitations may now be practical.

In the past, it was considered too dangerous to operate at these extreme high temperatures, but with advances in tank technology, robotics, and chemical handling it is possible to operate chemical processes up to 300°C.

For process tanks, high quality quartz is the best material to keep Piranha etch in control.

The Imtec Accubath XE Series etch tank is designed specially for higher temperature applications. Here are some of the key features of the bath:

  • In the Piranha etch process, the liquid in the Accubath XE tank comes in contact with nothing but high purity quartz.
  • No Teflon connections, sensors, or any other parts are used in the process area. Even the built in condensing coils and automated lid are quartz.
  • For reliability, a special version of Imtec's patented Aquaseal was developed to prevent a failure of the critical seal material that isolates the heater components from the environment.
  • Even the housing is special, utilizing Halar for its superior temperature and chemical compatibility as well as special construction techniques to isolate the heat inside.
  • For redundant safety, a sacrificial secondary containment liner is incorporated to prevent high temp chemical contained in the event of quartz breakage.

For more information about the special aspirator designed for the system, the “cool-down module”, and the secondary containment system, please click here for additional product details.