A new plasma ash method to prevent resist from flowing into contact and via holes

With downstream ashers, the wafer is heated to provide the necessary activation energy to remove the photoresist, using a chemical reaction with free oxygen radicals. Two factors, temperature and the presence of a hardened crust layer on top of the resist; can contribute to flowing resist into open contacts or vias. When the wafer is loaded into the process chamber, the transient temperature of the wafer surface will become hot enough to exceed the glass transition temperature (Tg) of the resist, causing it to soften and flow. The hardened top resist crust prevents the resist from being stripped at a rate that is fast enough to avoid viscous flow into the opening. Residual resist will remain in an opening, if the ash process ends before the “flowed” volume of resist can be consumed. A novel post contact and via etch resist strip process that prevents resist from flowing into the openings, has been developed using a plasma resist strip tool. The dry strip process is followed by a DI-water-only clean. This avoids the use of toxic wet chemicals that are costly. The DI- water clean is designed to remove fluorinated polymers from the holes and to remove polymer and particles from the ILD surface.