Plasma-assisted cleaning by metastable-atom neutralization

Plasma-assisted cleaning by metastable atomic neutralization (PACMAN) is a process that can clean hydrocarbon from extreme ultraviolet photo masks and dissolve hydrocarbon particles. It was developed with semiconductor manufacturing and cleaning in mind. The PACMAN process works by utilizing helium metastable atoms to break apart the contamination to be cleaned. As helium metastables interact with the contaminant surface, bonding electrons from the surface are "stolen" by the metastable helium resulting in "holes" where a bonding electron used to be. In this way, the structure of the contamination is compromised and allows for the removal either through desorption of C_xH_y molecules or by chain scission of the hydrocarbon backbone. A model of the helium metastable density within the processing chamber has been developed in addition to experimental measurements of the metastable density at the sample surface. Cleaning efficiency has been linked to both helium metastable density as well as electric field in the plasma sheath. Electric field calculations in the plasma sheath reveal that an electric field pointing into the plasma is needed for achieving high cleaning rates of hydrocarbons since it pins the holes that are created to the surface and stops the hydrocarbon bonds from re-forming. Operating the PACMAN process in this fashion allows for cleaning rates of approximately 1.2 × 10 ⁷ ± 5.1 × 10⁵ nm³/min from a particle without causing damage to the surrounding structure of the sample being cleaned. Carbon contamination in the form of carbon films on lithographic material has been shown to clean at rates of approximately 11.4 ± 0.3 nm/min.