Removal of organic contaminants from lithographic materials

One of the critical issues still facing the implementation of extreme ultraviolet lithography (EUVL) into mainstream manufacturing for integrated circuit (IC) production is cleanliness. EUV photons at 13.5 nm are easily absorbed by many species, including dust, thin-film layers, and other debris present in the path of the photons. Carrying out EUVL inside a vacuum helps reduce the amount of photon loss for illumination, however contamination in the sys- tem is unavoidable, especially due to carbon growth on the multilayer mirror collectors and to soft defects in the form of organic contamination on the mask. Traditional cleaning methods employ the use of wet chemicals to etch contamination off of a surface, however this is limited in the sub-micron range of contaminant particles due to lack of transport of sufficient liquid chemical to the surface in order to achieve satisfactory particle removal. According to the International Technology Roadmap for Semiconductors (ITRS), the photomask must be particle free at inspection below 30 nm. However, when analyzing the ability of traditional methods to meet the cleaning needs set forth by the ITRS, these methods fall short and often add more contamination to the surface targeted for cleaning. With that in mind, a new cleaning method is being developed to supplant these traditional methods. Preliminary research into a plasma-based method to clean organic contaminants from lithographic materials constructed an experimental device that demonstrated the removal of both polystyrene latex nanoparticles (representing hydrocarbon contamination) in the range of 30 nm to 500 nm, as well as the removal of 30 nm carbon film layers on silicon wafers. This research, called the Plasma-Assisted Cleaning by Metastable Atomic Neutralization (PACMAN) process is being developed with semiconductor manufacturing cleaning in mind. 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 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. Operating the PACMAN process in this fashion allows for cleaning rates of approximately 1.2x10^7 ± 5.1x10^5 nm^3/min without causing damage to the surrounding structure of the sample being cleaned. Carbon contamination in the form of carbon films on lithographic material have been shown to clean at rates of approximately 3.0x10^6 ± 1.3x10^5 nm^3/min. The PACMAN process works by utilizing helium metastable atoms to break apart the contamination to be cleaned. As helium metastables interact with the surface of contamination, 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 desorbtion of C_xH_y molecules or by chain scission of the hydrocarbon backbone. The ultimate goal of this research is to understand the removal mechanism and provide ranges for the important parameters that lead to contamination removal from lithographic materials