Adhesion of Contaminant Particles to Advanced Photomask Materials

A challenge to the use of extreme ultraviolet lithography (EUVL) is contamination of the mask during its processing and storage since current techniques for contamination prevention, such as pellicles, cannot be used with the mask materials under EUVL conditions. This paper characterizes the adhesion forces between model contaminant particles and photomask and thin film surfaces relevant to extreme ultraviolet and deep ultraviolet lithography. Specifically, experimental studies of the adhesion of nanoscale and microscale silica and silicon nitride particles to surfaces such as chromium oxynitride, quartz, MoSi, tantalum oxynitride, and ruthenium in air are described. In addition, an adhesion force mapping scheme, the “force band diagram,” is presented. This method allows upper and lower limits to be imposed on the adhesion force between given particle-substrate pairs. Two extremes of behavior are considered in the force bands, a rough prolate spheroid (minimum adhesion force) and a smooth oblate spheroid (maximum adhesion force). Experimental data are compared with the force band models and in all cases the measured adhesion forces fall within the established boundaries for particle sizes ranging from the nanoscale to the microscale. This technique is general, and can be applied to other surfaces of interest to the microelectronics manufacturing community.