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Cryogenic CO2 aerosol cleaning being a dry, chemically-inert and residue-free process is used in the
production of optical lithography masks. It is an attractive cleaning option for the mask industry to achieve the
requirement for removal of all printable soft defects and repair debris down to the 50nm printability
specification. In the technique, CO2 clusters are formed by sudden expansion of liquid from high to almost
atmospheric pressure through an optimally designed nozzle orifice. They are then directed on to the soft defects
or debris for momentum transfer and subsequent damage free removal from the mask substrate. Unlike
aggressive acid based wet cleaning, there is no degradation of the mask after processing with CO2, i.e., no
critical dimension (CD) change, no transmission/phase losses, or chemical residue that leads to haze formation.
Therefore no restriction on number of cleaning cycles is required to be imposed, unlike other cleaning methods.
CO2 aerosol cleaning has been implemented for several years as full mask final clean in production
environments at several state of the art mask shops.
Over the last two years our group reported successful removal of all soft defects without damage to the
fragile SRAF features, zero adders (from the cleaning and handling mechanisms) down to a 50nm printability
specification. In addition, CO2 aerosol cleaning is being utilized to remove debris from Post-RAVE repair of
hard defects in order to achieve the goal of no printable defects. It is expected that CO2 aerosol cleaning can be
extended to extreme ultraviolet (EUV) masks.
In this paper, we report advances being made in nozzle design qualification for optimum snow properties
(size, velocity and flux) using Phase Doppler Anemometry (PDA) technique. In addition the two new areas of
focus for CO2 aerosol cleaning i.e. pellicle glue residue removal on optical masks, and ruthenium (Ru) film on
EUV masks are presented. Usually, the residue left over after the pellicle has been removed from returned
masks (after long term usage/exposure in the wafer fab), requires a very aggressive SPM wet clean, that
drastically reduces the available budget for mask properties (CD, phase/transmission). We show that CO2aerosol cleaning can be utilized to remove the bulk of the glue residue effectively, while preserving the mask
properties. This application required a differently designed nozzle to impart the required removal force for the
sticky glue residue. A new nozzle was developed and qualified that resulted in PRE in the range of 92-98%.
Results also include data on a patterned mask that was exposed in a lithography stepper in a wafer production
environment. On EUV mask, our group has experimentally demonstrated that 50 CO2 cleaning cycles of Ru
film on the EUV Front-side resulted in no appreciable reflectivity change, implying that no degradation of the
Ru film occurs.
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