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As the mask technology moves towards production of 36 nm and 22 nm DRAM half
pitch nodes, printing features and sub-resolution assist features (SRAF) shrink below 80
nm. These narrow features become more fragile and place new demands on cleaning
processes for a physically non damaging solution. These challenges include compatibility
with new materials, oxidation, chemical contamination sensitivity, proportionally
decreasing printable defect size, and a requirement for a damage-free clean. CO2
cryogenic aerosol cleaning has, for many years, shown potential to offer a wide process
window for meeting some of these new challenges. CO2 cryogenic aerosol cleaning for
post AFM repair debris cleaning has been used for many years on masks greater than 90
nm DRAM half pitch nodes. Until recently, CO2 purity and delivery hardware issues
resulted in foreign material adder (FMACO2) contamination and SRAF damage below 150
nm critical feature size. Some key desirable properties of CO2 cryogenic aerosol cleaning are the non-oxidizing
and non-etching properties when compared to current chemical wet clean processes. In
this paper, recent advancements of CO2 cryogenic aerosol cleaning technology are
presented, highlighting improvements in the areas of FMACO2 reduction, lowering the
critical feature size without damage, and electrostatic discharge (ESD) mitigation. Key
aspects of successful CO2 cryogenic aerosol cleaning include the spray nozzle design,
CO2 liquid purity, and integrated system design. The design of the nozzle directly
controls the size, flux, and velocity of the CO2 snow particles. Methodology and
measurements of the solid CO2 particle size and velocity distributions will be presented,
and their responses to various control parameters will be discussed. FMACO2 mitigation
can be achieved only through use of highly purified CO2 and careful materials selection
of the delivery hardware. Recent advances in CO2 purity will be discussed and data
shown. The mask cleaning efficiency by CO2 cryogenic aerosol and damage control is essentially
an optimization of the momentum of the solid CO2 particles and elimination FMACO2.
Data on CO2 tribocharge mitigation, the main cause of ESD, will be presented and
application to current technology nodes discussed. The previous damage threshold of 150
nm SRAF structures have been reduced below 60 nm and data will be shown indicating
sub-50 nm is possible. The tool capability has been improved from previously doing
local cleaning of AFM repair sites to a full mask clean with prospects of replacing certain
wet clean steps where phase and transmission are degraded.
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