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As the mask technology matures, critical printing features and sub-resolution assist features
(SRAF) shrink below 100 nm, forcing critical cleaning processes to face significant challenges.
These challenges include use of new materials, oxidation, chemical contamination sensitivity,
proportionally decreasing printable defect size, and a requirement for a damage-free clean.
CO2 cryogenic aerosol cleaning has the potential to offer a wide process window for meeting
these new challenges, if residue adder issues and damage can be eliminated. Some key
differentiations of CO2 cryogenic aerosol cleaning are the non-oxidizing and non-etching
properties compared to conventional chemical wet clean processes with or without megasonics.
In prior work, the feasibility of CO2 cryogenic aerosol in post AFM repair photomask cleaning
was demonstrated. In this paper, recent advancements of CO2 cryogenic aerosol cleaning
technology are presented, focusing on the traditional problem areas of particle adders,
electrostatic discharge (ESD), and mask damage mitigation.
Key aspects of successful CO2 cryogenic aerosol cleaning include the spray nozzle design, CO2
liquid purity, and system design. The design of the nozzle directly controls the size, density,
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. Adder control can be achieved only through use of highly
purified CO2 and careful materials selection. 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 of
adders. The previous damage threshold of 150 nm SRAF structures has been reduced to 70nm
and data will be shown indicating 60 nm is possible in the near future. Data on CO2
tribocharge mitigation, the main cause of ESD, will also be presented and application to current
technology nodes discussed.
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