Extreme Ultraviolet Lithography (EUVL) is a leading lithography technology for the sub-32 nm chip
manufacturing technology. Photomasks, in a mask carrier or inside a vacuum scanner, need to be
protected from contamination by nanoparticles larger than the minimum feature size expected from
this technology. The most critical part with respect to contamination in the EUVL-system is the
photomask. The protection is made more difficult because protective pellicles cannot be used, due to
the attenuation of the EUV beam by the pellicle. We have defined a set of protection schemes to
protect EUVL photomasks from particle contamination and developed models to describe their
effectiveness at atmospheric pressure (e.g. in mask carriers) or during scanning operation at low
pressure. These schemes include that the mask is maintained facing down to avoid gravitational
settling and the establishment of a thermal gradient underneath the mask surface to
thermophoretically repel particles. Experimental verification studies of the models were carried out
in atmospheric-pressure carriers and in a vacuum system down to about 3.3 Pa. Particles with sizes
between 60 (for experiments, isn't it 125 nm?) nm and 250 nm were injected into the vacuum
chamber with controlled speed and concentration to validate the analytical and numerical models. It
could be shown that a deterministic approach using free molecular expressions can be used to
accurately describe particle deposition at these low pressure levels. Thermophoresis was found to be
very effective at both atmospheric and low pressure against the diffusional particle deposition,
whereas inertial particle deposition of large and/or fast particles can likely not be prevented. A
review of the models and their verification will be presented in this paper.
Particle standard is important and widely used for calibration of inspection tools and process characterization and
benchmarking. We have developed a method for generating and classifying monodisperse particles of different
materials with a high degree of control. The airborne particles are first generated by an electrospray. Then a tandem
Differential Mobility Analyzer (TDMA) system is used to obtain monodisperse particles with NIST-traceable sizes.
We have also developed a clean and well-controlled method to deposit airborne particles on mask blanks or wafers.
This method utilizes electrostatic approach to deposit particles evenly in a desired spot. Both the number of particles
and the spot size are well controlled. We have used our system to deposit PSL, silica and gold particles ranging from
30 nm to 125 nm on 193nm and EUV mask blanks. We report the experimental results of using these particles as
calibration standards and discuss the dependency of sensitivity on the types of particles and substrate surfaces.