Mask protection from particulate contaminants is one of the most serious concerns for the success of deploying extreme ultraviolet lithography (EUVL) technology for future semiconductor manufacturing. Standard pellicles are not applicable for an EUVL mask surface because of the high absorption of the EUV beam by any material. Therefore, methods such as thermophoresis and electrophoresis are desirable for protection, instead of using organic membrane pellicles. A reverse flow concept by introducing differential pressure between different operating zones (mask zone with higher pressure and optics zone with lower pressure) is introduced for protection of critical surfaces against particulate contamination. In this study, we show systematic investigations of the differential pressure effect on the protection of critical surfaces using 125 nm polystyrene latex spheres at a chamber pressure of 50 mTorr, whereas the critical surface zone pressure was varied up to 550 mTorr. We found that a higher particle speed needs a higher differential pressure between the mask zone and the optics zone to protect the mask from incoming particles. The higher differential pressure and higher relative flow velocity for the reverse flow provide more drag force for particles to stop before reaching the critical surface. The particles having an injection speed of up to 180 m/s could be completely repelled using the differential pressure approach, but the deposition of particles with a speed of 270 m/s is hardly prevented by this reverse flow concept. Different gases showed different amounts of protection efficiency due to the change of gas properties such as density and molecular weight. Argon showed better protection efficiency against particle contamination than air. By drag force comparison, argon has about 18% higher drag force than air. The experimental results showed that argon reduced the particle deposition between 6% and 50%, depending on the particle injection speed and the differential pressures. 0 2006 American Vacuum Society.