Experimental study of particle-free mask handling

One of the critical issues for EUVL masks is clean and particle-free mask handling. We reported that the number of particle adders on the front side of a mask in the dual pod during the process from the load port to putting on the Electrostatic chuck (ESC) in vacuum could be reduce to less than 0.01 particle/cycle (>= 46 nm). In addition, we found that chucking the mask on the ESC caused two serious issues. The first is that many particles stick to on the backside of the mask after chucking on the ESC, raising the question of whether the particle adders on the backside will travel to the front side. We examined the travel of these particles using the substrates after chucking and polystyrene latex (PSL) substrates that were dispersed on the backside. These experiments show that there is very little probability that particles on the backside will travel to the front side. The second issue is whether the mask blanks will charge up by chucking on the ESC and some particles will add on the front side. We measured the electric potential of the back and front sides of the mask and examined the particle adders. Our experiments revealed that to protect the mask from the particles, the mask must be grounded from the beginning to the end. For these two issues, we confirmed that a dual pod system works effectively to protect the mask from particles. This work is supported by NEDO as a part of the EUV mask program.