AVS 71 Session PS2-FrM: Plasma in EUV Scanner Technology

The extended exposure of tin (Sn) particles to hydrogen radicals has demonstrated the ejection of sub-micron sized particles from a surface covered in micron sized Sn droplets. This work experimentally investigates the three mechanisms, spitting, etching, and particle lift off, behind the mass transport of Sn, focusing on measuring the size, velocity, and directionality of the particles and the frequency of particle migration. Using a quartz crystal microbalance (QCM), measurements of the mass flux from a surface covered in micron sized droplets of Sn were performed with plasma radical densities similar to and greater than in the scanner. While etching rates of Sn by hydrogen were measured similar to other reports, significant, acute decreases in the mass were also observed throughout the duration of testing associated with either spitting or particle lift off. Further imagery, on a SEM, of the surface before and after exposure to hydrogen plasma confirmed a change in surface morphology. Larger Sn droplets appeared to have cratering along the surface, believed to be the result of cold spitting Sn. This is due to buildup of hydrogen beneath the surface forming a pressure gradient within the Sn particles leading to surface fracturing and particle ejection. An additional test was designed with a silicon wafer placed above the surface of the Sn droplets that captured cold spit particles. An SEM was then used to measure the size and position, allowing for the interpretation of mass and directionality of Sn leaving the surface. Velocity of spit particles is being determined by measuring the deflection of the cold spit particles through an electric field, having a known charge buildup on the surface. Based on the distance traveled before reaching the chamber walls, the incident velocity of particles is calculated. Measurement of liquid spit particles is underway using aerogel, with diagnostic techniques like ballistics work. The liquid particles are captured in the aerogel before using a uCT to generate a 3D rendering of the surface. Based on the cratering characteristics such as depth and width found at the surface of the aerogel, velocity of each particle is calculated. Additionally, this work is being performed on various surface materials to determine how the preferential recombination of hydrogen radicals with various surface materials impacts the rate of degradation of the Sn. Initial tests revealed a reduction in mass loss rates for gold when compared to aluminum. This is likely due to the greater recombination coefficient of gold when compared to aluminum.