Study On Counting Error In Particle Inspection

In EUV lithography (EUVL) it is important to protect a mask from the adhesion of particles because it is difficult to use a pellicle. At Selete, we evaluated a dual-pod carrier and reported on its ability to protect a mask from particles. In the evaluation the average number of particles added to the mask during several hundred handling-cycles was 0.4. Therefore, it is very important to precisely count the number of particle adders. However, according to the specification of the inspection tool, the counting error was greater than the average number of particle adders in the evaluation. In addition, it is known that the error increases for particles with a size near the detection limit. In the evaluation, we inspected a mask substrate four times and regarded signals detected multiple times as real particles. We studied the counting error by assuming that the detection probability followed a static statistical fluctuation. We found that the expected value of counting error was represented with the equation by the number of initial particles, particle adders, capture rate, and inspection times. Under our evaluation condition, even if no quasi-particles existed, the counting error by a single inspection was estimated to be approximately 4. However, the counting error by our evaluation (four inspections) was estimated to be approximately 0.05. Therefore, we found that the reliability by multiple inspections was much higher than that by a single inspection and that the number of particles near the detection limit could be found precisely by multiple inspections. * This work was supported by NEDO.