Formation factors of watermark for immersion lithography

In immersion lithography, some defects such as watermarks and nanoscale bubbles have been focused on as serious problems to be solved. To clarify the formation mechanism of the watermarks, the in-situ observations of the drying behaviors of water drops containing with particles and without particles, are conducted on Si substrates. In static watermark formation on a flat substrate, we can classify the watermark formation processes on the basis of the watermark shapes. From a surface energy balance analysis, particles dispersed in deionized (DI) water adhere on a Si substrate. In addition, owing to the Laplace force balance, the particles adhered on the Si substrate will attract surrounding particles. Hence, we can confirm the formation mechanism of the static watermark condensed in a ring shape. On the other hand, in dynamic watermark formation, we can observe clearly that a condensed watermark is formed on a Si substrate and particles move to a lower region in an inclined drop. In an actual immersion lithography system, the particles are more likely to remain in the immersion liquid under the lens system.