Sticking behavior and transformation of tin droplets on silicon wafers and multilayer-coated mirrors

Abstract

Silicon wafer and multilayer-coated mirror samples were exposed to impact of drops of molten tin to examine the adhesion behavior and cleaning possibilities. The sticking of tin droplets to horizontal substrates was examined for different surface conditions in a high vacuum chamber. Silicon wafers without a coating, with thick oxide top layer, and also with differently capped Mo/Si multilayer coatings optimized for reflection at a wavelength of 13.5 nm were exposed to tin dripping. Dependent on substrate temperature and coating, adhesion as well as detachment with self-peeling and self-contraction of spreaded drops was observed. The adhesion strength of solidified tin splats decreased strongly with decreasing substrate temperature. Non-sticking surface conditions could be generated by substrate super-cooling. The morphology of non-sticking tin droplets was analyzed by profilometry. Adhering deposits were converted in-situ via induction of tin pest by infection with gray tin powder and cooling of the samples. The phase transition was recorded by photographic imaging. It caused material embrittlement and detachment after structural transformation within several hours and enabled facile removal of tin contamination without coating damage. The temperature-dependent contamination behavior of tin drops has implications for the preferred operating conditions of extreme ultraviolet light sources with collection optics exposed to tin debris.2

In situ transformation and cleaning of tin-drop contamination on mirrors for extreme ultraviolet light

“…This behavior is attributed to the roughness of the SiC surface which promotes splat self-peeling even at elevated temperatures. 24 Nevertheless, similar phase conversion of tin drops dripped on the surface could also be achieved within about 10 hours in this case during continuous substrate cooling in the temperature range of -35 °C to -50 °C.…”

“…Heating of the examined substrates resulted in higher contact temperatures at the smooth surface during tin dripping, leading to stronger sticking of solidifying droplet splats due to increased adhesion. 24,28 Strong sticking was also found when tin drops were incident on already existing splats on the surface. In the case of EUV sources a large heat flux is originating from the hot plasma.…”

“…Table 1 At the bottom side of delaminated or non-sticking droplets we generally observe imprinted radial flow patterns pointing from the center of the tin splat to its circumference (see figure 7), in agreement with earlier findings. 24,32 This indicates that solidification occurs very fast during the spreading of the tin drops on the substrates which happens in just a few milliseconds after central impact. 28,32 Nevertheless, the initially liquid tin fully replicates the grating structure, as demonstrated by the profilometric analysis shown in figure 8.…”

“…However, many different aspects have to be taken into account during the operation of an LPP source. For example, in addition to the sticking of incident Sn particles and droplets, which decreases with decreasing temperature, 24 the surface temperature will also affect the efficiency of continuous in-situ etching and cleaning by hydrogen radicals that takes place during source operation 4,7 . If cooling to negative Celsius temperatures is desired the cooling water circuit for the collector mirror could be replaced by a circuit using a cooling fluid based on ethylene glycol in order to reach lower temperatures.…”

“…After contact with α-Sn seed-particles, phase transformation to gray tin was induced during substrate cooling to temperatures in a range between -25 °C and -40 °C. 23,24 The recovery of EUV reflectance after removal of detached converted tin pieces was reported for an MLM sample with Mo/Si coating. 23 The temperature dependence of droplet sticking to the substrates was studied for different sample types.…”