It is known that the photomask substrate deflects when the mask is set on the frame and the deflection is an obstacle
to light exposure. In this study, we introduce “the bending mask” to cancel out the deflection. The surface of the bending
mask has the height distribution in advance to cancel out the deflection, owing to Nikon’s accurate polishing technology
and Nikon’s accurate measurement machine.
Artificially grown excimer grade calcium fluoride is one of key optical materials used in microlithography applications. Such calcium fluoride is required for optical components requiring high laser durability and laser induced bulk damage threshold (LIDT). The mechanical properties of calcium fluoride can vary depending on the crystal axis, <111>, <110> and <100>. For example, material hardness is highest in the {100} crystal orientation. Furthermore, it is also known to cleave in the {111} plane. Therefore there is a possibility of a property that originates in such a crystal structure that influences LIDT. In this study, we investigated the relationship between crystal structure, laser durability and LIDT. The influence in the relation between the polarization plane of the ArF excimer laser and the crystal orientation of calcium fluoride in regards to LIDT was examined. The samples were all prepared from the same CaF2 crystal with optical axis's of <111>, <110> and <100>. The azimuth of the samples was measured by the reflection Laue method. For the experiment, the samples were rotated to the polarization plane of the ArF excimer laser, and the change in the number of irradiation pulses that damage was observed and measured. As a result, we determined the position of the crystal orientation of the calcium fluoride relative to the polarization plane of the ArF excimer laser that produced the highest LIDT.
The artificially grown calcium fluoride is one of key materials for microlithography and used for excimer laser optics etc. Such
calcium fluoride is required high laser durability and laser induced bulk damage threshold (LIDT). However, the artificially grown
calcium fluoride is not a complete crystal, and there are a lot of sub-grain boundaries inside the crystal that have the possibility of
causing degradation of laser durability and LIDT. Moreover, mechanical properties of calcium fluoride are different according to the
crystal axis, therefore there is a possibility that mechanical properties influences LIDT. In this study, we examined the relation
between crystal structure and LIDT.
First, we examined the relation between the crystal axis and LITD of single crystal calcium fruoride. The relation between the
crystallographic axis and LIDT that the laser enters was examined. The ArF excimer laser and the fifth high harmonic of the Nd:YAG
laser at 213nm were used for the irradiation source of light. We prepared samples that optical axes were <111>, <110> and <001>
from the same crystal. From the result of this examination, when the laser irradiated in <111> axis, LIDT was the highest.
Next, we observed the damage with polarizing microscope and optical microscope. The result of this observation suggested that the
laser damage of calcium fluoride was related to the crystal orientation.
Finally, we investigated the damage mechanism of calcium fluoride. It is thought that the laser irradiation induced stress is relaxed
most easily when the optical axis is <111>. Therefore, LIDT of calcium fluoride is supposed to be highest when the optical axis is
<111>.
The artificially grown calcium fluoride is used as materials of the optics such as the lenses of the illumination optical
system and the projection optical system of the lithography equipment that use the sources of light such as excimer
lasers. Such calcium fluoride is required high transmittance. However, there are very small scatterers and absorbers
inside the crystal and they cause degradation of transmittance. In this study, we examined these defects and clarified the
process how they occur.
Haze is characteristic optical defect in the artificially grown calcium fluoride. It is thought that haze is an aggregation of
very small scatterers and this scatterer is void or calcium oxide crystal. When we irradiate the light into a crystal with
much haze, the path of the light looks white. However, we were not able to clarify neither the structure nor components
of haze.
First, we examined how the scatterers were distributed by an infrared tomography method. The result pointed out that the
scatterers were located along sub-grain boundary and dislocation network. We prepared a surface sample for TEM
(Transmission Electron Microscopy) with FIB (Focused Ion Beam) from the point where it seemed that the scatterers
were located in the dislocation network, and observed it with TEM and analyzed grain boundary region and the grain
inside with EDS (Energy Dispersive X-ray Spectroscopy). From the EDS spectrum of the grain boundary region, a very
small amount of oxygen was detected, but no oxygen was detected from the grain inside. This suggests that oxygen is
located in the grain boundary. From these results, it is suggested that scatterers of haze are made of oxygen voids or
calcium oxides crystals.
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