Characterization of optical depth for laser produced plasma extreme ultraviolet source
SIOM
Abstract
This study investigates the emission mechanism of laser-produced plasma extreme ultraviolet source (LPP-EUV). In the experiment, the EUV radiation is generated by a 1064 nm laser interacting with slab Sn target. The EUV emission is characterized by EUV energy monitors and an EUV spectrometer. The dependency of CE on laser intensity and plasma relative optical depth is explored by a plasma emission-absorption layer model. The dependency is confirmed by an optical interferometry measurement of plasma electron density for the first time. Our work provides a method for characterizing and optimizing the optical depth of laser driven EUV source.
Experimental setup
The experimental layout of the laser interacting with the slab Sn target is shown in Fig. 1. A 1064 nm Nd: YAG drive laser is focused by a convex lens (F = 400 mm) onto the target at normal incidence. The Sn target is mounted in a vacuum chamber, which is pumped to 10−5Pa. After each laser shot, the target is moved to a fresh surface to guarantee identical laser-target interaction. The FWHM pulse width τ0 of the drive laser is 4.8 ns. The laser energy ranges from 4 mJ to 170 mJ with an FWHM
Results and discussion
The dependency of the EUV spectrum on laser intensity is shown in Fig. 3(a). The laser is kept at best focus on the target. The laser peak intensity is varied by changing laser energy. The EUV spectrum is normalized to the laser energy. In-band EUV emission grows rapidly and reaches a peak at 5.2×1011W/cm2, then decreases slightly as laser intensity further increases to 1.8×1012W/cm2. This tendency agrees well with the results measured by the EUV energy monitors.
Conclusions
EUV in-band emission is characterized at different angles relative to the drive laser direction. An emission-absorption layer model is proposed to explain the difference between emission angles and the dependency on laser intensity, which shows that the optical depth is crucial to the total EUV emission. Plasma scale lengths deduced from electron density measurements representing relative optical depth are consistent with the theoretical model.
