7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Measurements of electron density, electron temperature, and velocity field inlaser-produced EUV source plasmas using collective Thomson scattering

Kentaro Tomita1, Yiming Pan1, Atsushi Sunahara2,3, Katsunobu Nishihara2
1Division of Quantum Science and Engineering, Graduate School of Engineering, Hokkaido
University, 2 Institute of Laser Engineering, Osaka University, 3 Center for Materials Under eXtreme
Environment (CMUXE), School of Nuclear Engineering, Purdue University,e-mail: tomita.kentaro@eng.hokudai.ac.jp
Measurements of electron and ion dynamics are
essential to diagnose laser-produced multi-ionized lightsource plasmas. Generally, these plasmas are based byplasma physics, atomic physics, and radiationhydrodynamics. Therefore, measurements of plasma
parameters, such as electron density (ne), electron
temperature (Te), and averaged ionic charge (Z) are
important. In addition, measurements of plasma flow
velocity fields (vflow) may give us significant findings toknow plasma dynamics.
However, it is highly challenging to observe such local
plasma parameters due to their small (< 0.5 mm) scaleand short (<50 ns) lifetime. Here, we report
time-resolved two-dimensional profiles of Te, ne, Z, andoflaser-produced
Snplasmas
forvflowextreme-ultraviolet (EUV) lithography light sources
using a collective Thomson scattering (CTS)
measurements.
Figure 1 shows schematics of the plasma generation
system. For plasma generation, a pre-pulse laser (14 pspulse) and the main laser (CO2 laser with a 15 ns pulse
width, 100 mJ energy) irradiated droplet tin target [1].
The CTS system with custom-build spectrometer (notshown in the Fig.1) was used to measure the ion-term
spectra, with which time-resolved (5 ns resolution) local
values of ne, Te, Z, and vflow of the Sn plasmas wererevealed. To measure vflow, Doppler shift of the ion-term
spectra observed from two scattering angles wereanalyzed [2]. Detailed procedures to determine vflow field
are described in Ref. [2]. The way to determine ne, Te, Z
from the shape, spectral width, and absolute intensity of
the ion term spectra is shown in Ref. [1].
Figure 2 shows an example of two-dimensional
profiles of vflow and pressure P, which was calculatedusing measured ne, Te, and as P=neκ (Te+Ti/Z), where κ isBoltzmann constant and Ti is ion temperature (Ti isassumed to be same as Te in this study). Regarding Fig. 2,
two points should be emphasized, (i) both the direction
and magnitude of vflow varied with positions, (ii) there
exist the plasma flows toward the central plasma axis
(x-axis).
After further analyses, we found that the plasma
inflows exceeding 104 m/s toward a plasma central axis
play an important role in improving the total EUV lightemission, i.e., the plasma inflows maintain the EUV
source at a temperature suitable for EUV light emission
for a relatively long time and at a high density.
References[1] K. Tomita et al., “Time-resolved two-dimensional
profiles of electron density and temperature oflaser-produced tin plasmas for extreme-ultraviolet

lithography light sources”, Scientific Reports 12328, 7(2017).
[2] K. Tomita et al., “Observation of plasma inflows inlaser-produced Sn plasma and their contribution to
extreme-ultraviolet light output enhancement”, Scientific
Reports 1825, 13 (2023).

Fig. 1. (a) Side view and (b) top view of experimental layout.

Fig. 2. 2D-profiles of pressure and plasma flow-velocity field(vflow)