28pC07
協同的トムソン散乱法によるレーザー生成EUVプラズマの電子密度・電子温度・イオン温度計測
Measurements of Electron Density, Electron Temperature and Ionic Charge ofLaser Produced EUV Plasmas using Collective Thomson Scattering
富田 健太郎、井上 和哉、下田 諒、内野 喜一郎
Kentaro Tomita, Kazuya Inoue, Ryo Shimoda, Kiichiro Uchino
九州大学大学院総合理工学研究院
Interdisciplinary Graduate School of Engineering and sciences, Kyushu University
Extreme ultraviolet (EUV) lithography withwavelength of 13.5 nm is a promising technology
for next-generation microprocessors whose nodesize is less than 22 nm. For the practical use ofEUV lithography, an improvement of output powerand efficiency of EUV light source is indispensable,
and measurements of their fundamental plasma
parameters, such as electron density (ne), electrontemperature (Te) and ionic charge (Z) are
prerequisite to control and improve EUV plasmas.
In order to achieve precise measurements ofthese plasma parameters, we have applied collective
Thomson scattering technique to laser-produced
EUV light source plasmas. When the second
harmonics of Nd:YAG laser (λ = 532 nm) is used as
the light source, LTS spectra observed from EUV
plasmas ( ne = 1024 - 1025 m-3, Te = 30 - 50 eV) arein the collective regime and they consist of theelectron term and the ion term. In this study, only
the ion term were focused, because the electronterm was easily overwhelmed by plasma emission
when the energy of the probing laser is set to besmall to avoid plasma perturbations.
Figure 1 shows a schematic of the LTS system.
Three lasers were used in this study: a driving laser
to produce plasmas (fundamental of Nd:YAG laser,
λ = 1064 nm, 10 ns pulse width, laser energy Ep =40-260 mJ), a probing laser for LTS measurements(the second harmonics of Nd:YAG laser, λ = 532
nm, 10 ns pulse width, laser energy EL = 5-15 mJ,
spectral width<0.1 pm), and a heating laser to heat
plasmas (fundamental of Nd:YAG laser, 10 ns pulsewidth, laser energy EH = 0-440 mJ). The driving
laser was injected perpendicular to a target, which
has 0.5 mm width and is located in a vacuum
chamber (<10-2 Torr). The probing laser wasinjected perpendicular to both the driving laser andthe target as shown in Fig. 1. The heating laser wasinjected into the same axis and the same time as
those of the probing laser. Typical spot size of thetwo lasers at the focusing point was 50 μm. LTS
signals were collected by achromatic lenses (f = 300mm, effective diameter 46 mm) and focused on anentrance slit of a triple grating spectrometer (TGS).
The TGS has three gratings and reduces stray lights
efficiently. Scattered lights dispersed by the TGS
were detected by an intensified-CCD (ICCD)
camera(Princeton,PI-MAXUNIGENII,1024×1024 pixel, 10 ns gate width). To determinene, calibration of the absolute value of the LTS
signals was performed by using Rayleigh scattering
measurements. Rayleigh scattering lights fromatmospheric pressure nitrogen gas were observed in
the same experimental condition.
Thomsonscattering
measurements
wereperformed at a position of 200 μm from the surfaceof the carbon target and at a time of 15 ns after theplasma production. From the observed ion term
spectrum, ne, Te, Ti and Z were evaluated to be
1.5×1025 m-3, 14 eV, 11 eV and 3.8, respectively. In
order to heat plasmas, the other laser (thefundamental of Nd:YAG laser, power density(0-2.3) ×1016 W/m2) was injected to the LPP, and it
was found that ne, Te, Ti and Z changed in theranges of (0.4-1.5) ×1025 m-3, 14-53 eV, 11-24 eV,3.8-4.1, respectively. The measurement error wasestimated to be ±10%. Referring these results, anew LTS system for Sn plasmas has been designed.
Nd:YAG Laserω (1064 nm)
2ω (532 nm)
Target(Carbon)

ω/2ω
laser achromat
f=150 mmTarget (Carbon)
500 μm
600 μm (FWHM)

45°kiksk
f=300 mm

f=250 mm

f=300 mm2ω laserω laser

ω laser

Scatteredlights

Triple Grating
Spectrometer(TGS)ICCD Camera

Nd:YAG Laserω (1064 nm)

Fig. 1. The schematic of the collective Thomson scattering system forlaser produced plasmas. The inset shows the detailed diagram aroundthe target. ki and ks mean the wave-numbers concerning the probing
laser and scattering angle, respectively. k means the differentialwave-number.