Research On Laser-produced Gd Target Plasma Source At 6.7nm For Lithography

Extreme ultraviolet（EUV） lithography is promising technique to be introduced in industry for manufacturing features size below 22 nm, and EUV source is key part of extreme ultralviolet lithography（EUVL） system. Laser-produced plasma（LPP） is a table top with excellent performance and suitable for EUVL due to its small size, high brightness and tunable operating wavelength. Recent years, a systematic investigation of EUV source at λ=13.5 nm has been carried out and this can lay a solid foundation for its application. EUVL at operating wavelength of 13.5 nm will be introduced recently in high-volum industrialization manufacturing as the key technologies related to EUVL being grasped. To enable EUVL to manufacture even further smaller node size of integrated circuits to keep the Moore Law, recent years research on EUV source at wavelength of 6.7 nm has been performed. Similar to the initial phase of EUV source at 13.5 nm, a soruce at 6.7 nm with high in-band spectral brightness and low debris is the interest points in the development of EUV source.This thesis dealt with EUVL source based on laser-produced Gd plasma with intention for EUVL at λ=6.7 nm, and the thesis summarized the research status of 6.7nm source as well as the related physics processes. A flat-field grating spectrograph was designed and set up. The effect of laser parameters and target condition on the characteristic of operating spectra from LPP source have been studied. The spatio-temporal evolution characteristic of electron temperature and density of LPP from Gd₂O₃ nanoparticle doped glass target has been also investigated by using plasma spectrosocpy. Mitigation of energetic ion debris from laser-produced Gd plasma has been systematically investigated. The main contents of the thesis include:Firstly, a flat-field grating spectrograph has been designed and built for the diagnosis of Gd target plasma light source at operating wavelength of 6.7 nm. The spectral measurement rang of the spectrograph covers a range of 5-50 nm with spectral resolution of 0.02 nm Several methods for calibration the spectrograph included a high-order harmonic source based on interaction of low pressure Ar gas with femtosecond laser pulse, absorption edge of Zr filter film, and the line spectrum radiated from Si ions of laser-produced quartz plasma were used.Secondly, EUV emission spectrum from LPP based on Gd metal target and Gd₂O₃ nanoparticle doped glass target was studied. The LPP sources based on both target were with strong emission around 6.7 nm. In the case of LPP from Gd metal target, the impact of laser intensity on EUV spectrum has been studied, and it showed that there were two obvious peaks in the spectral intensity at wavelength of 6.76 nm and 7.14 nm, respectively when the laser intensity was increased to 6.4×10¹¹ W/cm² and above. The two lines around 6.76 nm and 7.14 nm were identified as resonance lines from Ag-like Gd17+ and Pd-like Gd18+ ions, respectively. It was found that the line intensity at longer wavelength around 7.0 nm was obviously higher than that around 6.7 nm as the laser intensity increased. this implied that the spectral intensity at longer wavelength around 7.0 nm increased faster with the increasing of laser intensity. The observed features that variation in intensity for the lines around 6.7nm and 7.0 nm with laser irradiance or electron temperature of plasma was strongly related to the nature of the Gd material, and it was in agreement with the theoretical results from S. Churilov et al.. Meanwhile, there was an obvious dip in the profile of EUV spectrum from LPP based on Gd metal target, which was due to the self absorption by dense LPP. Besides, it was also found that spectral width around 6.7 nm by using laser-produced Gd pre-plasma scheme with 50 ns delay between the pre-pulse and the main laser pulse was only one fifth of that achieved with one pulse scheme.On the other hand, the effect of doping density of Gd₂O₃ nanoparticle in glass target on EUV spectrum around 6.7 nm was carried out with a laser intensity of 5×10¹¹W/cm². It was found that the spectral intensity around 6.7 nm increased rapidly as the nanoparticle density increasing from 1% to 10%. The spectral intensity showed no further increasement if nanoparticle doped density was futher to be enlarged. This nature behavior of EUV radiation intensity around 6.7 nm was manily attributed to the self absorption by dense plasma. It was found that the spectral width around 6.7 nm with Gd₂O₃ nanoparticle doped glass target was about two fifths of that achieved by Gd metal target. Meanwhile, it was also found that spectrum radiation ranging from 250 nm to 900 nm based on the Gd₂O₃ nanoparticle doped glass target was much lower than that of the Gd metal target. As a result, the out-of-band radiation, which was harmful to mulatilayer optics as well as to the pattern exposure, has been reduced significantly when a Gd₂O₃ nanoparticle doped glass target was employed in an EUV source.Furthermore, the spatial-temporal evolution of electron temperature and density of the LPP with Gd₂O₃ nanoparticle doped glass target were measured by using line emission spectrum from Si（I）250.7 nm and Si（I）263.2 nm assisted with Boltmann double line. On one hand, the results showed that the electron temperature and electron density of LPP at a distance of 6 mm from the target surface respectively decreases from 4 eV and 1.2×1018 cm-3 to 1.5 eV and 8.5×1017 cm-3 as the time delay changing from 120 ns to 400 ns after the laser pulse irradiation of the target. On the other hand, with fixed delay time of 200 ns, both the electron temperature and density increased rapidly with the increasing distance until it reached to 6 mm from the target surface, and then their values started to decrease as the distance further increased. The maximum electron temperature and density are 2.6 eV and 8.5×1017 cm-3, respectively.In the last part of this thesis, characteristic of ion debris from laser-produced Gd metal target plasma and mitigation the velocity of ion debris were also experimentally investigated by time of flight（ToF） method. The peak velocity of of the Gd ion debris was measured to be 7.14×106 cm/s, and the corresponding kinetic energy was 3.7 eV when a laser pulse with intensity of 7×10¹¹W/cm² was used. Furthermore, mitigation of energetic ion debris from Gd plasma was carried out by ambient gas, magnetic field, dual laser pulse technique, and combined effect of ambient gas and dual laser pulse technique. The results showed that the yield of ion debris from the plasma was significantly reduced at pressure of 41 mTorr for argon or 310 mTorr for helium respectively. Comparison of the angular distribution of light source ion debris in the absence and presence of magnetic field which was 0.9 T, it was found that the ion debris can be effectively mitigated by magnetic field at all the direction with respect to the target surface. The influence of pre-pulse energy, the delay time between pre-pulse and main pulse and the wavelength of pre-pulse（selecting from wavelengths of 1064 nm, 532 nm and 355 nm） on the ion debris energy was investigated in dual laser pulses debris mitigation technique. The results showed that the thermal ion debris can be mitigated effectively with the wavelength of 355 nm irradiation, with pulse energy of 40 mJ, and with separation of main and pre-pulse of 550 ns. As a result, peak of the ion energy was reduced to about 0.2 keV, which corresponds to about value of 1/18 of the ion energy without any mitigation measured being taken.