Sách Part 4 EUV Lithography and Resolution Enhancement Techniques

Part 4
EUV Lithography and Resolution
Enhancement Techniques


18
Laser-Plasma Extreme Ultraviolet Source
Incorporating a Cryogenic Xe Target
Sho Amano
University of Hyogo
Laboratory of Advanced Science and Technology for Industry (LASTI)
Japan
1. Introduction
Optical lithography is a core technique used in the industrial mass production of
semiconductor memory chips. To increase the memory size per chip, shorter wavelength
light is required for the light source. ArF excimer laser light (193 nm) is used at present and
extreme ultraviolet (EUV) light (13.5 nm) is proposed in next-generation optical lithography.
There is currently worldwide research and development for lithography using EUV light
(Bakshi, 2005). EUV lithography (EUVL) was first demonstrated by Kinoshita et al. in 1984
at NTT, Japan (Kinoshita et al., 1989). He joined our laboratory in 1995 and has since been
actively developing EUVL technology using our synchrotron facility NewSUBARU. Today,
EUVL is one of the major themes studied at our laboratory.
To use EUVL in industry, however, a small and strong light source instead of a synchrotron
is required. Our group began developing laser-produced plasma (LPP) sources for EUVL in
the mid-1990s (Amano et al., 1997). LPP radiation from high-density, high-temperature
plasma, which is achieved by illuminating a target with high-peak-power laser irradiation,
constitutes an attractive, high-brightness point source for producing radiation from EUV
light to x-rays.
Light at a wavelength of 13.5 nm with 2% bandwidth is required for the EUV light source,
which is limited by the reflectivity of Mo/Si mirrors in a projection lithography system. Xe
and Sn are known well as plasma targets with strong emission around 13.5 nm. Xe was
mainly studied initially because of the debris problem, in which debris emitted from plasma
with EUV light damages mirrors near the plasma, quickly degrading their reflectivity. This
problem was of particular concern in the case of a metal target such as Sn because the metal
would deposit and remain on the mirrors. On the other hand, Xe is an inert gas and does not
deposit on mirrors, and thus has been studied as a deposition-free target. Because of this
advantage, researchers initially studied Xe. To provide a continuous supply of Xe at the
laser focal point, several possible approaches have been investigated: employing a Xe gas
puff target (Fiedrowicz et al., 1999), Xe cluster jet (Kubiak et al., 1996), Xe liquid jet
(Anderson et al., 2004; Hansson et al., 2004), Xe capillary jet (Inoue et al., 2007), stream of
liquid Xe droplets (Soumagne et al., 2005), and solid Xe pellets (Kubiak et al., 1995). Here,
there are solid and liquid states, and their cryogenic Xe targets were expected to provide
higher laser-to-EUV power conversion efficiency (CE) owing to their higher density
compared with the gas state. In addition, a smaller gas load to be evacuated by the exhaust
We have also studied a cryogenic Xe solid target. In that study, we measured the EUV
emission spectrum in detail, and we found and first reported that the emission peak of Xe
was at 10.8 nm, not 13 nm (Shimoura et al., 1998). This meant we could only use the tail of
the Xe plasma emission spectrum, not its peak, as the radiation at 13.5 nm wavelength with
2% bandwidth. From this, improvements in the CE at 13.5 nm with 2% bandwidth became a
most critical issue for the Xe plasma source; such improvements were necessary to reduce
the pumped laser power and cost of the whole EUV light source. On the other hand, the
emission peak of a Sn target is at 13.5 nm; therefore, Sn intrinsically has a high CE at 13.5 nm
with 2% bandwidth. The CE for Sn is thus higher than that for Xe at present, in spite of our
efforts to improve the CE for Xe. This resulted in a trend of using Sn rather than Xe in spite
of the debris problem. Today, Cymer (Brandt et al., 2010) and Gigaphoton (Mizoguti et al.,
2010), the world’s leading manufacturers of LPP-EUV sources, are developing sources using
Sn targets pumped with CO2 lasers while making efforts to mitigate the effects of debris.
In the historical background mentioned above, we developed an LPP-EUV source composed
of 1) a fast-rotating cryogenic drum system that can continuously supply a solid Xe target
and 2) a high-repetition-rate pulse Nd:YAG slab laser. We have developed the source in
terms of its engineering and investigated potential improvements in the CE at 13.5 nm with
2% bandwidth. The CE depends on spatial and temporal Xe plasma conditions (e.g., density,
temperature, and size). To achieve a high CE, we controlled the condition parameters and
attempted to optimize them by changing the pumping laser conditions. We initially focused
on parameters at the wavelength of 13.5 nm with 2% bandwidth required for an EUV
lithography source, but the original emission from the Xe plasma has a broad spectrum at 5–
17 nm. We noted that this broad source would be highly efficient and very useful for many
other applications, if not limiting for EUVL. Therefore, we estimated our source in the
wavelength of 5–17 nm. Though Xe is a deposition-free target, there may be sputtering due
to the plasma debris. We therefore investigated the plasma debris emitted from our LPP
source, which consists of fast ions, fast neutrals, and ice fragments. To mitigate the
sputtering, we are investigating the use of Ar buffer gas. In this chapter, we report on the
status of our LPP-EUV source and discuss its possibilities.