**6. Xe plasma debris**

In this section, we report the characteristics of the plasma debris that damages mirrors (Amano et al., 2010b). First, we investigated fast ions, fast neutrals and ice fragments, which constitute the debris.

When we found that EUV radiation was greater for a rotating drum than for a drum at rest, we also found that the number of fast ions decreased simultaneously. Figure 7(a) shows ion signals from a charge collector (CC) with laser pulse energy of 0.5 J and optimal intensity of 1010 W/cm2, for different drum rotation speeds. The ion signal reduces rapidly after the drum starts to rotate (> 4 rpm), after which the signal is almost independent of rotation speed. Ion energy spectra were obtained as shown in Fig. 7(b) using the time-of-flight signals shown in Fig. 7(a). Here, we assume that all ions were doubly charged because we measured the principle charge state of Xe ions to be two with an electrostatic energy analyzer (Inoue et al., 2005). Under the rotation condition, the maximum ion energy decreases to 6 keV and the number of high-energy ions (with energy of a few dozen kiloelectron-volts) also decreases. These are favorable characteristics for the debris problem. The decrease in the ion count under the rotation condition can be explained by a *gas curtain effect* that originates from the Xe gas localized at the target surface. The pressure of this localized Xe gas can be roughly estimated from the peak attenuation () in Fig. 7(a); we estimated the product of pressure and thickness to be about 10 Pamm.

Fig. 7. (a) CC signals of ions and (b) their energy spectra at rotation speeds of 0, 4, 10, 60 and 130 rpm. in (a) is the loss rate of ions due to the drum rotating. The ion number in (b) was calculated assuming the charge state was two.

Fast neutral particles were measured by the microchannel plate (MCP) detector when the number of fast ions decreased under the rotation condition. The MCP is sensitive to both ions and neutrals, making the use an electric field obligatory to repel ions so that the MCP detects only neutral particles. From the measurement, we found the number of neutrals to be approximately an order of magnitude less than the number of ions.

In the case of solid Xe targets, ice fragments might be produced by shock waves of laser irradiation, whereas this is not the case for gas or liquid targets. In early experiments using a solid Xe pellet, ice fragments were observed and mirror damage due to these fragments was

In this section, we report the characteristics of the plasma debris that damages mirrors (Amano et al., 2010b). First, we investigated fast ions, fast neutrals and ice fragments, which

When we found that EUV radiation was greater for a rotating drum than for a drum at rest, we also found that the number of fast ions decreased simultaneously. Figure 7(a) shows ion signals from a charge collector (CC) with laser pulse energy of 0.5 J and optimal intensity of 1010 W/cm2, for different drum rotation speeds. The ion signal reduces rapidly after the drum starts to rotate (> 4 rpm), after which the signal is almost independent of rotation speed. Ion energy spectra were obtained as shown in Fig. 7(b) using the time-of-flight signals shown in Fig. 7(a). Here, we assume that all ions were doubly charged because we measured the principle charge state of Xe ions to be two with an electrostatic energy analyzer (Inoue et al., 2005). Under the rotation condition, the maximum ion energy decreases to 6 keV and the number of high-energy ions (with energy of a few dozen kiloelectron-volts) also decreases. These are favorable characteristics for the debris problem. The decrease in the ion count under the rotation condition can be explained by a *gas curtain effect* that originates from the Xe gas localized at the target surface. The pressure of this localized Xe gas can be roughly estimated from the peak attenuation () in Fig. 7(a); we estimated the

Fig. 7. (a) CC signals of ions and (b) their energy spectra at rotation speeds of 0, 4, 10, 60 and

Fast neutral particles were measured by the microchannel plate (MCP) detector when the number of fast ions decreased under the rotation condition. The MCP is sensitive to both ions and neutrals, making the use an electric field obligatory to repel ions so that the MCP detects only neutral particles. From the measurement, we found the number of neutrals to

In the case of solid Xe targets, ice fragments might be produced by shock waves of laser irradiation, whereas this is not the case for gas or liquid targets. In early experiments using a solid Xe pellet, ice fragments were observed and mirror damage due to these fragments was

be approximately an order of magnitude less than the number of ions.

in (a) is the loss rate of ions due to the drum rotating. The ion number in (b) was

product of pressure and thickness to be about 10 Pamm.

**6. Xe plasma debris** 

constitute the debris.

130 rpm.

calculated assuming the charge state was two.

indicated (Kubiak et al., 1995). Since these reports, liquid Xe targets have been preferred over solid Xe targets, with the exception of our group. It is therefore necessary to clarify characteristics of fragment debris from a solid Xe target on a rotating cryogenic drum. After exposing a Si sample to the Xe plasmas pumped by 100 laser pulses, we observed fragment impact damage on its surface using a scanning electron microscope. We observed damage spots on the samples at laser energy of 0.8 J irrespective of whether the drum rotates. Conversely, we did not observe spots at laser energy of 0.3 J. To explain these results, we consider that the fragment speed (kinetic energy) might drop below a damage threshold upon reducing the laser pulse energy because the fragment speed is a function of incident laser energy (Mochizuki et al., 2001). Observing the damage spots, we know that the fragment size was larger than a few microns, and the gas curtain might not be effective for such large fragments. This would explain why the fragment impact damage was independent of the state of drum rotation. From these results, we conclude that fragment impact damage, which occurs especially for the solid Xe target, can be avoided simply by reducing the incident laser pulse energy to less than 0.3 J.

The laser pulse energy was set to 0.3 J to avoid fragment impact damage and the laser repetition rate was 320 pps, giving an average power of 100 W. Next, we investigated damage to a Mo/Si mirror, which was the result of total plasma debris (mainly fast ions) from the laser multi-shots experiments. After 10 min plasma exposure, the sputtered depth was measured to be 50 nm on the surface of a Mo/Si mirror placed 100 mm from the plasma at a 22.5-degree angle to the incident laser beam. Because a typical Mo/Si mirror has 40 layer pairs and the thickness of one pair is approximately 6.6 nm, all layers will be removed within an hour by the sputtering. Although Xe is a deposition-free target, sputtering by debris needs to be mitigated. However, the major plasma debris component is ions, and we believe their mitigation to be simple compared with the case of a metal target such as Sn, using magnetic/electric fields and/or gas. We are now studying debris mitigation by Ar buffer gas. Ar gas was chosen because of its higher stopping power for Xe ions and lower absorption of EUV light, and its easy handling and low cost. After the vacuum chamber was filled with Ar gas, total erosion rates were measured using a gold-coated quartz crystal microbalance sensor placed 77 mm from the plasma at a 45-degree angle, and simultaneously, EUV losses were monitored by an EUV detector placed 200 mm from the plasma at a 22.5-degree angle. Figure 8 shows the erosion rates as a function of Ar gas pressure. The rates were normalized by the erosion *N0* at a pressure of 0 Pa. When the Ar pressure was 8 Pa, we found the erosion rate was 1/18 of that without the gas, but the absorption loss for EUV light was only 8%. The erosion rates (N/N0) in Fig. 8 can be fitted to an exponential curve:

$$N\left(P\_{Ar}\right) = N\_0 \cdot \exp\left(-\frac{P\_{Ar}}{kT}\sigma l\right) \tag{3}$$

where *PAr* is the Ar pressure, *k* is the Boltzmann constant, *T* is the gas temperature, is the cross section and *l* is the debris flight length. From this fitting, we obtain = 2.0 × 10–20 m2. The Ar buffer gas successfully mitigated the effect of plasma debris with little EUV attenuation. Increasing the Ar pressure, mirror erosion decreases but EUV attenuation increases. Compromising the erosion and EUV attenuation, an optimized pressure is achieved. We should localize the higher density Ar gas to only the debris path so that EUV attenuation is as small as possible. We can design the optimized pressure condition using

Laser-Plasma Extreme Ultraviolet Source Incorporating a Cryogenic Xe Target 365

already acquired. When making efforts to improve the CE at 13.5 nm, we noticed that emissions around 6 nm became strong at higher laser intensity. When laser energy is 0.8 J and LP = 0 mm (i.e., laser intensity is 4 1012 W/cm2 under the rotation condition), there is a hump around 6 nm as shown in Fig. 9. The spatially integrated CE at 6.7 nm with 0.6% bandwidth is estimated to be 0.1% from this spectrum. Because the bandwidth of 0.6% for the La/B4C mirror reflectivity is narrower than the 2% for the Mo/Si mirror, the available reflected power is intrinsically small. The CE of 0.1% was not obtained under optimized conditions and higher CE may be achieved in the future. In any event, our source is only one

LPP source at present that can generate continuously an emission at 6.7 nm.

Fig. 9. Spectra of EUV radiation under the rotation (bold line) and at-rest (narrow line) conditions with laser intensity of 4 1012W/cm2 for best focus (LP = 0 mm). The laser

Fig. 10. CE for a wavelength of 5–17 nm as a function of LP under the rotation (130 rpm)

energy was 0.8 J.

condition. The laser energy was 0.8 J.

the value obtained and we consider the use of an Ar gas jet. Through this mitigation, we expect that erosion will be reduced by more than two orders of magnitude and the lifetime of the mirror will be extended. We believe the debris problem for Xe plasma will thus be solved.

Fig. 8. Normalized erosion rate as a function of Ar pressure. The laser energy was 0.3 J and the rotation speed was 130 rpm.

#### **7. EUV emission at 5-17nm**

We began developing the LPP source for EUVL and characterized it at 13.5 nm with 2% bandwidth, but Xe plasma emission has originally a broad continuous spectrum as shown in Fig. 9. If the broad emission is used, our source will be very efficient, not limiting its applications to EUVL. We characterized the source again in the wavelength range of 5–17 nm. Figure 10 shows the CE at 5–17 nm as a function of LP (laser intensity) with laser energy of 0.8 J. The maximum spatially integrated CE at 5–17 nm was 30% for optimal laser intensity of 1 × 1010 W/cm2. The maximum CE depended on the laser energy and was 21% at 0.3 J. Therefore, high average power of 20 W at 5–17 nm has been achieved for pumping by the slab laser with 100 W (0.3 J at 320 pps). We consider this a powerful and useful source.

Recently, new lithography using La/B4C mirrors having a reflectivity peak at 6.7 nm was proposed as a next-generation candidate following EUVL using Mo/Si mirrors having a reflectivity peak at 13.5 nm (Benschop, 2009). This means that a light source emitting around 6 nm will be required in a future lithograph for industrial mass production of semiconductors. Because our source emits broadly at 5–17 nm as mentioned above, it can obviously be such a 6 nm light source. We thus next characterized it as a source emitting at 6.7 nm. Here we did not carry out new experiments to optimize the plasma for emitting at 6.7 nm but looked for indications of strong emission at 6.7 nm from the spectrum data

Fig. 8. Normalized erosion rate as a function of Ar pressure. The laser energy was 0.3 J and

We began developing the LPP source for EUVL and characterized it at 13.5 nm with 2% bandwidth, but Xe plasma emission has originally a broad continuous spectrum as shown in Fig. 9. If the broad emission is used, our source will be very efficient, not limiting its applications to EUVL. We characterized the source again in the wavelength range of 5–17 nm. Figure 10 shows the CE at 5–17 nm as a function of LP (laser intensity) with laser energy of 0.8 J. The maximum spatially integrated CE at 5–17 nm was 30% for optimal laser intensity of 1 × 1010 W/cm2. The maximum CE depended on the laser energy and was 21% at 0.3 J. Therefore, high average power of 20 W at 5–17 nm has been achieved for pumping by the slab laser with 100 W (0.3 J at 320 pps). We consider this a powerful and useful

Recently, new lithography using La/B4C mirrors having a reflectivity peak at 6.7 nm was proposed as a next-generation candidate following EUVL using Mo/Si mirrors having a reflectivity peak at 13.5 nm (Benschop, 2009). This means that a light source emitting around 6 nm will be required in a future lithograph for industrial mass production of semiconductors. Because our source emits broadly at 5–17 nm as mentioned above, it can obviously be such a 6 nm light source. We thus next characterized it as a source emitting at 6.7 nm. Here we did not carry out new experiments to optimize the plasma for emitting at 6.7 nm but looked for indications of strong emission at 6.7 nm from the spectrum data

 value obtained and we consider the use of an Ar gas jet. Through this mitigation, we expect that erosion will be reduced by more than two orders of magnitude and the lifetime of the mirror will be extended. We believe the debris problem for Xe plasma will thus be

the 

solved.

the rotation speed was 130 rpm.

**7. EUV emission at 5-17nm** 

source.

already acquired. When making efforts to improve the CE at 13.5 nm, we noticed that emissions around 6 nm became strong at higher laser intensity. When laser energy is 0.8 J and LP = 0 mm (i.e., laser intensity is 4 1012 W/cm2 under the rotation condition), there is a hump around 6 nm as shown in Fig. 9. The spatially integrated CE at 6.7 nm with 0.6% bandwidth is estimated to be 0.1% from this spectrum. Because the bandwidth of 0.6% for the La/B4C mirror reflectivity is narrower than the 2% for the Mo/Si mirror, the available reflected power is intrinsically small. The CE of 0.1% was not obtained under optimized conditions and higher CE may be achieved in the future. In any event, our source is only one LPP source at present that can generate continuously an emission at 6.7 nm.

Fig. 9. Spectra of EUV radiation under the rotation (bold line) and at-rest (narrow line) conditions with laser intensity of 4 1012W/cm2 for best focus (LP = 0 mm). The laser energy was 0.8 J.

Fig. 10. CE for a wavelength of 5–17 nm as a function of LP under the rotation (130 rpm) condition. The laser energy was 0.8 J.

Laser-Plasma Extreme Ultraviolet Source Incorporating a Cryogenic Xe Target 367

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