**3. Drive laser – Nd:YAG slab laser**

356 Recent Advances in Nanofabrication Techniques and Applications

speed. The wipers demonstrated a recovery speed of 150 m/s up to a rotation speed of

Fig. 2. Measured diameter and depth of a crater as a function of the irradiating laser

Next, operational parameters of the drum are discussed to achieve high-repetition-rate laser pulse irradiation. In Fig. 1(b), *R* is the rotation speed, *r* is the radius of the drum, and *L* is the range of motion (scanning width of the target) along the rotational axis (z-axis). When the laser pulses are irradiated with frequency *f*, craters form on the target with separation length *d* between adjacent craters. The recovery time of a crater is *T*. Under the condition that

> <sup>2</sup> *r R <sup>f</sup> <sup>d</sup>*

For example, if we assume laser energy of *EL* = 1 J, a formed crater has a diameter of *Dc* =

overlap. At *r* = 5 cm and *R* = 1000 rpm, we obtain *f* = 17 kHz from Eq. (1). When *f* = 10 kHz and *L* = 3 cm, *T* is calculated to be 10 s using Eq. (2), and we know that a recovery speed of

2 <sup>2</sup> *rL <sup>T</sup> f d* 

(1)

(2)

*<sup>c</sup>* = 160 m, and *d* must be at least 300 m for the craters not to

*<sup>c</sup>*/*T*) of 16 m/s is required. Here, we have already obtained *Vc* = 150 m/s

1000 rpm, at a Xe flow rate of 400 mL/min.

craters do not overlap, *f* and *T* can be written as

via the wiper effect and the required speed has been achieved.

300 m and a depth of

the crater (*Vc* =

energy.

High peak power and high focusability (i.e., high beam quality) are required for a driving laser to produce plasma. In addition, high average power is required for high throughput in industrial use such as EUVL. We express such a laser as a *high average and high peak brightness laser*, for which the average brightness and peak brightness are defined as average power/(M2)2 and peak power/(M2)2, respectively; we began studying such lasers in the 1990s (Amano et al, 1997,1999).

We attempted to realize a *high average and high peak brightness laser* using a solid-state Nd:YAG laser (Amano et al., 2001). The thermal-lens effect and thermally induced birefringence in an active medium are serious for such a laser; thus, thermal management of the amplifier head is more critical, and the design of the amplifier system must more efficiently extract energy and more accurately correct the remaining thermally induced wavefront aberrations in the pumping head. To meet these requirements, we developed a phase-conjugated master-oscillator-power-amplifier (PC-MOPA) Nd:YAG laser system consisting of a diode-pumped master oscillator and flash-lamp-pumped angularmultiplexing slab power-amplifier geometry incorporating a stimulated-Brillouin-scattering phase-conjugate mirror (SBS-PCM) and image relays (IR). The system design and a photograph are shown in Fig. 3. This laser demonstrated simultaneous maximum average power of 235 W and maximum peak power of 30 MW with M2 = 1.5. The maximum pulse energy was 0.73 J with pulse duration of 24 ns at a pulse repetition rate of 320 pps. We therefore obtained, simultaneously, both high average brightness of 7 × 109 W/cm2sr and high peak brightness of 1 × 1015 W/cm2sr.

This peak brightness is enough to produce plasma but the average brightness needs to be higher for EUVL applications. The maximum average power is mainly limited by the thermal load caused by flash-lamp-pumping in amplifiers. The system design rules that we confirmed predicted that average output power at the kilowatt level can be achieved by replacing lamp pumping in the amplifier with laser-diode pumping. Since our work, it seems that there has been no major progress in laser engineering for such *high average and high peak brightness lasers.* Average power of more than 10 kW has been achieved in continuous-wave solid-state lasers using configurations of fibers (ex. IPG Photonics Corp.) or thin discs (ex. TRUMPF GmbH). On the other hand, for the short-pulse lasers mentioned above, the maximum average power remains around 1 kW (Soumagne et al., 2005), which is more than an order of magnitude less than the ~30 kW required for an industrial EUVL source. This is one of the reasons why CO2 lasers have been preferred over Nd:YAG lasers as the driving laser. To further the industrial use of solid-state lasers, there needs to be a breakthrough to increase the average power.

Laser-Plasma Extreme Ultraviolet Source Incorporating a Cryogenic Xe Target 359

mm/s. The Xe target gas is continuously supplied at a flow rate of 400 mL/min. Under these operation conditions, we obtain continuous EUV generation with average power of 1

The driving pulse energy was determined to be 0.3 J under the optimal condition that higher CE and lower debris are simultaneously achieved, as detailed below. At present, the maximum achieved CE is 0.9% at 13.5 nm with 2% bandwidth for the optimal condition. Under drum-rotating operation, we found the good characteristics of increased CE and less fast ions compared with the case with the drum at rest. We next detail the EUV and debris

Fig. 4. Experimental setup and photograph of the laser plasma EUV source.

In this section, we report our studies carried out to improve the CE at 13.5 nm with 2% bandwidth required for the EUVL source (Amano et al., 2008, 2010a). To achieve the highest CE, we attempted to control the plasma parameter by changing the driving laser conditions. We investigated dependences of the CE on the drum rotation speed, laser energy, and laser wavelength. We also carried out double-pulse irradiation experiments

To obtain data of EUV emission, a conventional Q-switched Nd:YAG rod laser (Spectra-Physics, PRO-230) was used in single-shot operation. By changing the position of the focusing lens to change the laser spot, the laser intensity on the target was adjusted to find the optimum intensity. We note that the lens position (LP) is zero at best focus, negative for in-focus (the laser spot in the target before the focus) and positive for out-of-focus (beyond

W at 13.5 nm and 2% bandwidth.

characteristics of the EUV source.

**5. Conversion efficiency for EUVL** 

to improve the CE.

the focus).

Fig. 3. Experimental setup and photograph of the PC-MOPA laser system.
