**2. High repetition rate picosecond Yb:YAG thin disc-laser in LPP EUV source**

Continuous shrinking of the microcircuit is the natural law for lower cost, higher yield, short time to the market in the semiconductor industry. The microlithography has been the central manufacturing technology, and the continuous shrinking of the wavelength is the principal architecture. The proposal of the application of EUV wavelength appeared long before the perspective of the light source itself. The shift of source technology to the ArF excimer is followed by immersion technology and the ArF laser is the long-life light source technology. The EUV lithography is now entering into the mass production phase in the 22 nm node, and the wavelength is 13.5 nm (92.5 eV) supported with Mo/Si high reflectivity mirrors. 13.5 nm wavelength is the first generation of ionizing radiation in the mass production of semicon‐ ductor industries. The laser-produced plasma (LPP) EUV source has been established as the basic architecture of the EUV source technology, after one decade of focused research and engineering. The present concern is the stability and cleanness of the source itself and further engineering is continued [22]. The EUV light source is essentially incoherent spherical emission from highly ionized Tin plasma. The source is composed of three parts, namely driving laser, plasma generation/exhaust, and EUV light collector. A large Mo/Si collector mirror has peak reflectivity at 13.5 nm with 2% bandwidth. It is located close to the high-power plasma source and the extension of the lifetime is the most critical engineering concern. It is reported in a recent conference that the power available at the intermediate focus (IF) in the field is 125 W, and a test source is operated in a company laboratory aiming at 250 W [23]. A typical config‐ uration of the LPP EUV source for high volume manufacturing (HVM) is shown in **Figure 2**, where a train of 100 kHz Sn droplet is injected and irradiated by a solid-state laser prepulse (purple), dispersed into a mist bunch, and irradiated by a CO2 laser main pulse (red). A discharge pumped EUV source is now employed for metrology purpose in less than 100 W level. The typical configuration is shown to the right of the LPP system. A small laser pulse initiates Sn vapor for main discharge from a rotating disc immersed in Sn liquid [24]. It is called as laser-assisted discharge plasma (LDP).

**Figure 2.** Configuration of double pulse method in LPP (left) and LDP (right) EUV sources.

of the lasers, it was also initiated high-power harmonic frequency generation and high-power mid-IR picosecond system consisting of an OPG followed by double OPA systems (**Figure 1**).

**2. High repetition rate picosecond Yb:YAG thin disc-laser in LPP EUV**

Continuous shrinking of the microcircuit is the natural law for lower cost, higher yield, short time to the market in the semiconductor industry. The microlithography has been the central manufacturing technology, and the continuous shrinking of the wavelength is the principal architecture. The proposal of the application of EUV wavelength appeared long before the perspective of the light source itself. The shift of source technology to the ArF excimer is followed by immersion technology and the ArF laser is the long-life light source technology. The EUV lithography is now entering into the mass production phase in the 22 nm node, and the wavelength is 13.5 nm (92.5 eV) supported with Mo/Si high reflectivity mirrors. 13.5 nm wavelength is the first generation of ionizing radiation in the mass production of semicon‐ ductor industries. The laser-produced plasma (LPP) EUV source has been established as the basic architecture of the EUV source technology, after one decade of focused research and engineering. The present concern is the stability and cleanness of the source itself and further engineering is continued [22]. The EUV light source is essentially incoherent spherical emission from highly ionized Tin plasma. The source is composed of three parts, namely driving laser, plasma generation/exhaust, and EUV light collector. A large Mo/Si collector mirror has peak reflectivity at 13.5 nm with 2% bandwidth. It is located close to the high-power plasma source and the extension of the lifetime is the most critical engineering concern. It is reported in a recent conference that the power available at the intermediate focus (IF) in the field is 125 W, and a test source is operated in a company laboratory aiming at 250 W [23]. A typical config‐ uration of the LPP EUV source for high volume manufacturing (HVM) is shown in **Figure 2**, where a train of 100 kHz Sn droplet is injected and irradiated by a solid-state laser prepulse

**Figure 1.** Building of the HiLASE R&D Centre in Dolní Břežany, Czech Republic.

**source**

106 High Energy and Short Pulse Lasers

The initial state of the injected Sn droplet is liquid phase of 10–20 μm diameter in the LPP system, and the direct laser irradiation results in a lower conversion efficiency (CE) and messy split of liquid Sn inside the chamber. The solution is the double-pulse method, as the initial pulse converts the liquid Sn droplet into nanocluster bunch (mist) for better laser absorption and ionization. An experiment demonstrated that the prepulse is much efficient in the case of picosecond pulse length compared to the nanosecond one. **Figure 3** shows the experimental results reported in a conference [25].

**Figure 3.** Ionization rate of Sn and CE depending on the pulse length of pre-pulse. Left: Ionization rate. Right: Conver‐ sion efficiency (CE).

The picosecond laser has typical parameters as pulse energy more than mJ, pulse length is 10 ps or less, and focusing diameter is a few times larger than the droplet diameter of 10–20 μm. The average power is more than 100 W at the repetition rate of 100 kHz. The laser specification is not easily covered by any commercial products and must be specifically developed. Thindisc laser is suitable for the required specification among other types of advanced lasers such as fiber or thin slab with its larger beam diameter. HiLASE project was dedicated in a research and development of kW class picosecond thin-disc lasers in the period of 2012–2015. One of the laser beamlines is PERLA (Pearl) C, which is aimed to realize a compact, stable 500 W picosecond thin-disc laser with 100 kHz repetition rate [26]. The research and development of the laser system is briefly described in the following.

Design of the laser comes from the thin-disk laser concept. **Figure 4** shows the configuration of the thin-disc laser module with a parabolic mirror that collimates and images the pump radiation from laser diodes. The parabolic mirror images several times the unabsorbed pumping radiation with a set of roof mirrors. The thermal lensing is limited minimum due to the axial thermal flow from the gain medium to the water cooled heat sink. The nonlinear effects in the solid-state medium (self-phase modulation, B-integral) are controlled at low level in the multiple optical passes in the thin disc. The cooling is efficient due to the small thickness of the disc. The typical discs are characterized by the gain thickness as 100–300 μm and the disc diameter as 8–30 mm. Special optical design is required to compensate the low single-pass amplification gain together with pump absorption. Regenerative amplifier is selected for medium-power amplifier, and multipass amplifier is designed for higher average power or higher pulse energy amplifier. Regenerative amplifiers allow very compact and robust laser systems. High-power regenerative amplifier concept is based on a ring cavity, which is in fact a new approach. High average power and high repetition rate regenerative amplifiers usually suffer from Pockels cell issues. A new kind of large aperture BBO Pockels cell was developed to overcome this obstacle (**Figure 4**). A kW-class regenerative amplifier with a ring cavity is a novel approach in the field of picosecond thin disk lasers.

**Figure 4.** Left: Concept of efficient pumping (blue beam) of thin-disk lasers. Right: in-house developed large-aperture and water-cooled BBO Pockels cell.

Various solid-state materials are applied in thin disc modules, and the Yb:YAG is the most favored one due to high quality in fabrication and picosecond pulse generation. Yb:YAG is studied for more than two decades in its growing, cutting, and polishing, and its thermome‐ chanical characteristic is well fitted for picosecond and subpicosecond pulse generation. One of the disadvantages of the thin-disc laser is the bonding technology of large diameter thin Yb:YAG disc to the heatsink basement to be robust in high-temperature and high optical fluence environment. Several bonding methods are available to 10 mm diameter and further new techniques are still tested for higher reliability. In the present stage, HiLASE Centre uses two types of bonding methods, namely soldering to a copper-tungsten heatsinks and bonding to a diamond substrate. The diamond substrate is advantageous for its higher thermal conductivity for lower disc temperature under high pumping fluence. Popular pumping source is a laser diode with 940 nm center wavelength where the absorption has a broader bandwidth. Yb:YAG has a narrower but high-peak absorption wavelength at 968.8 nm, which is called as zero-phonon line, and a specific laser diode at this wavelength is used for efficient pumping. The quantum defect decreases from 8.7% with 940 nm pumping to 5.9 % with zerophonon line pumping. Zero-phonon line pumping is also better in its suppression of nonlinear phonon relaxation in the Yb:YAG medium. The resulting steady-state disc temperature is kept lower compared to 940 nm pumping, and better stability of the amplification and higher output pulse energy is the positive result [27]. Pump diodes should have bandwidth <1 nm. Since the absorption line near 968.8 nm is very narrow, the diodes are stabilized by volume Bragg gratings (**Figure 5**).

The picosecond laser has typical parameters as pulse energy more than mJ, pulse length is 10 ps or less, and focusing diameter is a few times larger than the droplet diameter of 10–20 μm. The average power is more than 100 W at the repetition rate of 100 kHz. The laser specification is not easily covered by any commercial products and must be specifically developed. Thindisc laser is suitable for the required specification among other types of advanced lasers such as fiber or thin slab with its larger beam diameter. HiLASE project was dedicated in a research and development of kW class picosecond thin-disc lasers in the period of 2012–2015. One of the laser beamlines is PERLA (Pearl) C, which is aimed to realize a compact, stable 500 W picosecond thin-disc laser with 100 kHz repetition rate [26]. The research and development of

Design of the laser comes from the thin-disk laser concept. **Figure 4** shows the configuration of the thin-disc laser module with a parabolic mirror that collimates and images the pump radiation from laser diodes. The parabolic mirror images several times the unabsorbed pumping radiation with a set of roof mirrors. The thermal lensing is limited minimum due to the axial thermal flow from the gain medium to the water cooled heat sink. The nonlinear effects in the solid-state medium (self-phase modulation, B-integral) are controlled at low level in the multiple optical passes in the thin disc. The cooling is efficient due to the small thickness of the disc. The typical discs are characterized by the gain thickness as 100–300 μm and the disc diameter as 8–30 mm. Special optical design is required to compensate the low single-pass amplification gain together with pump absorption. Regenerative amplifier is selected for medium-power amplifier, and multipass amplifier is designed for higher average power or higher pulse energy amplifier. Regenerative amplifiers allow very compact and robust laser systems. High-power regenerative amplifier concept is based on a ring cavity, which is in fact a new approach. High average power and high repetition rate regenerative amplifiers usually suffer from Pockels cell issues. A new kind of large aperture BBO Pockels cell was developed to overcome this obstacle (**Figure 4**). A kW-class regenerative amplifier with a ring cavity is a

**Figure 4.** Left: Concept of efficient pumping (blue beam) of thin-disk lasers. Right: in-house developed large-aperture

Various solid-state materials are applied in thin disc modules, and the Yb:YAG is the most favored one due to high quality in fabrication and picosecond pulse generation. Yb:YAG is

the laser system is briefly described in the following.

108 High Energy and Short Pulse Lasers

novel approach in the field of picosecond thin disk lasers.

and water-cooled BBO Pockels cell.

**Figure 5.** Left: Diamond-heat spreader-bonded thin disc. Right: Absorption cross section of Yb:YAG.

The high repetition rate beamline PERLA C operates at 100 kHz and provides picosecond pulses from 1 to >4 mJ in a compressed pulse. The seeder of the laser system is a commercially available Yb-doped fiber laser from Fianium. The pulse length is 12 ps at 50 MHz repetition rate, and the pulse energy is 6 nJ with 20 nm broad bandwidth. The pulses are stretched to 0.5 ns pulse length by a small Bragg grating. The pulse bandwidth is filtered to 2.2 nm by the bandwidth of the grating, and the bandwidth limited pulse length is less than 2 ps. The seeder pulses are amplified by a semiconductor optical amplifier (SOA) and a single-mode fiber amplifier before injection into a regenerative amplifier. The advantage of the SOA is its electric controllability of the gain time window and used as a pulse picker to reduce the repetition rate from 50 to 1 MHz. The average power is 300 mW before the regenerative amplifier.

The regenerative amplifier is composed of a single Yb:YAG module with a standing wave cavity for 100 W operation (**Figure 6**). The total footprint is compact as 900 × 1200 mm including a pulse compressor. The pump spot size of the thin disc is 2.7 mm in diameter with cavity length of 2 m. A double Pockels Cell system optically switches the input and output pulses. The size of the BBO crystal is 8 × 8 mm2 . The crystal holder is engineered to avoid damages to the BBO by piezo ringing in high repetition rate switching. The maximum available BBO aperture is 12 × 12 mm2 , and the repetition rate is 1 MHz and the voltage is 10 kV. As described earlier, the pumping is by zero-phonon line continuous fiber-coupled laser diodes. The maximum amplified pulses are 1.2 mJ of energy at 100 kHz repetition rate with M2 = 1.3 beam quality

**Figure 6.** Left: 100 W regenerative amplifier. Right: Optical scheme including a CVBG pulse compressor (HR, highly reflective mirror; TD, thin disk; LD, pump laser diodes; PC, Pockels cell; TFP, thin-film polarizer; FR, Faraday rotator; PBS, polarizing beam splitter; CVBG, chirped volume Bragg grating; λ/2 and λ/4, half- and quarter-wave plates; SMF, single-mode fiber; PLD, pump laser diode; WDM, multiplexer; CRC, circulator; FBG, fiber Bragg grating; OSC, oscilla‐ tor).

This is critically important for precise irradiation like prepulse in an LPP EUV source. Pulses are compressed by a chirped volume Bragg grating (CVBG) compressor, which is a very robust, compact, and easy to align bulk compressor with 8 × 82 mm aperture. The CVBG compressor was tested for long time operation and demonstrated a reliable pulse compression of high average power pulse train with >85 % diffraction efficiency under optimized cooling condition. Compressed pulses measured by intensity autocorrelation (**Figure 7**, right) have temporal width 1.6 ps (sech2 ). The pulse-to-pulse energy stability measured over 4 million pulses was better than 1.7%, and the long-term average power stability measured over 1 h was <1.5 % (RMS value). Better housing and active stabilization can even improve the stability.

bandwidth of the grating, and the bandwidth limited pulse length is less than 2 ps. The seeder pulses are amplified by a semiconductor optical amplifier (SOA) and a single-mode fiber amplifier before injection into a regenerative amplifier. The advantage of the SOA is its electric controllability of the gain time window and used as a pulse picker to reduce the repetition rate

The regenerative amplifier is composed of a single Yb:YAG module with a standing wave cavity for 100 W operation (**Figure 6**). The total footprint is compact as 900 × 1200 mm including a pulse compressor. The pump spot size of the thin disc is 2.7 mm in diameter with cavity length of 2 m. A double Pockels Cell system optically switches the input and output pulses.

the BBO by piezo ringing in high repetition rate switching. The maximum available BBO

earlier, the pumping is by zero-phonon line continuous fiber-coupled laser diodes. The maximum amplified pulses are 1.2 mJ of energy at 100 kHz repetition rate with M2 = 1.3 beam

**Figure 6.** Left: 100 W regenerative amplifier. Right: Optical scheme including a CVBG pulse compressor (HR, highly reflective mirror; TD, thin disk; LD, pump laser diodes; PC, Pockels cell; TFP, thin-film polarizer; FR, Faraday rotator; PBS, polarizing beam splitter; CVBG, chirped volume Bragg grating; λ/2 and λ/4, half- and quarter-wave plates; SMF, single-mode fiber; PLD, pump laser diode; WDM, multiplexer; CRC, circulator; FBG, fiber Bragg grating; OSC, oscilla‐

This is critically important for precise irradiation like prepulse in an LPP EUV source. Pulses are compressed by a chirped volume Bragg grating (CVBG) compressor, which is a very robust,

was tested for long time operation and demonstrated a reliable pulse compression of high average power pulse train with >85 % diffraction efficiency under optimized cooling condition. Compressed pulses measured by intensity autocorrelation (**Figure 7**, right) have temporal

better than 1.7%, and the long-term average power stability measured over 1 h was <1.5 %

(RMS value). Better housing and active stabilization can even improve the stability.

). The pulse-to-pulse energy stability measured over 4 million pulses was

compact, and easy to align bulk compressor with 8 × 82

. The crystal holder is engineered to avoid damages to

mm aperture. The CVBG compressor

, and the repetition rate is 1 MHz and the voltage is 10 kV. As described

from 50 to 1 MHz. The average power is 300 mW before the regenerative amplifier.

The size of the BBO crystal is 8 × 8 mm2

aperture is 12 × 12 mm2

110 High Energy and Short Pulse Lasers

quality

tor).

width 1.6 ps (sech2

**Figure 7.** Left: 1.2 mJ of the output pulse energy at 100 kHz repetition before compression has been achieved from the 100 W PERLA C in a nearly diffraction-limited beam. Right: the pulses were compressed to 1.6 ps (FWHM) by a CVBG as shown by the intensity autocorrelation trace.

A higher average power regenerative amplifier was developed with a ring cavity (**Figure 8**). The amplifier is switched by a reliable Pockels cell, which is in-house design for 10 × 10 mm2 aperture with effective cooling. The fundamental spatial mode operating cavity is designed for a 5.2 mm pump spot and the cavity contains a single diamond-bonded Yb:YAG thin disc. The disc is zero-phonon line-pumped by VBG-stabilized fiber-coupled diodes. Laser cavity was tested in the CW regime to evaluate the thermal distortion. 550 W output was observed with almost 50% optical-optical efficiency and >4 mJ was achieved in a 100 kHz pulse train with a nearly diffraction-limited output beam (**Figure 8**).

**Figure 8.** Left: ring cavity of the 500 W PERLA C laser system. FM, folding mirror; PCX M, planoconvex mirror; PCV M, planoconcave mirror; TFP, thin-film polarizer; PC, Pockels cell; TDLH, thin-disc laser head; DL, pump diodes; λ/2, half-waveplate. Right: performance of the 500 W ring cavity in CW and pulse mode (PERLA C).
