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A narrow-linewidth, high average power deep-ultraviolet (DUV) coherent laser emitting at 193 nm is demonstrated by frequency mixing a Yb-hybrid laser with an Er-fiber laser. The Yb-hybrid laser consists of Yb-fiber lasers and an Yb:YAG amplifier. The average output power of the 193 nm laser is 310 mW at 6 kHz, which corresponds to a pulse energy of 51 μJ. To the best of our knowledge, this is the highest average power and pulse energy ever reported for a narrow-linewidth 193 nm light generated by a combination of solid-state and fiber lasers with frequency mixing. We believe this laser will be beneficial for the application of interference lithography by seeding an injection-locking ArF eximer laser.
© 2015 Optical Society of America
1. Introduction
Coherent laser radiation below 200 nm is of great significance to applications such as photolithography [1], metrology [2], and photoelectron spectroscopy [3]. Particularly, narrow-linewidth and high coherence ArF eximer laser at 193 nm is a useful light source for the interference lithography before extreme ultra violet light soure is ready for sub-10 nm node [1]. The ArF eximer laser system for the state-of-art lithography is an injection locking system with two ArF eximer lasers in order to realize both narrow-linewidth and high power. The power consumption of the current injection-locked ArF eximer laser is however high, beyond tens of kilo-watt. Hence, changing the seed of the ArF eximer amplifier in the injection-locked system to a solid-state or fiber laser based narrow-linewidth source is a potential way to decrease the power consumption. Using a solid-state laser as a seed source would be also beneficial in terms of spatial coherence, as high spatial coherence enables to realize narrower interference lithography patterning. In addition, a DUV light source combining both high-power with high spatial coherence is ideal for laser machining and realizing minute drilling or cutting tasks of unworkable materials by use of high photon energy (6.4eV).
In the past ten years, the generation of DUV radiation by solid-state lasers, including fiber lasers, has been extensively studied. Sakuma et al. reported the generation of 40 mW of average output power at 193 nm with a linewidth below 200 MHz laser system made of three Nd:YLF lasers and CsLiB6O10(CLBO) crystal [4]. Kawai et al. achieved the generation of DUV pulses with 140 mW of output power at 193 nm and 200 kHz repetition rate in 2003, by using the 8th harmonic of a fundamental wave at 1547 nm [5]. In the same year, Umemura et al. reported the results of 220 mW average power at 193 nm produced by a K2Al2B2O7(KABO) system which included an OPO cavity in the setup [6]. Zhang et al. developed a 2.23 mW average output power narrow-linewidth laser at 193 nm using a KBe2BO3F2 (KBBF) [7]. The generation of 1.05 W output power at 193.5 nm using the same crystal was later reported by Kanai et al., however the pulse repetition rate was of 5 kHz and the pulse duration was of picoseconds, which implies the linewidth is not narrow [8]. Employing a KBBF crystal and in a narrow-linewidth capacity, Ito et al. demonstrated the 193 nm coherent radiation pulses with a repetition rate of 6 kHz and 0.2 W average power [9]. Recently, Koch et al. described a DUV light source with excellent beam quality emitting 240 mW of output power at 191.7 nm, using the 7th harmonic of a Nd:YVO4 laser [10]. In the continuous-wave (CW) laser generation, Sakuma et al. detailed a 10 mW CW laser emitting at 193.4 nm, which consisted of a CLBO crystal in an enhancement cavity seeded by fiber amplifiers [11]. Scholz et al outlined a 193-nm narrow-linewidth CW laser with 15 mW of output power arising from the fourth-harmonic generation (FHG) of a 772 nm extended-cavity diode laser employing a KBBF crystal recently [12].
Evidently, the most popular fundamental laser source for frequency conversion to DUV wavelength is based on the high power Nd-doped lasers, Ti:sapphire lasers or fiber lasers. Fiber lasers are the most sought after option, owed to their compactness and the simplicity of their alignment. Currently, on the other hand, up to 120 W of average power at 193 nm can be obtained; this was demonstrated by using two ArF eximer laser chambers [13]. Exchanging the seed source of the ArF eximer laser for a solid-state or fiber laser based DUV 193-nm laser should allow both the requirements for high coherence and high power from the final ArF eximer amplifiers output to be met. This places a lower-limit on both the linewidth and the output power of the 193-nm seed source, which should be several GHz and 200 mW, respectively. Since the ArF eximer laser operates at 6-kHz repetition rate, the seed of 193 nm laser for injection locking should have the same repetition rate. Finally, the solid-state laser based 193 nm seed should have a good beam profile compared to that of the previous ArF laser, which will be also beneficial for lithography.
In this paper, we demonstrate the generation of a high-power, narrow-linewidth, high-coherence 193-nm laser by frequency mixing between an Yb-hybrid laser and an Er-fiber laser using CLBO crystals.
2. Laser systems
The schematic diagram for the generation of 193-nm laser radiation by frequency mixing is shown in Fig. 1. To begin with, the Yb-doped fiber amplifier consists of a pulsed distributed feedback (DFB) diode laser, which is utilized to seed subsequent fiber amplifiers as depicted in the schematic diagram of Fig. 2. The DFB laser produces pulses that have a repetition rate of 96-kHz, a duration of 10-ns, and at 1030 nm, the linewidth is of 1.5 GHz approximately and the output power is 0.1 mW. This pulsed laser is firstly pre-amplified to 16 mW by a single mode fiber (SMF) amplifier with the repetition rate of 96 kHz. An acoustic-optic modulator (AOM) was placed after the first pre-amplifier in order to lower the repetition rate down to 6 kHz and simultaneously the power decreased to about 0.1 mW. From this 0.1 mW seed, the second SMF amplifier and the third SMF amplifier are sequentially applied to amplify the power, respectively, in excess of 1 mW and 10 mW at the repetition rate of 6 kHz.
Fig. 2 Schematic diagram of the different components in the Yb-doped fiber amplifier; ISO: Isolator; AOM: Acoustic-optics modulator.
A photonic crystal fiber (PCF) is applied for the next amplifier. The core diameter of the PCF fiber is 40 μm and the inner cladding diameter is 200 μm. The pump laser diode (LD) operates at the wavelength of 976 nm with an output power of 10 W. The PCF amplifier increased the signal power to 1.2 W, corresponding to a pulse energy of 0.2 mJ, which in turn seeded the Yb:YAG single crystal fiber (SCF) amplifier. The output power and spectrum from the PCF amplifier used to seed the Yb:YAG amplifier are shown in Fig. 3. A maximum average power of 1.2 W was obtained for a pump power of 7.7 W [Fig. 3(a)]. The optical spectrum of Fig. 3(b) reveals a signal to amplified spontaneous emission (ASE) contrast ratio of nearly 30 dB (0.02 nm resolution) and no obvious signs of nonlinear effects, such as self-phase modulation (SPM) or stimulated Brillouin scattering (SBS) [14].
Fig. 3 (a) Average output power of the PCF amplifier at varying pump power; (b)Spectrum of the PCF amplifier at 1.2 W
In recent years, solid-state amplifiers have been demonstrated with average output powers in excess of hundreds of watts, such as the slab amplifier [15], the thin disk amplifier [16] and the SCF amplifier [17, 18]. Thanks to a simple alignment, the ability to provide high beam quality, and being cost-effective, a 40-mm long, 1-mm in diameter water-cooled Yb:YAG SCF was chosen as the final power amplifier in the Yb amplifier system. The Yb:YAG SCF amplifier was operated in a double pass configuration and pumped by a high brightness fiber-coupled laser diode at 940 nm with a maximum power of 70 W.
Two optical isolators were placed between the PCF and Yb:YAG SCF amplifiers, which reduced the power from 1.2 W to 0.9 W. With the careful alignment of the pump coupling optics of the Yb-YAG SCF amplifier, the highest output power achieved was 4.55 W and 9.12 W in the single-pass and the double-pass configurations, respectively, as shown in Fig. 4(a). Hereafter, the Yb:YAG SCF amplifier was used in a double-pass configuration and the optical spectrum Fig. 4(b) reveals the ASE coarsely estimated to be no more than 7% of the total power from the area calculation of the optical spectrum analyzer, which should come from the Yb-doped fiber pre-amplifiers.
Fig. 4 (a)Average output power of Yb:YAG SCF amplifier at varying pump power in the single- and double-pass configurations; (b) Spectrum of the Yb:YAG SCF amplifier at 9.12 W
3. Frequency conversion
The 1030-nm emission from the Yb amplifier system underwent frequency conversion as shown in Fig. 1. First, green light at 515 nm was produced by SHG using a non-critically phase-matched (NCPM) lithium triborate (LBO) crystal (5×5×30 mm3, Input: Anti-reflection (AR) coating for 1030 nm, Output: AR coating for 515 nm) maintained at a set temperature of 187.9 °C. The green light was in turn converted to 258 nm by successive SHG using a CLBO crystal. For this stage, BBO and CLBO crystasl are both available candidates. However, the walk-off angle of BBO crystal is nearly three times more than that of CLBO crystal, although its effective nonlinear coefficient is two times more than that of CLBO [20]. In order to obtain a good beam quality, a CLBO crystal (5×5×20 mm3) with no coating was used for FHG. The output power of SHG at 515 nm from the 1030 nm light reached a maximum power of over 6 W, corresponding to a frequency conversion efficiency of more than 70% [Fig. 5(a)]. The ensuing fourth-harmonic at 258 nm (from the 515 nm beam) resulted in output powers in excess of 3 W, corresponding to a conversion efficiency of nearly 50% [Fig. 5(b)]. The 1030-nm laser from the Yb:YAG SCF amplifier had a beam quality factor of M2∼1 as we reported previously [19], and therefore both the 515-nm and 258-nm lasers inherited the good beam quality from the 1030-nm laser.
Fig. 5 Average output power and frequency conversion efficiency for varying pump power resulting from (a) SHG to 515 nm; (b) FHG to 258 nm.
An erbium-doped fiber amplifier (EDFA) system developed in-house and producing pulses of up to 1.2 W at 1553 nm was synchronized to the Yb-hybrid system. The EDFA consisted of a DFB laser operating at 1553 nm and three stages of Er-doped fiber amplifier. The pulse duration from the EDFA was 5 ns and the linewidth was of 300 MHz. The synchronization between the Yb-hybrid and EDFA system output pulses was obtained, using a signal generator (Agilent 86110A) and digital delay generators (DG645, Standford Research Systems). The timing jitter was estimated to an upper-value of approximately 200 ps, which falls within the acceptable range for frequency conversion. This value was determined from the uncertainty of the electronic equipment and environmental factors were not taken into account. The synchronized 1553-nm and 258-nm pulses served as pump beams in the sum-frequency generation (SFG) process and created a beam at 221 nm by use of a first CLBO crystal, which in turn, in conjunction with the residual 1553-nm beam pumped a second CLBO crystal to produce the final 193-nm laser.
Several nonlinear crystals have been shown to efficiently generate narrow-linewidth, highly-coherent DUV radiation, namely BBO, LBO, CLBO, and KBBF [4, 5, 8, 21]. The choice of the nonlinear crystal for both SFG processes was based on various aspects. Table 1 indicates the values of the walk-off, phase matching (PM) angle, and effective nonlinear coefficient of these four crystals for the SFG to generate 221 nm with pump beams of 258 nm and 1553 nm and the SFG between 221 nm and 1553 nm to produce 193 nm [20]. While BBO crystals have the highest efficient nonlinear coefficient, they present a large walk-off angle and absorption at 193 nm. In the case of KBBF crystals, they are the only ones so far capable of generating vacuum ultra violet laser by SHG, but they have large walk-off angle and low conversion efficiency as well as the lack of commercial availability. Moreover, the CLBO crystal has a larger acceptance angle than that of the BBO crystal. Hence, the CLBO crystal seemed like the most suitable choice in terms of high efficiency and good beam quality. In this work, type I (o+o→e) CLBO crystal with no coating was used in both SFG processes. The experimental configuration for both SFG processes is illustrated in Fig. 6.
Fig. 6 Schematic diagram of the two stages of SFG; DM1: 258HR/1553HT mirror; DM2: 221HR/1553HT mirror; DM3: 221HR/1553HT mirror; HWP: Half Wave Plate.
Table 1. Nonlinear Optical Properties of different crystals for SFG
A first dichroic mirror (DM1), which was highly reflective (HR) for 258 nm and highly transmissive (HT) for 1553 nm, combined the two beams to pump the nonlinear crystal. 1 W average output power of ultra violet (UV) light at 221 nm was obtained at the first SFG with the careful alignment in space and time in a CLBO crystal (5×5×20 mm3). A second dichroic mirror (DM2), which is HR for 221 nm and HT for 1553 nm, separated the generated UV radiation from the residual 1553 nm laser. The two separated beams were recombined by a third dichroic mirror (DM3) at the second CLBO crystal (5×5×20 mm3) for the second SFG process, which produced the final DUV 193-nm output. The residual pump beams were separated from the generated DUV radiation using a CaF2 prism (93%–94% estimated transmission) and occluded by an iris to single out the desired wavelength at 193 nm for analyses. Generally, a same confocal parameter for both SFG beams would lead to the maximum conversion efficiency [22]. In our experiment, however, two beam sizes were set to be almost the same to obtain the highest conversion efficiency in each SFG stage. This discrepancy would be caused by the insufficient power of 1553 nm laser in our case. To avoid hydration of the CLBO crystals (and potential distortion of their refractive index), both of them were placed in gas cells which received a steady flow of Ar gas and were maintained at a temperature of over 150 °C [23, 24]. In addition, the CLBO crystal for FHG stage was placed in the same situation as that of the CLBO crystals for SFG to avoid hydration.
The output power of the generated 221-nm UV light as a function of varying 258-nm pump light is shown in Fig. 7(a). A maximum average output power of 1 W was achieved, which corresponds to a conversion efficiency of nearly 40% from the 258-nm beam. The residual power of 1553-nm laser from the SFG process was over 600 mW. The maximum average power of the 193-nm generated light was 310 mW [Fig. 7(b)], which corresponds to a pulse energy of 51 μJ. The conversion efficiency from 258-nm laser to 193-nm laser was about 12.4%. Both the SFG powers at 221 nm and 193 nm were proportional to the 1553-nm laser power.
As the power measurements of 193-nm laser were acquired after the CaF2 prism (Fig. 6), the maximum output power is estimated to 330 mW. The pulse duration of the resulting 193-nm light was measured to be approximately 4 ns using a biplanar phototube [Fig. 8(a)]. Each of the 221-nm and the residual 1553-nm pump beams presented high quality beam profiles [19], which in turn led the 193-nm generated beam to have a high beam quality itself. The beam profile was verified by a back illumination type CCD camera placed after the prism [Fig. 8(a) inset]. With the ability to produce such a spatially coherent 193-nm DUV light, this laser system seems to be a suitable contender for amplification by a ArF eximer laser in the application of interference lithography patterning. For obtaining high conversion efficiency and avoiding degradation of CLBO crystals, the peak power intensity was set to be no more than 30 MW/cm2 in every SFG stage including the FHG stage [25]. Both CLBO crystals for SFG were operated successfully for over half a year as well as the CLBO crystal for FHG to 258 nm, during which there was no evidence of UV laser induced degradation. Hence, the generated 193-nm beam was able to maintain stable high power output over long periods of time. Figure 8(b) shows the stability of the output power around 200 mW in free running, over a period of 30 minutes. The observed power fluctuations were caused by the air disturbances due to the air-conditioning system. In addition, room temperature changing would also affect the power stability in a longer time period. Hence, the power stability could be further improved by adding airtight boxes to the laser systems and adding feedback system to maintain the temperature of the laser systems.
Fig. 8 (a)Pulse duration of 193-nm measured by a oscilloscope; Inset: Beam profile of the 193-nm laser; (b) Power stability of the 193-nm laser over 30 minutes.
The linewidth of the pulsed DFB seed laser at 1030 nm and the Er-fiber laser was 1.5 GHz and 300 MHz, reseptively. Narrow linewidths on both laser systems helped to achieve higher conversion efficiency to DUV by using long nonlinear crystals. The linewidth of the DUV 193-nm laser was estimated to be 4 GHz (0.5 pm), which meets the requirement for our target of using this laser as seed source to ArF eximer lasers for the application of interference lithography. Moreover, the total electric power consumption of this narrow-linewidth 193-nm laser was 10% of that of ArF eximer laser oscillator. The electric power consumption could be decreased by 40% if this 193 nm laser is applied to be the seed of the 100 W injecgtion-locking ArF eximer laser.
4. Conclusion
In conclusion, we have developed a high-power, narrow-linewidth DUV coherent laser source at 193 nm based on Yb-hybrid and Er-fiber lasers and frequency-mixing using CLBO crystals. The time-synchronized free-running Yb- and Er-fiber laser systems had simple alignments, thanks to their experimental configurations being free of parametric cavities and feedback parts, and they both demonstrated the ability to operate stably over long periods of time. The generation of DUV laser for long term was achieved without the CLBO crystals showing obvious signs of hydration or UV induced degradation. The maximum output power of the 193-nm laser was 310 mW at 6 kHz repetition rate, corresponding to a pulse energy of 51 μJ. To the best of our knowledge, these are the highest ever reported values for the average output power and the pulse energy of a solid-state or fiber laser based DUV 193-nm laser with narrow-linewidth of about 4 GHz. Not only does this 193-nm laser fulfill the requirements necessary for seeding an injection-locked ArF eximer laser for the application of interference lithography, but we believe it should also considerably decrease the electric power needed for the ArF eximer laser to operate. What is more, full coherent and high-power 193-nm light would be also valuable for a laser machining.
Acknowledgments
This work was financially supported by the New Energy and Industrial Technology Development Organization(NEDO).
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