We have obtained an average output power as much as 1.2 W at 200 nm by using a 2.71-mm thick KBe2BO3F2 crystal optically contacted by CaF2 and SiO2 prisms on the both sides. Watt-level average-power was generated tunably in the deep ultraviolet region from 185 nm to 200 nm. The average power is the highest, to our knowledge, ever obtained by nonlinear crystals in the wavelength region below 200 nm.
©2009 Optical Society of America
Coherent light sources in the deep ultraviolet (DUV) and vacuum ultraviolet (VUV) have been required for photo-lithography and photoelectron spectroscopy. An 177-nm source was successfully applied to high resolution photoelectron spectroscopy . In particular, ArF lasers have become a main source for the 45-nm node lithography for semiconductor processing . For ArF lithography, coherent light sources besides ArF lasers are required for the inspection of mask patterns, and the potential use as seeding to ArF lasers.
The DUV and VUV coherent light sources can be realized by the frequency conversion with nonlinear crystals. As for the nonlinear crystals, (i) wide transparency range to the shorter wavelength side, (ii) large birefringence to achieve the angle-tuned second harmonic generation (SHG), and (iii) sufficiently large nonlinear susceptibility are entirely important. An LiB3O5 (LBO) crystal, for example, can transmit the VUV light of 160 nm, while it requires an infrared laser field for the sum-frequency generation (SFG) due to the relatively small birefringence . Even though the large nonlinear susceptibility of a β-BaB2O4 (BBO) crystal is feasible to generate DUV~VUV light with the SFG, we should take care to lower the temperature because the wavelength of the generated light in near the absorption edge  KB5O8·4H2O (KB5) has a limitation due to a relatively small birefringence similar to LBO . Among nonlinear optical crystals, KBe2BO3F2 (KBBF) crystal has an obvious advantage. KBBF exhibits transmission of VUV light down to a wavelength of 155 nm, and we generated 156 nm by sum-frequency mixing . The large birefringence allowed the shortest SHG wavelength (170 nm) . In addition, the nonlinear susceptibility is relatively large .
As reported by Chen et al. [8–10], KBBF crystal is the only nonlinear optical crystal so far that is able to produce VUV coherent light through SHG. However, the bulk KBBF crystal has a plate-like form along the z-axis of the crystal, so a special prism coupling technique [8,9] is necessary to avoid cutting the crystal along the phase-matching direction for producing VUV harmonic generation beyond 200 nm. Chen and Watanabe’s groups first achieved 6th harmonic generation (177.3 nm) of a 1064-nm Nd:YVO4 laser by using an optically-contacted KBBF-CaF2 prism-coupled device (KBBF PCD) . Later, we reported an effective average output power of about 4.5 mW at 197 nm just by SHG from 394 nm to 197 nm using a similar device . At that time the thickness of the KBBF crystal was only 1.2 mm, so the conversion efficiency from 394 nm to 197 nm was only about 0.64%. In the previous paper, we have reported to generate an average output power as much as 360 mW at 200 nm from an input power of 4.7 W by using a 2.3-mm thick KBBF PCD . The conversion efficiency from 400 nm to 200 nm was 7.7%.
In this paper, we report that watt-level VUV tunable light source was produced by KBBF-PCD as a fourth harmonic of Ti:sapphire laser. A new prism-coupled device with a 2.71-mm thick KBBF crystal has been newly fabricated. The quality of optical contact was considerably improved. The largest thickness (2.71 mm) improved the conversion efficiency significantly for the picosecond parallel beam. By this device together with a high average power Ti:sapphire laser (16 W at 800 nm) we have produced an average power as much as 1.2 W at 200 nm with a repetition rate of 5 kHz. Furthermore we obtained an output average power of 1.05 W at 193.5 nm, and sub-W at 188 and 185 nm.
2. Optically-contacted, KBBF prism-coupled device
Figure 1 is the optically-contacted SiO2-KBBF-CaF2 prism-coupled device. The crystal plate has a 2.71 mm thickness and was stacked by calcium fluoride and quartz prisms with an apex angle of 55°. The damage threshold of quartz is higher than that of calcium fluoride. Therefore, the quartz prism was placed at an incidence side (right). This may be because the fused quartz can be polished more perfectly. The use of CaF2 in the exit is inevitable due to the absorption in quartz. Both the KBBF and prism surfaces were polished to a flatness better than λ/10 at 632.8 nm with a surface roughness < 2 nm. The interfaces between the KBBF and prism surfaces were optically contacted. The total transmittance of the device at 632.8 nm is about 88%. “Optical contact” refers to the molecular adherence between the prism and KBBF surfaces and indeed after they are contacted we cannot separate them. The aluminum clamping is only to protect the KBBF prism-coupled device and to prevent KBBF from cleavage.
3. Generation of the fourth harmonic at 197 nm by a femtosecond laser
We first tried to generate DUV light as a fourth harmonic of a femtosecond Ti:sapphire laser.
Figure 2 shows a setup of a Ti:sapphire chirped pulse amplification (CPA) laser system to produce the fourth harmonic at 197 nm. The system consists of a 130-fs oscillator (IMRA femtolite-780 model: A-10-WS), an Offner type stretcher, a regenerative amplifier, a multi-pass amplifier, a grating pair compressor and a BBO crystal for frequency doubling. The repetition rate is 1 kHz. The center wavelength of the oscillator is fixed to 788 nm.
The output power of the oscillator is 5 mW at 47.2 MHz. The 130-fs pulses are stretched temporarily to 200 ps by the pulse stretcher. These stretched pulses were amplified over 106 times by the regenerative amplifier. The output power of the regenerative amplifier was 300 mW at 1 kHz. The pulses were sliced by an Ultrafast Pockels cell (the rise time 200 ps, fall time 1.5 ns)(Leysop, UPC068) to eliminate the temporal pedestal which comes from amplified spontaneous emission of the regenerative amplifier. The pulses were amplified up to 6 W by the multi-pass amplifier. The output beam was compressed by a grating-pair compressor, and the final average output power at 788 nm became 3 W. The amplifiers were pumped by 4 frequency-doubled, Q-switched Nd:YLF lasers (Quantronix, 527DR). Among them two 12-W lasers were used to pump the regenerative amplifier and final amplifier, and another two 18-W lasers were used to pump the multi-pass amplifier and final amplifier as shown Fig. 2.
The fundamental 788-nm beam was directed to a 0.5-mm thick type-I BBO crystal without any focusing to produce a 394-nm beam, resulting in the maximum average output about 1 W. The diameter of the second harmonic (SH) beam was about 4.7 mm. The spectra of the fundamental (788 nm) and second harmonic (394 nm) are shown in Fig. 3 .
The second harmonic was then directed to the KBBF PCD, also without focusing. An average output power as high as 68 mW was obtained at 197 nm from an input power of 1 W. For a power level of 1 W, the conversion efficiency from 394 nm to 197 nm was 6.8%. Figure 4 shows 197-nm output power and conversion efficiency vs. 394-nm input power. In this experiment, we use a femtosecond laser as a pump laser. If we use the laser with a longer pulse (narrow band) width to avoid the group velocity mismatch in the KBBF crystal, the conversion efficiency will be increased. The pulse duration of the fundamental was measured by single-shot autocorrelator and was 130 fs. This is the transform-limited pulse, because the time-band width product Δν·Δτ = 0.441 calculated with a spectrum width of Fig. 3(a) and a pulse duration of 130 fs. The group velocity mismatch in the 0.5-mm thick BBO crystal is ~70 fs, then the SH pulse duration becomes ~200 fs. The decrease of the conversion as increasing input power may be due to two photon absorption of KBBF in addition to linear absorption of impurity because the conversion efficiency increases in a picosecond laser as shown later in Fig. 6 . The group velocity mismatch of the KBBF crystal is calculated to ~500 fs/mm by using the newest Sellmeier Eq. (7). The optical path length in the 2.71-mm thick KBBF crystal is 4.6 mm by the phase matching angle of 54° at 197 nm. As a result, the group velocity mismatch of the KBBF became 2 ps. This is the main reason why the conversion efficiency was relatively poor. In order to improve the frequency conversion efficiency of the KBBF crystal to obtain higher VUV optical power, a picosecond pulse laser is required.
4. Generation of watt-level VUV light below 200 nm by a picosecond laser
A picosecond laser was prepared to produce high power VUV light source. Figure 5 shows a layout of a picosecond Ti:sapphire laser system . The system consists of a 350-ps oscillator (Spectra Physics, Tsunami), a regenerative amplifier and a multi-pass amplifier. The repetition rate is 5 kHz. The wavelength of the present oscillator is tunable from 800 nm to 740 nm.
The output of the oscillator, typically 600 mW of power at a repetition rate of 80 MHz is used to seed the regenerative amplifier. This supplies just seed pulses to the regenerative amplifier. The output of the regenerative amplifier was 1.5 W at 5 kHz. The final average output power, after the multi-pass amplifier, was 16 W. The amplifiers were pumped by a LD pumped, frequency-doubled, Q-switched Nd:YAG laser (Mitsubishi MEL-Green 100). The absolute wavelength and spectral width were measured by means of a wavemeter (High Finesse Model WS/6L). The spectral width was 2.8 pm at the center wavelength of 800 nm in agreement with the measured pulse width of 340 ps in the transform limit. The spatial beam profile is Gaussian and M2 is about 1.4 both in the s- and p-plane .
The fundamental 800-nm beam was directed to a 20-mm thick LBO crystal cut for type-I SHG phase-matching without any focusing to produce SH, resulting in the maximum average output about 8 W. The second harmonic was then directed to the KBBF prism-coupled device, also without focusing. The SH beam diameter was about 3 mm and the peak intensity was 90 MW/cm2 where the pulse width was 250 ps at 400 nm. An average output power as high as 1.2 W was obtained at 200 nm from an input power of 8 W. At a power level of 8 W, the conversion efficiency from 400 nm to 200 nm was 15%, which is the highest to date for a high repetition rate Ti:sapphire laser system operating at 400 nm. The walk off angle of KBBF is 4.2°. The optical path length in KBBF is 4.6 mm, resulting the spatial walk off of 190 µm at the exit surface. This displacement is negligible to a 3-mm diameter beam.
The fundamental wavelength was changed to 774 nm to generate 193.5 nm (ArF) by fourth harmonic (4ω). The maximum output power reached to 1.05 W at 193.5 nm as shown in Fig. 6.
We generated two other wavelengths in the VUV region by the shorter fundamental wavelengths (752 nm and 740 nm). The combination of mirrors in the amplifier was changed to generate these wavelengths. The output power was 0.72 W at 188 nm, and 0.2 W at 185 nm, respectively. To avoid the absorption by oxygen, this experiment was conducted in a nitrogen atmosphere.
Figure 7 shows the output power of 4ω below 200 nm and input power of corresponding second harmonic (2ω) vs. wavelength of 4ω (lower axis) and 2ω (upper axis). The wide wavelength region below 200 nm was tunably covered at watt-level (from 200 nm to 190 nm) and sub-W level (from 190 nm to 185 nm). The generation of the shorter fundamental wavelength is possible down to ~700 nm. However the generation of 4ω was not attempted because the apex angle (55°) of the prisms in the KBBF-PCD corresponds to the phase matching at 193.5 nm for the normal incidence to the entrance prism surface, and the shorter wavelength beam, which need the large incidence angle to the prism surface, cannot go through the 5 mm-window of the KBBF-PCD at the phase-matching condition. If the apex angle of the prisms becomes larger, the shorter wavelength can be generated. We believe the average power is the highest ever obtained by nonlinear crystals in the wavelength below 200 nm.
We have demonstrated the high average power, watt-level tunable VUV light source by the fourth harmonics of the Ti:sapphire laser. The fourth harmonic was obtained by the optically contacted, KBBF prism-coupled device consisting of a 2.71-mm thick KBBF crystal stacked by CaF2 and quartz prisms. The output powers were 1.2 W at 200 nm, 1.05 W at 193.5 nm, 0.72 W at 188 nm, and 0.2 W at 185 nm, respectively. The conversion efficiency from 400 nm to 200 nm reached to 15%. The average powers are the highest, to our knowledge, ever obtained by nonlinear crystals below 200 nm. These results can be applied to laser processing and inspection in the DUV and VUV such as ArF photolithography, photoelectron spectroscopy, mastering of optical discs (Compact Disk or Digital Versatile Disk).
References and links
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