We present a source of line-narrowed continuous-wave (CW) radiation at 193.4 nm with over 10 mW of output power for the first time, to our knowledge. The system configures four successive frequency conversions of outputs from three single-frequency fiber amplifiers at 1064, 1107, and 1963 nm. The 266-nm beam produced by frequency quadrupling of 1064-nm light is sum-frequency mixed with the 1963-nm light to generate 234.3-nm radiation, which is consequently mixed with the 1107-nm light to generate 193.4-nm radiation. Both mixings are achieved in temperature-tuned non-critically phase-matched (NCPM) crystals.
©2011 Optical Society of America
A demand for an increase in integration and density of semiconductor devices has stimulated the development of deep ultra-violet (DUV) lasers. While the 193.4-nm line-narrowed argon fluoride (ArF) excimer lasers have been widely used for illumination light sources of photolithography, true continuous-wave (CW) or quasi continuous-wave (QCW) DUV lasers based on nonlinear frequency conversion of visible or infrared (IR) sources are also playing important roles for inspection tools. In 1982, Hemmati et al. demonstrated the generation of CW DUV light at 194 nm by sum-frequency mixing (SFM) of the 257-nm second harmonic of a 515-nm argon-ion laser with the 792-nm radiation of a dye laser . After that work, there have been a lot of efforts to realize CW or QCW DUV lasers at or near 193 nm [2–4]. One of the most successful approaches to 193 nm would be the eights harmonic generation of a 1547-nm erbium-doped fiber amplifier (EDFA), with which the generation of 140 mW of 193-nm light at a 200 kHz was reported [5, 6]. As far as we know, however, a CW DUV laser at 193.4 nm in vacuum (i.e. 193.3 nm in air) has not been demonstrated yet. The actinic inspection is advantageous in accurate imaging of the defects on masks onto the sensor plane. In addition, the CW laser can be spectrally-narrowed enough to use with a high-NA objective lens requiring small chromatic aberration and is thus favorable for achieving high resolution.
In this paper, we report the generation of line-narrowed CW light at the center wavelength of ArF excimer laser based on frequency conversion of radiations produced by fiber amplifiers. We obtained 11.6 mW of 193.4-nm radiation by mixing only 19 mW of 234.3-nm light with resonantly enhanced 1107-nm radiation from an Yttribium-doped fiber amplifier (YDFA) in a CsLiB6O10 (CLBO) crystal under non-critically phase-matched (NCPM) condition. The 234.3-nm beam was generated by SFM of 1 W of 266 nm from a frequency-quadrupled YDFA at 1064 nm and 2.6 W of 1963 nm from a thulium doped fiber amplifier (TDFA) in a LiB3O5 (LBO) crystal that is also NCPM. We also show the output wavelength can be tuned slightly by adjusting the input wavelength and crystal temperature.
2. Nonlinear optical crystals to generate 193-nm light
In order to obtain 193-nm radiation with a practical power by nonlinear frequency conversion, the crystal must be not only transparent down to 193 nm but also phase-matchable for frequency doubling or SFM process to attain it. As crystals for such an application, CLBO, LBO, BBO (β-BaB2O4), and KBBF (KBe2BO3F2) have been used to date. KBBF is the only crystal in which phase-matching has been reported for the generation of 193-nm radiation by frequency doubling of a fundamental wave at 386 nm . However, there is no prospect of practical use for KBBF crystals which are extremely difficult to grow. To obtain phase-matching for the generation of 193-nm (=λ 3) radiation in the other crystals, it is necessary to rely on SFM of two beams with different wavelengths of λ 1(>2λ 3) and λ 2(<2λ 3):
Further, the phase-matching condition is expressed as a following Eq. (2) in the case of collinear interaction that is usually required for efficient SFM:
Here n 1, n 2, and n 3 are the refractive indices of crystal at wavelengths of λ 1, λ 2, and λ 3, respectively. Table 1 shows combinations of λ 1, λ 2 to generate λ 3 ~193 nm which have been previously reported and applicability of nonlinear optical crystals to each SFM.
Any SFM process listed in Table 1 can be achieved in a BBO crystal. However, BBO has a significant absorption at 193 nm, which essentially makes high-power operation at that wavelength difficult. Both CLBO and LBO have shorter band edges leading to smaller absorptions at 193 nm than those of BBO. Owing to its larger nonlinear coefficient and moderate birefringence, CLBO has ever proven its better capability of high-power DUV generation than LBO [3, 6, 10]. Although combinations of input wavelengths for SFM are limited, use of CLBO is apparently desired for this application. Deki et al reported phase-matched SFM of IR (λ 1 = 1064 nm) and UV (λ 2 = 236.3 nm) radiations to reach 193.4 nm in a CLBO crystal . They had to decrease the temperature of crystal down to −185°C to adjust the refractive index so as to achieve NCPM condition. However, the CLBO crystal is usually utilized with its temperature heated to 120°C or higher as 170°C to prevent deterioration due to hygroscopicity. In order to realize phase-matching at or over 120°C in CLBO, it is obviously required to apply λ 1 that is somewhat longer than 1064 nm and λ 2 that is shorter than 236.3 nm. According to the most reliable Sellmeier’s equations for CLBO , NCPM SFM to generate λ 3 = 193.4 nm is expected under such conditions as λ 1 = 1100 nm and λ 2 = 234.6 nm at 120°C, or λ 1 = 1107 nm and λ 2 = 234.3 nm at 170°C. The CW laser at 1100–1107 nm is available by use of an YDFA. Thus the issue is how to generate the other input wavelength of 234.3-234.6 nm, which possibly needs be relied on another SFM process. If one wants to utilize a matured, 266-nm source obtained by frequency quadrupling of 1064 nm as one of the input sources, the other input wavelength satisfying Eq. (1) is between 1966 and 1987 nm:
The most commonly used Sellmeier’s equations for LBO  produced calculations showing that type-II NCPM SFM is realized in a z-cut LBO crystal at a temperature of 33°C for (3) and 14°C for (4). The IR beam at 1966–1987 nm can be produced by a TDFA. Thus DUV radiation at 193.4 nm is expected to be generated with a 266-nm source by two subsequent SFMs both of which are performed in crystals under NCPM condition. In addition, all of sources can be operated in single-frequency, CW mode, with which externally-resonant SFM could also be configured to enhance conversion efficiency .
A schematic of the CW 193.4-nm laser source is shown in Fig. 1 .
Three single-frequency laser sources were employed. One was a Frequad-HP (Oxide corporation), which generated 1-W output at 266 nm with about 400-kHz linewidth. The source comprises a 1064-nm YDFA with two externally-resonant cavities for frequency quadrupling. One cavity converts 1064 nm to 532 nm in a LBO crystal, and the other successively converts 532 nm to 266 nm in a BBO crystal . The M 2 values were measured as 1.3 in vertical and 1.2 in horizontal directions, respectively. The second laser source was a thulium-doped fiber (TDF) master-oscillator power-amplifier (MOPA) source at 1963 nm. The source produced 2.6 W of IR beam with measured M 2 values of 1.03 for both directions. The third source was an ytterbium-doped fiber (YDF) MOPA generating 2.7 W at 1107 nm with measured M 2 values of 1.3. Because a linewidth of each MOPA output is much smaller than that of 266 nm, the linewidth of mixed wave is approximately estimated as 400 kHz.
The first SFM process, 1963 + 266 -> 234.3 nm was performed by a 60-mm long, uncoated LBO crystal mounted in a holder attached to a thermoelectric element. The big LBO crystal we used had a measured absorption coefficient at 1963 nm of 3.2%/cm, which would not allow use of an external cavity even with a shorter-length crystal as 20 mm. However, such a few percent of absorption could be reduced enough to make it possible with a high-quality crystal. The two beams were made collinear by using a dichroic mirror transmitting 1963 nm while reflecting 266 nm. The polarizations of IR and UV beams were aligned in parallel to X and Y-axis, respectively, for type-II (eo-e) interaction in the XY plane of LBO crystal. Each beam was loosely focused into the LBO for single-passing SFG to form calculated waist sizes of 74 μm for UV and 102 μm for IR. The ξ parameters defined in the famous BK theory  are 1.18 and 0.27, respectively, both of which are smaller than 2.84 of the optimum value for B = 0 (i.e. NCPM). The reduction factor h m for SFM with unequal ξ parameters as above is roughly estimated as 0.5 . The phase matching was observed at a crystal temperature of 22.7°C with a 1963-nm power of 0.09 W. The maximum power of SFM output at 234.3 nm was measured as 24 mW for input powers of 0.99 W at 266 nm and 2.5 W at 1963 nm with the temperature of LBO decreased to 22.1°C so as to compensate heating effects by absorption of IR beam. Considering that the SFG power for the optimum condition (i.e. ξ=2.84 with confocal focusing) is calculated to be 42.7 mW, the obtained power agrees reasonably well with that by taking into account 0.5 of reduction factor described above. The measured M 2 values for SFG beam was 1.9 and 2.0 for vertical and horizontal directions, respectively. The deterioration of beam quality might be due to an effect of absorption of IR light or to different spot sizes of input beams.
The 234.3-nm UV beam was used to sum-frequency mix with resonantly enhanced IR beam at 1107 nm in a 25-mm long, Brewster-cut CLBO crystal to generate DUV radiation at 193.4 nm. The crystal was cut at θ = 90° for type-I (oo-e) NCPM SFM. The enhancement was accomplished with a bow-tie ring cavity that was locked by the Hänsch and Couillaud method . The spot size of both input beams in the crystal was 40 μm in the vertical direction giving ξ parameters of 1.84 for IR and 0.37 for UV, respectively. Again we applied loose focusing because it was difficult to perform SFM with tightly-focused invisible beams in the Brewster-cut crystal with its phase-matching temperature unconfirmed. The UV beam was focused by two uncoated spherical lenses (L4, L5) and the power launched into CLBO was 19 mW. The collimated IR beam was mode-matched into the cavity with another two spherical lenses (L6, L7). In order to make two beams collinear inside the crystal, the UV beam was arranged to have a relative angle of 5.2° with respect to the cavity axis. The 193.4-nm beam generated as an extraordinary ray in the crystal was directly measured as 11.6 mW at a crystal temperature of 111°C. Taking 19.6% of Fresnel reflection at the exit surface into account, the generated 193.4-nm power inside the crystal was 14.4 mW indicating that 68% of 234.3-nm input wave was depleted.
For the generation of 193.4 nm by SFM of 234.3-nm and 1107-nm radiation in NCPM CLBO, the temperature of crystal was needed to be tuned at 111°C while a calculated temperature was 170°C. In order to investigate the cause of discrepancy, we also tried to generate the same wavelength in the same crystal by SFM of 234.7-nm and 1100-nm radiation, in which a predicted phase-matching temperature was 120°C. The phase matching for this SFM was experimentally obtained at 78°C. Figure 2 shows the calculated phase-matching temperature as a function of input wavelength along with the measured results described above. As is shown, there is a significant discrepancy between the numerical and experimental results.
Table 2 shows measured temperatures of NCPM CLBO previously reported by three groups along with calculated values for each case. It should be noted that the measured phase-matching temperatures are lower than the calculated values also for those cases.
As for the temperature of NCPM LBO, we again observed a small discrepancy between the measured and calculated values. We additionally performed SFM of 1991 nm with 266 nm to generate 234.7 nm. A fiber-coupled DFB laser at 1991nm was applied to the master oscillator in the MOPA system. While the calculated NCPM temperature is 37.3°C, we observed phase-matching at 45.3°C. Figure 3 shows measured and calculated phase-matching temperatures as a function of input wavelength with the other one fixed at 266 nm. On the contrary to CLBO, the measured values were higher than the calculations.
Two results show that the accurate prediction of temperature for NCPM SFM is not straightforward for some cases even with proven temperature-dependent dispersion equations. They also indicate the source can be tuned to a specific wavelength around 193.4 nm.
We have demonstrated the generation of line-narrowed CW DUV radiation at 193.4 nm with over 10 mW of output power converted from a matured source at 266 nm. The source consisted of single-frequency, all- fiber lasers and frequency conversion stages. The 193.4-nm beam was generated by SFM of resonantly enhanced 1107-nm radiation with a 234.3-nm beam in a Brewster-cut CLBO in NCPM condition. Over 20 mW of 234.3-nm light has been generated just by single-passing SFM of two beams at 266 and 1963 nm in a NCPM LBO. It appears that using an externally-resonant SFM configuration with an improved quality of LBO crystal would easily boost the 234.3-nm power leading to a CW 193.4-nm source with several hundreds of milli-watts of output power.
The authors greatly thank Prof. Yushi Kaneda at University of Arizona and Dr. Yasunori Furukawa of Oxide Corporation for helpful advice and discussion.
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