DUV Lasers Made with Nonlinear Crystals Enhance Lithography Performance

Argon-fluoride (ArF) excimer lasers — deep UV (DUV) lasers emitting light with a wavelength of 193 nm — are used in lithography to create precise patterns. However, the limited coherence of conventional ArF excimer lasers hinders their effectiveness in applications requiring high-resolution patterns, like interference lithography.

To generate highly coherent DUV for interference lithography and other applications, researchers at the Chinese Academy of Sciences (CAS) developed a hybrid ArF excimer laser that delivers both narrow linewidth and high coherence at 193 nm. In the hybrid ArF excimer laser, the ArF oscillator has been replaced with a narrow linewidth, solid-state, 193 nm laser seed. The laser seed, which exhibits good beam quality, improves coherence and maintains high output power in the hybrid ArF excimer laser system.

The hybrid ArF excimer laser’s intensified photon energy and coherence facilitate the direct processing of various materials, including carbon compounds and solids, with minimal thermal impact. This versatility could extend the usefulness of the hybrid ArF excimer laser to a range of applications, from lithography to laser machining.

Researchers at the Chinese Academy of Sciences developed a 193 nm DUV laser generated by cascaded lithium triborate crystals. Courtesy of H. Xuan, GBA branch of Aerospace Information Research Institute, Chinese Academy of Sciences.

The conventional way to build a DUV source based on a solid-state laser is through multistage frequency conversion from the NIR, using solid or fiber lasers as pump sources. In addition to a pump laser, a nonlinear optical crystal is used to achieve a DUV source that is reliable and economical.

The 193 nm laser developed by the researchers is based on two stages of cascaded sum frequency generation in lithium triborate (LBO) nonlinear crystals between an erbium (Er)-doped fiber laser and a ytterbium (Yb)-hybrid laser. The low-cost, low-complexity LBO crystal is considered a good candidate for 193-nm lasers for industrial applications.

The pump light of the 193 nm, narrow linewidth, nanosecond-pulsed laser is generated from two actively synchronized pulsed lasers. The pump lasers, at 258 and 1553 nm, are derived from a custom built Yb-hybrid laser that uses fourth-harmonic generation and an Er-doped fiber laser, respectively. A 2 mm × 2 mm × 30 mm Yb:YAG bulk crystal for power scaling completes the setup.

The maximum output power of the laser at 193 nm and at 221 nm is 60 mW and 220 mW, respectively.

To the best of the researchers’ knowledge, the 193 nm DUV laser demonstrates the highest average power ever reported for both 193 nm and 221 nm lasers by frequency mixing in LBO crystals. The conversion efficiency is 27% for 221 nm to 193 nm and 3% for 258 nm to 193 nm, which is the highest DUV conversion efficiency ever reported using LBO, according to the team.

The 193 nm laser has a pulse duration of 4.6 ns and a repetition rate of 6 kilohertz (kHz), corresponding to a pulse energy of 10 μJ. The linewidth of the 193-nm laser is estimated to be about 640 megahertz (MHz), which is the narrowest linewidth from a solid-state, pulsed laser using LBO crystal reported so far, according to the team.

The laser output is suitable for seeding ArF excimer amplifiers to generate highly coherent DUV for lithography. Although the LBO crystal has a small nonlinear coefficient in the DUV region, it could be grown and cut to a large dimension while still maintaining a low cost, which allows for compensation through a longer interaction length.

Professor Hongwen Xuan said that the results of the research confirm the viability of pumping LBO with solid-state lasers to achieve reliable, effective generation of a narrow linewidth laser at 193 nm. The results could lead to a way to fabricate a cost-effective, high-power DUV laser system using LBO. The research shows the potential of LBO crystals to generate DUV lasers at power levels ranging from hundreds of mW to W, and could encourage exploring the use of LBO to generate lasers at other DUV wavelengths.

Coherent light sources in the DUV region are used in defect inspection, metrology, and spectroscopy, in addition to lithography. By pushing the boundaries of DUV laser technology, the 193 nm DUV laser from CAS could benefit many applications across the scientific and industrial domains.

The research was published in Advanced Photonics Nexus (www.doi.org/10.1117/1.APN.3.2.026012).