Development of high coherence, 200mW, 193nm solid-state laser at 6 kHz

The high coherent, high power 193-nm ArF lasers are useful for interference lithography and microprosessing applications. In order to achieve high coherence ArF lasers, we have been developing a high coherence 193 nm solid state laser for the seeding to a high power ArF laser. We used the sum frequency mixing of the fourth harmonic (FH) of a 904-nm Ti:sapphire laser with a Nd:YVO₄ laser (1342 nm) to generate 193-nm light. The laser system consists of a single-mode Ti:sapphire oscillator seeded by a 904-nm external cavity laser diode, a Pockels cell, a 6-pass amplifier, a 4-pass amplifier, a 2-pass amplifier and a wavelength conversion stage. The required repetition rate of 6 kHz corresponding to the ArF laser, along with a low gain at 904 nm induces serious thermal lens effects; extremely short focal lengths of the order of cm and bi-foci in the vertical and horizontal directions. From the analysis of thermal lens depending on pump intensity, we successfully compensated the thermal lens by dividing a 527-nm pump power with 15, 25 and 28 W to 3-stage amplifiers with even passes, resulting in the output power above 10W with a nearly diffraction limited beam. This 904-nm output was converted to 3.8 W in the second harmonic by LBO, 0.5 W in FH by BBO sequentially. Finally the output power of 230 mW was obtained at 193 nm by mixing the FH with a 1342-nm light in CLBO.