An all-solid-state, deep-UV light source alternative to excimer lasers is achieved through cascaded high-harmonic and sum-frequency generation of neodymium yttrium vanadate lasers—with the added bonus of high beam quality and stability.
Coherent radiation in the deep-ultraviolet (DUV) spectral range, with a wavelength of less than 200 nm, is of great interest for several applications, such as lithography, metrology, spectroscopy, and the fabrication of fiber Bragg gratings (FBGs).
Most of these applications benefit from a narrow linewidth and a diffraction-limited beam profile. For example, using an interferometric inscription technique, the maximum length of a FBG is limited by the bandwidth of the inscribing laser because of self-apodization. While there are no solid-state laser materials available that provide direct laser emission in the DUV spectral range, argon-fluoride (ArF) excimer lasers provide direct emission at 193 nm at output power levels of more than 100 W, dominating high-power applications such as lithography.
Conversely, all-solid-state DUV light sources offer advantages for low-power applications like metrology and the fabrication of FBGs. These all-solid-state sources are realized by frequency conversion of infrared (IR) lasers and provide superior beam quality and excellent spectral properties compared to ArF excimer lasers, but their realization is still challenging. For metrological applications in the semiconductor industry, such as characterization of complex stepper optics, an emission wavelength of 193.368 nm and a narrow bandwidth are imperative. However, for the fabrication of FBGs, any wavelength in the DUV spectral range below 200 nm is sufficient.
All-solid-state DUV sources are generally based on cascaded second-harmonic generation (SHG), sum frequency generation (SFG), and difference frequency generation (DFG) of IR solid-state lasers. So, the possible concepts—consisting of the choice of fundamental wavelength and the conversion scheme—are defined by the materials that provide phase-matching and transparency in the DUV spectral range.
Readily available nonlinear crystals that provide these necessary properties include barium borate (BBO), lithium triborate (LBO), lithium tetraborate (LTB), cesium lithium borate (CLBO), and more recently, potassium fluoro-beryllo-borate (KBBF).
Because of the low birefringence of BBO, LBO, LTB, and CLBO, DUV radiation below 200 nm can only be obtained by SFG of UV sources above 200 nm using visible or IR radiation with these materials. And because LTB offers only a small nonlinearity, most narrowband all-solid-state DUV sources use BBO, LBO, or CLBO in the last sum frequency mixing stage.
Previously, DUV light sources based on titanium sapphire (Ti:sapphire) master oscillators and power amplifiers, fiber amplifiers, and 1 μm neodymium (Nd) lasers, or a combination of those, have been demonstrated. Because of the reliability and maturity of Nd laser technology, many approaches for an all-solid-state frequency conversion chain to the DUV start with a fundamental wavelength of 1064 nm. However, sum frequency mixing to the DUV between 190 and 200 nm requires a parametric conversion stage, which limits the efficiency and necessitates bandwidth narrowing.