What is an Excimer Laser?

Excimer lasers are a class of lasers that operate in the ultraviolet (UV) spectral region and produce nanosecond pulses. Their gain medium consists of a gas mixture containing rare gases (such as argon, krypton, or xenon) and halogens (such as fluorine or chlorine/hydrogen chloride), along with helium and neon as buffer gases. The excimer gain medium is typically pumped by high-voltage discharges—or occasionally by short (nanosecond) current pulses from an electron beam—to create so-called excimers (excited dimers). These molecules exhibit a bound state only in their excited electronic states, while remaining unbound in the electronic ground state.

Pulse Parameters, Beam Quality, and Power Efficiency

Excimer lasers are unable to achieve continuous-wave (CW) operation, partly due to the inability to maintain a stable discharge with suitable characteristics. Pulse durations are typically a few nanoseconds, though they can occasionally reach approximately 100 nanoseconds. A typical excimer laser emits pulse energies ranging from 10 mJ to 1 J. Repetition rates vary from as low as 10 Hz to 1 kHz or higher, resulting in average output powers ranging from less than 1 W to several hundred watts. Consequently, excimer lasers are the most powerful laser sources in the ultraviolet region, particularly at wavelengths below 300 nm.

The beam quality of excimer lasers is generally low, as achieving high beam quality is difficult given the short resonators, large gain volumes, and rapid pulse buildup. Before such beams can be utilized in applications, a beam homogenizer is typically required. Without specific measures, the emission linewidth is usually on the order of 1 nm; however, by using wavelength-selective elements like diffraction gratings within the laser resonator, this can be reduced to well below 1 pm.

Device Lifespan

The lifespan of early excimer lasers was quite limited due to various issues, such as the corrosiveness of the gases used, ablation of electrode materials, degradation of optical materials by intense UV light, and contamination of the gas by chemical by-products and dust generated during discharge. The latter issue is typically resolved by periodic replacement of the gas mixture, for example, every 30 million pulses.

Maintaining electrodes and optical components in good working condition has required the development of complex measures. Many engineering designs, such as the use of corrosion-resistant materials and advanced gas recirculation and purification systems, have largely mitigated the inherent challenges of the excimer laser concept. The lifespan of modern excimer lasers is primarily limited by the durability of the UV optical components, which must withstand high-flux, short-wavelength radiation for approximately billions of pulses.