Dynamics and manipulation of the dominant 13.5 nm in-band extreme ultraviolet emitting region of laser-produced Sn plasmas

Laser-produced Sn plasmas have been extensively studied as an efficient soft x-ray light source for next generation high volume manufacturing of semiconductor microchips. One of the main areas of focus is to increase the conversion efficiency, laser to 13.5 nm 2% bandwidth extreme ultraviolet (EUV) light. Previous experiments have been conducted without a tight control of the initial parameters and are not thoroughly diagnosed, leading to assumptions that only exist under unique conditions and/or parameter spaces. This becomes important, as a new need for a much smaller EUV light source for metrology has emerged. The focus is the size of the dominant in-band EUV emitting region, which is not typically measured. The target geometry, laser focal spot size and laser intensity are numerically and experimentally investigated focusing on the underlying physics that governs the ability to modify the size and properties of the EUV emitting region.;A combination of experimental data coupled with numerical simulations are used to show the impact of a 1.064 mum laser's absorption depth on the heating and EUV emission of a Sn plasma. Most notably, the core of the plasma is "overheated" beyond that required to efficiently emit EUV light, to allow the optimal temperature to expand to a region with lower density that is more optically thin.;The ratio of Sn sphere diameter to laser focal spot size did not change the size of the EUV emitting region. However, the electron densities in the emitting region as well as the surrounding plasma were manipulated by target geometry.;Reducing the laser focal spot size from one that is required for lithography to one that may be used for metrology produces a shorter electron density scale length due to increase lateral expansion. This generates a higher conversion efficiency, smaller and slightly brighter EUV light source.;Varying the laser intensity using a small laser spot size affected the peak electron temperature and the size of the EUV emitting region. The amount of lateral expansion also varied with the laser intensity, thus affecting the brightness, conversion efficiency and duration of emission of EUV light.