Experimental Study on the Temporal Evolution Parameters of Laser–Produced Tin Plasma under Different Laser Pulse Energies for LPP–EUV Source

Abstract

The laser–produced plasma extreme ultraviolet (LPP–EUV) source is the sole light source currently available for commercial EUVL (extreme ultraviolet lithography) machines. The plasma parameters, such as the electron temperature and electron density, affect the conversion efficiency (CE) of extreme ultraviolet radiation and other critical parameters of LPP–EUV source directly. In this paper, the optical emission spectroscopy (OES) was employed to investigate the time–resolved plasma parameters generated by an Nd:YAG laser irradiation on a planar tin target. Assuming that the laser–produced tin plasma satisfies the local thermodynamic equilibrium (LTE) condition, the electron temperature and electron density of the plasma were calculated by the Saha–Boltzmann plot and Stark broadening methods. The experimental results revealed that during the early stage of plasma formation (delay time < 50 ns), there was a significant presence of continuum emission. Subsequently, the intensity of the continuum emission gradually decreased, while line spectra emerged and became predominant at a delay time of 300 ns. In addition, the evolution trend of plasma parameters, with the incident laser pulse energy set at 300 mJ, was characterized by a rapid initial decrease followed by a gradual decline as the delay time increased. Furthermore, with an increase in the incident laser pulse energy from 300 mJ to 750 mJ, the electron temperature and electron density of laser–produced tin plasma exhibiting a monotonically showed increasing trend at the same delay time.

Research on High-Precision Position Detection Based on a Driven Laser Spot in an Extreme Ultraviolet Light Source

“…In the EUVL process, the best way to produce extreme ultraviolet light is laserproduced plasma (LPP) [6], that is, a high-power, high repetition frequency, narrow-pulse width CO 2 laser is used to bombard a droplet tin-target, which is filtered and collected to produce 13.5 nm extreme ultraviolet monochromatic light [7][8][9]. This requires that the driving-laser pointing stability is accurate so that the CO 2 laser energy is efficiently concentrated on a tin droplet with a diameter of tens of µm to improve the 13.5 nm optical conversion efficiency.…”