J. R. Freeman

Dynamics of femto- and nanosecond laser ablation plumes investigated using optical emission spectroscopy

We investigated the spatial and temporal evolution of temperature and electron density associated with femto- and nanosecond laser-produced plasmas (LPP) from brass under similar laser fluence conditions. For producing plasmas, brass targets were ablated in vacuum employing pulses either from a Ti:Sapphire ultrafast laser (40 fs, 800 nm) or from a Nd:YAG laser (6 ns, 1064 nm). Optical emission spectroscopy is used to infer the density and temperature of the plasmas. The electron density (ne) was estimated using Stark broadened profiles of isolated lines while the excitation temperature (Texc) was estimated using the Boltzmann plot method. At similar fluence levels, continuum and ion emission are dominant in ns LPP at early times (<50 ns) followed by atomic emission, while the fs LPP provided an atomic plume throughout its visible emission lifetime. Though both ns and fs laser-plasmas showed similar temperatures (∼1 eV), the fs LPP is found to be significantly denser at shorter distances from the target su...

Enhancements of extreme ultraviolet emission using prepulsed Sn laser-produced plasmas for advanced lithography applications

Laser-produced plasmas (LPP) from Sn targets are seriously considered to be the light source for extreme ultraviolet (EUV) next generation lithography, and optimization of such a source will lead to improved efficiency and reduced cost of ownership of the entire lithography system. We investigated the role of reheating a prepulsed plasma and its effect on EUV conversion efficiency (CE). A 6 ns, 1.06 μm Nd:yttrium aluminum garnet laser was used to generate the initial plasma that was then reheated by a 40 ns, 10.6 μm CO2 laser to generate enhanced EUV emission from a planar Sn target. The effects of prepulsed laser intensity and delay timings between the prepulsed and the pumping pulse were investigated to find the optimal pre-plasma conditions before the pumping pulse. The initial optimization of these parameters resulted in 25% increase in CE from the tin LPP. The cause of increased EUV emission was identified from EUV emission spectra and ion signal data.

Laser wavelength dependence on angular emission dynamics of Nd:YAG laser-produced Sn plasmas

We investigated the laser wavelength effect on angular atomic and ionic emission from laser-produced Sn plasma, since it is regarded as a viable candidate for an EUV lithography source. For producing plasmas, the fundamental, second and fourth harmonics radiation from a Nd?:?YAG laser were used. The angular variation of atomic and ionic particle analysis was carried out using quartz crystal microbalance and Faraday cups by moving them in a circular path at a constant distance from the target normal. Along with atomic and ionic emission, we also compared the plasma emission features in the visible and EUV spectral regions. Results indicate strong forward bias in atomic and ionic plasma debris at all wavelengths. Shorter wavelength plasmas are found to generate more atomic particles while ion flux showed a similar trend irrespective of the excitation wavelength.

Wavelength dependence of prepulse laser beams on EUV emission from CO2 reheated Sn plasma

Extreme ultraviolet (EUV) emission from laser-produced plasmas (LPP) centered at 13.5 nm is considered a leading candidate for the light source in future lithography systems. Tin is currently the best material for generating this EUV emission since it emits strongly within the 13.5 nm region due to its various ionic states (Sn8+-Sn14+). Highly efficient and low-debris LPPs are a pre-requisite for their use as light sources for EUV lithography. Tin plasmas generate debris that can damage collection optics over time. Techniques to mitigate debris are needed to extend the lifetime of these components and the system. Optimization of plasma conditions is necessary for increasing EUV emission and enhancing conversion efficiency (CE). Improving the source CE is necessary in order to reduce the cost of ownership and hence, develop a commercially viable lithography system for the semiconductor industry. One method to accomplish this is to reheat pre-formed plasma with a laser pulse to enhance EUV emission. This enhancement is achieved by controlling those plasma conditions necessary for optimizing EUV emission. We investigated the role of prepulse laser wavelength on prepulse plume formation and EUV in-band signal enhancement. A 6 ns Nd:YAG laser operating at 1064 nm and 266 nm was used for generating the prepulse plume. The expanding plume was then reheated by a 35 ns CO2 laser operating at 10.6 μm. The role of prepulse wavelength and energy on EUV conversion efficiency is discussed.

Effects of pre-pulses on extreme ultraviolet conversion efficiency in laser-produced tin plasmas

Extreme ultraviolet (EUV) lithography is being considered for manufacturing the next generation of computer chips. However, a suitable source for EUV emission at 13.5 nm must be identified. The source must be able to provide reliable, clean, and powerful EUV emission at 13.5 nm with 2% bandwidth and be capable of meeting the demands of high volume manufacturing. Laser produced plasmas (LPP) have emerged as a promising source for EUV emission, but higher in-band conversion efficiency (CE) and debris control must first be realized.1 Tin is considered the material of choice for producing this plasma, as its plasma emits strongly in the EUV in-band region, contributed by various ionic stages (Sn8+ – Sn14+). However, the net emission of 13.5 nm photons is controlled by plasma opacity, which depends on level populations of different ionic states, ionization balance, and electron density. For obtaining the highest CE, ideal plasma temperatures and densities should be created for the longest possible period of time with the maximum collectable size.2 Reheating of a pre-formed plasma is one of the methods for controlling density and hence optimizing plasma opacity for maximum EUV emission.