Plasma and X-UV Source Characteristics for Al Targets Heated by 40 Nsec Nd-laser Pulses.
NRL Memorandum rept.
NAVAL RESEARCH LAB WASHINGTON DC
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The total laser energy 3-32 J, temporal history 40 nsec FWHM and focal energy distribution 3-10exp 13 Wsq cm peak, 1.5 x 10exp 13 Wsq cm average for lambda 1.06 microns Ndglass laser interactions with planar Al targets were measured. Laser-produced plasma emissions within the 10 eV - 3 keV ultraviolet and x-ray range were recorded. Primary emphasis was given to obtaining the dependence of kilovolt x-ray emission on irradiance varied by altering the energy on target or the lens-target spacing. A maximum efficiency for conversion of incident laser energy to radiation above 1.5 keV was 0.25 into 2 pi sr. Emitted x-ray intensity over this range of photon energy was found to increase as the 3.2 power of laser energy at best focus. X-ray emission decreased when a prepulse preceded the main pulse by a controlled amount exceeding 0.2 microseconds but 5 microseconds, with a minimum emission at 0.5 microseconds separation. Plasma temperatures were estimated from x-ray line intensity ratios and continuum slopes a value near 230 eV was obtained from free-bound continuum slope for 3 x 10exp 13 Wcu cm peak irradiance. Temperatures decreased at lower irradiances, as inferred from x-ray line ratios. This report covers systematic experimental measurements of laser plasma x-ray output. The work formed the basis for the later demonstration of microlithography with this same x-ray source, in support of the NRL patent for laser plasma x-ray lithography. The patent was licensed by a 100-person startup company, Hampshire Instruments, Inc., who offered a technically viable method to achieve submicron critical linewidths for the production of integrated circuits.
- Atomic and Molecular Physics and Spectroscopy
- Plasma Physics and Magnetohydrodynamics