Abstract

X ray production in the region around 130 Å from laser-produced plasmas has been investigated for using them as a possible source for projection x-ray lithography. The dependence of production efficiency on target material, intensity, and pulse length has been extensively studied using a 0.53 µm wavelength laser with a maximum output of 0.3 J. Production efficiency of 1% into a 3 Å bandwidth has been demonstrated from a Sn target for intensities around 10¹¹ W/cm² using an 8 ns pulse. Absolute x-ray production in the region around 130 Å is measured using two different techniques for wavelength discrimination. A broad band channel defined by a grazing incidence carbon x-ray mirror and a Be transmission filter coupled with a Si XUV solid state diode measures x-rays in the 177 Å to 111 Å region. We also measure x-ray production in a narrow band around 130 Å defined by a near normal incidence synthetic multilayer mirror. Filters, mirrors, and detectors have been absolutely calibrated at synchrotrons to obtain absolute yields. In addition, we measure spectra using a transmission grating spectrometer. The spectra show that a relatively narrow feature in Sn near 130 Å enhances x-ray production making this element an optimum choice for this wavelength region. The intensity scaling data suggest that laser spot size and two-dimensional expansion of the plasma play an important role in optimizing x-ray production for these sub-joule irradiation conditions. In future experiments we plan to extend these measurements to larger spot sizes using a higher power laser to better understand the effects of two-dimensional plasma expansion on x-ray production. We also plan to extend these studies using a 1 µm laser to investigate the wavelength dependence on conversion efficiency.

© 1992 Optical Society of America