Characterization and control of laser plasma flux parameters for soft-x-ray projection lithography.

Laser plasmas are intrinsically an attractive soft-x-ray source for projection lithography. Compact, flexible, and small enough to be dedicated to a single installation, they offer an alternative to costly multi-installation synchrotron sources. For laser plasmas to provide ideal sources of soft x rays for projection lithography, their properties must be tuned to optimize several critical parameters. High x-ray conversion in the spectral band relevant to projection lithography is obviously required and has already received the attention of several studies. However, other features, such as the spectral content and direction of the x-ray emission, the plasma and particulate emission, the technology of the target, and efficient laser design, must also be optimized. No systematic study of all these features specifically for projection lithography has yet been made. It is our purpose to optimize these parameters in a coordinated approach, which leads to the design of a source that satisfies all the demanding requirements of an operating lithographic installation. We make an initial investigation of the plasma and particle emission of plasmas that have previously been shown to be good x-ray converters to the 13-nm band. The importance of the results reported may well force new approaches to the design of laser plasma soft-x-ray sources for projection lithography.