Laser copper plasma X-ray source debris characterization

Laser copper plasma sources are a compact, economical means of producing high intensity X-rays at the correct wavelengths for X-ray lithography. Copper debris in the form of vapor, ions, dust, and high speed particles is an unwanted byproduct of the laser copper plasma technique. Improved methods for debris mitigation are essential for production X-ray lithography using laser copper plasma sources. The main objective of this project is to characterize the debris, particularly the size, amount, and velocity spectrum of high speed particles. The measurements used a source-laser-pulse-synchronized high speed spinning disc. An optical scanning boom microscope analyzed debris collected on a target. Kinematic triangulation using the location of the debris indicates the velocity of debris particles. Velocities of copper debris particles were found to be in the hundreds of meters per second, roughly one order of magnitude slower than previously believed. The precision of results was limited by a mismatched sampling rate for particle collection. This discovery opens up the possibility of a host of mechanical debris blocking countermeasures previously thought to be too slow to effectively stop high speed debris particles. This study also suggests that X-rays emitted at high laser pulse rates could be blocked by debris generated during the previous laser pulse. An area requiring further refinement and study is in the mechanics of plasma generation. The target location and laser spot size and focal point are critical factors in the process and have a low tolerance for variation.