Online

Hurley, David

Mechanical Engineering

2008

MS

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 objective of this project was to develop and implement a tool for the study of the size, amount, and velocity spectrum of high speed particulate debris. The measurements used a source-laser-pulse-synchronized high speed spinning disc. An optical scanning boom microscope analyzed debris collected on a target. Debris target imagery was analyzed using an image processing and pattern recognition program. This provided an unbiased assessment of debris accumulation. The position of debris particles was used to determine their velocity using kinematic triangulation.
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 accuracy of these results was compromised by multi-pulse aliasing. The new understanding of debris velocities suggests reconsideration of a host of 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 the low speed debris generated during the previous laser pulse. The target location and laser spot size and focal point are critical elements in the plasma generation process and were found to have a low tolerance for variation. This finding identified the mechanics of plasma generation and parameter control as areas requiring further refinement and study.