Optical Inspection of EPL Stencil Masks

摘要

We are now at a major junction in lithography where non-optical lithographies, such as Electron Projection Lithography (EPL), are being introduced. The mask used in EPL is a silicon substrate with a thin silicon (～2μm) membrane, with openings for electrons to pass through, acting as a scatterer. This must be inspected as mask defects may print effecting pattern fidelity. Initial mask inspection work has used SEM inspection to find these defects. However, we have historically used optical mask inspection tools, utilising wavelengths at or above those we are using for imaging, to qualify masks. This technology has been increasingly difficult to sustain as we have moved from imaging using mercury lamp based sources to pulsed excimer laser based sources that are not very suited to the inspection imaging. Indeed, review of defects found has moved from optical microscopes to SEM based tools. Inspection tools have also evolved, with the first SEM based mask inspection tools being developed to find the smallest defects, however these have the penalty of very low throughput. We will show the potential of using optical systems for the transmissive inspection of EPL masks. The high potential of existing tools will be shown together with the need for a next generation of inspection tools. We will show that simulations indicate that an inspection source with 193nm wavelength would be required for the detection of 50nm defects on a mask. It will also be shown how the position of the defect within the membrane greatly influences detection as well as the implications of moving to a thinner silicon membrane.