Inspection of stencil mask using transmission electrons for character projection electron beam lithography

Character projection (CP) lithography is one of the candidates for reliably fabricating fine patterns below 0.15 mu m. To achieve a high CD accuracy of resist patterns, it is very important to establish the inspection method of electron beam (EB) masks for CP lithography systems. The EB mask is one of the stencil masks, which has typically a 20 mu m-thick Si membrane structure.(1)) A 0.1 mu m pattern size on the wafer corresponds to 2.5 mu m on the mask (magnification 1/25). Then, the aspect ratio of the mask aperture becomes eight. It is difficult to detect defects on the side-wall of the aperture with normal SEM, because a secondary electron from the deep side-wall can rarely emerge up to the mask surface. To resolve the problem, we have developed a novel inspection method for EB stencil masks using transmission electrons. Setting the multi-channel-plate (MCP) detector under the mask, we have successfully observed 0.1 mu m-level defects on the side-wall.