Fabrication Process of Character Projection Mask for EB Lithography

A new structure for a character projection (CP) mask has been developed for the electron-beam (EB) lithography system, EX-8D. The mask comprises an electroplated Au membrane on which apertures are formed and a (110) oriented Si substrate. The Au membrane is 4 µm thick, which is thick enough to stop 50 keV electrons. The aspect ratio of the Au aperture is much less than that of the conventional Si aperture, so that the size of the apertures can be well controlled. By a repair process using a focused ion beam (FIB), the edge roughness and the curvature radius at the corner of the aperture are reduced to less than 0.2 µm. Since the demagnification ratio of EX-8D is 1/36, these errors are 0.006 µm on the wafer and sufficiently small for 0.15 µm lithography. On the CP mask, we arranged a 7×7 array of apertures in a 1650×1650 µm 2 area with a small amount of dead space. This arrangement can be realized by the use of a Si (110) substrate on which back-etched holes with ve rtical sidewalls were formed. In addition, this CP mask has sufficient mechanical rigidity and heat conductivity, because the aperture membrane is supported by Si struts.