◆ 　In order to fabricate sub-micron structures with high aspect ratio, we are studying about sub-micron LIGA (Lithographie, Galvanoformung, Abformung) process.

◆ One of the most important things for sub-micron LIGA process is fabrication of an high contrast X-ray mask with sub-micron resolution. It is realized by combination of electron direct lithography using thick resist and Au electroforming (Fig.1).

◆ In order to fabricate sub-micron structures, a warp in a sample and heat generation under SR X-ray irradiation was investigated. It is important for X-ray lithography with sub-micron resolution to control a gap between a resist and an X-ray mask precisely, because Fresnel diffraction has influence on lithography accuracy by the gap. In order to control the gap precisely, PMMA resist was polymerized without residual stress and a warp in a sample by controlling of polymerization process (Fig.2). Figure 3 shows profiles of the measured surface temperature of an Au absorber, a SiC membrane and a PMMA resist under SR X-ray irradiation. The surface temperature indicated an increase under SR X-ray irradiation. In order to avoid these influences, density of SR X-ray irradiation energy is reduced by means of scanning exposure. As a result, sub-micron Ni structures with an aspect ratio of 75, corresponding to 0.2 um width and 15 um height, were fabricated (Fig.4).

Fig.1　An fabricated X-ray mask.

Fig.2　Residual stress of PMMA resist.

Fig.3　Profiles of the measured surface
temperature.

Fig.4　sub-micron Ni structures with
an aspect ratio of 75.