Fabrication techniques of microstructures with high resolution and high aspect ratio are necessary for practical microelectromechanical systems (MEMS) that have high performance and integration. In order to fabricate microstructures with sub-micron resolution and high aspect ratio, deep X-ray lithography has been investigated using the compact synchrotron radiation (SR) light source called``AURORA''. An X-ray mask for sub-micron deep Xray lithography, which is composed of 1 lm thick Au as absorbers, 2 lm thick SiC as a membrane and 625 lm thick Si as a frame, was designed. In preliminary experiments, the following results were achieved: EB resist microstructures with an aspect ratio of 22 corresponding with 0.07 lm width and 1.3 lm height were formed; a 10 lm thick PMMA resist containing no warp was formed by direct polymerization, enabling more precise gap control. IntroductionDeep X-ray lithography using a SR light source, such as the LIGA (German acronym for Lithographie, Galvanoformung, Abformung) process [Bley et al. 1994], is one of the most promising techniques in the microelectro-mechanical systems (MEMS) ®eld because this technique can fabricate microstructures with high aspect ratio and sub-micron resolution. The microstructures with submicron widths and gaps (lines and spaces), which were fabricated by deep X-ray lithography, are expected to be applied to practical MEMS devices with integration and high performance, such as highly sensitive micro sensors, high power microactuators, high accuracy microstructures, and so on. Especially, PMMA microstructures fabricated by deep X-ray lithography, which are characterized by high permeability to visible light, can be applied to optical MEMS devices directly. For example, a PMMA grating structure with a pitch of k=4 and height of more than 20 lm can be applied to ®lter and switch devices for optical ®ber [Du et al. 1998]. In this paper, we present investigation and fabrication of sub-micron structures with high aspect ratio by deep X-ray lithography using SR light source. ResolutionTwo important factors that may in¯uence the resolution of X-ray lithography are the creation of photoelectrons and Fresnel diffraction [Ehrfeld et al. 1995]. The in¯uence of the creation of photoelectrons will be increased at shorter wavelength. However, it has been reported that creation of photoelectrons did not in¯uence resolution of X-ray lithography [Bley et al. 1994]. On the other hand, in¯uence of Fresnel diffraction will be increased at longer wavelengths. The resolution limit caused by Fresnel diffraction, R d , is given bywhere k and G are wavelength and the gap between the Xray absorber and a PMMA resist respectively. In order to fabricate sub-micron structures, the value of k and G must be reduced. In our experiment, a beamline for X-ray lithography of the SR light source``AURORA'' at Ritsumeikan University, which is composed of a 200 lm thick layer of Be and a 50 lm thick layer of Kapton as band pass ®lters, was used. As shown in Fig. 1, critical wavelength and wavelen...
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