Fabrication of Three-Dimensional Nanostructures by Electron Beam Lithography
Kenji Yamazaki and Hiroshi Yamaguchi
Physical Science Laboratory
Three-dimensional (3D) nanostructures are now attracting great interest because of their possible applications, such as to nanomechanical devices and nanorobotics. To build various 3D nanostructures, we have devised and are developing 3D electron beam (EB) lithography (3D-EBL). Although our technique has some advantages, such as high resolution and fast fabrication, it has so far shown drawbacks related to the electron scattering/proximity effect and to the poor precision of structure/3D alignment, compared with some other 3D fabrication techniques. We have newly devised two methods in 3D-EBL to significantly improve the 3D alignment and reduce the proximity effect.
When we make complicated 3D structures with this technique, the EB writes from largely different directions and therefore should be well positioned three-dimensionally; high accuracy of 3D alignment is necessary. The high accuracy is achieved by using a transmission electron image to accurately control sample rotation (< 1 mrad) and obtain sufficient accuracy of 2D positioning of EB writing on a rotated sample . In a 3D nanostructure we created in negative resist (hydrogen silsesquioxane, HSQ), the 3D alignment accuracy is on the order of 10 nm (Fig. 1). When we use positive resist, the proximity effect is very serious. However, we have succeeded in suppressing the proximity effect by leaving buffer areas (not exposed to EB) that surround the target structure and have sizes similar to the range of fast secondary electrons. As a result, we have reduced undesirable dissipated energy, which changes vertically in the resist . Figure 2 shows SEM images of a 3D nanostructure in positive resist [poly(methyl methacrylate), PMMA], which demonstrates a high aspect ratio and the great flexibility of structures that can be created by the technique.
These methods will accelerate the development of 3D-EBL, making it promising for various nanotechnology applications.
This work was supported in part by KAKENHI (20246064).
 K. Yamazaki and H. Yamaguchi, J. Vac. Sci. Technol. B 26 (2008) 2529.
 K. Yamazaki and H. Yamaguchi, Appl. Phys. Exp. 1 (2008) 098001.
Fig.. 1. 3D nanostructure in HSQ, made by writing arrays of dots from +/-X, +/-Y, and Z directions.
Fig.. 2. 3D nanostructure in PMMA, made by leaving buffer areas to suppress the proximity effect.
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