Independent-Exposure Method in Electron-Beam Lithography

Abstract

Lithography technologies can be distinguished by mask lithography and scanning lithography so-called direct-writing process. The representative for mask lithography is photo-lithography, while the representative for scanning lithography is electron-beam lithography. Electron-beam lithography as scanning lithography has three major processing in the lithography procedure: (1) resist deposition, (2) alignment & exposure and (3) development. Electron-beam lithography is one of the most important fabrication technologies for the topdown miniaturization and high-accuracy 2D and 3D surface profile because the wavelength of electron-beam calculated by De Broglie’s wavelength is much shorter than that of the source of other lithography (Sinzinger & Johns, 2003; Kley, E.B & Schnabel, B. 1995; Hirai et al., 2000). In the photo-lithography, there is the limitation of pattern resolution by optical diffraction, but electron beam lithography does not have the limitation of pattern resolution by diffraction due to the much shorter wavelength. Compared to photo-lithography, electronbeam lithography has some attractive advantages including: (1) small spot size under 10nm, (2) no mask, (3) the precise control of electron beam by electrostatic or magnetic lens, (4) lower defect densities (Madou, 2002). However, electron beam lithography also has some disadvantages including: (1) strong scattering of electrons in solid affecting the increase of pattern resolution, (2) high cost of system and maintenance, (3) more complex machine system than photo-lithography system due to the requirement of vacuum and electron optics (Madou, 2002). For the fabrication of micro optical elements such as a multi-level lens and a binary structure in electron beam lithography, it was suggested that the alignment method should be used. It consists of a sequence of binary scanning pattern with L step which results in N=2L number of level (Dammann, 1970; Goodman & Silvestri, 1970; Kong, et al., 2004; Woo et al., 2009). Fig. 1 shows the alignment method for the fabrication of a 4-level lens in which L step is 2. Thus, there are 2 times of repetitive processes between electron beam lithography and etching process. In addition, alignment marks and technology in this method have to be required in order to reduce the fabrication error. Thus, it is also important to design and fabricate the alignment structure. Due to the sequence of binary fabrication step and repetitive process, it requires more time and cost for the fabrication of such a multi-level lens.