Fabrication of a needle array using a Si gray mask for x-ray lithography

Abstract

The authors fabricated a gray mask for x-ray lithography using a silicon-on-insulator wafer by employing microelectronic mechanical system fabrication technologies. Generally, when a three-dimensional (3D) resist structure is fabricated using x-ray lithography, an x-ray mask is rotated or scanned to adjust the exposure time of x ray irradiating the resist. However, the authors maintain that 3D resist structure can be made not only by manipulating the x-ray exposure time but also it could be accomplished rather more effectively by making basic changes in the very structure of x-ray masks. Here they present a procedure for making such a mask. Moreover, the use of such a mask could simplify the x-ray lithography process and make it operator friendly. The structure proposed here is known as gray mask and comprises of Si x-ray absorber on SU-8 membrane. Such a structure, unlike stencil mask structure, allows the formation of pattern with isolated features. The x-ray gray mask was evaluated on a beam line BL-4 at TERAS Synchrotron Radiation Facility at the AIST. By using the x-ray gray mask, the authors fabricated cone shaped and gimlet shaped polymethylmethacrylate (PMMA) resist structures. The base diameter and the maximum height of the cone shaped structure were 32 and 75μm; the base width and the maximum height of the gimlet were 34 and 76μm. They also showed that cross-sectional shape of PMMA resist structures matched quite well with the simulation results.