Fabrication of 3-D Nanoscale Structures of Arbitrary Shape by a Single-step E-beam Grayscale Lithographic Process

Abstract

Three dimensional (3-D) structures are widely utilized and critical to device functionality in many applications, such as PBG (photonic band gap) crystals, DOE's (diffractive optical elements), blazed gratings, MEMS, NEMS etc. The performance characteristics of such structures are highly sensitive to their dimensional fidelity, which is strongly influenced by the fabrication process. In most cases, the fabricated structures were of microscale and regular shapes and their applications were therefore prohibited accordingly. In this study, grayscale electron beam lithography and reactive ion etching were employed to fabricate complex 3-D structures. With the aid of a unique algorithm, PYRAMID which aims at correcting proximity effect of EBL, three arbitrary shape 3-D structures were successfully fabricated on photoresist and Si substrate respectively. A series of major factors involved in fabrication process, dose coefficient, base dose, spot size, line spacing, developing time, developer concentration and ultrasonic agitation were studied one by one along with fabrication results. Design of Experiment was used to statistically determine the significance of individual factor and the cross effect of multiple factors and perform an optimization and prediction based upon existed experimental results. It is revealed that base dose, developing time, developer concentration and their interactions are significant factors in terms of as-fabricated structure geometry and resolution. A response optimization based upon experimental results were performed and 20 solutions were found consequently. Compare these solutions with the experimental factors used to achieve optimum response, it is concluded that the optimized solutions agree with experimental data.