Fine undercut control in bilayer PMMA-P(MMA-MAA) resist system for e-beam lithography with submicrometer resolution

Abstract

Lift-off process is proved to be one of the simplest and most reliable technique for fabrication of microstructures [1], including those with submicrometer sizes [2]. The control of resist image profiles becomes of increasing importance if desired image size is decreased. The easiest way to control the profile is to involve multilayer resist systems with a bottom planarizing layer more soluble by the developer than a top image recording layer. In trilayer systems intermediate layer usually of inorganic matter serves as a separator preventing the two resist layers mixing. The intermediate layer's drawback is necessity of its removal in openings before the development of the bottom layer. Bilayered resist system for submicrometer E-beam lithography based on PMMA as the top imaging layer and P(MMA-MAA)-copolymer as planarizing underlayer has been recently reported by Kuzmin et al [3]. Earlier Dolan [4] proposed offset mask technique for lift-off photoprocessing in order to obtain metal lines as narrow as 0.25 pm. To get appropriate amount of undercut in his trilayer system Dolan applied blanket exposure of the bottom layer of photoresist to provide its solubility by the developer. Image transfer has been made into top layer of the photoresist spun on an opaque separating metal film. In E-beam lithography similar approach has been applied for fabrication of nanometer scale structures [3]. The authors of [3] used two selective developers for the top and the bottom layers to control the undercut. In order to improve E- beam resist profile control we combined methodic from [3] and the Dolan's blanket exposure of the copolymer bottom layer before spinning on of the top PMMA image layer in the bilayer system.