1x stencil masks fabrication and their use in Low-Energy Electron-beam Proximity Lithography (LEEPL)

Abstract

Thirty years ago it was the common believe of most of the lithographer that the limit end for the optical lithography will be at about 1 μm ground rule. So NGL tools were developed to go in the 500nm and 250nm regions. 15 years later the different optical lithography techniques were still alive exposing feature sizes down to 200nm and the NGL tool developer had to move to 100nm and below. Today 100nm features made by optical lithography is world wide a common technique in most of the modern chip manufacturing plants, and feature sizes beyond the used wavelength are state of art. So do we really need NGL or will the optical lithography lives forever? Well, there are already optical system available or will be soon delivered to the manufacturing lines which are able to expose feature sizes down to 70nm and even to 50nm if they use 193nm immersion lithography. But for what price? The optical lithography became extremely expensive. Reticles for the 70nm technology full with OPC structures may cost up to $ 500 K and an optical reticle set up to $ 2 Million. So in my understanding the introduction of any NGL technique will only happen if such a technique can demonstrate at least the same performance as the optical lithography but at a lower cost level. The best understood NGLs are the electron beam lithography techniques used in e-beam direct writing tools for the exposure of masks and reticles or e-beam techniques which expose the wafers using masks like E-beam Projection Lithography (EPL) or Low Energy E-beam Proximity Lithography (LEEPL) respectively. Both EPL and LEEPL require a similar mask technique so called stencil masks. The first 1x stencil masks (a silicon wafer with a thin membrane area containing the pattern as physical holes) were developed by IBM Germany more than 25 years ago and perfected in the Advanced Mask Facility (AMF) at IBM Vermont. Today, these 1x stencil masks used for LEEPL are mainly produced by Hoya, DNP, Toppan and NTT-AT in Japan. This paper will explain the different fabrication processes for the stencil masks, their different shapes and their performance used for LEEPL.