Direct write electron beam lithography: a historical overview

Abstract

Maskless pattern generation capability in combination with practically limitless resolution made probe-forming electron beam systems attractive tools in the semiconductor fabrication process. However, serial exposure of pattern elements with a scanning beam is a slow process and throughput presented a key challenge in electron beam lithography from the beginning. To meet this challenge imaging concepts with increasing exposure efficiency have been developed projecting ever larger number of pixels in parallel. This evolution started in the 1960s with the SEM-type Gaussian beam systems writing one pixel at a time directly on wafers. During the 1970s IBM pioneered the concept of shaped beams containing multiple pixels which led to higher throughput and an early success of e-beam direct write (EBDW) in large scale manufacturing of semiconductor chips. EBDW in a mix-and match approach with optical lithography provided unique flexibility in part number management and cycle time reduction and proved extremely cost effective in IBM's Quick-Turn-Around-Time (QTAT) facilities. But shaped beams did not keep pace with Moore's law because of limitations imposed by the physics of charged particles: Coulomb interactions between beam electrons cause image blur and consequently limit beam current and throughput. A new technology approach was needed. Physically separating beam electrons into multiple beamlets to reduce Coulomb interaction led to the development of massively parallel projection of pixels. Electron projection lithography (EPL) - a mask based imaging technique emulating optical steppers - was pursued during the 1990s by Bell Labs with SCALPEL and by IBM with PREVAIL in partnership with Nikon. In 2003 Nikon shipped the first NCR-EB1A e-beam stepper based on the PREVAIL technology to Selete. It exposed pattern segments containing 10 million pixels in single shot and represented the first successful demonstration of massively parallel pixel projection. However the window of opportunity for EPL had closed with the quick implementation of immersion lithography and the interest of the industry has since shifted back to maskless lithography (ML2). This historical overview of EBDW will highlight opportunities and limitation of the technology with particular focus on technical challenges facing the current ML2 development efforts in Europe and the US. A brief status report and risk assessment of the ML2 approaches will be provided.