Automatic Registration in an Electron-Beam Lithographic System

In the fabrication of integrated circuits, electron-beam systems are increasingly used to directly expose circuit patterns on resist-covered semiconductor wafers. These systems are made attractive by their inherent capability for writing patterns at high resolution, which eliminates the need for the masks used in optical exposure systems. To compete economically in production, electron-beam systems must attain a high level of wafer throughput. To achieve this, the system must be fully automated and spend the minimum of time in performing overhead functions such as various calibrations, pattern registration, and mechanical positioning of the wafers. This paper describes pattern registration, a process by which the circuit patterns required at a particular level of device fabrication are mapped to those of the preceding level. It discusses the considerations taken as basic in designing an accurate, high-speed registration process for a production-type electron-beam exposure system. This automatic registration system operates in 150 milliseconds per integrated circuit chip, allowing the system to achieve a throughput of 2000 5-mm chips per hour with overlay error of less than 0.75 µm (3σ). The operation of this system, its performance characteristics, and measurements of its pattern-matching accuracy are presented.