Electrodynamics of fast beam blankers

Abstract

Performance characteristics of an advanced electron beam blanker for lithography are presented. Various electrodynamic effects are discussed, which must be eliminated to achieve high beam placement accuracy during and after blanking. These electric and magnetic field effects have been measured over six orders of magnitude in time. The fast beam jitter characteristic of transit time effects in a double-deflection blanker is captured with nanosecond time resolution. Eddy current effects measured in the micro- to millisecond time domain are shown to be an inherent problem in earlier double-deflection blanker designs. A consequence is beam misplacement after the unblank transition, which can be 0.05 μm even after 500 μs. Several examples of pattern artifacts in purposely underdeveloped resist are given to illustrate graphically the lithographic consequences of the eddy current effect. All of these electrodynamic effects have been addressed with a new ‘‘virtual ground’’ blanker design. The MEBES IV-TFE maskmaker has been upgraded with this new blanker to meet its accuracy requirements throughout the nano- to millisecond time scale. This enables pattern generation at 160 MHz with 30 nm (3σ) critical dimension (CD) control for 2.0 μm features with addresses up to 0.25 μm. The CD control is tested with patterns that are fractured with a single pixel placed across the scan boundary. This situation provides the most extreme test of blanker accuracy at the nanosecond time scale. Blanker eddy current effects are eliminated from all exposures, including those in underdeveloped resist. The blanker design upgrade has facilitated the fabrication of reticles for production 64 Mbit DRAMs and developmental 256 Mbit DRAMs.