High-precision reticle making by electron-beam lithography

Abstract

We have developed an electron‐beam (e‐beam) lithography system named ‘‘NOWEL’’ which utilizes very accurate pattern writing method and high‐speed data processing. Accordingly NOWEL makes it possible to manufacture reticles for 16‐Mbit dynamic random access memories (DRAM’s) or more densely integrated very large scale integrated circuits (VLSI’s). The NOWEL system has three key features: (1) Vertical landing deflection system. This consists of three yokes in series and one short‐working‐distance lens (M=0.86). The landing angle at the corner of the 5‐mm2 main field is <0.0025 rad. Therefore the butting error arising from a 10‐μ height variation of the substrate is <0.05 μ. Deflection aberration is about 0.2 μ. (2) Double exposure method called ‘‘A/B mode.’’ We adopted a variable shaped beam. The first exposure is made by serial writing with constant‐size rectangular spots, and the second one is made on the previously exposed area by shifting the beam position and reshaping the beam spots. One‐half dosage is given in each exposure. By this method the edge roughness of each pattern is improved to be <0.05 μ. (3) Pattern data compression. A hierarchical pattern data structure was developed. In the memory cell area the data can be divided into only a few pattern data for cell units and their repetition data, i.e., start positions, pitch data, and numbers of cell units in the cell area. According to the repetition data in the main field the main deflector moves the positions of cell units, written by the subdeflector, with the same pattern data. We call this exposure method ‘‘main deflection matrix mode.’’ For fine positioning especially in this mode we developed a 20‐bit digital‐analog convertor (0.005‐μ least significant bit). These reduction techniques should compress the amount of total pattern data to 1/10 for a 16‐Mbit DRAM. By using NOWEL we made a test reticle which contained 16‐Mbit cell patterns with total accuracy within 0.1 μ. In this simplest case only one cell unit with the patterns for 128‐bit cells and their repetition data were needed.We have developed an electron‐beam (e‐beam) lithography system named ‘‘NOWEL’’ which utilizes very accurate pattern writing method and high‐speed data processing. Accordingly NOWEL makes it possible to manufacture reticles for 16‐Mbit dynamic random access memories (DRAM’s) or more densely integrated very large scale integrated circuits (VLSI’s). The NOWEL system has three key features: (1) Vertical landing deflection system. This consists of three yokes in series and one short‐working‐distance lens (M=0.86). The landing angle at the corner of the 5‐mm2 main field is <0.0025 rad. Therefore the butting error arising from a 10‐μ height variation of the substrate is <0.05 μ. Deflection aberration is about 0.2 μ. (2) Double exposure method called ‘‘A/B mode.’’ We adopted a variable shaped beam. The first exposure is made by serial writing with constant‐size rectangular spots, and the second one is made on the previously exposed area by shifting the beam position and reshaping the beam spots. One‐half dosage i...