Experimental results of the stochastic Coulomb interaction in ion projection lithography

Abstract
Throughput and resolution are connected in ion and electron projection lithography (IPL and EPL) because of the space charge and Coulomb interaction between the particles in the beam. Due to the lack of experimental data it was not possible to estimate this effect accurately. Therefore an experiment setup has been developed which has the most significant parameters close to planned IPL exposure tools. These parameters are the linear particle density and the crossover shape and size. The stochastic Coulomb interaction blur, depending on the total beam current, has been measured in about 100 settings of the beam current, beam energy, and crossover shape. The results show that the stochastic Coulomb interaction blur scales to the power of 0.587±0.101 (1σ) of the linear particle density in a system with a uniform crossover of 400 μm. To decrease the current density in crossover IPL systems can have an aberrated crossover. In case of this type of crossover of 670 μm the current dependency is 0.820±0.072 (1σ). With an aberrated crossover the stochastic Coulomb interaction is not uniform over the exposure field; it decreases towards the edges of the exposure field and plateaus near its center. This reflects the shape of the crossover. The experimental results can be used to make predictions of the allowable current in planned IPL exposure tools. The process development tool can have 1.00±0.25 (1σ) μA He+ ion current at 100 nm resolution. The allowable H+ ion current in the beta tool at 100 nm resolution is 9.09±2.14 (1σ) μA and 3.69±0.81 (1σ) μA at 50 nm. This corresponds to a throughput of 40.1±0.4 (1σ) wafers per hour (300 mm) at 100 nm and 26.1±2.0 (1σ) wafers per hour (300 nm) at 50 nm.

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