Design of an electron projection system with slider lenses and multiple beams

Abstract

The commercial applicability of electron beam projection lithography systems may be limited at high resolution because of low throughput. The main limitations to the throughput are:• Beam current. The Coulomb interaction between electrons result in an image blur. Therefore less beam current can be allowed at higher resolution, impacting the illumination time of the wafer. • Exposure field size. Early attempts to improve throughput with "full chip" electron beam projection systems failed, because the systems suffered from large off-axis aberrations of the electron optics, which severely restricted the useful field size. This has impact on the overhead time.A new type of projection optics will be proposed in this paper to overcome both limits. A slider lens is proposed that allows an effective field that is much larger than schemes proposed by SCALPEL and PREVAIL. The full width of the die can be exposed without mechanical scanning by sliding the beam through the slit-like bore of the lens. Locally, at the beam position, a "round"-lens field is created with a combination of a rectangular magnetic field and quadrupoles that are positioned inside the lens. A die can now be exposed during a single mechanical scan as in state-of-the-art light optical tools.The total beam current can be improved without impact on the Coulomb interaction blur by combining several beams in a single lithography system if these beams do not interfere with each other. Several optical layouts have been proposed that combine up to 5 beams in a projection system consisting of a doublet of slider lenses. This type of projection optics has a potential throughput of 50 WPH (300 mm) at 45 nm with a resist sensitivity of 6 µC/cm 2 .