U.S. Pat. No. 6,946,662 discloses a lens assembly for directing a multitude of charged particle beamlets onto an image plane. The lens assembly comprises a plurality of electrodes having a beam passing region where a plurality of charged beam apertures are formed. The electrodes are stacked along an optical path with insulator members spacing the electrodes apart. At the edge of the electrodes, said electrodes are clamped together for forming a lens assembly. The image provided by the lens assembly is reduced and projected onto a wafer via a reduction electron optical system. The lens assembly is used to correct beam aberrations which occur later on in the path of projection when the beams are demagnified.
A disadvantage of the system disclosed in U.S. Pat. No. 6,946,662 is, that the lens assembly needs to be complex in order to provide for the required corrections.
U.S. Pat. No. 7,091,504 to Applicant discloses an electron beam exposure apparatus comprising a focusing electron optical system comprising an array of electrostatic lenses, wherein each lens focuses a corresponding individual beamlet to a cross-section smaller than 300 nm, directly onto the wafer. Since this system does not need a reduction electron optical system, the effects of beam aberrations due to this reduction electron optical system can be circumvented.
A disadvantage of the system disclosed in U.S. Pat. No. 7,091,504 is that the electron optical system needs to be arranged much closer to the target.
Furthermore, in order to provide a stable electrostatic field in the downstream direction, the electrode substrates of the prior art, as for example disclosed in U.S. Pat. No. 6,946,662, are thin and arranged close to each other, i.e. spaced apart by small distances using insulator members at the sides of the electrodes. Such thin electrodes form the weakest part of a projection lens assembly; the electrodes may break or deform easily when handled, and when sparks occur between the electrodes as a result of the high potential difference there between, the electrodes are usually damaged too severely to be of further use. Replacement of the electrodes can result in significant downtime of a lithography apparatus in which they are used.
It is a goal of the present invention to provide a structurally robust compact modular projection lens assembly for projecting a multitude of charged particle beamlets, e.g. tens of thousands of beamlets or more, directly onto a target. It is a further goal of the present invention to provide a modular projection lens assembly which can be easily handled and maintained and which may be placed as one unit.
It is a further goal of the present invention to provide a compact modular projection lens assembly that is easily assembled to within precise specifications, and a method for assembling such a projection lens assembly.