Patent Number: 045256292
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a deflective focusing system for a charged particle beam, the system having a magnetic lens and an electrostatic deflector. In general, deflective focusing systems for charged particle beams (referred to merely as "beams", hereinafter) are widely used in cathode ray tubes, television camera tubes, electron beam processing equipment, electron beam exposure equipment, scanning type electron microscopes, or the like. For example, as VLSI (Very Large Scale Integrated Circuit) techniques evolve, the development of an electron beam exposure equipment with high speed and high accuracy is strongly desired. In order to realize such an exposure equipment, it is essentially necessary to develop a high-performance deflective focusing system. In an electron beam exposure equipment, a beam produced from an electron gun is shaped to a beam with a square section. This square beam is then demagnified. The demagnified beam is then focused and deflected to project at a desired postion on a target plane or specimen wafer on a table. In the deflective focusing system, aberrations due to the deflection of the beam, i.e., chromatic aberration, astigmatism coma, field curvature and distortion, are required to be small and the landing angle at which a beam is incident to the target must also be small. If the aberrations and the landing angle are large, resolution and accuracy of patterning are lowered. Further, from a viewpoint of the high speed deflection of beam, electrostatic deflection is preferable to magnetic deflection. Generally speaking, when a beam is focused and deflected by a magnetic focusing field and an electrostatic deflection field which overlap each other and these fields are distribute uniformly over the whole of the deflective focusing space, the aberrations are extremely low and the landing angle is small enough that the beam is incident vertically to an image plane or target. In the electron beam exposure equipment, however, a demagnifying lens is disposed on the object plane side of the deflective focusing system and a wafer or stage is disposed on the image plane side, so that it is difficult to obtain a completely uniform electromagnetic field over the whole deflective focusing space. There are fringes on the object plane side and the image plane side of the deflective focusing system where the electric field and magnetic field abruptly change. If the electromagnetic field has fringes in this way, electron optical properties of the deflective focusing system are different from the properties in case of the uniform distribution. It follows that both of the aberrations and landing angle increase. For instance, an in-lens type magnetic deflector is disclosed by J. L. Mauer et al. in "Electron Optics of an Electron-Beam Lithographic System", IBM J. RES. DEVELOP., pp. 514-521, November 1977. This deflector has large aberrations and landing angle and, since magnetic deflection is employed in this deflector, the deflection speed is slow. There have also been some proposals where a plurality of stages of deflectors are provided and adjusted in a manner so that the deflective aberrations due to the respective deflectors cancel each other out to realize small aberrations and small landing angle throughout the system as a whole. See, for example, "Advanced deflection concept for large area, high resolution e-beam lithography" by H. C. Pfeiffer et al., J. Vac. Sci. Technol., 19(4), November/December 1981, pp. 1058-1063. In the disclosed variable axis lens, four-stage deflectors and one dynamic stigmator are used to reduce the deflective aberrations and landing angle. The deflective aberrations are completely removed and the vertical landing condition is also satisfied by such a multi-stage deflection system. These facts are theoretically proved by T. Hosokawa in "Systematic elimination of third order aberrations in electron beam scanning system", Optik, Vol. 56, No. 1 (1980), pp. 31-30. In this case, however, the number of power sources for driving deflectors is increased because of the multi-stage deflectors. Since a power source of such a deflective focusing system is very expensive, the cost of this multi-stage deflection system is very complicated. In addition, high manufacturing techniques are required, as the number of deflection stages is increased. This requirement also constitutes a barrier against the realization of a multi-stage deflection system. SUMMARY OF THE INVENTION In view of the above, it is an object of the present invention to provide a deflective focusing system for a charged particle beam the system having a simple arrangement which attains the reduction of the aberrations and landing angle. It is another object of the present invention to provide a deflective focusing system for a charged particle beam in which a uniform electromagnetic field without fringes is not employed; instead fringes produced in the electromagnetic field distribution of the deflective focusing system are utilized and the fringes are adjusted in such a way that an electromagnetic field distribution having small aberrations and landing angle, as in the case of a uniform electromagnetic field without fringes is obtained with a simple construction. It is a further object of the present invention to provide a deflective focusing system for a charged particle beam in which parameters of the deflective focusing system are so selected that a magnetic focusing field and an electrostatic deflection field have substatially uniform distributions in a central portion of a magnetic lens, so as to obviate the above-described disadvantages. In order to achieve these objects, a deflective focusing system according to the present invention comprises a magnetic lens for focusing a charged particle beam, a plurality of rings made of magnetic material arranged substantially concentrically with the magnetic lens inside of the magnetic lens, the rings being arranged dividedly (that is, at spaced apart positions) in the direction of the central axis of the magnetic lens so as to form a predetermined magnetic focusing field distribution, and a one-stage electrostatic deflector having a plurality of deflection electrodes which are spaced apart in a circumferential direction of the magnetic lens, the electrodes being arranged substantially concentrically with the magnetic lens inside of the magnetic lens and extending in the direction of the central axis so as to form a predetermined electrostatic deflection field distribution, so that the charged particle beam passes through the concentrically arranged deflection electrodes to be deflected in accordance with a voltage applied to the deflection electrodes. In a preferable embodiment of the present invention, ring-like grounding electrodes are disposed substantially concentrically with the magnetic lens on the object plane side and the image plane side of the electrostatic deflector along the passage of the charged particle beam. It is preferable to insert ring-like spacers between the rings of magnetic material, the spacers being made of non-magnetic material and having substantially the same diameter as the rings of magnetic material so that the magnetic focusing field distribution is adjusted by the thicknesses of the rings in the direction of the central axis and the thicknesses of the ring-like spacers. It is also preferable to provide a gap for adjusting the electrostatic deflection field between the ring-like grounding electrodes and the electrostatic deflector, so that the electrostatic deflection field distribution is adjusted in accordance with the length of the gap. For example, the electrostatic deflection field has an abrupt fringe if the gap is narrow. Further, in order to have an abrupt fringe in the field distribution, the inner diameter of the ring-like grounding electrode may be smaller than the inner diameter of the deflection electrode. It is also preferable that the end portion of the electrostatic deflector on the image plane side be shifted from the end portion of the rings of magnetic material on the image plane side toward the object plane side, in the direction of the central axis. Preferably, the electrostatic deflector may have an inner diameter substantially equal to that of the ring-like grounding electrode. It is also preferable to provide a shielding electrode, for example, in the form of a hollow cylinder, around the outer periphery of the electrostatic deflector. In a preferred embodiment of the present invention, there is provided a case having a first room for accommodating the coil of the magnetic lens, a second room for accommodating the rings and a third room for accommodating the electrostatic deflector and the ring-like grounding electrodes. The case may have a flange extended inwardly to cover the ring-like grounding electrode disposed on the object plane side of the electrostatic deflector. It is preferable to provide a sealing member made of non-magnetic material between the first and second rooms so that the second room is vacuum-tightly sealed and that the rings are fixed at predetermined positions. In addition, it is preferable that the ring-like grounding electrode on the image plane side has a flange for supporting the rings. Further, a stigmator coil can be wound on the periphery of a portion of the electrostatic deflector which is protrudes from the rings toward the object plane side. A dynamic focusing coil can be wound around the stigmator coil.