Radiant energy – With charged particle beam deflection or focussing – Magnetic lens
Reexamination Certificate
1999-03-29
2002-07-02
Anderson, Bruce (Department: 2881)
Radiant energy
With charged particle beam deflection or focussing
Magnetic lens
C250S3960ML
Reexamination Certificate
active
06414319
ABSTRACT:
The invention concerns a lens system, particularly for focusing electrons, with a cylindrical lens with pole shoes or electrodes, between which an aperture is located. This aperture has a slot-shaped cross section perpendicular to the optical axis of the lens.
With the aid of electron-beam lithography, structures of a minimal size can be created, a substantial field of application being the production of electronic components and integrated circuits on the surface of disk-shaped semiconductor crystals (wafers). To focus the electrons, electron lens are required that have a short focal distance and whose usable writing field (i.e. the range in which the focal point can be moved parallel to the lens axis) is as large as possible.
Known for this purpose are so-called ‘variable axis’ lenses (H. C. Pfeiffer and G. O. Langner, J. Vac. Sci. Technol., 1983, pages 3 to 14), where a suitable a correction field is dynamically superimposed onto a field of a circular lens which determines the position of the flying spot. However, it proves disadvantageous that the usable field of action of these lenses is limited by the bore radius of the pole shoes which is limited by the size of the focal distance. In practice, there are thus writing regions with diameters in the range of one millimetre. In order to be able to process integrated circuits of a standard size or even a whole wafer, conventional lithographic systems thus require, in addition to the electron optics, a table with workpiece clamping devices which can be moved in a plane perpendicular to the electron beam. In this case the efficiency is limited by the mechanical parameters of the table, where the minimum size of the creatable structures depends on the guide accuracy and the speed of movement of the table determines the maximum writing speed.
Furthermore, known in the state-of-the-art are both electrostatic as well as magnetic cylindrical lenses to focus the electrons (H. Rose, Optik 36, 1972, pages 19 to 36). Cylindrical lenses are provided with electrodes or pole shoes which serve to guide the electric or magnetic field, and between which there is a slot-shaped aperture through which extend the possible trajectories of charged particles in the lens. Here the longitudinal axis of the slot-shaped axis is aligned perpendicular to the optical axis of the lens; the plane formed by the optical axis and the longitudinal axis is termed hereinafter the centre plane. For stigmatic imaging, i.e. the punctual imaging of punctual electron emitters and parallel incident rays, cylindrical lens are however unsuitable, as they focus only perpendicular to the direction of the slot, while movable components of the particles parallel to the direction of the slot remain uninfluenced. Thus bar-shaped, astigmatic punctual patterns obtain which are insufficient for lithographic purposes.
Finally the movement of charged particles can also be influenced via quadripole fields. Perpendicular to its plane a magnetic quadripole also acts as a lens. Standard images, however, cannot be generated in this way because the influence of a quadripole on a planar, parallel beam cluster depends on its angular position to the magnetic field. While the quadripole lens focuses parallel beams in a plane, which comprises the optical angle, it acts in the plane perpendicular thereto, which comprises the optical axis, so as to defocus a parallel beam cluster. Solely the amount of the focus distances can be set equally in both planes. Owing to these characteristics, quadripole lenses are unsuitable for standard optical imaging.
Against this background, the object of the invention is to develop a lens system in which the apertures for the electron beam are of optional length in one dimension and which produces stigmatic imaging.
This task is solved therein that the cylindrical lens is combined with a magnetic lens with which a quadripole field can be produced, the lenses are arranged with little or no distance between them and their optical axes run parallel to one another, the quadripole lens has a slot-shaped aperture which is oriented parallel to the opening of the cylindrical lens, the focusing plane of the quadripole lens, which comprises the optical axis, is oriented parallel to the longitudinal axes of the apertures and its defocusing plane perpendicular to the longitudinal axes and the refractive power of the cylindrical lenses can be set at twice that of the quadripole lens.
The central idea of the invention consists therein to combine one cylindrical lens with one magnetic quadripole lens, where the term quadripole lens comprises all lens within the meaning of the invention by means of which fields of quadripole symmetry can be produced, for instance also eight-pole lens. The optical axes of both lens extend parallel to each other and their principal planes match or are at a small distance to each other. The term ‘small’ is to be understood in the sense that the distance is small compared with the focusing length of both lens and the lens system can be described in good convergence by a single lens. Between its pole shoes the quadripole lens has a slot-shaped aperture, which is oriented parallel to the opening of the cylindrical lens, the length of both openings being optional. The projections of the two openings preferably match in the direction of the optical axis. The orientation of the quadripole lens is such that its focusing plans extends parallel to the longitudinal axis of the apertures and its defocusing plane perpendicular to the longitudinal axes. The refractive power of the cylindrical lens is suitably chosen so as to be twice that of the quadripole.
Resulting from this configuration, the quadripole lens provides a focusing effect in the plane of the slot where the focal distance is twice that of the cylindrical lens. Perpendicular to the direction of the slot the negative refractive power of the quadripole is overcompensated by the twice as high refractive power of the cylindrical lens. As a result a focal distance obtains which is also equal to twice that of the cylindrical lens. The result is thus a stigmatically focusing lens. Decisive for the coordination of the lens focal distances is solely that the sum of the refractive powers of the quadripole perpendicular to the slot longitudinal axis and the cylindrical lens is equal to the refractive power of the quadripole lens parallel to the slot longitudinal axis.
The advantage of the lens system consists therein that limitations of the beam path in the direction of the slot, as they exist in the case of circular lenses because of the bore radius of the pole shoe, are eliminated or substantially reduced. Nonetheless the system permits stigmatic imaging comparable to conventional circular lenses. In particular, no optical lens errors occur which are less than third order. Suitably in the majority of application cases the optical axis of the quadripole lens extends in the centre plane of the cylindrical lens. Furthermore it proves a significant advantage that along the slot, neglecting influences of the endwise slot edges, largely translation symmetry exists. Therefore the imaging characteristics are independent of the position of the optical axis of the quadripole in respect of the longitudinal axis of the slot. With suitable adjustment of the lens parameters, in particular the field intensities, imagings can of course also be produced with any other charged particles, for example ions which for instance are usable for the doping of semiconductors. Finally, it is conceivable for special applications that the coordination of the lens focal distances deviates from the above-mentioned setting, for instance to produce line-like structures on a workpeice with an astigmatic focal spot.
A preferred quadripole lens consists of one aperture on or at whose edges there extend current-carrying conductors. The conductors are oriented more or less parallel to the optical axis. Suitably the conductors here comprise coils wound on a yoke. The conductors or windings extend on the inside edges of the aperture, which face
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