This invention relates to an electron lens equipment for use in an apparatus for electron beam lithography, and the like.
In general, in an apparatus for electron beam lithography for use in semiconductor production, for example, defocusing occurs when an electron beam is widely deflected. Therefore, it has hitherto been the practice to provide a so-called dynamic focusing correction according to which only one coil or subcoil is arranged within the magnetic field of a lens and is used to change the focal distance thereof. At this time, however, a rotation of the electron beam by the lens develops, and when the electron beam is deflected from the center axis of the lens, a displacement thereof develops. In a conventional system in which the electron beam is caused to pass on only the center axis of the lens, the rotation poses no problem.
In recent years, an electron-optical system in which the electron beam is caused to pass outside the axis of the electron lens has been proposed. However, in such a system, the displacement due to rotation of the electron beam becomes an important problem. Especially in the case where the positioning needs to be made at high precision, as in apparatus for electron beam lithography, it becomes a serious problem.
Figure 1 is a diagram for explaining a prior art electron lens system. Letting B.sub.L denote a magnetic field established by a lens 2 and B.sub.C denote a magnetic field established by a coil 3, the focal distance f and the rotation .theta. at this time are as follows: ##EQU1## where e denotes the electron charge, m the mass of electron, and .PHI..sub.o the accelerating voltage.
Now, considering the variations .DELTA.of the respective quantities from the time when B.sub.C =0, ##EQU2## provided that B.sub.L&gt;&gt;B.sub.C. Here, the half-width of the magnetic field of the lens 2 is denoted by L.sub.L, and the distance between the lens 2 and a focusing plane 1 is denoted by b. When the respective integrals in the above expressions (l)-(4) are expressed by the maximum values and half-widths of the magnetic fields, and the resultant expressions are arranged on the supposition that only the focusing plane is moved by the change of the focal distance, the following approximate expression holds: ##EQU3## where .DELTA.b denotes the variation of b, and f.sub.L denotes the focal distance at B.sub.C =0. In the figure, B.sub.Lmax and B.sub.Cmax indicate the maximum values of the magnetic fields of the lens 2 and the coil 3, respectively, L.sub.C the half-width of the magnetic field of the coil 3, and a the distance between an objective point 4 and the lens 2.
Now, assuming by way of example that b=80 mm, .DELTA.b=0.1 mm, f.sub.L =50 mm and L.sub.L =50 mm, then .DELTA..theta.=0.4 mrad=milliradians holds. Supposing that the electron beam has been deflected in a stage preceding the lens, and letting r denote the deflection on the focusing plane, the displacement .delta. is expressed by .delta.=r.multidot..DELTA..theta.. To cite examples in a typical practical range, when r=1.4 mm (2 mm.sup..quadrature. field), a displacement (in a direction perpendicular to the Z-axis illustrated of .delta.=0.56 .mu.m develops, and when r=2.1 mm (3 mm.sup..quadrature. field), a displacement of .delta.=0.84 .mu.m develops. These displacements pose a serious problem when it is intended to attain a high precision with 0.1 .mu.m as in the apparatus for electron beam lithography.