1. Field of the Invention
The present invention relates to a method and an apparatus for positioning adapted for use in an exposure apparatus for manufacturing semiconductor devices, and more particularly to a method and an apparatus, in positioning a substrate provided with at least a registration mark and other patterns, for positioning by distinguishing said mark from said other patterns.
2. Description of the Prior Art
With the progress toward finer patterns in semiconductor devices, particularly in large-scale integrated circuits, there are commonly employed reduced projection exposure apparatus for circuit pattern printing in order to meet the requirements of fine pattern definition and a high productivity. Such conventional apparatus projects a reticle pattern, which is larger for example 5 times than the pattern size desired on the silicon wafer, in a reduced size through a projection lens, thus exposing a square area with a diagonal length of 21 mm or shorter on the wafer in one exposure. Consequently, in order to print the circuit patterns on the entire surface of a wafer of a diameter of ca. 125 mm, there is employed a so-called step-and-repeat process in which the wafer is placed on a movable stage and is subjected to repeated exposures with stepwise movement.
In the manufacture of large-scale integrated circuits, there are formed patterns of at least several layers on a wafer in succession, and the desired function cannot be attained due to deficient conductivity or insulation unless the error in superposition or positional aberration between the patterns of different layers is maintained under a determined limit. For example, in a circuit with a minimum line width of 1 .mu.m, there is only permitted a positional aberration of 0.2 .mu.m at maximum.
In the reduced projection exposure process, the pattern registration, namely the registration of a projected reticle pattern with a pattern already formed on the wafer, is achieved either by the off-axis method or by the through-the-lens method. In any case, the wafer is subjected to a rough alignment, called pre-alignment, when placed on the stage, and, if said pre-alignment is not precise enough, the registration mark formed on the wafer is significantly displaced from the central detecting position in an alignment microscope in case of the off-axis method or in a detecting system in case of the through-the-lens method. This leads to an erroneous detection of another pattern as the registration mark and eventually gives rise to an error in registration.