1. Field of the Invention
This invention relates to a projection optical apparatus for controlling, in an apparatus provided with a projection optical system, particularly an exposure apparatus for the manufacture of semiconductor elements, the pattern image of a projected mask to a desired projected state irrespective of any variation in the optical characteristic of the projection optical system.
2. Related Background Art
In recent years, exposure apparatuses, particularly reduction projection type exposure apparatuses have become indispensable for the production of integrated circuits. In such an exposure apparatuses is usually made of a projection lens for reduction-projecting a circuit pattern on a reticle onto a semiconductor wafer to 1/5 or 1/10 and obtaining a resolution of 1 .mu.m or more for the line width.
Especially at present, to achieve an improvement in the degree of integration of semiconductor devices, a projection lens which will increase the resolving power while keeping a great projection exposure area has become desired.
Generally, in such an exposure apparatus, a focus detector utilizing a gap sensor which will detect the spacing between a projection lens and a wafer is incorporated to cope with the variation in the thickness of the wafer and the unevenness of the surface of the wafer when a predetermined pattern is formed by the lithography process. Such focus detector is shown, for example, in U.S. Pat. No. 4,650,983. Automatic focusing in which the imaging plane of the projection lens, i.e., the image surface of the projected pattern, is made coincident with the surface of the wafer on the basis of the detection signal of the focus detector is effected.
However, it is known that by the projection lens itself absorbing heat energy or by the environmental temperature and the atmospheric pressure changing, the optical performance of the projection optical system is varied and the best imaging position fluctuates. The fluctuation of the imaging position caused by the variation in the projection lens itself cannot be corrected by only the conventional focus detector which utilizes a gap sensor.
Such fluctuation can be eliminated if use is made of optical focus detecting means through the projection lens, but there are various problems in it. A pattern-like film layer of the order of 1 .mu.m is present on the wafer which is a substrate to be projected, and the pattern-like film layer includes a layer of optically transparent material such as SiO.sub.2 and a layer of optically opaque material such as Si or aluminum. Further thereon, there is present a photoresist film of maximum several microns. In addition to such a situation, the use of a printing light in such optical focus detecting means would cause the photoresist on the wafer to be sensitized, and heed must be given to this point. Means using a printing light of nonsensitizing wavelength for the photoresist is also possible, but in such case, the aberrations of the projection lens are bad and image formation of good quality cannot be accomplished.
In addition, such a thermal influence is exerted not only to the fluctuation of the imaging position but also to the magnification of the projection optical system, and this forms a cause that prevents image formation of good representability from being accomplished.
To solve the above-noted problems, a system for automatically modifying the focus detector utilizing a gap sensor correspondingly to any variation in the atmospheric pressure is proposed in U.S. application Ser. No. 826,979 filed on Feb. 7, 1986, abandoned in favor of a continuation application, Ser. No. 931,013 filed on Nov. 17, 1986, now U.S. Pat. No. 4,687,322 granted Aug. 18, 1987, now abandoned.
Further, U.S. Pat. No. 4,666,273 proposes the technique of stabilizing the magnification of the projection optical system for any variation in the physical factors inside or outside the projection optical system, particularly any variation in temperature caused by illuminating energy.