In the making of micro-circuits the general process followed is that of generating an oxide film on the semiconductor substrate; coating the oxide film with a photoresist and then illuminating the photoresist through a mask to expose selected portions of the resist. After exposure, the photoresist is developed, etched and further processed. Once this is done the same steps are repeated a number of times. The exposure pattern on the photoresist is determined by masks which are prepared for the purpose. Separate masks are used for each of the successive steps. If everything is to appear in the proper place on the micro-circuit a high degree of alignment is necessary between steps. Typical alignment systems employed heretofore are described in U.S. Pat. Nos. 3,975,364; 4,011,011; 4,006,645 and U.S. patent application Ser. No. 19,964, for example.
Almost all the current automatic alignment systems require that the mask and wafer be essentially fixed with respect to the alignment system during the alignment process. This is no problem in a step-and-repeat type system, but it is far from ideal in a scanning projection printer since misregistration can occur when the mask and wafer move with respect to the projection system. Furthermore, better alignment is achieved if the alignment is done continuously during scanning and exposure, rather than at a single place over the scan field. Not only does this eliminate the time lost during static alignments thereby increasing throughput, but it will do a better job, especially on large subfields. Further, the system provides a high signal-to-noise ratio even with a moderately high alignment system bandwidth.
Most focusing systems are indirect and bring the wafer to a fixed location. Thermal changes in the projection system which cause the best focus location to move are not accommodated by indirect systems. The system according to the present invention is direct, is ideally suited for a scanning system, and does not require any vibrating or oscillatory motions that could degrade system performance. A direct working system is very important if a glass cover is used to protect the mask since such covers could be expected to have a considerable variation in thickness requiring a different focus setting for each mask.
While a number of different types of alignment and/or focusing systems have been employed heretofore with moderate success, my contribution to the art is a new system, which is an improvement over such prior art systems, as will become apparent as the description proceeds.