LITHOGRAPHY CONTROL ON UNEVEN SURFACE

A reticle pattern is transferred to a semiconductor wafer with a coated resist film by using a reduction (demagnification) exposure system having a focus sensor and an autofocus mechanism operating in response to an output of the focus sensor. An average level of a plurality of autofocus points in a chip area is calculated while the semiconductor wafer is mounted on a stage. The semiconductor wafer is scanned and levels of the surface in the chip area are sampled. From the distributions of sampled surface levels, a stepped surface level to be focussed is derived. An offset value between the average level and the stepped surface level to be focussed is derived and an autofocus point is corrected by the offset value to expose the chip area of the semiconductor wafer. A method of exposing a semiconductor wafer is provided which can perform an exposure most suitable for each process, by identifying a stepped surface level to be automatically focussed.