Semiconductor fabrication using optical projection apparatus is enhanced in an arrangement having means for producing exposures tailored to the patterns on the conventional photo-mask. A predetermined correction photo-mask capable of producing different exposure levels according to the original mask pattern is superimposed with the original mask on the semiconductor wafer by sequential double exposure using an additional mask change and alignment.
A better arrangement provides two beams simultaneously illuminating the two masks; the two beams are recombined with a high quality beam splitter arrangement before reaching the imaging lens. The two masks only have to be aligned to each other once. Afterwards, the wafer is exposed regularly. Therefore, an uncorrected image has the lowest threshold for an isolated opening, medium threshold for equal lines and spaces, high threshold for an isolated opaque line. After the correction scheme, all thresholds are made equal.
The correction mask is made of grainless dyes of different light transmission or of one semitransparent substance of different thickness, such as thin layers of iron oxide, chromium or silicon. Standard lithographic patterning techniques, such as subtractive etching, lift off or plating are used to fabricate the correction mask.