Improvement of alignment accuracy for scaled exposure field

As the reduction of design rules, improvement of the overlay accuracy for intra exposure field comes to be important. We traced the transition of the exposure field size for each process steps (e.g.,isolation layer etch, poly-si deposition), and found that the LOGOS oxidation has fairly large effect on the expansion of exposure field. For the range of 400 similar to 1000nm of LOCOS oxidation thickness, the expansion ratio was about 3.8 similar to 4.2 ppm. This expansion ratio is proportional to the oxidation thickness within that range. At the length of a side of 20mm square exposure field, this value (4ppm) is equal to 0.08um. So the overlay error at the corners of exposure field is amount to 0.04um. This amount is not negligible for the quarter micron design rule. For the purpose of improving the overlay accuracy for extended field which is caused by LOGOS oxidation, we investigated the method of reducing the projective magnification for the isolation layer. The overlay accuracy with the reduction of -3ppm shows the 0.065um as the 3sigma value, and is improved to the half of control wafer with the reduction of 0ppm.