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 b important. We traced the transition of the exposure field size for each process steps, and found that the LOCOS oxidation has fairly large effect on the expansion of exposure field. For the range of 400-1000nm of LOCOS oxidation thickness, the expansion ratio was about 3.8-4.2 ppm. This expansion ratio is proportional to the oxidation thickness within that range. At the length of a side of 20mm exposure field, this value is equal to 0.08micrometers . So the overlay error at the corners of exposure field is amount to 0.04micrometers . 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 LOCOS oxidation, we investigated the method of reducing the projective magnification for the isolation layer. The overlay accuracy with the reduction of -3 ppm shows the 0.065 micrometers as the 3 sigma value, and is improved to the half of control wafer with the reduction of 0 ppm.