Projection moire alignment technique for mix and match lithographic system

Abstract

Projection moire alignment is a highly useful technique for mix and match lithographic system. The same alignment marks are used for both photo and X-ray photolithography and high sensitivity is obtained using simple alignment optics. This paper deals with a projection moire alignment system which applies to a g-line or i-line stepper that is part of a mix and match lithographic system. A He-Ne laser is used as the light for alignment. By using two pairs of gratings which are 1/2 grating pitch out of phase with each other, we obtain two moire signals which are also 1/2 grating pitch out of phase. Using difference of these two moire signals, high sensitivity is obtained. In order to achieve high overlay accuracy in automatic alignment, it is important to consider the shape of the alignment marks. Asymmetrical resist coverage caused by spin coating causes an offset error and in case of aluminum coated wafers, a rough surface causes a random error. Some defects in the gratings caused during etching process also produce error. For the purpose of analyzing some effects of alignment marks on projection moire alignment, we calculated alignment error using Fresnel diffraction integral. We obtained the following results. An aberration method using reflected light from wafer marks is useful for rough surfaces. Defects in a grating are eliminated by average effects. And we calculated an alignment error caused by wafer tilt. By tilting 5 second to 10 second, output waves become asymmetry and contrast is reduced. Tilt of 5 sec. makes an alignment error of about 0.03 tm.