Abstract: A wafer alignment system aligns a wafer by checking the alignment marks formed on the back surface of the wafer. A number of guiding rays are used to determine the corresponding alignment mark on the back of the wafer to ensure that the wafer is properly aligned. The alignment system of the invention also includes a wafer stage and a fixed base, wherein the wafer stage and the fixed base contains a number of apertures that allow the guiding rays to pass through and strike on the alignment marks on the wafer.

Abstract: An apparatus and method for determining local de-focus problems on a real time basis in a wafer exposure system. The distance of a wafer from an objective lens and rotation of the wafer about two orthogonal axis at each field position is adjusted to achieve optimum focus for each field of the wafer. The rotational data is fed to a computer and analyzed to determine if any of the rotational angles or if the difference between individual rotational angle and the mean rotational angle exceed critical angles. If any of the critical angles are exceeded local de-focus will occur and the exposure system must be checked for defects. If none of the critical angles are exceeded processing continues with the next wafer. The comparison of the rotational angles to the critical angles is performed for each wafer before continuing with the next wafer so that problems are discovered on a real time basis.