FC2: focus control technique for critical level i-line photolithography

In order to satisfy the focus control requirements for critical level i-line photolithography, the benefits of the focus calibration 2 (FC2) system was studied. This feature is standard on Nikon Step and Repeat (NSR) model 10 and above i-line steppers. As features shrink towards 0.35 μm, so to will the focus control requirements on the steppers used to print them. For these types of geometries, typical usable DOF values of <1 μm are seen. As a result, we have targeted focus control of +/- 0.05 μm in order to satisfy these needs in a production environment. The FC2 system uses an in-situ sensor placed on the wafer stage which is capable of measuring the real focus of the projection lens. This focus position is cross-referenced to the auto focus system which maintains wafer best focus. By accurately measuring this position and updating the wafer auto focus system, the wafer is exposed closer to the best focus of the projection lens. In this manner the wafer auto focus system is calibrated to the best focus of the projection lens