Photomasks, referred to for short as masks or else as reticles, are used in lithographic methods for chip production. Structures that serve for imaging desired structures on wafers are formed on said masks. Said structures on the mask are predefined as mask design. Lithographic scanners are used for the exposure of wafers. A light-sensitive layer, the resist, is formed on a wafer. Said resist is developed after exposure by the scanner and the structure is etched into the wafer surface.
Structures produced on the wafer are becoming smaller and smaller. In order to produce the structures, the resolution limit of the optical units used in scanners has to be exploited further and further. The optical unit of a scanner acts similar to a low-pass filter between mask and wafer. This low-pass filtering has the effect that structures on the mask mutually influence one another during the imaging. An imaging of a structure onto the wafer is influenced not only by itself but also by its neighborhood. The imaging quality is influenced by further mask properties such as, e.g.: Refractive index of the mask substrate, thickness of the absorber, side wall angle, corner rounding, dimensions of structure features such as, for example, changes in the line widths of structures, line end shortening.
Since the influences are attributable to optical effects and since they are locally delimited, these effects are referred to as optical proximity effects (OPC effects for short).
Besides the OPC effects mentioned, the mask structures on the way to the structure on the wafer are changed by further effects. These may be, for example: optical aberrations as a result of the optical system of the scanner, apodization and vector effects, effects during the exposure and development of the resist and during the etching of the wafer.
Nowadays, all factors which change the structure on a mask until the representation thereof on the wafer are referred to as OPC effects.
In order to counteract these undesired changes in the structures on the wafer, the structures on the mask are changed in a targeted manner such that the imaging of the changed structures with the effects brought about by the lithographic system produces a structure on the wafer which comes as close as possible to the desired structure. This correction of a mask is called Optical Proximity Correction (abbreviated to OPC).
The change experienced by a structure on a mask proceeding from a predefined mask design until the formation of a wafer structure is described in a so-called OPC model.
An OPC model has numerous parameters for adaption to the lithography process used. This adaptation is effected empirically by a calibration mask being fabricated, on which test structures are formed, which are exposed onto a wafer. The wafer structures produced in the process are measured by use of a wafer CD-SEM. With this information, the parameters of the OPC model are then adapted such that, proceeding from the known mask designs, they can determine the resulting wafer structures with high possible accuracy. The OPC models are implemented in commercially available software solutions, such as, for example, the Calibre or nmWorkFlow software from Mentor Graphics.