1. Field of the Invention
This invention relates to integrated circuit design. More specifically, the invention relates to a method and an apparatus to determine a proximity correction using a visible area model.
2. Related Art
The relentless miniaturization of integrated circuits has been a key driving force behind recent advances in computer technology. This miniaturization has been made possible by a number of technologies, such as OPC (Optical Proximity Correction). OPC uses process models to determine proximity corrections which allow the system to generate the desired feature shapes on the wafer. The accuracy of these process models is becoming increasingly important as semiconductor integration densities continue to increase at an exponential rate.
A process model models the behavior of one or more semiconductor manufacturing processes which typically involve complex physical and chemical interactions. Since it is almost impossible to find exact analytical formulae to predict the behavior of these complex interactions, statistical models are usually used to model these processes.
Once these statistical process models are found, they can be used to make corrections to layouts to compensate for undesirable effects of a semiconductor manufacturing process. Since corrections are typically computed based on nearby features, these correction techniques are usually called proximity correction techniques. Proximity correction techniques can be used for any semiconductor manufacturing process whose effect can be predicted (or simulated) using statistical models.
For example, optical proximity correction techniques can be used to compensate for the idiosyncrasies of a lithography process by correcting the polygons in a layout. The corrected layout can then be used to generate the desired feature shapes on the wafer.
Prior art techniques for proximity correction typically use a process model that linearly superimposes contributions from neighboring polygons. A neighboring polygon's contribution at an evaluation point is typically computed by convolving a statistical process model with the polygon. These linear-convolution based proximity correction techniques are sufficiently accurate at large feature dimensions. However, these techniques are largely insensitive to the specific positioning of polygons relative to one another.
Unfortunately, as semiconductor integration densities continue to increase at an exponential rate, it is becoming increasingly difficult to use linear-convolution based techniques to perform proximity correction effectively.
Hence what is needed is a method and an apparatus to determine a proximity correction, especially when the proximity effects depend (partially or fully) on the relative positions of polygons.