1. Field of the Invention
The present invention generally relates to methods and systems for generating simulated images from design information.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Fabricating semiconductor devices such as logic and memory devices typically includes processing a substrate such as a semiconductor wafer using a large number of semiconductor fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a resist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer and then separated into individual semiconductor devices.
As design rules shrink, the design that is formed on a specimen such as reticles and wafers, even when formed using an optimally performing process, can look much different from the actual design. For example, due to the inherent limitations of the physical processes involved in forming a design on a physical specimen, features in the design formed on the physical specimen typically have somewhat different characteristics than the design such as different shapes (e.g., due to corner rounding and other proximity effects) and can have somewhat different dimensions (e.g., due to proximity effects) even when the best possible version of the design has been formed on the specimen.
Sometimes, it is not possible to know how the design will appear on the specimen and in images of the specimen, on which the design information has been formed, generated by tools such as inspection tools, defect review tools, metrology tools and the like. However, it is often desirable to know how the design will appear on the specimen and in images generated by such tools for a number of reasons. One reason is to make sure that the design will be formed on the specimen in an acceptable manner. Another reason is to provide a reference for the design, which illustrates how the design is meant to be formed on the specimen, that can be used for one or more functions performed for the specimen. For example, in general, a reference is needed for defect detection so that any differences between the design formed on the specimen and the reference can be detected and identified as defects or potential defects.
Much work has therefore been done to develop various methods and systems that can simulate how the design will be formed on the specimen and will appear in images of the specimen. There are several currently used methods for generating such simulated images. For example, one currently used method is forward electromagnetic (EM) modeling. In this method, design information such as computer aided design (CAD) and material information for a specimen is used as input, and the method simulates the physical interaction between light and material and generates a near field or far field image. Another example of a currently used method is rule-based approximation. Given some practical constraints, this method handcrafts a collection of rules that approximate transformation from CAD to observed image.
There are, however, several disadvantages to the currently used methods for generating simulated images. For example, forward EM simulation is extremely computationally intensive, making it infeasible for use in real production, e.g., the simulated images cannot be generated fast enough using forward EM simulation for the production use case. This is true for mask inspection, wafer inspection, and wafer metrology applications. In addition, forward EM simulation cannot simulate CMP and etching effects captured by electron beam and optical images. In another example, rule-based approximation tends to result in a large collection of very complex rules, making it generally inapplicable and causing poor prediction in reality.
Accordingly, it would be advantageous to develop systems and methods for generating simulated images from design information that do not have one or more of the disadvantages described above.