Lithography is a widely used technique for the manufacture of various patterned structures such as integrated circuits, magnetic devices, and other devices. In lithography, a final product is manufactured in a multi-step process, where a “resist” material (sensitive to electromagnetic radiation) is produced for creation of an initial pattern. A pattern is formed in the resist material by exposing different regions of the resist material to different radiation doses. The radiation, e.g., optical radiation in the ultraviolet (UV) or deep UV (DUV) range, or x-rays regimes, is projected through a mask (e.g., reticle) onto the “resist” material.
There are several types of masks that are used in patterning and/or inspection systems. This includes, for example, binary reticles and phase-shift masks. Defects in a mask pattern result in a defected pattern created on an object. Defects in a reticle/mask may be a result of incorrect designing of the mask pattern. Defects may also be generated during a reticle's fabrication process as well as during subsequent processing and handling. A process for manufacturing a reticle/mask is typically similar to an object (e.g., wafer) patterning process. For example, the goal of reticle manufacturing is forming a pattern in an opaque material such as a relatively thin chrome layer on a substantially transparent substrate such as glass (e.g., chrome on glass COG reticles).
Various techniques have been developed for optimizing reticle/mask design and patterning techniques. Some examples of these techniques are described in the following publications:
US 2002/140920 describes a system and method for lithographically printing patterns on a semiconductor using combinations of illumination and mask patterns, which are optimized together to produce the desired pattern. The method of optimizing both illumination and mask patterns allows the development of mask patterns that are not constrained by the geometry of the desired pattern to be printed. Thus, the method provides high quality images even when the desired printed patterns have critical dimensions that approach the resolution limits of a lithographic system. The resulting mask patterns using the method do not obviously correspond to the desired patterns to be printed. Such masks may include phase-shifting technology that use destructive interference to define dark areas of the image and are not constrained to conform to the desired printed pattern.
U.S. Pat. No. 5,326,659 describes a method for making a mask for optical lithography or other projection printing, wherein the mask is represented by a mask pattern. The mask provides a substantially binary output image on the surface of a wafer as light is applied to the mask. Light passes through the mask and onto a wafer at varying intensities, such intensities represented by output intensity values, the threshold values of which produce output images within predetermined constraints. The method includes the steps of defining sampling points, which are representative of the binary output image. These sampling points are used in defining local objective functions, which are combined to give a total objective function.