In electronic design automation, a design rule is a geometric constraint imposed on circuit board, semiconductor device, and integrated circuit (IC) designers to ensure their designs function properly, reliably, and can be produced with acceptable yield. Design rules for production are developed by process engineers based on the capability of their processes to realize design intent. Electronic design automation is used extensively to ensure that designers do not violate design rules; a process called design rule checking (DRC). DRC is a major step during physical verification signoff on the design, which also involves LVS (layout versus schematic) checks, XOR checks, ERC (electrical rule check), and antenna checks. The importance of design rules and DRC is greatest for ICs, which have micro- or nano-scale geometries; for advanced processes, some fabs also insist upon the use of more restricted rules to improve yield. Design rules Design rules are a series of parameters provided by semiconductor manufacturers that enable the designer to verify the correctness of a mask set. Design rules are specific to a particular semiconductor manufacturing process. A design rule set specifies certain geometric and connectivity restrictions to ensure sufficient margins to account for variability in semiconductor manufacturing processes, so as to ensure that most of the parts work correctly. The most basic design rules are shown in the diagram on the right. The first are single layer rules. A width rule specifies the minimum width of any shape in the design. A spacing rule specifies the minimum distance between two adjacent objects. These rules will exist for each layer of semiconductor manufacturing process, with the lowest layers having the smallest rules (typically 100 nm as of 2007) and the highest metal layers having larger rules (perhaps 400 nm as of 2007). A two layer rule specifies a relationship that must exist between two layers. For example, an enclosure rule might specify that an object of one type, such as a contact or via, must be covered, with some additional margin, by a metal layer. A typical value as of 2007 might be about 10 nm. There are many other rule types not illustrated here. A minimum area rule is just what the name implies. Antenna rules are complex rules that check ratios of areas of every layer of a net for configurations that can result in problems when intermediate layers are etched. Many other such rules exist and are explained