Designers may find the flow from the design rule specification tool to the design rule check tool to be surprising (spoiler: there is no design rule specification tool).
The cornerstone of physical verification is DRC (design rule checking): checking that the design adheres to the technology design rules. As we all know, DRC has become very challenging in advanced technologies, as there are thousands of rules and their complexity grows exponentially. As a result, programming and verifying the DRC check code becomes a very hard and challenging task.
Designers are very familiar with DRC tools. These tools have been around for more than 30 years, and each major EDA company offers one. What designers are less familiar with is how these checks are being constructed -- the flow from the design rule specification tool to the design rule check tool. What they may find to be really surprising is that actually there is no design rule specification tool.
The prevalent practice is to start programming DRC check code without such formal specification. This practice has held on for years, but -- with rules becoming so complex -- programming checks for them without a formal spec will be fraught with issues: the code writing takes a long time, and hand-programming is error prone and very tough to maintain and update. Errors or inaccuracies in the check code vs. the rule intent may result in too much slack in density in the better case and lower yield in the worst case. This practice is equivalent to writing complex code for a system or component right away without a specification of the system or the component behavior. It doesn't sound very reassuring, does it?
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