3.4  Library-Cell Design

The optimum cell layout for each process generation changes because the design rules for each ASIC vendor’s process are always slightly different—even for the same generation of technology. For example, two companies may have very similar 0.35 m m CMOS process technologies, but the third-level metal spacing might be slightly different. If a cell library is to be used with both processes, we could construct the library by adopting the most stringent rules from each process. A library constructed in this fashion may not be competitive with one that is constructed specifically for each process. Even though ASIC vendors prize their design rules as secret, it turns out that they are similar—except for a few details. Unfortunately, it is the details that stop us moving designs from one process to another. Unless we are a very large customer it is difficult to have an ASIC vendor change or waive design rules for us. We would like all vendors to agree on a common set of design rules. This is, in fact, easier than it sounds. The reason that most vendors have similar rules is because most vendors use the same manufacturing equipment and a similar process. It is possible to construct a highest common denominator library that extracts the most from the current manufacturing capability. Some library companies and the large Japanese ASIC vendors are adopting this approach.

Layout of library cells is either hand-crafted or uses some form of symbolic layout . Symbolic layout is usually performed in one of two ways: using either interactive graphics or a text layout language. Shapes are represented by simple lines or rectangles, known as sticks or logs , in symbolic layout. The actual dimensions of the sticks or logs are determined after layout is completed in a postprocessing step. An alternative to graphical symbolic layout uses a text layout language, similar to a programming language such as C, that directs a program to assemble layout. The spacing and dimensions of the layout shapes are defined in terms of variables rather than constants. These variables can be changed after symbolic layout is complete to adjust the layout spacing to a specific process.

Mapping symbolic layout to a specific process technology uses 10–20 percent more area than hand-crafted layout (though this can then be further reduced to 5–10 percent with compaction). Most symbolic layout systems do not allow 45° layout and this introduces a further area penalty (my experience shows this is about 5–15 percent). As libraries get larger, and the capability to quickly move libraries and ASIC designs between different generations of process technologies becomes more important, the advantages of symbolic layout may outweigh the disadvantages.


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