2.1 Full Custom IC Design
Full custom design flow is depicted in below figure.
Based on the schematic transistor level circuit is drawn. W/l ratio is decided; layout does mask set is required for manufacturing the transistors. A full custom IC includes logic cells that are customized and all mask layers that are customized. Full custom IC design and manufacturing take large lead time. Typically full custom design flow includes CMOS sizing and manual layout. Even today full custom design flow is used for building analog blocks and analog Ics. There are no efficient HDLs available yet to model analog blocks owing to inherent characteristics of analog circuitry.
Thus in full custom flow every transistor is designed and drawn by hand. Till date this is most successful (only) way to design analog portions of ASICs. Full custom design flow can give the highest performance but the longest design time.