This is the traditional method of designing integrated circuits. With a standard cell and gate array the lowest level that the design is performed at is the logic gate level, i.e. NAND, NOR, D-Type, etc. No individual transistors are seen. However, full custom design involves working down at this transistor level where each transistor is handcrafted depending upon what it is driving. Thus a much longer development time occurs and consequently the development costs are larger. The production costs are also large since all masks are required and each design presents new production problems.
Full custom integrated circuits are not so common nowadays unless it is for an analogue or a high-speed digital design. A mixed approach tends to be used which combines full custom and standard cells. In this way designer can use previously designed cells and for those parts of the circuit that require a higher performance then a full custom part can be made.
CAD tools for full custom
The CAD tools follow the general form described for a standard cell. However, since the design of full custom parts involves more manual human involvement then the chances of error are increased. The designer thus relies very heavily on simulation and verification tools. In addition since cells are designed from individually handcrafted transistors then they must be simulated with an analogue circuit simulator such as SPICE before being released as a digital part. Needless to say, the choice of a design route that incorporates full custom design is one that should not be taken lightly.