An overview of ASIC design methodology

The approach now most often used with low-cost prototyping of gate arrays and custom designs is that of the Multi-Chip Wafer (MCW). Here different designs are mixed on the same wafer using electron beam lithography. A further refinement of this technique is to use an electron beam to directly write the pattern onto the wafer surface. This, in theory at least, provides the fastest prototype turnaround. It is often erroneously thought that production quantities must be in the thousands before custom chips can be considered. This is certainly not the case especially with gate arrays where a production requirement of 50 can be economic. In estimating the relative costings of a custom chip versus a number of catalogue part ICs, the saving in board space, number of cards required, number of PCB layers required, etc. should be taken into consideration. Even if the custom chip costs more than the catalogue parts it often will be cheaper to use the ASIC because of these other savings