Full custom

Full-custom design is a methodology for designing integrated circuits by specifying the layout of each individual transistor and the interconnections between them. Alternatives to full-custom design include various forms of semi-custom design, such as the repetition of small transistor subcircuits; one such methodology is the use of standard cell libraries (standard cell libraries are themselves designed using full-custom design techniques). 
Full-custom design potentially maximizes the performance of the chip, and minimizes its area, but is extremely labor-intensive to implement. Full-custom design is limited to ICs that are to be fabricated in extremely high volumes, notably certain microprocessors and a small number of ASICs. 
The main factor affecting the design and production of ASICs is the high cost of mask sets and the requisite EDA design tools. The mask sets are required in order to transfer the ASIC designs onto the wafer.