One IC type is known as a full custom IC. A full custom IC is a chip created specifically to implement the gates of the desired circuit. The design flow for a full custom IC is shown. Digital designers don't usually build full custom ICs themselves but rather they send the desired circuit to a group or company whose engineers specialize in transforming digital circuits into full custom ICs.

Those engineers, assisted by computer aided design (CAD) tools, convert the desired digital circuit design into a circuit of transistors and decide where to place each transistor on the surface of the chip, how to orient each transistor, ow big to make each transistor, how to place the wires that connect the transistor and so on. All that information about how the transistors and wires should be layed out on a chip's surface is known as a layout. The full-custom IC engineers then send that layout information to a factory that specializes in fabricating ICs, known as a fabrication plant or fab for short.

Fabricating an IC is a complex error prone process utilizing state of art photographic, laser and chemical equipment that each can cost hundreds of millions of dollars. The transistors and wires are formed as dozens of layers on the surface of a chip, the lower layers define the transistors and connections within logic gates while the upper layers define the wires that exist between gates.

An IC may have tens of layers. Each layer requires a set of masks that allows iht to reach specific regions of the chip to modify chemicals on the chip surface during the formation of layers. This cost for creating the masks required for IC fabrication is known as nonrecurring engineering (NRE) cost. The cost is called non recurring because it occurs once, before chips are made and then does not recur, no matter how many chips are subsquently manufactured. Because a full-custom IC type requires masks to be made for every layer of the chip, the NRE cost for a full custom IC type is very high, typically tens of millions dollars.