Application-specific Integrated Circuit - Gate-array Design

Gate-array Design

Gate-array design is a manufacturing method in which the diffused layers, i.e. transistors and other active devices, are predefined and wafers containing such devices are held in stock prior to metallization—in other words, unconnected. The physical design process then defines the interconnections of the final device. For most ASIC manufacturers, this consists of from two to as many as nine metal layers, each metal layer running perpendicular to the one below it. Non-recurring engineering costs are much lower, as photolithographic masks are required only for the metal layers, and production cycles are much shorter, as metallization is a comparatively quick process.

Gate-array ASICs are always a compromise as mapping a given design onto what a manufacturer held as a stock wafer never gives 100% utilization. Often difficulties in routing the interconnect require migration onto a larger array device with consequent increase in the piece part price. These difficulties are often a result of the layout software used to develop the interconnect.

Pure, logic-only gate-array design is rarely implemented by circuit designers today, having been replaced almost entirely by field-programmable devices, such as field-programmable gate arrays (FPGAs), which can be programmed by the user and thus offer minimal tooling charges non-recurring engineering, only marginally increased piece part cost, and comparable performance. Today, gate arrays are evolving into structured ASICs that consist of a large IP core like a CPU, DSP unit, peripherals, standard interfaces, integrated memories SRAM, and a block of reconfigurable, uncommited logic. This shift is largely because ASIC devices are capable of integrating such large blocks of system functionality and "system-on-a-chip" requires far more than just logic blocks.

In their frequent usages in the field, the terms "gate array" and "semi-custom" are synonymous. Process engineers more commonly use the term "semi-custom", while "gate-array" is more commonly used by logic (or gate-level) designers.