A custom integrated circuit (IC) is sometimes called a full-custom IC. To understand the specific requirements of this type of IC design, we first look at the more typical design process called semi-custom design. Sometimes this process is called application-specific IC, or ASIC design.
Memories, standard cells, and larger macro functions such as processor cores are all examples of these building blocks. Using pre-defined building blocks increases designer efficiency. The design process is one of assembly of the building blocks and verification that the collection of blocks used will satisfy the design requirements. Sophisticated EDA tools are used to create specific versions of the building blocks to address unique requirements. A memory compiler that creates the correct memory size for a given design is an example of this process. Descriptions of digital circuits expressed in high-level languages such as Verilog are automatically “compiled” into the logic elements needed to implement these functions. This is called logic synthesis and is another example of this process. The entire collection of design elements is then placed and routed on the silicon substrate using automated technology.
While this process is very efficient, it creates a circuit that is a “best fit” for the requirements based on the standard building blocks that are available. Sometimes, the “best fit” approach will not meet the goals of the design. For high-precision requirements, where the performance, accuracy, power, or size of the IC is critical, a custom IC design approach is needed.
A custom, or full-custom IC, design process uses unique building blocks that are created specifically for the function required. Many of the same tools are used in a custom IC design process that are used in the previously described semi-custom, or ASIC, design process. The difference is, the building blocks that are assembled are often custom built to deliver specific capabilities. An IC layout editor is an important tool for this kind of design process. The methods used to assemble these devices can also be different to accommodate the unique requirements of the IC or block being developed.
In cases where the performance, accuracy, power, or size of a particular circuit is critical, a custom IC design approach is the only method that will deliver the required capability. Many such demanding requirements exist for analog or power ICs. For example, a high-frequency receiver for a 5G cell phone infrastructure will require custom design due to the specific requirements for signal integrity and accuracy. ICs that switch large amounts of power, such as those found in smart electric meters, also require special design considerations to accommodate the extreme heat and electric fields of these applications.
Custom IC design also finds significant application in the development of the building blocks used in semi-custom, or ASIC, design. The standard cells used in this methodology are typically designed using full custom techniques to ensure maximum density and performance. Since these cells are re-used many times, the extra work to build an optimal design is justified. The low-level storage elements used to build memories (i.e., bit cells) are also built using a custom design process. Another example is the datapath in an embedded processor core. These elements must perform at a high speed to support the operating clock frequency of the processor core. In this case, the elements of the datapath are custom designed so they can be routed by abutment (i.e., all connections line up on the left and right sides of the datapath blocks). This method reduces routing delays to a minimum but requires custom design to facilitate the approach.
Below are the steps that are required in a custom IC design flow:
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