Standard cells versus Full-custom Layout
The use of standard cells versus full-custom layout is a critical consideration in the physical design of integrated circuits. Here are some key points and insights related to this topic:
Standard cells Layout
Standard Cell Approach:
In standard cell design, there is a fixed cell height, but the cell width can vary to accommodate different circuit elements. Standard cells are designed to abut (align) on two sides, typically left and right, in a regular manner to prevent design rule violations. This approach provides flexibility and is a good starting point for layout design.
Economic Considerations:
The area on a semiconductor wafer directly impacts production costs and profitability. Maximizing the number of dies (individual chips) per wafer is economically advantageous, as wafer costs are relatively fixed regardless of the amount of circuitry on them.
Technical Advantages of Smaller Layouts:
Smaller layouts have shorter interconnecting wires, reducing parasitic loading and crosstalk effects. Minimizing layout size is essential for optimizing circuit performance.
Density Metric:
To quantify layout density, a common metric is the number of transistors per square millimeter of area. This metric allows for comparisons between different circuit layouts and processes.
Standard Cell Density:
Standard cell layouts typically achieve a density of around 5,000 transistors per square millimeter, depending on the process technology.
Full-custom Layout
Full-Custom Layout Density:
Full-custom layouts can achieve higher density due to tailored designs but come with increased complexity. For example, a full-custom digital filter layout achieves a density of approximately 17,500 transistors per square millimeter, representing a 3.5-fold increase compared to standard cells in the same process.
Interconnect Burden:
Interconnect wiring, often referred to as the “interconnect burden,” impacts both physical space and electrical parameters, including parasitic loading. It’s a significant consideration in layout design.
Contextual Aspect Ratios:
Unlike standard cells, full-custom layout cells have contextual aspect ratios that depend on neighboring cells. Width and height constraints are adjusted based on the specific requirements of adjacent cells.
Custom Layout Advantages:
Full-custom layouts provide greater design flexibility and can achieve higher density. They allow for precise optimization of performance and area.
Trade-offs:
The choice between standard cells and a full-custom layout involves trade-offs in design time, area efficiency, and performance. Automated place and route tools are emerging that offer a compromise between the two approaches.
Future Trends:
As process technology advances, the role of hand-rendered, full-custom layout may evolve. Software tools are expected to continue improving to automate tedious layout tasks, allowing designers to focus on creative aspects.
In summary, the choice between a standard cell and a full-custom layout depends on the specific requirements of the integrated circuit, with considerations for economics, performance, and design complexity. Advances in technology and tools continue to shape the landscape of semiconductor design.
What is the primary advantage of the standard cell approach to physical design?
The primary advantage of the standard cell approach to physical design is its regular and fixed cell height, which allows variable width for implementing circuits. It also facilitates regular abutment of cells, ensuring that any cell can be placed next to another without violating design rules.
Why is it essential to minimize the layout size in semiconductor manufacturing?
Minimizing layout size is crucial in semiconductor manufacturing because it directly impacts profitability. The cost of wafer production is relatively fixed, whether the wafer is densely packed with circuitry or not. Smaller layouts enable more dies per wafer, increasing cost efficiency. Additionally, smaller layouts lead to shorter interconnecting wires, reducing parasitic loading and crosstalk effects, which can improve circuit performance.
What metric is commonly used to quantify layout density in semiconductor design?
The number of transistors per square millimeter of area is a common metric used to quantify layout density in semiconductor design. It serves as a raw number and is applicable when comparing different types of circuitry or processes.
How does the density of a full-custom layout compare to that of a standard-cell layout, both using the same process?
In the provided example using the same 0.8 µm process, a full-custom layout achieved a significantly higher density, approximately 3.5 times that of the standard-cell layout. This difference is primarily attributed to the interconnect wiring in full-custom layouts, referred to as the “interconnect burden.”
What distinguishes the contextual aspect ratio of cells in a full-custom layout from the fixed aspect ratio of standard cells?
In a full-custom layout, the aspect ratio of cells depends on that of their neighboring cells. It is not fixed like the standard cells. The width and height constraints of each cell are influenced by the maximum allowable width of the widest cell in the group, making them contextual. This flexibility allows for more efficient custom layouts but requires more effort in design