Analog layout, its an art as some people say, but I would say that it's a technical art filled with colorful layers and delicious complications that makes it different than the digital world.
The analog layout is the process of designing a mask gds from the symbol-based schematic for analog circuits. This is done only to circuits that have complex constraints and need a manual touch. It has various steps like the floorplan, placement, routing, and verification. The people who do analog layouts are analog layout engineers. In every step of the way in engineering, there's always a testing phase, In analog layout also there are steps like drc and lvs to make sure everything is good, along with these an experienced analog layout engineer possesses a common sense that can't be tested, which makes circuits perform well. Despite these advances in technology, layout engineers still make use of experience in order to generate the correct layout.
The common sense that the analog layout engineers apply to layouts differs with different engineers and circuits and sometimes it doesn't have a standard way that's why we also call it a custom layout.
As I mentioned earlier, the custom layout is very broad and differs by radio-frequency layouts, memory layouts, and mixed-signal layouts.
The layout design is responsible for implementing the design on silicon wafers, resulting in a physical representation of the design that meets the performance and reliability specifications.
Designers must consider several factors while laying out analog circuits in VLSI, including floorplanning, signal routing, power distribution, ESD protection, and layout verification.
Floorplanning is the process of determining the optimal size and location of each block in the chip layout. This process also involves determining the aspect ratio of the chip, which is the ratio of the length to the width. In general, a chip with a square shape is preferable, as it minimizes the parasitic effects of the circuit.
Signal routing is another critical aspect of analog layout design. Analog signals must be routed in such a way as to minimize noise and parasitic effects. Careful consideration must be given to the placement of signal wires to ensure that they are not routed too close to each other or to power wires.
Power distribution is also essential in analog layout design. The power distribution network must be designed to ensure that the power supply is stable and noise-free. This process includes the placement of power and ground wires, the use of decoupling capacitors, and the design of a robust power distribution network.
ESD protection is necessary to ensure the reliability of the circuit. ESD protection circuits are incorporated into the layout to protect the chip from ESD events.
Layout verification is the final step in analog layout design. The designer must run various verification checks on the layout, including DRC (Design Rule Checking), LVS (Layout vs. Schematic), and ERC (Electrical Rule Checking), to ensure that the layout meets design requirements and is error-free.
In conclusion, analog layout design is a crucial aspect of designing analog circuits in VLSI. The layout design process involves several critical considerations, including floorplanning, device sizing and placement, signal routing, power distribution, ESD protection, and layout verification. The designer must have a deep understanding of the underlying physics and design rules, as well as hands-on experience with layout tools and techniques, to achieve optimal performance and reliability in the design.
On this website, we collaboratively try to create learning resources available, to help future-generation engineers and also working professionals in improving their skills in analog layout.