1. Specify your design targets
The first step towards designing an integrated circuit is determining what it has to do. Does it multiply signals? Filter signals? Or even multiply and filter signals? And how much power can it use? Femtowatts? Microwatts? Megawatts? These goals are not carved in stone – it could happen that in a later stage, for example, the local fire department advises against power consumption in the megawatts range.
2. Design your electronic circuit
With the design goals in mind, you can start designing the actual circuit. This usually starts by determining what output signal you want to generate under which circumstances. Secondly, you decide which functional blocks of components you will use to achieve that. For instance, you want to multiply a signal by a factor two, but only if its value is higher than a certain threshold. There is virtually an unlimited number of ways to achieve this; you can first multiply by two and then compare to the threshold, first compare and then multiply by two, multiply by four and divide by two and then compare, accidentally solve Schrödinger’s equation and forget to compare the signals, etc. Some methods are more efficient than others. Often, there is a tradeoff between the merits and the vices of the solutions you find, so you will have to make choices here. Try a few different solutions to see what happens.
3. “Paint” the layout
When you have designed the circuit of your dreams, it is time to convert it into a layout. On the micro scale, an integrated circuit consists of stacked layers of different materials. Some materials conduct currents, some do not, and some materials conduct currents only under specific circumstances. Because of the different properties of these materials and the way they are connected, an input signal is filtered, multiplied, transformed or processed in any other way.
You need to draw layers of different materials such that the layout represents your circuit – this is sometimes called layer painting. Luckily, you can often tweak standard solutions. Software can also often generate parts of your design.
4. Check the layout vs the schematic (LVS)
You can simulate the behavior of your layout and compare it to the behavior of the circuit. If they do not match, tweak the layout such that it better represents the circuit. You can shorten or broaden layout wires to lower their resistances, or use different materials to tune the behavior of your layout.
You can also decide to take the behavior of the layout into account in the circuit simulations. For instance, layout wires that are close to one another influence each other, which you have to model in your circuit. Often, you tweak both circuit and layout until a reasonable match between the both is found. If the circuit and layout behavior match the first time, … well, that never happens.
5. Check the Process Design Kit (PDK)
Not every layout that you can think of can and should be produced. There are dozens of design rules (PDK rules) that you have to adhere to, and some are more abstract than others. For instance, the production accuracy is limited, and as a result the metal layer width is restricted to certain sizes. Additionally, metal plates should not be too closely spaced; passing electric currents generate electric fields, which interfere with nearby wires. Moreover, the path lengths of different signals should not differ too much if the signals should be processed at the same time. The PDK check of your software will check these rules such that you can adapt your layout and your circuit … again.
6. Tweak the circuit and the layout until both the PDK and LVS pass
There is plenty of time to rethink your life choices during this process.
7. Send the layout to a foundry and let it stew for 3 to 6 months
Your integrated circuit works – in theory! Quick, send it to the manufacturer. You will receive a real-life version of your IC after only three to six short months. Afterwards, you can finally use it and hope it behaves just as nicely as it does in simulations.
8. Boast
You have designed an actual chip, congratulations! You are now permitted to brag about it at every possible opportunity.