Fabricate a prototype of a chip is expensive and it needs from several months up to 1 year to be finished. Therefore, it is very important to ensure that you take advantage of your money with each fabrication opportunity you have.
1- Design the circuit schematic in Cadence Virtuoso. Ensure that all the simulation are correct and the circuit behaves as desired. On the picture below, you can see an small example of how a simple Cadence schematic look like:
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2- Layout generation. When fabricating the circuit, each electronic device has a physical structure in real life. For example a resistor is a long wire of POLY, a MIM capacitor are two metal layer with a insulator in between or the transistors have more complex structure.
All the devices are set together and routed following the schematic connections. Matched transistors or devices need special care and some additional structures have to be added like guard rings or dummies.
3- DRC Check. This test is performed usually with the tool Calibre, which is fully integrated in the Cadence Layout Environment. Calibre is a software from Mentor Grpahics and not from Cadence. DRC stands for Design Rules Check.
In this check rules for a proper layout are check, like space between polygon of the same layers or layers densities. Layers densities (labeled as ‘R’) are ignored here because they are going to be solved later, as it can be see in the pic below.
4- LvS Check. Here the Layout is compared versus the Schematic. The connections between components are exactly as it is drawn in the schematic. Here is where the possible electric shorts are found.
5- Pass the antenna and Bonding Wire rules. After both checks (DRC and LvS) are passed, these rules ensure that the chip can be manufactured properly without technical problems. For example long wires in a single metal layer are not desired or vias from the top layer directly to the gate of a transistor neither.
6- Floor Planing. The electric pads are connected to the previously designed block. Also if the chip is composed from various blocks, here they are put together. Also, the number of pins and the pin-out is defined.
7- Dummy filling. The previously chip layout does not fill the 100% of each metal layer. During fabrication it is desired that all the layers are more or less equally filled. This is made, excepting critical regions, automatically with a SKILL script. Here is where the DRC rules errors for “densities” are solved.
8- Last DRC and LvS. This check is not strictly necessary, if the previous 3 steps were performed right. But it is a good practice to double check everything after you may make small changes. A LvS (Layout versus Schematic) with the all the dummies is strongly recommended.
9- Export to a GDS file. The final version is exported in Cadence and sent to the manufacturer.
To do that, go to the CIW window. Then File>> Export >> Stream…
10- Send to the Manufacturer. In our case we are part of the EuroPractice. This is a program, launched and supported by the EU, aiming to facilitate the access to chip fabrication, reducing the entrance costs, reducing the risks and giving support.
Once the gds file is exported, we send it to our EuroPractice partner which checks the DRC rules and other aspects again. Usually they find errors or they suggests some changes. After 2 or 3 iterations. The final gds file is ready.
In our case, the partner EuroPractice, sum up many prototypes into a Multi Project Wafer (MPW) and send it to TSMC, which is the real manufacturer.
At the process, we received and email like this:
11- Receive and test the chip. After some months you will receive a low number of dies. The elapsed time ranges from 2 to 10 months depending on many factors. Between 20 and 40 units to test. You can decide if you want to encapsulate it into a package or not. Packaging a prototype die, it is not really cheap, and could cost almost the same as the fabrication.
For an early stage of prototype, you may want to only measure the fabricated die on-chip with needles. When you have a more developed prototype, you usually want to include it to a PCB, etc. So in this case it makes more sense to package.
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