Full-Custom FAQ
I'm confused about the layers...
The "active" layer determines the source-gate-drain locations of both NMOS and PMOS devices. The opposite of active determines where the field oxide is grown. The overlap of the active layer and the poly layer determines where the gate oxide is grown.
The "P-select" layer covers the PMOS device with photoresist so that the NMOS can be ion implanted with n-type ions to form the doped source and drain.
The "N-select" layer covers the NMOS device with photoresist so that the PMOS can be ion implanted with p-type ions to form the doped source and drain.
Some processes (i.e. not MOSIS) do not have the select layers and instead have separate n and p active layers. The select layer is then derived from this. (This means there are only 2 instead of 3 drawn layers in these processes.)
Also note that the video today showed an additional implant from what we described in class. The implant was before the source and drain implant and before poly. It is the threshold implant which adjusts the characteristics of the transistor. We will discuss this more on monday when I get into the electrical characteristics of MOS devices.
My LVS doesn't match vdd! and/or gnd!
This has many possibilities. The most common are:
- You didn't make an inputOutput pin in the layout.
- vdd and vcc are not the same components. The "body" (well) of the transistors in our library (nmos/pmos) are assumed to automatically connect to gnd! and vdd!, respectively. If you use vcc instead of vdd, this will cause a floating net problem and LVS won't run. You can intentionally change this by using the nmos4/pmos4 devices that let you explicitly connect the body, but that should be unnecessary for this class.
Why does my extracted netlist seem disconnected?
Your designs WILL NOT PASS LVS if you use Create->Via. It will appear that the nets are disconnected if you try to highlight them in the extracted view. For example, your first level inverter output should connect to all the other FO4 poly gates. If you select the metal net in the extracted view and it does not highlight all the poly gates, you have this problem!
