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It will no doubt be helpful if we also take a plane or "bird's eye" view of what this circuit looks like aswell. There are, in fact, some interesting things we can gain by looking at some of them.
We have been looking at the development of the circuit from a cross-sectional point of view, watching theformation of the various levels which make up the finished CMOS inverter. This is, in fact, not the way a circuit designer looksat things. A circuit designer sees things from above, and only worries about the placement of transistors, and how they will beconnected together. In fact, the only factor in the actual design of the layout engineer has any choice on is thetransistor width, W. All other parameters are decided upon beforehand by the process engineer. So what does the layoutengineer see? We start with the n- implant to make the n-tank, as shown in . (You should go back and follow along with the cross-sectional views of the process, aswe review looking at things from the top.) A mask opposite to that of the n-tank allows us to an n-channel adjust. We next deposit and pattern the nitride for the active regions, and grow the field oxide (FOX) . We remove the nitride, and deposit and pattern the poly., as seenin shows what the two masks look like for the n+ and p+ source/drain implants: Note that the gate poly extends beyond where the implant is being performed (inside the dotted line). This is a design rule which is the way the circuit designer takes into account the fact that the manufacturingprocess must have some tolerance built in, because things will not always be lined up just perfectly. Now we make some contactholes, seen in : And finally, we sputter and pattern the metallization, which is depicted in . You should go back to MOSFETs , and convince yourself that the circuit shown in is indeed what has been constructed in . See if you can identify all of the correct parts. Note that there is a connectionbetween (ground) and the p-substrate very close to the n-channel source. There is also a contact between the n-moatand which is very close to the p-channel source. What advantage would this have? Hint: review thediscussion of latch-up .
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