SRF Tech
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Arizona
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Hi Jeff, If I understand you, your concern is that by connecting a MIM cap directly to the pad, you are violating a classic ESD rule in which no gate oxides should be directly connected to a pad, but rather should be somewhat isolated by a secondary resistor and secondary ESD clamps (some people refer to this as a "CDM resistor" or "CDM protection", since it was first proposed by Maloney as a form of protecting gate oxides from CDM failures.)
So what to do with the MIM, do you need the secondary protection?
The answer is convoluted....it depends on your process and the type of MIM your using. Some MIMs have a very nice, thick and robust nitride layer, and can tolerate overvoltage stresses of 15-25V...in these case you may probably get away without using secondary protection, and thus keep up your performance by avoiding series resistance. If you do this, I still strongly recommend secondary ESD clamps on the back side of your MIM.
If your MIM is actually a very high performance, thin insulator, you may need to consider treating it as a gate-oxide and protect it as such...yes the resistance will hurt you, and you will need to tune the resistance to both meet the manufacturing needs of ESD and the performance needs of your product.
In the end, you will need to know your process, and it reliability windows, as well as what your product can allow. I have many times used the robustness of a MIM cap as my ESD isolation, and its worked great, but in other processes, my MIM was nearly as delicate as my oxides and it was out of the question.
One way around this is if you can substitute a MOM cap (metal finger/overlap cap) for your MIM in this particular application. Such a cap would be plenty robust to not worry about ESD. But I do not know how big your cap needs to be and and so forth, but its something to consider.
I am assuming of course that you are using reasonable primary ESD protection, either provided for by the foundary or by some third party.
If I misunderstood your concern please let me know. Stephen
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