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Schottky diode in CMOS process

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leg1234

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Hi, all

I am confused about the schottky diode in standard N-well CMOS process. Ideally, it is possible to build a n-wll to mental-1 schottky diode. Why I hardly see this in chip? and what is the difference of normal schottky diode and the schottky built in CMOS process? Why schottky is not that popular in regular process?

thanx.
 

Mistico_Pereyra

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Yes, it is posible, but you have to break some design rules of the process and you have to speccify them to your foundry, see layout figures of the chapter 3 of this document.
 

pit1000

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I think, that it is connected to necessity of use in addition at least one mask. Fabs, which use modular processes (for example XFAB XC06 modular CMOS) suggest diodes schottky, but it increases cost.
 

leg1234

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Thank you both. First, I cannot read that file.
In "Analog Integrated Circuit Desing" by Martin. It does mention about the schottky, and it shows the profile Metal1 -> N+ implant-> N- well -> Metal1. diode interface forms in between N-well and Metal1. It looks good and reasoonable. It does not need extra mask. (N+ implant, N- well, and Metal1)
My question is:
1. Why I hardly see this in chip?
2. What is the difference of normal schottky diode and the schottky built in CMOS process?
3. Why schottky is not that popular in regular process?
 

pit1000

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2. From Alan Hastings
PSD guard ring require NOPLDD mask.
 

    leg1234

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yakyao

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Hi, my understanding for the additional mask NOPLDD in XFAB process is to increase the break down voltage (19V), the NOPLDD is used to prevent the LDD form together with P+ guard ring.
 

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