Deep N-well (DNW) ---?????

deep n well

Hi all



Can somebody plz explain why a DNW is used in analog layouts with appropriate documentation.Thanks in advance.


Regards
Swathi

deep nwell

DNW is used as a well for PMOS. It also serves as an isolation from onther type of devices. In a twin well process, the subsrate serves as the P well of NMOS.
sorry, i could not provide any document for that...

deep n well process

Deep N-well is a special layer used to supress Substrate Noise coupling injected by Digital Logic in Mixed Signal environment.During the digital logic switches from high to low or viceversa it injects noise which will be propagated through the substrate.Since the sensitive Analog circuit will be on same substrate, the noise can degrade the performance of the analog circuit.For example this noise can be amplified by an op-amp and its output will vary.

Sensitive analog blocks are surrounded by Gurad Rings and Deep N-well to suppress this kind of noise.

deep nwell layout

Generally DeepNwell is used to isolate NMOS from the substrate of other NMOS.
For example suppose all the NMOS in a amplifier is kept in PWELL (common substrate ) and that is connectet to gnd. and there is other NMOS whose substrate is connected to other potential not to gnd. In that case you need to isolate that NMOS from other NMOS, otherwise substrate will get short. So to avoid this DeepNwell is used.

So, for that you have to put that particular NMOS in a DeepNwell Layer and that MOS should sorround with a NWELL guardring.
Here DeepNwell is act as a bottom side and the NWELL guardring is act as a sidewall to seperate that particular NMOS from common P-type substrate.

For Example:- Suppose there is a Pond full of water and you want to put some instrument in that pond but at the same time water should not touch to that instrument. So for that you will construct something like BOX (with five faces, top side should open) and kept that instrument inside that BOX and put that BOX in the water, so that water will not able to touch that instrument. Similarly the bottom layer of the BOX is act as DEEPNWELL and the side wall of that BOX act as NWELL guardring. Hence the Psubstrat of that particular NMOS get isolated with other NMOS.

difference between nw resistor & dnw resistor

Deep N Well Process is a foundray method to islolate NMOS or for that mater PMOS.
this is done to prevent/reduce noise generated from substrate spl. during switching etc.
its like u have a n-well(N-) on 2 sides and bottom part is dep n-well(N+).
now with p-substrate u can form NMOS.

This structure isolates p-substate of the MOS from others. Hence less noisy.
This method is also used in digital ckts. spl. for deep submicron technologies.

PS: Kindly see figure attached for better understanding

Thanks
Manu

should dnwell be inside nwell

Hi all


Thank you very much for the response. DNW concept is clear right now.

Hav a great day !!!!


Cheers
Swathi

dnw nmos

Deep Nwell may not only used for mos transistors but also used for resistances...
In resistance it is used for the same concept to isolate the resistance from any noise generated in analog circuits...

n buried layer vs dnw

kumard35b said:

Deep Nwell may not only used for mos transistors but also used for resistances...
In resistance it is used for the same concept to isolate the resistance from any noise generated in analog circuits...

Click to expand...

This is rigth for diffusion resistors. For poly resistors the shilding can be done only using nwell. It is not needed to used deep nwell.

Bastos

deep nwell

Another benifit of the N-device build inside deep-N well is that it can serve as a negtive voltage switch when the N-well itself is shorted to a higher voltage, as we used them a lot in flash memory design.
while normal N-device can not do this since the grounded P-substrate will inject current into negtive source/drain, normal P-mos is the only choice for negtive voltage, however as you know P-device is not a good choice for a low voltage switch.

dnw guard ring

Very clear here,
D-Nwell can be used as

1) noise isolation
2) nmos which needs different sub potential

deepnwell

Does anyone has tried to cover a whole digital block with DeepNwell and surround it with Nwell connected to quiet VDD ?
It should greatly reduce the noise source from digital :?:

Is there any drawback or pitfall in that method ?

related:

Hi

you can refer to the attached document for Deep N-Well concept.

deep n well ic mask layout

I think "Deep Nwell" in tarun_taurus's pic is called N+ buried layer sometimes. However, in some other process, Deep Nwell is like normal Nwell. Only it will be deeper. It can be use in isolated NMOS or HVMOS.

cmos poly resistor noise nwell layout

it also can be used to construct the drain of HVMOS.

why nmos not in well

Hi,

What about pmos transistors? Is there any reason for having/not having deep n-well underneath pmos transistors. nmos transistors should be put in dnw to isolate their bulk from the global substrate. But, is there any difference between having nwell only (without dnw) and having dnw for pmos transistors?

Thanks

deep n well guard ring

I think DNW is employed for HV PMOS in most of cases. Both symmetrical and asym HV PMOS can be generated with DNW.

d - nwell

Hi,
DNW is employed for critical analog circuits where you want to isolate substrate noise to couple to your analog circuits. PMOS are less noisy than NMOS since PMOS has its nwell which isolates the substrate noise, but such is not valid for NMOS . So actually to reduce noise on NMOS we put Deep nwell. But adding DNWELL there is extra cost involved for extra mask requirement.

Thanks,
rajesh

deep nwell isolated nmos

What difference would it make if we have DNW for PMOS as it's already in its own Nwell?

What about the DNW doping compared to NW doping?

deep n well layout

okguy said:

Does anyone has tried to cover a whole digital block with DeepNwell and surround it with Nwell connected to quiet VDD ?
It should greatly reduce the noise source from digital :?:

Is there any drawback or pitfall in that method ?

Click to expand...

gr8 idea.....
anyone tried??
i suppose it is useful if we have smal digital block and large analog block... the noise will get supressed before it reach the substrate......

Added after 20 minutes:

In conclusion we can say that Nwell can be used for
1) Noise isolation
2) Seperate bulk connection for NMOS..... but how do we provide this in bulk CMOS process??


Deepak...

nmos dnwell

I think all points are covered by others.

But one must understand that nwell & DNW are shorted.
So if some bulk of pmos is at different potential, then that should have not be put under the same DNW. bcos nwell is connected to VDD and nwell and DNW are shorted to VDD.

So PMOS bulk if not VDD, should not be put under DNW.

regards,
JT.