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Why we should fill all the empty spaces with substrate contact in ICs?

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bouchy

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Why In IC it is recommended to fill all the empty spaces with substrate contact?
Thanks
 

Substrate contact

latch-up improvement?
 

    B

    bouchy

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Re: Substrate contact

working in ic custom analog layout for over 8 years I have not and will not place substrate in empty area. Its serves no advantage in an empty area. Further more what happens if a modification is required and the area that you need metal 1 over is already being used to tie down the substrate! more mask changes more money!
Spare space should be at a minimum and can be used up for layer ids or spare cap or nothing. The only time I would even remotely think about is if I had sensitive signals and I needed to sheild them from the substrate.
 

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    bouchy

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Re: Substrate contact

bouchy said:
Why In IC it is recommended to fill all the empty spaces with substrate contact?
Thanks
Click to expand...

I think empty spaces should be filled by decap rather than substrate contacts.
The basic DRC rules usually take care of the number of substrate contacts required
at regular intervals.
 

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    bouchy

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Re: Substrate contact

Yup it's correct. Substrate contact at empty places woudn't help to anybody. I mean any transistor.
It's better to fill the empty space with De-coupling cap and tye them b/w sensitive net and AVSS.
 

    B

    bouchy

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Substrate contact

how does this help in latch up improvement..... i'm not able to visualise the factor which reduces the problem of latchup through this....
 

Re: Substrate contact

why do ppl put silly comments up?
latch up...no. No effect.

Use empty space for either De-coupling cap

Or

Layer IDs, logo etc.

Or

Nothing!

End of!
 

Re: Substrate contact

You need to be very careful when placing substrate contacts down:

1. Filling empty space with PTAP reduces the effect you naturally get from field oxide, i.e. substrate isolation. If your substrate is noisy ( and it can be!), you have just brought all that noise up and injected it elsewhere.

So if are on a high omhic substrate and you place lots of contacts down, what are you really doing? You've paid for high omhic but you've now made it low ohmic

2. Also you can short out guard rings by filling them with taps.

ALWAYS draw a cross section of what you've done when you place down substrate contacts as this will make it easier to see what you've actually done.

- A
 

    B

    bouchy

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Substrate contact

substrate contacts decreases substrate resistance which in turn reduces chances of latchup.. but it is useful around Nmos not just anywhre in the chip.. so i prefer decoupling cap.
 

Substrate contact

How do you say that decrease in substrate resistance decreases chance of latchup... I don't see a link between the two... can u explain???
 

Substrate contact

hi,

Basically till i know latchup problem arrises when there there will be potantial difference arrises at nwell and due to that it triggers bipolar transistor structure.

So basically to avoid latch up you have to only reduse resitance of well and substract near mos.

you can achive by high doping but it will also increase leackage current in device so by adding more taps you can achive that.

i hope you now got it how latch up is reduced by more contact.

Can any one please tell me concept of decoupling capacitor and how to use and buit it in lay out, or from where i can got information about it?

i will be very thankfull.

shitansh_vaghela@yahoo.co.in
 

Re: Substrate contact

Hi All,

1/ To increase layout's robustness, the bulk must be strongly biased with
PTAP and NTAP strips

2/PTAP must be as far as possible than mos source,

A+
 

Re: Substrate contact

k_90 said:
working in ic custom analog layout for over 8 years I have not and will not place substrate in empty area. Its serves no advantage in an empty area. Further more what happens if a modification is required and the area that you need metal 1 over is already being used to tie down the substrate! more mask changes more money!
Spare space should be at a minimum and can be used up for layer ids or spare cap or nothing. The only time I would even remotely think about is if I had sensitive signals and I needed to sheild them from the substrate.
Click to expand...

One of the substrate tap function is to collect substrate current... so, if the design is sensitive to the substrate current i.e RF, u better put ur empty space with substrate tap other than decap.....
 

Substrate contact

But this is recommended in Art of Layout. In my experience, filling it for spare space will reduce many strange problems according to noise, parasite NPN, and etc. It makes your circuit much stronger.
 

Re: Substrate contact

deepak242003 said:
substrate contacts decreases substrate resistance which in turn reduces chances of latchup.. but it is useful around Nmos not just anywhre in the chip.. so i prefer decoupling cap.
Click to expand...

how does De-coupler cap helps?

And why we require substrate contacts ? Why can we use direct metal contact to feed to substrate?
 

Hi,

1. Empty space can be filled up with substrate connection.
- Help in reducing the substrate resistance
- Absorbing the minority charge carriers generated from other noisy devices
- reduction in the sub resistance helps in the preventing Latch Up

2. Usually empty spaces are filled with De coupling caps
- useful in filtering the glitches/noise on power/ground nets due to other noisy circuits/blocks/devices

Depending upon the requirement use the empty space.

Thanks and regards,
Basu
 

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