Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
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.
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.
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.
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.
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.
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?
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.
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.....
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.
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.
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.
This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
By continuing to use this site, you are consenting to our use of cookies.
