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Design >> Analog Design >> Big Contact vs Multiple Contacts
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Message started by qiushidaren on Jul 31^st, 2007, 12:17am

Title: Big Contact vs Multiple Contacts
Post by qiushidaren on Jul 31^st, 2007, 12:17am

Hi all,

Why we don’t use big contact rather than multiple contacts in layout? What is the reason?

Regards,
Terry

Title: Re: Big Contact vs Multiple Contacts
Post by vivkr on Jul 31^st, 2007, 2:29am

Hi Terry,

This has to do with reliability. If you use a very large contact, then the metal may penetrate all the way to the active region (diffusion).
This would then result in a short. Hence, one uses multiple contacts instead. Usually, the dimensions of individual contacts and vias
are fixed for a given technology.

Razavi discusses this and other issues in the last chapter of his textbook on analog IC design.

Regards
Vivek

Title: Re: Big Contact vs Multiple Contacts
Post by kwkam on Jul 31^st, 2007, 6:43am

Use of multiple contact result lower contact resistance than a single big contact

Title: Re: Big Contact vs Multiple Contacts
Post by sheldon on Aug 1^st, 2007, 4:36pm

Terry,

One other reason is manufacturability. If the layout has large vias and small vias mixed
in a layout. Then to resolve the small via windows, the large vias are overexposed. This
results poor control over the large via windows and makes DRC checks more complex.
The enclosure and overlap rules for via windows become highly area dependent. In addition
to photo effects, the etch control also tends to suffer. As a result, the manufacturing guys
strongly prefer layouts with a single fixed via window size.

Best Regards,

Sheldon