Why do we use multiple vias and contacts instead of a single via and contact?

Hi,
I'm a beginner in layout designing. I wanted to know why do we use multiple vias n contacts instead of a single via and contact..

Re: multiple vias

people use multiple vias to connect to different layers of a board
foristance three layers may need to talk to one point
but not at the same time
via current limit resistors

you can use a single via and contact if you like
or use only top and bottom etc...

multiple vias

usually a via will introduce ~20ohm of resistance (90ns process), so if high current presents or you really care about the routing voltage drops, then multiple via will help. especially for cross layer power bus routing connection, you shall see tens of vias or even hundres of vias to reduce the resistance.

Re: multiple vias

Aditionally to the above answers, yield is also a factor for placing multiple vias or contacts. Especially contacts often cause yield problems in sectors where failures are practically unacceptable (eg. automotive). Some place&route tools are capable of doing post- routing via optimization in order to place double vias where possible to increase yield.

Regards,

C.

Re: multiple vias

Lets look at this query from a different perspective...
Via is a connection between two metals.
Imagine there is a metal 2 layer carrying 300uA of current which has to be connected to the source/drain of a transistor. Lets say, you put a single via and a single contact. This will usually have a size of around 0.2x0.2 ie 0.4um²... So this actually means the 300uA of current has to reach the diffusion regions of the transistor through this small region only...
This will lead to problems as this area gets heated up when compared to other areas due to high current density which will lead to a very short life for the device. Also, as long as the device works, it'll see a lot more resistance than expected which might lead to the failure of functionality as well...
So one should not be happy to have put a wide metal at places carrying more current but also suitable increase the contacts and vias to enable easy flow of currents between the different layers.
Hope this helps...
I am a design guy. So layout guys, please correct me if I am wrong...

Re: multiple vias

thanks

what is the advantage of using smaller vias than using one large via.. n contact.

Re: multiple vias

The contact and via sizes are usually fixed for a particular technology and fab... (Again, layout guys correct me if I am wrong)...
Every via/contact will have some resistance.... By using many of them, the resistances will become parallel and hence effective resistance decreases...
hope this helps...

Re: multiple vias

hi srieda... i agree with ur point
but im working on BJT... n the foundry does not provide with fixed via or contact size. it just provides the minimum size.... i can go for a larger one... so i want to know which is preferable??? single large via/cont or multiple small vias??

Re: multiple vias

hi all

why isn`t anybody talking about reliability concerns with single vias and contacts???

Regards
Brittoo

Re: multiple vias

How do we calculate the number of via required.

Re: multiple vias

anoopk said:

How do we calculate the number of via required.

Click to expand...

I dont there exists a fixed formula to calculate the number of vias required. All you can get is the "minimum" number of vias reqired for a given metal width (this is given by the foundry).
To get to a maximum value, you need to take into consideration the charge flow throught the vias and try to reduce the resistance offered to the flow of current as much as you can.

Added after 1 minutes:

darshitha said:

thanks

what is the advantage of using smaller vias than using one large via.. n contact.

Click to expand...

A Large via will provide great stress to the metal layer below. Hence it is always advisable to have multiple smaller vias rather than going in for one large via.

--Cmos_Dude

Re: multiple vias

we use multiple vias to increase the reliabilty/yield of the design at the time of fabrication.
the larger the number of vias lesser is the resistivity.

but at the same time instead of placing the large number of contacts, we try to place few vias but in a relaxed manner.

as far as the larger via is considered, we generally try to avoid large structures as at the time of fabriction there is more possibilty of etchant getting collected over the site of large structure and hence effects the smaller devices near to it.

Added after 1 minutes:

we use multiple vias to increase the reliabilty/yield of the design at the time of fabrication.
the larger the number of vias lesser is the resistivity.

but at the same time instead of placing the large number of contacts, we try to place few vias but in a relaxed manner.

as far as the larger via is considered, we generally try to avoid large structures as at the time of fabriction there is more possibilty of etchant getting collected over the site of large structure and hence effects the smaller devices near to it.

Re: multiple vias

via size is fixed in a process because different sized vias will etch at different rates. If the process allows multiple sizes, it is more difficult to control via size or even control that all vias are open. Instead of bigger vias, we use multiple vias

since you are using a BJT process it is probably not critical, you could make long vias if you want to, but it is more work.