Why contact and via have exact size ?

[dream_taker]

via size ic

Why contact and via have exact size (say == 0.22um) but not minimum size like Metal 1 should be >0.23um (just I have given numbers. It can be in any technology). Is there any specific reason for it ?

 

[sergeyr77]

Re: contact and via size

This is due to yield problems. Actually it is minimum opening size.
PS. If you need more contact/via area ( for reducing voltage drop on it, for elecromigration issues, etc) icrease number of contacts.

 

[dream_taker]

Re: contact and via size

But they dont give in Design Rule that its a min. size. They mention it has EXACT size. If its yield problem then it can be minimum size rather than exact size. Can anyone have idea abt this rule ?

 

[dreamteam]

contact and via size

This must be exact size because if they only said that is minimum size, you could make a big via (we can imagine 1u/1u). In this case, metal stress will be too important and can have negative effect on your design. In all fundary, via have exact size.

 

[uladz55]

Re: contact and via size

It's not only metall stress problem.
If you have CONTs and VIAs with different size, you can get over-etching problem for CONTs and VIAs with smaller size and damage structure.

 

[okguy]

Re: contact and via size

If you have good relation with the wafer fab, you can wave it to reduce their resistance for some critical application.
But, then, it becomes under your own risk and control!

 

[Colbhaidh]

Re: contact and via size

For technologies 0.35um and smaller, the contact and Via sizes are usually exact. This is because of the process. The process itself may vary the contact size but only by about +/-0.05um. This is due to the way global planarisation (CMP) is used for inter metal dielectrics and the formation of contact and via tungsten plugs.
If you vary the size outside the design rules, it would have severe implications for the tungsten plug process - e.g. it might not work at all. Some foudries allow you to change the dimemsion in one direction only, although I would not advise that as this can cause yield issues (apart from edge die seal).

 

[okguy]

Re: contact and via size

You are right. The 2 times I did it it was only an extention in 1 direction and much less than twice the contact size. But, it helped to lower contact resistance.

 

[kwkam]

contact and via size

In nowadays technology, via and contact are exactly size are because of the control of the process and yield issue. However, in some old Bipolar process. via and contact can be vary.