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what is the reason of the top metal layer...

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daffo123

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Top metal layer...

Do you know the reason??

Why is a top metal width wider than other lower metals in semiconductor
process?
 

koggestone

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Re: Top metal layer...

Top Metal Layers (like M7 , M8 , M9 , ...) are Wider bcoz to decrease Resistance.

Equation for Resistance is
R = rho * Length / (Width * Thickness)

User has Control over Width .
Thickness is a process Parameter (i.e fixed) and is also big for Top Metal layers
to decrease Resistance.
 

    daffo123

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daffo123

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Re: Top metal layer...

koggestone said:
Top Metal Layers (like M7 , M8 , M9 , ...) are Wider bcoz to decrease Resistance.

Equation for Resistance is
R = rho * Length / (Width * Thickness)

User has Control over Width .
Thickness is a process Parameter (i.e fixed) and is also big for Top Metal layers
to decrease Resistance.

Thank you for your explanation

But,why only top metal is wide??

If M1,M2,M3,M4,M5 are also wide, All metal would have low resistance.

Please let me know...
 

ankitgarg0312

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Re: Top metal layer...

lower metals are kept for nets connection that are not conducting heavy current.
top metals are for carrying heavy currents like clock,vdd,vss etc...

if we have wider metals in lower levels , you will face issues due to heavy capacitve effects of wide metals and biggest issue will be routing and die area.

e.g.

for a small inverter output having average current of 10uA , if u put a big metal your rise time fall time ,delays will not improve :
1. resistance will decrease due to increased width of metal
2. capacitance of that metal wire will also increase cap = (E * Area / D )
so overall your rc time contant will not decrease

but increasing metals width will increasr area of inverter due to more routing area required and overall your die size will increase ( cost increase )

hope this helps !!
 

    daffo123

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daffo123

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Re: Top metal layer...

That helps me a lot

Thank you~!!!

Added after 15 minutes:

I have one more question.

If I use 8metals process, Which metal is preferred on CTS.

Only metal8(top and wide metal) is preferred?

I really really want to know.

Thank you in advance.
 

jitendravlsi

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Re: Top metal layer...

Dear,

if have total 8 metal layers then you should opt for 7&8 for power routing and next highier metals 5&6 for CTS.


THANX
 

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