Sheet resistance of metals

[dinesh hegde]

sheet resistance of metals

Hi,
Why sheet resistance of matsl varies as the width of metal increases. (According to theory sheet resistance should remain constant).
Reasons that I could think of,
1. Thickness variation because of CMP.
2. Width reduction factor may be different for different width of metals.

Regards,
Dinesh

 

[erikl]

site:edaboard.com al cupper

I think your 2. item is correct: But not the "difference" of the "reduction factor" (which includes under-etching, optical resolution errors and other process-dependent deviations), but its constant contribution, which by this creates the width-dependency.

For very small structure sizes, also the width-dependent relative (negatively counting) part of the non-conducting oxide skin (Al2O3 on Al) may contribute to this effect (width-dependency due to the relatively different "contribution" of the oxidized side walls).

 

[dinesh hegde]

sheet resistance ic design

Sorry, I could not understand second point. Please explain more.

 

[erikl]

Sorry, I could not understand second point. Please explain more.

Non-noble metals like Al or Cu (which are the main constitutents of IC on-chip connections) unavoidably (in the IC production process) wear a thin oxide layer (skin) around them, which is an isolator and thus doesn't contribute to the metal layer conductance (during the via creation this insulating layer is being removed by etching, or bruised by ultrasonic action during the wire bonding process). The "thick"ness of this layer is in the order of only 5 .. 10 nm, depending on temperature and environment after the termination of the wire etching.

Thin as this non-conductive layer is, for very narrow process features (say ≤ 45nm) their side-wall part plays a relative role dependent on the width of the wire connections, so more as the height of these wires - at very small feature sizes - is sometimes larger than their width.

 

[sandeep_torgal]

Dinesh,
I feel it is due to the CMP(chemical mechanical polishing). The copper lines suffer dishing and erosion due to CMP. Hence the sheet resistance varies. Let me know if you are still unclear.

Regards,
Sandeep

 

[dinesh hegde]

I feel both of points contribute for this. Thanks.

Regards,
Dinesh