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 Semiconductor Electromigration In-Depth
Aluminum Vs Copper
By: Sverre Sjøthun, July 24, 2001  Print this article

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In the mid -90ies, IBM found a way to use copper as interconnect instead of aluminum without having problems with the atoms migrating out of a copper wire into surrounding chip material. This technique is known as dual Damascene Cu.

As most CPU designs nowadays are moving over to the copper interconnect technology because of speed and price considerations, this changeover also carries a hidden bonus. Research has shown that the dual Damascene Cu has a much higher resistance to electromigration than the previously utilized aluminum interconnect-technology. This is clearly illustrated in the image below:

The two metals are exposed to the exact same conditions, yet the aluminum interconnect has lost almost 1/3 of its length while the copper interconnect is barely affected.

This is an extract from an IBM report comparing copper and aluminum interconnect wiring:

"The integrity of the damascene-copper process was evaluated by fabricating 288-Kb SRAMs and performing the standard package thermal cycle and temperature/humidity/bias stresses. In addition, we performed electromigration and stress-migration lifetime measurements of 300 nm wide / 400 nm high wires. For all stresses, subtractive-aluminum control wafers and chips were also fabricated as a reference. The yields for the packaging stresses were excellent, with 0% fails for the damascene-copper chips and 0.4% for the subtractive-aluminum-control chips. Figures 1 and 2 show electromigration and stress-migration data for the 300 nm multilevel wires. Compared with the subtractive-aluminum wires, the damascene-copper wires exhibited more than two orders of magnitude improved reliability. Based on these reliability data, we believe that damascene-copper wiring is fundamentally more reliable than subtractive-aluminum wiring."

StressCopperAluminium

 

n

Yield

n

Yield

2.3V 100°C 5hrs

149

100%

247

100%

2.7V 140°C 5000hrs

149

100%

244

96.7%

0 - 125°C thermal cycle 20X

149

100%

244

100%

-40 - 150°C thermal cycle 200X

149

100%

244

100%

-65 - 150°C thermal cycle 1000X

11400

100%

-

-

-160 - 300°C thermal cycle 200X

11400

100%

-

-

288-Kb SRAM Functional Stress Results for Subtractive-Aluminum and Damascene-Copper Interconnects.

Fig. 3 Electromigration Data at 295°C and 2.5 MA/cm2 for Damascene-Copper vs. Subtractive-Aluminum.

Fig. 3 Electromigration Data at 295°C and 2.5 MA/cm2 for Damascene-Copper vs. Subtractive-Aluminum.


Fig. 4 Stress-Migration Test Structure Resistance vs. Time for Damascene-Copper and Subtractive-Aluminum

Fig. 4 Stress-Migration Test Structure Resistance vs. Time for Damascene-Copper and Subtractive-Aluminum


The important data for us is shown in figure 1. The mean time to failure of copper interconnects compared to aluminum is nearly 100 times less! (150 hours compared to 1.3 hours).

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  Introduction
  Aluminum Vs Copper
  How does this affect my chips?
  Forced Air Convention
  Case cooling
  Conclusion


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