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

What is electromigration? A lot of people know that an increase of the bus speed gives you higher CPU speed, higher bandwidth on RAM, and in fact an overall increase in bandwidth of your other peripherals as well, but what happens to a chip when you overclock it, run it at over rated speeds and don't provide it with proper cooling? What are the long-term damages? This is what we'll cover in our electromigration in-depth analysis.

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When overclocking a chip, there are a few considerations to make. A lot of people know that an increase of the bus speed gives you higher CPU speed, higher bandwidth on RAM, and in fact an overall increase in bandwidth of your other peripherals as well. Many of you are also aware of the result of way-beyond spec PCI and AGP speeds; corrupt data on your harddrive, even defective drives, and general system instabilities. Some are probably also familiar that when you have a chip that "almost does 850MHz" you can increase the voltage a bit to compensate for that, and it will probably run at the desired speed -- for a while.

But what happens to a chip when you overclock it, run it at over rated speeds and don't provide it with proper cooling? What are the long-term damages?

This is what will be discussed in depth in this article.

What is electromigration?

Harris Semiconductor Lexicon of technical terms puts it this way:

"Motion of ions of a metal conductor (such as aluminum) in response to the passage of high current through it. Such motion can lead to the formation of "voids" in the conductor, which can grow to a size where the conductor is unable to pass current. Electromigration is aggravated at high temperature and high current density and therefore is a reliability "wear-out" process. Electromigration is minimized by limiting current densities and by adding metal impurities such as copper or titanium to the aluminum."

Electromigration is an effect that occurs when an extremely dense electron flow knocks off atoms within the wire and moves them, leaving a gap at one end and high stress at the other. In a chip, the formation of such a void will cause an open circuit and result in a failure. At the other end, the increase of stresses can cause fracture of the insulator around the wire and shorting.

Fig. 1 Schematic diagram showing drifting of the cathode and mass accumulation at the anode.

Fig. 1 Schematic diagram showing drifting of the cathode and mass accumulation at the anode.


What this amounts to is the fact that when an electrical current passes through a conductor some of the metal atoms is swept along with the flow of electrons.

Fig. 2 The scanning electron microscope (SEM) picture below shows the real world effects of electrom

Fig. 2 The scanning electron microscope (SEM) picture below shows the real world effects of electrom


The smaller we make the conductor, the bigger the effect will appear. And, as the trend in CMOS technology is, they are getting smaller and smaller every year. As the cores are shrinking, the operating frequencies are increasing and the CPU speeds have increased by 30% per year the last 15 years on average according to the Semiconductor Industry Association.

The increase will continue exponentially. In 1999, the Semiconductor Industry Association predicted shrinkage in the core from the current 0,18 micron to 0,13 micron by the end of 2003. Even as I type this, I read that Intel and AMD seem to be a little bit ahead of the SIA's predictions.

So far, it seems as if the problems regarding electromigration will increase at the same ratio as the CPU frequencies.

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


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