E migration independent from temperature. Why, when it's a quantum effect?

[JonJ678]

I know theres a materials scientist on these boards somewhere, but I can't remember the fellows name. Hopefully he'll spot this.

As far as I know, electron migration is electrons tunnelling from one place to another. So directly proportional to current, and a function of voltage. Everyone in overclocking seems convinced that it's independent of temperature, and I can't for the life of my imagine why it would be. Either the community has missed the point, the effect is considered negligible, or I've thoroughly misunderstood tunnelling.

Information would very much be appreciated

 

[JonJ678]

I hadn't drawn a distinction between forced ion diffusion and electron tunnelling. Thank you.

Black's equation is lifespan before death by electromigration. It puts mean time before failure as proportional to exp(1/T). For one thing that's not independent, for another its a fairly strong relationship, with higher temperatures leading to faster death by electromigration. So we assume this dependence is negligible.

Electon tunnelling is what I have misnamed in the title, I ran into it when looking into why the q6600 had a far higher allowable voltage than the q9550. So you also think this is dependent on temperature?

If anything I don't think it helps, what I thought was one effect is in fact two, both of which kill processors faster at higher temperature. I may start pointing people at Black's equation. Thanks for your post

 

[JonJ678]

I have a reasonable grasp of tunnelling, enough to know that at any applied voltage it'll happen but the odds of it happening become rather higher as voltage is increased. I also suspect the ions moving qualify as tunnelling as well, as they're moving through an energy barrier which classically would be considered too great.
Do you know the form of the relationship between temperature, voltage and electron tunnelling, and if not where I should go to find out? Something analogous to Black's. Proportional to temperature seems a fair guess, but I base this on absolutely nothing. I'd trying to work out how I translate a safe voltage at 320 degrees to one with the same electron damage at 220.

I am starting to gather the resources needed to start overclocking further. So the better the theoretical understanding of overclocking I have, the further I'll go. There's a nasty gap in my knowledge that needs to be filled before I start making things

 

[JonJ678]

1st class physics degree

How goes job hunting /phd funding with that? I wish you luck, some of my friends are struggling.

Very good post, thank you. I've had to write the equation out on paper as I couldn't read the one line version, but it's all good. I wont try running any numbers based on it for the obvious reasons, but the relationships are useful. Temperature independent makes sense at last, your patience is appreciated.

You're quite right, this is only useful for design. However before sending the chip below zero it would be wise to grasp at least the basics of how one kills processors. I'll head over to xtreme once I'm done trying to follow the theory and talk to the people who build fridges, but I'd rather make informed decisions about voltages than rely solely on trial and error and replacing dead components


So, electromigration is classical; ions exchanging places when energy is sufficient. Statistical still? This is strongly temperature dependent, so keeping the system appreciably below ambient will allow higher electron energies without death from this effect. I'm currently viewing this as solid state diffusion accelerated by hammering electrons into the ions, is this fair? If so I might be able to actually run some numbers on this, I think below a certain temperature even the most energetic electron just isn't going to make any difference.


Electron tunnelling is a separate effect. Keeping the system cold wont help with this, increasing voltage will steadily increase this. Memories of the maxwell-boltzmann distribution suggests that this is always going to occur, and can be safely ignored up to an approximate voltage beyond which all hell breaks lose as a significant fraction of the electrons have sufficient energy to move to where they aren't expected. I can see this ruining stability, but rogue electrical fields don't seem to be dangerous as such.

Is it a fair statement that I won't damage anything with this, but stability suddenly starting to drop off with increased voltage is a sign that it's becoming an issue? This suggests an ultimate limit, beyond which no further voltage nor cooling will help.

Cheers again man

 

[JonJ678]

Good fun. I'm glad you've found funding

Not quite, if they exchanged places, there would be no overall movement. Rather the ions are pushed out of the way by the electrons, and sort of 'squish up' to make room. Gaps then appear in the track, breaking the circuit

This is the first point you've made that I think you're wrong on.

The lattice is fixed, and the bonding energy between the ions is rather high. Momentum transfer will deform the lattice, but not much. Similarly this would be temporary were it the active mechanism, and would make electromigration badly named. Collisions transferring energy to ions with consequent increase in probability of exchange sounds more plausible. Note that this does make a difference, as the region in question is a mix of silicon, phosphorous and boron (or whichever two doping ions are popular these days). So exchanging doping ions with lattice would explain the name of the effect, why its permanent, and why it's dangerous.

It's the other way around really, the high temps are CAUSED by the collisions! When an electron transfers energy to an ion, it can be given as vibrational energy, causing a temperature increase. Keeping the temps VERY low will decrease the temperature rise, as the cross section of the ions blocking the electrons will be reduced, as they will be moving a lot less. It will still happen though, just more slowly

Not the point I was aiming at re cooling. Higher energy electrons transfer more energy to the ions, leading to more frequent electomigration events. Cooling the lattice removes energy from it, leading to less frequent migrations. Therefore sufficient cooling will offset the increase in voltage in terms of electromigration?

It has a lot more to do with the sheer number of electrons rather than their energy. I think. Need to read up a bit on that to be sure.

This sounds about right, though Id expect current density and electron energy to both matter. Going to have a look through some of my old textbooks I think.

What you will see as you approach the voltage limit (if this is causing the instability) is only occasional errors initially, but as you increase the volts, the frequency of these errors will increase greatly

Happy days

Quantum mechanically however, electrons can 'borrow' energy from the surrounding vacuum

Not very convinced by this. Looks suspiciously like lies aimed at making sense from a classical perspective. I'll have to get back to you on this one.