Aug 13, 2013

Laying n-Type Epi

posted by Bryon Moyer

Dopants used to be there just for their doping. But stress is now an important aspect as well, which means the dopant atoms must be sized appropriately as compared to their silicon hosts. This has worked for p-type, where compressive stress is desired. Germanium, which is larger than silicon, compresses the silicon, increasing hole mobility.

n-type should be the reverse: tensile stress is needed, meaning smaller dopant atoms. Phosphorus and carbon are both smaller and can work. Sounds simple, right?

Well, apparently not so. The n-type dopants have a tendency to migrate, and so far increased border security hasn’t worked. OK, kidding. About the security, that is. The migration has remained to be solved.

At Semicon West, Applied Materials announced that they had found a way to create a stable n-type epi layer. How do they manage it, you ask? Keep asking… they’re not telling. There was a mention of millisecond anneals helping to tweak any vagabonds before they get too far. And whatever they do sets up a strict thermal budget, although not so low that it affects the back-end interconnect processing.

Details aside, if this is all working as promised, then we have more control over how to optimize the performance of n- and p-type devices. You can read more in their release.

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Aug 08, 2013

Interconnect @ 7 nm

posted by Bryon Moyer

IC interconnect is supposed to do two things: provided a path for electrons with as little resistance as possible and ensure that different paths don’t interact with each other. The first is about metal, the second about the dielectric between metal lines.

Copper is a good, low-resistance metal, but you can’t simply put copper on silicon or it can diffuse in. So you have to put down a barrier layer first, some sort of metal that will block the copper from contacting the silicon directly. Then you need a seed layer to enable the copper to adhere to the barrier and grow from there when deposited.

So far, we’ve been using Ta and TaN as barriers, about 16 nm of it. With a thick metal stack, that’s not an issue. The barrier itself may not have the lowest-possible resistance, but when it’s a small percentage of the stack, with copper making up the bulk, then, for the most part, you don’t notice.

Problem is, copper is getting thinner, and the barrier isn’t. This means that the resistance is going up as the percent of copper goes down. Which suggests the need for a new barrier that can be made thinner (putting off the day of reckoning).

As related in a discussion coincident with Imec’s Technology Forum and Semicon West, Imec has found that a 4-nm layer of Mn can reduce the resistance of 40-nm half-pitch lines by 45%, suggesting that this could be a good next step. It’s not a done deal yet, since they haven’t demonstrated reliability, nor have they completed the other duties needed to get a new material into the manufacturing flow; those efforts are underway.

Meanwhile, on the dielectric side of things, low-Κ dielectrics get their low Κ from the fact that they’re porous and have embedded carbon. The problem is, during an etch cycle, the etchant starts to invade the pores and remove the carbon. When done only on the fringes of a large expanse of oxide, that might not have a discernible effect. But on thin strips of oxide between metal lines, it essentially turns what are supposed to be low-Κ lines into normal-Κ lines.

In order to explore what might happen if the etch operation was done at very low temperatures, Imec did some experiments under cryogenic conditions. As expected, the ion mobility went down, slowing depletion, but a surprise effect occurred: some sort of barrier layer developed on the oxide, protecting it from the etchant. They’re not really sure what this barrier consists of.

They are also not sure what temperature enables the effect. If it truly requires cryogenic conditions, then it’s likely going to be too expensive to put into production. But if simply lowering the temperature to something more accessible can cause the effect, then we may have something interesting to pursue.

The thing is, Imec says that this is the only solution to the etch issue currently under study. So if it doesn’t work, then we’re back to square 1. Obviously, their fingers are crossed.

You can find out more about their metal announcement in one release and their dielectric announcement in another.

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Aug 06, 2013

Are Diamonds the Coolest?

posted by Bryon Moyer

It’s not every day you get to hold a diamond whose size is on the order of inches (or cm, for those of you that require simpler math). But there it was, and when I touched it to ice, my fingers went cold in no time.

This surreal flirtation with a vaunted Girl’s Best Friend (thanks to what has to be one of the most successful marketing campaigns ever) came not as I lounged in the VIP room of some swanky event, baubles ablaze, but in the rather pedestrian setting that is the Sensors Expo exhibit hall. I was sitting with Element Six (don’t worry, DeBeers hasn’t lost control of this aspect of diamondry – this is one of their divisions) discussing diamonds’ role in cooling.

We’ve actually covered their diamonds before in the contest of EUV windows. This time the agenda was completely different: They had samples and simple demos showing the effects of diamond being the best known cooling substance. Which is weird: we’ve all experienced thermally-conductive materials before (even if stopping short of licking a flagpole), but the speed at which this stuff works is uncanny. And that’s specifically because most of us have never handled diamonds of this size and shape before, and even if we were lucky enough to be around a multi-carat rock, we typically don’t geek out and say, “Ooo, let me dip that in this here ice water – feel what happens!” Definite date-killer.

In this case, I was able to watch a typical metal coin slowly carve its way through a piece of ice and then repeat with a diamond wafer. The coin looks like it’s working hard to melt the ice (because it is). The diamond goes through as if it were butter.

We normally associate good thermal conductivity with good electrical conductivity, and that’s not an accident: electrons that move easily can also ferry away heat easily. But diamond is an insulator, and it works differently: through phonons. Phonons are a quasi-particle invented to deal with physical vibration, especially when it comes to crystal lattices.

With Rube Goldberg-style mechanical linkages, if there’s lots of play in the joints, then when you move lever A which is loosely connected to rod B and thence sloppily to rod C which is poorly connected to the peg on wheel D, your initial movement gets attenuated by the time it gets to the wheel, and lots of the energy is lost. You work hard just using up that play.

Crystals are the same way: the tighter the bond, the more efficiently they transmit vibration. And this vibration can carry heat away. Of course, imperfections can mess that all up, causing reflections and other interference effects that result in lower thermal conductivity, so purity of crystal helps.

Graphene is the second-best conductor, but it only conducts in the x and y directions, since it’s not bonded to the layer below it. And even so, these are only sp2 bonds. Diamonds have tighter sp3 bonds in x ,y, and z directions, which accounts for its current reign as the King of Cool.

For certain applications, these guys are trying to get diamond inserted under your dice: they say a 10 °C reduction in temperature can double device lifetime. This isn’t likely going to help extremely price-sensitive devices like memories, but from a system standpoint, they say that, overall, costs can come down by more than the added price of using diamond. Specific target apps are RF, GaN power devices, and LED lighting.

But before you go running to Jared, there’s new news afoot. A shadow may be appearing over diamond’s cooling primacy. The Navy Research Lab (NRL) has announced that they’ve found a material with better thermal conduction than diamond. But before you go running to the NRL, bear in mind – this material hasn’t actually been tested yet. This is the work of “ab initio” simulation. Such simulations are what I refer to as a “first principles” approach, where you model things like atoms and electrons and such at a low level and then build stuff with them to simulate what will happen. This is done regularly for electronic analysis; using it for thermal analysis is apparently newer.

In the case of boron arsenide (BAs), its calculated simulated thermal conductivity is on the same scale as – and possibly higher than – diamond. And the reason is due to an effect that isn’t usually considered: the scatterings that can impede heat dissipation apparently have a hole at certain frequencies; vibrations at those frequencies can carry heat away extraordinarily effectively.

Or so the simulations say. Before you can indulge in the new Cooler than Cool, they have to actually build the stuff and test it and then, if it works, commercialize it. Or, rather, since this is a navy lab, spin it out and commercialize it.

So diamond is safe for now, but it might just be having a few 3:00 AM tossing-and-turning sessions over the next few years as it stresses over whether it will remain the coolest thing in town.

You can find more about Element Six’s cooling diamonds here; you can find the NRL announcement here.

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