Tube Tech

Published Date
01 - Mar - 2007
| Last Updated
01 - Mar - 2007
 
Tube Tech


Nanotechnology is the future! We've heard this a million times before, but now it finally seems that nanotubes have a foot grounded in reality


Tech buzzwords come and go, some more long-lasting than others. Like "convergence". But it isn't often that so many researchers in so many areas get hooked on a thing. That thing today is nanotubes. These miraculous structures are promising to deliver in so many areas that big companies-IBM, Intel, and the likes included-and small companies alike are rushing in to invest in nanotechnology in general, and nanotubes in particular.

If you missed out on what carbon nanotubes are, check out Move Over Silicon, Digit, May 2006. But here's a one-line recap: they are elegant-looking structures made of carbon (duh): hollow columns of carbon atoms. One of their most important properties is that they can be both semi-conducting and conducting; the conducting ones conduct very well indeed, well surpassing copper and aluminium in that area. You'll find out more of their properties as you read ahead.

As our regular disclaimer goes, we can't cover everything that nanotubes are good at in this space, but we look at their applications in things ranging from displays to batteries to chip interconnects. And more.

And Nanotubes Come In…
…to save the battery. They're running out so often these days, people are getting tempted to give up on battery-powered devices altogether and take up farming. But would you believe that someone is planning on making batteries last longer by using nanotube-turbo-charged capacitors?

In case you never attended engineering college (whether you were enrolled or not), a capacitor is a crucial component of almost anything electronic, and it holds charge, which it can release very quickly. (That's why you're not supposed to open up a TV set: it has a lot of capacitors that hold high voltages.) How it works, briefly, is that two conductive surfaces are separated by an insulator. If the electrodes have a voltage difference, and are then connected by a wire, current flows so as to equalise that voltage difference.

Sounds like a recipe for a battery. Why not just use capacitors? The problem is they can't hold much charge. They'd have to be huge to hold as much charge as a standard battery. Why? Because the surface area of the conductive surfaces (the electrodes) would have to be very large.

Enter the nanotube, to the sound of a boisterous orchestra. Joel Schindall, an engineer at MIT, thought up this one: cover the electrodes with millions of nanotubes. And how would this work, please?

We said the surface area of the electrodes would have to be much larger. Now, why does a Turkish towel-with its coating of water-absorbing threads-work better than a plain cloth towel? Because it has more surface area. Just so, the nanotubes increase the surface area of the electrodes to a very high degree, without taking up too much space themselves. The result is a capacitor that can rival a traditional battery in terms of charge.

Timeline: five years. (Schindall's estimate, not ours.)

Motorola And Their "Hybrid" Display
We begin with a disclaimer here: the use of nanotubes in displays does not promise world peace. It doesn't even promise to revolutionise displays-not too many companies have yet bought into the idea. But we must mention this one so you get an idea of the sheer range of applications nanotubes have!

 There are too many new display technologies, and nanotubes have jumped in to add to the confusion. Will they, won't they deliver? Motorola's nano-emissive display design has been touted (by Motorola-who else?) to be superior to all flat-panel technologies. In what way? Well, all the usual suspects: very good brightness, very quick response times, colours almost as good as those on a CRT, longer life, and-the best part-they're cheaper to manufacture.

How does it work? If you remember engineering college lessons-or even if you don't-it's simple. CRTs sweep an electron beam across the screen, which is coated with phosphors that glow when excited by electrons. The nano-emissive display uses an array of nanotubes to fire the electrons, instead of whatever fires them in regular CRTs, and there are clusters of nanotubes behind each pixel. Think of an NED as a cross between CRT and flat-panel technologies.

Motorola's prototype is 4.7 inches diagonally, with a resolution of 128 x 96 pixels. It was designed to be one piece of a 42-inch HDTV screen with a resolution of 1280 x 720. Well, they're getting there.

There's always a catch to any nanotube application, and it always goes like this: "how to get the nanotubes to work in <some> way." Here, the problem is attaching the nanotubes to a glass substrate. Motorola says they're managing pretty well.

There's always a catch to any nanotube application, and it
always goes like this: "how to get the nanotubes
to work in <some> way"


Nantero…

…is a small company that has big plans for NRAM (nano RAM), saying it will replace all kinds of memory altogether, including the hard disk, once and for all. They say we'll have a universal memory of sorts in the form of NRAM. It's again about nanotech coming to the rescue and making possible the dream of Universal Memory.
There happen to be several contenders for Universal Memory-notably MRAM ("M" for "magnetic"), FRAM ("F" for "Ferroelectric Materials"), and more. We're hoping to see something concrete soon-these terms have been bandied about for too long now! In any case, to understand how NRAM works doesn't require a PhD.

Refer the figure below, and look at each component, specially the interconnects, the ribbon-like things, and the grey electrodes. The nanotube ribbons, anchored to the interconnects, stay suspended over the electrodes. Now when a voltage is applied between the ribbon and the electrode, the ribbon bends and touches the electrode.

That's one key to the working. The other key is that the ribbon stays stuck to the electrode even when the voltage is turned off, due to certain molecular surface attraction forces. This makes for non-volatility, meaning that power need not be continuously supplied to the device.

And thus are obtained the ones and zeroes: when the ribbon is far from the electrode, it's a zero. When the ribbon is touching the electrode, the resistance between them is much lower, and it's a one.

How does this compare with SRAM and DRAM? It turns out that the switching can be as fast as that of SRAM, making NRAM have the "fast" characteristic of SRAM; at the same time, the density of the assembly approaches that of DRAM. And the whole thing is non-volatile, like we mentioned, so it's got the goodness of Flash.


RAM, in sum, trounces all three-DRAM, SRAM, and Flash-except for one little thing: it isn't as fast as SRAM. So we have the best of all three worlds, give and take a little. Visit nantero.com for more on NRAM. There's a movie illustrating the principles of operation of NRAM

Seagate Innovates… Again

Seagate is obsessed with hard disks. Which is quite in order, considering that's their business, but they have this habit of coming up with innovations ever so often. They're now dabbling in nanotubes, and are researching a rather complex process that can make hard disks denser.

We're saying "complex," but we can break it down into steps to see where the nanotubes come in.

First, we need denser hard disks. Because. Now, techniques such as perpendicular recording do make disks denser, but we need more density. Just because.

Now, if one just packs the bits on a hard disk platter too close together, there's a chance that a bit can flip its adjacent bit. In other words, it's demagnetisation at work, because of too high a density.

Now how does one remedy this? By using a recording material of high "anisotropy," meaning that it's harder to demagnetise, and so will reduce the chances of bits flipping each other. Problem: such material is also harder to magnetise as well.

This has an answer, too. Use a laser beam to heat the spot being recorded on, because when a spot is hot, it's easier to record on. This happens to be a better solution than using a stronger recording head.

Still with us? OK. When the heating is taking place, the lubricant film on top of the recording surface (yes, there is one) could evaporate or decompose. That, in turn, considerably reduces the life of the disk.

And here's where our hero-the nanotube-comes in. Spread nanotubes all over the surface of the platter-nanotubes filled with lubricant. The nanotubes slowly release the lubricant over the life of the disk, keeping the head spinning happily.

So that's how it is-they can hold lubricant as well, we've now learnt!

Aiding In Cooling…
…are the nanotubes of tomorrow. Cooling of chips, that is. Fujitsu and Intel are into this one, and it was reported more than a year ago.

What Intel is doing is simple. It just involves putting nanotubes into the thermal grease that makes up the layer between a microprocessor and its heatsink. A nanotube layer works orders of magnitude better than regular thermal paste, it's been claimed.

Why nanotubes? Because they conduct heat extremely well, and because they lend themselves to suspension in polymers and coatings. This second point is important: Intel will either design a polymer film containing billions of nanotubes, or try and find a way to deposit the nanotubes onto the (silicon) substrate.

Fujitsu's system is a heatsink made up of millions of nanotubes "grown" on a wafer substrate. The structure of the heatsink is such that it matches the pattern of the electrode bumps on the base of the chip to be cooled. Fujitsu is now working to improve the nanotube density around the bumps which, of course, will lead to even higher heat dissipation.

In independent research at Purdue University, "lightning" (!) produced by nanotubes could generate tiny air currents that can cool chips. This is air cooling that the researchers say works as nicely as water cooling. The technique hinges around the fact, as usual, that nanotubes are small. Ions (electrically-charged atoms) are generated using electrodes separated by as little as 10 microns. The cathode is made of nanotubes with tips just as wide as 5 nm.
The ionisation of the air leads to an imbalance in the charges on the electrodes, which results in "lightning bolts." This nano-lightning is responsible for the tiny breezes that do the cooling.

Normally, lightning requires thousands of volts, but here it's been done with a hundred volts and less-because the nanotube tips are so narrow, and because the electrodes are so close together.

These three examples aside, there are many people doing different kinds of work on cooling using nanotubes, and nanotube cooling could be one of the first major "solutions to a real-world problem" that nanotech rolls out.

At one million amperes per square centimetre, copper just burns out.
Enter the nanotube: they can carry a thousand times that much

 
Also Good As Interconnects
Now we get into our "As chips get smaller and faster" mode of speaking. And so, as chips get smaller and faster, copper interconnects get more difficult and expensive to fabricate; their electrical properties degrade at the scales we're approaching.

Electromigration is the name for the phenomenon whereby the reliability of integrated circuits degrades-because it makes nanometre-sized copper interconnects unreliable, and can even lead to wire failure. Visit http://en.wikipedia.org/ wiki/Electromigration for an explanation of the phenomenon.

Finally, at one million amperes per square centimetre, copper just burns out. Enter the nanotube, as usual: they can carry a thousand times that much. In addition, bundles of densely packed nanotubes can have significantly lower resistance than copper-and as you're aware, lower resistance is a much sought-after attribute.

In addition, with nanotubes, there is no need to create deep, narrow trenches on silicon wafers in which copper conductors are traditionally buried-further increasing the potential for miniaturisation.

With all these advantages to be had, and with all the difficulties that come in the way of nanotubes being used as interconnects, it's only natural that almost every nanotech organisation-and some non-nanotech companies as well-are in a race to bring out nanotube-interconnect-enabled chips.

Scientists at the NASA Ames Research Center (ARC) are amongst those working on the problem. One of the problems is that of the nanotubes being entangled; when they are, they don't display their fantastic current-carrying prowess. The idea at the ARC is to develop a process to untangle them.

In January of this year, it was reported that researchers at Rensselaer Polytechnic Institute had created hybrid structures that "combine the best properties of carbon nanotubes and metal nanowires." These structures could help overcome some of the main hurdles to using nanotubes in chips, displays, sensors, and such.

Of particular interest is the fact that the researchers' approach allows "the precise attachment of nanotubes to individual metal pins, offering a practical solution to the problem of using carbon nanotubes as interconnects."

As a final word, Intel, too, is looking at nanotubes as a replacement for copper interconnects. The company has managed to create prototype interconnects out of nanotubes, and measure how well the interconnects perform.

The whole nanotube interconnect thing points, essentially, to what you'd expect: shrinkage. And we are, naturally, going to see a lot of that-just like we've been seeing the past couple of decades-so expect the nanotube to begin taking over a few years from now.
The Rise Of The Nanotube comes down to three things: innovate and think up new uses for carbon nanotubes, given all their wonderful properties; find more of those wonderful properties; and most important as of now, find how best to manufacture and interface them.

Manufacture is notoriously difficult; for example, in one process, they come out as a mixed bundle of conducting and semi-conducting, and it's hard to separate them. Another problem, for example, is how to un-bundle them.

Some of what we've talked about above could just turn out to be that much more thin air; some of it could turn out to be groundbreaking. In any case, look forward to the "nanotube computer," which, by the way, is also being talked about.  



Ram Mohan RaoRam Mohan Rao