Thursday, September 21, 2000

Asides from the problem of shrinking the line widths of devices, is the problem of making sure that they work correctly in these hyper-miniaturized formats. Transistors, capacitors, and the many wires that connect, and supply them, all face problems as IC's are pushed further down the nanometer scale. As R&D continues on the miniaturization course, they bring their devices further into the heady world of quantum physics, and of a troublesome effect known as quantum tunneling.

Tunneling is the effect or ability of an elementary particle, such as an electron, to "jump" from one place to the next without bothering to move through the space in-between those two points. In our everyday, macroscopic lives, tunneling - or quantum tunneling - is of no great concern. The vast, overwhelming majority of particles in our bodies, after all, are held firmly in place in the form of atoms, and molecules. Those few rebels caught jumping around the structures of our fingernails, our cutlery, and the neighborhood dog - with their whimsical, nanometer leaps from one point to the next - are hardly even a passing significance to the unbelievably gigantic macro existence of the greater world.

IC's, however, rely on shuttling around large numbers of free-wheeling electron particles in a well choreographed manner; zipping them across wires, through transistors, and storing them in the capacitors that represent the 1s, and 0s of the computer world. Up until fairly recently, this could all be done without being overly worried about the weird world of quantum physics. Now, though, as transistors, and capacitors being to scale down to smaller, and smaller sizes, those nanometer leaps become more, and more significant. Computer chips are rather finicky devices, and when their parts are only a few nanometers wide, a few tunneling electrons can make the difference between an electric charge being in the right place, or the wrong place all together. When a charge is in the wrong place, it can result in data loss, or in transistors going kablooie.

Tunneling is quickly becoming an immediate problem where logic gates are concerned. Logic gates are the tiny structures inside transistors that are "switched" to allow electrons to flow. When the gate is shut the electrons should stay at the source, where they belong. With transistors continually being scaled down, logic gates must be scaled down as well, right down to being a few nanometers thick. At this level, it becomes possible for errant electrons to tunnel past the gate, even when it is shut. This can cause a decided imbalance in the internal charge of a transistor, and wreak all kinds of havoc.

To counteract this inevitable result of miniaturization, researchers are experimenting with new materials, and constructs to stem the tide of unwanted electrons. One method is to make the gate oxide itself out of silicon nitride. This approach would net fabricators a 50% increase in insulating efficiency over the current use of silicon dioxide, at sizes of 25-50nm.

Another approach being taken by an IBM team is to construct double-gate transistors. The double-gate transistor, as its name implies, is simply a transistor with two gates. In traditional MOSFETs (Metal-Oxide Semiconducting Field Emission Transistors), the source and drain are separated by a single gate. The double-gate approach creates a sandwhich-like structure, which has a gate on both sides of the silicon. In this manner, it is possible to apply more voltage, more evenly across the region separating source, and drain, thus greatly reducing the number of electrons that can sneak past. This technology, of course, has fundamental differences from existing MOSFET design. Effectively, double-gate technology adds an entirely new layer to IC design in order to incorporate the extra gates, and the way they must be produced is very much different. Double-gate technology, then, may take some time to become widely available in marketplace, though it shows a great deal of promise as circuits continue to shrink.

part 4: A Heated Exchange