The shrinking insulator

Our Other

Sites

• PC
  
  Quest
• Voice&Data
• Computers@Home
• CIOL
• DQ
  
  Week
• DQ
  
  Channels India

The shrinking insulator

The success of IBM’s low-resistance copper technology introduced last year highlights the fact that integrated circuit chips consist of more than just the semiconductors, such as silicon, that most people associate with them. The chips also require conducting materials, such as aluminum and now copper, to carry electric current around the chip, and insulators, which block current flow.



Insulators are used to create the memory cells that store data on chips. In a field-effect transistor–the basic component of computer chips–a layer of insulator isolates the gate from the channel that connects the source and the drain. And insulators are used to separate the layers of wiring on a chip, preventing current from leading from one circuit to another.

In today’s most widely used chip technology– complementary metal-oxide semiconductor

(CMOS)–the insulator is silicon dioxide. It is well suited for its job not only because it is a good insulator but because it is compatible with silicon and can easily be ‘grown’ where needed during the fabrication of a chip. Furthermore, the fabrication process has been optimized for silicon dioxide, and any replacement will naturally force a reworking of that process. But in spite of the central role the material has played in the semiconductor industry, the consensus is that replacements must be found.

This is the case, for instance, with the memory cell on a dynamic random access memory (DRAM) chip. The cell includes a capacitor–a pair of conductors separated by a silicon dioxide insulator–which is used to store a tiny electric charge; the presence or absence of a charge in the cell corresponds to a 1 or a 0 in the memory. To squeeze more memory onto a chip, the cells have had to shrink, while continuing to hold the same amount of charge. To date, designers have compensated by modifying the shape of the cells, making them narrow and deep to maximize their charge-storage capacity. But the diminishing size of cells makes this more and more difficult.

Researchers are looking at

perovskites, a family of minerals either found in the earth’s crust or created synthetically, which have some of the highest dielectric constants known. Unfortunately, unlike silicon dioxide, the dielectric constants of perovskites vary with their form. A solid crystal of barium

titanate, for example, can have a dielectric constant between 5,000 and 10,000, compared with a dielectric constant of four for silicon dioxide, but when barium titanate is made into a thin film–the form it would take in a computer chip–its dielectric constant drops precipitously. A typical film that is 20 nanometers (Nm) thick has a dielectric constant of only about 200, higher than silicon dioxide but disappointing relative to bulk barium

titanate. Furthermore, since barium titanate reacts strongly with silicon during processing, it cannot easily be formed into the three-dimensional trench structures used to increase the storage capacity of silicon dioxide memory cells. To turn barium titanate into a useful material for memory cells, the researchers will have to find some way to substantially increase the dielectric constant in thin films of the stuff.

To that end, atomic spacing in the thin films are modified by growing them layer by layer on a substrate whose own atomic spacing is slightly larger or smaller than that of the perovskite. As the spacing of the perovskite film tries to align with that of the substrate, it is stretched or squeezed. Such manipulation of the atomic spacing can have far-reaching effects, such as transforming a metal into an insulator, in materials such as perovskite superconductors. Assuming the researchers can learn to increase the dielectric constant as well, barium titanate could be a promising material to replace silicon dioxide in memory cells.

The problem with insulators in CMOS technology affects more than just the capacitor in memory cells. Beneath the gate of every CMOS transistor is a thin insulating layer of silicon dioxide. “As we have scaled down our CMOS devices further and further,” explains Susan Cohen, Manager, Thomas J Watson Research Center, IBM, “the silicon dioxide layer has had to become thinner and thinner.” But as that happens, the layer–typically 3 to 4 Nm thick in current manufacturing–loses its insulating capacity, and the amount of current that leaks through the insulator grows exponentially. “We are approaching the limit, thought to be somewhere below approximately 2 Nm, where silicon dioxide will no longer be useful as a gate insulator,” Cohen says. Researchers are exploring alternatives. The candidates include aluminum oxide, zirconium oxide, yttrium oxide and silicon dioxide mixed with transition or rare-earth metals such as zirconium or lanthanum. Scientists are testing the materials’ electrical properties, including their dielectric constants, as well as their physical and chemical characteristics, such as how they interact with silicon.