The batch-processing equipment vastly increases the time it takes for a wafer to pass through the production line. The reason for the delay is that the furnace or wet bench cannot process a single wafer—or even two, or 20—when it arrives from a previous manufacturing step. The equipment's temperature, gas-flow rate, or chemical concentration is set to handle many wafers at once. To handle fewer wafers would require different settings. So a wafer must wait until dozens of other wafers are ready to go in. This constraint means that some wafers may wait around for days before they move into the equipment for the next step.

The greatest technical advantage of single-wafer manufacturing is that it produces wafers with more good chips than batch processing does. It also produces ICs that are faster and more reliable. There are several reasons for these improvements, but the most important ones can be summed up in two words: tighter tolerances. Single-wafer equipment is smaller than batch equipment, so it is possible to have better control over conditions like temperature and gas flow. In batch furnaces, for example, the annealing temperature or gas flow may vary from place to place inside the furnace. These differences create variations in the electrical properties of the circuits from wafer to wafer, across a single wafer, and even on an individual IC. And these properties, in turn, affect how the circuits work. For example, if the voltage at which the transistor turns on is too high, the transistor switches more slowly than one with a lower threshold voltage. And slower transistors mean slower circuits.

If threshold voltages vary from wafer to wafer, the circuits on those wafers will run at different speeds. And, even worse, if the threshold voltages stray too far from their nominal values, the circuit may not work at all.

The trick in single-wafer manufacturing is to devise processes that handle a single wafer efficiently enough to compensate—through speed, for example—for the economies of scale that are lost with the elimination of batch processing. In the annealing step, for example, the single-wafer alternative to batch processing is rapid thermal processing. It occurs in a chamber in which lamps heat the wafer directly, so the temperature to which each wafer rises is more uniform over time, and the electrical properties of the circuits are more uniform as well.

To speed the wafers through a single-wafer facility, engineers cluster several manufacturing steps into a single unit, which also helps to control ambient conditions more precisely. For example, forming the layers that make up the transistor's gate takes three separate substeps: growing of the silicon dioxide gate insulation, adding nitrogen to the silicon dioxide to protect the insulation against the infusion of impurities from the gate material, and depositing the polycrystalline silicon that will eventually form the gates of individual transistors. In a single-wafer facility, all three of these substeps can occur inside a single machine, as they do at most of the 47 facilities in the world where the most advanced commercial chips are fabricated.

These combined machines, known as cluster tools, include a separate chamber for each of the steps of the process [see photo, "Get It Together"]. The wafer passes from one chamber to the next through a vacuum region located at the center of the unit. Connecting the process chambers through a vacuum prevents the surface of the wafer from being exposed to the atmosphere, which could contaminate it, throughout the entire gate-formation process.

The extreme isolation available with this cluster machine allows the silicon dioxide thickness to be controlled uniformly to a few monolayers of atoms—a control not possible with batch machines. Such uniformity is critical to the fabrication of the most advanced circuits today—those with wires only 90 nm across—because the total thickness of the silicon dioxide layer that insulates the gate from the channel in these devices is only 1 or 2 nm thick. Cluster equipment will become even more critical in the next semiconductor generation, which will produce wires only about 65 nm wide.

In some cases a single-wafer machine is fundamentally better than its batch counterpart because of differences in the physical principles through which they work. For example, both the rapid-thermal-processing machines used in single-wafer manufacturing and the furnaces used for batch processing heat the wafers by irradiating them with photons. But the wavelengths of the photons in the two processes are different. The batch-processing furnace produces only photons with wavelengths above 800 nm—in the infrared region of the optical spectrum.

The single-wafer alternative, on the other hand, uses rapid-thermal-processing lamps that produce photons both above and well below 800 nm—even some photons in the visible and ultraviolet parts of the spectrum. These shorter-wavelength photons are more energetic than the infrared photons and can excite the atoms of the wafer electronically. As a result, they permit a lower processing temperature while nevertheless shortening the processing time from hours to minutes. The lower temperatures also make it easier to introduce new materials into the manufacturing process, for example, new gate insulators that will lower the leakage current between the gate and the channel and cut down considerably on the power that the ICs consume. The bottom line is improved performance, reliability, and yield for the chips.