Ellipsometry Finds Application for Thin SOI Layer Measurement

Much recent R&D effort has gone into developing SOI wafers as a serious alternative substrate material to bulk silicon. The material promises to either boost speed performance by 30% or reduce power consumption by 30%. Because most of the process-related thickness uniformity issues for thin (2000 Å) SOI have been resolved, it has become available as a substrate material. R&D is further progressing on ultrathin (300-1000 Å) SOI.

There is nothing new about silicon-on-insulator (SOI) use — it has been a semiconductor technology fixture for a number of years. Until recently, however, its applications were limited to very thick layers — on the order of microns — where the devices were actually built. During that period, the need for accurate measurements was more than adequately met by tools using reflectivity. But, with leading-edge companies such as Intel, AMD, IBM and others beginning to implement thin SOI applications for their high-performance microprocessors, the application range of SOI is altering quickly and considerably, as are the measurement requirements that must be met to deal with more demanding applications.

New device technology is fueling a move to very thin SOI layers. Designers want to ensure that the device's junction's depletion layer is well inside the complete thin SOI region. The reason is that, by doing this, the parasitic capacitance effects of the junction can be eliminated, resulting in considerably higher switching speeds. However, this requires that the actual silicon layer be as thin as possible — in this case in the 200 nm region. Some manufacturers want to go even thinner than that.

Laser ellipsometry wafer maps can be analyzed to provide a “go/no go” indication in a production environment. (Source: Philips Analytical)

The measurement requirement in SOI's case requires that the tool be able to transmit through the silicon. Because the bandgap of silicon is ~1.14 eV, this means that the energy — the wavelength — of the light used must be less than that. Thus, the actual wavelengths needed lie more in the infrared and red areas than beyond that — as in the case of the more energetic blues — which would be absorbed by the silicon, rendering it opaque.

Because the silicon's top layer is transparent to these redder wavelengths, the user can go through it and also measure the underlying, buried oxide. Therefore, with only one measurement it becomes possible to take a one-shot multistack SOI thickness measurement of the buried oxide and the thickness of the silicon layer that is on top of it. If there is a gate oxide on top of it, it can be measured as well. Of course, different layers are measured with varying degrees of accuracy; however, the measurements' accuracy repeatibility sigma range is <0.03 Å for gate oxide, <0.5 Å for the silicon top layer, and <0.1 Å for the buried silicon dioxide layer. Compared with SOI process control parameters, these metrology inaccuracy figures can be considered negligible.

Philips Analytical (Almelo, Netherlands) has developed a method for measuring very thin SOI layers. Based on its PQ Ruby platform, the PQ Ruby thin SOI system has been enhanced with the addition of two more optimized lasers, as well as a rework of the complete optical train, enabling it to work for those particular wavelengths. Changes also have been made to the quarter-wave plate, creating less thermal hysteresis and resulting in more stable measurements.

The system was optimized through work done with a major SOI manufacturer. The result is (for the moment) a unique ellipsometry-based tool that enables rapid and accurate measurement of very thin SOI layers completely across the wafer. The platform is well-suited to act as a backup for other metrology equipment, determining thickness and optical refractive indices, as well as absorption constants and the reflectivity of thin films.

For additional information on inspection, measurement and test, go to www.semiconductor.net/imt