Semiconductor measurements come clean
04/01/2005
Advances in metrology tools combat nanometer contamination
When measuring semiconductor features, recent industry developments illustrate that the linkage between metrology and contamination control is getting tighter as features shrink.
Therma-Wave Inc. (Fremont, Calif.; www.therma-wave.com) has rolled out a thin-film and critical dimension metrology tool that features an improved, built-in version of the company’s molecular contamination cleaning system. This desorption unit removes the airborne molecular contamination build-up on wafers so that thin-film measurements can be made.
KLA-Tencor Corp. (San Jose, Calif.; www.kla-tencor.com) also includes a desorption module in its optical thin-film measurement products. According to KLA-Tencor staff technologist Arun Srivatsa, “This is used to clean and measure selectively on metrology targets on scribe lines.” He adds that the technique, which involves heating up a small area on the wafer so that organics boil off, is of most value when measuring the gate oxides and other ultrathin films for 65-nanometer (nm) and below processes.
These thin films are less than 50 angstroms (Å)-or 5 nm-thick. Studies by KLA-Tencor have revealed that the build-up of airborne molecular contaminants can exceed 0.5 Å within two hours after a gate oxide is grown. That’s large enough to throw off optical measurements of the thin film by more than the few tenths of an angstrom that the measurement tools have in their error budget.
So, metrology companies must introduce point-of-use cleaning for contamination control. Only by removing contaminants just prior to measurement can the tools ensure accurate results.
Recently, Xidex Corp. (Austin, Texas; www.xidex.com) revealed that its carbon nanotubes would be used in semiconductor metrology tools. The industry wants to use carbon nanotubes to form the tips of scanning probe microscopes. These tips act like a microstylus, tracing out molecular-sized features. Scanning probe microscopes, which are also known as atomic force microscopes, can profile deep vias, high aspect ratio trenches and critical dimensions in advanced semiconductor processes (like the upcoming 32-nm process node).
The research consortium SEMATECH (Austin, Texas; www.sematech.org), which is interested in carbon nanotubes because of their characteristics, is working with Xidex in developing its technology. Nanotubes, unlike other materials, are very resistant to wear and can be made with diameters that measure in nanometers. That radius of curvature is tighter and the resulting tips smaller than are possible with other materials.
Even though carbon nanotube tools have yet to be deployed, scanning probe microscopes face measurement challenges that are contamination-driven. The Xidex solution involves the use of what’s known as tapping mode. “The tip only momentarily contacts the surface,” says Paul McClure, president and CEO of Xidex. “It drives itself in and out of any surface layers so quickly that they have no effect.”
At present, the tips are not made in a cleanroom but that may change as production ramps up. “A cleanroom will be required to increase yield and to protect the tips prior to packaging,” predicts John Allgair, project manager for lithography metrology at SEMATECH. III