Ellipsometer Focuses on Ultrathin Gate Measurement
Alexander E. Braun, Senior Editor -- Semiconductor International, 7/1/2001
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Philips Analytical (Almelo, Netherlands) has introduced an ellipsometry platform dedicated to ultrathin gate oxide and high-k metrology. The platform, the PQ Ruby Sub'tomic, is designed to measure gate oxides at 16 Å and below. Philips developed it from its existing ellipsometers, which are used in in-line process control for thick films and multi-layer stacks. The technology was modified to do transparent thin-film measurements. The optical path, including the electronics and hardware, was optimized and tuned to perform best at ultrathin gate oxide measurements.
In producing the new platform, the company studied every aspect of the ellipsometer system, including the optics and electronics, to identify the components that contribute the most to inaccuracy. This included items such as polarization angle, l/4 plate position, basal positioning and overall optical alignment. Because the system must concentrate on microspots for in-line process control, aligning and optimizing subsystems and detector electronics such as analyzers and motion control — basically every piece of technology that makes up an ellipsometer — required the development of a different perspective and goals, as well as some compromises. Philips identified the most sigma calculation, based on all the contributing factors to inaccuracy, and concentrated on it to improve ultrathin gate measurement accuracy.
| Gate oxide thickness reduction progression according to ITRS. (Source: Philips Analytical) |
However, the tradeoff to achieve these levels required that a modicum of accuracy for other applications be surrendered. This is a truly dedicated system, for use with gate oxides and relevant applications, and therefore has the most stable single laser source available: the 633 nm HeNe laser. To enable it to be used for other applications, a UV/VIS-based reflectometer has been added. Recent hardware and software developments have enabled a substantial increase in reflectometer measurements, especially for the thicker films (1000 Å and up).
Although designed for ultrathin gate oxide measurements, and regardless of the compromises mentioned, the PQ Ruby Sub'tomic tool can also be used for other applications. In the area of process control, the UV/VIS-based reflectometer is sufficiently capable to fill an occasional backup role. In the case of in-resist measurements, for example, if 5000 Å layers are considered, tool-to-tool reproducibility must be below 10 Å; however, process accuracy is ±100 Å. This provides the operator with upper and lower limits to determine whether there is a problem. The reflectometer can function comfortably down to 10 Å, still well within process limit measurement. The combination of the 633 nm laser (0-1500 Å film thickness range at very high repeatability) and the UV/VIS-based reflectometer (1000 Å-30 µm film thickness range), while making the platform a dedicated ultrathin gate dielectric metrology tool, keeps it versatile.
For additional information on inspection, measurement and test, go to www.semiconductor.net/imt
