CD Measurement System Discards Libraries

Scatterometry has been considered as a solution to in-line measurement requirements, applying two basic modes: single-wavelength multi-angle and multiwavelength single-angle ellipsometry. However, both systems share one problem: They require an extensive and costly model-based library to enable them to match the diffraction pattern that they detect, and match time is influenced by library size. Also, whenever a pattern parameter is changed — pitch or CD — the library must be regenerated, often requiring third-party assistance. Another issue with ellipsometry is that, because it tends to use a large spot, measurements are sensitive to grating orientation.

Nanometrics Inc. (Milpitas, Calif.) has introduced its NanoSpec 9000cd system, which provides spectroscopic ellipsometry at normal incidence, using real-time regression analysis to eliminate the need for libraries. The system does regression analysis to actually measure structures. Its spot size is 40 µm, allowing an average measurement of all the lines under it — unlike the CD-SEM operation mode — permitting its use as a production monitor. It is also compatible with other Nanometrics platforms in use for CMP and CVD cell thickness measurement.

In production mode, the system collects data in 1.5 sec and analyzes it within 0.5 sec, enabling the taking of several data points per wafer while keeping up with the process tool. The data provides information on the individual structure's top and bottom CD, the sidewall angle, the grating pitch and height, and film and sublayer thicknesses.

When a problem is detected in production mode, the system is capable of further, off-line structure analysis. In the top and bottom CD analysis the structure is "sliced" horizontally into two pieces, providing two results. Presumably, there will be some sidewall angle, and these results will differ from each other. In a more analytical mode the system takes more slices through the feature, providing more sidewall information. When drawing profiles, any unusual characteristic becomes obvious.

If desired, the platform does not preclude the use of the model library analytical method. Although designed to do measurements on the fly, it can use models in much the same way it does film thickness measurements, by curve-fitting to theoretical, ideal model curves.

The system's main differentiation from other platforms is that it is not library-dependent. Libraries can be cumbersome, expensive and, at times, difficult to transfer. Another issue with a library is that considerable support may sometimes be required from its manufacturer, particularly if the library is something that the user is unwilling to generate himself.