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Scatterometry-based critical dimension and profile metrology

Posted: 16 Sep 2002 ?? ?Print Version ?Bookmark and Share

Keywords:microscopy? metrology? cd metrology? se method? cd-sem?

The continuing evolution of smaller feature sizes in semiconductor device manufacturing is driving the need for advanced metrology methods. As geometries are pushed below 0.15m, critical dimensions and feature profile metrology has become key to overall control of lithography and etch processes that looks to enhance device yield.

Conventional CD metrology methods such as critical-dimension scanning electron microscopy (CD-SEM) and conventional imaging-based optical methods are near or at the effective limits of resolution for advanced semiconductor devices. They also have the severe limitation that their top-down view provides little or no data concerning the sides and bottoms of features, and are thus unable to profile anomalies and artifacts. Profile data can be obtained by cross-section SEM, but this technique must be performed off-line and is destructive, costly, and time consuming.

Critical dimension scatterometry
In response to the limitations of CD-SEM and traditional imaging-based optical CD methods, scatterometry based optical profilometry techniques have emerged as a promising new technology for CD metrology. Scatterometry utilizes the optical signals produced by reflectometry or ellipsometry measurements taken on a grating structure consisting of a symmetrical line/space array patterned in transparent or non-opaque films. The interaction of light reflection, diffraction, and refraction striking the target produces phase and intensity information from the signal reflected off the target that can be used along with computational modeling techniques based on Rigorous Coupled Wave Analysis (RCWA) to reconstruct the profile shape of the patterned feature.

Currently available scatterometry CD and profile metrology tools use a variety of optical signal acquisition methods with varying degrees of complexity and abilities to detect faults. In addition, modeling of the scatterometry data to reconstruct CD and profile can take two main pathsoff-line modeling combined with a solution database, or an on-line real-time direct regression solution calculation.

Three methods of optical signal acquisition are widely used in scatterometry critical-dimension CD applications:

  • Reflectometry at a normal angle of incidence. In this method, intensity is compared to wavelength.

  • 2-angle single wavelength ellipsometry. In this method, intensity and phase are compared to the change in the angle of incidence.

  • Spectroscopic ellipsometry at grazing incidence angle. In this method, intensity and phase are compared to wavelength.

    There are significant differences among these three methods, in terms of the quality and information content in the optical signals produced. Normal incidence reflectometry permits fast data acquisition, but phase information is absent and the technique has low sensitivity to re-entrant or undercut sidewall profiles. Two variable-angle ellipsometry and spectroscopic ellipsometry are somewhat slower in data acquisition, but produce more information about shape detail and profile due to both greater information content provided in the signal and the use of near-grazing angles of incidence.

    Modeling of the data to reconstruct features is performed either on-line, using real-time regression for solution finding; or off-line by comparing acquired spectra to a stored database of possible solutions. RCWA is used to calculate the diffraction of light by the grating structures on the wafer to simulate SE or reflectometry signals corresponding to the unique features (profile, CD, angle, film thickness) of the grating structure. RCWA models used for normal-incidence reflectometry and for 2-theta ellipsometry are less complex because there is less information content in the optical signal. RCWA models used for spectroscopic ellipsometry are more complex because of the higher data content of the output signal, and are more capable of reconstructing detailed profile shapes.

    Until recently, real-time regression has been limited to modeling normal-incidence reflectometry and 2-angle theta ellipsometry signals. These two approaches have been able to perform calculations and provide measurement results in seconds, but only because they begin with less data content. The results have in turn been largely restricted to non-complex structure approximations such as rectangles and trapezoids, and cannot provide the more comprehensive rich profile detail that is needed for effective process control in advanced etch and photolithography processes of advanced devices.

    Off-line modeling combined with a solution database, by contrast, can perform more complex calculations because it is not directly tied to the measurement throughput of the metrology tool. The off-line modeling approach compares the acquired spectral data to a stored database library of possible solutions, each comprising a previously computed set of optical spectra covering the known variations in process parameters. The best match between the acquired data and the stored data is selected as the measurement result. Low-cost PC hardware and a customized search engine can match measured spectra against the pre-computed solutions in under a second.

    Until very recently, tools combining off-line library-based modeling with spectroscopic ellipsometry optical measurement have provided the best overall performance. The combination of high information content data collection optical signal data with complex modeling capability provides the highest quality measurement results in terms of CD and profile shape detail. Reconstructed profiles show excellent correlation with images obtained from cross-section SEM. Features below 0.15m have been measured with subnanometer precision. However, this approach remains problematic in situations with rapidly changing conditions, due to database generation time and parameterization issues.

    Breakthrough scatterometry developments
    The ideal scatterometry metrology tool should combine high information content spectroscopic ellipsometry and real-time regression-based modeling with complex profile capabilities. A new metrology system with this capability has been recently introduced. By employing breakthrough proprietary software algorithms, the computational load can be handled by currently available CPUs, an achievement that represents an order-of-magnitude advance over current techniques. Neither performance, speed or profile complexity are compromised. Solutions are calculated in a matter of seconds and provide full profile shape detail with subnanometer precision.

    Because this new tool allows the recipe to be modified quickly, and because the setup for new structures is also rapid, it has strong benefits as a standalone optical metrology platform for engineering and process development or foundry/mixed manufacturing environments. For integrated metrology applications, where cost is a more significant factor, the library-based spectroscopic ellipsometry tool can provide the needed speed and precision at reasonable cost.

    In shallow trench isolation and in damascene integration, process variation can take the form of reentrant angles, notching, t-topping and other anomalies that will cause voiding and cracking of deposited film in later process steps. For critical gate patterning processes, anomalies that can reduce yield include undercut, micro-trenching, and notching. In these and other applications, photolithography and etch process engineers require CD metrology tools that provide comprehensive and precise shape profiling rather than simple rectangular or trapezoidal profile approximations.

    Leading semiconductor device manufacturers have used the SE based tool combined with off-line database modeling for Gate Develop Inspect (DI) and Final Inspect (FI) CD applications in the past 2 years. The new SE tool with real-time regression modeling has successfully demonstrated similar levels of performance on these applications as well as for STI and damascene trench applications.

    As feature sizes in advanced semiconductor devices move to and beyond the 0.15m technology node, conventional CD metrology tools cannot provide the shape detail required by engineers for effective process control and yield management. A comprehensive scatterometry solution combining both real-time modeling and library-based modeling delivers the high-performance CD scatterometry capability required for fab-wide deployment.

    Andrew H. Shih
    Therma-Wave Inc.

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