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5 April 2007 Advanced CD-AFM probe tip shape characterization for metrology accuracy and throughput
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As semiconductor and data storage industries apply Critical Dimension Atomic Force Microscopy (CD-AFM) for their metrology needs in research and production, (1) measurement accuracy/repeatability and (2) measurement throughput are the major criteria for acceptance. However, these two requirements are usually contradictory for a metrology instrument. For example, a scatterometer can take a snapshot of a wafer in seconds, but such indirect CD measurements are biased by the availability of library models and uncertainty of computer simulations. Transmission Electron Microscopy (TEM) provides an atomic-scale resolution that is traceable back to the lattice structure of atoms, yet the cross-section data is highly localized and can take days or weeks to acquire. In the case of CD-AFM, since the scanning probe physically interacts with the structure of interest at a close proximity, the determination of sample morphology comes from direct measurements. Therefore, the measurement uncertainty can be attributed to: (1) AFM probe tip shapes and (2) system control and scan algorithms. For the former, past efforts have been mainly focused on improving metrology accuracy and repeatability by reducing the dimensional uncertainty of a tip shape. This approach includes characterizing the probe tip shape periodically. Inevitably, such tip shape calibration procedure takes time (approximately 5 min) and burdens production throughput. In this paper, we introduce several new methods for AFM probe tip shape characterization with different designs of tip shape characterizers. The new tip shape characterizers were designed to address the limitation of current structures. First, a single silicon overhang structure with wear-resistant coatings was used as the characterizer for both tip width and tip shape profile. Tip-to-tip scan repeatability data (0.7 nm 3 Sigma) and measurement statistics suggest an improvement over present state-of-the-art practice. Tip shape profiles of several high aspect ratio (20:1 to 25:1), low lateral stiffness probes were successfully characterized with this method. Furthermore, the use of single characterizer provides an opportunity to shorten tool calibration time, and consequently, increase measurement throughput. In addition, a carbon nanotube characterizer prototype is proposed for CD-AFM. When scanning probe geometry shrinks with semiconductor technology nodes, it has become a challenge to characterize a probe with a few tens of nanometer of width with a micrometer-size characterizer. Using a comparable or smaller size of characterizer for a small (20 to 50 nm) AFM probe not only reduces the dimensional uncertainty, but also expands the 2-D profiling capability of current tip shape characterization. We will discuss limitations of current tip shape profiling techniques, proof-of-concept experiments for new characterizers, implementation of new tip shape characterization methods, and approaches to increasing measurement throughput.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Hao-Chih Liu, Jason R. Osborne, Marc Osborn, and Gregory A. Dahlen "Advanced CD-AFM probe tip shape characterization for metrology accuracy and throughput", Proc. SPIE 6518, Metrology, Inspection, and Process Control for Microlithography XXI, 65183K (5 April 2007);


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