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Technical Highlights in Nanotechnology
 

Critical Dimension Metrology of Nanoscale Structures with Small Angle X-ray Scattering
 
The continued reduction in pattern sizes throughout the semiconductor industry will soon require new metrologies capable of high throughput non-destructive measurements of dense, high aspect ratio patterns with subnanometer resolution. In collaboration with industrial partners, we are developing a metrology based on Small Angle X-ray Scattering (SAXS) to quickly, quantitatively, and non-destructively measure the smallest, or "critical", dimensions expected in the next two technology nodes with subnanometer precision. Quantities of interest include critical dimension, pattern sidewall angle, statistical deviations across large areas, and quantitative measures of pattern sidewall roughness. These efforts are driving toward the specification of a laboratory scale device capable of providing pattern dimensions during routine tests of fabrication processes.
 
Ronald L. Jones and Wen-li Wu
 
The drive to reduce feature size to the sub-100 nm regime continues to challenge traditional metrology techniques for pattern characterization. Determination of the quality of the patterning process currently depends on the production of test patterns, such as line gratings, and evaluating dimensional control and the number of defects. As pattern sizes decrease, existing metrologies based on scanning electron microscopy and light scatterometry face significant technical hurdles in quantifying pattern dimensions and defects in dense, high aspect ratio patterns used in modern semiconductor circuitry. There are currently no clear solutions to pattern quality measurement for future technology nodes with dimensions on the order of 30 nm, and dimensional control to less than a nanometer.
 
To address these issues, NIST is developing a transmission x-ray scattering based method capable of angstrom level precision in critical dimension evaluation over large, (50 x 50) mm, arrays of nanoscale periodic structures. In contrast to light scatterometry, Small Angle X-ray Scattering (SAXS) is performed in transmission using a sub-Angstrom wavelength. With a wave-length more than an order of magnitude smaller than the pattern size, the patterns can be analyzed using methods traditionally employed in crystallographic diffraction. The high energy of the x-ray beam allows the beam to pass through production quality silicon wafers without requiring a specialized sample environment (ultrahigh vacuum). While current measurements utilize the flexibility and wide array of instrumentation available at a synchotron source, initial results and the commercial availability of x-ray sources and detectors suggests the technique is portable to a laboratory scale device.
 
As shown in the schematic below, the patterned sample is placed in an x-ray beam where the transmitted intensity is measured as a function of the scattering angle, 2theta, on a 2-D detector. The intensity is then fit to models describing the average pattern shape. To save valuable space within the total patterned area, industrial test patterns are typically smaller than (50 x 50) mm. Using a monochromator to define the wavelength and two sets of beam defining slits, beam footprints of approximately (40 x 40) mm have been successfully demonstrated. In contrast to 1-D techniques such as light scatterometry, the use of a 2-D detector allows the simultaneous characterization of all dimensions for patterns such as vias (arrays of cylinders standing perpendicular to the surface) and via pads (arrays of rectangles). In addition to measuring the pattern dimensions along the substrate plane, measurements at varying sample rotation angles, w, can be used to reconstruct the average 3-dimensional pattern shape.
 
Figure 1: Schematic of SAXS geometry, showing incident and scattered beams (red lines), sample with pattern oriented at rota-tion angle w, and 2-D detector (right).
Figure 1: Schematic of SAXS geometry, showing incident and scattered beams (red lines), sample with pattern oriented at rotation angle w, and 2-D detector (right).
 
In collaboration with International SEMATECH, recent SAXS measurements of line gratings have demonstrated the capability of SAXS to measure fabricated patterns of thermally grown oxide with overall dimensions designed for current metrologies. The resulting detector image is a series of diffraction peaks along a single diffraction axis perpendicular to the orientation of the grating. The number of observable diffraction orders is in part determined by the pattern quality. The observation of over 30 orders of diffraction suggests the relatively high quality of these patterns and provides ample data for high precision analysis. In addition, the number of observable diffraction orders serves as an immediate measure of pattern quality, termed a SAXS "fingerprint." With a measurement time on the order of a second, dose exposure matrices commonly used to evaluate optical imaging conditions can potentially be evaluated in a matter of minutes.
 
In addition to the SAXS "fingerprint", quantitative information is obtained through model fitting of the data. In contrast to reflection based techniques, the models used here are relatively simple. Quantitative measures of the pattern repeat period and the line width are therefore obtained rapidly and precisely. For these samples, SAXS data have provided sub-nanometer precision in both average repeat distance and average line width. The precision of this measurement is in part dictated by the number of observable diffraction orders, which is in turn limited by the instrumental resolution and pattern quality. Ongoing studies will compare these results with measurements from light scatterometry, atomic force microscopy, and scanning electron microscopy.
 
Figure 2. Top down SEM image (top left) of a photoresist grating on a silicon wafer compared to the resulting 2-D SAXS image (top right). Also shown are the intensities of the diffraction peaks
Figure 2. Top down SEM image (top left) of a photoresist grating on a silicon wafer compared to the resulting 2-D SAXS image (top right). Also shown are the intensities of the diffraction peaks as a function of scattering vector,?q (=4pi/wavelengthsintheta q, where wavelength is the x-ray wavelength) fit with a simple model (solid blue line).
 
In collaboration with the IBM T.J. Watson research center, the effects of specific types of defects in patterns on the resulting SAXS image are being explored. Shown in Figure 2, a simple model of an ideal line grating captures the main features of the diffraction spots, however additional information is observed in the 2-D image perpendicular to the diffraction axis. Streaks of intensity emanating from diffraction peaks are visible near the beamstop. These streaks are indicative of deviations from the ideal grating and may provide information about defects such as long wavelength line edge roughness.
 
Figure 3: Schematic of via-pad patterns (a), where the blue rec-tangles represent etched regions in a low-k film, and (b) the re-sulting SAXS detector image.
Figure 3: Schematic of via-pad patterns (a), where the blue rectangles represent etched regions in a low-k film, and (b) the resulting SAXS detector image.
 
In addition to precisely measuring dense patterns of etched silicon oxide, the application of SAXS has been demonstrated for a wide range of materials that are currently used or are being explored for use in the microelectronics industry. We demonstrated the capabilities of SAXS on dense patterns of organic photoresist, silicon oxide, and nanoporous low-k samples (detector image from 2-D array of via pads shown in Figure 3). Additionally, measurements on samples of copper filled low-k patterns demonstrate the capability to probe multiple component and metallic patterns. Finally, the transmission geometry allows the probing of densely packed structures whether buried or exposed, providing a potential to measure other nanoscale 3-D structures being explored for future microelectronic devices and photonic crystals.
 
Continuing efforts will focus on developing quantitative measures of line edge and sidewall roughness, isolating specific types of long range order defects arising from masks, and reconstructing an average 3-D pattern shape. In addition, further investigations will provide precise specifications for a laboratory scale device capable of similar measurements.
 

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R. Jones, T. Hu, W. Wu, E. Lin (Polymers Division, NIST); G. Orji, T. Vorburger (Precision Engineering Division, NIST); A. Mahorowala (T. J. Watson Research Center, IBM); G. Barclay (Shipley Co.); D. Casa (Advanced Photon Source, Argonne National Laborotory)
R. L. Jones, T. J. Hu, E. K. Lin, W.-L. Wu, D. M. Casa, N. G. Orji, T. V. Vorburger, P. J. Bolton, G. G. Barclay, Proc. SPIE 5038, 191 (2003)
 
 
 
 
 
NIST Material Science & Engineering Laboratory - Polymers Division