John Henry J. Scott

Objective: To evaluate cross-sectional high resolution transmission electron microscopy (HRTEM) as a gate dielectric thickness measurement technique and to quantify its accuracy and precision for comparison against alternative techniques.

Approach: Ultrathin films of silicon dioxide (SiO2) and silicon oxynitride (SiOxNy) on silicon substrates are used widely in the semiconductor industry as gate dielectrics in transistors. Gate oxide fabrication tools produce films with a tolerance of less than 0.3 nm, and require metrology tools with a precision of better than 0.1 nm. To evaluate HRTEM as a thickness measurement technique, blanket films of dielectric grown on silicon were obtained from SEMATECH. TEM samples were prepared in cross section by mechanical dimpling and ion milling. High resolution micrographs were acquired at 300 keV using a CCD camera (Figure 1, top). Digital image processing was used to calibrate the magnification (using the silicon substrate as an internal standard), identify the dielectric film boundaries, and extract the film thicknesses.

Figure 1. (Left) High resolution transmission electron micrograph of single crytsal silicon beneath an ultrathin dielectric film. (Right) Crosses mark the recorded position of each intensity peak. Weighted-centroid fitting allows atomic spacings to be determined with sub-pixel precision.

Results and Impact: New methods were developed for obtaining 2-dimensional calibration information from a lattice image of the silicon substrate. The spacings between atomic planes could be measured with sub-pixel accuracy by locating peaks using a weighted center-of-mass algorithm (Figure 1, bottom), followed by a least-squares fit of lattice basis vectors. The film/substrate interface was located using integrated intensity profiles. Films nominally 2 nm thick were measured with an estimated precision of 0.2 nm.

Future Plans: Multislice simulations of HRTEM images as a function of beam tilt, defocus, thickness, and surface roughness are planned to explore the factors limiting precision. Further developments in automated image processing are also underway.

Publications:
1. Chemical and Structural Characterization of Ultrathin Dielectric Using AEM; J.H. Scott and E.S. Windsor, Structure and Electronic Properties of Ultrathin Dielectric, MRS Symp. Proc. vol 592, (in press).
2. Gate Dielectric Thickness Metrology Using Transmission Electron Microscopy; John Henry J. Scott, E.S. Windsor, D. Brady, J. Canterbury, A. Karamcheti, W. Chism, A.C. Diebold, 2000 International Conference on Characterization and Metrology for ULSI Technology, June 26-29, 2000.
AIP Conference Proceedings (in press).
3. Silicon Oxynitride Film Thickness Measurements Using HRTEM and Grazing Incidence X-ray Photoelectron Spectroscopy (GIXPS); John Henry J. Scott, E.W. Landree, T. Jach, E.S. Windsor; Microscopy and Microanalysis, volume 6, supplement 2, edited by G.W. Bailey, S. McKernan, R.L. Price, S.D. Walck, P.-M. Charest, R. Gauvin, Springer (2000), pp. 60-61.

Web Contact micro@nist.gov