Accurate dimensional metrology with atomic force microscopy

Atomic force microscopes (AFMs) generate three dimensional images with nanometer level resolution and, consequently, are used in the semiconductor industry as tools for sub-micrometer dimensional metrology. Measurements commonly performed with AFMs are feature spacing (pitch), feature height (or depth), feature width (critical dimension), and surface roughness. To perform accurate measurements, the scales of an AFM must be calibrated. We have designed and developed the calibrated AFM (C-AFM) to calibrate physical standards for other AFMs. The C-AFM has displacement metrology for all three axes traceable to the 633 nm wavelength of the Iodine-stabilized He-Ne laser. This is accomplished through the integration of a flexure x-y translation stage, heterodyne laser interferometers, and a z-axis piezoelectric actuator with an integrated capacitance sensor. This capacitance sensor is calibrated with a third interferometer. We have performed both pitch and height measurements for external customers. Recently, we performed pitch measurements on holographic gratings as part of an ongoing international comparison driven by BIPM (Bureau International des Poids et Measures). We have also completed a preliminary design of a prototype pitch/height standard and are evaluating preliminary test samples. Additionally, we are working toward the development of linewidth standards through the comparison of C-AFM width measurements with values obtained from other methods. Our step height and linewidth measurements are in good agreement with those obtained by other methods, and we are working to improve the lateral resolution and hence the uncertainty of our probe-based linewidth measurements by studying the use of nanotubes and other types of sharp tips as linewidth probes.