Dimensional metrology with the NIST calibrated atomic force microscope

Atomic force microscopes (AFMs) are increasingly used in the semiconductor industry as tools for sub-micrometer dimensional metrology. The scales of an AFM must be calibrated in order to perform accurate measurements. We have designed and developed the calibrated AFM (C-AFM) at the National Institute of Standards and Technology (NIST) to calibrate standards. Specifically, our primary calibrations are expected to be of combined pitch/height, or three dimensional magnification standards for AFM. The C-AFM has metrology traceable to the International System of Units (SI, from Systeme International d'Unites) meter for all three axes. 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. Our first pitch measurements for an outside customer were recently completed, in which we were able to report relative expanded uncertainties (k = 2) as low as 1% on sub-micrometer pitches. The uncertainty budget for these measurements includes the effect of sample non-uniformity, which is the dominant contribution for some of the reported uncertainties. Four samples were measured -- two with grid patterns and two with orthogonal linear scales. The results as reported to the customer are included in this paper. We have also recently made considerable improvements in our uncertainty budget for step height measurements. For example, we recently achieved 0.2 nm expanded uncertainty (k = 2) on a 20 nm step, and achieved 0.008 nm expanded uncertainty in the measurement of the approximately 0.3 nm single atom step on Si (111). We also participated in the recently completed first round of the NIST linewidth correlation project, in which linewidth measurements obtained from different methods (including the C-AFM) are compared. In this paper, we will report on the current status of the C-AFM, and on our plans for further development.