Calibration of a dual probe NanoCaliper AFM for CD metrology

Results are presented which demonstrate the feasibility of calibrating a critical-dimension atomic force microscope (CD-AFM) without the use of a reference artifact in such a way that high-precision critical dimensions can be generated independently of changes in probe tip shape (including effects of tip wear), and in the presence of surface force uncertainties and stage uncertainties. Experiments were conducted using a dual-probe NanoCaliper CD-AFM architecture. The results support an estimate of 0.2 nm for single-point (static) repeatability of tip-tip calibration achievable in a commercial tool. A comprehensive method developed for calibration and measurement using a dual probe system can remove other dimensional drifts that have effects similar to tip wear. We also found that three different "calibration events" can potentially be used to compute nondimensional interaction strengths that determine a surface force bias needed to compute CDs from noncontact mode scans. Verification of this predicted result will make it possible to build a dual probe system that is self-calibrating not only with respect to tip length and other dimensional drift, but also with respect to tip radii, cantilever stiffnesses and other parameters. We have also demonstrated the feasibility of a new diffraction-based method for directly measuring the cantilever-edge-to-cantilever-edge separation and showed that this method is capable of improving CD measurement precision still further.