Critical Dimension AFM (CD-AFM) is a widely used reference metrology technique. To characterize modern semiconductor devices, small and flexible probes, often 15 nm to 20 nm in diameter, are used. Recent studies have reported uncontrolled and significant probe-to-probe bias variation during linewidth and sidewall angle measurements. To understand the source of these variations, tip-sample interactions between high aspect ratio features and small flexible probes, and their influence on measurement bias should be carefully studied. Using theoretical and experimental procedures, one-dimensional (1D) and two-dimensional (2D) models of cylindrical probe bending were developed and tested. An earlier 1D bending model was refined, and a new 2D distributed force (DF) model was developed. Contributions from several factors were considered, including: probe misalignment, carbon nanotube (CNT) tip apex diameter variation, probe bending before snapping, and distributed van der Waals-London force. A method for extracting Hamaker probe-surface interaction energy from experimental probe bending data was developed. Comparison of the new 2D model with 1D single point force (SPF) model revealed a difference of about 28% in probe bending. A simple linear relation between biases predicted by the 1D SPF and 2D distributed force models was found. The results suggest that probe bending can be on the order of several nanometers, and can partially explain the observed CD-AFM probe-to-probe variation. New 2D and three-dimensional (3D) CD-AFM data analysis software is needed to take full advantage of the new bias correction modeling capabilities.
Citation: Journal of Micro/Nanolithography, MEMS, and MOEMS
Pub Type: Journals
CD-AFM, reference metrology, accuracy, measurement bias, probe bending, CNT