The use of critical dimension atomic force microscopes (CD AFMs) in semiconductor manufacturing, both for process control and as a reference metrology tool, is increasing, If the tip width is calibrated consistently between measurements, a CD AFM can function as an excellent width comparator. Relative widths can be measured with uncertainties of 1 nm or less. However, to perform accurate measurements, the absolute tip width must be accurately calibrated. Until recently, conventional methods for accomplishing this had standard uncertainties on the order of 5 nm. Recently developed CD reference materials now make it possible to calibrate absolute tip width with uncertainties at the 1 nm level. The highlights of our method are: ( 1) the use of single-crystal silicon and preferential etching to pattern well-defined and highly uniform features; (2) the use of high resolution transmission electron microscopy (HRTEM) to access the Si lattice spacing directly as a source of traceable width information, and (3) the use of CD AFM to transfer width information from the HRTEM samples. These standards are known as single crystal critical dimension reference materials (SCCDRM), and prototype SCCDRMs have recently been delivered to SEMATECH Member Companies for evaluation.