Comparison and uncertainties of standards for critical dimension atomic force microscope tip width calibration

Since the advent of critical-dimension atomic force microscopes (CD-AFMs) in the 90s, these tools have enjoyed growing acceptance in semiconductor manufacturing both for process development and to support in-line critical dimension (CD) metrology. The most common application of CD-AFMs has been to support critical-dimension scanning electron microscope (CD-SEM) and scatterometer metrology as a reference for tool matching or as a nondestructive alternative to transmission electron microscopy (TEM) and scanning electron microscopy (SEM) cross sections. For many years, CD-AFM users typically developed in-house reference standards for tip width calibration - often based on SEM or TEM cross sections. But the uncertainty of such standards was often large or unknown. Tip characterizer samples - which used a sharp ridge to calibrate the tip width - are commercially available. However, scanning such samples can result in tip damage, and the uncertainty of tip calibrations based on this method is at least 5 nm. In 2004, NIST, SEMATECH, and VLSI Standards collaborated on the development and release of single crystal critical dimension reference materials (SCCDRMs) to SEMATECH member companies. These specimens, which are fabricated using a lattice-plane-selective etch on (110) silicon, exhibit near vertical sidewalls and high uniformity and can be used to calibrate CD-AFM tip width to approximately 1 nm standard uncertainty (k = 1). Also in 2004, commercial critical dimension standards (CCDS) were introduced. Using CD-AFM instruments at both NIST and SEMATECH, we have performed a comparison of nominal 45 nm and 70 nm CCDS specimens with the SCCDRM calibration. Our observations show that these two independently performed calibrations are in agreement.