CD reference materials for sub-tenth micrometer applications

Prototype linewidth reference materials with Critical Dimensions (CDs) as narrow as 70 nm have been patterned in (110) silicon-on-insulator films. The sidewalls of the reference features are parallel, normal to the substrate surface, and have almost atomically smooth (111) surfaces. Linewidth calibration begins with the measurement of the electrical CDs of multiple reference features located at a selection of die sites on a wafer. The absolute widths of the cross sections of a sub-set of reference features on several chips that are diced from the wafer are then subjected to high resolution transmission electron microscopy (HRTEM) imaging to determine their physical CDs by lattice-plane counting. Sample preparation for lattice-plane counting by HRTEM is destructive, and other reference features on the same chip become unusable for reference-material purposes. However, a calibration curve for converting the measured electrical CDs of reference features on other chips on the wafer, known as "product reference features," to their physical values is obtained. The uncertainty attributed to the physical CD values of the product reference features generally varies inversely with the linear correlation between the cross-section lattice-plane counts and the corresponding electrical CD measurements of the sub-set of reference features that were selected for HRTEM imaging. A linear correlation value of approximately 0.97 has been obtained from a sub-set of 12 HRTEM measurements. In this case, the uncertainty attributed to the physical CD values of the product reference features was typically 13 nm. An apparent time-dependence of the electrical CD of the as-pattemed reference features is believed to be responsible for most of the product reference feature uncertainty. However, it has now been found that a forming-gas annealing treatment appears to prevent the referenced time dependence and thus has the potential for reducing the uncertainty level.