Applications of High Resolution Immersion Lens Scanning Electron Microscopes to Sem-Based Defect Review at 250nm Design Rules

Abstract

IntroductionThe use of SEM-based defect review tools has increased dramatically over the past five years as the semiconductor industry moved from 0.7 micron to 0.25 micron design rules. During this period, a dramatic inflection occurred at the 0.5 micron node; optical microscopy lacked sufficient resolution to determine even if a simple etch step was properly performed. Accordingly, many “inspection SEMs” were introduced into the wafer fabrication facility. With ever increasing focus on yield improvement, defect review SEM's proliferated in the fab in an effort to drive down both baseline defects and process excursions. In order for such an effort to be successful, a clear improvement in the low voltage image resolution performance of the scanning electron microscope was required. Commercial vendors have responded with impressive tools achieving image resolutions of 2.5–4nm @1 kV. At this level of performance, routine observation of semiconductor wafers is possible at 100,000X magnification. This dramatic improvement in resolution is the result of many factors, by far the biggest of which is the use of “immersion lens” designs which employ a strongly excited objective lens operated at short working distances (∼2mm). This electron optic design was first introduced into the laboratory SEM market, with instruments capable of producing approximately 1.2nm resolution at 20kV. These high performance lenses in which the magnetic field extends below the bottom of the polepiece, were fitted onto 200mm platforms allowing whole wafer inspection/defect review. The features of such tools and their application to the IC industry is the subject of this paper. An example of the superb imaging performance of such a 200mm tool is illustrated in figure 1.