Chiwoei Wayne Lo

Productive application of voltage contrast for detection of optically undetectable defects

Open contacts and vias, gate oxide shorts, metal stingers, approximately 0.1 micrometers pattern defects and small particles are widely reported to be optically undetectable using conventional bright-field optical imaging-based inspection system. These critical yield limiting defects often represent > 50 percent of the defect distribution on advanced submicron processes. Many of these defects are optically undetectable because they lie beneath the device surface. Even advanced, high resolution UV inspection tools cannot detect them, primarily because of limited depth of focus. High aspect ratio copper dual damascene structures and opaque low k dielectrics are expected to exacerbate the problem. E-beam-based inspection system have been available for some time but have not so far received widespread adoption because of their relatively slow throughput and high nuisance defect rates. This paper reviews e-beam-based voltage contrast defect detection technology and its limitations. Several new approaches to voltage contrast defect detection are presented that allow practical application to a wide eliminate stage move time as a facto in throughput. An improved technique is presented here that makes use of a very large field of view magnetic objective lens to eliminate stage overhead. This approach allows literally any die on a wafer to be compared to any other die with virtually zero impact on the total area coverage rate, thus allowing extremely flexible, targeted sampling. Voltage contrast defect detection application examples are presented that highlight the advantages of this approach.