1 June 1990 Determination of and compensation for wafer bow and warp in a scanning electron microscope requiring precise feature locating and variable tilt
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Precise and accurate feature positioning in SEMs is becoming more critical. Moving the stage to a predetermined location must be done with accuracy and precision that put the feature ofinterest in the field ofview at a magnification high enough to detect orrecognize the same feature. Ifthis is notdone, some sort ofsearch, either automatic ormanual must be performed. This may not only be bothersome, but detrimental to inspection or measurement throughput performance. Ultra precise stages - for example, those using laser interferometers or linear encoders - are capable of positioning precisions, if not accuracies, to 0. 1 micron. In both optical and SEM systems where inspection is normal to the plane ofthe waler(cailed zero tilt), precise locating of features is possible without serious attention being paid to the bow or warp of a wafer. From the SEMI Standards Manuals, it is seen that a 200 mm wafer may have up to 65 microns of bow. In optical lithography tools and optical inspection or measurement systems, a vacuum chuck may alter or reduce the bow. However, in the vacuum chamber of the SEM this technique does not work. The bow or warp remains. The problem occurs in going to a particular numerical address whenthe waferis tilted, ifthat numerical address was determined at some different tilt -themost probable, of course, being zero iilt. Tilting of the wafer will cause the initially observed feature to move through an arc of "unknown" extent (unknown because it is a function of the bow and the bow is not known at that point). A 60 degree tilt of awafer with 40 microns of bow can cause about 35 microns oflaten.l displacement of a feature from where it would be expected for a wafer with no bow. The effect of this displacement on detectability is discussed. Actual displacement measurements on a 125 mm wafer ait plotted. These plots are compared with those derived from measurements made by optical and SEM systems specially set up to measure bow magnitudes. Bow-magnitude data obtained from a separate bow-measuring insirument or from data taken in-situ in the SEM itself can be used to correct the positioning error that would occur with that particular wafer. Bow related effects may be a practical limitation on the open-loop positional precision capabily of the SEM at non-zero tilts.
© (1990) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Duane C. Holmes, Duane C. Holmes, "Determination of and compensation for wafer bow and warp in a scanning electron microscope requiring precise feature locating and variable tilt", Proc. SPIE 1261, Integrated Circuit Metrology, Inspection, and Process Control IV, (1 June 1990); doi: 10.1117/12.20069; https://doi.org/10.1117/12.20069

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