High-throughput mapping of short-range spatial variations using active electrical metrology

Spatial variations of parameters in semiconductor manufacturing, such as critical dimension (CD) and overlay, have significant impact on the performance and yield of integrated circuits (IC). Among these spatial variations, the variations of parameters between transistors separated by a very short spatial distance such as 1 mum to 100 mum (intertransistor variations) can be particularly hazardous for those types of ICs that require exact transistor-transistor matching. To measure these intertransistor variations, both high-throughput and high-spatial-sampling-density beyond the scope of currently available metrology tools are needed. We have thus developed an active electrical metrology method of measuring intertransistor variations using on-chip, active, electrically addressable arrays of test structures to provide the high-throughput (5 mus/data point) and high-density (3 mum/grid spacing) needed. Test chips were designed and fabricated on a HP 0.35-mum process, and the testing configuration was set up to optimize throughput and precision. This method was verified with the measurements of on-chip calibration arrays. The spatial variations of both intertransistor CD (effective gate length.) and overlay (between poly/diffusion) within the test chips were mapped with this method. For these circuits, the intertransistor CD variations were found to depend primarily on the layout, whereas the intertransistor overlay variations were found to be dominated by errors of the pattern generator used to fabricate the masks.