As device dimensions shrink the number of parameters influencing CD increases (PEB dispersion, development uniformity, resist thickness, BARC thickness, +/- scan focus control, scanner focus control at edge of the wafer...). Separation between all these contributors is not easy using only CD-SEM measurement, and particularly with isolated lines. For high volume manufacturing (where "time is money") and in the case of litho cluster drift, a quick and accurate diagnostic capability is an advantage for minimizing tool unavailability. An important attribute of this diagnostic capability is that its implementation is on standard production wafers. The use of production wafers enables continuous monitoring and also allows a direct correlation between monitoring measurements and the impact on product.
The technology that enables this type of diagnostic capability makes use of a compact dual tone line-end-shortening based target. A key benefit to this technology is that it provides a separation of the dose and focus parameters, which leads to quicker route cause determination.
After building a calibration model and determining minimum dose and focus sensitivity, both short term and long term stability of the model is investigated. The impact of wafer topology on model prediction is also investigated in order to assess on-product monitoring capability. The main error contributors are then identified for both track and scanner and the impact on CD control is evaluated. These cluster error contributors are then varied, first separately, and then combined. Measurement results are compared to the input parameters in order to determine error detection ability, measurement accuracy and separation capability.