Effect of setpoint on CD measurement in CD-AFM:TB plausibility study

The amplitude setpoint affects the critical dimension measurement with CD-AFM. The setpoint amplitude is the amplitude of the resonant oscillation of the AFM tip maintained by the feedback loop as it scans the surface. The Setpoint therefore decides the tip-surface distance, and the tip-surface interaction force as well. Normally, the tip moves an unknown distance away from the sample surface. Such a tip-sample distance on the top and bottom surface is cancelled out in height measurement. In width measurement, however, the tip-sample distance on the left and right sidewall will add up to produce a bias in the measured CD values. The bias will appear in the opposite way and by the same amount in line and trench measurement. We conducted the experiments to see the effect, and found out there exists the dependence of the measured linewidths on the setpoint in the consistent behavior as our hand-waving predicts. The effect may be a significant uncertainty source in the CD-AFM metrology.