**2.4 Range of linearity**

438 Recent Advances in Nanofabrication Techniques and Applications

Fig. 5. Spectral response (signal to noise ratio) for corresponding overlay errors (shown in the legend box). The plots (a) and (b) also show response (black line) calculated from the

Fig. 6. (a) shows Pads 1-15 with increasing programmed positive overlay shifts and Pads (-1)-(-15) with increasing negative overlay shifts, (b) and (c) show TE spectra from corresponding vertical and horizontal gratings pad sequence respectively, and (d) shows

measured spectral response in arbitrary units.

calibration pad spectra.

The linearity range is tested by printing a sequence of pads with varying overlay shifts. Pads in the right half of the sequence (pads 1-15) have increasing positive overlay shifts (Fig. 6(a)) in 15 nm steps. Pads (-1) to (-15) have increasing negative overlay shifts in 15 nm steps. Fig. 6b and 6c shows raw TE spectra collected from these pads. The difference signal is calculated by subtracting the pad 1 spectrum for pads 2-15, and the pad -1 spectrum for pads -2 to -15. The range of linearity observed (~±70 nm) is significant for this process as shown in Fig. 6(d). A similar linearity range is observed for the horizontal gratings.
