Figure 3

Alignment calibrator (nano-ruler). In order to compensate for alignment errors caused by mechanical errors (e.g. stage movement) we design a calibrator and measure the alignment errors. (a,b) The calibrator of the first and the second layer, respectively. (c) SEM image of a fabricated calibrator. The calibrator or ruler is used to measure alignment errors and each scale on the ruler represents 50 nm. After making the first layer calibrator structures, we can fabricate the second pattern on top of the first calibrator. If there are no alignment errors, then the second patterns should fit onto the first pattern exactly. Thus, by observing the fabricated patterns, we can find the proper offset to compensate for the alignment errors induced by mechanical errors. (d,e) Statistical results of the translational errors. By applying offset properly into the mask (CAD pattern) design, we can find the intrinsic alignment errors that occur from the EBL system. (f) Error data set after imposing the optimum offset. We test three samples and three alignment processes are done at different positions on each sample. Measured errors are sub-20 nm.