A digital nano-moiré method with wavelet transformation is explored to measure nanoscale
in-plane displacement fields. By applying e-beam lithography, a periodic PMMA
nanostructure array is fabricated directly on the specimen and used as the specimen
grating. Moiré patterns are generated by overlapping the images of the PMMA specimen
grating obtained from AFM scanning and the virtual reference grating produced by a
digital image generating process. Then, the overlapped images are filtered by the 2D
wavelet transformation (WT) to capture the target moiré patterns. Existing methods, by
overlapping the monitor-generated scanning lines with the image of the specimen
grating, cause a mismatch problem. Previously, the carrier moiré method was
explored with the aim of curing the mismatch problem. Unfortunately, the carrier
moiré method, in addition to suffering from increased complexity of mathematical
calculations, is incapable of directly obtaining the displacement field. Thus, the
mismatch problem will result in inconveniences and restrictions in the practical
application. Instead of using monitor-generated scanning lines, the proposed method
applies the virtual reference grating, and thus puts the mismatch problem to rest.
Nevertheless, the resultant moiré image suffers from low contrast which, if left
untreated, might distort the measurement result. Therefore, the WT, known for its
sharpened abilities of characteristic and edge detection, is used to capture the target
moiré patterns and improve the measurement accuracy. The proposed method has
been carried out in the laboratory. Experimental results have shown that the
proposed method is convenient and efficient for nanoscale displacement measurement.