**4. Problems and perspectives**

64 Recent Advances in Nanofabrication Techniques and Applications

The anisotropy of etching rate of silicon induced obvious difference of the spatial resolution of patterning of Si along different crystal orientation. In our experiments, the anisotropic etching rate seemed to improve the edge resolution, and the anisotropic etching process of silicon strongly affected the patterning results including the edge step resolution. It was easier to obtain a large aspect ratio and a better resolution for Si(110) rather than for Si(100). The sharpest step edge resolution obtained presently in Si(110) samples was only 41nm with 1 pixel and the step edge resolution of about 120 nm, indicating an aspect ratio of 3:1. In principle, the estimated value of geometrical blurring was ~240 nm with the geometry described in scheme 3, and the spatial resolution was improved by reducing the distance between the source and the sample, which was related to the divergence of the metastable helium beam. The achievable width in our experiments was much smaller than the calculated one because of the hardening effect in the process of chemical etching. If the experimental parameters in both the exposure and etching processes (e.g., the exposure time, etching time, etc.) were optimized further, higher resolution of the patterning onto the

Fig. 9. Tapping-mode AFM images with sectional analysis of periodic arrays of square silicon microwell fabricated on Si(100) substrate with DDTS SAM (exposure time: 20min, etching time: 10min): (a) AFM images with sectional analysis of arrays of square microwell with a depth of about 180 nm; (b) 3D views of the arrays of silicon microwell over a 80

Athough the interaction between the outermost surface of the SAM and the irradiation of metstable atoms has been investigated for about ten years [34-41], the detailed change in surface chemistry of the outermost of the SAM molecules under the irradiation of helium atom beam is still difficult to predict and remains to be explored. What we demonstrated in the present study was that both the positive-tone and the negative-tone sensitivity of the SAM were related to largely different dosage of MAB and the length of the alkyl chains of SAM molecule (as illustrated in scheme 1 (IV) and (V)). When we optimized the experimental parameters, we accidentally obtained a novel patterning with both negative

um×80 um area and with a periodicity of 12.5 um.

silicon substrates could be obtained.

Atom lithography based on He\*-MAB and SAMs has been demonstrated to have significant potential in fabricating arrays of micro- and nanostructures. This new fabrication technique has opened a novel way in the practical application of the atom lithography in micro- and nanofabrication of silicon. Atom lithography using He\*-MAB and SAMs to pattern the surface of silicon wafer without coating intermediate layer to create the arrays of silicon microstructures has been realized successfully. In order to improve the spatial resolution of the patterning, new etching method instead of KOH wet etching method need to be developed to meet the requirments of nanopaterning. At present, though the image of a pattern is in micrometers range due to TEM grids, a sharp edge in the tens-of-nanometers range indicates a potential of submicrometer-sized patterning. The new masks with nanopattern instead of present TEM masks should be used in order to generate nanostructure arrays. The mechanism of positive and negative patterning on silicon wafer has been investigated. Our results suggest that both the positive-tone and the negative-tone sensitivity of the SAM are related to largely different dosage of MAB and the length of the alkyl chains of SAM molecule. More experiments need to be carried out in order to conclude the detailed parameters to alter patterning type.

In the future, further studies on this subject can be listed as following:

