**3.2 Arrays of Si(110) microstructures**

60 Recent Advances in Nanofabrication Techniques and Applications

hydrophilic area might not be much weaker in resisting the etchant than the unexposed

Fig. 4. Tapping-mode AFM images with sectional analysis of square silicon microwell array created on Si(111) substrate ( Negative patterning, exposure time: 40min; etching time:

Since the ODTS SAM resist comprised highly oriented 18C-long-chain hydrocarbon molecules chemically bonded to the silicon substrate, its surface became hydrophobic [30-32]. Previous studies revealed that, under irradiation of He-MAB with a 20 eV internal energy, C-H bond scission and C-C bond scission occured and a C=C double bond could be formed at the outermost surface of the SAM [34-37]. At low doses, breaking of the C-H bond caused H loss, leaving a C-rich film that could be oxidized in air. Consequently, the exposure region was changed from hydrophobic to hydrophilic, leading to a positive pattern [38,39]. At high doses, further degradation of the hydrocarbon molecules caused fragmentation of the carbon chain, and cross-linking polymerization between adjacent fragmented SAM molecules might occur, giving rise to the formation of a highly resistant carbonaceous layer and leading to a negative pattern [36]. In addition, the contamination resist, which was induced by hydrocarbon molecules remaining on the inner wall of the vacuum chambers, remained a concern [40-41]. When the metastable atoms impacted the surface, they transferred their internal energy to these physisorbed hydrocarbon molecules and induced a polymerization change within the exposed region. To clarify whether these resist materials could remain on the surface even after immersion in a KOH solution, a clean Si(111) sample without an SAM was used. Only a trace of negative contrast was observed, indicating a negligible contamination effect. In our negative pattern case, the exposure region of the SAM resist became etching-resistant, which might be mainly ascribed to the cross-linking

60min, SAM: ODTS) (a): 50×50 µm2; (b) 3D image; (c): 21.7×21.7 µm2).

polymerization of the SAM molecules.

hydrophobic area.

The AFM images of the square micromesa arrays formed on the Si(110) substrate with ODTS SAM was shown in figure 5, which corresponded to negative patterning with an exposure time of 20 min and an etching time of 5 min. A square silicon micromesa array with a micromesa size of ~7×7 µm2 (e.g., the size of one mesh of TEM stencil mask) was fabricated successfully on the substrate. The average height of the step was 27 nm, indicating an etching rate at 5 nm/min. The sharpest step edge had a width of 1 pixel (41 nm) and a height of 26 nm, indicating a steep sidewall with an aspect ratio of 1:1.7. The sectional analysis shown in Figure 5(a) indicated that the rms roughness of the top area of the micromesa and the pedestal base area were 2 nm and 15 nm, respectively. The flat top area of the micromesas could be observed clearly, suggesting the strong durability of the exposed SAM resist against chemical etching. The flaw pedestal base area of the micromesa was also very conspicuous, and this rough characteristic could be attributed to the imperfect SAM formation on the silicon substrate, which induced a local fluctuation in the etching rate of transferring the latent image in SAM resist onto the underlying silicon substrate and, consequently, produced a rough surface. As described in scheme 1, the negative-tone sensitivity of the SAM might be mainly due to the cross-linking polymerization of ODTS molecules induced by the irradiation of the He\*-MAB [19,20]. In addition, it was noteworthy that the etching time had a notable influence on the production of high-quality patterning. In our practical runs, the following etching times were chosen: 5, 10, and 20 min with identical exposure times. The sectional analysis of AFM images of these samples indicated that the rms roughness of top area of the micromesa in the case of overetching was bigger than that in the case of underetching.

Fig. 5. Tapping-mode AFM images of periodic arrays of square silicon micromesa fabricated on Si(110) substrate with ODTS SAM (exposure time: 20min, etching time: 5min): (a) AFM image with sectional analysis of the arrays of silicon micromesas with a height of about 26 nm ; (b) 3D views of the arrays of silicon micromesa over a 80 um×80 um area and with a periodicity of 12.5 um.

Atom Lithography: Fabricating Arrays of Silicon Microstructures

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

um×50 um area.

clearly in figure 8(a) and 8(b).

Using Self-Assembled Monolayer Resist and Metastable Helium Beam 63

the positive patterning. The rough microwell with different diameters can also be observed

Fig. 7. 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 220 nm; (b) 3D views of the arrays of silicon microwell over a 80

Fig. 8. Tapping-mode AFM images with sectional analysis of periodic arrays of square silicon microwell fabricated on Si(100) substrate with HTS SAM (exposure time: 30min, etching time: 10min): (a) AFM images with sectional analysis of arrays of square microwell with a depth of about 220 nm; (b) 3D views of the arrays of silicon microwell over a 50

Fig. 6. Tapping-mode AFM images with sectional analysis of periodic arrays of square silicon microwell fabricated on Si(110) substrate with DDTS SAM (exposure time: 40min, etching time: 10min): (a) AFM images with sectional analysis of arrays of micromesas with a height of about 27 nm ; (b) 3D views of the arrays of silicon micromesas over a 60 um×60 um area.

When a TEM holey carbon grid was used as a mask, an interesting positive patterning result was obtained. The AFM images of the microstructures fabricated on the Si(110) substrate with DDTS SAM were displayed in figure 6, which corresponding to the positive patterning (exposure time: 30 min; etching time: 10 min). The rough micromesa with different diameters can be observed clearly in figure 6(a) and 6(b).

#### **3.3 Arrays of Si(100) microstructures**

Compared with the negative patterning shown in figure 5, a positive patterning could be obtained by adjusting the dosage of He\*-MAB irradiation and the length of alkyl chains of SAM molecule. The AFM images of the square microwell arrays fabricated on the Si(100) substrate with DDTS SAM was shown in figure 7, corresponding to the positive patterning (exposure time: 20 min; etching time: 10 min). A square microwell arrays with a microwell size of ~7×7 µm2 was fabricated on the Si(100) substrate. The depth of the microwells was about 220 nm. The sharpest wall edge had a width of 1 pixel (41 nm) and a depth of 100 nm, indicating a steep sidewall with an aspect ratio of 2.5:1. The sectional analysis shown in figure 7(a) indicated that the rms roughness of the bottom area of the microwell and the top area of the wall were 31 nm and 22 nm, respectively. The rough characteristic could also be attributed to the imperfect SAM formation on the silicon substrate, as mentioned in 3.2. The positive-tone sensitivity of the SAM should be mainly due to the loss of hydrophobicity of DDTS SAM (with shorter alkyl chains, 12C) induced by the irradiation of the metastable helium atoms. The topography and line profile of the microwells array indicated a much rougher top surface of the sidewall with an rms roughness of 22 nm. This large rms value suggested that the hydrophobic area might be much weaker in resisting the KOH etchant, and the DDTS SAM might not be a good resist for atom lithography. The corresponding 3D views of the array wae given in figure 7(b). Additionally, we also used TEM holey carbon grid as a mask to obtain a positive pattern result. The AFM images of the microstructures fabricated on the Si(100) substrate with HTS SAM was shown in figure 8, corresponding to

Fig. 6. Tapping-mode AFM images with sectional analysis of periodic arrays of square silicon microwell fabricated on Si(110) substrate with DDTS SAM (exposure time: 40min, etching time: 10min): (a) AFM images with sectional analysis of arrays of micromesas with a height of about 27 nm ; (b) 3D views of the arrays of silicon micromesas over a 60 um×60

diameters can be observed clearly in figure 6(a) and 6(b).

**3.3 Arrays of Si(100) microstructures** 

When a TEM holey carbon grid was used as a mask, an interesting positive patterning result was obtained. The AFM images of the microstructures fabricated on the Si(110) substrate with DDTS SAM were displayed in figure 6, which corresponding to the positive patterning (exposure time: 30 min; etching time: 10 min). The rough micromesa with different

Compared with the negative patterning shown in figure 5, a positive patterning could be obtained by adjusting the dosage of He\*-MAB irradiation and the length of alkyl chains of SAM molecule. The AFM images of the square microwell arrays fabricated on the Si(100) substrate with DDTS SAM was shown in figure 7, corresponding to the positive patterning (exposure time: 20 min; etching time: 10 min). A square microwell arrays with a microwell size of ~7×7 µm2 was fabricated on the Si(100) substrate. The depth of the microwells was about 220 nm. The sharpest wall edge had a width of 1 pixel (41 nm) and a depth of 100 nm, indicating a steep sidewall with an aspect ratio of 2.5:1. The sectional analysis shown in figure 7(a) indicated that the rms roughness of the bottom area of the microwell and the top area of the wall were 31 nm and 22 nm, respectively. The rough characteristic could also be attributed to the imperfect SAM formation on the silicon substrate, as mentioned in 3.2. The positive-tone sensitivity of the SAM should be mainly due to the loss of hydrophobicity of DDTS SAM (with shorter alkyl chains, 12C) induced by the irradiation of the metastable helium atoms. The topography and line profile of the microwells array indicated a much rougher top surface of the sidewall with an rms roughness of 22 nm. This large rms value suggested that the hydrophobic area might be much weaker in resisting the KOH etchant, and the DDTS SAM might not be a good resist for atom lithography. The corresponding 3D views of the array wae given in figure 7(b). Additionally, we also used TEM holey carbon grid as a mask to obtain a positive pattern result. The AFM images of the microstructures fabricated on the Si(100) substrate with HTS SAM was shown in figure 8, corresponding to

um area.

the positive patterning. The rough microwell with different diameters can also be observed clearly in figure 8(a) and 8(b).

Fig. 7. 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 220 nm; (b) 3D views of the arrays of silicon microwell over a 80 um×80 um area and with a periodicity of 12.5 um.

Fig. 8. Tapping-mode AFM images with sectional analysis of periodic arrays of square silicon microwell fabricated on Si(100) substrate with HTS SAM (exposure time: 30min, etching time: 10min): (a) AFM images with sectional analysis of arrays of square microwell with a depth of about 220 nm; (b) 3D views of the arrays of silicon microwell over a 50 um×50 um area.

Atom Lithography: Fabricating Arrays of Silicon Microstructures

**4. Problems and perspectives** 

the detailed parameters to alter patterning type.

etching) for atom lithography.

**5. Acknowledgement** 

**6. References** 

1. To prepare more perfect SAMs for atom lithography;

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

2. To make new mask with sub-100nm pattern for patterning transfer; 3. To improve the precision to position and adjust the mask during exposing;

4. To investigate both other substrates (e.g. GaAs) and other etching processing (e.g. dry

The author gratefully acknowledges support from the National Natural Science Foundation of China (Grant No. 20777072, 91023040), the Fundamental Research Funds for the Central Universities (Grant No. WK2030020018) and would like to thank Prof. Yasushi Yamauchi, Dr. Mitsunori Kurahashi for their help with experiments and their helpful comments. The

[1] Xia, Y.; Rogers, J. A.; Paul, K. E. and Whitesides, G. M. (1999). Unconventional methods for fabricating and patterning nanostructures. *Chem. Rev.* 99 1823-1848. [2] Campbell, C. J.; Smoukov, S. K. M.; Bishop, K. J.; Baker, E. and Grzybowski, B. A. (2006).

substrates with sub-micrometer resolution. *Adv. Mater.* 18 2004-2008.

Direct printing of 3D and curvilinear micrometer-sized architectures into solid

author also notes that part of the work was completed while he was at the NIMS.

interesting flexibility of atom lithography in micro- and nanofabrication.

Using Self-Assembled Monolayer Resist and Metastable Helium Beam 65

and positive pattern on one Si(100) substrate with DDTS SAM (as discribed in figure 9), which was very different from previous results. In this case, a novel array of square micromesas with microholes in it was fabricated successfully. The mechanism to induce this dual patterning is remained to be explored at present. This dual array also indicated some

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 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 silicon substrates could be obtained.

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

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 and positive pattern on one Si(100) substrate with DDTS SAM (as discribed in figure 9), which was very different from previous results. In this case, a novel array of square micromesas with microholes in it was fabricated successfully. The mechanism to induce this dual patterning is remained to be explored at present. This dual array also indicated some interesting flexibility of atom lithography in micro- and nanofabrication.
