Optical Science Laboratory
Recently, nanoprint-nanoimprint technologies or scanning probe microscope (SPM) lithography is attracting attentions for the fabrication of nanostructures used in electronic or optical devices. Compared to conventional "projection-type" lithography, these "contact-type" lithographies have advantages in size and ease of operation. However, because the basic concept involves transferring the surface shape of a mold, the pattern cannot be modified in nanoprint-nanoimprint technologies. SPM lithography, in which an electrochemical reaction is induced by a conductive probe tip, can provide pattern flexibility. However, the throughput is still low. We are trying to develop a lithography technique that can provide good throughput and flexibility simultaneously.
Figure 1 shows the principle of nanoelectrode lithography. A nanoelectrode surface consists of a conductive area and an insulating area. The nanoelectrode makes contact with the surface of a target. When a voltage is applied, current flows between the nanoelectrode and the target material. Then, an electrochemical reaction occurs on the target surface. For example, anodic oxidation of the semiconductor, Si or GaAs, transfers the pattern to the target [1, 2]. Figure 2 shows an example of Si substrate patterning by nanoelectrode lithography. In Fig. 2, a nanoelectrode with a 300-nm-pitch dot pattern was used. The fabricated oxide pattern worked well as a mask for wet and dry etching, which means that resist-less patterning is possible. In addition, nanoelectrode lithography does not deform the surface of the target during the patterning process. Therefore, it will enable us to overwrite a pattern to fabricate a more complex pattern. Figure 3 shows checked pattern fabricated by repeating the process with a line-and-space (L/S) pattern .
As shown here, nanoelectrode lithography has some advantages, such as direct fabrication of the etching mask and modification of fabricated patterns by combining this approach with another lithographic technique. In addition, the lithography process can directly fabricate patterns defined by chemical characteristics such as hydrophilic or hydrophobic properties. These pattern may be used as templates for selective growth of semiconductors. We will apply nanoelectrode lithography for patterning of metal layers or resist layers on a substrate to prove the generality of the technique.
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 A. Yokoo, S. Sasaki, Jpn., J. Appl., Phys., 44, 1119 (2005)
 A. Yokoo, J. Vac. Sci. Technol. B, 21, 2966 (2003)
Fig. 1 Nanoelectrode lithography Fig. 2 Fabricated pattern on Si Fig. 3 Multiple patterning
(a) after 1st procedure
(b) after 2nd procedure
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