Narrow features in metals at the interfaces between different etch resists
Applied Physics Letters
The ability to create structures on length scales below 100 nm easily is a challenging feat. We report here a facile technique for the fabrication of such structures in gold ͑Au͒ with feature sizes smaller than 50 nm, utilizing two families of Au etch resists in conjunction. The first resist family consists of self-assembled monolayers ͑SAMs͒ of alkane thiols on Au, which provide substantial resistance against cyanide etch solutions. The second class consists of metals deposited on the surface
... ted on the surface of Au, which also provide similar resistance of the Au film to CN etchants but are not conducive for the formation of SAMs. Selective etching is initiated at the interface between these resists, proceeds into the Au layer, and results in narrow trenches in the Au film. Our protocol allows for the sequential removal of both resists and thus permits the creation of planar Au surfaces with well-defined sub-50-nm etch patterns. Nanostructure fabrication on length scales below 100 nm is a challenging goal, especially when it needs to be done in a cheap and scalable fashion. 1-3 While techniques exist to create such features with remarkable fidelity using scanning microscopy like e-beam writing 4 or dip-pen lithography, 5 such techniques are inherently serial and are not amenable for reproduction on large scales. More recent work has concentrated on the development of nontraditional lithographic methods such as imprint and contact lithography as techniques for the formation of smaller features. 6,7 Recently, active research has been done in utilizing self-assembled monolayers ͑SAMs͒, formed from long-chained alkanethiols, as resists on the surfaces of coinage metals.     While their high degree of order enables these SAMs to function as highly efficient resists, 12 it has been also suggested to exploit disordered regions in SAMs, at the interface of two topologically patterned coinage metals, to create narrow trenches much below the diffraction limit encountered in optical lithography or the capabilities of the standard microcontact printing with SAM resists. 13,14 However, this approach, also known as topologically directed etching ͑TODE͒, leaves behind a nonplanar metal surface, which is impractical for many applications. We expand on the idea of using the active edge regions in SAMs and demonstrate the formation of sub-50-nm lines on planar surfaces of Au. The key concept here is utilizing not only SAMs, but also non-SAM-forming metals ͑e.g., Ti͒ as etch resists. At the interface of these two families of resists, there exists a region of disorder in the SAM, from which the more labile SAM thiols are easily removed and nucleate etch pits. As the region of disorder is present only on the Au side, the corresponding etch lines are narrower than those formed in TODE. Finally, the lack of SAM formation on Ti allows for Ti removal and yields flat Au substrates patterned with trenches. A schematic outline of the various processing steps in our approach is presented in Fig. 1 . The deposition of an Au layer on a silicon substrate ͓Fig. 1͑a͔͒ is followed by the patterning of a photoresist layer ͓Fig. 1͑b͔͒. Deposition of a second, SAM-resistant metal ͑e.g., Ti͒ ͓Fig. 1͑c͔͒ and liftoff leads to an inverse pattern being formed, with the Au being covered by Ti in some areas only ͓Fig. 1͑d͔͒. The next step is the formation of a SAM layer by dipping the substrate in a hexadecane thiol ͑HDT͒. Ordered SAMs are formed on Au covered regions and not on those covered by the Ti ͓Fig. a͒ Author to whom correspondence should be addressed; electronic mail: email@example.com FIG. 1. Schematic presentation of multiresist fabrication strategy. ͑a͒ Deposition of Au ͑20 nm͒ by e-beam evaporation is followed by spin-coating of Shipley 1805 photoresist ͑4 K rpm͒. ͑b͒-͑d͒ Photolithographic patterning ͑b͒, Ti deposition ͑3 nm͒ ͑c͒, and liftoff ͑d͒ result in the formation of a patterned Au substrate. ͑e͒ SAM formation on the exposed Au regions is accomplished by dipping the substrates in a 10 mM HDT solution in ethanol for at least 2 hs. Right ͑magnified view͒: The disordered SAM region in this approach is expected to be narrower than the disordered SAM region in TODE, as the HDT can only self-assemble on top of the Au surface. ͑f͒ Au substrate is etched in cyanide solution with 1 M KOH, 10 mM ferricyanide, 1 mM ferrocyanide, 100 mM sodium thiosulfate. Right ͑magnified view͒: As a result of the narrower disordered SAM region, the etched linewidths are thinner in this approach than in the case of the TODE approach. ͑g͒ Both etch resists are removed by sequential treatment in a 3% HF solution ͑30 s͒ and 10 s oxygen plasma etching. Right ͑magnified view͒: The presence of a SAM layer on both metals in TODE prevents the removal of one metal and results in a nonplanar topography.