The fabrication of patterned devices often requires high-precision lithography. As the dimensions of the features decrease, so do the number of techniques available for fabricating them. The problem is particularly acute when the necessary patterns are of nanometer size. In this case, the only lithography available is electron-beam (e-beam) lithography. However, according to Kelly; Roberta, in “Electron Beam Lithography” the results can be imprecise because of uneven distribution of the resist on the wafer surface, which results in improper focusing of the electron beam and imprecise deviations of line thickness' from what is desired. The nanostructure wires thus formed can vary from the desired thickness'.
Since current methods for preparing nanometer-size structures are not precise, the structures are created by a hit-or-miss type of technique. For example, the lithographic preparation of a metal-gap-metal structure with a 4-nm gap is accomplished by e-beam lithography (1). According to the prevailing method, a series of metal lines is prepared first and these lines are paired with a commensurate series of lines across gaps. To ensure the formation of the metal-gap-metal structure with the desired gap size, the gaps formed when the pairing series is deposited are varied in the form of a wedge, from larger to smaller than the required gaps, in an attempt to hit the right gap size by chance. This imprecise technique is wasteful and time-consuming, and it is not suited for manufacturing processes. In addition, it is necessary to select the correct pair by closely examining the entire series with electron microscopy or atomic force microscopy (with nanotube tips) and ultimately by tunneling measurements. Usually the pairs which do not fit the desired gap dimensions are ignored.
While structures containing 4-nm size gaps have been obtained using the above described technique (such as in FIG. 1b herein), the process for fabricating them is complicated.
The fabrication of metal-gap-metal patterns with gaps below 4 nm is rarely attempted by lithographic techniques. Such small gaps are fulfilled by break-junction techniques, and not by any lithographic method.
The prior art reference of U.S. Pat. No. 4,058,430 of Suntola et al, (6) describes a very accurate method for growing thin films of great accuracy in the nanometer range using layer by layer deposition.