SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.5 Systems: 2.5.8 Other E-Beam System Research
2.5.8.1 STM writing
The scanning tunneling microscope (STM) has been used to write nanometer-sized patterns in research experiments. It simply consists of a sharp tip used as a field emission cathode that is scanned a few nanometers above the surface of the sample. Resolution is obtained not by lenses but rather by keeping the tip so close to the surface that the electrons do not have a chance to diverge.
However, the technique is severely limited in writing speed and the resist thickness it can expose, and has seen only a few very limited applications. STM lithography is discussed in Sect. 8.8.3, and in the review article by Shedd and Russel. [92]
2.5.8.2 Parallel beam architectures - microcolumns
In addition to the projection systems described above, several other new architectures have been proposed for increasing the parallelism of e-beam lithography. One proposal is to build an integrated matrix of electron sources, producing an array of parallel beams within one column. [93-94] In contrast, researchers at NTT have proposed the use of an array of micromachined beam blankers and objective lenses, illuminated by a single high-current electron gun. [95] Other researchers are developing discrete components for miniaturized single-beam electron sources and columns. [96-98]
In an ongoing effort at IBM, researchers are seeking to shrink the lenses and other optical components to micrometer sizes using micromachining techniques, thereby building a high-performance, low voltage electron beam column. [96] [99-100] Low-voltage has both advantages and disadvantages over high-voltage lithography (see Sect. 2.5.4.3) but is required here simply because of the small size of the components. In this design an entire e-beam column is only several millimeters high, assembled from micromachined silicon membranes supported on anodically bonded silicon and pyrex wafers. This concept is still in the early development stages.
Microcolumn research seeks to provide exposure parallelism by building an array of small columns. If they can be produced cheaply enough, maintenance would be simplified by the use of disposable electron optics. Although the optics may be inexpensive, the control system for a large array of columns may be very expensive. While many technical hurdles have already been overcome, the ultimate success of beam arrays may be decided solely by economics.
Next Sub-Section: 2.5.9 Electron Beam Fabrication Services
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