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SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography

Section 2.7 Resists: 2.7.5 Inorganic and Contamination Resists

2.7 Resists
2.7.1 Charge Dissipation
2.7.2 Positive Resists
2.7.3 Negative Resists
2.7.4 Multilayer Systems
2.7.5 Inorganic and Contamination Resists
2.7.6 Other Research: Scanning Probes and Thin Imaging Layers
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2.7.5 Inorganic and Contamination Resists

Some of the first high-resolution e-beam exposures were made with "contamination lithography" -- by simply using the electron beam to crack contaminants sorbed onto the substrate. These carbonaceous and silicaceous contaminants are produced from oil in the vacuum pumps or from organic residue on the sample surface. By using the contamination as a mask for ion milling, wires as narrow as 50 nm were made in the 1960s. [181] Later, the technique was used for fabricating nanometer-scale superconducting devices [182] and metal lines for the study of electron transport in mesoscopic devices. [183]

The dose required for the deposition of contamination depends on how much oil and other contaminants are in the vacuum system (an untrapped diffusion pump provides an ample supply), but the dose is very high, typically in the range of 0.1 to 1 C/cm2. The high dose limits its application to very sparse patterns. Cracked hydrocarbons provide poor selectivity for etching or milling, so the choice of metals is also limited (for instance, it is not practical to pattern aluminum this way). The contamination can be easily cleaned by heating the substrate to ~100 C.

Another technique for producing nanometer-scale patterns - again using doses on the order of 1 C/cm2 -- is the use of metal fluorides. A high current density of electrons causes the dissociation of materials such as AlF3, MgF2, NaCl, LiF, KCl, and CaF2 [184] at doses around 10 to 20 C/cm2. At lower doses (1 to 3 C/cm2) AlF3 acts as a negative resist, developed in water. [185]

One reason for the very high resolution is that these materials are modified by the primary beam of electrons and are insensitive to the much larger spread of secondary electrons. The highest resolution patterns were formed in NaCl crystals, where 50 keV electrons were used to drill holes of ~1.5 nm diameter, [186] but the patterns could not be transferred to any useful material. While negatively exposed AlF3 makes an excellent etch mask [185] for fluorine-based RIE, the process has not been applied to any useful devices. Recent research in metallic compound resists [187-188] has concentrated on mixing AlF3 and LiF to reduce the dose needed for dissociation, to provide more uniform films, and to expose these films with the lower current density and lower voltage (20 to 50 kV) available in common e-beam exposure tools. Slots in these films of width 5 nm have been made with 30 keV electrons. [188] At doses similar to those of the metal fluorides, silicon dioxide [189] has also been used for nanometer-scale patterning.

Next Sub-Section: 2.7.6 Other Research: Scanning Probes and Thin Imaging Layers

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