SPIE Handbook of Microlithography, Micromachining and Microfabrication, Volume 1: Microlithography
Section 2.7 Resists: 2.7.1 Charge Dissipation
2.7.1 Charge Dissipation
A common problem is the exposure of resist on insulating substrates. Substrate charging causes considerable distortion when patterning insulators and may contribute significantly to overlay errors even on semiconductors.  A simple solution for exposure at higher energies (>10 kV) is to evaporate a thin (10nm) layer of gold, gold-palladium alloy, chrome, or aluminum on top of the resist. Electrons travel through the metal with minimal scatter, exposing the resist. The film is removed before developing the resist. When using Au or Au/Pd, the metal film is removed from the top of the resist with an aqueous KI/I solution.  A chrome overlayer would be removed with chrome etch.  Aluminum can be removed from the resist with an aqueous base photoresist developer. Acid mixtures or photoresist developer for removing aluminum will sometimes react with exposed e-beam resist; therefore, aluminum is not the best choice for charge dissipation. When evaporating any metal, it is important not to use an electron gun evaporator since x-rays and electrons in the evaporator will expose the resist.
Another approach to charge dissipation is the use of a conducting polymer, either as a planarizing layer under the resist or as a coating over the resist. The commercial polymers TQV (Nitto Chemical Industry) and ESPACER100 (Showa Denko) have been used for this purpose. [129-130] Both are coated at a thickness of about 55 nm and have a sheet resistance around 20 M/. TQV uses cyclohexanone as the casting solvent, which swells and dissolves novolac resins (present in most photoresists and SAL), and so a water-soluble PVA (polyvinyl alcohol) layer is needed to separate the resist from the TQV. ESPACER100 has the advantage that it is soluble in water and so can be coated directly onto many resists. TQV is removed with methyl isobutyl ketone/isopropanol (MIBK/IPA), the developer used for PMMA. ESPACER is removed in water. Other water soluble conducting polymers can be prepared from polyaniline doped with onium or triflate salts. [131-132]
Next Sub-Section: 2.7.2 Positive Resists
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