9 June 1995 High-contrast process using a positive-tone resist with antistatic coating and high-energy (100-keV) e-beam lithography for fabricating diffractive optical elements (DOE) on quartz
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Diffractive optical elements (DOE) are becoming important as optical signal processing elements in increasingly diverse applications. These elements, fabricated on quartz, may be used as phase shift type masks or as embedded components that implement a transfer function within a processing network. A process is under development for the fabrication of a DOE implementing a Jervis error diffusion kernel for research in half tone image processing. Dry etching is performed after lithography and pattern transfer through a nickel mask. This results in etched areal features on the substrate. An optical diffraction medium is thus created. Lithographic patterning is done by e-beam lithography (EBL) to realize small features, but also offers the important advantage of a large depth of field which relaxes the problem of complex surface topology. The recent availability of high energy (100 KeV) lithography tools provides a capability for precision overlay, small feature resolution, and enhanced image contrast through a lower induced proximity effect. Patterning by EBL on insulating substrates is complicated by the necessity of providing a vehicle for the avoidance of charge buildup on the surface. In a previously presented paper a methodology was shown for the use of TQV-501 (Nitto Chemical) antistatic compound as a final spin on film for use with PMMA and SAL-601 (Shipley). In this current work, a process is described using EBL and a high performance positive resist working with a final film layer of antistatic TQV-501 on a nickel coated wafer. The process may then be reapplied to realize additional lithographic levels in registration, for multilevel DOE components. High energy (100 KeV) EBL is used to provide high quality pattern definition. The e-beam sensitive resist, ZEP-320-37 (Nagase Chemical) in dilution, together with a top film layer of TQV-501 serves as a bilevel resist system and is used for patterning the desired image before definition of the nickel mask through a wet etch. ZEP- 320-37 in 1:1 dilution with the native solvent is spin applied and softbaked for 2 hours at 120 degree(s)C, after which the antistatic film is spin applied and softbaked for 10 minutes at 70 degree(s)C. After exposure, the TQV film is removed within the development procedure itself. The image is developed in xylene followed by a thorough IPA rinse. The pattern is then transferred to the nickel film via a dilute HCl etch bath. Finally, magnetron RIE is performed on the substrate through the defined nickel mask, after which the nickel film is completely removed in HCl.2438
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Louis C. Poli, Louis C. Poli, Christine A. Kondek, Christine A. Kondek, Barry L. Shoop, Barry L. Shoop, George F. McLane, George F. McLane, "High-contrast process using a positive-tone resist with antistatic coating and high-energy (100-keV) e-beam lithography for fabricating diffractive optical elements (DOE) on quartz", Proc. SPIE 2438, Advances in Resist Technology and Processing XII, (9 June 1995); doi: 10.1117/12.210416; https://doi.org/10.1117/12.210416

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