Submicron e-beam lithography process using an overcoating conducting polymer for the reduction of beam charging effects on lithium niobate and quartz

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This paper presents a processing methodology for direct write E-beam lithography on insulating substrates. Processes suitable for use with negative tone SAL-601-ER7 and positive tone PMMA are presented. Both are three level processes and are used for fabrication of surface acoustic wave (SAW) devices with transducer electrode line space dimensions of 0.7 and 0.4 micron, respectively. The authors method involves spinning a film of a commercially available conductive polymer, known as TQV-501, as the final coating. A Leica Instruments EBMF 10.5 electron beam tool operating at an energy of 30 keV is used to perform lithography. SAW devices were patterned with these processes and then realized by metallization with aluminum via lift-off. A two level positive tone process was used consisting of a film of P(MMA-MAA) followed by a film of PMMA. This is followed by a final spin coating of TQV-501 to provide a discharge medium. The negative tone process consists of a film of SAL-601-ER7 which is followed by a film of polyvinyl alcohol (PVA). A final film layer of TQV-501 is then applied. The PVA serves as both a diffusion barrier to prevent intermixing of the TQV-501 and SAL-601-ER7 during layering and later to prevent stripping the SAL-601-ER7 during the solvent strip of the TQV-501. Patterns are written in reversed tone form on negative resist to provide the proper final positive image tone suitable for lift-off. TQV-501 offers the advantage of simple application via spin coating and a soft bake and easy removal by stripping in solvent.