Jürgen Brugger, Nao Takano, Marc Antonius Friedrich van den Boogaart, Oscar Vazquez Mena
Stencil lithography is a surface patterning technique that relies on the local deposition of material through miniaturized shadow mask membranes. This method has been used since many years in various implementations for the formation of patterns, mainly structured thin metal films, in situations when lithography equipment is not available or when the surfaces don’t allow the harsh process steps typically involved in photolithography such as spin coating of photoresist, high temperature baking, development, wet or dry etching and resist stripping. Stencil lithography is down-sizable into the sub-100-nm scale which makes it very interesting as method for (in-vacuum) rapid-prototyping of nanostructures without the risk of contamination, and for laboratories without access to high-end nanolithography equipment. The major challenges in stencil lithography are following: i) the fabrication of large area nanostencils requires one high-resolution lithography step and sophisticated MEMS processes, ii) the mechanical properties of thin membranes (typically ~ 100 nm thick) requires careful handling, iii) the effect of deposited material on stencil leads to aperture clogging and membrane bending which limits the resolution capabilities, iv) the precise positioning of nanostencils to prefabricated surfaces structures for aligned multiple layer patterning, v) and eventually the recycling of used stencils. Despite these difficulties, stencils are extremely useful tools for flexible and reliable patterning of surface structures across multiple length-scales from mm to sub-100 nm. In collaboration with our project partners we have recently progressed in several of the above mentioned challenges: the fabrication of stencils is now based on a set of advanced silicon micromachining steps including DUV exposure for >200 nm apertures and a combination of DRIE and wet etching. Focused beams are used to fabricate stencils with sub-100 nm apertures. In particular the mechanical stability of the ultra-thin low-stress silicon nitride membranes could be considerably improved by topographic reinforcement rims. As a result, the membranes deform less under the stress of deposited film and consequently the surface patterns are better defined. We have studied patterning by stencil lithography of metals (e.g. Al, Au, Bi, Cr, Ti, Cu) on various surfaces (e.g. Si, SiO2, SU-8, PDMS, PMMA, freestanding SiN cantilevers, and curved surfaces. Pulsed laser deposition of metal on self-assembled monolayers (SAMs) has resulted in novel micro/nanopattern for molecular electronic devices. Parallel fabrication of thousands of sub-micrometer resonating beams that are integrated with CMOS circuitry have been realized by stencil lithography and sacrificial etching. The talk will present the current state-of-the-art of the nanostencil lithography , will highlight the strength of the method but will also discuss the current limits and challenges ahead to make it a truly reliable nanofabrication method.
2006
