Stencil lithography: an ancient technique for advanced micro- and nano-patterning

This work describes the advancements made in micro and nano patterning via stencil lithography. Stencil lithography is a resist-less and direct pattern technique where a controlled amount of material is deposited through apertures on to a substrate in a vacuum environment. This unique approach enables large scale micro and nano structuring of unconventional materials and on a large range of substrates otherwise not possible using conventional photolithography. A theoretical approach is presented for correct implementation of stencil lithography with the aim of giving the reader sufficient insight in stencil lithography and associated phenomena, such as the deposition method, stencil aperture clogging, and the resulting pattern distortion (blurring). The developed "zone theory" allows the reader to estimate the size of the resulting surface patterns and pattern uniformity over a large area. The fabrication of thin-film stencils consist of a few basic process steps such as the choice of membrane material. The motivation for use of specific materials or processes are described followed by the description of a basic planar stencil fabrication scheme. A detailed fabrication description for a 100 mm stencil containing a large number of membranes with apertures ranging from 200 nm up to several 100 μm is given. The utilization of stencils show that the patterns are accurately transferred on to a substrate and that the size of the resulting structures is correctly approximated using the "zone theory". However, deposition of material with a high residual stress through mechanically unfavorable membrane geometries results in stencil membrane deformation and loss of pattern resolution. Two additional fabrication schemes for mechanically reinforced stencil membranes are presented. The utilization of improved stencils shows increased membrane stability, while comparisons of deposited patterns through planar and improved stencils show that the patterns deposited through improved stencils have better pattern resolution and definition. A Finite Element Method model is successfully used to predict stencil deformation under influence of deposited, high stressed material. This model allows the determination of optimum stabilization structures and there placement, enabling a shorter design and fabrication delay. The full integration stencil lithography into existing and established processes requires not only a precise transfer but also a precise pattern registration. A stencil alignment procedure is presented which enables a sub-micrometer alignment accuracy. A set of application examples is given where stencil lithography can be useful or even necessary for successful device fabrication. These examples show the potential of stencil lithography for use as a reliable micro and nano patterning technology in a large range of applications fields. Furthermore, a new approach for the dynamic utilization of stencils is given where the presented system might be useful for rapid prototyping of micro- and nano-patterns. This system has a low-tech threshold and enables nanopatterning without the need for a cleanroom environment.


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