Silicon Supported Membranes for Improved Large-Area and High-Density Micro/Nanostencil Lithography

In this paper, the fabrication and use of stencils for full-wafer scale shadow mask (stencil) lithography is described. The stencils fabricated via microelectromechanical systems are mechanically stabilized and show clearly reduced stress-induced membrane deformation, which translates into a more accurate surface pattern definition. Solid-state SiN membranes 500 nm thick and up to 1 mm2 in size having a 20- m-thick silicon support rim following the outline of the stencil apertures were fabricated in a 100-mm Si wafer. The minimum aperture size presented in this paper is 3 m. The increase of membrane stability was confirmed by depositing a highly stressed 35-nm-thick chrome layer. The results demonstrate a stability increase of the Si-supported compared to nonsupported membrane with identical shape by up to 89% as measured by the reduced out-of-plane deflection of overhanging membrane sections. Comparison by scanning electron microscopy and atomic force microscopy of the resulting micropatterns obtained by Cr deposition through both unsupported and Si-rim supported stencils shows better edge sharpness and clearer spatial details for surface patterns deposited through the stabilized stencil compared to those deposited through the nonsupported stencil. The improved stabilized stencils allow for large-area high-density surface patterning while maintaining membrane stability and pattern definition during stencil lithography

Corrugated membranes for improved pattern definition with micro/nanostencil lithography

We present a MEMS process for the fabrication of arbitrary (adaptable to specific aperture geometries) stabilization of silicon nitride membranes to be used as miniature shadow masks or (nano) stencils. Stabilization was realized by the fabrication of silicon nitride corrugated support structures integrated into large-area thin-film solid-state membranes. These corrugated support structures are aimed to reduce the membrane deformation due to the deposition-induced stress and thus to improve the dimensional control over the surface patterns created by stencil lithography. We have performed physical vapor deposition (PVD) of chromium on unstabilized (standard) stencil membranes and on stabilized (corrugated) stencil membranes to test the proposed stabilization geometry. Both the membrane deformation and the surface structures were analyzed, showing reduced deformation and improved pattern definition for the stabilized stencil membranes. The structures have been modeled using a commercial finite element method (FEM) software tool. The simulation and experimental results confirm that introducing stabilization structures in the membrane can significantly (up to 94%) reduce out-of-plane deformations of the membrane. The results of this study can be applied as a guideline for the design and fabrication of mechanically stable, complex stencil membranes for direct deposition

Predicting mask distortion, clogging and pattern transfer for stencil lithography

One of the ultimate tasks for stencil lithography is the ability to fabricate arrays of structures with controlled dimensions on the nanometer scale precisely positioned on a suitable surface. The race to shrink feature sizes requires the limits of conventional lithography to be extended to high-throughput, low cost, reliable and well-controlled processes of which stencilling is a promising candidate for nanoscale applications. Identifying, predicting and overcoming issues accompanying nanostencil lithography is critical to the successful and timely development of this technique for a wide range of potential applications. This paper addresses phenomena associated with stencil nanopatterning and presents the results of modelling and simulation studies for predicting the deleterious effects of mask distortion and clogging during pattern transfer. It is shown that degrading effects of stress-induced deformation of stencils can be dealt with via optimal design of corrugation structures which in turn reduce stencil deformation and significantly improves pattern definition. Modelling results are validated by comparison to experiment. The corrugation structures can be used to define practical design rules for fabrication of stable large area (‘‘full scale’’) purpose-designed stencil membranes. The accurate modelling of the clogging phenomenon combined with gradually evolving stencil deformation, also presented in the paper, can be used for prediction of pattern distortion, to calculate maximum thickness of a deposited layer and/or for prediction of the stencil lifetime

Permalloy thin films exchange coupled to arrays of cobalt islands

Periodic arrays of elongated cobalt islands exchange coupled to continuous Permalloy thin films were fabricated using silicon nitride stencil masks and the magnetic spin configurations during magnetization reversal were studied with photoemission electron microscopy. The presence of cobalt islands results in a spatial modulation of the magnetic properties of the Permalloy films and domain walls positioned at the island boundaries. While magneto-optical Kerr effect measurements indicate differences depending on film thickness, the direct observations reveal two reversal mechanisms: formation of domains running between the islands and coherent rotation followed by propagation of a large domain

Fabrication of metallic patterns by microstencil lithography on polymer surfaces suitable as microelectrodes in integrated microfluidic systems

Microstencil lithography, i.e. local deposition of micrometer scale patterns through small shadow masks, is a promising method for metal micropattern definition on polymer substrates that cannot be structured using organic-solvent-based photoresist technology. We propose to apply microstencil lithography to fabricate microelectrodes on flat and 3D polymer substrates, such as PMMA or SU-8, which form parts of microfluidic systems with integrated microelectrodes. Microstencil lithography is accompanied by two main issues when considered for application as a low-cost, reproducible alternative to standard photolithography on polymer substrates. In this paper we assess in detail (i) the reduction of aperture size (clogging) after several metal evaporation steps and corresponding change of deposited pattern size and (ii) loss in the resolution (blurring) of the deposited microstructures when there is a several micrometers large gap between the stencil membrane and the substrate. The clogging of stencil apertures induced by titanium and copper evaporation was checked after each evaporation step, and it was determined that approximately 50% of the thickness of the evaporated metals was deposited on the side walls of the stencil apertures. The influence of a gap on the deposited structures was analyzed by using 18 um thick SU-8 spacers placed between the microstencil and the substrate. The presence of an 18 um gapmade the deposited structures notably blurred. The blurring mechanism of deposited structures is discussed based on a simplified geometrical model. The results obtained in this paper allow assessing the feasibility of using stencil-based lithography for unconventional surface patterning, which shows the limits of the proposed method, but also provides a guideline on a possible implementation for combined polymer-electrode microsystems, where standard photoresist technology fails

Nanostenciling for fabrication and interconnection of nanopatterns and microelectrodes

Stencil lithography is used for patterning and connecting nanostructures with metallic microelectrodes in ultrahigh vacuum. Microelectrodes are fabricated by static stencil deposition through a thin silicon nitride membrane. Arbitrary nanoscale patterns are then deposited at a predefined position relative to the microelectrodes, using as a movable stencil mask an atomic force microscopy (AFM) cantilever in which apertures have been drilled by focused ion beam. Large scale AFM imaging, combined with the use of a high precision positioning table, allows inspecting the microelectrodes and positioning the nanoscale pattern with accuracy better than 100 nm

The diminished effect of index rebalances

The author revisits the strategy of trading S&P 500 index re-compositions under the pre- and post-crisis financial environments, proving that the return structure has significantly changed. The results show for the first time, that there are currently no tradable abnormal returns between announcement and event dates in the post-crisis sample period, indicating smoother rebalancing mechanisms by bank’s client facing desks and better services for passive end-investors. The newly added firms inflate the S&P 500 index by less than 10 basis points per year. The results could be attributed to improved execution algorithms used by the banks, and potentially to the new regulatory reforms in the sector, which prevents financial institutions from taking large trading positions with their balance sheets