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

Reverse transfer of nanostencil patterns using intermediate sacrificial layer and lift-off process

We propose a new process by which patterns produced by nanostencil lithography can be reversed, so that the final pattern on the substrate has the same contrast (filled or empty) as that of the stencil. In this process, the stencil pattern is first formed on an intermediate sacrificial layer, and then transferred onto the underlying substrate in a reverse manner. Using this process, we can form various pattern structures that cannot be produced by the normal stencil process, such as an array of pores or multiple parallel bridges. Because a bridge in the stencil is transferred also as a bridge on the substrate, we can not only avoid the widening of a narrow bridge pattern by the stress-induced bending of the membrane, but also reduce the width of the bridge even further using the pattern blurring. Using SiO2 as an intermediate layer, we have fabricated various reversed Cr patterns on Si, including an array of 800 nm circular pores and a 100-nm-wide and 150-nm-long nanobridge

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

Reusability of nanostencils for the patterning of Aluminum nanostructures by selective wet etching

One of the major advantages of stencil lithography is the possibility to use stencils many times. However, when stencils contain nanoapertures, the clogging of the membranes limits the useful life time of the stencils. The clogging is due to the accumulation of material deposited inside the apertures of the stencil. Here, we report a study on the effect of the clogging on the life time of stencils after Al depositions through the stencils. Then we present a method to clean the stencils based on Al wet etching to eliminate the clogging. We show that this method allows the reusability of stencils for the repeatable depositions of Al nanostructures

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

Metallic Nanowires by Full Wafer Stencil Lithography

Aluminum and gold nanowires were fabricated using 100 mm stencil wafers containing nanoslits fabricated with a focused ion beam. The stencils were aligned and the nanowires deposited on a substrate with predefined electrical pads. The morphology and resistivity of the wires were studied. Nanowires down to 70-nm wide and 5 µm long have been achieved showing a resistivity of 10 µΩcm for Al and 5 µΩcm for Au and maximum current density of ~108 A/cm2. This proves the capability of stencil lithography for the fabrication of metallic nanowires on a full wafer scale

Sub-100 nm-scale Aluminum Nanowires by Stencil Lithography: Fabrication and Characterization

We present the fabrication process and electrical characterization of sub-100 nm scale Al nanowires (NWs) fabricated by stencil lithography (SL). We use a stencil with sub- 100 nm wide nanoslits patterned by focused ion beam (FIB) milling. The stencil is aligned and clamped onto a substrate containing predefined electrical contacts. Then a 60 nm-thick layer of Aluminum (Al) is deposited through the stencil producing NWs with lengths of ~1, 2 and 5 μm and widths down to 65 nm. The NWs show an ohmic behavior with values varying from 30 Ω up to 300 Ω, depending on the dimensions of the structures. We have extracted a resistivity for the Al NWs of ~10 x 10-8 Ωm. We also show that stencils can be cleaned and reused, proving that SL is a cost-efficient and scalable manufacturing method for the direct fabrication of metallic NWs on a full wafer scale

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