Veronica Savu

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 mum long have been achieved showing a resistivity of 10 microOmegacm for Al and 5 microOmegacm for Au and maximum current density of approximately 10(8) A/cm(2). This proves the capability of stencil lithography for the fabrication of metallic nanowires on a full wafer scale.

High-Resolution Resistless Nanopatterning on Polymer and Flexible Substrates for Plasmonic Biosensing Using Stencil Masks

The development of nanoscale lithographic methods on polymer materials is a key requirement to improve the spatial resolution and performance of flexible devices. Here, we report the fabrication of metallic nanostructures down to 20 and 50 nm in size on polymer materials such as polyimide, parylene, SU-8, and PDMS substrates without any resist processing using stencil lithography. Metallic nanodot array analysis of their localized surface plasmon spectra is included. We demonstrate plasmon resonance detection of biotin and streptavidin using a PDMS flexible film with gold nanodots. We also demonstrate the fabrication of metallic nanowires on polyimide substrates with their electrical characteristics showing an ohmic behavior. These results demonstrate high-resolution nanopatterning and device nanofabrication capability of stencil lithography on polymer and flexible substrates.

Analysis of the blurring in stencil lithography.

A quantitative analysis of blurring and its dependence on the stencil-substrate gap and the deposition parameters in stencil lithography, a high resolution shadow mask technique, is presented. The blurring is manifested in two ways: first, the structure directly deposited on the substrate is larger than the stencil aperture due to geometrical factors, and second, a halo of material is formed surrounding the deposited structure, presumably due to surface diffusion. The blurring is studied as a function of the gap using dedicated stencils that allow a controlled variation of the gap. Our results show a linear relationship between the gap and the blurring of the directly deposited structure. In our configuration, with a material source of approximately 5 mm and a source-substrate distance of 1 m, we find that a gap size of approximately 10 microm enlarges the directly deposited structures by approximately 50 nm. The measured halo varies from 0.2 to 3 microm in width depending on the gap, the stencil aperture size and other deposition parameters. We also show that the blurring can be reduced by decreasing the nominal deposition thickness, the deposition rate and the substrate temperature.

High Throughput Nanofabrication of Silicon Nanowire and Carbon Nanotube Tips on AFM Probes by Stencil-Deposited Catalysts

A new and versatile technique for the wafer scale nanofabrication of silicon nanowire (SiNW) and multiwalled carbon nanotube (MWNT) tips on atomic force microscope (AFM) probes is presented. Catalyst material for the SiNW and MWNT growth was deposited on prefabricated AFM probes using aligned wafer scale nanostencil lithography. Individual vertical SiNWs were grown epitaxially by a catalytic vapor-liquid-solid (VLS) process and MWNTs were grown by a plasma-enhanced chemical vapor (PECVD) process on the AFM probes. The AFM probes were tested for imaging micrometers-deep trenches, where they demonstrated a significantly better performance than commercial high aspect ratio tips. Our method demonstrates a reliable and cost-efficient route toward wafer scale manufacturing of SiNW and MWNT AFM probes.

Reliable and Improved Nanoscale Stencil Lithography by Membrane Stabilization, Blurring, and Clogging Corrections

The reliable and reproducible fabrication of nanostructures by stencil lithography (SL) faces three main challenges: the stability of the thin stencil membranes with nanoapertures, the blurring of the deposited structures, and the clogging of the nanoapertures in the stencil membranes. This study reports on these three important issues and presents corresponding solutions for the patterning of nanostructures by SL. To increase the stiffness and the stability of the membranes, we have used a hexagonal array of corrugations to globally reinforce the stencil membranes. To correct the blurring, we have used a corrective etching that improves the definition of Al nanostructures. Using this corrective etching, we have fabricated poly-Si nanowires. Finally, we have used metal wet etching to remove the material accumulated on the membranes as a remedy to the clogging.

Sputtering of (001)AlN thin films: Control of polarity by a seed layer

The authors report on the ability to control the polarity of sputter deposited AlN(001) thin films using seed layers. Reactive sputter deposition leads to N-polarity on any substrate hitherto applied, i.e., Si(111), sapphire, SiO2, and polycrystalline metals such as Pt(111), Mo(110), and W(110). A site-controlled polarity allows for an efficient excitation of shear modes of surface, bulk, and Lamb waves by interdigitated electrodes. The authors were able to introduce the Al-polarity through a metal-organic chemical-vapor deposition seed layer. By subsequently patterning the substrate surface, it was possible to define the desired film polarity of sputter deposited AlN film. Polarities were determined by selective etching with KOH solutions and by piezoresponse force microscopy.

Dynamic stencil lithography on full wafer scale

In this paper, the authors present a breakthrough extension of the stencil lithography tool and method. In the standard stencil lithography static mode, material is deposited through apertures in a membrane (stencil) on a substrate which is clamped to the stencil. In the novel dynamic mode, the stencil is repositioned with respect to the substrate inside the vacuum chamber and its motion is synchronized with the material deposition. This can be done either in a step-and-repeat or in a continuous mode. The authors present the first results proving the accurate x-y-z in situ positioning and movement of our stages during and in between patterning.

Stenciled conducting bismuth nanowiresa)

Stencil lithography is used here for the fabrication of bismuth nanowires using thermal evaporation. This technique provides good electrical contact resistance by having the nanowire structure and the contact pads deposited at the same time. It has also the advantage of modulating nanowires’ height as a function of their width. As the evaporated material deposits on the stencil mask, the apertures shrink in size until they are fully clogged and no more material can pass through. Thus, the authors obtain variable-height (from 27 to 95 nm) nanowires in the same evaporation. Upon their morphological (scanning electron microscopy and atomic force microscopy) and electrical characterizations, the authors obtain their resistivity, which is independent of the nanowire size and is the lowest reported for physical vapor deposition of Bi nanowires (1.2×10−3 Ω cm), only an order of magnitude higher than that of bulk bismuth.

Stencil-Nanopatterned Back Reflectors for Thin-Film Amorphous Silicon n-i-p Solar Cells

We fabricated amorphous silicon n-i-p solar cells with two types of nanopatterned back reflectors using stencil lithography. One reflector type has a plasmonic grating that is embedded in the ZnO layer; the other one has a metallic grating patterned on top of the Ag layer. From comparing the short-circuit current densities of the two device types, we conclude that light trapping through grating coupling is more efficient than coupling of light through the excitation of localized surface plasmons. The back reflectors were patterned with dot arrays by evaporation of Ag through millimeter-size stencil membranes. The stencils themselves were patterned by wafer-scale nanosphere lithography. The dot arrays have a periodicity of 428 nm and efficiently scatter light in the near-infrared wavelength range. Both back reflectors types lead to the same morphology for the silicon films. This allows us a fair comparison of the two light coupling mechanisms. We found a 14% and 19% short-circuit current density enhancement for the plasmonic and for the metallic grating, respectively. The external quantum efficiency gains between 550 and 650 nm show similar guided modes resonances for both device types, but the excitation is stronger for the device with the metallic grating.

Quick and Clean: Stencil Lithography for Wafer-Scale Fabrication of Superconducting Tunnel Junctions

This paper presents a resist-less process for parallel fabrication of sub-micrometer Al-AlOx-Al superconducting tunnel junctions. A custom stencil is fabricated containing 200 nm low-stress SiN membranes with micro-apertures. The stencil is aligned and clamped with a 1 mum accuracy to a substrate wafer containing Ti-Au contact electrodes. The junctions are fabricated by evaporating Al from two different angles, with an intermediate in-situ oxidation step. Measurements of the devices down to 0.3 K show stencil lithography is a good candidate for parallel, resist-less patterning of sub-micrometer area tunnel junctions. Challenges are addressed and further developments are proposed.