Making the connection from the macroscopic realm of cables and measurement devices to the nanometer scale of individual molecules requires a creative mixture of micromachining techniques and understanding of the growth processes at atomic scales. In order to make electrical measurements at the single molecule level a stencil mask based on a silicon membrane has been developed to deposit metals onto atomically flat insulating substrates in ultra high vacuum (UHV). Stencil deposition has the advantages of being a resist free deposition process, and compatible with UHV-clean substrates, such as vacuum cleaved KBr. This is particularly critical for molecular electronics measurements where a detailed knowledge of both the contact geometry and chemical nature are crucial for theoretical agreement.
The stencil masks are created using standard micromachining techniques at the McGill Nanotools facility. Openings as small as 50 nm were created in a region of membrane 50-100 nm thick using the focused ion beam at the University of Sherbrooke. This aspect ratio was chosen to minimize clogging and shadowing effects found with openings of this size in a region 500 nm thick due to the high aspect ratio. Masks are now being used to deposit 60-150 um long tantalum and gold nanowires on KBr without removing the sample from vacuum. In order to make the external contacts to these structures, a secondary mask possessing openings with a gap larger than 100 um in the middle has been fabricated The contacts are deposited with this mask at a 30-40 degree angle to the substrate, allowing the metal films to contact both the freshly cleaved crystal face as well as the side mounted metal contacts to the sample holder. Using two sets of masks allows a separation of length scales, where nanometer scale openings are present solely in the membrane but the interface to the macroscopic is accessed by the mm contact mask.
To further allow the positioning of molecules near electrodes, nanometer size pits have been created in the surface using electron irradiation of a heated substrate. We will also discuss the use of these pits as a template to control the growth of metals and molecules on nanometer length scales.
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