Sharp Needles: Fabrication of Tungsten Nanotips by AC Electrochemical Etching and Laser Enhanced Etching for Nanoprobing
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Sharp Needles: Fabrication of Tungsten Nanotips by AC Electrochemical Etching and Laser Enhanced Etching for Nanoprobing on Interconnects of Advanced Technology Nodes. M. K. Dawood1, Z.H. Mai1, T. H. Ng1, H. Tan1, P.K. Tan1 and J. Lam1 1 GLOBALFOUNDIRES Singapore Pte. Ltd, 60 Woodlands Industrial Park D Street 2 Singapore 738406 ABSTRACT Sharper nanotips are required for application in nanoprobing systems due to a shrinking contact size with each new transistor technology node. We describe a two-step etching process to fabricate W nanotips with controllable tip dimensions. The first process is an optimized AC electrochemical etching in KOH to fabricate nanotips with a radius of curvature (ROC) down to 90 nm. This was followed by a secondary nanotip sharpening process by laser irradiation in KOH. High aspect ratio nanotips with ROC close to 20 nm were obtained. Finally we demonstrate the application of the fabricated nanotips for nanoprobing on advanced technology SRAM devices. INTRODUCTION Technology scaling results in ever shrinking device and contact size and increases the challenges of failure analysis (FA). Nanoprobing is used for electrical failure analysis (FA) for fault isolation [1]. There are generally two types of nanoprobing systems (i) scanning electron microscope (SEM) based nanoprobers and atomic force based nanoprobers [2-3]. A typical SEM based nanoprobing system comprises of an SEM integrated with up to 8 nano-manipulators with tungsten nanotips connected to a parametric analyzer. Such a system is capable of direct transistor characterization at the contact level. As contact size decreases, nanotips with sufficiently small radius of curvature (ROC) are required to serve as probes in a nanoprobing system. Tungsten (W) is desired due to its material hardness, conductivity and slow oxidation in air. The sharper the nanotip apex, the smaller is the ROC. Various tip sharpening methods have been developed such as field emission deposition, oxidation in oxygen environment or high electrical field in air [4]. These methods require complicated and expensive set-ups. There is therefore a need for low cost methods of fabricating sharp nanotips. In this work, a two-step process was used to fabricate W nanotips with controllable tip dimensions. First, an optimized AC electrochemical etching in KOH process was used to fabricate nanotips with a radius of curvature down to 90 nm [5]. Second, laser irradiation in KOH [6] was used to further sharpen the nanotips. High aspect ratio nanotips with ROC close to 20 nm were obtained. Finally, the application of these fabricated nanotips for nanoprobing on advanced technology nodes is demonstrated.
DISCUSSION Experimental Setup Polycrystalline W wires of ~0.25 mm thickness and 99.9 % purity were selected in order to fit into the tip holder in the nanomanipulator. The wires were cut into lengths of ~30 mm. AC-Electrochemical Etching The W nanotips are initially formed by a downward AC electrochemical etching in dilute KOH (0.5 M) solution with an additional integrated lift
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