Field ion microscope evaluation of tungsten nanotip shape using He and Ne imaging gases (original) (raw)
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Physical Review B, 2005
Detailed knowledge of the tip apex structure is necessary for quantitative comparison between theory-based simulations and experimental observations of tip-substrate interactions in scanning probe microscopy ͑SPM͒. Here, we discuss field ion microscopy ͑FIM͒ techniques to characterize and atomically define SPM tungsten tips. The tip radius can be estimated from field emission data, while FIM imaging allows the full atomic characterization of the tip apex. We find that when FIM is applied to tips with a radius of a few nanometers ͑as is desirable for high-resolution atomic force microscopy imaging͒, limitations not apparent with less sharp tips arise; successful resolution of these limitations will extend the utility of FIM. Field evaporation can be used to atomically engineer the apex into a desired atomic configuration. Starting from a W͑111͒ wire, a tip terminating in three atoms can reproducibly be fabricated; due to its geometry and stability, this apex configuration is well suited for application as an atomically defined electrical contact in SPM experiments aimed at understanding contact mechanics at the atomic scale.
Fabrication of [001]-oriented tungsten tips for high resolution scanning tunneling microscopy
Scientific reports, 2014
The structure of the [001]-oriented single crystalline tungsten probes sharpened in ultra-high vacuum using electron beam heating and ion sputtering has been studied using scanning and transmission electron microscopy. The electron microscopy data prove reproducible fabrication of the single-apex tips with nanoscale pyramids grained by the {011} planes at the apexes. These sharp, [001]-oriented tungsten tips have been successfully utilized in high resolution scanning tunneling microscopy imaging of HOPG(0001), SiC(001) and graphene/SiC(001) surfaces. The electron microscopy characterization performed before and after the high resolution STM experiments provides direct correlation between the tip structure and picoscale spatial resolution achieved in the experiments.
MRS Proceedings, 2013
ABSTRACTSharper 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.
Materials Science and Engineering: A, 1999
In a probe-hole field emission microscopy experiment, doses of nickel have been in situ deposited onto a thermally cleaned tungsten tip held at 300 K in clean ultra high vacuum conditions. The deposited doses of nickel were annealed successively at temperatures ranging from 340 to 2215 K to cause diffusion of the deposited amount on various tip surfaces. The changes in the work function (DF) and in the pre-exponential function (ln B) have been calculated from ten point Fowler-Nordheim (F-N) plots for the total tip surface, the W(121) and the W(111) surfaces at each successive stage of annealing. These variations are discussed in terms of diffusion of nickel atoms on the tungsten tip surface.
Reproducible Electrochemical Etching of Tungsten Probe Tips
Nano Letters, 2002
An electrochemical procedure in KOH electrolyte has been developed to reproducibly produce ∼5 nm radius tungsten probe tips. It has been found that a spurious electrochemical etching process, driven by the natural potential difference between an Ir electrode and the W tip, causes rapid tip blunting at the end of the electrochemical etching period. By electrically reversing this potential difference within 500 ns following tip separation, the blunting process is eliminated yielding sharp tips with varying cone angles.
e-Journal of Surface Science and Nanotechnology, 2007
We present a method to prepare tungsten tips for use in multi-tip scanning tunneling microscopes. The motivation behind the development comes from a requirement to make very long and conical-shape tips with controlling the cone angle. The method is based on a combination of a "drop-off" method and dynamic electrochemical etching, in which the tip is continuously and slowly drawn up from the electrolyte during etching. Its reproducibility was confirmed by scanning electron microscopy. Comparison in tip shape between the dynamic and static methods was shown.
Reactive-Ion-Etching of 100nm Linewidth Tungsten Features Using SF6:H2 and a Cr-Liftoff Mask
MRS Proceedings, 1993
Reactive ion etching of features down to 100 nm in linewidth in tungsten has been studied using an SF6 based chemistry. The studies were carried out in a PlasmaTherm 500 etcher operated at low pressure (2 mTorr) and power (100 mWatts/cm2). Key processing parameters have been identified to achieve the resolution and aspect ratio required for high contrast x-ray masks. The critical parameters include sample temperature, gas dilution and end point detection. However, even with all of these parameters optimized, additional sidewall passivation is required to obtain the necessary 6.5:1 aspect ratio. A novel method of achieving such passivation based on an intermittent etching process is described.
Preparation and characterization of electrochemically etched W tips for STM
Measurement Science and Technology, 1999
Abstract. We have investigated methods for cleaning dc-etched polycrystalline tungsten tips for scanning tunnelling microscopy (STM). The cleaning methods include Ar-ion sputtering, heating, chemical treatments and Ne-ion self-sputtering. We correlate transmission electron microscopy images of the tip, field-emission data from the tip and images of a clean Cu (111) surface to find an optimum procedure for STM imaging. Clean and sharp tips are made by sputtering, combined with careful heating by electron bombardment. We found that ...
Thermally Driven Self-Limiting Atomic Layer Etching of Metallic Tungsten Using WF6 and O2
ACS Applied Materials & Interfaces, 2018
The semiconductor industry faces a tremendous challenge in the development of a transistor device with sub-10 nm complex features. Self-limiting atomic layer etching (ALE) is essential for enabling the manufacturing of complex transistor structures. In this study, we demonstrated a thermally driven ALE process for tungsten (W) using sequential exposures of O 2 and WF 6. Based on the insight gained from the previous study on TiO 2 thermal ALE, we proposed that etching of W could proceed in two sequential reaction steps at 300°C: (1) oxidation of metallic tungsten using O 2 or O 3 to form WO 3 (s) and (2) formation and removal of volatile WO 2 F 2 (g) during the reaction between WO 3 (s) and WF 6 (g). The O 2 /WF 6 etch process was experimentally studied using a quartz crystal microbalance (QCM). We find that both the O 2 and WF 6 ALE half reactions are self-limiting, with an estimated steady-state etch rate of ∼6.3 Å/cycle at 300°C. We also find that etching of W proceeds readily at 300°C, but not at temperatures lower than 275°C. Thermodynamic modeling reveals that the observed temperature dependence is likely due to the limited volatility of WO 2 F 2. The use of WF 6 with O 3 in place of O 2 also allows W etching, where the stronger oxidant leads to a larger mass removal rate per cycle. However, we find O 2 to be more controllable for precise metal removal per cycle. In addition, etched W films were examined with ex situ analytical tools. Using spectroscopic ellipsometry (SE) and scanning electron microscopy (SEM), we confirm etching of tungsten film on silicon substrates. Surface analysis by X-ray photoelectron spectroscopy (XPS) revealed a minimal fluorine content on the W film after partial etching and on the silicon surface after full etching. This suggests that W ALE does not significantly alter the chemical composition of W films. This work serves to increase the understanding of ALE reactions and expand the base of available ALE processes for advanced material processing.