Abstract
Quartz Tuning Fork (QTF) based sensors are used for Scanning Probe Microscopes (SPM), in particular for near-field scanning optical microscopy. Highly sharp Tungsten (W) tips with larger cone angles and less tip diameter are critical for SPM instead of platinum and iridium (Pt/Ir) tips due to their high-quality factor, conductivity, mechanical stability, durability and production at low cost. Tungsten is chosen for its ease of electrochemical etching, yielding high-aspect ratio, sharp tips with tens of nanometer end diameters, while using simple etching circuits and basic electrolyte chemistry. Moreover, the resolution of the SPM images is observed to be associated with the cone angle of the SPM tip, therefore Atomic-Resolution Imaging is obtained with greater cone angles. Here, the goal is to chemically etch W to the smallest possible tip apex diameters. Tips with greater cone angles are produced by the custom etching procedures, which have proved superior in producing high quality tips. Though various methods are developed for the electrochemical etching of W wire, with a range of applications from scanning tunneling microscopy (SPM) to electron sources of scanning electron microscopes, but the basic chemical etching methods need to be optimized for reproducibility, controlling cone angle and tip sharpness that causes problems for the end users. In this research work, comprehensive experiments are carried out for the production of tips from 0.4 mm tungsten wire by three different electrochemical etching techniques, that is, Alternating Current (AC) etching, Meniscus etching and Direct Current (DC) etching. Consequently, sharp and high cone angle tips are obtained with required properties where the results of the W etching are analyzed, with optical microscope, and then with field emission scanning electron microscopy (FE-SEM). Similarly, effects of varying applied voltages and concentration of NaOH solution with comparison among the produced tips are investigated by measuring their cone angle and tip diameter. Moreover, oxidation and impurities, that is, removal of contamination and etching parameters are also studied in this research work. A method has been tested to minimize the oxidation on the surface and the tips were characterized with scanning electron microscope (SEM).
Highlights
Scanning Probe Microscopy (SPM) is a branch of imaging technique that scans the surface with the varying sharp probe whose diameter vary from size of the atom up to 10 nm [1]
The latest attempts have come in the form of a basic electrochemical “drop-off” process to optimize tip manufacturing [21]
Some of the important experimental results for 0.4 mm tips showing tip diameter, etching time and cone angle in 2 M and 4 M NaOH solution on Alternating Current (AC), Meniscus and Direct Current (DC) Etching, which we explored from our experimental work, are given in the subsections
Summary
Scanning Probe Microscopy (SPM) is a branch of imaging technique that scans the surface with the varying sharp probe whose diameter vary from size of the atom up to 10 nm [1]. The custom etching procedure allows one to create larger cone angles and proved superior in reliably producing high-quality tips. This shows an ideal atomically sharp yet stable tip having a cone angle of about. In the electrochemical process, tungsten wire is connected with the positive voltage through which etching takes place within the meniscus on the wire surface and below the marginal air/electrolyte interface [9]. A necking effect is found in the meniscus where the etching rate is increased At some level, this portion of the wire becomes so thin that its lower end weight cannot be sustained due to less tensile strength, so the latter drops off and a sharp tip is remained. (OM) analysis confirmed the high success rate of the sharp tips [13]
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