Very Sharp Platinum Tips by Electrochemical Etching

Very sharp platinum tips Martin Kupper 08309652 State of the art2.1 Electrochemical etchingThe electrochemical etching of very sharp tips is an established procedure for scanningtunneling microscopy [5]. It is possible to make tips out of different metals, butbasically the process is the same every time. A wire is dipped in an electrolyte, anda voltage is applied between the wire and a counter electrode [11]. The voltage - andwhether it is AC or DC - depends on the metal and electrolyte used and has to bedetermined by experiment. Then the oxide layer, whitch appears during the etching,has to be removed by another electrochemical step. This is usually done with a small,negative DC voltage, where the tip is the ground. It is possible to do a further stepbetween the etching and the oxide removal, called micropolishing. This is actually asecond etching with an other electrolyte or an acid, and usually a lower voltage. Thisation of cobalt and nickel 2565reduces the diameter of the head of the tip again and produces a smooth surface [2].3560 I. H. Musselman and P. E. Russell: Platinumliridium tips(a) A nickel tip etched with a (b) A Pt/Ir tip etched with CaCl 2 /H 2 O/HCl.KCl solution. [6][12]FIG. 4. SEM images of Ni tips a lower tip; b upper tip; Magnifications:FIG. 5. SEM images of the same Ptllr tip shown in Fig. 3 after micropolishing.a Radius 11000, of curvature b < SOO 300, A. Cone b half angle 800, of 8". b 3000.a 200, a 2000,Figure 2.1: Different kinds of tips.The current I flowingtips, however,duringPt peaksthewereetchingnot observedprocesssuggesting thatis shown inthe surface C layer was in excess of the escape depth ofthe PtFig. 2.Auger electron.The relative thicknesses of the surface contamination layerscan for the becut identified versus the etched inPt/Ir thetips electrochemical were determined etch-Two regimesby monitoring the intensity of the C Auger signal as a functionof sputtering time (3 keY, 25 mA emission current,ing of Me: Point 1→2 and Point 2→3. This behavior is dueto a decreased 4 charge X 10 - 5 Pa Ar). carriers For the cut concentration, tip, the C signal reached backgroundafter 45 s of sputtering, whereas the sputtering timeas a consequenceof the precipitation varied from 30 s to 3 of min the for the Me three etched complexes. Pt/lr tips. Thislarge variation in C thickness among the etched tips wasWhen the electrochemical reflected in their STM imaging etching unpredictability. starts Out Point of 20 1, hydratedchemicaletched species4Pt/Ir tips,formsapproximatelyaround20% (thosethepresumablywire, spreadingwith minimal surface C) successfully imaged sputter-deposflame.Unfortunatelyat least 30 min per tiflame temperature, iC without altering thzation. The extent oprecautions were takthe etching setup. Calevel observed for thminated from the etcreadily dissolves in wvapor pressure thanby boiling and subsethe tips and to prepaing step, the contactthe thin film of etchabox under a N2 atmAuger survey speCO 2was removed frfavorably with that awerc observed for allA special method for etching tips on the is tip reverse surface etching composition. [3]. Carbon Here the was wire observed is not in all dipped offor the cut and etchein the etch solution from above, the but survey instead spectra.Anode: Mes +4Cl − from For underneath.the cut→ MeCl 2 tip, Pt peaks This4 +2e − were is carried also observeddefining the upper thickness of the carbon . contamin­outof the Pt Auger electAr + ion beam to spwith a layer of high-density electrically insulating liquid under the electrolyte. Theation layer to be between 20 and 50 A. For two of the etchedwhereas a range of 2wire is in this liquid and protudes a little in the electrolyte. This way the tips get anextremely slim shape (see Fig.2.2) [4]. However, because of the setup, doing this kindc. Image acquisitiof etching is a lot more complex then the etching from above.Two sets of nine Petching procedure wwere tested by imagspikes or streaks weusing tips from eithacquired using 80%pearance with rounhigh ranging from 1unusual features obsremaining 20% of thdesirable tip shape rface chemistry.