We acknowledge the services of Stanley Flegler, Carol Flegler, and Abigail Tirrell at the MSU Center for Advanced Microscopy. We thank Ray Clark and Mark Wilson for technical assistance with the set-up of the electrochemical etching apparatus. Earlier contributions from Anne Benjamin, Georg Bollen, Rafael Ferrer, David Lincoln, Stefan Schwarz and Adrian Valverde, and technical assistance from John Yurkon are also acknowledged. This work was partially supported by the National Science Foundation contract no. PHY-1102511 and PHY-1307233, Michigan State University and the Facility for Rare Isotope Beams, and Central Michigan University.

1. Electrochemical Etching
<ol>
	<li>Experimental set-up
	<ol>
		<li>Apparatus 
		Note: The electrochemically etching set-up requires a standard 0 - 30 V direct current (DC) benchtop power supply and appropriate cables, a separatory funnel, a wide base glass beaker, and standard rod and utility clamp with electrically insulating grips. Small screws, insulated stand-offs, and alligator clips will also be required. Additional items, described below and shown in the picture of the et...

<ol>
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B. 9, 601-608 (1990).</li><li>Shi, W., Redshaw, M., Myers, E. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+masses+of+32,33S,+84,86Kr,+and+129,132Xe+with+uncertainties+0.1+ppb.">Atomic masses of 32,33S, 84,86Kr, and 129,132Xe with uncertainties 0.1 ppb.</a> Phys. Rev. A. 72, 022510 (2005).</li><li>Van Dyck, R. S., Zafonte, S. L., Van Liew, S., Pinegar, D. B., Schwinberg, D. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ultraprecise+Atomic+Mass+Measurement+of+the+&#945;+particle+and+4He.">Ultraprecise Atomic Mass Measurement of the &#945; particle and 4He.</a> Phys. Rev. Lett. 92, 220802 (2004).</li><li>Hobara, R., Yoshimoto, S., Hasegawa, S., Sakamoto, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamic+electrochemical-etching+technique+for+tungsten+tips+suitable+for+multi-tip+scanning+tunneling+microscopes.">Dynamic electrochemical-etching technique for tungsten tips suitable for multi-tip scanning tunneling microscopes.</a> e-J. Surf. Sci. Nanotechnol. 5, 94-98 (2007).</li><li>Klein, M., Schwitzgebel, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+improved+lamellae+drop-off+technique+for+sharp+tip+preparation+in+scanning+tunneling+microscopy.">An improved lamellae drop-off technique for sharp tip preparation in scanning tunneling microscopy.</a> Rev. Sci. Instrum. 68, 3099-3103 (1997).</li><li>Kerfriden, S., Nahl&#233;, A. H., Campbell, S. A., Walsh, F. C., Smith, J. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+electrochemical+etching+of+tungsten+STM+tips.">The electrochemical etching of tungsten STM tips.</a> Electrochim. Acta. 43, 1939-1944 (1998).</li><li>Lemke, H., G&#246;ddenhenrich, T., Bochem, H. P., Hartmann, U., Heiden, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improved+microtips+for+scanning+probe+microscopy.">Improved microtips for scanning probe microscopy.</a> Rev. Sci. Instrum. 61, 2538-2538 (1990).</li><li>Song, J. P., Pryds, N. H., Glejb&#248;l, K., M&#248;rch, K. A., Th&#246;l&#233;n, A. R., Christensen, L. 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P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+electrochemical+etching+of+tips+for+scanning+tunneling+microscopy.">On the electrochemical etching of tips for scanning tunneling microscopy.</a> J. Vac. Sci. Technol. A. 8, 3570 (1990).</li><li>Ekvall, I., Wahlstr&#246;m, E., Claesson, D., Olin, H., Olsson, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Preparation+and+characterization+of+electrochemically+etched+W+tips+for+STM.">Preparation and characterization of electrochemically etched W tips for STM.</a> Meas. Sci. Technol. 10, 11-18 (1999).</li><li>Schiller, C., Koomans, A. A., van Rooy, T. L., Sch&#246;nenberger, C., Elswijk, H. 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We have described straightforward procedures to electrochemically etch sharp field emission points (FEPs) in a NaOH solution, and to test the FEPs by operating them in field emission mode. The etching procedure described is a variation of existing techniques-the lamella drop-off technique7,8 and the floating layer technique9,10. However, we found it to be more convenient and reliable to implement than the aforementioned methods.
Before starting the etching procedure, to m...

Sharp tips or points have long been used in microscopy applications, such as the field ion microscope (FIM)1 and the scanning tunneling microscope (STM)2, and a range of techniques for producing sharp tips of various materials have been developed3. These sharp tips can also be operated as field emission points (FEPs) by applying a high voltage to them, and serve as a convenient electron beam source. One application of such as source is ion production via electron impact ionization (EII). The FEP is particularly advantageous in applications where temperature fluctuations produced by thermal emitters are undesirable. For example, ion production via EII of background gas or vapor in high precision Penning traps4,5.
A simple method for fabricating FEPs is to electrochemically etch tungsten rods in a sodium hydroxide (NaOH) solution. This technique is relatively straightforward to implement with modest equipment and has been shown to be quite reproducible and reliable. A number of methods are described in the literature and improvements to these techniques continue to appear6. Here we describe a method for the electrochemical etching of tungsten tips in a NaOH solution. Our method is a variation of the lamella drop-off technique7,8 and the floating layer technique9,10. Like these two methods it enables the production of two tips from a single etching procedure. A picture of the experimental apparatus for etching the tips is shown in Figure 1.
<img alt="Figure 1" src="/files/ftp_upload/54030/54030fig1.jpg" /> 
Figure 1. Etching apparatus. Photograph of the experimental apparatus used for electrochemical etching of tungsten rods with NaOH solution. <a href="https://www.jove.com/files/ftp_upload/54030/54030fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Electrochemical etching of tungsten in the aqueous NaOH base occurs via a two-stage process. First, intermediate tungsten oxides are formed, and second, these oxides are non-electrochemically dissolved to form the soluble tungstate anion. This process is described, in simplified form, by the two reactions
(1) W + 6OH- &#8594; WO3(S) + 3H2O + 6e-, and
(2)&#160;WO3(S) + 2OH- &#8594; WO42- + H2O.
The etching current and the NaOH solution molarity used affect the time and voltage required to etch through the tungsten rod. Studies of these effects are presented and discussed. More importantly, the etching parameters have an effect on the geometry of the tips and, as such, on their operation in field emission mode. The geometry of the tips we produced were characterized by imaging them with a scanning electron microscope (SEM). These images can be used to estimate, for example, the tip radius. In addition, the tips were operated in field emission mode by applying a negative voltage of typically a few hundred volts to a few kilovolts to them and monitoring the resulting electron emission current. The relationship between field emission current, I, and applied bias voltage, V, can be described by the Fowler-Nordheim equation11
(3) I = AV2 e-Creff/V,
where reff is the effective radius of the tip, A is a constant, and C is the second Fowler-Nordheim constant <img alt="Equation 9" src="/files/ftp_upload/54030/54030eq9.jpg" />, in which b = 6.83 eV-3/2V/nm, <img alt="Equation 11" src="/files/ftp_upload/54030/54030eq11.jpg" /> is the work function of tungsten (<img alt="Equation 11" src="/files/ftp_upload/54030/54030eq11.jpg" /> &#8776; 4.5 eV), k is a factor that depends on geometry (k &#8776; 5), and <img alt="Equation 12" src="/files/ftp_upload/54030/54030eq15.jpg" /> is the Nordheim image correction term (<img alt="Equation 12" src="/files/ftp_upload/54030/54030eq15.jpg" /> &#8776; 1)12. Hence, the effective radius of the tip can be determined by measuring the electron current as a function of bias voltage. Specifically, it can be obtained from the slope of a so-called Fowler-Nordheim (FN) plot of ln(I/V2) vs 1/V.

<table border="0" cellpadding="0" cellspacing="0" width="1230">
	<tbody>
		<tr height="20">
			<td height="20" style="height:20px;width:325px;">Tungsten Rod 0.020&#34; x 12&#34;</td>
			<td style="width:159px;">ESPI Metals</td>
			<td style="width:451px;"><a href="http://www.espimetals.com/index.php/online-catalog/467-Tungsten">http://www.espimetals.com/index.php/online-catalog/467-Tungsten&#160;</a></td>
			<td style="width:297px;">3N8 Purity</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">NaOH salt</td>
			<td>Cole-Parmer</td>
			<td>Item # WU-88404-71</td>
			<td>100 g</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Separatory funnel</td>
			<td>Cole-Parmer</td>
			<td>Item#&#160;WU-34506-03</td>
			<td>250 mL&#160;</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">DC Power supply</td>
			<td>BK Precision</td>
			<td>1672</td>
			<td>Triple Output 0 - 32 V, 0 - 3 A DC Power Supply</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Acetone</td>
			<td>Cole-Parmer</td>
			<td>Item#&#160;WU-88000-68</td>
			<td>500 mL</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Data Acquisition Card</td>
			<td>National Instruments</td>
			<td style="width:451px;">NI PXI-6221</td>
			<td>16 AI, 24 DIO, 2 AO</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Relay</td>
			<td>Magnecraft</td>
			<td>276 XAXH-5D</td>
			<td>7 A, 30 V DC Reed Relay</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">6-way 6&#34; conflat flange cross</td>
			<td>Kurt J Lesker</td>
			<td>C6-0600</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">6&#34; to 2-3/4&#34; conflat zero length reducer flange&#160; (x3)</td>
			<td>Kurt J Lesker</td>
			<td>RF600X275</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">2-3/4&#34; conflat flange SHV feedthrough</td>
			<td>Kurt J Lesker</td>
			<td>IFTSG041033</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">2-3/4&#34; conflat flange BNC feedthrough</td>
			<td>Kurt J Lesker</td>
			<td>IFTBG042033</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">2-3/4&#34; conflat flange linear feedthrough</td>
			<td>MDC</td>
			<td>660006, REF# BLM-275-2</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">6&#34; conflat flange blankoff</td>
			<td>Kurt J Lesker</td>
			<td>F0600X000N</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">6&#34; conflat flange window</td>
			<td>Kurt J Lesker</td>
			<td>VPZL-600</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">HV Power supply</td>
			<td>Keithley Instruments</td>
			<td>Keithley Model #2290-5</td>
			<td>0 - 5 kV DC HV Power Supply</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Picoammeter</td>
			<td>Keithley Instruments</td>
			<td>Keithley Model #6485</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">Faraday Cup</td>
			<td>Beam Imaging Solutions</td>
			<td>Model FC-1 Faraday Cup</td>
			<td></td>
		</tr>
	</tbody>
</table>

electrochemical etching and characterization of sharp field emission points for electron impact ionization

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. The results of studies in which the etching current and the molarity of the NaOH solution used in the etching process were varied are presented. The investigation of the geometry of the tips, by imaging them with a scanning electron microscope, and by operating them in field emission mode is also described. The field emission tips produced are intended to be used as an electron beam source for ion production via electron impact ionization of background gas or vapor in Penning trap mass spectrometry applications.

Study of etching parameters
During the etching process the power supply is operated in constant current mode. The voltage required to maintain this constant current increases slightly as the tungsten rod is etched away (due to the increase in resistance of the rod). The current drops almost to zero when the tip etches all the way through. A small current continues to flow due to the fact that the upper tip is st...

Watch this Scientific Journal Video about Electrochemical Etching and Characterization of Sharp Field Emission Points for Electron Impact Ionization at JoVE.com

Electrochemical Etching and Characterization of Sharp Field Emission Points for Electron Impact Ionization

A method for electrochemically etching field emission tips is presented. Etching parameters are characterized and the operation of the tips in field emission mode is investigated.

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. ...