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

The overall goal of this procedure is to fabricate field emission points or FEPs by electrochemically etching tungsten rods, and to then use these FEPs to produce an electron beam by operating them in field emission mode. The FEP produced using this method can provide a convenient electron source that can be used, for example, in electron impact ionization to produce ions for applications such as mass spectrometry. The main advantage of this technique is that it is reliable and relatively straightforward to implement with modest equipment.

Demonstrating parts of the procedure will be Eranjan and Nadeesha, graduate students from my lab. Begin with preparing lengths of 0.5 millimeter diameter tungsten rod. Use a pair of wire clippers to cut the rod into 25 milimeter lengths.

About a dozen small rods can be made from one 12 inch rod. Then, using sonication, clean the rods in acetone for 15 minutes. After the acetone bath, rinse the rods with deionized water for a few seconds.

Next, prepare the sodium hydroxide solution using all safety precautions. The process is highly exothermic and can release heat that may cause burns or ignite flammables, and could cause the solution to splash out of the container. Once made, transfer the sodium hydroxide solution to a wash bottle and then use it to fill a separatory funnel with a 1.5 molar sodium hydroxide solution.

Set up the apparatus with the copper FEP catcher block placed inside a wide-based, glass beaker, and position a copper cathode above it spaced off with insulating standoffs. Next, set the current on the DC power supply to the desired value, typically 200 milliamps. Then, place a tungsten rod in the holder and connect the positive terminal of the power supply to the 4-40 holding screw.

Now, insert the tungsten rod through the hole in the copper cathode. Extend the rod approximately 12 milimeters beyond the hole. Then, connect the negative terminal of the power supply to the copper cathode.

Then, adjust the drip rate from the separatory funnel to match the drip rate through the hole. Try for about one drip every three seconds. Wait for the reservoir in the copper cathode to fill before proceeding.

Next, turn on the DC power supply to start the etching. The automatic shut-off circuit will cut the current once the rod etches through. To inspect the FEPs, carefully remove the bottom FEP from the catcher block using a pair of pliers or tweezers.

Remove the upper FEP from the holder by loosening the 4-40 screw and gently pulling it out. Then, briefly rinse the etched rod with acetone, and then deionized water. Store the FEPs in a desiccator.

First, select and examine an FEP under a microscope. The tip should taper to a fine point. FEPs that are bent or have an irregular structure should be discarded.

The field emission test set up includes a vacuum chamber, electrical feedthroughs, pumps, a high voltage power supply, a picoammeter, and a Faraday cup, or a simple conducting collection plate on a linear feedthrough. The set up also requires an FEP holder. The Faraday cup should be placed about two centimeters from the tip of the FEP.

Next, connect the high voltage supply to the SHV feedthrough attached to the FEP holder. Then, connect the picoammeter to the BNC feedthrough that the Faraday cup is attached to. Now, pump down the set up to a pressure of one ten-thousandth of a millibar or lower.

With the pressure dropped, gradually increase the bias on the FEP and monitor the electron beam current on the Faraday cup. When a current registers, the field emission has begun. Now, increase the high voltage current in incremental steps, such as 50 volts.

At each step, record the average electron beam current. Do not raise the current above one microamp here. The test is complete when the voltage has reached its maximum value with the current at a few hundred nanoamperes.

After the first FEP test, use conditioning to clean the tip. To do this, operate the testing set up in field emission mode at about five nanoamperes for an hour. Later, repeat the current versus voltage scan.

The voltage required to maintain a constant etching current increases slightly as the tungsten rod is etched away. Then, the current drops to almost zero when the rod etches all the way through. The time required to etch through the rod depends on the current's strength and the molarity of the solution.

The etching rate increases linearly with the current. As expected from Ohm's Law, the etching voltage is linearly proportional to the current and the voltage required to deliver the constant current decreases with increasing molarity due to the increased conductivity of the solution. When operated in field emission mode, electrons from the FEP strike the Farady cup and the current is recorded.

After the conditioning process, the FEP fired at a lower voltage indicating that a lower bias potential on the FEP could remove electrons and that the effective radius of the FEP had decreased. After watching this video, you should have a good understanding of how to fabricate FEPs via electrochemical etching, how to use them to produce and electron beam, and how to characterize the FEP.