The Manufacture Of Electrochemical Corrosion Method STM Tungsten Tip Preparation Instrument

STM is one of the three atomic-level resolution electron microscopies in theworld today, its horizontal resolution can achieve0.1nm, and vertical resolution0.01nm. It is used to aim at the tested samples atoms with a tip probe of atomic levelto scan point to point. Therefore, the consumable part tip probe has a critical influenceon its image quality. This tip is usually made of tungsten or platinum. The ideal needleprobe has only one atom on its tip, but the usual has a small cluster of atoms.Therefore, this kind of probe is very difficult to produce.The usual methods of making a tip are manual method, grinding, ion milling, thedirectional blasting method, evaporation method and electrochemical etching method.The experimental principle of the electrochemical etching method is simple, thedevice is easy to prepare and easy to control and with a strong repeatability, whichwill helps the manufactured instruments in automatic operation.The principle of electrochemical corrosion is to use tungsten in NaOH solution tocorrode under energized conditions. The operation method is to vertically inserttungsten into the electrolyte into a certain depth and corrosion will take place in thepart that tungsten contacts the liquid. As the corrosion proceeds, the tungsten belowthe liquid will be thinner and thinner, finally, the part under the liquid will be pulledoff due to gravity, and therefore, the single-atom tip is formed. Because of theexistence of the electrolysis voltage, there will be a current flowing through theelectrolytic cell poles. With the tungsten gradually tapering, this current will graduallyreduce evenly. And when the tungsten pulled off (that is when the tip forms), thiscurrent will suddenly change from around20mA to less than1mA. Then if we canquickly cut off the electrolysis voltage, the single-atom tip just formed will beretained, if not, the tip will be corroded to blunting by the residual current. Themethod described in this paper is to use sampling resistor to convert this current signalinto a voltage signal, to compare it with the reference voltage by voltage comparatorLM393, and with the logic LM393’s output, to control an IRF9640FET to cut thepower off. The LM393conversion output time is less than300ns, while the IRF9640turn-off time is no more than75ns. Thus, the entire power-off time will not exceed375ns. Now there are articles about the making of electrochemical etching of STMtungsten tip preparation instrument published at home and abroad, e.g. the systemdesigned by Niu Jian-long, Wu Xue, Li Feng,Zuo Zhen-tao and others in NankaiUniversity, College of Physics Science. But set system does not have the time controlunit and voltage control unit and therefore, can not change the experimentalparameters easily; and there is no protective isolation between current loop andcontrol loop, which we have to say is a hidden danger.My design consists of four main parts: mechanical bracket, power supply, controlcircuit and electrolytic cell. The purpose of mechanical stent is that it not only enablesthe tungsten to insert into the electrolytic cell vertically, but also adjust the tungstendepth immersed under the liquid surface; I adopted the dual power supply design inthe power part, and to isolate the low voltage, low current power supply of the controlcircuit and the high voltage, high current of the electrolytic cell with solid-state relayin order to protect the control circuit.In addition, in the electrolysis process, if the electrolysis voltage is high, the tipaspect is relatively small; if the electrolysis voltage is relatively low, the tip aspect isbig. However, if the aspect ratio is too big, the vibration would be relatively big,which is not conducive to testing; if we blindly pursuit small aspect ratio to increasecell voltage, then in the short time of turning off the off-switch, the needle justgenerated would be corroded more by the residual voltage. Therefore, I used themethod of secondary corrosion. First, to conduct electrolysis with larger cell voltagefor a period of time to obtain the desired aspect ratio; then to corrode with low voltageuntil it is completed, then an ideal tip is formed. Therefore, in my design, I set atiming unit and control the high voltage and the low voltage with two outputs. And thelength of time is get in accordance with the experiment.With the tip made by this method, we scaned the fine structure of the STM imagewith ranges500*500nm.