Fabrication of Tungsten nanoprobes by Electrochemical etching: Role of cathode geometry and their use as electrical probe
LLow cost electrical probe station using etched Tungsten nanoprobes: Roleof cathode geometry
Rakesh K. Prasad a and Dilip K. Singh a* a Department of Physics, Birla Institute of Technology Mesra, Ranchi-835215.*Email: [email protected]
Abstract
Electrical measurement of nano-scale devices and structures requires skills andhardware to make nano-contacts. Such measurements have been difficult for number oflaboratories due to cost of probe station and nano-probes. In the present work, we havedemonstrated possibility of assembling low cost probe station using USB microscope(US $ 30) coupled with in-house developed probe station. We have explored the effectof shape of etching electrodes on the geometry of the microprobes developed. Thevariation in the geometry of copper wire electrode is observed to affect the probe length (0.58 mm to 2.15 mm) and its half cone angle (1.4 ° to 8.8 ˚) . These developed probeswere used to make contact on micro patterned metal films and was used for electricalmeasurement along with semiconductor parameter analyzer. These probes show lowcontact resistance (~ 4 Ω) and follows Ohmic behavior. Such probes can be used forlaboratories involved in teaching and multidisciplinary research activities and AtomicForce Microscopy. Keywords: electrochemical etching, tungsten tip, DC voltage, Low cost probe station. . INTRODUCTION
Advancement in the field of nanofabrication has led to miniaturization of devices tonanometers . Research labs and teaching efforts in the field of electronics and opto-electronicdevices to such small dimensions, require probes for micron or smaller size. Additionally,these factors have limited the access of experts from various domains of science andengineering to explore nanoscale structures for multi-disciplinary applications. Variousresearch groups have attempted to devise methods of fabricating metallic nano-probes usingcost effective techniques to achieve reproducible tip geometry. There are various methods forthe formation of tungsten tip like cutting (1, 2), mechanical pulling (3-11) , grinding (12, 13) ,ion milling (14-18) , ion beam – induced deposition (19) , electrochemical etching (20-35) andelectrochemical machining (36). Recently in 2019 Yamaguchi et al. introduced a new methodcalled flame etching to fabricate tungsten tip. (37)
In 1951, Miller et al. reported aboutpossibility of fabrication of sharp metal tips by electrochemical process (38).
With time therehave been refinements in the methods to get sharp, smooth and long taper tip with perfectlyconical geometry.The driving force for the research in this direction has been the concern aboutreproducibility of probe geometry and their immense application in nano characterizationtools for topography, electrical and optical measurements (34, 39, 40).
Few notable improvedtechniques for electrochemical etching are drop-off methods with direct current (DC) voltage (20, 41) dynamic etching technique (39), reverse chemical etching (24, 42, 43) . Chemicaletching is one of the most effective methods for fabricating various types of nano-probes withdifferent geometry. For the purpose of chemical etching, Sodium hydroxide (NaOH) orPotassium hydroxide (KOH) as electrolytes are used with varying molar concentration in therange 0.1 M -10 M (26, 27, 29, 30, 44).
Tungsten wire was used as an anode during etchingwhile a variety of materials like stainless steel (25), chromium- nickel stainless steel (44), ridium (26) , platinum (28) and tungsten wire (29) have been used as cathode . Althoughdifferent researchers have used cathode of varying geometry ( wire or rod, circular loop, ringand L-shaped etc) (26, 28, 29) there has not been any study on the effect of cathode geometryon the resultant nano-probes formed. There are various parameter which affect the rate ofelectrochemical etching, formation of probe and their geometry like electrolyte concentration,immersion depth of wire, size and position of cathode, applying DC or Alternating current(AC) varying voltages and diameter of tungsten wire (45). Applying AC voltage is usuallymore difficult to control and it affects the smooth formation of probes, in comparison to DCvoltages (46).
The meniscus formed between the wire-electrolyte interface due to capillaryforces directly influences the probe-tip shape (44).
Low DC voltage during etching wassuggested by Ibe et al. to give less oxide film formation as in comparison with the use of highvoltage (47).
Till now, the initial tungsten wire diameter for nano-probe formation has beeninvestigated to vary from 50 µm to 250 µm (28-30, 34, 37, 39) , however a very few haveused higher diameters such as 1000 µm. (35, 44)
Most groups have used drop-off methodwith some modification in the circuit for automatic cut-off in the power supply after fall ofthe tungsten wire into the solution, (25, 35, 44) , even though it is more complicated andexpensive.Nowadays for fine fabrication of nano-probes the complete electrochemical etching issub-divided into a few step processes involving certain parameter changes. Two step etchingprocess involving coarse and fine etching was demonstrated by Olivier L. Guise et,al in 2002,where the DC voltage was changed at each step (26) . In 2012 Yasser khan et al. introduced atwo dynamic electrochemical etching in which simple etching of the probe was followed by asecondary step of cleaning, drying and re-installing the probe but in a smaller immersiondepth than in the first etching (30) . Recently in 2018 Danish Hussain et al. used a trapezoidalpotential for micro needle formation followed by a DC potential for further probe tipormation (34) . Shanli Qin in 2019 also followed a two-step technique and fabricated Nano-probes tips (35) .Cleaning is essential after the fabrication mechanism of nano-probes to avoid thedevelopment of a thin oxide layer of H O, H , O , CO , and other hydrocarbons (48, 49) due todissolution of tungstate anions (W ) in water during the growth (50) . These oxide filmsincrease the thickness of the probes and reduce their performance as a good electrical contactfor electrical measurements (25) . Different techniques are used to reduce or remove the oxidelayer including annealing inside ultra - high vacuum scanning tunneling microscopy chamber (25) , chemical cleaning by Hydrofluoric acid (51) , immersing in deionized water, acetone,ethanol or methanol (26). Sometimes post-cleaning, the tip is dried gently either by cleannitrogen gas or by sputtering (25) .
In our study, we have explained the role of electrodes with different shapes andreproducibility of sharp, smooth and long tapered tip by using different geometrical pattern ofcopper wire that is used as another electrode. Using a two-step method, we etched thetungsten tip by NaOH solution for one hour, followed by further etching in KOH solution tillthe etching process is completed. We have also explored the effect of cathode geometry onthe formation of sharp and smooth probes. Formation of tungsten tip is then analyzed by theoptical microscope and field emission scanning electron microscope (FESEM). These nano-probes were used in developing a low-cost, effective probe-station that can be utilized forelectrical measurements in micro-electronics.
I. EXPERIMENTAL DETAILS
For the electrochemical etching process to fabricate metallic nano-probes, we used 2MNaOH and 2M KOH solution sequentially during two step etching process. During etchingcopper wire of diameter 0.3 mm was used as cathode while tungsten wire of diameter 1.0mm (purity 99.9 %, Sigma Aldrich) was used as anode. DC power supply of 5V(Scientech, Model 4073) was used as driving source for etching process. Etching wasperformed using cathode of varying shape: straight, circular, triangular, square andpentagon folded wire. Two-step process with different etchant solution was taken due todifference in their etching rates. Etching with NaOH was first performed (referred as 1 st step) and followed by etching with KOH (2 nd step). After etching with NaOH, the tungstenwire was cleaned by ethanol and de-ionized water. During first step, etching with 5 voltsDC offers optimum etching rate resulting into smooth outer surface of the tips, whileetching at higher voltages like 8 volts results into relatively much faster etching rate withirregular rough surface. NaOH leads to faster etching rate as compared to KOH, but resultsinto rough outer surface. Initially etching with NaOH was performed to reduce the wirediameter to few mm (step 1) and subsequently the wire was etched with KOH solutionwith 3 volts DC power to smoothen the outer surface as the outer diameter reduces to ~ 50nanometers (step 2). Further we explored the effect of copper cathode geometry on theetching rate and roughness of outer surface of formed nano probes. For comparison ofresultant geometries during the etching process other parameter like separation betweentwo electrodes (2 cms) and portion of the tungsten rod dip inside the solution (3 cms)along with DC voltage was kept fixed. For anode with varying geometrical shape the sidesof each shape was kept constant 3 cms (triangular shape, rectangular shape and pentagon) ,while for circular shape the diameter was kept at 3 cms. Visibly the tungsten anode in theform of straight wire was located at the centre of the copper cathode of variouseometries. The surface morphology and the diameter and length of probes were analysedusing a high – resolution ZEISS sigma 300 field emission scanning microscope (FESEM).During the process of electrochemical etching, following reaction takes place: (47) O + 6e - → (g) + 6 OH - On cathodeW(s) + 8 OH - → W + 4 H O + 6e - On AnodeW(s) + 2 OH - + 2 H O → W + 3 H (g) (over all reaction)Etching with NaOH solution was performed for 1 hour and after that etching was doneusing KOH solution till the etched portion of the wire drop-off known as drop off method.The entire process was monitored by digital microscope to elucidate the effect of minusesformed on the neck formation on the wire’s outer surface leading to formation of tip . Result and discussion
Figure 1(a) shows the schematic of the experimental setup used for fabrication of metallicnano-probes. Figure 1(b) shows the photograph of the etched anode with NaOH for 01hour. Etching leads to formation of neck at the air-solvent interface, schematically asshown in figure 1(c). Figure 1(d) shows the photograph of tip formed before drop-off.During etching a meniscus is formed around the anode after application of DC voltage,clearly visible with naked eye. Figure 1(d) and 1(e) shows the photographs of process ofelectrochemical etching using anodes of circular and square geometry respectively,progress of etching process leads to formation of bubble at the cathode surface as seen infigure 1(d) and 1(e). Etching with anode of different geometry results into difference inshape of etched neck of varying sizes as estimated using Vernier caliper (table-1). igure.1 (a) shows schematic of the experimental setup used for electrochemical etching ofTungsten wire to fabricate nanoprobes. (b) Shows photographs of formation of neck duringetching at air-solvent interface. (c) Shows schematically diagram of formation of neck duringetching. (d) and (e) shows photograph of etching using circular and square cathodes.Formation of meniscus is clearly visible around the anode resulting into neck creation on thetungsten wire surface.Table-1: Effect of anode geometry on the neck formation during electrochemical etching.
Shape ofelectrode Area(cm ) Top(mm) Neckwidth(mm) Bottom(mm)
Straight
Circular
Triangular
Square
Pentagon urther continued etching using NaOH results into drop-off of etched portion after 90minutes. A closer look using FESEM indicates rough surfaces at the probe tip, making themunsuitable to be used as electrical contact probe and Atomic force microscopy probes.Prolonged etching using NaOH alone leads to drop-off for all the cathode geometry andresults into rough outer surface of the tip edge (see in table-S1, supplementary information).Figure 2 shows the relative effect of etching with NaOH and KOH separately using triangulargeometry of cathode. Figure 2(a) and (b) shows etched probe using NaOH at two differentmagnifications, while figure. 2(c) and (d) shows etching with KOH alone. Etching with KOHresults into smooth outer surface with much smaller probe length.Figure.2 shows relative effect of etching with NaOH and KOH. (a) & (b) shows etchingof tungsten wire using NaOH only for about 90 minutes at two different magnifications.(c) & (d) shows etching using KOH only for 100 minutes. KOH etching results intosmooth surface.ince NaOH based etching occurs at much faster rate but results into undesirable outersurface of the probes, while etching using KOH gives rise to smooth surface with muchslower rate. A combined process of etching first with NaOH for 01 hour followed byetching with KOH till drop-off results into probes with tip radius in nanometer rangeand smooth outer surface.Figure 3 shows the effect of cathode geometry on the shape and tip radius of fabricatednano-probes. The tip length and tip radius was estimated schematically as shown in thefigure (a). Probes fabricated under similar conditions with two step process andcathodes of different geometry are shown in figure (b) straight electrode, (c) circularelectrode, (d) triangular electrode, (e) square electrode and (f) pentagon electrode. Insetof the fig. (b)-(f) shows the apex of the fabricated respective probes.Above figure shows the FESEM images of probes etched with two step process andusing cathodes of different geometry. Figure 3(a) shows the schematic of estimation oftip radius and probe length. he FESEM images of probes fabricated using two-step process under similarconditions using cathodes of varying geometry using straight, circular, triangular,square and pentagon electrode are shown figure 3 (b)- (f) respectively. The estimatevalues of tip radius and probe length are summarized in table-II.Table-II The geometrical parameters of probes obtained with electrodes with varyinggeometry.
ShapeOf cathode Length(mm) Half angle(degrees) Radius of tip(nm)Straight
Triangular
Circular
Square
Pentagon m) while,square electrode results into probes with length 2.15 mm. A closer look into the half angles ofthe probes formed shows etching with square electrode results into sharpest probe, whileetching with triangular electrode results into probe with half angle of 5°. Among all thecathode shapes, the best probe tip shape results from triangular electrode. Triangularelectrode results into probes with symmetric shape with single mode, while other shapes ofelectrodes into multimode structure, which may give rise to multiple resonance frequenciesfor their end use as scanning probe microscopy. The eteching with varius cathode geometrywere repeated before summarizing the shape of resultant probes. igure 4: (a) Shows photograph of developed low cost probe station. (b) & (c) shows contactmade over one of the connecting pads of a portion of RAM using devloped probes at lowerand higher resolutions. (d) I-V measurement for a metallic strip of micrometer size by usingdevloped probe- station.A nano-probe station with the benefit of minimalistic contact area for micro and nanoelectrical characterisation was established using the developed nano-probes and a low costUSB based microscope with a magnification upto 1000×. The nano-probe station (as shownin figure 4(a)) was assembled with the help of four screw gauges, each mounted on amagnetic stand, that aids in the fine mechanical movement of each nano-probe. The nano-probes that are linked with the screw gauge, have an intermediatery insulating pipe to avoidany type of metallic contact. Then, each nano-probe end is coupled to a single wire connector hat is utilised in giving an electrical response that is determined using Keitheley 2410SourceMeter. An approximate visualisation of the contact between metallic sample strip withthe nano-probe is determined by using a digital microscope. This nano-probe station isemployed to determine the I-V characterisitics of a micrometer size metallic strip as shown inFigure 4a. The linear fit of the electrical response gave us the resistance of the metallic stripto be 3.8 ohms. Conclusion
Etching with NaOH results in rough probe surface and results in probe apex size of ~ 2 mrange, whereas etching with KOH results in shorter probe length with smoother surface andprobe apex size of ~ 500 nm range. There has been no report about the formation ofnanoprobes (probes with tip apex size of nm) using direct etching with DC sources, withoutany additional circuit breaker or modification in circuit. As such, the combined process ofsequential etching using NaOH followed by KOH results in the formation of nanoprobesusing DC source alone making it inexpensive. For the first time, geometry of the cathodeused for etching is found to affect the probe apex angle and probe length. Cathode oftriangular geometry is found to form sharp probe- tip with single mode. Multiple groove onthe tip edge results into multimode giving rise to multiple resonance frequencies: which is notdesirable for probe spectroscopy and microscopy measurements. Using the developednanoprobes the possibility to use them as low cost probe station for nanoelectronics has beendemonstrated. cknowledgments One of the authors Rakesh K. Prasad thanks TEQIP-III for fellowship. Dilip K. Singh thanksDST, Government of India (Fellowship IFA13-PH65) and Seed Money Scheme, BirlaInstitute of Technology, Mesra for funding.
Data Availability
The data that support the findings of this study are available from the corresponding authorupon reasonable request.
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Low cost electrical probe station using etched Tungsten nanoprobes: Roleof cathode geometry
Rakesh K. Prasad a and Dilip K. Singh a* a Department of Physics, Birla Institute of Technology Mesra, Ranchi-835215.*Email: [email protected]
When we etched 1.5 hour for 1 st step by 2M NaOH solution and 2 nd steps by using 2M KoHsolution by further etching we obtain irrugular surface with tip radius in 2.07 µm havinglength 1.78mmFig. S1 FESEM image of probe develped by 1step etching for 1.5 hours and furtheretching with KoH Solution . able S1: Result of the etching with different shape of electrode only by one stepprocess but we are able to form micro/submicro tip radius with irrugular surface . Shape ofelectrode Length(mm) Half angle(in degree) Radius ofTip( m) Straight
Circular
Triangular
Square