Electrochemical Etching of Tungsten for Fabrication of Sub-10-nm Tips with a Long Taper and a Large Shank
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From the perspective of maximizing the practicability of tungsten nano-tips, a sharp tip, long taper, and large shank are usually expected. However, simultaneously satisfying the requirements for tip radius, taper length, and shank diameter is theoretically impossible with the conventional drop-off tip fabrication method, which is based on lamellae electrochemical etching. In this study, a two-step etching method is proposed to fabricate a sub-10-nm tungsten tip directly from a 1-mm rod. First, a floating electrolyte-based drop-off process is carried out to fabricate a tungsten needle with a long length taper of 10 mm. Then, an inversed lamellae drop-off process is conducted to realize fine etching of the needlepoint. As a result, an ultra-sharp tungsten nano-tip with a radius of 5.5 nm and taper length of 10 mm is successfully fabricated from a 1-mm tungsten rod.
KeywordsTungsten tip Nano tip Drop-off etching Liquid metal Electrochemical etching
Miniaturization is an important issue for the development of science and technologies in micro- and nanoscales. As a typical case of miniaturization, the fabrication of nano-tips has drawn increasing attention because of their numerous applications in various fields like high-resolution microscopy [1, 2], micro- and nano-machining , and nanolithography . Although several materials can be used for the fabrication of nano-tips , tungsten has been most widely studied and utilized owing to its superior mechanical and chemical properties, such as high hardness, high melting temperature, strong oxidation resistance, and that it does not mix readily with other metals .
To solve the mentioned problems in the conventional drop-off method in which the lower parts are the targeted tips, some approaches based on making use of the upper tips have been recently proposed [6, 10, 14, 15, 16]. Theoretically, if the upper tip and the lower tip have the same radius, then it is possible to make the lower wire as light as possible to make a sharper upper tip, and the shank of the upper tip can be much larger and longer. However, this approach is faced with other challenges. First, the positive potential is applied on the upper tungsten wire. When the lower tip drops and is cut off from the circuit, the upper tip keeps reacting with the electrolyte. Hence, the upper tip is usually blunt compared with the lower tip [13, 17]. Moreover, the wire usually breaks at the air–liquid interface, so the upper tip usually has a short and fixed taper length, which restricts its practical applications to some specific areas .
Klein et al.  proposed an improved lamellae etching method. They interrupted the etching process at a suitable time and turned the tungsten wire upside down. By the upside down reversal, the cut-off time for the upper tip was greatly shortened, and a sharper tip was fabricated, though the taper was very short. As reported, a tungsten tip with a radius of 12.5 nm was produced . Guise et al.  developed an instantaneous cut-off device, which could turn the power off within a minimal delay of about 500 ns. On the basis of this instrument, an ultra-sharp tungsten tip with a radius of 3.6 nm has been successfully achieved with a shank of 0.35 mm. Although these modified drop-off methods have successfully minimized the tip radius, they rely on specialized setups, and the taper length of the tip has not been considered [17, 18]. Furthermore, the fabrication of tungsten nano-tips with a shank larger than 1 mm has not yet been reported.
In this communication, we propose a combined electrochemical etching process to fabricate tungsten nano-tips with a long taper and a large shank. The process consists of two steps. First, a floating electrolyte-based drop-off process is carried out to fabricate a tungsten needle with a long taper from a large rod. Then, an inversed lamellae drop-off process in which the needle is further etched by applying the positive potential on the lower tip (rather than the upper tip as in the conventional drop-off method) is conducted to minimize the tip radius. It is expected that nano-tungsten tips with a long taper and a large shank can be fabricated using this two-step etching method.
In the tungsten drop-off etching process, the taper was formed owing to the nonuniformly distributed thickness of the mucous layer [6, 13]. Thus, in the first step, the thickness of the floating NaOH electrolyte determined the taper length of the needle, and it was set to about 10 mm in this study. The inert and nontoxic fluorocarbon ether beneath the electrolyte was used to protect the tungsten shank. The length of the tungsten rod immersed in the inert solution was about 5 mm.
A positive potential varying from 2 to 8 V was applied on the tungsten rod while the platinum plate (10 mm × 10 mm × 0.1 mm) worked as the cathode. As the etching process continued, the diameter of the tungsten rod around the air–solution interface decreased. Finally, the lower part fell off, and a tungsten needle was formed. Through this process, tungsten needles with tapers of different lengths were supposed to be achieved by regulating the thickness of the floating NaOH electrolyte.
In the second step, to further reduce the tip radius of the tungsten needle, an inversed lamellae drop-off process in which etching of the upper tip could be instantaneously cut off was conducted. A schematic of the experimental setup is shown in Fig. 2b. A closed platinum loop with a diameter of 4 mm served as the counter electrode and the supporter of the lamellae NaOH electrolyte. To avoid violent bubble release, 2 M NaOH electrolyte was used in this step. The tungsten needle penetrated the center of the membrane without contacting with the platinum loop, and a droplet of 1 ml of liquid metal gallium (Ga) sustained on a copper plate was used as the connector of the vulnerable tip and the power supply.
In the second fine etching step, the vertical position of the tungsten needle was precisely adjusted using a numerically controlled Z axis with stepping accuracy of 1 µm. A detection potential of 0.1 V was charged between the needle and the copper plate and the downward movement of the needle was stopped immediately once it made contact with the Ga droplet to complete the electric loop. Then, the position of the Pt loop was manually adjusted to be close to the gallium droplet without contact. With this approach, the length of the lower tip, which was finally discarded, could be minimized. A positive potential of 3 V relative to the platinum loop was applied on the copper plate. In this process, the weight of the lower part of the needle was extremely small owing to its small radius and length. Meanwhile, once the lower tip dropped off from the tungsten needle, the electric loop (Pt loop—NaOH membrane—W needle—Ga droplet—Cu plate) became disconnected immediately. Thus, etching of the upper tip was immediately terminated as it was completely separated from the etching circuit. Thus, nano-tips were fabricated using this method.
3 Results and Discussion
Figure 3a*, b* shows the high-magnification SEM images of the tungsten needlepoints etched with applied potentials of 8 V and 2 V, respectively. The tip radiuses were about 418 nm and 60 nm. As shown in Fig. 3a, b, the mass removed with 2-V etching was much larger than that with 8-V etching; thus, a smaller needlepoint was formed. For the following inversed lamellae drop-off process, the smaller the needlepoint, the sharper the tungsten nano-tip that can be achieved. To optimize the etching potential for needle fabrication, tungsten needles etched with a potential varying from 2 to 8 V were fabricated, and their sectional diameters at the point located 1 mm from the needlepoint were measured as a characterization parameter. Figure 3c shows the measured sectional diameters of the tungsten needles etched by different potentials varying from 2 to 8 V. It is obvious that the diameter is positively related to the potential. As a lower etching potential resulted in a tungsten needle with a smaller needlepoint, 2 V was then selected as the potential for the first needle etching step.
Figure 5b shows the high-resolution SEM image of the tip. A conical nano-tip with a taper length of 120 µm was formed by the fine etching process. Figure 5b* shows the high-resolution SEM image of the tungsten tip. The tip radius was 5.5 nm, which is even smaller than that of the sharpest tungsten tip made by the conventional drop-off method with a radius of 10 nm . In the inversed lamellae drop-off process, the location of the Pt loop is important because it determines the mass of the lower tip. When the loop was at location II, which was lifted by 4 mm relative to location I, the radius of the tip was about 16 nm, as shown in Fig. 5c, c*. When the loop was at location III, which was further lifted by 4 mm, the radius of the tip was about 42 nm, as shown in Fig. 5d, d*. It is obvious that the tip radius increased with the increase of the drop-off length. It is noteworthy that the smallest nano-tips fabricated by the second fine etching step may have different tip radii in different etching operations as the position of the Pt loop was determined by naked-eyes and manually lifted. However, it can be well repeated to fabricate nano-tips with radius of sub-10 nm.
To fabricate the tungsten nano-tip with a large shank as shown in Fig. 5b, the two combined etching steps are both indispensable. The first floating electrolyte-based drop-off process makes it possible to fabricate tungsten needles with a long taper and a large shank. Meanwhile, the fabricated needle has a small needlepoint, which makes it possible to greatly reduce the drop-off mass in the second fine etching step. As for the second inversed lamellae drop-off step, the key point is the use of liquid metal to apply the positive potential on the lower tip of the tungsten needle instead of the upper tip as in the conventional drop-off process. At the moment of dropping, the upper tip and the lower tip separate from each other under the influence of gravity. The potential on the upper tip vanishes naturally after a very short time, which is similar to what happens to the lower part in the conventional drop-off process. In this way, the problem of the conventional method that the upper tip is usually less sharp than the lower tip owing to the delay of disconnecting the power has been solved. Considering that we do not use any specialized instruments, we believe this simple, versatile, and cost-saving method can be an alternative approach for the fabrication of tungsten tips with a sub-10-nm radius, a long taper, and a large shank.
In conclusion, we proposed a two-step electrochemical etching method to fabricate ultra-sharp tungsten nano-tips with a long taper and a large shank. The first floating electrolyte-based drop-off process was carried out to fabricate tungsten needles with a long taper and a large shank. Then, as the second fine etching step, an inversed lamellae drop-off process further etched the needlepoint to form a sub-10-nm nano-tip. In this way, an ultra-sharp nano-tungsten tip with a small radius of 5.5 nm, a long taper of 10 mm, and a large shank of 1 mm was finally obtained without the use of specialized instruments. Compared with the tips fabricated by conventional drop-off methods, it is believed that the tungsten tips fabricated by the proposed two-step method have more practical values.
This work was financially supported by the research fund for Basic Research (Free Exploration: JCYJ20180302174311087) and the research fund for International Cooperation from the Science and Technology Innovation Committee of Shenzhen Municipality (GJHZ20180928155412525 and GJHZ20180411143558312).
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