electron beam fabricated platinum and tungsten nano wires

Materials Today: Proceedings xxx (xxxx) xxx

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On the electrical characterization of focused ion/electron beam fabricated platinum and tungsten nano wires Abhishek Kumar Singh ⇑, Anil Kumar Choudhary Department of Physics, Darbhanga College of Engineering, Mabbi, P.O. Lal Sahpur, Darbhanga, India

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Article history: Received 13 January 2020 Accepted 24 January 2020 Available online xxxx Keywords: FIB-CVD Nanostructure fabrication FIB induced deposition Nano wires Platinum Tungsten

a b s t r a c t Focused Ion Beam (FIB) has been found to be a rapid, powerful and versatile tool for the maskless fabrication of nano/micro structures. Focused Electron Beam (FEB) assisted deposition and inspection, with FIB, expands the capability of the fabrication technique. An e-beam is less harmful than an ion beam. The fabricated structures have high potential in making Nano Electro-Mechanical Systems (NEMS) and Micro Electro-Mechanical Systems (MEMS). FIB/FEB made structures can also be fruitfully exploited as a gas sensor, temperature sensor, mass sensor, etc. Characterizing and analyzing different properties of FIB/FEB made nano/micro structures also carries importance from the point of view of understanding basic science in the nano regime. In this paper, we report the electrical properties of W and Pt nano wires fabricated using FIB and FEB. Pt and W nano wires were fabricated using FIB/FEB assisted Chemical Vapour Deposition (CVD) process with different dimensions, ranging from 45 nm to 300 nm, and lengths, ranging from 2 lm to 10 lm. In situ resistivity measurements of the nano wires were carried out. The resistivity of the nano wires was found to be higher than their bulk values. These results are understood on the basis of the O, C and Ga incorporation in the nano wires, as characterized from (EDS) analysis. Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.

1. Introduction Focused ion beam (FIB) assisted fabrication process has found to be of importance in the manufacturing and development of prototype nano/micro sensors [1,2], actuators [3], micro and nano electro-mechanical systems (MEMS and NEMS) [4,5], and biological applications [6]. FIB/FEB fabricated structures can also be used as functional building blocks for various nano-optoelectronic devices [7] and are destined to play an important role in futuristic nano/micro systems. One of the important applications of FIB is that of providing electrical contacts to connect structures of the nano regime to the macro world [8]. However, there are several problems and phenomena which are associated with FIB/FEB, namely, incorporation of C [9] and other elements, such as Ga (the primary beam used in FIB), substrate atoms deposition due to the secondary ions [10,11], structural deformation occurring under intense ion beam power [12], etc. All these phenomena adversely affect the electrical properties of the FIB fabricated nano

⇑ Corresponding author. E-mail address: [email protected] (A.K. Singh).

structures. In this paper we measure the resistivity of Pt and W nano wires fabricated using Ga ion FIB deposition as well as FEB induced deposition and report the effect of the growth parameters, viz. ion beam current and beam dwell time, on the resistivity.

2. Experimental A dual beam FIB system (FEI Nova NanoLab 600) employed for the fabrication consists of a field emission Ga + ion source, high resolution ion optics, precision milling and gas chemistry functionalities for the deposition of carbon, platinum and tungsten, along with a scanning electron microscope (SEM) and energy dispersive spectrometer (EDS). The energy resolution of the X-ray Si (Li) detector provided is 132 eV. The energy and beam current of focused gallium ions can be varied in the range 5–30 keV and 0.3 pA–20nA, respectively. The spot size depends on both the beam current and the accelerating voltage and can be realized down to 7 nm under optimal conditions. A high speed turbo molecular based pumping module provides a clean vacuum of 5  10–6 mbar in the microscope column. Naphthalene (C10H8), methyl cyclopentadienyl trimethyl

https://doi.org/10.1016/j.matpr.2020.01.439 2214-7853/Ó 2020 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the scientific committee of the International Conference on Advanced Materials and Nanotechnology.

Please cite this article as: A. K. Singh and A. K. Choudhary, On the electrical characterization of focused ion/electron beam fabricated platinum and tungsten nano wires, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.439

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nano wire is 250 nm and 300 nm and for the W nano wire it is 100 nm and 130 nm, respectively. Initially, Pt nano wires of thickness and height in the range of 45 nm to 100 nm were deposited and the corresponding resistance was measured in situ within the FIB chamber. Subsequently, the thickness was increased by depositing over layers in steps of 50–70 nm. After each deposition the dimensions and I-V characteristics were measured in situ. Fig. 2(a) and (b) depict deviation from linear behaviour and decrease in current before it breaks out. Similarly, W nano wires have been deposited and the corresponding dimensions and resistances were carried out for them. From these data taken from the nano wires fabricated under different growth conditions the dependence of resistivity on the deposition parameters, namely beam current and beam dwell time, was obtained. Figs. 3 and 4 show the dependence variation of resistivity as a function of beam current and dwell time, respectively. It may be noted that each point represents an average value obtained from at least 3 structures of identical size. Gas injection system (GIS) is used to discharge precursor gas molecules over the surface of the substrate, which gets physisorbed. Then by using incident ions the organometallic compounds have been cracked into volatile and

platinum ((CH3)3Pt(CpCH3)) and tungsten hexacarbonyl (W(CO)6) precursor gases are used for the in situ deposition of carbon, platinum and tungsten, respectively. The gas needles (diameter 500 mm) are held at a distance of 250 mm above the substrate. I-patterned mask has been used for Au deposition on which the electrical connections have been made. 30 keV ion energy with current ranging from 10 pA to 0.1nA and dwell time varying from 100 ns to 1 ms have been used for the Pt and W depositions. The width and height of the fabricated nano wires have been kept in the range of 45 nm to 300 nm and the lengths have been kept in between 2 mm and 10 mm. For the measurements of all the dimensions, the FIB cross-section method has been used. Keithley’s remote source meter (model 6430) has been utilized for the resistivity measurements. All the measurements have been performed at room temperature (293 K) and at a pressure of less than 5 x 10 6 mbar.

3. Results and discussions 3.1. Fib fabricated pt and w nano wires Fig. 1(a) and (b) show the SEM images of the fabricated Pt and W nano wires. In order to have a good contact with the Au pads, nano wires of sufficient length were deposited so that they extend well on the Au pads. The measured thickness and width of the Pt

Resistivity (10-3Ω.cm)

2.8

1μm

Tungsten Platinum

2.4 2.0 1.6 1.2 0.8 0.4 0

20

40

1μm

6.0x10

-4

5.0x10

-4

4.0x10

-4

3.0x10

-4

2.0x10

-4

1.0x10

-4

100

0.35

(a)

0.30

(b)

0.25 Current (mA)

Current (A)

-4

80

Fig. 3. Resistivity vs. current for FIB fabricated Pt and W nano wires at dwell time of 100 ns.

Fig. 1. (a) and (b): SEM image of FIB fabricated Pt and W nano wires.

7.0x10

60

Current (pA)

0.20 0.15 0.10 0.05

0.0

0.00 0

2

4 Voltage (V)

6

8

10

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Voltage (V)

Fig. 2. (a) and (b): I-v characteristics of Pt and W nano wires, showing a non-linear behavior before it breaks out.

Please cite this article as: A. K. Singh and A. K. Choudhary, On the electrical characterization of focused ion/electron beam fabricated platinum and tungsten nano wires, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.439

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(a)

1μm

(b)

1μm Fig. 4. Resistivity vs. dwell time for FIB fabricated Pt and W nano wires at current of 10 pA.

Focused ion beam

Volatile products produced by ion impact

Scan C, Pt or W Deposited film Gas Nozzle Precursor gas molecules Substrate Fig. 5. Schematic picture of the deposition process.

non-volatile parts. Volatile elements (carbon, hydrogen etc.) are removed from the chamber and the metallic non-volatile element gets deposited over the substrate. The schematic picture of the deposition process is shown in Fig. 5. The deposition process, which is basically a competition between the cracking of the adsorbed gas molecules by mostly the secondary electrons and the sputtering caused by the incoming beam, should depend upon the rate of gas delivered to the surface and the pixel dwell time of the beam. Fig. 3 shows that Pt nano wires deposited at low beam current of 10 pA have a resistivity of 2.2  10 3 X-cm, which reduces and maintains an approximately constant value of 1.2  10 3 X-cm for ion beam current of 30 pA or higher. On the other hand, the resistivity of W nano wires does not show any systematic dependence on the beam current and exhibits an average value of 0.5  10 3 X-cm. The graph of variation of resistivity with the beam dwell time (Fig. 4) shows that the resistivity of Pt nano wires decreases with beam dwell time. In contrast to this result, Table 1 Atomic percentage composition of FIB & FEB fabricated Pt and W nano wires. Elements

pt W C Ga O

Pt nano wire

W nano wire

FIB

FEB

FIB

FEB

45–55% – 35–50% 7–12% –

25–35% – 65–75% – –

– 50–65 35–50% 7–15% Negligible amount

– 45–55% 35–45% – 7–14%

Fig. 6. SEM image of the burned nano wires at higher current.

lower values of resistivity for W nano wires have been seen for low beam dwell time. These varying results may be because of the variations of the compositions in the nano wires, which consist mainly of impurities such as C, Ga and O, apart from the metallic element, and these mixing of the impurities could depend on the cracking process. It may be possible that at low current or low beam dwell time cracking may not be sufficient as compared to high current or high dwell time, or vice versa, and this affects the compositions in the nano wires. The chemical compositions of the nano wires, determined by EDS for the FIB assisted CVD deposited Pt and W nano wires is shown in Table 1, which shows a large amount of C incorporation along with Ga content of the nano wires, which have come from the precursor gases that have been used for the depositions and the incident beam, respectively. The higher resistivity of the FIB fabricated wires can be understood from the compositional analysis. The higher resistivity and chemical composition of FIB fabricated structures using different precursor gases and at various deposition parameters has been reported in the literature [9,13,14,15], from which the independence of the order of resistivity for the same kind of element on the precursor gases can be seen. It has been noticed that during the current passage the structure often burns out (Fig. 6) and this is because of high temperature rise in the nano wire. During the phenomenon agglomeration occurs and spherical droplets of different sizes evolve. 3.2. Feb fabricated pt and w nano wires Fabrication of the nano wires can also be performed by the focused electron beam induced CVD process. The deposition rate has been found to decrease gradually as we go above and below 5 keV beam energy and the optimized value has been found at 5 keV energy and 0.14 nA current. The deposition of the structures has been done using these optimized parameters. The depositions and measurements of the structures have been done in the same way as in the case of the FIB fabrications. The resistivity of the FEB fabricated Pt nano wires has been found in the order of 1– 10 X-cm, which is much higher (102–103) than for the FIB fabricated nano wires. On the other hand, the resistivity of the FEB fabricated W nano wires has been found in the order of 10 3 X-cm, which is of the same order as that of the FIB deposited nano wires. Table 1 shows the chemical compositions of the elements in the nano wires. The FEB fabricated Pt nano wires have been found to

Please cite this article as: A. K. Singh and A. K. Choudhary, On the electrical characterization of focused ion/electron beam fabricated platinum and tungsten nano wires, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.439

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incorporate larger amounts of C element than the FIB fabricated nano wires, from which the higher resistivity of the FEB fabricated Pt nano wires than the FIB fabricated nano wires can be well understood. Higher C incorporation for FEB fabricated Pt structures than for FIB fabricated Pt micro structures has also been reported in the literature [9,16]. The same order of resistivity of the FIB and FEB fabricated W nano wires can be explained also from Table 1, which shows that the elemental composition of the nano wires is the same. In the literature, the study of resistivity of electron beam fabricated W micro structure has also been studied by a few groups [17,18]. 4. Conclusions The electrical resistivity of FIB/FEB assisted Pt and W nano wires have been measured and compared with the bulk values as well as the values found in the literature. The higher resistivity of the nano wires can be understood from the EDS analysis. The EDS result shows a high percentage of C incorporation in the nano wires. The FIB fabricated Pt nano wires have been found to possess higher resistivity than the FEB fabricated Pt nano wires. On the other hand, the resistivity of the FEB fabricated W nano wires has been found to be of the same order as that of the FIB fabricated W nano wires. All these observations can be understood from the EDS result, which shows the variation of the C composition. Moreover, the variation of the resistivity of the nano wires with the beam current as well as the beam dwell time has been reported. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Please cite this article as: A. K. Singh and A. K. Choudhary, On the electrical characterization of focused ion/electron beam fabricated platinum and tungsten nano wires, Materials Today: Proceedings, https://doi.org/10.1016/j.matpr.2020.01.439