Available online at www.sciencedirect.com
ScienceDirect
Procedia Chemistry 19 (2016) 619 – 625
5th International Conference on Recent Advances in Materials, Minerals and Environment
(RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer
(MAMIP), 4-6 August 2015
Structural and Electrical Characterizations of CuNi Thin Film
Resistors
Nurul Khalidah Yusopa, Nurulakmal Mohd Shariffb , See Chin Chowc, Ahmad Badri
Ismail a,b
a
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia,14300 Nibong Tebal, Pulau Pinang,
Malaysia
b
School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia,14300 Nibong Tebal, Pulau Pinang,
Malaysia
Abstract
This work studied the development of Cu-Ni for thin film resistor; specifically reported in this paper is the surface
morphology and resistance. Films were deposited on the substrate of commercialized flame retardant (generally known as FR4)
printed circuit board by using thermal evaporator. The thickness of the film was set to 100 nm and the dc current was maintained
at 26 A. The sheet resistance and bulk resistivity of the thin films were measured via four-point probe test. The stability of thin
film resistor was measured through temperature coefficient of resistance (TCR). According to the experimental results, it can be
seen that the sheet resistance of the thin film resistor decreased with increasing copper composition.
©
Published
by Elsevier
B.V. B.V.
This is an open access article under the CC BY-NC-ND license
©2016
2016The
TheAuthors.
Authors.
Published
by Elsevier
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia.
Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia
Keywords:CuNi thin film, four-point probes test, thin film resistor
* Corresponding author. Tel.: +6012 - 5210544
E-mail address:ahmadbadri@usm.my
1. Introduction
Thin films technology in the electronic industries especially as resistor are developing very fast since they have
potential applications in computers, smartphones, tablets and other electronic devices 1–7. Copper nickel (Cu-Ni), an
alloy made up between copper and nickel either physically mixing or chemically mixing to form homogeneous solid
solution7,8, is one of the material that is commercially used in fabrication of resistor since it is cheaper compared
toother materials such as nickel chromium. Different sizes, shapes and composition of Cu-Ni alloys contribute to
1876-6196 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia
doi:10.1016/j.proche.2016.03.061
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Nurul Khalidah Yusop et al. / Procedia Chemistry 19 (2016) 619 – 625
different properties however Cu-Ni alloys generally have low resistivity and low temperature coefficient of
resistance (TCR) which are guaranteed to provide low and constant resistance over a temperature range 7,9. These
properties are required in microelectronic industries to improve batteries consumption. The most common copper
alloy used in resistor is known as ‘constantan’ which is Cu alloyed with 45% Ni with low resistivity less than 50
μΩ.cm and extremely low temperature coefficient resistance (TCR) value less than 50 x 10 -6 /K. There are a lot of
techniques have been used in order to form Cu-Ni alloys such as mechanical alloying7,9, sol gel method10 and
electrochemical process11. In this current work, we prepared Cu-Ni thin film resistor by using evaporation process
and examined the surface morphology, and the dependence of electrical properties such as resistivity and TCR.
2. Methodology
A mixture of copper and nickel with appropriate composition were mixed according to the intended weight
percentage. The alloy was milled for 3 hours using zirconia ball with ratio 1:10 of powder to ball before it was then
palletized using pressing method. The pallet was used as the target source in evaporation technique using EMITECH
K950X Turbo Evaporator. A 3.5 mm x 3.5 mm printed circuit board was used as the substrate in the deposition
process. In order to see the effect of composition on the thin film resistor, the composition of copper was set to 50
wt. %, 60 wt. %, 70 wt. % and 80 wt. % of Cu. The electrical properties of the film such as sheet resistance and bulk
resistivity were analyzed using JANDELL Cylindrical four-point probes with the samples were divided into 9 sites.
In order to examine the stability of film against temperature, a set of TCR experiment was set up using the
Wheatstone bridge theory as shown in Figure 1. The value of TCR can be obtained from the slope of temperature
against resistance. Scanning electron microscope (SEM) model S-36; Leica Cambridge Ltd. with a Leo Supra 35VP
system and X-ray fluorescence (XRF) were used for material characterization and verification including verification
of samples composition, surface morphology and thickness cross section. Phase analysis was carried out using XRD
(Philip PW 1820) to confirm the presence of Cu-Ni coated film after sputtering method with the addition of Xpert
Highscore Software.
Fig. 1. Circuit used as TCR test
3. Results and Discussion
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621
From four-point probes testing the results showed that the sheet resistance decreased as the composition of
copper in the alloy was increased (see Table 1). The film resistance of Cu-Ni alloy coatings also decreased with
increasing Cu content of the coatings. The resistivity values of thin film was plotted against Cu composition and
presented in Figure 2. The trend is as expected as higher amount of Cu, which has higher electrical conductivity than
Ni, would increase the conductivity of thin film. The results of sheet resistance obtained as in Table 1 with 9 sites
were measured on each samples. Overall all the sheet resistances recorded were much lower in values due to the
films surfaces as shown in Figure 4; which can carry more current for application purpose as reported by Hur and
coworkers9. Other factors such as the effect of substrate condition prior to the deposition highly influence the sheet
resistance as such reported by Yang and fellow researchers12. All films showed a small gradient increased of
resistance in TCR test as shown in Figure 3. The resistance of the thin film changed as the temperature was
increased and reverts to the original values once the power supply was turned off. This indicates that the films
experienced thermal expansion that usually happened in copper alloyas reposted by researcher6 but due to the small
changes the resistors stability are good up to temperature 100 0C.
Table 1. Electrical properties of different copper compositions.
Copper content (%)
Sheet resistance(Ω/sq)
Bulk Resistance (Ω)
50
5.7075
51.37
60
4.3086
38.78
70
3.8344
32.19
80
1.8465
16.62
Resistivity x10-7 (Ω.cm)
7
6
5
4
3
2
1
0
0
20
40
60
80
Cu composition (wt. %)
100
Fig. 2. Resistivity values as a function to 50 wt. %, 60 wt. %, 70 wt. % and 80 wt. % of copper composition
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Nurul Khalidah Yusop et al. / Procedia Chemistry 19 (2016) 619 – 625
1.1
0.035
0.03
1
0.025
0.02
0.8
0.015
R//Ro
TCR (oC-1)
0.9
0.7
0.01
0.6
0.005
0
0.5
40
45
50
55
60
65
70
75
80
85
Cu (wt. %)
Fig. 3. The variation of TCR and R/R0 as a function of copper composition
Images obtained from the analysis by SEM for thin films with respective composition of copper produced by
evaporation method are shown in Figure 4 (a), (b), (c) and (d). Thin film produced using thermal evaporation
method has a rough and porous surface. The surfaces displayed beehives-like deposits which contributed to the low
values of sheet resistance since the roughness of morphology increased 9. As tabulated in Table 2 all the samples
were successfully deposited on substrate with their respective thicknesses.
Nurul Khalidah Yusop et al. / Procedia Chemistry 19 (2016) 619 – 625
(a)
(b)
(c)
(d)
Figure 4: Morphologies of CuNi alloys at a different composition (a) 50 wt. % at magnification x500 (b) 60 wt. % at magnification x500 (c) 70
wt. % at magnification x1000 and (d) 80 wt. % at magnification x500
Table 2. Thickness of coating achieved by sputtering method
Copper composition (wt. %)
Thickness achieved (nm)
50
102.0
60
126.8
70
115.3
80
102.9
Figure 5 presents the XRD patterns for the Cu-Ni samples with different copper compositions. XRD analysis
revealed all samples exhibited the cubic structure from the two planes of (111) and (200) of Cu-Ni9–11,13 There are
no impurity peaks detected in all the samples as only two peaks existed. It is noticed that the planes reflection shift
towards the lower angles as the Cu content increased similarly reported by Wu and colleques 11. It was assumed due
to the composition of the copper hence changes the lattice occupy by Cu and Ni causing sample with higher copper
content to small shrinkage. The lattice constant (a) obtained from the interplanar spacing (d) value corresponding to
(111) plane is 0.8 A which is lower than the actual lattice constant of Cu and Ni hence contributed to the lower
reading in sheet resistance. Verification of element composition that existed in the samples by EDX testing was
done and the amount of each element was tabulated in Table 3 according to their respective weight percentages and
atomic percentages.
623
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Nurul Khalidah Yusop et al. / Procedia Chemistry 19 (2016) 619 – 625
(111)
CuNi
(200)
Intensity (counts)
Cu80Ni20
Cu70Ni30
Cu60Ni40
Cu50Ni50
30
35
40
45
50
55
2ϴ (degrees)
Fig. 5. XRD patterns for the CuNi samples deposited by different Cu compositions
Table 3: Parameters for EDX testing according to copper composition
Copper Composition (%)
Cu
Ni
Weight percentages (%)
Atomic percentages (%)
Weight percentages (%)
Atomic percentages (%)
50
70.20
68.52
29.80
31.48
60
65.20
62.30
34.80
37.70
70
74.61
73.09
25.39
26.91
80
81.41
80.19
18.59
19.81
4. Conclusion
CuNi thin films were successfully deposited using thermal evaporator obtaining thickness of 102 – 126.8
nm with minimal impurities. The composition remains in the range of its original composition upon deposited using
thermal evaporation method. The Cu content controlled the electrical properties of the resistors. Deposited CuNi
thin film deposited on the FR4 substrate produced a beehive-like structure with the sheet resistance range between 1
– 6 Ω/sq and recorded the highest resistance of 50 Ω.
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