1. Introduction
One of the fundamental passive components is the thin film resistor, which is applied primarily in electronic circuits. Thin film resistors have exceptional properties, such as a low temperature coefficient of resistance, high precision, high stability and low noise, and are commonly used in precision electronic equipment [
1,
2,
3].
Copper–manganese alloy films with high thermal stability and low resistance are usually used in mobile electronic devices. Copper allows for low electrical resistivity and manganese produces thermal stability in this Cu–Mn alloy system [
4]. Based on the equilibrium phase diagram, there are no intermetallic phases in the Cu–Mn system. Copper can serve as a substitute species for Mn in the FCC (Face Centered Cubic) lattice [
5]. A characteristic feature of Mn in Cu is its larger activity coefficient compared to other elements that have limited solubility, such as Al and Mg [
6]. This feature makes the Cu–Mn solid solution less stable than other Cu alloys when a stable reaction, such as Mn oxidation, can take place. Thus, Mn does not tend to precipitate or segregate within the Cu film but can easily diffuse out to the surface and interface under oxidative conditions [
7].
Misjak et al. reported on the specific resistivity of Cu–Mn films, which they measured over the whole composition range [
8]. The resistivities of pure Cu and Mn films were 1.7 μΩ·cm and 174 μΩ·cm, respectively. The curve increased monotonically when the Mn content was 0–80 at.%, with a maximum of 205 μΩ·cm at 80 at.% Mn that corresponds to a temperature coefficient of resistance (TCR) of −308 ppm/°C [
8]. Focusing on the electrical properties of Cu
0.5–Mn
0.5 alloy films, the resistivity was about 137 μΩ·cm with a TCR of −377 ppm/°C. However, the TCR value was too high, which would have to be improved for applications in mobile electronic devices.
In order to minimize the resistance change in Cu–Mn alloy films, foreign elements are added to improve the TCR. The addition of dysprosium was attempted to improve the electrical properties of Cu–Mn resistive films since rare earth doping has been used as an effective way to regulate the electrical properties of oxides [
9,
10,
11]. Dysprosium has a larger resistivity (92.6 μΩ·cm) and a higher melting point (1407 °C). Dy may be beneficial for the enhancement of resistivity and the minimization of TCR in thin films. The effects of Dy content and annealing temperature on the phases, microstructural and electrical properties of Cu–Mn thin films are investigated in this study.
3. Results and Discussion
Cu–Mn–Dy alloy films were deposited onto the substrates using a DC and RF magnetron co-sputtering system. The compositions of the as-deposited film were examined using the electron probe micro-analyzer (EPMA). The relative concentrations of copper, manganese and dysprosium were analyzed at three points in the deposited films. The measured data for copper, manganese and dysprosium are listed in
Table 1. The dysprosium content was increased with an increase in the RF sputtering power in the Cu–Mn films. For example, Dy increased from 20.5 at.% at 40 W to 40.0 at.% at 70 W.
Figure 1 shows X-ray diffraction patterns of the as-deposited and annealed Cu–Mn films with different added amounts of dysprosium, which were deposited on glass substrates. Annealing was performed in a N
2 atmosphere at a temperature up to 450 °C. All of the Cu–Mn–Dy films annealed at ≤350 °C displayed an amorphous structure, except for alloy films without Dy addition (
Figure 1a,b). When the annealing temperature was set at 450 °C, a Cu crystallization phase with a (111) peak was clearly observed in the Cu–Mn–Dy films. An additional (200) diffraction peak of the Cu
2O phase appeared in the Cu–Mn–Dy films after annealing at 450 °C as shown in
Figure 1c. This demonstrates that none of the elements were crystallized or oxidized in Cu–Mn films with Dy addition after annealing at 300 °C in a N
2 atmosphere. However, two crystallization phases (Cu and MnO) appeared in Cu–Mn alloy films without Dy addition as shown in
Figure 1. The crystallization strength of the MnO peaks also increased with increased annealing temperatures. This is believed to be due to the oxidation of the Cu–Mn alloy film during annealing. Iijima et al. reported that Mn does not tend to precipitate or segregate within the Cu film but can easily diffuse out to the surface and interface under oxidative conditions [
6,
12]. In contrast, Mn at the interface can be selectively oxidized by reacting with oxygen under proper conditions. In our study, the samples were annealed in a N
2 atmosphere. Why did the MnO cause oxidation in the alloy films? Further studies of the microstructure of alloy film with transmission electron microscopy will be performed and discussed later.
Figure 2 shows a cross-section TEM micrograph, SAD and Energy Dispersive X-ray Analysis EDX analysis of Cu–Mn films without Dy addition that was annealed at 300 °C. The film thickness is about 60 nm as shown in
Figure 2a. This shows that there are different crystalline structures between the upper and lower layers in Cu–Mn alloy films. In the lower layer, MnO crystallites are present as demonstrated using SAD analysis (
Figure 2c). Regarding MnO formation, it is believed that oxygen from the Al
2O
3 substrate reacted with Mn during annealing because the MnO phase exists in the lower layer of the alloy film. In the upper layer, the Cu
2O phase was found according to SAD analysis as shown in
Figure 2d. This result indicates that the Cu–Mn alloy film surface can be oxidized to cause the formation of the Cu
2O phase during annealing in a N
2 atmosphere. Under N
2 annealing, the surface of Cu–Mn alloy films was oxidized due to the presence of oxygen and/or moisture inside the chamber/furnace [
13,
14]. Copper only forms two thermodynamically stable oxides, namely CuO and Cu
2O. From the Gibbs free energy point of view, the Cu
2O phase likely formed first because the Cu
2O phase (
ΔG0 = −122 KJ/mol) formation has a lower Gibbs free energy than the CuO phase (
ΔG0 = −99 KJ/mol) [
15,
16]. Moreover, Luo et al. reported that only Cu
2O is expected to form at very low oxygen partial pressures [
17]. A similar oxidation phenomenon occurred for Cu–Mn alloy films annealed in pure Ar at 350 °C for 1800 s. On the other hand, the Cu
2+ ion has a greater charge density than the Cu
+ ion; thus, it forms much stronger bonds that release more energy. Cu
2+ is more stable than Cu
+ in an aqueous medium. Wang et al. reported that Cu
2+ cations react with metallic Cu to form Cu
+ through a disproportionation reaction, with these unstable Cu
+ cations subsequently rapidly reacting with O–O
(adsorb) to form Cu
2O [
18].
The sample surface shows the formation of a discontinuous oxide layer [
6]. The EDX analysis conducted using different positions in the Cu–Mn film is listed in
Figure 2e. The Cu/Mn ratios differed greatly between the bottom, middle and surface areas of the film as they were 0.24, 3.4 and 0.54, respectively. This result indicates that the copper atoms are concentrated in the middle of the film while the manganese atoms are distributed on the surface and bottom. Haruhiko Asanuma et al. [
12] reported that Mn migrates toward the interface and reacts with a surface oxide layer until, finally, a Mn complex oxide layer is formed during subsequent annealing [
19,
20].
Figure 3 displays a cross-section TEM micrograph, EDX and SAD analysis of Cu–Mn alloy films with 40 at.% Dy addition that was annealed at 300 °C. There is an oxidation layer with a thickness of 3 nm on the surface as shown in
Figure 3a. The oxidation layer can also be called a passivation layer. However, an amorphous structure was observed in the alloy films, which is evident from the halo SAD patterns and high-resolution transmission electron microscopy as shown in
Figure 3b,c. This result is consistent with the XRD analysis (
Figure 1). It was noted that the Cu–Mn–Dy thin films had an amorphous structure at 300 °C. Unlike the Cu–Mn alloy films, there were some crystalline phases observed after annealing at 300 °C This may be attributed to the multiple element alloy effect, which could be explicated by the kinetics theory because of slow atomic diffusion [
21,
22]. This means that dysprosium addition in Cu–Mn films leads to the formation of an amorphous structure and oxidation resistance after annealing at 300 °C. The EDX analyzed positions in the Cu–Mn–Dy film are listed in
Figure 3d. This table shows that the element distribution for Cu, Mn and Dy was more uniform between the top and bottom parts of the film.
Figure 4 shows a cross-section TEM micrograph, EDX and SAD analysis of Cu–Mn films with 40 at.% Dy addition that was annealed at 350 °C. When the annealing temperature was increased to 350 °C, there was an oxidation layer with a thickness of 5 nm on the surface as shown in
Figure 4a. A 5-nm crystalline layer on the film bottom was observed using high-resolution transmission electron microscopy analysis as shown in
Figure 4b. According to the SAD pattern analysis, this crystalline layer belongs to the Cu crystallization structure, as shown in
Figure 4c. It is known that amorphous thin films are in a metastable state. The heat treatments can initiate ordering processes, such as structural relaxation and crystallization [
23]. To further observe and determine the size of precipitated particles, HRTEM was used. Some microcrystallites appeared on the surface films, as shown in
Figure 4d. The precipitated particles were distributed on the amorphous matrix and had a size smaller than 5 nm. The nanobeam electron diffraction pattern is shown in
Figure 4e, which demonstrates that these microcrystallites belong to the Cu
2O phase. The EDX was analyzed using different positions in the Cu–Mn–Dy film, as shown in
Figure 4f. Unlike Cu–Mn alloy films (Mn atoms are distributed on the surface and bottom), we found that Mn atoms had a more uniform distribution in the Cu–Mn–Dy annealed films compared to Cu–Mn annealed films.
The X-ray photoelectron spectroscopy (XPS) analysis included survey scans to understand the elemental composition at different depths. The compositions of the Cu–Mn–Dy film with 40 at.% Dy addition that was annealed at 300 °C were analyzed using electron spectroscopy for chemical analysis (ESCA) as listed in
Table 2. It was noted that the oxygen concentration significantly differed between the film surface and interior area (at a depth of 28 nm). The oxygen concentrations for the film surface and interior area were 68.9 at.% and 1.6 at.%, respectively. This result indicates that the surface of Cu–Mn–Dy film was oxidized. The same result was obtained by TEM analysis as shown in
Figure 3a.
Figure 5 shows the effects of dysprosium addition on the electrical properties of annealed Cu–Mn films. The resistivity of Cu–Mn films increased with an increase in dysprosium. Dy addition enhanced the resistivity of Cu–Mn films. There are two reasons to explain this phenomenon: (1) dysprosium has a higher resistivity (92.6 μΩ·cm) that contributes to the alloy film resistivity and (2) an amorphous structure in the alloy film can be obtained by Dy addition. However, it was noted that the resistivity of Cu–Mn–Dy film increased significantly after annealing at 350 °C. This is due to the increase in the oxidation of the alloy film after annealing at 350 °C compared to 300 °C since Cu
2O microcrystallites exist at 350 °C (
Figure 4e). The resistivities of Cu–Mn films with 40 at.% Dy addition were ∼2100 and ∼1200 μΩ·cm after annealing at 250 °C and 300 °C, respectively. Generally speaking, the resistivity of alloy film increases with an increase in the annealing temperature because the grain boundaries, crystal defects and oxides generation are increased after annealing [
3]. Nevertheless, many scattering behaviors are believed to affect the resistivity of Cu–Mn–Dy films, which includes surface scattering, grain boundaries scattering, rough surfaces scattering, impurities scattering and oxides generation [
24]. However, we hypothesize that the generation of oxides plays an important role in determining the electrical properties of Cu–Mn–Dy films.
Figure 6 shows the effects of dysprosium addition and annealing temperature on the temperature coefficient of resistivity (TCR) of Cu–Mn films in a nitrogen atmosphere at ambient temperatures for 2 h. The annealing treatment is an important factor in determining the resistor stability of the thin film. When the annealing temperature is ≤300 °C, the TCR changes significantly from approaching zero to having a positive value with increasing Dy content, except for specimens with 40 at.% Dy addition. As the annealing temperature was increased to 350 °C, the TCR values rapidly increased, except for specimens with 40 at.% Dy addition, which may be caused by the presence of Cu microcrystallites. As the alloy films crystallized, the TCR became positive because the majority of all metals have a positive TCR [
25,
26]. However, the TCR values suddenly changed from positive to negative when the Dy addition was increased to 40 at.%. This may be due to the increased Cu
2O formation on the surface (producing an oxidation layer), as shown in
Figure 1 and
Figure 4. This result implies that the TCR value is strongly dependent on Dy content in the Cu–Mn films. The Cu–Mn films with 40 at.% Dy addition displayed a resistivity of ∼2100 μΩ·cm with the smallest TCR (−85 ppm/°C) after annealing at 300 °C in a N
2 atmosphere.