Synthesis of Copper Oxide Aided by Selective Corrosion in Cu Foils

Copper oxide films aided by selective corrosion have been synthesized. Using copper (Cu) foils of commercial printed circuit boards (PCBs), chemical attack in small areas on Cu surfaces and their oxidation with heating process in air atmosphere at low temperature have allowed to produce Cu oxide layers. Phase formation of Cu oxide samples was validated by X-ray diffraction studies, and their conduction properties were registered and evaluated by current-voltage curves. Stability analysis of the Cu oxide samples was conducted by a correlation profile between average strain and Cu oxide percentage grown, which confirms that the structure defects dependent on plastic deformation define the existence of both CuO and Cu2O phases beneficial to mitigate corrosion and advance in Cu oxide-based devices to build electronic circuits as components on PCBs.


Introduction
It is well known that copper (Cu) is a nature-abundant material and is the third metal more employed in the world, after iron and aluminum.It has an important biological role in photosynthesis, and it is essential for human life.Despite the standard negative electrode potential with −0.345 V, its corrosion can occur at a significant rate under strenuous conditions [1].Both cupric oxide (CuO) and cuprous oxide (Cu 2 O) are the two chemical compounds formed by oxygen and copper in the nature only.Such oxides are semiconductors, where Cu 2 O has a direct bandgap of 2.1 eV and reddish appearance, while CuO with a bandgap of 1.3 eV is black in color [2].
Due to the miniaturization in the integrated circuits (ICs), Cu has been widely used for microelectronic device industry, including wiring technology, electromagnetic interference shielding, and electrostatic dissipation technology [3].However, the reduced spacing between IC components on a printed circuit board (PCB) can produce various types of corrosions, although the underlying mechanism in all cases has been electrochemical migration (EM) as one of the most severe problems, where events such as temperature cycles during soldering at temperature above 150 °C, degradation of the thermal interface in contacts, existence of dust particles between contacts, and application of high voltage for operation of the electronic devices have made it easy for corrosive environments [4,5].e high-temperature corrosion occurs when gradual EM by Cu ions along an interconnection under high current density (current crowding) increases the resistance of the Cu lines with reduction in their cross-sectional area [6].High-temperature corrosion can be able to assist the oxidation in the Cu lines where their surface will remain safe, and small fluctuations will degrade the oxide in few critical points due to the pit formation under humidity conditions.e interior of a pit is naturally poorer in oxygen, and pH locally decreases to very low values, thus pitting corrosion increase due to the catalytic process where dendrite growth from cathode to anode produces an electric short and premature failure.Figure 1 shows pitting corrosion and dendrite growth on a PCB when high electric field was applied [7].
Previously, studies have shown that for maximizing reliability in IC design and PCB fabrication, width scaling of Cu lines must be investigated by EM phenomena where to reduce failures by corrosion environments, two scenarios have been recommended with respect to the mechanisms governing the Cu ion migration with the suggested spacing between IC components satisfying a width <1 μm, whereas a width >1 μm is for components on PCBs [6,8].
Nowadays, researchers have synthesized p-channel Cu 2 O thin-lm transistors on exible polyethylene terephthalate (PET) substrates by using magnetron sputtering and vacuum annealing techniques [9,10].Such novel electronic devices provide modest transport properties with hole mobility (µ h ) in the range of 1 to 10 cm 2 /Vs and hole concentration (n h ) from 10 16 to 10 17 cm −3 .Its conduction mechanism is attributed to the electronic structure of intrinsic defects such as Cu vacancies [V Cu ], which create trap states [11].Conversely, the operation characteristics of the Cu 2 O thin-lm transistors are of no use today to engineering applications, and the market for Cu 2 O-based devices has been delayed by setback in its scalability, degradation of their electronic properties at high-temperature conditions, and requirements for high quality of Cu 2 O with a minimum in structure defects.Consequently, to achieve useful engineering applications using Cu oxide-based devices compatible with silicon technology and controllable operation characteristics, the formation and growth of CuO on the Cu surface can increase the chemical bonding and path of the speci c thermal resistance in Cu 2 O-based devices for electronic circuits designed as components on PCBs [5].
e layer CuO is accompanied by defects such as dislocations and grain boundaries where reactivity of these is dependent on intrinsic structure disorder (elastic strain e ects) [12].us, transformation in the microstructure of the Cu surface (process known as plastic deformation) can be a strategy to adjust defect density and mitigate corrosion issues [3].
In the past, oxidation procedures at high temperature around 1000 °C in air atmosphere have been developed to oxidize Cu foils in order to obtain polycrystalline Cu 2 O crystals [11], where Cu oxidation has taken place by di usion process of the Cu ions which migrate because there is an opposite di usion of [V Cu ] from oxide layer to the Cu foil [1].Nevertheless, as CuO is less stable than Cu 2 O and its formation energy is lower, the presence of CuO appearing together with Cu 2 O can be attained by plastic deformation on the Cu surface at low temperatures [2,4].ereby, the work presents the synthesis of Cu oxide lms aided by selective corrosion of Cu foils with the subsequent heating process in air atmosphere at low temperatures.Such experimental procedure will be discussed in Section 2. Phase formation and conduction properties in Cu oxide lms and stability analysis as a function of average elastic strain originated by selective corrosion will be covered in Section 3. Conclusions about this research are presented in Section 4.

Experimental Procedure
Cu foils with an average thickness of 15 µm were obtained from commercial PCBs with copper joined in a berglass epoxy polymer.Before, Cu foils were cut in a cross-sectional area of 1 cm 2 ; after, cleaning process to remove nonpolar substances was developed with acetone; nally, washing process was completed in deionized water and successively dried with nitrogen.In the synthesis of Cu oxide aided by selective corrosion, the following four stages are involved: (a) choosing the area for chemical attack, (b) immersing Cu foils in FeCl 3 /H 2 O solution, (c) removing residual FeCl 3 /H 2 O by soaking the chemically attacked Cu foils, and (d) heating the Cu foils in air atmosphere inside of quartz tube furnace.Figure 2 shows the stages in the synthesis to obtain Cu oxide lms.
A nontoxic permanent marker typically engaged in the design of PCBs was used to synthesize Cu oxide samples.An area of 4 mm × 4 mm was de ned on each Cu foil under study.A permanent marker is resistant at FeCl 3 /H 2 O solution; therefore, during corrosion, the outside area was protected as shown in Figure 2. Iron chloride (FeCl 3 ) solution activated with H 2 O was employed as the electrolyte.Once Cu foils were immersed in the FeCl 3 /H 2 O solution to obtain CuCl 2 and soaked with water, heating process at 35 °C with a duration of 2 min at atmospheric pressure was completed.In accordance with the following reaction mechanism, the formation of Cu oxide lms took place in two stages: where x is the chemical composition in Cu oxide samples.
e heating temperature for the Cu samples was chosen as a function of the boiling point for HCl of 48 °C, where Cl − ions and residual water are fully evaporated.e surface of Cu oxide samples resulted with opaque gray color after thermal process.e presence of ionic species such as copper hydroxide (CuOH) and acidic chloride (HCl) determines the oxidation degree of the CuCl 2 surfaces because displacement interactions between OH − and H + ions initially synthetize CuO by surface migration of such ionic species and after Cu 2 O by di usion of Cu + ions and reaction with atmospheric oxygen [3,14].Nevertheless, as the activity of these species is poorer under heating process within humidity, the Cu oxide lms were nonstoichiometric.
Using X-ray di raction with PANalytical di ractometer of CuK α radiation (λ 0.1541 nm), phase formation of Cu oxide lms was validated.Conduction behavior in the samples was registered and evaluated by current-voltage curves at room temperature.
e electrical parameters of the samples were collected by employing a digital storage oscilloscope (Tektronix, TDS1012C).To know the performance of the Cu oxide samples as a function of structure  Advances in Materials Science and Engineering defects involved, a stability analysis was done by a correlation pro le between elastic strain and Cu oxide percentage.

Results and Discussion
is section discusses phase formation and conduction properties in the Cu oxide samples.Here are the revised XRD patterns and electrical characterization to demonstrate the potential of the selective corrosion technique with emphasis in the oxidation degree of the Cu foils dependent on their plastic deformation.

X-Ray Di raction in Cu Oxide
Samples. Figure 3 shows XRD patterns of Cu oxide lms synthesized with the heating process.e samples labeled as CS-1, CS-2, and CS-3 exhibit monoclinic phase (CuO) with peaks located at 33.9 °and plane (110), 58.1 °and plane (202), and 65.2 °and plane (022) according to PANalytical Card number 00-041-0254.Also, peak positioned at 69.7 °and plane (310) corresponds to cubic phase (Cu 2 O) according to PANalytical Card number 00-005-0667.As a reference, the XRD pattern of Cu foil also is shown.e larger peak-position displacement between XRD patterns of the Cu oxide lms and XRD pattern of the Cu foil was observed which ensures presence of the elastic strain in Cu oxide surface induced by high density of the defects in chemically attacked Cu foils.

Conduction Properties in Cu Oxide Samples.
e performance of Cu oxide samples has been con rmed with the schematic diagram of Figure 4(a).To ensure linear response, a function generator (Matrix, MFG-8250A) was used to produce a linear ramp signal at low frequency (f 100 Hz) with voltage ranging from −2 V to 2 V to conserve lowvoltage operation across the aluminum (Al) contacts.Also, a resistor of R 1 kΩ was chosen to monitor the current signal.e log current-voltage plots for samples labeled as CS-1, CS-2, and CS-3 are shown in Figure 4(b), where such current-voltage curves have shown ohmic behavior.
To investigate conduction parameters, a model based on the point form of Ohm's law [15] has been proposed by ΔV ΔI where σ o 5.8 × 10 5 Ω −1 cm −1 is the Cu conductivity at room temperature, E a is the activation energy of Cu oxide, k B is the Boltzmann constant, T is the temperature in samples under test, l c 0.77 nm is the interatomic distance into CuO crystalline lattice associated with minimal free path of charge carries [16], and S is the cross-sectional area (S t • d), where d 0.5 mm is the diameter of aluminum (Al) contact and t the thickness of the samples.e ΔV/ΔI rate was extracted from Figure 4(b), with ΔV as the di erential voltage and ΔI as the di erential current, respectively, while E a was estimated with the slope of the dotted line in Figure 4(b) which crosses at the voltage axis.Using MATLAB program, the cross-sectional area S t • d is computed by solving (5) and by the Arrhenius conductivity dependent on temperature [17].[V Cu ] was estimated by where σ s (E a ) [V Cu ] • e • μ Cu is the conductivity for the Cu oxide layer, while μ Cu corresponds to the mobility of Cu vacancies.e quantity [V Cu ]μ Cu from (6) was computed, con rming that [V Cu ] is attained in the range of 10 14 to 10 15 cm −3 and mobility from 1 to 40 cm 2 /Vs, which con rms that the operation characteristics in Cu oxide samples would be useful in engineering applications because their speci c thermal resistance chemically controlled by CuO and electronic properties driven by Cu 2 O allow advances in design of Cu oxide-based devices whose principle of operation similarly to previous studies by other authors is dependent on strain e ects [18,19].Table 2 summarizes the conduction parameters found.Advances in Materials Science and Engineering deformation in Cu foils, average strain along each crystallographic plane on the oxidized Cu surface with respect to the Cu foil as substrate is approximately estimated by (Δd/d) Δθ/ tanϕ [12], where Δθ is the di erence between measured incident angle θ m and reference angle θ r , while the angle of each Cu foil plane corresponds to ϕ. e parameter (x ≈ Δd/d) can be linked with the chemical composition in Cu x O 1−x which de nes its nonstoichiometry [20].e elastic strain must be analyzed as a function of Cu oxide grown, being it proportional to the intensity in percentage of each peak [12,20].Figure 3 allows evaluating Δθ at each crystallographic plane of both oxides (Cu 2 O and CuO), as well as tanϕ at each crystallographic plane in the Cu foil; thus, a correlation pro le between x and Cu oxide percentage is built in Figure 5.

Stability in Cu Oxide
e structure defects can be validated by stability analysis, which determines the ability of the Cu oxide layers to remain structurally stable during electrical conduction.
e stability in the Cu oxide samples is a function of both tensile and compressive stress into a limit where Cu oxide (Cu x O 1−x ) is fully grown into two phases.In Figure 5, both oxides (Cu 2 O and CuO) give a slight deviation from their nonstoichiometry into the range of −1 < x < 1, which corresponds to the limit where the Cu oxide percentage from 30 to 100% is stable.In contrast, anomalous Cu oxide is lower than30%, because its severe nonstoichiometry is associated with intergranular local attack in uenced by poor quality of the initial Cu surface.
Selective corrosion followed with the heating process at 35 °C had allowed obtaining both Cu 2 O and CuO phases in accordance with the experimental procedure detailed in Section 2, where oxidation in Cu foils occurs beginning with a rst layer of CuO grown on Cu, and subsequently a layer of Cu 2 O grows between Cu and CuO.ese two layers increase with time because the Cu 2 O has a very small expansion coe cient at lower temperatures of 300 °C where the lattice constant changes to less than 0.5% and the dissociation pressure of CuO is lower than the atmospheric pressure [11,13].As Cu vacancies agglomeration or trapped oxygen happen into grain boundaries, intrinsic concentration of defects corresponding with [V Cu ] and the opaque gray color in Cu oxide samples explicate the stability of the CuO phase.

Conclusions
Cu oxide samples synthesized by corrosion and heating processes at low temperature have been studied.Selective corrosion was achieved by chemical attack in smaller areas from Cu foils of commercial PCBs.Both CuO and Cu 2 O phases were formed under air atmosphere.
e study conducted by X-ray di raction and electrical characterization found that phase formation and conduction properties are linked with structure defects into Cu foils.Stability analysis has con rmed that the structure behavior in Cu oxide samples is strongly dependent on its intrinsic disorder.us, the selective corrosion technique has signi cant bene ts as physical and mechanical properties are in uenced by plastic deformation, which has resulted in stable Cu oxide samples with next electronic engineering applications for Cu oxide-based devices.

Figure 1 :
Figure 1: Corrosion phenomena on a PCB based on EM under high electric eld conditions.

Figure 2 :
Figure 2: Stages employed in the synthesis of Cu oxide lms.

Figure 3 :
Figure 3: XRD patterns in Cu oxide lms grown on Cu foils.

Figure 4 :
Figure 4: (a) Schematic diagram of practical circuit implemented to measure the current-voltage curves in Cu oxide samples.(b) Log current-voltage plots of each sample.

Figure 5 :
Figure 5: Correlation pro le for di erent Cu oxide samples.
3 activated with H 2 O are Fe 2+ Cl − 2 , Fe + (OH) − , and H + Cl − which indicate that corrosion in Cu foils occur because initially Cu + ion in the presence of H + ion produces cupric chloride (CuCl), and later cuprous chloride (CuCl 2 ) is fully formed by the reaction of CuCl with Cu 2+ ion [1, 13, 14].

Table 1 :
Parameters for selective corrosion in Cu foils.