Electroless nickel–phosphorus (Ni–P) films were produced on the surface of p-type monocrystalline silicon in the alkaline citrate solutions. The influences of bath chemistry and plating variables on the chemical composition, deposition rate, morphology, and thermal stability of electroless Ni–P films on silicon wafers were studied. The as-deposited Ni–P films were almost all medium- and high-P deposits. The concentrations of Ni2+ and citric ions influenced the deposition rate of the films but did not affect P content in the deposits. With increasing
Electroless process of metal and alloy films makes nanometer-scale structures on silicon (Si) wafers due to its simplicity and ability to fill in fine patterns. The mass production of metallic structures on Si wafer is one of key techniques in micro- and nanoscale device applications [
Electroless M–phosphorus (M=Ni, Co) deposition is the most important catalytic deposition process, due to its simplicity in operation, low equipment cost, and excellent properties in wear and corrosion. More recent work about M–P nanomaterial could be prepared by electrodeposition [
In the present work, the Ni–P film was electrolessly produced in an alkaline bath solution on the surface of p-type monocrystalline Si substrates from alkaline citrate solutions. The influences of bath chemistry and plating parameters on the chemical composition, deposition rate, and morphology of Ni–P film were investigated in detail; at the same time, the thermal stability and proposed mechanism of the deposit were studied.
The p-type monocrystalline Si wafers were used as the substrates (size: 10 × 10 × 0.2 mm). One surface of Si substrate was etched by reactive ion etching process. Yoo et al. [
Bath composition and operation conditions for electroless Ni–P films.
Bath chemistry (g·L−1) | Value | Plating variables | Value |
---|---|---|---|
NiSO4 | 15~50 | pH | 8~12 |
NaH2PO2 | 10~40 | Temperature ( |
50~90 |
Na3C6H5O7 | 10~40 | Time ( |
15~90 |
The mass variation of the samples before and after each deposition was measured by an analytical balance (FA2004B, resolution 0.1 mg). The microstructure and morphology of the top surface and fracture surface of the deposits were observed by a scanning electron microscopy (SEM, JSM-6360) operated in a high vacuum mode and electron beam acceleration voltage of 20 kV, equipped with the attached liquid-nitrogen-cooled Oxford Si X-ray energy dispersive spectroscopy (EDS) detector. Each sample was tested at three locations, to confirm uniformity and present representative average values.
Thermal stability of the deposit was measured using thermogravimetric analysis and differential scanning calorimetry (TG-DSC, DSC 404F3 A00 Pegasus) applied from room temperature to 650°C. The high pure nitrogen gas of 20 mL·min−1 was used as the carrying gas. The vacuum of the working environment was 10−4 Pa. The heating rate was 10 K·min−1.
Figure
Plots of the P content in the deposit and weight variations of samples as a function of the concentration of Ni2+(NaH2PO2 = 20 g·L−1, Na3C6H5O7 = 20 g·L−1, deposition conditions: pH = 10.0,
Figure
Plots of the P content in the deposit and weight variations of samples as a function of the concentration of
Figure
Plots of the P content in the deposit and weight variations of samples as a function of the concentration of citric ions (NiSO4 = 25g·L−1, NaH2PO2 = 20 g·L−1, the deposition conditions: pH = 10,
Figure
Weight variation of samples (a) and chemical composition of the deposits (b) as a function of plating time (NiSO4 = 25 g·L−1, NaH2PO2 = 40 g·L−1, Na3C6H5O7 = 20 g·L−1, pH = 9.0,
Figure
Plots of the P content in the deposit and weight variations of samples as a function of the pH (NiSO4 = 25 g·L−1, NaH2PO2 = 40 g·L−1, Na3C6H5O7 = 20 g·L−1,
Figure
Plots of the P content in the deposit and weight variations of samples as a function of the temperature (NiSO4 = 25 g·L−1, NaH2PO2 = 40 g·L−1, Na3C6H5O7 = 20 g·L−1, pH = 9, plating time = 30 min).
The bath composition and characteristics of representative samples for discussion are listed in Table
Bath composition and characteristics of selected samples.
Sample | Ni2+ (g·L−1) | Cit− (g·L−1) | pH | Ni (wt%) | P (wt%) | Figure | ||||
---|---|---|---|---|---|---|---|---|---|---|
Number 14 | 25 | 20 | 40 | 15 | 4.0 | 70°C | 10 | 89 ± 0.2 | 11 ± 0.2 | |
Number 17 | 25 | 20 | 40 | 60 | 11.5 | 70°C | 9 | 90 ± 0.2 | 10 ± 0.2 | |
Number 19 | 25 | 20 | 20 | 15 | 0.3 | 70°C | 9 | 95 ± 5.4 | 9.3 ± 0.3 | |
Number 21 | 25 | 20 | 20 | 15 | 0.4 | 70°C | 12 | 87 ± 0.3 | 13 ± 0.3 | |
Number 23 | 25 | 20 | 40 | 15 | 0.1 | 60°C | 9 | 89 ± 0.6 | 11 ± 0.6 | |
Number 25 | 25 | 20 | 40 | 15 | 2.2 | 90°C | 9 | 89 ± 0.2 | 9 ± 0.2 |
Note:
SEM images of the top surface of the alloy films. The samples are substrate (a), number 14 (b), number 19 (c), number 21 (d), number 23 (e), and number 25 (f).
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SEM images of the top surface of the alloy films deposited with different plating times: (a) 15 min, (b) 30 min, (c) 60 min (NiSO4 = 25 g·L−1, NaH2PO2 = 40 g·L−1, Na3C6H5O7 = 20 g·L−1, pH = 9.0,
Figure
SEM image (a, b) and line scanning analysis (c, d) of the EDS of the fracture surface of sample numbers 19 (a, c) and 14 (b, d) deposited for 90 min.
Figure
In the alkaline solutions, the chemical reactions for electroless Ni–P film would be expressed by the following equations, reported by Zhang et al. [
According to (
The hydrogen evolution (
However, the metal Ni mainly resulted from the chemical reactions between the Ni ions and the reductant radicals [
However, the Si wafers could be oxidized in the alkaline solution at the plating conditions, such as bath temperature. The chemical reaction was as follows:
Hsu et al. [
During this spontaneous reaction, the Si substrate surface was oxidized and became a catalytic surface inducing further codeposition of Ni–P film. Therefore, the oxidation and reduction reactions of
In this study, the P content and weight variation of the sample steadily increased with increasing
Island growth was evident and remarkable sparse sphere particle nucleation was present. Then, islands grew and then met one another and drove to form a continuous film. Adhesion of the Ni–P film with Si substrate is naturally poor because of the sparsely distributed Ni nuclei particles in the initial nucleation stage (see Figures
Figure
DSC curves of the Ni–P films from sample numbers 19 and 14 at the same heating rate of 10 K·min−1.
In this work, the Ni–P film was electrolessly plated on the surface of p-type monocrystalline Si wafers in the alkaline bath solutions. The influences of bath chemistry and deposition variables on the chemical composition, morphology, deposition rate, and thermal stability of the Ni–P deposits were investigated in detail. The following results have been shown:
The concentration of Ni2+ ions has an effect on the deposition rate of the film because of the increase of free Ni2+ in solutions but did not influence P content in the deposits. With the increasing A bath was composed of 25 g·L−1Ni2+, 40 g·L−1 The pH and temperature had an impact on the chemical composition and the deposition rate of the films. When the pH was low, at 8.0, the deposition rate was close to zero. The gained weight slightly increased and then decreased with increasing pH. The gained mass reached a maximal value of 1.5 mg at pH = 10.0. The P content increased with increase in pH. As the temperature was about 50°C, the reduction of Ni could not occur. With increasing temperature from 60 to 90°C, the P content increased significantly. The deposition rate was greatly changed and the gained mass reached a maximal value at the temperature of 80°C. The film growth rate is much faster than the initial nucleation rate of the deposits. Island growth was evident, and remarkable sparse sphere particle nucleation was present. The concentration of The thermal stability of the medium-P film was better than that of the high-P deposit. An exothermic peak is detected with the crystallization peak temperature located at 339°C for high-P deposit and 341°C for medium-P deposit, respectively.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work has been supported by the Natural Science Foundation of Jiangsu Province (Grant no. BK20150260) and the funding of Changzhou High Technology Research Key Laboratory of Mould Advanced Manufacturing (Grant no. CM20173001). The authors gratefully acknowledge the Testing Center of Changzhou University for providing SEM/EDS facilities.