Laser-assisted localized zinc deposition

Pulsed lasers can be used to achieve localized zinc coatings without masking by focusing the laser beam on the cathode in the electroplating bath. At the Dnipro University of Technology (Dnipro/Ukraine), an acceleration of galvanic zinc deposition by a factor of 9.4 was observed at a laser power density of ^{70-107 W/m2}. The scanning of the cathode surface by laser radiation leads to the localized formation of a metallic coating whose configuration corresponds to the path of the laser beam.

Recently, the electrodeposition process stimulated by laser irradiation to accelerate the electroplating process has been the subject of increased interest in the metal coating industry [1-4].

In microelectronic device manufacturing, existing processes for producing complex metallized structures require several particularly complex steps for mask fabrication, photolithography process, etc. This increases the complexity of process control and production costs. The development of laser-assisted metal deposition processes is a promising technology for fast maskless selective deposition.

The application of localized electrolytic zinc coatings is promising for microelectronics to protect critical components from corrosion and to apply micro-dosed zinc-containing solders for small parts.

The aim of this work was to determine the optimal conditions for laser-assisted local electrolytic deposition of zinc in the form of dots and lines for the formation of contact areas or images of the integrated circuit topology.Fig. 1: Schematic representation of the experimental setup: 1 - radiation source (solid-state ruby laser KVANT-12: l=694 nm, ω=(70-95)-107 W/m2), 2 - rotating mirror, 3 - electrolytic cell, 4 - cathode, 5 - anode, 6 - rectifier, 7 - coordinate table

Materials and methods

Electrolytic deposition of zinc coatings was carried out from a standard sulphuric acid electrolyte with the following composition (g/L):

_ZnSO4-7Н2О- 250, _Na2SO4 -75, Al(_SO4)3 - 30, pН- 4

The experimental system (Fig. 1) was set up on the basis of a KVANT-12 solid-state ruby laser. The generation on ruby was carried out in pulsed mode at a wavelength (λ) of the laser beam of 694 nm, a generation frequency(f) of 10 Hz, a radiation energy per pulse (W) of 2.2-3.0 J, a pulse duration(_tp) of 2-5 ms and a radius(_r0) of the focused laser beam of 0.1 mm. At a beam energy of 2.2 J, the intensity of the laser radiation (ω) was 70-107 W/m2. The work table was used to fix and position the cathode surface relative to the tightly focused laser beam. The scanning speed (υ) of the laser beam during the deposition of the zinc tracks was 3 mm/s.

The microstructure of the zinc layers was analyzed with an optical microscope "Neophot-21".

The temperature of the aqueous electrolyte solution was determined using a copper constantan thermocouple. The steady-state value of the thermoelectric force was determined for a time of 150-200 s and measured with a digital voltmeter.

Results and their discussion

According to the results obtained by examining the transmission spectra of a series of aqueous electrolyte solutions with the NIR 61 spectrophotometer(Fig. 2), the zinc electrolyte exhibits a relatively high transmission (55.8 %) at the length of the laser radiation used (λ=694 nm).

The analysis of the cathodic polarization curves (Fig. 3) showed that with increasing cathodic potential, the curve of zinc reduction with the laser radiation source switched off (curve 2) lies below the curve of the laser-assisted electrodeposition process (curve 1). In the laser-assisted electrodeposition mode, an increase in current density and a shift of the cathode potential towards positive values are observed compared to the deposition mode with direct current without laser irradiation.

The efficiency of the laser influence can be estimated using the laser acceleration coefficient, which is also a measure of the selectivity of the process and corresponds to the ratio of the current density in laser-assisted electroplating mode(J) to the current density when the laser radiation source is switched off(_J0). From the course of the Fig. 4: Dependence of the laser acceleration coefficient on the cathode potential curve K(E) (Fig. 4) it can be seen that the highest value of K = 9.4 is at a cathode potential E = -0.97 V for a laser power density of ^{70-107 W/m2}.

Investigations have shown that at a potential E > -0.97 V, no zinc deposition is visible when the laser radiation source is switched off. At E < -0.99 V, which corresponds to a cathode current density with laser irradiation of 0.8 ^A/dm2, uniform coverage of the entire surface of the copper cathode with a layer of metallic zinc can be observed.

The analysis of the morphology of the electrodeposited zinc surface showed that the use of external laser exposure of the cathode area in the electroplating process led to a change in the surface morphology compared to coatings produced by direct current (DC) without laser exposure.

The process of cathodic zinc reduction from the sulphate electrolyte is accompanied by the release of hydrogen, which passivates the cathode surface by adsorption over the entire surface. Analysis of the electrodeposited coatings (Fig. 5 left) showed the formation of surface bubbles that look like "frozen" gas bubbles and represent such superficial defects.

In the laser-assisted electroplating mode, the internal stress of the zinc coating decreases in the radiation field and the coating shows a developed surface (Fig. 5 right).

According to the experimental data in Figures 3, 4 and 5 , the process of localized laser-assisted electrodeposition of copper-based zinc is best performed at E = -0.97 V and J = 4.6 ^A/m2.

Fig. 5: Morphology of the laser-assisted electroplated zinc surface: left - in the radiation zone; right - outside the radiation zone

The increase in deposition current density under laser irradiation is associated with a localized heating of the interface between the solution and the electrode and an associated increase in mass transfer caused by the mixing of the solution near the cathode surface [5-8]. The laser acceleration of the zinc deposition process is due to the thermal effect detected by examining the laser irradiation zone with a copper constantan thermocouple. It showed a heating of the central area of the focused laser beam with a radius _r0 = 10 mm from 296 K (22.85 °C) to 332 K (58.85 °C).

The current density and the laser acceleration coefficient of the electrodeposition process are functions of the electrolyte solution temperature (Arrhenius dependencies):

<1>

<2>

where _Т0 - temperature of the aqueous electrolyte solution, ΔТ-temperature change of the aqueous electrolyte solution, W - energy of the metal ions, R - universal gas constant.

The energy of the discharged metal ions is determined by the following expression [9, 10]:

<3>

where Е - the current potential value, _Е0 - the equilibrium potential value(Е0₌ -0.96 V relative to the silver chloride electrode).

Table 1 shows the energy values of the discharged zinc ions, calculated according to formula <3>.

Using the data on the steady-state radial temperature distribution in the laser irradiation zone (Fig. 6), we can calculate the radial distribution of the current density during zinc deposition:

<4>

where J0₌ 1 ^A/m2 - current density of zinc deposition at T0₌ 296 К; Δt(r) - temperature, counted from _T0; Ea₌ 39 kJ/mol - activation energy of zinc deposition.

The values of the current density calculated using equation <4>, taking into account the data on the temperature in the laser irradiation zone (Fig. 6) and the value of the metal ion discharge energy (Table 1), are close to the values of the current density for the laser-assisted process in the central part of the laser irradiation zone. However, for r > _r0 (where r is a radial variable and _r0 is the laser beam radius), the temperature decreases, so the following modification of the equation is proposed to describe the radial dependence of the current density of laser-assisted electrodeposition:

<5>

Fig. 6: Radial temperature distribution in the laser irradiation zone (l = 694 nm, ω = 70,107 W/m2) where and r* is the stationary radius of the local coating of the deposited metal.

The application of expression <5> at temperatures corresponding to the shape of the curve (Fig. 6) and the value of the metal ion discharge energy (Table 1) results in a radial profile of the current density (Fig. 7), from which a decrease in the current density from the center to the edge of the laser irradiation zone can be seen.

In connection with the practical application of laser-assisted electrodeposition in the technology of protective coatings [11-15], it is important to determine the radial profile of the local layer thickness of zinc coatings under the selected optimum conditions. The local layer thickness (d) is proportional to the current density and deposition time:

<6>

where k - electrochemical equivalent(_kZn = 0.^34-10-6 kg/A-s), ρ - metal density (ρZn = 7130 ^kg/m3), А - atomic mass(_AZn = 65.^37-10-3 kg/mol), τ - time of the deposition process, F - Faraday's constant.

The results of the calculation of the local layer thickness as a function of the radial variables are shown in Figure 8.

The calculated data (Fig. 8) show that the thickness of the local coating decreases with increasing distance r from the center(r = _r0) to the boundary of the focused laser beam and drops sharply beyond this boundary(r > _r0).

In local laser-assisted electrodeposition of zinc, the average zinc deposition rate in the area of the focused laser beam is 6.1 μm/h under optimal conditions of laser-assisted electrodeposition(J = 4.6 ^A/m2, E = -0.97 V), and the coating with a thickness of 0.08 μm obtained at this potential at direct current is deposited in laser-assisted mode for a time τ ≈ 47 s.

Fig. 9: Localized coatings in the form of: Left - dot; Right - lines (× 20)

To form a localized metallic coating whose surface exceeds the diameter of the focused laser beam (Fig. 9 left), it is necessary to scan. This was realized by a translational movement of the cathode relative to the stationary laser beam (Fig. 9 right).

If υ is the linear velocity of the uniform scan, the formula for calculating the thickness of the local zinc line is as follows:

<7>

where _r0 - radius of the focused laser beam, N - number of beam passes along the line path. At N = 30 and υ = 0.1 mm/s, the thickness of the local zinc coating is 0.05 µm, which corresponds to 188 atomic monolayers of zinc (zinc atom diameter _dzn = 0.266 nm).

At high radiation powers, the temperature of the electrolyte solution in the area close to the cathode clearly exceeds the temperature of the main volume of the electrolyte solution. As a result, the heated layers of the irradiation area are replaced by cooler neighboring layers of the electrolyte solution and intensive mixing of the solution occurs.

The convection flow leads to a reduction in the thickness of the diffusion layer on the cathode surface and to a reduction in the concentration of metal ions. This significantly increases the layer growth rate.

Conclusions

The optimum conditions for the local laser deposition of zinc on copper from sulphuric acid electrolyte were determined, the parameters of the local zinc spots and lines were calculated and the speed of the electrodeposition process was estimated.

After technological elaboration, the obtained results can be used in maskless process for local electrodeposition of zinc to repair microdefects in protective coatings of conductive elements of printed circuit boards.

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