1. Introduction

In the past decade, femtosecond laser microfabrication has demonstrated its great potential for applications in material microprocessing and microdevice manufacturing [1–3]. Femtosecond laser pulses offer extraordinarily high peak powers in extremely short pulse durations, which provide a new approach for local modification of transparent materials through nonlinear optical processes. The femtosecond laser micromachining technique is capable of integrating three-dimensional (3D) microfluidic, microoptical or photonic components inside transparent materials, which are important for developing optofluidic technology [4–6]. For the purpose of developing monolithic and compact chip-sized multifunctional systems, it is desirable to integrate more functions other than the microfluidics and the microoptics in a single glass chip. From this point of view, there is an urgent need to incorporate the electrical and thermal functions into microdevices using femtosecond laser micromachining. Therefore, selective metallization on the surfaces of insulators such as glasses or crystals becomes necessary.

Conventional metallization process for nonconductive materials largely depends on metal deposition by vacuum evaporation, sputtering, and chemical vapor deposition, which are widely utilized in microelectronic industry today [7–8]. However, these methods require complex photolithographic processing steps for selective metal micropatterning, consequently increasing the complexity and cost of implementation. Laser direct write technique is an alternative way for selective metallization of insulators with the aid of other technologies [9], such as laser induced forward transfer [10], laser assisted chemical vapor deposition [11], laser assisted deposition from liquid solutions [12], laser-induced plasma-assisted ablation [13], direct laser writing on glass surface from metal powders [14], femtosecond laser direct writing on the surface of embedded microfluidic channel from electroless copper solution [15], and so on. In this paper, we propose a new approach for realizing selective metallization of insulators by femtosecond laser modification combined with electroless plating process. Using this approach, conductive copper microstructures with good adhesion are deposited in the femtosecond laser irradiated area on the insulator surfaces. Furthermore, this approach will provide great potential for integrating microelectronics, microoptics and microfluidics on a single microchip.

2. Experimental

The fabrication process mainly consists of four steps illustrated in Fig. 1, including (1) formation of silver nitrate thin films on insulator substrates; (2) modification of insulator surfaces by femtosecond laser direct writing; (3) removal of unirradiated silver nitrate films by acetone; and (4) selective copper coating by electroless plating.

 

Fig. 1. Schematic illustration of the fabrication process for the selective metallization of insulators: (1) formation of silver nitrate thin films on insulator substrates, (2) modification of insulator surfaces by femtosecond laser direct writing, (3) removal of unirradiated silver nitrate films by acetone, and (4) copper coating by selective electroless plating.

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Commercially available microscope slides (borosilicate glass) with a thickness of 2.5 mm and lithium niobate (LiNbO₃) crystal with a thickness of 3 mm were used as insulator substrates. The substrates were first cut into small coupons of a size of 9mm × 4.5 mm and then polished on four sides and cleaned with acetone in ultrasonic bath before use.

To form the silver nitrate thin films on the sample surfaces, a 1.0 mol/l aqueous solution of silver nitrate was prepared in the dark at room temperature. The samples were first rinsed with distilled water, and then immediately immersed in the fresh silver nitrate solution for 10 min. After that, the samples were taken out and naturally dried in the dark at room temperature. The thickness of the thin films could be controlled to some extent by adjusting the immersion duration.

A Ti: Sapphire laser system (Legend USP, Coherent Inc.) with an operating wavelength of 800nm, a pulse width of 40fs, and a repetition frequency of 1 kHz was used for femtosecond laser direct writing. The laser beam was focused by an objective lens (20X, NA=0.45) onto the insulator surface coated with silver nitrate films. The sample was placed on a computer-controlled XYZ stage, which could precisely control the scanning speed and the pattern. The laser pulse energy was first continuously adjusted using a combination of a half-wave plate and a Glan-Taylor prism polarizer as an attenuator, followed by a stack of neutral density filters. The laser power was monitored using an energy meter. The entire laser direct writing process was performed in air.

After laser modification, electroless copper plating was carried out. The plating solution contained copper (II) sulfate pentahydrate (CuSO₄·5H₂O), ethylenediaminetetraacetic acid disodium salt (EDTA disodium salt) and formaldehyde (HCHO) at concentrations of 5g/l, 14g/l, and 5g/l, respectively. The stability and the surface tension of the solution were maintained by adding 2–2’bipyridy (0.02g/l) and polyethylene glycol (PEG, 0.05g/l). In addition, the PH value of the solution was controlled to be ~12.5 by adding proper amount of sodium hydroxide (NaOH) solution. The plating temperature was hold at 40 °C throughout the plating duration around 20min.

An optical microscope and a digital camera were used to capture the images of copper microstructures on the glass and crystal surfaces.

3. Results and discussion

Figure 2 shows optical micrographs of the copper microstructures selectively deposited on the glass surface with a laser scanning speed of 60μm/s. One can see that continuous metal lines with smooth edges are deposited on the glass substrates, and the linewidths of deposited Cu films are approximately 6μm and 5μm when the laser powers are set at 5mW and 3mW. Particularly, it should be noticed in Fig. 2(a) that there is a defect (as indicated in the blue circle) right above the metal line fabricated by femtosecond laser direct writing which is not metallized, providing a clear evidence of the good selectivity. In fact, even when the laser power is reduced to as low as 1mW, the Cu line can still be fabricated by this technique. Basically, the features of the fabricated microstructures can be controlled by femtosecond laser direct writing parameters such as the laser power, scanning speed, focal conditions, and so on. The thickness of copper microstructures mainly relies on the electroless plating process, which has been described elsewhere [16]. In addition, the thickness of the coated silver nitrate films before laser irradiation is also important for selective copper deposition.

 

Fig. 2. Optical micrographs of deposited copper microstructures on the glass substrates at the scanning speed of 60μm/s: (a) Laser power: 5 mW. (b) Laser power: 3 mW.

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To examine the electrical properties of copper microstructures, we fabricate some micro-electric circuits on the glass substrates. A typical image captured by the digital camera is shown in Fig. 3(a). The resistances of pattern (I) and pattern (II) are approximately 15 Ω and 6 Ω, respectively. These results confirm that the copper microstructures formed on the glass substrates are electrically conductive. In addition, these copper lines selectively deposited on the glass surface shows good adhesion to the glass substrates, as they can remain intact after a cleaning process with distilled water in ultrasonic bath. The good adhesion is further confirmed by a peel test using scotch tape.

 

Fig. 3. Photographs of metal patterning on the glass when laser power is 8mW: (a) Electric circuits fabricated on glass surface. (b) Cu lines in pattern (I). (c) Cu lines in pattern (II).

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Furthermore, we perform the selective metallization on LiNbO₃ crystal surface. Copper microelectrodes embedded in the LiNbO₃ crystal are fabricated for demonstrating the three-dimensional integration capabilities of this technique. Figures 4(a) and 4(b) shows the top view and end view images of the electrodes embedded in the crystal, respectively. Two grooves with 10μm width and 16μm interval from center to center are ablated by femtosecond laser with an average power of 10mW at a scanning speed of 200μm/s. The micrograph of the V-shaped cross-section of the electrodes, as shown in Fig. 4(b), exhibits that both the two grooves of ~10μm depth have nearly been fulfilled by deposited copper after electroless plating of 120min. Since optical waveguides can also be directly written inside LiNbO₃ crystal using femtosecond laser pulses[17], the embedded micro-electrodes can therefore be easily integrated with the buried optical waveguides, opening up the potential for fabricating 3D micro-electro-optic system devices including optical switch, optical modulator, and so on.

When the femtosecond laser beam is focused on the insulator surfaces coated with silver nitrate films, Ag particles can be formed from the decomposition of silver nitrate films in the irradiated area [18, 19]. The decomposition reaction formula of silver nitrate is as follows:

The femtosecond laser direct writing has high spatial resolution and small heat affected area, facilitating the selective deposition of the Ag particles in the irradiation area. Moreover, after the laser irradiation, although the sample was ultrasonically cleaned in distilled water for 10 min for removal of unirradiated silver nitrate films, selective electroless copper plating still can be achieved. We speculate that the femtosecond laser may have played a role of localized melting and quasi-welding at the interface of Ag particles and insulator surfaces [20–21], which promotes the adhesion between the reduced Ag particles and the substrates. In the subsequent electroless plating process, these particles can serve as seeds for in-situ selective copper deposition [22–24].

 

Fig. 4. Optical micrographs of micro-electrodes embedded in LiNbO₃ crystal. (a) Top view; (b) end view.

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Although selective metallization of insulators using nanosecond lasers has been reported before [12, 25, and 26], the replacement by the femtosecond laser brings several unique advantages. First, as clearly shown in Fig. 4(b), the microelectrodes are actually deeply embedded in the insulators, which is a desirable characteristic for many novel microdevices requiring 3D configuration. Second, when the femtosecond laser intensity is well-controlled near the threshold of the ablation, selective metallization of glass could be realized at nanoscale spatial resolution, since tiny microstructures (i.e., nanometer-sized holes and grooves) have been reliably produced in dielectrics using a femtosecond laser [27]. Lastly, this technique offers a capability to directly incorporate microelectric elements into microoptic and/or microfluidic circuits in a single glass chip. In our previous work, we have demonstrated the integration of 3D photonics and microfluidics using femtosecond laser micromachining [28]. Further development of this technique has great potential for rapid and cost-effective fabrication of monolithic 3D micro-electro-opto-fluidic devices.

4. Conclusion

In conclusion, we have demonstrated the selective metallization on insulator substrates using femtosecond laser direct writing followed by successive electroless copper plating. The geometrical parameters of the obtained copper microstructures can be controlled by adjusting the parameters of the laser direct writing and/or the chemical plating process. These metallic microstructures show good electrical conductivity and strong adhesion. In comparison with the conventional approaches of selective metal deposition on the insulator surface which usually require complex lithographic process, this technology is simpler, faster, and more cost-effective. For these reasons, this novel technology has great potential for integrating electrical functions into a variety of microdevices.