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Precise micro-scale patterning of metal thin-films with semiconductor materials is a critical step in the fabrication of any micro-devices. Conventional metal patterning methods such as photolithography, laser-induced direct printing techniques, and micro-contact printing methods all have different disadvantages such as high capital equipment cost and low throughput efficiency. In this article, an optically-controlled digital electrodeposition (ODE) method for direct patterning of metal thin-films on a semiconductor substrate has been demonstrated. This method allows for dynamic patterning of custom micro-scale silver structures with high conductivity of 2 × 107 S/m in large scale within 10 seconds and could reach a smallest line width of 2.7 μm. The entire process is performed at room temperature and atmospheric pressure conditions, while requiring no photolithographic steps or metal nanoparticle inks. Utilizing this direct structural formation technique, a bottom-up protocol for rapidly assembling nanowire-based field-effect transistors has been demonstrated, which shows that this novel technique could potentially become an alternative, low-cost and flexible technology for fabricating integrated nano-devices.
© 2015 Optical Society of America
1. Introduction
Both semiconductor layers and metal electrodes are necessary components of micro/nano scale devices such as sensors, solar cells, and field-effect transistors. The ability to rapidly design and cost-effectively pattern micro metal electrodes along with semiconducting materials is vital for fabricating novel devices in micro/nano scale, and has attracted considerable attentions from researchers worldwide. In the past few decades, conventional metallization techniques based on thermal deposition, sputtering, and photolithographic patterning process for fabricating micro-scale electrodes have been well developed. However, these techniques still suffer from the disadvantage of relatively high costs due to the requirements for specialized equipment and complex manufacturing processes such as multiple masking and etching steps. Consequently, many researchers are now exploring technologies that are able to directly write or print conductive micro structures at atmospheric pressure and room temperature.
Direct patterning methods based on metal nanoparticle inks—such as screen printing [1–3], micro-contact printing [4, 5], nanoimprinting [6, 7], inkjet printing [8–10], laser-induced printing [11–13], and electrodeposition patterning [14–16] have been found to be particularly useful in fabricating electrically conductive, micro-scale structures on a substrate. However, each of these methods has its own limitations. Screen printing is a low-cost, large-area fabrication process, but yields poor resolution (50 – 150 μm). Inkjet printing can digitally fabricate metal electrodes on a substrate in one rapid step, but it also suffers from low resolution (a few tens of micrometres) due limitations in nozzle size and droplet formation. In comparison, techniques based on micro-contact printing and nanoimprinting can realize high-resolution conductive structures, but master molds, fabricated using a conventional photolithography process, are still necessary in these techniques. Recently, direct metal patterning via a femtosecond laser has been proposed as an alternative maskless, non-vacuum, low temperature method for fabricating high resolution silver electrodes [17–19]. However, this technique is still relatively complex, because a metal nanoparticle ink must be spin coated onto the substrate first and then all non-melted particles need to be washed away using a special solvent. The delicate, patterned structures might be damaged during this washing process. Additionally, this technique is also time-consuming, especially over large areas since it is a serial process. Light-guided electrophoresis, another technique that is capable of fabricating arbitrary patterns of nano-materials, has also been reported recently [20]. However, photo masks are still necessary with this recent technique. In general, specially-prepared inks that must satisfy specific viscosity and surface tension constraints are required in all these techniques. Thus, manufacturing cost is further increased as these inks can be only obtained after a series of complicated chemical reactions.
On the other hand, electrodeposition is a widely-used method for manufacturing metal films [21, 22] and alloy films [23] due to its relatively low-cost and adjustable film thickness. For example, the electrodeposition of gold has been commonly used to metalize contact pads in microelectronic devices [24]. However, patterning metal film utilizing traditional electrodeposition is also limited by the same disadvantages associated with metal evaporation and sputtering processes, i.e., lithographic and chemical etching steps are required to define the metal film patterns.
In order to address these limitations of existing methods, we present, herein, a maskless rapid and bottom-up metal patterning method called optically-controlled digital electrodeposition (ODE). Compared to other method for patterning metal film, this proposed technique is capable of fabricating arbitrary, micro-scale, metal structures directly onto a semiconductor surface at room temperature and at atmospheric pressure conditions, while requiring no mask-based lithography process or metal nanoparticle inks. Moreover, utilizing this method, silver electrodes owning high electrical conductivity and arbitrary shapes were fabricated in a rapid speed. Furthermore, taking advantage of this method, micro/nano logic devices such as nanowire-based field-effect transistors could be assembled rapidly, and hence this method could potentially become an alternative, low-cost and flexible technology for fabricating integrated nano-devices in the future.
2. Methods
2.1. Materials
Silver nitrate (≥ 99.8% purity) was dissolved into deionized water to obtain silver nitrate solutions with concentrations of 10 mM, 50 mM and 100 mM, respectively. For assembling nanowire-based field effect transistors, CuO nanowires were synthesized through thermal oxidation of copper (Cu) foils (1 cm × 1 cm) on a hotplate at 550 °C for 12 hours in stationary air [25]. Before heating, the Cu foils were pre-treated using the following steps: 1) sonicated for 10 minutes in an acetone solution to degrease; 2) rinsed twice, for 1 min each time, in hydrochloric acid and deionized water to remove any surface oxides. After thermal oxidation, the Cu foil with grown CuO nanowires were immersed in 2 ml 99% ethanol. A three-second sonication was performed to release the CuO nano wires into the ethanol for further application.
2.2. Experimental system
As shown in Fig. 1, a typical ODE chip is a microfluidic chip consisting of a photoconductive lower substrate, a microfluidic chamber for containing and transporting the electrolyte, and an ITO (indium tin oxide) upper electrode. The photoconductive electrode is composed of a hydrogenated amorphous silicon layer (a-Si:H) deposited on an ITO-coated glass substrate by means of a plasma-enhanced chemical vapour deposition (PECVD) process. Specifically, the height of the micro fluidic channel is 50 μm, the thickness of the ITO film is 120 nm and the thickness of the a-Si:H film is 1 μm. To drive the electrodeposition process on the a-Si:H surface in the microfluidic chamber, a sinusoidal signal generated from a signal generator (Agilent 33522A, U.S.A.) is applied between the top and bottom ITO electrodes. When patterning a thin-film metal, the ODE chip containing a solution of metal ions, such as silver nitrate and copper sulphate, is fixed on a three-dimensional movable platform. Image patterns designed using commercial personal computer software (Flash 11, Adobe, US), are projected onto the a-Si:H substrate via a digital LCD projector (Sony VPL-F400X, Japan) through a condenser objective (Nikon 50X/0.55). An image acquisition module, including a charged coupled device (CCD; DaHeng Image DH-SV1411FC, China), a microscope (OPTEM Zoom 160, USA), and a personal computer equipped with an image acquisition card, was used to monitor the associated silver micro-structure deposition processes and FET device assembling process in real time.
Fig. 1 (a) Illustration of the mechanism for fabricating metal electrodes utilizing the optically-controlled digital electrodeposition (ODE) technique. As shown, the light patterns are projected onto an a-Si:H substrate, which creates localized regions of high electrical conductivity. Around the illuminated a-Si:H surface, metal ions in the solution are reduced and deposited on the substrate to create the metal electrodes. (b) Illustration of the OEK experimental system for application of the ODE technique. As shown, light patterns, generated using computer software, are projected from a digital LCD projector through a condenser lens. An alternating electric field is established across the chip using a signal generator. (c) The computer-generated patterns and the corresponding silver electrodes fabricated on the chip’s surface. As shown, the green areas on the a-Si:H substrate illuminated by the square patterns have metal electrodes deposited on them once the silver ions reduction starts. (Scale bar: 50 μm)
2.3. Patterning mechanism
To begin with, the solution with metal ions was injected into the assembled ODE chip. Digitally projected images and alternating electric field are simultaneously applied to perform the pattering process. Due to the photogeneration of electron-hole pairs under irradiation, the electrical conductivity of the illuminated a-Si:H could increase by several orders of magnitude from 10−11 S/m to approximately 10−4 – 10−3 S/m. Thus, compared to the non-illuminated a-Si:H area, the incident light generates localized regions of high conductivity, resulting in the creation of localized virtual working electrodes. Under an exerted alternating electric field, a higher electrical potential could be dropped on the virtual electrode/solution interface than the non-illuminated a-Si:H/solution interface. Only upon these projected a-Si:H, metal ions on the interface are reduced in situ through trapping electrons from the a-Si:H when an alternating electric field is applied, as shown in Fig. 1(b) and 1(c). That is, the reduced metal atoms attach onto the illuminated surface and aggregate into metallic micro-structures which have the same shapes as the projected images. Thus, the shape and size of the patterned micro-structures are directly controlled by the digitally designed and projected LCD images. Within a deposition time of 10 seconds, customized metallic micro-structures could be fabricated at room temperature and atmospheric pressure. During this direct assembly process, no preformed metal inks and photo masks are required, because the patterns of the metal film are directly adjusted by the projected light images.
3. Results and discussion
3.1. Silver film
A silver nitrate solution was selected for fabricating a silver-based thin-film due to its nontoxicity and common availability. The location and shape of the silver structures are determined by the light patterns, so the number and orientation of the micro structures could all be controlled via software design of the patterns and the presence of the projected light. Utilizing ODE technique, silver electrodes with designed shapes and sizes could be rapidly fabricated in parallel, on a large scale (500 μm × 500 μm each time), on the semiconductor surface. As shown in Figs. 2(b)-2(e), electrodes with kinds of patterns could be rapidly fabricated. The minimum line width resolution could reach 2.7 μm using the current experimental system. However, this minimum resolution could be significantly improved by using a higher resolution LCD projector and an optical condenser with a higher numerical aperture. Theoretically, the minimum line width resolution of this ODE technique could reach the half-wavelength diffraction limit of visible light. Besides fabricating silver electrodes, this ODE technique is able to fabricate other metal electrodes such as copper, gold, etc., by using the corresponding metal ion solutions.
Fig. 2 (a)-(e) SEM pictures of different electrodes and conduction paths fabricated using ODE technique. As shown in the AFM-scanned measurement in (f), a conducting line with a width of ~2.7 μm can be obtained.
3.2. Effect of alternating signal
AC voltage signal plays a vital role in metal film patterning process using ODE method. In order to investigate how the applied AC voltage signal affects the reduction and deposition process during silver patterning, the entire ODE chip was modelled and analysed using an equivalent circuit, as shown in Fig. 3(a). Ra-Si:H and Ca-Si:H denote the resistance and the capacitance of the a-Si:H, respectively; When illuminated, the a-Si:H has a high electrical conductivity, thus the impedance contribution from the capacitance is negligible. CEDL denotes the capacitance of the electric double layer (EDL), which is a spatial region of finite thickness at the interface. The structure of the EDL has an important bearing on the rate of charge transfer and the processes of nucleation and growth of metallic crystals, because the electrodeposition of metal atoms takes place in the interface [26]. Rct and ZW represent the resistance at the interface induced by the reduction of silver ions and the transfer of silver ions, respectively. The values of CEDL, Rct and ZW are variable depending on the applied electrode voltage potential and the ions concentration. For simplification, the value of the CEDL for the AgNO3 solution could be calculated by the Eq. (1) and Eq. (2) [27]. In which, the c is the concentration of the silver ions; the ε is the permittivity of the solution, λD is the thickness of the electrical double layer, T is the temperature.
Fig. 3 (a) The equivalent circuit for the ODE chip which can be considered as a series connection of the a-Si:H film, the solid/solution interface, and the metal ion solution. The interface can be represented as a parallel connection of the electrical double layer and Zc. The figure on the right shows the numerically simulated distribution of the electrical potential in the 100 mM silver nitrate solution when a 50 kHz alternating signal with amplitude of 10 Vpp is imposed. (b) Simulation result shows the impedance in each section of the equivalent circuit as a function of the input electric field frequency across the ODE chip. Here, the thickness of the solution layer and the a-Si:H is set at 50 μm and 1 μm, respectively. The area of each section is set as 1 cm2 to simplify the calculation. The conductivity and relative permittivity values of the 100 mM solution are 1.1 S/m and 78, respectively. The conductivity of intrinsic and illuminated a-Si:H is 10−11 S/m and 10−3 S/m, respectively. The relative permittivity of the a-Si:H is 11. (c) Optical images of the silver patterns deposited under different AC electric fields using a 100 mM silver nitrate solution. As shown, silver electrodes fabricated using low frequency (<50 kHz) electric fields could not form the square shapes as following the illumination patterns. And, no metal electrodes could be formed if a DC electric field is used. The results indicate that AC electric field with frequency of 50 – 100 kHz and amplitude of 5 – 10 Vpp is optimum for fabricating silver electrodes using the ODE technique. (Scale bar: 50 μm)
RL and CL represent the resistance and capacitance of the silver solution. The effect of CL is negligible due to the high conductivity of the electrolyte.
Based on this circuit, the distribution of the potential in the ODE chip containing a 100 mM silver nitrate solution was numerically simulated using a commercial finite element method software package (Multiphysics, COMSOL AB, Sweden). As shown in Fig. 3(a), the colours indicate the distribution of the electrical potential while the arrows indicate the amplitude and direction of the electric current when fabricating silver patterns under an AC voltage with amplitude of 10 Vpp and a frequency of 50 kHz. This result suggests that the voltage dropped on the solution atop the illuminated a-Si:H is larger than the voltage potential dropped on solution atop the non-illuminated areas. The current in the solution above the illuminated a-Si:H is also larger, which means there are more electrons available for the reduction of the silver ions. This has explained why the metal ions in the solution are only reduced near the surface of the illuminated a-Si:H when the AC voltage is applied. Figure 3(b) illustrates the contribution to the impedance from each part as a function of the applied frequency when using a 100 mM silver nitrate solution. According to the analysis of the equivalent circuit, both the amplitude and the frequency of the AC electric field affect the metal deposition process; this is due to their direct impact on controlling the electrical potential on the EDL layer, because silver deposition only occurs when the potential dropped across the solution/a-Si:H interface is large enough to reduce the silver ions in the solution.
In order to investigate the optimum alternating electric field for fabricating silver micro-electrodes, the patterning process under various sinusoidal signals with different amplitudes and frequencies were monitored and analysed in real-time. Optical images of the silver patterns after 30 seconds of deposition under different electric fields, using a 100 mM silver nitrate solution, are displayed in Fig. 3(c). The results suggest that electrical fields in the frequency ranging from 50 – 100 kHz and amplitude ranging from 5 – 10 Vpp are optimal for fabricating well-defined silver micro-structures. Low frequency electric fields cannot fabricate stable silver structures with well-defined edges. Also, frequencies much higher than 100 kHz are not desirable. This is because, as analysed above, the potential drop across the EDL decreases with increases in the signal frequency, thus higher voltage amplitude is required to activate the reaction at high frequencies. It is important to note that a direct current, as used in the traditional electrodeposition process, cannot be used for the ODE technique. The metal ions would be reduced across the entire surface of the a-Si:H substrate due to the strong electrode polarization under a direct current, i.e., the electric field at the interface of the non-illuminated a-Si:H/solution would also reduce the silver ions.
3.3. Adjustable thickness
The thickness of the metal electrodes could be controlled by the deposition time (i.e., the duration that the alternating electric field is applied). According to Faraday’s law of electrolysis show in Eq. (3), the number of moles of a substance (e.g. nsilver) produced on an electrode during an electrochemical reaction is directly proportional to the number of electrons transferring from the electrode (Q), where the z denotes the valence of the silver ions and F denotes the Faraday constant.
Since an alternating current is used in the ODE process, silver ions are only reduced on the a-Si:H substrate, when it works as a cathode. The amount of silver ions reduced during any elapsed time period could be calculated by Eq. (4), in which the jave represents the time-averaged electric current density, A represents the area of the light patterns, and t is the deposition time.
Considering Eq. (5) and Eq. (6), we can obtain a relationship between the deposition time and the current density, as shown in Eq. (7). Where the Nsilver, msilver, Vsilver, ρ and h are the molar mass, mass, volume, density and thickness of the silver, respectively.
Therefore, if the electric field frequency and amplitude, and silver ion concentration used in the ODE process are constant, the value of jave will be also constant. And then the thickness of the micro-structures is completely determined by the deposition time.
Figure 4 demonstrates that the conclusion matches well with the experimental results. Based on this ODE technique, a series of 4 × 4 arrays of 30 μm × 30 μm silver electrodes were fabricated using elapsed times of 10, 20, 30, 40, 50, and 60 seconds, respectively. As shown in Fig. 4(a), thickness of the silver structure increase as the deposition time increases. The processes were replicated using solutions with concentrations of 10 mM, 50 mM and 100 mM. The voltage across the ODE chip was kept at 10 Vpp and 50 kHz during these fabrication processes. The thickness of each fabricated electrode was measured using an atomic force microscope (AFM). The measured results shown in Fig. 4(c) reveal that the thickness of the electrodes is linearly proportional to the deposition time, which also suggests the deposition process is mainly dominated by the electrode potential rather than the mass transfer process. The results also demonstrate that the speed of silver deposition is related to the initial concentration of silver ions. That is, a higher solution concentration facilitates a more rapid deposition of silver electrodes. This result can be explained by the fact that discharging current is larger in the solution with a higher silver concentration. As shown, the thickness of the electrodes could reach over 2 μm after 60 seconds under a 50 kHz AC electric field at 10 Vpp using a 100 mM silver nitrate solution. The surface roughness of the fabricated silver electrodes was also characterized using AFM and SEM, as shown in Fig. 4(b) and Fig. 4(d). The results indicate that the silver structures have a flat surface and the roughness of the silver electrodes could be decreased by lowering the deposition speed.
Fig. 4 (a) SEM images showing growth of the silver electrodes as a function of time (after 10, 20, 30, 40, 50, and 60 seconds; scale bar: 10 μm) (b) The surface roughness of an electrode as characterized using an AFM and an SEM. (c) Silver electrode thickness as a function of deposition time and solution concentration when the AC electric field is 50 kHz and amplitude of 10 Vpp was used. These results demonstrate that the thickness of the silver pattern is linearly proportional to the deposited time and the deposition speed also increases with the increase of solution concentration. (d) Surface roughness of the silver electrodes as a function of the deposition time and solution concentration.
3.4. Characterization of electrical conductivity
It is worth noting that metal electrodes fabricated by this ODE technique do not require any post-process sintering in order to obtain a high electrical conductivity. As demonstrated before, the thickness of the silver structures increases with the increase of the deposition times. In order to characterize the conductivity of the silver films, square structures of 130 μm × 130 μm were fabricated at 10s and 30s using a 100mM solution. Using the Van der Pauw method [28], four-point probes were applied to measure the sheet resistance (Rs) of these silver structures (Fig. 5(a)) using an analytical probe station (Everbeing DB-8, Taiwan). Based on the measurements shown in Fig. 5(b), the resistivity of the patterned silver can be calculated using the Eq. (8), in which the ρ is the resistivity and h is the thickness of the silver film. The patterned silver has a measured conductivity of 2 × 107 S/m, which is on the same order of magnitude as that of bulk silver (6.2 × 107 S/m). Additionally, the patterned silver films have an excellent mechanical flexibility, i.e., the electrodes did not break even when deflected over a wide range.
Fig. 5 (a) Schematic of the Van der Pauw method for measuring the sheet resistance. (b) The measured sheet resistance of the silver film with different thickness, the inserted Figure shows the SEM picture of a silver film with a size of 130 μm × 130 μm.
3.4. Assemble nanowire-based FET
Controlled bottom-up metal-film patterning process based on this ODE method could make it very convenient to assemble micro devices such as micro sensors, solar cells and field-effect transistors. Utilizing ODE technique, sensor elements such as nanowires, nano particles and CNTs can be manipulated and assembled utilizing the optically-controlled electrokinetics [29–31]. Previously, the completion of such a device would traditionally require a photolithography step to fabricate the metal electrode [32]. The combination of the optically-induced electrokinetics and the optically-controlled electrodeposition could lead to integrated fabrication of micro devices. Herein, in order to demonstrate the feasibility of the technique, copper oxide (CuO) nanowire-based FETs were fabricated as an example. The process flow is illustrated in Fig. 6(a), i) inject solution containing nano materials, e.g. CuO nano wires, into the microfluidic space in the ODE chip, ii) assemble the CuO array using optically induced electrokinetics, iii) replace the CuO nanowire solution with silver nitrate solution and fabricate silver electrodes via ODE on the CuO nanowire in situ, to assemble a field-effect transistor, iv) In the FET, the patterned silver electrodes are the source electrode and the drain electrode, the CuO nanowire is a p-type semiconductor, the a-Si:H functions as dielectric film, and the ITO electrode functions as a gate electrode. Figure 6 shows the SEM picture of the assembled FET. The inset figure shows the connection between the silver electrode and the CuO nanowire. The characteristic IV curve output has been measured using a semiconductor parameter analyser (Agilent 4155C, U.S.A). As shown in Fig. 6(c), IDS is the current passing through a single CuO nanowire, VDS is the voltage across the source and drain electrodes, and VGS is the voltage drop between the source and gate electrodes. In a FET, the VGS induces an electric field and affects the resistance of the CuO. To investigate the effect of VGS on the IDS, the values of IDS was measured under applied VDS ranging from 0 to 25 V and VGS ranging from −30 V to 20 V in increments of 10 V. The result demonstrates that a p-type FET was fabricated using ODE chip. CuO is a p-type semiconductor and the main charge carriers are holes. As the holes concentration in the CuO nanowire decreases when the VGS decrease, its resistance increases. Vice vasa, the concentration of the holes increases when the VGS is negative, causing the nanowire resistance to decrease.
Fig. 6 (a) The process for successful fabrication of an integrated CuO nano wire-based field effect transistor, i) The CuO nanowire solution was injected into the ODE chip, ii) manipulating the CuO nanowire utilizing optically-controlled electrokinetics, iii) patterning a silver film on the CuO nano wire in situ, iv) a schematic side view of the CuO nanowire-based FET. (b) SEM image of the fabricated FET, the inserted figures show the CuO nanowires and the detailed features of the connection between the silver and CuO, respectively. (c) The output characteristic curves, IDS-Vds, with the effect of VGS, which indicates that the electrical conductivity of the CuO increases as the VGS decreases.
4. Conclusion
We have successfully demonstrated a novel, bottom-up, metal electrode fabrication process based on optically-controlled digital electrodeposition. This proposed technique was demonstrated by fabricating custom-designed, silver micro-electrodes rapidly without using a silver nanoparticle ink, conventional vacuum deposition techniques, or microlithographic masks. Micro-electrodes with customized geometries, high conductivity of 2 × 107 S/m and a smallest line width of 2.7 μm, were directly fabricated onto a substrate in a digitally-controlled manner at atmospheric pressure and room temperature conditions. Furthermore, the process combining this method with optically-induced electrokinetics for fabricating nanowire-based FET has been demonstrated, which could potentially become an alternative manufacturing process for integrated nano devices in the future. Moreover, since this method enables direct fabrication of metal films onto a semiconductor film surface, our future work will investigate the direct patterning of sheet electrodes for solar cells in a low-cost and effective way.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Project No. 61302003), the NSFC/RGC Joint Research Scheme (Project No. 5141101088), the CAS-Croucher Funding Scheme for Joint Laboratories (Project No. 9500011), and the CAS-FEA International Partnership Program for Creative Research Teams.
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