Optically-controlled digital electrodeposition of thin ... - OSA Publishing

Optically-controlled digital electrodeposition of thin-film metals for fabrication of nano-devices Na Liu,1,2 Fanan Wei,1,2 Lianqing Liu,1 Hok Sum Sam Lai,3 Haibo Yu,1 Yuechao Wang,1 Gwo-Bin Lee1,4 and Wen J. Li1,3,* 1

State Key Laboratory of Robotics, Shenyang Institute of Automation, Chinese Academy of Sciences, Shenyang, 110016 China 2 University of Chinese Academy of Sciences, Beijing, 100049 China 3 Department of Mechanical and Biomedical Engineering, City University of Hong Kong, Kowloon, Hong Kong SAR 4 Department of Power Mechanical Engineering, National Tsing Hua University, Taiwan 5 [email protected] 6 [email protected] 7 [email protected] 8 [email protected] 9 [email protected] 10 [email protected] 11 [email protected] * [email protected]

Abstract: Precise micro-scale patterning of metal thin-films with semiconductor materials is a critical step in the fabrication of any microdevices. 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 thinfilms 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 OCIS codes: (160.5140) Photoconductive materials; (220.4610) Optical fabrication; (230.2090) Electro-optical devices; (230.4000) Microstructure fabrication.

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Received 27 Jan 2015; revised 12 Mar 2015; accepted 12 Mar 2015; published 19 Mar 2015 1 Apr 2015 | Vol. 5, No. 4 | DOI:10.1364/OME.5.000838 | OPTICAL MATERIALS EXPRESS 838

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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

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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 highresolution 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, speciallyprepared 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

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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

Fig. 1. (a) Illustration of the mechanism for fabricating metal electrodes utilizing the opticallycontrolled 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 computergenerated 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)

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

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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. 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 aSi: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

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.

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

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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. 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.

λD = 1.764 × 10−11 CEDL =

ε λD

T c

(1) (2)

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 microelectrodes, 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

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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.

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 aSi: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 (