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Laser Sweeping Lithography: Parallel Bottom-up Growth Sintering of Nanoseed-organometallic Hybrid Suspension for Eco-friendly Mass Production of Electronics Jinho Yun, Minyang Yang, and Bongchul Kang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04468 • Publication Date (Web): 22 Feb 2018 Downloaded from http://pubs.acs.org on February 24, 2018

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ACS Sustainable Chemistry & Engineering

Laser Sweeping Lithography: Parallel Bottom-up Growth Sintering of Nanoseed-organometallic Hybrid Suspension for Eco-friendly Mass Production of Electronics

Jinho Yun†§, Minyang Yang†* and Bongchul Kang††*§

† Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Daejeon, 34141, Korea

†† Department of Mechanical System Engineering, Kumoh National Institute of Technology, 61 Daehak-ro, Gumi, 39177, Korea *Corresponding authors. [email protected] (Tel. 82-42-350-3224, Fax 82-42-350-3210) and [email protected] (Tel. 82-54-478-7400, Fax. 82-54-478-7319) §

J.Y and B.K. contributed equally to this work.

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ABSTRACT We present a novel laser sweeping lithography (LSL) process, revolving around an ecofriendly mass production method, to fabricate conductive patterns in parallel with no restriction on the type of substrate. Particle-free organometallic solution is reformulated inprocess into a nanoparticle/organometallic hybrid suspension, via an incomplete thermal decomposition using radiative heating. The growth sintering undergoing a series of ion precipitation, clustering, growing, and agglomeration procedures is then initiated by irradiating a line-modulated diode laser of a near infrared wavelength through a thermally enforced laser mask on the hybrid suspension. This leads to the concurrent parallel production of silver conductors with a high-conductivity (2.9 µΩ·cm), -durability, and resolution of 5 µm on the corresponding to mask openings, without the need of any additional steps and corrosive chemicals. This method is highly effective for large-area fabrication of high-density electronics, as the production time proportionally decreases with increased pattern density and area, compared to conventional laser fabrication methods based on a single laser spot. Therefore, the LSL process is suitable for eco-friendly mass production of various electronic devices in industrial environments.

KEYWORDS:

Laser

sweeping

lithography,

Growth

Nanoparticles, Parallel printing.


sintering,

Organometallics,

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Introduction Photolithography is widely used as the main electrode patterning process for the mass production of microelectrodes because of its high-resolution and fidelity, outstanding throughput, well-verified mechanism, and high controllability. However, it has critical limitations, such as the complicated steps caused by the indirect lithography mechanism, high vacuum facilities required to deposit materials for patterning, and need of corrosive and harmful etchants [1]. All of the aforementioned factors result in a high initial investment, large emission of pollutants, and high production costs. To resolve these issues, numerous alternative fabrication methods have been studied in various science and engineering fields [2-4]. Unlike the typical photolithography based on indirect patterning using photoresist, alternative printing method involving inkjet and roll printing, which is based on a direct patterning regime, is able to directly deposit functional metal solutions on the necessary area of substrate in air [5-8]. This printing methods also have limitations for the practical mass production of microelectronics, as industrial printing methods have lower resolutions than conventional photolithography [5,6]. Even if advanced printing methods, such as electrohydrodynamic-inkjet printing, improved the resolution of less than 10 µm [9,10], the production time increases by square of the increase of printing area and pattern density due to the nature of serial printing regime (e.g. inkjet printing); the costly engraved rolls and consumable components required for the parallel printing regimes of roll printing increase production costs and hinder process flexibility [11-13]. This limitations can be overcome by combining the printing method with other technologies, such as laser, electric discharging, and chemical treatments [14-17], resulting in the development of novel hybrid printing method. One such technique is laser-focused sintering (LFS) where the printing of metallic nanoparticle suspensions has been studied in order to overcome the fundamental limitations of the printing method. It has shown that LFS can effectively minimize the process ACS Paragon Plus Environment


complexity, as its ability to concentrate the energy is at least one order of area smaller than the printable droplet size, and both patterning and sintering (usually sequential procedures) can be performed by irradiating the laser [18-19]. However, this method also has critical limitations that need to be addressed before it can be applied in an industrial setting. For example, ultrafine nanoparticles (diameter less than 5 nm) that are required to improve the sinterability of nanoparticles, according to the thermodynamic size effect of the nanoparticles, are quite expensive and chemically unstable compared to commercially available nanoparticles (with diameter of 20~100 nm) [20]. In addition, since the production time of the LFS method also increases dramatically with increasing pattern density and entire processing area, the method is not suitable for practical mass production pursuing continuous permanence of production but is only advantages for manufacturing customized production with small batch sizes [18-20]. And the reliable patterning resolution and position accuracy is not lasting as well as is more than 20 µm, because of the accumulated system uncertainties (e.g. the motion accuracy of the laser system, instability of laser operation, and other timedependent factors) [20]. Since the expansive laser source with short wavelength of 400~700 nm was required to enhance the material absorption, configuring multiple lasers in parallel to increase the productivity of the process is neither an effective nor an economically feasible solution [18-20]. Although parallel photonic printing methods, such as intensive pulse light (IPL) sintering and laser-induced thermal desorption, was introduced, the pattern resolution was broadened by the combination effect of enhanced diffraction by incoherent light and thermal diffusion by metal nanoparticle precursor or adhesion to glass substrate weak due to film transferring under non-sintered state [21,22]. Therefore, as an alternative to conventional photolithography, a parallel laser printing method based on a low-cost material and energy source is required to enable the eco-friendly large-scale production of durable high resolution electronics without the use of vacuum/etching processes, photoresist, and complicated


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process steps. In this paper, a laser sweeping lithography (LSL) process is presented as a novel sustainable mass-production method for fabricating conductive micropatterns in parallel on any types of substrate. By linearly scanning a line-modulated laser with a near-IR (NIR) wavelength

though

a

thermally

enhanced

reusable

laser

mask

to

a

coated

nanoparticle/organometallic hybrid suspension, which is reformulated in-process from a particle-free organometallic solution, various silver micro-conductors with controllable specifications can be concurrently formed by the parallel growth sintering following a series of precipitation, clustering, growth, and sintering interactions. It is possible to continuously replicate the micro-electrodes corresponding to the open spaces in the mask on various substrates including glass and polymer films without additional processes.

Materials and methods Fabrication of reusable laser mask. A 3-inch quartz wafer was used as the substrate of the designed mask. A chrome layer of 10 nm, which was used to improve interfacial adhesion, was sputtered onto the wafer. The chrome layer does not affect the transmission of the laser passing through the mask due to its small thickness (10 nm) than the laser penetration depth. Next, a silver masking layer of 100 nm thickness was deposited on the chrome layer using an e-beam evaporator. Thereafter, the prepared specimen was followed by a photolithography and reactive ion etching in order to prepare an open mask area corresponding to the predesigned pattern (which included micropatterns with a resolution varying from 5 µm to 100 µm). To form a SiO2 pellicle layer, Si was deposited using a chemical vaporization deposition process and then thermally oxidized to SiO2 at 300 °C. The mask specimen was baked at 600 °C for 60 min to enhance the interfacial adhesion between the substrate of laser mask



(quartz) and the protective silica layer. This was achieved through the interfacial merging between neighboring atoms induced by thermal diffusion [23,24]. Specimen preparation. A soda-lime glass slide of 1 mm thickness and a polyimide film of 100 µm thickness (SKC Co.) were used as substrates. Organometallic solution (CO-011, Inktec Co.) of a silver weight ratio of 10 % is composed of a chelate bonding structure of a silver ion and carbamate-amine complex, which is formed by another chelate bonding between carbamate and amine [25]. The organometallic solution is transparent because the silver ion completely dissolved in organics. As the chelate bonding is weak, the bonding could be easily broken by thermal energy. The transparent organometallic solution was spun on the substrates at a rotation rate of 1000~3000 rpm. The solution-coated substrate was irradiated with an incoherent infrared source of a power of 300 W by linearly translating the light source (velocity: 10~50 mm/s). Here, the thermal treatment condition was determined by adjusting the distance between the light source and substrate, the optical power and the translation rate. Laser sweeping lithography. The pre-fabricated laser mask was placed on the prepared specimen. Here, the assembly was clamped by applying light load along the edge of the mask in order to maintain close contact between the mask and substrate. A line-modulated continuous wave (CW) diode laser (LIMO Gmbh.) of a wavelength of 930 nm and a maximum power of 500 W was used as the light source in the LSL method. The laser spot had a linearly uniform intensity distribution, which is characterized by a 12 mm length along the fast axis and a 0.1 mm width along the slow axis, respectively. This laser spot was placed on the mask assembly, as shown in Fig. S2, and was used to perpendicularly irradiate the specimen assembly. The entire area of the assembly was covered by scanning the linear spot in zigzag patterns through synchronous control of a three axes linear motion stage. Here, the


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laser power and scanning rate were tuned in accordance with the solution thickness and the type of substrate. A CW fiber laser (IPG Photonics Gmbh.) of a wavelength of 1070 nm, which was used for a light source of the counter method (LFS), was focused to 25 µm in diameter by employing two galvanometer mirrors and an f-theta lens of 163 mm focal length. The patterns were fabricated by varying a laser power at a scanning rate of 25 mm/s. Post-processing. After detaching the mask from the substrate, the specimen was immersed in an n-hexane solution in an ultrasonic bath, in order to remove the complex not irradiated and modified by the laser. This was followed by drying the sample in nitrogen. To further improve the conductivity of fabricated conductor, the specimen was baked at 250~300 °C, depending on the thermal resistance of the substrate, for 10 min on a hot plate.

Results and discussion The original organometallic solution does not contain any solutes, as shown in the transmission electron microscopy (TEM) inset of Fig. 1-(a), but it completely decomposed into solid silver and evaporable organics owing to sufficient thermal input and time [26]. A nanoparticle/organometallic hybrid suspension is intermediately formed during this procedure, as shown in the inset of Fig. 1-(a). Since this transient phase involves the growth-limited ultrafine nanocrystals of 2~3 nm in size, the optical absorption dramatically increases throughout the ultraviolet (UV) to the NIR wavelength ranges, and a surface plasmon resonance peak at a wavelength around 500 nm emerged, as shown in the inset of Fig. 1-(a) [27]. Thus, the particle-free organometallic solution is reformulated in-process to a nanoparticle/organometallic hybrid suspension to enhance the absorption efficiency of laser irradiation through a uniform thermal treatment induced by the radiation of an incoherent infrared (IR) source, as shown in Fig. 1-(a). Various silver micro-conductors corresponding to



mask opens are created in parallel by irradiating the hybrid solution with a line-modulated diode laser through a thermally enhanced reusable laser mask, as shown in Fig. 1-(b). Even if a photomask with multilayered dielectric structures could be used with a surface plasmon mode laser, a laser mask based on single layered silver, which is depicted in Fig. 1-(b), is more preferable for the fabrication of the microelectrode because of its low fabrication cost and practical limitation on fabricating mask opens below 20 µm [28]. Thickness of the silvermasking layer was carefully determined by considering the optical density (OD) for a laser wavelength used in the laser sweeping lithography (LSL). An OD of more than 2, which corresponds to less than 1 % of the total incident radiation, was estimated for film thickness above 53 nm (Fig. S1 in Supporting Information). Protective layer is needed for the repetitive and reliable reuse of the laser mask. Silica, which was chosen as a protective layer due to its high hardness, inertness with organics, and heat resistivity, was additionally deposited on the laser mask to protect the masking layer from any physical damage and chemical contamination that could occur whenever repetitively mounting the mask on the solutioncoated substrate. The thickness of the silica layer was also determined because it critically affects the final fidelity of patterning process. Although the maximum thickness of the silica layer is limited by the laser diffraction from the narrow openings of 5 µm in the mask [29,30], it should also be larger than the heat diffusion length [31,32]. According to theories related to Fresnel diffraction and thermal diffusion, the thickness corresponding to a near-field diffraction zone is approximately 980 nm, which is identical to the laser wavelength, but the minimum thickness to insulate a thermal diffusion was calculated to be 825 nm (see Eq. S1 and S2 of Supporting Information). As a result, the thickness of the silica layer was determined to be 900 nm. The ultrafine nanocrystals, which were formed in the hybrid suspension via incomplete thermal decomposition, work as nanoseeds to trigger the thermal decomposition of the ACS Paragon Plus Environment

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adjacent organometallic matrix when the laser irradiates the hybrid suspension, as depicted in Fig. 1-(c). The Ag nanoseeds are preferentially heated by absorption of the laser energy, and then the organometallic matrix adjacent these is decomposed simultaneously and collectively. Here, the Ag ions that precipitated from the solution matrix gather around the nanoseeds and are tightly grouped. Through a series of this process, the Ag clusters are thermally grown, Ag grains are formed, and a highly densified polycrystalline Ag film is finally created, as depicted in Fig. 1-(c). In addition, residual volatile organics, such as decomposed organics and solvents, are evaporated and removed from the Ag film during the laser irradiation. As this growth sintering process involves a series of precipitation, clustering, growth, and sintering steps, it is certainly different from conventional laser sintering methods that use thermal diffusion of achieve the interfacial aggregation of adjacent metal nanoparticles [33]. In addition, the VIS wavelength lasers used by conventional laser sintering methods correspond to the plasmonic resonance mode of metal nanoparticles, and are therefore not suitable for the growth sintering method, as they could induce the explosive evaporation of organics in the organometallic matrix. Furthermore, laser irradiation of the plasmon oscillation mode of a VIS wavelength causes thermal damage to the silver-based photomask as silver NP is very good absorber of visible radiation. Therefore, it was concluded that an NIR laser was required to maintain the stability of the sequential thermal phase transition procedure during the growth sintering operation and to prolong the lifetime of the laser mask. Fig. 2-(a) demonstrates the real experiment setup for performing the LSL and the pattern sample fabricated on glass. The linearly focused beam with a width of approximately 12 mm was rapidly swept onto the reusable laser mask (4 inch diameter). As a result, the highly complicated patterns with various widths had been created in 20 sec on a glass slide with 4 inch in diagonal (Supporting Movie). The pattern width generated by the LSL method depends on the incident laser power and scan rate, both of which determine the heat flux per ACS Paragon Plus Environment


unit volume. The width of the pattern usually broadens with increasing a laser energy input per unit time, as shown in Fig. S4. The excessive heat flux, which is generated by the intensive laser absorption of the nanoparticles, causes substrate damage such as carbonization, ablation, and thermal distortion. However, the patterns can be delaminated from the substrate during the washing procedure if the flux is lower than that required for sintering the nanoparticles, as shown in the insets of Fig. 2-(b). As the margin for the normal operation of laser sintering is quite narrow at a low scan rate of less than 10 mm/s, it is difficult to maintain stable processing conditions for a long period. However, because the processible range gradually becomes wider as the scan rate increases, a high scan rate of 50 mm/s is preferred for high process stability and productivity. Fig. 3-(a) shows the variation of resistivity of LSL-processed pattern, which was fabricated at a laser power of 115 W and a scan rate of 50 mm/s, depending on post-baking temperatures. The resistivity of the LSL-processed silver pattern was 15.2 µΩ·cm, which is approximately three times higher than that of LFS method (5.8 µΩ·cm) [34,35]. Here, the LFS method used a laser with a wavelength of 1070 nm, which is 140 nm longer than the 930 nm wavelength used in the LSL method. Irradiation of a long laser wavelength (1070 nm in LFS) results in the relative long time required to complete the growth sintering due to relatively low laser absorption. This slow growth sintering of LFS densifies the structure and increases the grain size. In contrary, the relatively high resistance of LSL using a short laser wavelength was caused by the increased number of grain boundaries and the small grain size (less than 100 nm) owing to the limited thermal growth of the silver nanoclusters, as shown in the inset of Fig. 3-(a) [36]. When the power of the incident laser increased to enhance thermal growth and improve electrical conductivity, the mask layer experienced damage, meaning that its lifetime would decrease dramatically. In addition, excessive thermal diffusion resulted in the formation of patterns outside the desired region, as shown in the inset of Fig. 2-(b). Because ACS Paragon Plus Environment

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of these negative effects, it is recommended that the laser power be limited to within a specific allowable level, so that the film resist delamination by tolerating hydraulic drag forces, and the cohesive force between the silver crystals to prevent their re-dispersion into the solution during the washing process. The final resistivity was further improved by a simple post-baking procedure after the washing and drying process, as shown in Fig. 3-(a). The post-baking temperature should be optimized considering the glass transition temperature of the substrates used, in order to prevent permanent distortions such as a warpage. Consequently, the resulting resistivity of 2.9 µΩ·cm, which is noteworthy and comparative to that of typical printing methods and close to the of bulk silver, was achieved by heating the sample to 300 °C for 10 min. This large decrease in resistivity resulted from the formation of larger grain in the range of 200~300 nm and a corresponding decrease in the number of grain boundaries, as shown in the inset of Fig. 3-(a) [11,37]. The morphology of the fabricated pattern is also an important factor to consider when evaluating the feasibility of LSL. Fig. 3-(b) compares the cross-sectional profiles obtained using the LSL method and the conventional laser sintering method based on a single laser spot. The LSL method shows more uniform profiles than those obtained using the conventional method due to the occurrence of horn-like edge. This accounts for the discrepancy in the distribution of the laser intensity between the two methods. In LSL method, the intensity of laser passing through the opens in the mask is spatially uniform and has flat distribution; however, in a typical laser sintering method, most of the laser energy focused on a spot is concentrated on the transversal axis, because of its spatial mode with a Gaussian distribution. In terms of the thermally induced flow dynamics of the hybrid complex studied here, the lateral temperature gradient along the surface, which occurred when a focused laser spot was used, is steep in comparison with the gradient observed in the LSL method, as shown in Fig. 3-(c). This produces a significant surface tension gradient, and, in turn, induces ACS Paragon Plus Environment


an outward Marangoni flow from the center of the spot to the edge of the border [35,38]. This crater-like profile caused by the thermal gradient-induced flow is usually observed when the typical laser sintering methods are employed. However, in terms of topography, the LSL method resulted in a rougher surface than the other method (i.e. root mean square roughness of LSL: 15 nm; FLS: 7 nm). This is attributed to the different laser wavelengths used. The surface became smoother when the wavelength increased, since both the growth sintering interaction and organic evaporation progress in a more stable manner due to the relatively low absorption property of longer wavelength [34,35,39]. Therefore, irradiating a laser with a 1070 nm wavelength on the hybrid suspension is suitable for achieving an optically smooth surface according to the laser sintering of hybrid suspension [35]. However, the scanning rate of the typical laser sintering method using a 1070 nm wavelength is limited to 25 mm/s; otherwise, the LSL method improves the scan rate and sintering efficiency because shorter wavelength leads to higher laser absorption by the hybrid suspension. However, a more uneven surface is inevitably generated by the explosive evaporation of surface organics, therefore the surface roughness increased. Nevertheless, the LSL method is appropriate for the fabrication of electrodes since the roughness produced by LSL is comparative to that obtained using typical printing methods of metal nanoparticle suspensions and is available level for most thin film applications, such as solar cell, and displays. [6,40-41]. Fig. 4 shows the comparison between the patterns on the mask and the corresponding LSLprinted patterns. When a mask with a minimum pattern width of 5 µm was used, the width of the corresponding printed pattern was 5.3 µm, as shown in Fig. 4-(a). The difference between these two values is approximately 6 %, which is much smaller than the selective sintering method using an intensive pulsed light (e.g. an error of 60 % in 10 µm mask open) [21]. The printed pattern agreed well with that of the mask, owing to the combination effect of low diffraction by using a coherent light source and limited thermal diffusion by low thermal ACS Paragon Plus Environment

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conductivity of the hybrid suspension. Because, however, the size error steeply increased and the patterns were disconnected intermittently when an open width of less than 4 µm was used due to the enhanced diffraction by narrower opens, the minimum resolution of LSL method exhibited 5 µm. A laser mask with arbitrary patterns, including various directional components, was applied to investigate the influence of the sweeping direction of the laser on the pattern fidelity; an understanding of this is critical for the realization of a single-step fabrication process. The position and width of fabricated patterns is also well matched with mask open, as shown in Fig. 4-(b). As a result, the LSL method enables to achieve a high resolution of 5 µm and a position accuracy of submicron, which is impossible values in typical laser direct writing methods using a high speed scanner system, and to maximize the pattern fidelity within the error level of 6 % without dependence on the scanning direction. To further improve the printing resolution and position accuracy, the laser mask should be rebuilt by employing a thinner pellicle and a more precise photolithography process. Fig. 5-(a) shows the arbitrary patterns fabricated on the glass substrate, which were finished by washing out the reminding hybrid suspension. Good adhesion between the printed pattern and substrate is required to ensure device reliability, even though achieving strong adhesion between the silver pattern and the glass substrate without the additional use of adhesion promoters is highly challenging in printed electronics [22]. The LSL-printed pattern on the glass substrate tolerated the scotch tape test up to 100 repetitions without any obvious defects. This outstanding adhesion was caused by the self-residual of the infinitesimal amount of organics at the interface between the silver layer and the glass surface, which is verified by the slight increase of intensity corresponding to carbon atom in the line-scanning energy-dispersive X-ray spectroscopy measurement (Fig. 5-(b)). Since the residual organics acts as a binder layer to physically combine the silver particles and glass surface, the LSL method can achieve the outstanding good adhesion for glass substrate without any adhesion ACS Paragon Plus Environment


promoters, which is challenging in typical printing methods including laser sintering. The amount of residual organics can be adjusted by changing the baking time to generate the nanoseeds [42]. This method can be used to fabricate flexible electronics by employing a polyimide film instead of a glass slide. Flexible conductive patterns were also produced successfully, as shown in Fig. 5-(c). The productivity of the LSL method was quantitatively compared with typical LFS method using a single spot [18,26,35]. Characteristic factors, such as the patterning area per second and the processing time for a specific substrate, were calculated as functions of the pattern density based on the experiment conditions (Figure S5 and Table S1 in Supporting Information). Here, the time required for subordinate steps, such as handling, pre-fabrication of laser mask, alignment, and post-processes, was excluded from this calculation to objectively evaluate the patterning speed between two methods. The results showed that the LSL method is more effective for fabricating high density and high-resolution patterns than the typical method. For example, the LSL can reduce the processing time for a pattern density of more than 90 % at least three orders of magnitude compared to the typical method using a laser spot of 10 µm. This productivity improvement is more significant with higher pattern densities, larger overall fabrication area, and higher required pattern resolution; hence LSL is a favorable method for industrial applications and satisfies key specifications required by practical production fields.

Supporting information The Supporting Information is available free of charge on the ACS Publications website.


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Acknowledgements This work was supported by National Research Foundation of Korea (NRFK) (grant No. 2017R1D1A1B03032268 and 2012-010307).



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D. All-inkjet-printed Flexible Electronics Fabrication on a Polymer Substrate by Lowtemperature

High-resolution

Selective

Laser

Sintering

of

Metal

Nanoparticles

Nanotechnology 2007, 18, 345202 DOI: 10.1088/0957-4484/18/34/345202. (16) Park, J. U.; Hardy, M,; Kang, S. J.; Barton, K.; Adair, K.; Mukhopadhyay, D. K.; Lee, C. Y.; Strano, M. S.; Alleyne, A. G.; Georgiadis, J. G.; Ferreira, P. M.; Rogers, J. A. Highresolution Electrohydrodynamic Jet Printing Nat. Mater. 2007, 6, 782-789 DOI: 10.1038/nmat1974. (17) Carlson, A.; Bowen, A. M.; Huang, Y.; Nuzzo, R. G.; Rogers, J. A. Transfer Printing Techniques for Materials Assembly and Micro/Nanodevice Fabrication Adv. Mater. 2012, 24, 5284-5318 DOI: 10.1002/adma.201201386. (18) Hong, S.; Yeo, J.; Kim, G.; Kim, D.; Lee, H.; Kwon, J.; Lee, H.; Lee, P.; Ko, S. H. Nonvacuum, Maskless Fabrication of a Flexible Metal Grid Transparent Conductor by LowTemperature Selective Laser Sintering of Nanoparticle Ink ACS Nano 2013, 7, 5024-5031 DOI: 10.1021/nn400432z. (19) Park, J. H.; Jeong, S.; Lee, E. J.; Lee, S. S.; Seok, J. Y.; Yang, M.; Choi, Y.; Kang, B. Transversally Extended Laser Plasmonic Welding for Oxidation-Free Copper Fabrication toward High-Fidelity Optoelectronics Chem. Mater. 2016, 28, 4151-4159 DOI: 10.1021/acs.chemmater.6b00013. (20) Kang, B.; Han, S.; Kim, J; Ko S. H.; Yang M. One-Step Fabrication of Copper Electrode by Laser Induced Direct Local Reduction and Agglomeration of Copper Oxide Nanoparticle J. Phys. Chem. C, 2011, 115, 23664-23670 DOI: 10.1021/jp205281a. (21) Kim, I.; Woo, K.; Zhong, Z.; Lee, E.; Kang, D.; Jeong, S.; Choi, Y. M.; Jang, Y.; Kwon, S.; Moon, J. Selective Light-Induced Patterning of Carbon Nanotube/Silver Nanoparticle


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Figure Captions

Figure 1. Schematics of laser sweeping lithography (LSL) process and growth sintering. (a) In-process reformulation of Ag nanoseeds-embedded organometallic hybrid suspension via radiative heating using incoherent IR source on particle-free organometallic solution. The insets are transmission electron microscope (TEM) images of organometallic solution before and after thermal treatment and spectral optical extinction of organometallic solution depending on thermal treatment. (b) Scanning of a line-modulated diode laser on an assembly of laser mask and hybrid suspension-coated substrate and selective formation of Ag electrode. The cross-sectional configuration of laser mask. (c) Left: Precipitation of Ag ions involved in adjacent organometallic matrix by selective laser absorption of Ag nanoseeds and Ag clustering by their self-migration toward the nanoseeds. Right: Formation of Ag polycrystalline film with large grain by thermal growth and aggregation of Ag clusters. Insets are the corresponding TEM images.


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Figure 2. Parametric study of parallel growth sintering. (a) Practical experimental method of scanning of a line beam laser and a specimen immediately after the laser scanning and separation of laser mask. (b) Correlations between a laser power and a scan rate in LSL process. The insets show the specimens when thermal damage and delamination occurs, respectively.



Figure 3. Characteristics of the LSL-processed pattern. (a) Improvement in specific resistance of LSL-processed conductor with respect to a post-baking temperature. The insets show the scanning electron microscope images without and with a post-baking at a temperature of 300 °C. (Scale bar: 300 nm) (b) Comparison of cross-sectional profiles between FLS and LSL method and AFM image corresponding to LSL method. (c) Schematics of the origin of surface profile discrepancy depending on distribution of laser intensity.


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Figure 4. Correspondence of LSL-processed pattern comparing mask and substrate images. (a) Microscope images of the mask opens and the corresponding patterns on glass substrate. The insets show the SEM images of the fabricated pattern. (b) Direct comparison between mask opens and printed patterns to show good position accuracy and resolution of patterning.



Figure 5. Demonstration of LSL-processed patterns on various substrates. (a) Arbitrary pattern sample fabricated on a glass substrate. (b) Scotch tape test and variations of main compositions (Ag, C) around interface using line–scanning energy-dispersive X-ray spectroscopy measurement and corresponding cross-sectional SEM image. (c) Samples using a polyimide film.


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For Table of Contents Use Only

Synopsis: Laser sweeping lithography enables the sustainable mass-production of microelectronics without the need of toxic chemicals and vacuum deposition.


Laser Sweeping Lithography: Parallel Bottom-up Growth Sintering of

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