Article pubs.acs.org/IECR
Additive Manufacturing Technologies Compared: Morphology of Deposits of Silver Ink Using Inkjet and Aerosol Jet Printing Tobias Seifert,*,†,‡,∥ Enrico Sowade,*,†,∥ Frank Roscher,§ Maik Wiemer,§ Thomas Gessner,§ and Reinhard R. Baumann†,§ †
Digital Printing and Imaging Technology, TU Chemnitz, 09126 Chemnitz, Germany Center for Microtechnologies (ZfM), TU Chemnitz, 09126 Chemnitz, Germany § Fraunhofer Institute for Electronic Nano Systems (ENAS), 09126 Chemnitz, Germany ‡
S Supporting Information *
ABSTRACT: We report about a detailed comparison of the additive manufacturing methods inkjet printing (IJP) and aerosol jet printing (AJP). Both technologies are based on the direct-writing approach enabling the non-contact deposition of various materials in flexible patterns, e.g., for printed electronic applications. The deposited pattern elements were classified as (i) drops (IJP) or splats (AJP), (ii) lines, and (iii) squares. These elements can be considered as basic elements of the deposition systems and also of printed electronics. The pattern elements were deposited with IJP and AJP using the same silver nanoparticle ink. After printing, the layers were characterized regarding their morphology by optical and topographical measurement methods as well as regarding their electrical characteristics. It turned out that drops deposited with IJP and splats deposited with AJP can have similar dimensions. However, the shapes of the deposits differ widely. In the case of lines, AJP enables narrower line widths and thinner line thicknesses in comparison to IJP. In IJP, the line morphology varies depending on the direction of the deposition. Finally, the morphology of the deposited lines determines the electrical conductivity. For printed squares, the IJP layers show much higher layer thickness and a different layer topography compared with AJP as result of a higher volume per area deposition of materials.
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Both technologies promise higher flexibility, scalability, and variability2 in comparison with traditional lithography-based technologies.2,14 Today, several publications can be found that present interesting examples of applications using IJP8,9,14−16 and also AJP3,17−20 as digital and additive manufacturing methods. AJP has been reported as a deposition technology for layers in electronic devices such as flexible displays,18 thin-film transistors, 21 circuits, 22 multilayer ceramic capacitors (MLCCs),23 and biological sensors.14,24 However, in comparison to IJP the number of scientific publications is much less. Comparisons of the two technologies concerning the characteristics of their deposits are underrepresented in the literature. Only a few publications that cover both technologies are available. Hon et al.2 and Zhang et al.25 introduced AJP and IJP as direct-writing technologies and compared them in a tablewise manner on the basis of datasheet parameters. In 2011, Kalio et al.26 evaluated the potential of IJP for fine line printing on n-type wafers in comparison with AJP. Gieser et al.27 demonstrated a first approach of combining IJP and AJP to synergize the characteristics of the two technologies without doing a comparison related to the morphology of the deposits. Hoey et al.3 reviewed research works about AJP and analyzed the physics behind different aerosol jet systems. Silver tracks
INTRODUCTION
Driven by the trend toward their steady miniaturization and enhanced complexity as predicted by Moore’s law, the related research and development of electronic and sensor devices or integrated power sources aims to optimize the involved production processes in regard to efficiency, high volume throughput, energy consumption, raw material reduction, and sustainability.1,2 Therefore, production methods such as additive manufacturing technologies, e.g., liquid deposition methods, are considered as promising alternatives to lithography-based approaches.3 Especially, direct-writing technologies such as inkjet printing (IJP) allow the precise and non-contact deposition of liquids containing functional materials3 and have attracted considerable interest during the last years. IJP is a well-known and established technology in graphical printing and is increasingly being applied in the field of printed electronics.4 Today, IJP has evolved as a trending technique to realize the manufacture of devices such as antennas,5 transistors and integrated circuits,6,7 thin-film solar cells8,9 and memories.10 Aerosol jet printing (AJP) is a new and thus less-established technology focusing primarily on applications in the area of printed electronics. The technology is considered as a potential competitor to IJP3,11 because it also allows the maskless and non-contact deposition of a wide range of functional liquids in flexible patterns.3 In contrast to IJP, AJP facilitates the use of three-dimensional nonplanar substrates in the millimeter range as a result of the focused stream of ink ejected.12,13 © 2015 American Chemical Society
Received: Revised: Accepted: Published: 769
September 17, 2014 December 11, 2014 December 12, 2014 January 8, 2015 DOI: 10.1021/ie503636c Ind. Eng. Chem. Res. 2015, 54, 769−779
Article
Industrial & Engineering Chemistry Research
Figure 1. Inkjet-printed droplets at a drop distance of 150 μm (nozzle diameter 21.5 μm) and aerosol-jet-printed splats at a splat distance of 150 μm with a sheath gas rate of 70 cm3·min−1 (nozzle diameter 200 μm).
°C and the ink was heated for both IJP and AJP to a temperature of 30 °C. The elevated ink temperature improved the drop formation in IJP because the relatively high initial ink viscosity declined. For both deposition systems, Cabot CSD 32 silver nanoparticle ink (Cabot Corporation, Boston, MA, USA) was used as received (density of 2.0 g·cm−3, solids content 45−55 wt %, viscosity 50−100 cP, all values according to the ink data sheet). Standardized 6 in. RCA-cleaned silicon dioxide (SiO2) wafers with an average thickness of 600 μm were applied as substrates in all of the printing tests. The pattern elements printed were classified as (i) single droplets or splats, (ii) lines, and (iii) squares. Single splats refer to AJP and represent the smallest pattern that can be deposited. Single droplets are the smallest element that can be ejected on a substrate using IJP. Lines and squares are formed from droplets deposited consecutively side-by-side in the x direction (lines) or in the x and y direction (squares). For a comparison of droplets deposited by IJP and splats deposited by AJP, arrays with sizes of 1.5 mm × 1.5 mm consisting of 100 droplets/ splats with a center-to-center distance of 150 μm were created for both systems. In order to print lines with IJP, the drop distance sD (i.e., the center-to-center distance of ejected drops) was varied between 5 and 150 μm in steps of 5 μm. The printing was performed in printing direction (IPD) and counter printing direction (CPD). In the case of AJP, lines were obtained by opening the shutter and uniformly moving the substrate under the active nozzle. To vary the line width, the sheath gas rate was adjusted in a range between 50 and 100 cm3·min−1 in steps of 5 cm3·min−1. Furthermore, the process velocity (corresponding to the axis motion speed) was varied among 1, 3, and 5 mm·s−1. Finally, squares with a side length of 5 mm were deposited by both IJP and AJP. A drop distance of 20 μm was chosen for IJP. The squares were developed unidirectionally in a line-by-line manner as usual for IJP. In the case of AJP, the squares were developed with the perimeter fill algorithm (PFA) and the serpentine fill algorithm (SFA). In PFA, the deposition starts from the outer border and lines are deposited continuously in the shape of size-decreasing squares moving inward. SFA is closer to the approach in IJP, but in contrast to IJP, SFA is bidirectional. The differences between the two approaches are described in more detail later on in the Results and Discussion. The deposits on the silicon substrate were characterized using a Nikon Eclipse 200 light microscope (Nikon Corporation, Tokyo, Japan). 2D and 3D profilometry was
deposited by AJP were investigated by Kopola et al.28 Mahajan et al.29 studied the influence of process parameters on the geometry of aerosol-jet-printed silver lines. Choppali et al.30 investigated aerosol-jet-printed line widths of zinc oxide as a function of the applied impact exhaust flow rate and atomizer gas rate. For IJP, a detailed morphology investigation of polymeric deposits was done by Soltman and Subramanian.31 Reflecting the existing literature, a comparison based on a comprehensive analysis of the two technologies under defined process conditions (using the same ink, substrate, and comparable processing conditions) has not yet been reported. Therefore, the focus of the present research work is set on the differences and similarities between AJP and IJP and their deposited layers. Furthermore, we also consider the influence of the layer morphology on the electrical performance to indicate application areas of IJP and AJP.
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EXPERIMENTAL SECTION IJP was performed using a drop-on-demand Dimatix Materials Printer 3000 (DMP-3000) from Fujifilm Dimatix (Santa Clara, CA, USA). The printer was equipped with a 16-nozzle development printhead (Fujifilm Dimatix DMC-11610) with a nominal droplet volume of 10 pl. The nozzle diameter was about 21.5 μm with a nozzle-to-nozzle distance of 254 μm. All of the experiments were carried out with one nozzle and a nozzle-to-substrate distance of 1 mm. The maximum jetting frequency was set to 5 kHz. The control signal for the piezo inkjet printhead was adjusted to obtain a stable drop ejection with a ball-shaped drop on a trajectory orthogonal to the nozzle plate. The maximum voltage applied to the piezo transducer was 36 ± 4 V. As main parameters of IJP, the printing direction and the drop distance (center-to-center distance between the deposited droplets) were varied in the experiments. For all of the experiments with AJP, an Optomec Aerosol Jet 300 CE printer (Optomec Design Company, Albuquerque, NM, USA) was used. The system was equipped with an extended nozzle tip having a nozzle orifice diameter of 200 μm. During the experiments, (i) the AJP process velocity and (ii) the sheath gas rate as factors of aerodynamic focusing of the aerosol mist were varied. Both deposition systems used are shown in photographs and compared in detail by technical specifications in Table S1 in the Supporting Information. All of the printings and layer post-treatments were carried out under ambient conditions. As many parameters as possible were set to constant values on both printing systems. During printing, the temperature of the substrate holders was set to 50 770
DOI: 10.1021/ie503636c Ind. Eng. Chem. Res. 2015, 54, 769−779
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Industrial & Engineering Chemistry Research
Figure 2. 2D and 3D surface profilometry of (A, C) an inkjet-printed droplet and (B, D) an aerosol jet-printed splat.
terms of the aerosol mist, which consists of very small, atomized droplets in a stream of a transport gas. Thus, the shape of the deposit is a function of the homogeneity of the aerosol mist and the shutter speed. The small sprinkle spots located around the centric large splat represent small portions of atomized ink outside the main focused stream of transport gas. These portions of atomized ink are separated as they do not merge with the main focused gas stream that forms the centric large splat. Renn32 explained this effect with droplets that converge inside the nozzle as a result of impact with the sheath gas and diverge after exiting the nozzle. This effect leads to overspray, which in turn causes the sprinkle spots next to the large splat. Mist droplets with smaller diameters tend to impact on the substrate with a larger distance to the main larger splat compared with droplets with larger diameter. Most of the sprinkle spots are situated at the top of the centric large splat, indicating the transferring path of the deposition head in relation to the substrate. The length of the splats of AJP measured along the transferring path (from bottom to top in Figure 1) is 120.9 ± 21 μm including the sprinkle spots. Considering only the centric main splat, the average splat diameter is 50.1 ± 7.2 μm. In contrast to the splats of AJP, the inkjet-printed droplets are uniformly circular-shaped with high edge sharpness, and no sprinkle splats or satellite drops (as they are called in IJP) are visible. All of the deposited droplets appear identically. They have a diameter of 61.5 ± 0.6 μm, which is slightly larger in comparison with the main splats in AJP. When the sprinkle splats of AJP are taken into account, the IJP deposits have smaller feature sizes. The small standard deviation of the inkjet-printed droplets indicates a low variation in drop volume and thus high process accuracy. Further details about the measurement procedure for the drops and splats are available in Figures S1 and S2 in the Supporting Information.
carried out with a Veeco Dektak 150 surface profiler (Veeco, New York, NY, USA). Electrical characterization was performed with a four-point measurement setup using a Süss Microtec probe system (Süss, München, Germany) and a Keithley 2612 SourceMeter (Keithley Instruments, Cleveland, OH, USA).
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RESULTS AND DISCUSSION Droplet and Splat Morphologies. The smallest element that can be deposited is a single droplet fabricated by IJP or a so-called splat fabricated by AJP.32 Single droplets can be easily ejected from the inkjet nozzle, as IJP is usually based on the ejection of individual drops with a high number of nozzles. A pattern is obtained by depositing drops next to each other. The printer performs a line-by-line approach to develop the desired pattern. Single droplets occur if the drop distance sD exceeds the arising sessile droplet diameter on the substrate. The size of the single droplet mainly depends on the drop volume (determined by the nozzle orifice diameter and the signal applied to the printhead) and the interaction of the droplet with the substrate. In contrast, the approach of a splat deposited by AJP is quite different. AJP is based on a continuous aerosol mist process. An electromagnetic shutter is responsible for separating the continuous flow of aerosol mist into determined amounts of aerosol mist. Therefore, the splat size on a substrate depends mainly on the aerosol nozzle size, the opening time of the shutter, the density of the aerosol mist flow, and its interaction with the substrate. Figure 1 shows droplets fabricated with IJP and splats deposited with AJP. Interestingly, the deposits of the two systems have similar dimensions despite the very large difference in nozzle orifice (21.5 μm for IJP vs 200 μm for AJP). The elements deposited by AJP exhibit smaller sprinkle spots around a centric large splat. This can be explained in 771
DOI: 10.1021/ie503636c Ind. Eng. Chem. Res. 2015, 54, 769−779
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Figure 3. Comparison of raster-based IJP in single-nozzle mode and vector-based AJP.
Figure 4. Microscopic images of printed lines with (A) varying drop distance sD and printing direction for the IJP process and (B) varying sheath gas rate Qs for the AJP process.
The morphologies of the deposited drops and splats were investigated by means of profilometry. Figure 2 shows the 2D cross-sectional profiles of the deposits measured in the middle region and a 3D surface profile. As depicted, similar morphologies were obtained despite the very different processes and nozzle orifices used. The two deposits are comparable regarding their diameters, shapes, and thicknesses. In both cases, the well-known coffee ring is formed, indicating that similar evaporation processes take place for the deposits of AJP and IJP. Convective flows transport materials from the centers of the drops and splats to the edges, where they agglomerate.31,33 Line Morphologies. The principle of drop coalescence is used for the creation of line patterns in IJP. The distance between the printed droplets is set smaller than the diameter of a single printed droplet on the substrate. In general, IJP is based on a raster scanning/processing mode (systems are also available for functional materials deposition based on vectorbased processing, but this is not very common for IJP). The pattern is defined in a grid of pixels and transferred to the
substrate in a line-by-line manner, e.g., starting from the bottom left and ending at the top right position of a substrate. Each pixel in the grid assigned to the pattern results in one ejected droplet. Different initial situations can arise for the raster-based processing method, as shown in Figure 3. If a line is printed by subsequently placing droplets parallel to the moving direction of the substrate/printhead, the line is deposited in printing direction (IPD) of the jetting system. If the line is printed perpendicular to the moving direction of the substrate/ printhead, the processing is carried out counter printing direction (CPD). In the latter case, more movements of the axis system and thus more processing time are required because the drop ejection frequency is limited. In the case of deposition IPD, the actual drop ejection frequency is higher, and the processing time is therefore shorter. Both deposition IPD and deposition CPD are used for more complex patterns expanding in the x and y directions, such as squares. In contrast to the line-by-line raster-based processing in IJP, a vector-based scanning mode is applied in AJP. The pattern is developed by defining pattern points and shapes (i.e., the printing path) 772
DOI: 10.1021/ie503636c Ind. Eng. Chem. Res. 2015, 54, 769−779
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Figure 5. Comparison of line morphologies of (A) IJP as a function of drop distance and printing direction and (B) AJP as a function of process velocity vp and sheath gas rate Qs.
Figure 6. Morphologies of lines deposited IPD and CDP with IJP as functions of drop distance: (A) average line width; (B) average line peak height.
sprinkle splats from the line at each side. The higher the sheath gas flow, the more prominent are these channels. Figure 5 shows cross-sectional profiles for all of the lines depicted in Figure 4. The difference in the IPD and CPD profiles for IJP is remarkable (Figure 5 A). The CPD layer thickness is nearly tripled relative to that for IPD at a drop distance of 5 μm. The coffee ring, which could be clearly seen in printed single drops, is no longer visible in the lines printed CPD. Lines printed IPD still feature a ringlike shape, but it is not as prominent as for the drops. Also, the line width varies massively. Lines printed CPD have a smaller line width than lines printed IPD. The lower the drop distance, the more pronounced is this behavior, as can be seen in the case of the 5 μm drop distance. The line widths for a drop distance of 5 μm were measured as 194.5 ± 7.7 μm for IPD and 66.1 ± 6.6 μm for CPD (further details about the line width and height measurements can be found in Figure S3 in the Supporting Information). For a drop distance of 45 μm, the line widths are 65.6 ± 11.4 μm for IPD and 52.7 ± 3.4 μm for CPD. These line widths are approximately equal to the diameters of single drops introduced above. The main reason for the different line morphologies IPD and CPD is the principle of stacked coins described by Soltman and Subramanian.31 The droplets deposited CPD start to dry before subsequent drops can coalescence with the already deposited droplets as in the case of lines printed IPD.31 As illustrated in Figure 3, much more movement of the deposition system is required for the CPD process. In this sense, the time delay between two drops deposited subsequently is significantly higher for deposition CPD than for deposition IPD. Thus, deposition IPD refers to a wet-in-wet approach whereas CPD refers to a wet-in-semidry or even wet-in-dry approach. To investigate the influence of the drop distance on the line morphology in IJP in more detail, different lines were deposited with drop distances ranging from 5 to 50 μm in 5 μm steps.
with start and end positions based on mathematical equations. The transferring path corresponds to these definitions, and thus, vector-based processing is much more flexible than the raster-based line-by-line approach. For the printing paths, the shutter is open and the aerosol mist is guided to the substrate. In the case of transfer paths, the shutter remains closed. For more complex patterns than single lines, fill algorithms are available that transfer the substrate under the head in a defined way. For example, a square can be filled perimeter-wise or in serpentine style, as shown in Figure 3. Figure 4 shows lines deposited by IJP and AJP as a function of drop distance and printing direction for IJP and as a function of sheath gas rate for AJP. The drop distance in IJP defines the amount of deposited material per unit area. For AJP, a similar parameter is the process velocity, as the deposition process is continuous. The lower the process velocity, the more material is deposited per unit area at a certain atomization rate. Additionally, the sheath gas rate in AJP influences the aerodynamic focus of the material stream. Variation of the sheath gas rate also influences the morphology of the printed line pattern. In the aerosol jet nozzle, the aerosol stream is surrounded by an annular flow of the sheath gas that is used to direct the materials to the substrate and to prevent clogging of the nozzle orifice. As is visible in Figure 4, the higher the drop distance and the sheath gas rate, the lower the line widths obtained. The printing direction in IJP has a dramatic influence on the line morphology. Smooth line shapes are obtained IPD for a drop distance of 45 μm, whereas individual drops are visible CPD, resulting in bulging and scalloping of droplets, as also described by Soltman and Subramanian.31 In comparison to IJP, the lines obtained by AJP have sprinkle splats at the edges similar to those of the single splats discussed above. The intensity of the sprinkle splats decreases at higher sheath gas rates. A channel in the lines deposited with AJP is also visible, separating the 773
DOI: 10.1021/ie503636c Ind. Eng. Chem. Res. 2015, 54, 769−779
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Figure 7. Morphologies of lines deposited by AJP at different process velocities as functions of the sheath gas rate: (A) average line width; (B) average line peak height.
Drop distances larger than 50 and 75 μm result in interrupted lines and individual droplets, respectively (see Figures S4 and S5 in the Supporting Information for further details). In all of these cases, printing was performed IPD and CPD with a maximum deposition frequency of 5 kHz. Figure 6 shows the measured average line widths and line peak heights (highest measured line thickness) as functions of drop distance. The data confirm the above observations. The line width and line peak decrease with increasing drop distance. Lines deposited IPD have larger line widths and smaller line peak heights than the lines deposited CPD. With the given ink and substrate a big range can be covered: lines with widths from about 56 to 195 μm and peak heights from about 0.6 to 6 μm can be obtained by varying only the drop distance and printing direction (see Figures S6 and S7 in the Supporting Information for further details). This is an important advantage for the field of printed electronics, where different line widths and layer thicknesses are required. Similar to IJP, process variations in AJP were also done. Two parameters were considered: the process velocity vp was set to 1, 3, or 5 mm·s−1, and the sheath gas rate Qs was varied from 50 to 100 cm3·min−1 in steps of 10 cm3·min−1. Figure 7 summarizes the obtained results. The process velocity has a bigger influence on the line height than on the line width. The line peak heights at 5 mm·s−1 are tripled or quadrupled compared with those at 1 mm·s−1. The higher the process velocity, the less material per unit area is deposited at a certain atomizer gas rate. A higher sheath gas rate results in a more focused deposition stream, and thus, smaller line widths and larger layer thicknesses are achieved. With a sheath gas rate of 50 cm3·min−1, a line width of 61.5 ± 3.9 μm is obtained, in comparison with 37.5 ± 0.8 μm for a sheath gas rate of 100 cm3·min−1. However, for higher sheath gas rates, channels separating the sprinkle splats from the main line can be observed, as shown before in Figure 4. The channels arise from the intense gas flow, which is able to move the deposit on the substrate as long as it is liquid. A gas flow with lower strength (e.g., 50 cm3·min−1) is not able to overcome the inertia of the liquid, and no complete separation takes place. However, the channels can be seen also for the lower gas flow rate (see the lines obtained by AJP with Qs = 50 cm3·min−1 in Figure 4). In contrast to IJP, the coffee ring in the lines printed with AJP is present at all sheath gas rates and process velocities investigated. The results show that IJP and AJP allow the deposition of lines with different morphologies depending on the process parameters. The important process parameters in IJP are the
drop distance and the printing direction. The drop distance determines the number of droplets and thus the amount of material deposited per unit area. The printing direction strongly influences the morphology of the deposited lines as a function of the time delay between the depositions of subsequent droplets. In contrast to the introduced drop-on-demand IJP process, AJP is based on a continuous deposition principle. Thus, the amount of material per unit area is determined by the process velocity/axis motion speed and the deposition rate of the ink at a given atomizer gas rate. The sheath gas rate influences the morphology of the deposited lines. As a conclusion, Table 1 shows the maximum and minimum values of the line width and line peak height for lines printed with IJP (IPD and CPD) and AJP. Table 1. Comparison of Widths and Peak Heights of Lines Deposited with IJP and AJP width [μm]
peak height [μm]
deposition technology
max
min
max
min
IJP IPD IJP CPD AJP
195 ± 8 70 ± 9 50 ± 2
57 ± 2 56 ± 6 33 ± 2
1.7 ± 0.1 5.8 ± 0.1 1.6 ± 0.1
0.6 ± 0.1 0.7 ± 0.1 0.3 ± 0.1
While inkjet-printed lines show good edge sharpness, comparable results could not be obtained with AJP. Here again, sprinkles and clusters of small splats are situated on both sides of the printed line. This effect is in accordance with sprinkles and clusters of splats already observed while printing single splats. These effects enhance the area on a substrate covered by a line fabricated with AJP. Overall AJP enables thinner lines with smaller cross-sectional areas compared with IJP. Square Morphologies. To obtain a continuous square, droplets are placed on the substrate in the x and y directions (see Figure 3) with a drop distance smaller than the drop diameter on the substrate. For the deposition of squares by AJP, PFA and SFA were used and compared. The distance between the lines was set to 20 μm for both fill algorithms. Figure 8 shows the squares produced by IJP and AJP. It can be seen that the square deposited by IJP looks much more homogeneous than the squares obtained by AJP. In the PFA pattern, individual lines are visible indicating the path of the printer during deposition. This leads to the conclusion that the lines used to cover the area within the pads dry before the deposition head passes again to deposit a successive part of the PFA, which is comparable to the effect of stacked coins in IJP. 774
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Figure 8. Microscopic images of squares deposited with (A) IJP (drop distance = 20 μm) and (B) AJP using PFA and SFA (with a 20 μm line distance). Squares were dried at a temperature of 50 °C.
Figure 9. Comparison of square morphologies obtained by (A) IJP and (B) AJP with PFA and SFA.
process velocity to 3 mm·s−1. Table 2 summarizes the line dimensions obtained for IPD and CPD IJP and for AJP. The
In contrast, the SFA pattern does not have individual lines despite the use of the same line distance of 20 μm. Additionally, the squares filled with PFA show bulging of silver ink at the corners of the printing path. This bulging effect arises because the corner speed is lower during the deposition, and thus, more material is deposited at the corner as the shutter does not close while its direction changes according to the printing path. Figure 9 shows a comparison of the profiles of the printed squares shown in Figure 8. In contrast to the first impression of the square deposited by IJP, the layer is not very homogeneous. Most of the silver material is accumulated at the center of the square, with peak heights of more than 7 μm. In contrast, much less material per unit area is deposited by AJP at an atomizer gas rate of 840 cm3·min−1 and a process velocity of 3 mm·s−1. The PFA results in a maximum layer peak height of 1.2 μm, and that for SFA is 0.75 μm. The roughness of the PFA layer is much higher in comparison with the SFA layer. However, the overall amounts of material deposited in the two cases are the same. In comparison, IJP allows a higher edge sharpness and overall a lower pattern homogeneity. Thinner and less wavy layers are obtained with AJP. Whereas in IJP the deposition rate of material per unit length is dependent on the drop volume and the drop distance, in AJP the amount of material deposited is dependent on the process velocity. The focused aerosol mist exiting the nozzle orifice at a certain atomizer gas rate is related to the atomization properties of the ink in combination with the trace width to determine the overlapping of successive deposited lines. Additionally, the fill algorithm influences the final printing result simply by varying the time between the interaction of successive deposited lines, as it is visible for squares done with PFA and SFA. Electrical Measurements. Lines were deposited with both IJP and AJP technology for electrical measurements. A drop distance of 20 μm was used for the IJP process. For AJP, the sheath gas flow rate was set to 70 cm3·min−1 and the axis
Table 2. Line Dimensions Used for the Electrical Characterization deposition technology
length [mm]
width [μm]
peak height [μm]
cross-sectional area [μm2]
IJP IPD IJP CPD AJP
11 11 11
83 ± 2 84 ± 4 30 ± 4
0.8 ± 0.1 2.3 ± 0.1 0.6 ± 0.1
46 ± 5 109 ± 8 11 ± 2
cross-sectional line areas were calculated on the basis of profilometry images and cross-sectional scanning electron microscopy (SEM) images. The lines were post-treated on a hot plate at 100, 150, 200, and 250 °C for 30 min and additionally at 250 °C for 120 min and 300 °C for 90 min. Figure 10 shows SEM images of the surfaces of the deposited silver inks without any temperature treatment and for the different hot-plate temperatures and treatment durations. The morphologies depicted in Figure 10 are typical for printed silver nanoparticle inks and demonstrate the grain size development as a function of temperature and treatment duration.34−36 Other printed materials (e.g., copper) also result in similar morphologies during sintering.37 On the basis of Figure 10A, the particle size of the Cabot CSD 32 ink was determined to be 35 ± 8 nm. After treatment at 100 °C (Figure 10B), no significant difference can be observed compared with the as-printed layer without any temperature treatment. Neck formation between the silver nanoparticles takes place at about 200 °C, as is visible in Figure 10C. Higher temperatures and longer heating durations result in further densification of the structure. Most of the boundaries between the silver nanoparticles disappear, and a crystalline structure can be observed. Longer heating times or temperatures higher than 775
DOI: 10.1021/ie503636c Ind. Eng. Chem. Res. 2015, 54, 769−779
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Figure 10. SEM images of the deposited silver layers (A) as printed without any temperature treatment and (B−F) temperature-treated at (B) 100 °C for 30 min, (C) 200 °C for 30 min, (D) 250 °C for 30 min, (E) 250 °C for 120 min, and (F) 300 °C for 90 min.
300 °C did not improve the layer density without damaging the structure. Thus, 300 °C and 90 min can be considered as the final sintering stage. Even at this temperature, pores in the structure start to develop, as shown in Figure 10F. The average line resistance of all deposited lines for IJP and AJP was measured for the different post-treatment temperatures and durations, as depicted in Figure 11. For better visibility, the standard deviation is not presented (the standard deviation was in most of the cases
