March 2007
Evaluation of Gravure Print Forms for Printed Electronics
Erika Hrehorova & Ramesh-Chandra Kattumenu
Department of Paper Engineering, Chemical Engineering & Imaging
Western Michigan UniversityThe implementation of printing technologies into electronics manufacturing is promising today, specifically in the production of low-cost and disposable electronics for various applications, such as displays and RFID tags.
Abstract
This paper is focused on the evaluation of gravure print-form quality and the capabilities of available engraving methods to engrave uniform grooves for the printing of fine lines. Different methods used for gravure print form engraving are discussed. Chemical etching, which today uses laser ablation of the mask resist, is the most promising engraving process when considering the printing of fine lines for circuitry. This process is capable of engraving fine lines as continuous grooves, rather than rows of individual cells as in the case of electromechanical engraving. Gravure print forms were characterized by using White-Light Interferometry in Vertical Scanning Mode. Based on the results, it is concluded that the studied engraving method is capable of producing very uniform cell dimensions over a large area and, thus, delivering a consistent volume of ink when printing functional layers. Considering engraved fine grooves, it was found that the quality of the engraved grooves depends on the orientation of the line to the imaging direction. Various line widths at five different angles to the imaging (print) direction were studied. A widening effect was observed for all measured line widths. The highest width gain was found for line widths lower than 100 microns.Introduction
There are two major applications that will account for printed electronics in the next 20 years1. One is display and lighting applications and the other is memory and logic circuits. For logic and memory applications, the major product is a radio-frequency identification (RFID) tag. An RFID tag consists of several types of devices—i.e., antenna, capacitor, resistor, transistor, diode, and memory. In general, these devices are produced by different processes to achieve the highest performance.Patterning issues that are crucial to electronics manufacturing include resolution, design protocols, accuracy, registration, and yield. A truly challenging task for printing techniques is achieving a one-micron accuracy level in order to become relevant to microelectronics. However, single-layer printing with reduced lateral-accuracy requirements (10 μm, for optical filters and gratings) and replication with overlay of larger patterns (micrometer scale) may soon be applied in niche markets. Triggered by promising applications with intermediate accuracy requirements, improvements might drive printing technologies to reduce pattern sizes from 30 μm to 100 nm and to improve overlay from 20 μm to submicron levels2.
Among conventional printing processes, gravure printing is the premier process due to its high quality and ability to print at high speeds. The robustness of its image carrier is advantageous, contributing to good printing stability over time. These advantages of gravure printing make it a promising process for electronics manufacture3, 4.
There are many factors influencing gravure print quality, such as substrate properties (smoothness, compressibility, porosity, ink receptivity, wettability, etc.) and ink properties (ink chemistry, viscosity, rheological behavior, solvent evaporation rate, and drying, etc.)5. Furthermore, process parameters, such as doctor blade angle and doctor blade pressure, impression pressure, and printing speed have a tremendous effect on the quality of the printed ink film. The heart of a gravure press is the gravure cylinder, which carries the image design to be printed. A gravure cylinder is composed of a steel base core with flanged steel journals. It is electroplated with copper and then polished to a predetermined diameter. Precise diameters of gravure cylinders are critical because any variances in diameter—as little as two thousandths of an inch—can significantly affect the print registration. Once engraved, cylinders are electroplated with a thin layer of chromium to ensure a surface hardness that protects the softer copper against scratches and abrasion by the doctor blade during printing6.
Different engraving methods can produce various shapes and dimensions of gravure cells, determining the ink release and, ultimately, the quality of the transferred ink film7. The quality and uniformity of engraved features is essential for the printing of functional layers for electronics. There are three basic types of engraving methods—the widely used electromechanical engraving, direct laser engraving, and traditional chemical etching. Electromechanical engraving uses a diamond stylus to cut gravure cells directly into the copper layer of the revolving cylinder. In general, one oscillation of the stylus produces one cell. The volume of the cell depends on the amplitude of oscillation. The frequency of oscillation is limited by mechanical resonance of the diamond stylus holder. Different tonal values are realized by engraving the cells with various diameters and depths; however, always with a fixed aspect ratio defined by the stylus geometry. Lines are typically produced as a row of dots and, thus, substrate wetting and ink spreading need to be optimized in order to produce continuous lines6. Ragged edges are common with this type of engraving. However, recent advances in electromechanical engraving are pushing toward higher resolutions (up to 2,000 l/cm for security applications) and edges are being smoothed by engraving them as closed continuous lines8, 9.
Direct laser engraving is based on focusing the laser beam onto the gravure cylinder surface and local vaporization of the image-carrier material. Since copper does not absorb laser energy efficiently, a zinc layer is added to the copper surface and used as a layer for engraving6. The geometry of the cells is determined by the diameter of the laser beam and the energy of the laser pulse. Each laser pulse creates one cell, and the pulse energy defines the depth and, ultimately, the cell volume. Advances in laser engraving allow for a wide spectrum of cells ranging from conventional (depth variable) to auto typical (halftone gravure), with a free selection of cell dimensions for optimized ink transfer10.
Laser technology has been used in the gravure industry not only for direct laser engraving, but also for imaging of the mask resist prior to chemical etching of gravure cylinders. Traditionally, the gravure print form was based on film analog exposure and developing of the mask layer followed by chemical etching with an appropriate etchant. Today, a laser beam is employed to ablate the mask resist—the process known as laser ablation11. The copper layer is not affected by the laser beam because the threshold of ablation is higher than that for the mask resist. The cell depth is controlled by the time that the etchant is in contact with the copper surface of the cylinder and, thus, the produced cells have the same depth and tones are controlled only by diameter of the cell12.
The typical workflow of gravure cylinder preparation using laser ablation followed by chemical etching is shown in Figure 1. The resolution of engraving is limited by the diameter of the laser beam focus. With a direct laser imaging system, a minimum beam size of about 40 microns is used and, therefore, the minimum line width is about 40 microns. Indirect laser systems use a laser beam split into four beams of equal power (sufficient to ablate the mask resist) and typically work with a beam diameter of 10-20 microns for gravure applications. The minimum line width is, therefore, also about 10-20 microns, or a little more due to the sidewall etching.
White-Light Interferometry
There are several different methods to characterize and evaluate the quality of engraving. These include optical microscopy, fluid volumetrics, replicates, confocal microscopy, and white-light interferometry. In addition, atomic force microscopy and scanning electron microscopy can be used to evaluate gravure cells; however, these are performed off line and are more complex13. White-light interferometry is a widespread method for optical surface profilometry, and it has already shown its applicability in the printing industry, more specifically in measuring of anilox rolls for flexographic printing14, engraved cells for gravure13, as well as screen-printed lines and patterns15.The principle of operation is based on the beam of polychromatic white light split into two parts. One part travels to a reference mirror and the other to the surface under study. Interference fringes are observed when the light reflected from the sample recombines with the light reflected from the reference mirror, where the best contrast fringes occur at the best focus16. Vertical-scanning interferometry (VSI) developed in WYKO17 is based on similar principles. However, in the VSI mode, the white light is filtered through a neutral density filter, which preserves only short coherence lengths of the light. The interferometric lens scans the surface at varying heights by vertical movement through the focus, capturing frames of interference data at fixed intervals. The interference signal for each point of the studied surface is recorded, providing information about the fringe modulation, which is consequently used to calculate the surface height profile16. It is possible to obtain the height profile of a surface in the course of one measuring procedure; therefore, the lateral scanning is not necessary. Another advantage of this method is its theoretically unlimited unambiguous measurement range. The longitudinal measurement uncertainty of the height profile measurement is related to the surface roughness; it is independent on the parameters of the imaging system. Because the illumination and the observation are coaxial, the height profile can be measured also in deep cuts and holes. The coaxial arrangement avoids the formation of shadows.
Materials and Experimental Methods
Gravure plates for this work were engraved by Schepers GmbH & Co. KG (MDC Max Daetwyler Group). A laser imager DIGILAS (MDC Max Daetwyler Corp.), using a 100 Watt YAG laser (1064 nm), was employed to image the mask resist. The screen used for imaging of the print form was 400 l/cm. After the laser imaging, the print form was placed into the spray etching machine, where the cells and grooves were etched into the copper surface exposed by laser ablation.The print form design was created in Adobe Illustrator CS2 with the intent to get information about the capability of the used engraving method to produce high-quality print forms with uniformly engraved lines. It included larger solid areas to evaluate the cell dimensions’ uniformity, various line blocks at five angles to the print direction, and patterns for measuring conductivity if used for printing with conductive inks. Line blocks incorporated into the design were designed in such a way so that the widest specified lines (300 microns) were at the edges and the narrowest lines (15 microns) were in the center. Figure 2 illustrates different positions of line blocks with regard to the print direction. The red boxes in Figure 2 demonstrate the areas that were investigated in this work.
A WYKO RST-Plus vertical interference microscope in VSI mode was employed in this study. This instrument is capable of measuring the surface topography over two-dimensional fields of view from 6x8 mm to 100x150 mm with a vertical resolution of 3 nm.
Results & Discussion
Engraving Quality of Large Solid Area
With white-light interferometry, it was possible to obtain accurate 2D and 3D profiles of engraved forms. Typical results from vertical scanning are presented in Figure 3a and 3b. In order to extract the actual dimensions of engraved cells and analyze their uniformity, X- and Y- cross-sections were made across multiple cells and rows of cells. A total of 40 individual cells was taken into account when calculating the average cell depth and diagonal width in both X- and Y- cross-sections. There is different scaling of the X and Y axes in cross-sections in Figure 3c and 3d; thus, they do not provide a real representation of width to depth ratio. For a better illustration, a detailed image of gravure cells showing their shape in 3D perspective and in cross-section is given in Figure 4.
Figure 3: 2D contour (a) and 3D profiles (b) of solid coverage area (100% tone) gravure cells engraved at 400 l/cm and the cross-sections made in X (c) and Y (d) axes (Note different scaling of X and Y axes in cross-section profiles).
Figure 4: Detailed image of engraved cells showing the shape in 3D perspective (left) and in cross-section (right).
When calculating dimensions using pixel data from the VSI scans, one must take into account the roughness of chromium surfaces (the average roughness of a chromium-plated cylinder is about 0.3 – 0.6 μm), which can lead to variations when defining the cell dimensions13. Moreover, the wall roughness is also a factor contributing to cell volume variations and, thus, the amount of ink transferred onto the substrate. In this work, the top surface height mean was used in the calculation of cell parameters.
It was found that the average depth calculated from X- and Y- cross-section profiles for solid coverage area gravure cells engraved at resolution 400 l/cm is very uniform. Average cell depths extracted from the X-profile and Y-profile were found to be 21.45 ± 0.31 (1.5 percent) and 21.72 ± 0.45 (2.1 percent), respectively. The diagonal dimensions for measured cells calculated from X- and Y- cross-sections are very similar, implying the square shape of engraved cells on the top surface. The diagonal length was measured to be 100.2 ± 3.6 μm (3.6 percent) as calculated from the X – cross-section and 102.6 ± 2.4 μm (2.3%) from the Y – cross-section.
Quality of Engraved Lines
Gravure printing tends to be a directional process when fine lines are printed. The smoothest edges are typically produced in the print direction. Printing in the perpendicular direction often results in more pronounced “sawtooth” edges and poor line contours when printing from electromechanically engraved print forms. Additionally, it is likely that more ink will be transferred onto the substrate in the print direction resulting in more conductive traces18, 19.
The situation is, however, different for lines engraved as continuous grooves. It has been reported that when the substrate comes in contact with the fluid in the groove, a strong eddy current can be observed, and as the substrate exits the groove, a recirculation region attaches to the moving substrate and follows it20 (Figure 5). Thus, when printing the lines oriented perpendicularly to the print direction, it is more likely that they will be wider than the lines printed parallel to the print direction. It was also reported21 that the strength of recirculation depends on the groove orientation relative to the print direction. As the angle increases from 0° (parallel) to 90° (perpendicular), the strength of recirculation also increases.
In this work, the quality of engraved grooves was evaluated in terms of line depth and top and bottom width uniformity. The effect of line orientation with regard to the laser imaging direction was first studied by measuring lines (specified width of 37.5 microns) imaged in parallel, perpendicular, and in a 45° angle to the imaging direction. Figure 6 shows the 2D and 3D plot of engraved lines and their depths. Figure 7 illustrates the X-profile cross-section used for calculation of groove parameters.
Figure 6: Contour (a) and 3D plot (b) of engraved lines used for evaluation of width and depth of the grooves.
Engraving the lines as continuous grooves might lead to increased uniformity of the printed line width and, consequently, reduced edge roughness. Though, the uniformity of width and depth along the groove is essential in order to assure a consistent amount of ink being deposited onto the substrate. Table 1 presents the average values for groove depth and top and bottom widths of measured grooves. The values are given in the form of an average ± standard deviation with the percent of standard deviation in brackets. It is evident that all the measured parameters are affected by the orientation of the line with respect to the laser imaging direction. The smallest depth was found for parallel orientation of the lines and the largest for 45° oriented lines. Generally, the depth of the grooves depends on the active etchant concentration and the etching timeFrom Table 1, it can be seen that with increasing groove width, the depth of the etched groove is also slightly increasing. The widest lines were engraved at the 45 ° angle, and the narrowest lines were measured for lines parallel to the imaging direction. Considering uniformity of measured parameters based on standard deviation of depth and width, it is obvious that the grooves’ depth and top surface width dimensions have a standard deviation of around 6 percent. However, the bottom width varies more along the grooves with a standard deviation of 23 to 28 percent. Figure 8 shows a more detailed image of engraved grooves. Decreased uniformity in dimensions along the line might lead to a variation of ink transfer. The varying amount of functional material transferred from the grooves can result in nonuniform conductivity of printed traces. More extensive research is necessary in order to find how such variation affects the functionality of printed functional features.
[microns]
[microns]
[microns]0°
19.35 ± 1.15 (5.9%)
50.69 ± 3.39 (6.7%)
19.07 ± 4.39 (23.0%)
90°
21.46 ± 1.39 (6.5%)
55.92 ± 3.47 (6.2%)
18.82 ± 5.10 (27.1%)
45°
22.12 ± 1.38 (6.2%)
59.41 ± 3.83 (6.4%)
19. 67 ± 5.60 (28.5%)
The line width specified in the digital file for measured grooves (Table 1) was 37.5 microns, although the measured width shows noticeable width gain. Width gain for engraved grooves can be calculated according to the following formula:
Figure 9: The width gain for different line widths specified in the digital file sent to the imaging system In order to study the widening effect of the engraving method in parallel and perpendicular orientations, specified line widths of 15, 25, 37.5, 50, 75, 100, 150, 250, and 300 microns were measured. It was found that the increase in line width is not consistent for different specified line widths, and higher width gain was observed for finer lines. Figure 9 shows the relationship between width gain and specified line width in parallel and in perpendicular orientation to the imaging direction. Evidently, there is a significantly higher increase in line width for lines below 100 microns, and the finer the line, the more width it will gain. For example, the specified line width of 15 microns was measured to be 29.1±3.0 microns in parallel and 31.6±1.9 microns in the perpendicular direction. This corresponds to the average width gain of 94 percent and 111 percent for parallel and perpendicular directions, respectively. Width gain for parallel orientation of lines was always lower than that for perpendicular; however, this difference lessened for higher line widths. Experimental results fit very well with power law function, which can help in predicting the widening effect for different specified line widths.
So far, only perpendicular and parallel orientation of lines and their effect on width gain have been shown. However, integrated circuit designs may require conductive traces at various angles to the print direction. Figure 10 shows the width gain results for engraved fine lines (below 100 microns) at different angles to the imaging/print direction. It can be seen that finer lines are more sensitive to changes in orientation than coarser lines. Width gain is the highest for the 15-micron specified line at a 45-degree angle to the print direction.
Conclusions
Direct printing of electro-active materials is receiving significant attention due to applications in emerging and next-generation technologies, such as organic light-emitting diodes, organic transistors, and other low-cost and disposable electronics. Among conventional printing processes, gravure printing is the most promising due to its high-quality print. Although, the process is very well optimized for graphical purposes, the requirements for electronics printing are more demanding in terms of resolution and registration.There are several different methods of gravure print form manufacturing. This work considers the quality of engraved print forms prepared by laser ablation followed by chemical etching. White-light interferometry in vertical scanning mode was employed to evaluate the quality of engraved features. White-light interferometry provides fast and accurate three-dimensional representation of surface profiles. It was found that the width of engraved lines is higher than the line widths specified in electronic files sent to the laser imager. The highest width gain was observed for the narrowest lines. There are several possible causes of such widening effects during the gravure print form manufacturing. These may include the scattering of the laser beam during laser ablation of the mask resist and sidewall etching during the engraving step. In order to determine the widening effects of individual processes involved in manufacturing, further study is necessary.
Furthermore, the effect of line orientation with regard to print direction was studied. It was found that the finer the line, the more sensitive it is to changes of orientation. The finest line that was engraved by the studied engraving method was 29.1±3 microns in orientation parallel with the print direction. The next step is to evaluate the ability of different materials to print such fine lines, where necessary.
Acknowledgements
The author acknowledges Mr. Walter Siegenthaler (MDC Max Daetwyler Corp.) for assistance with the production of gravure print forms. Additionally, appreciation goes to Dr. Philip Guichelaar from the Department of Mechanical & Aeronautical Engineering at Western Michigan University (WMU) and Dr. Alexandra Pekarovicova and Dr. Dan Fleming from the Department of Paper Engineering, Chemical Engineering and Imaging at WMU for all their help with this work.Literature Cited
1. Harrop, P., Das, R., “Organic Electronics Forecasts, Players, Opportunities 2005-2025,” IDTechEx Press, 2005, p. 220
2. Michel, B., et al., “Printing Meets Lithography: Soft Approaches to High-resolution Patterning, IBM J. Res. & Dev., 45, No. 5, September (2001), p. 697 - 719
3. Pudas, M., Halonen, N., Granat, P., Vahakangas ,J., “Gravure Printing of Conductive Particulate Polymer Inks on Flexible Substrates,” Progress in Organic Coatings, 54, (2005), p. 310 - 316
4. Fenoll, M., Catusse, R., Rousset, E., “Gravure Printing: Material Characterization for All-Organic Capacitor,” IARIGAI’s 32nd International Research Conference on Digitalization and Print Media, Porvoo, Finland, September 4 – 5 (2005) p. 83 - 92
5. Bohan M. F; Claypole T. C; Gethin D. T, “The Effect of Process Parameters on Product Quality of Rotogravure Printing,“ Proc. of the Institution of Mechanical Engineers, Part B, Journal of Engineering Manufacture, 214, 3 (2000), p. 205 - 219
6. Gillett, K. et al, Gravure – Process and Technology, GAA and GEF, Rochester, 2nd Edition, New York (2003)
7. Rong, X , Pekarovic, J., Pekarovicova, A., “Gravure Printability from Laser and Electromechanically Engraved Cylinder,” Proceedings of International Printing and Graphic Arts Conference, Vancouver, British Columbia, Canada, October (2004) p. 151-154
8. Ammann, R., “Cylinder Engraving in your Living Room: GRT-Gravurtechnik tests ‘Xtreme’ Engraving from Hell Gravure Systems,” Verpackungs – Rundschau, July (2004), p. 28 - 29
9. “High speed VISION 3 engraving head takes the specialty engraving method for linework reproduction to unprecedented new speeds!“ Daetwyler News, No. 4, MDC Max Daetwyler AG, CH-3368 Bleienbach · Switzerland, August (2003), http://www.daetwyler.com/news/030812daetwyler_news_4eng.PDF (last visited on February 12, 2007)
10. Henning G., Frauchiger J., “New engraving technique opens up new perspectives for rotogravure printing,” Flexo & Gravure Asia, 1 (2002), p. 6 - 9
11. http://www.daetwyler.com/prod/digilas/gravure.htm (last visited on February 12, 2007)
12. Henning G., Selbmann K. H., Brocklet A., “Laser Engraving in Gravure Industry,” Proc. of SPIE: Workshop on Laser Applications in Europe, 6157, 61570C (2006)
13. Bohan M. F. J., Clist A. M., Claypole T. C., Gethin D. T., “Characterisation of gravure cylinders,” Proc. of TAGA’s 53rd Annual Technical Conference, (2001) p. 1-12
14. Caber P. J., “Interferometric profiler for rough surfaces,” Applied Optics, 32, 19 (1993) p. 3438 – 3441
15. E. H. Jewell, T. C. Claypole, and D. T. Gethin, “Two- and Three-Dimensional characterization of Screen-Printed Lines,” Proc. of TAGA’s 57th Annual Technical Conference (2005)
16. Veeco Metrology Group, WYKO Profilers Technical Reference Manual, Version 2.2.1, September (1999)
17. Cohen, D. K. et al., U.S. Patent 5,355,221, October 11 (1994)
18. Claypole, T. C., Jewell, E. H., Davies, G., Vigne, V., “The Effect of Speed and Viscosity on Line Quality in Rotogravure Printing with Reference to Printed Electronics,” IARIGAI’s 32nd International Research Conference on Digitalization and Print Media, Porvoo, Finland, September 4 – 5 (2005) p. 93 - 99
19. E. H. Jewell, T. C. Claypole, and G. R. Davies “Print Line Quality in the Gravure Process,” Proc. of TAGA’s 58th Annual Technical Conference, (2006)
20. Yin, X., Kumar, S. “Flow visualization of the liquid emptying process in scaled-up gravure grooves and cells,” Chemical Engineering Science, 61, Issue 4 (2006), p. 1146-1156
21. Yin, X., Kumar, S. “Flow visualization studies in scaled-up gravure grooves and cells,” Proc. of TAGA’s 55th Annual Technical Conference, (2003) p. 540-545About the Authors:
Erika Hrehorova is a PhD student in the Department of Paper Engineering, Chemical Engineering and Imaging at Western Michigan University, where she also received her MS in Paper and Imaging Science. Moreover, she received the MS degree from the Department of Chemical Technology of Wood Pulp and Paper at Slovak University of Technology. Her main research focus is concentrated on the suitability of gravure printing in electronics manufacture and the development of new functional materials based on conductive and semiconductive polymers for printed organic electronics.Ramesh Kattumenu is currently a PhD student at Western Michigan University in the Department of Paper Engineering, Chemical Engineering and Imaging. His research interests include materials and processes for RFID, printed electronics, applications of organic polymers in printed electronics, structure and properties of polymer nanocomposites. He received his Master of Science degree in Materials Science and Engineering from Western Michigan University with a major focus on polymer nanocomposites.
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