Enhancing Stretchability of Flexible Electronics - Research Paper Example

Executive summary The work describes a simple and reliable technique used to fabricate gold patterns in a polymethylsiloxane (PDMS) substrate using peel off process. Two surface modifications that involved self assembled monolayers were utilized for reliable and easy transfer of gold patterns onto PDMS. Photolithography is an optical method used to transfer micrometer scale patterns on the substrate. The process is similar to the lithography printing. Patterns are transferred to a photoresist layer, which is a thin film than easily spread over the substrate. This resulted in successful transfer of 2 micrometer size patterns with dots and lines shapes from the silicon substrate to PDMS substrate. Introduction The realization of stretchable, flexible electronics has been one of the most challenging processes in the field of electronics [1]. Stretchable electronics involves deposition of stretchable electronic components on stretchable substrates or stretchable materials like polymers. Stretchability development is expected to increase significantly, thus providing solutions in filed like medical and production industry. The technology can be applied in electronics that require large surface area, since it enables electronics to be spread over a curved surface and on moving parts [1], [2]. Recent advancement has resulted in large area electronic devices to become lightweight and thin, and thus allows, for instance, displays and solar cells to be placed on walls and roofs respectively. The development along this line is expected to advance with increased rollability and bendability of displays [1], [3]. As a result, large surface sensors, memory and actuators are being developed for application in intelligent surfaces. This is due to the fact that more development in large surface are electronic devices will enable them to be more elastic, making it possible to place flexible sheet device on a curved surface [3]. One of the most significant challenges in the development of stretchable flexible electronics is the fabrication of stretchable electrodes and wiring that can exhibit good electronic properties and the best mechanical robustness. Various types of high quality good conducting materials have been proposed. One of them includes 3D micro-patterning on a polydimethylsiloxane (PDMS), including metal coated net films, thin metals, single walled carbon nanotubes and grapheme films [1], [4], [5], [6]. The materials produce net- and novel-like structures that can produce excellent mechanical and conductive stretchability. Different manufacturing processes have been developed for manufacturing these microstructures in the industry. They include mechanical punching or cutting, vacuum evaporation and photolithographic patterning [7], [8]. In this study we demonstrate a simple and reliable fabrication technique used to produce a micrometre size gold patterns in PDMS substrate. The gold patterns produced through lift off technique were transferred to PDMS through peel off process that required peeling off of the layers. A permanent 2 micrometer line and dotted gold patterns was fabricated through lift off process and UV lithography. After this, the patterns were transferred to PDMS using surface self assembled monolayer (SAM) surface modification. Photolithography is an optical method used to transfer micrometer scale patterns on the substrate. The process is similar to the lithography printing. Patterns are transferred to a photoresist layer, which is a thin film than easily spread over the substrate. Literature review The motive behind the development flexible electronics is the realization of stretchable, flexible and high speed performing devices that consumes less power that can be used in the next generation of integrated circuits. Research by Kahng et al., (2006) focussed on the stretchability of silicon components applied in high speed performing electronic devices in rubber surfaces [9]. They found out that electronic devices can exhibit bendable properties if they are manufactured from a thin film on a substrate. These devices experience strains during bending below the fracture point. The devices that experience bend, flex or stretch on curved surfaces should have a full stretchability. If the elastic limit is exceeded, fracture may occur. Silicon is one of the material which offer stretchable properties. The researchers utilized single silicon ribbon on rubber substrate. They applied photolithography process to transfer patterns onto the substrate using etching process, before removing the exposed parts. They found that bonding is formed between the substrate [9]. Similar research was conducted by Locour et al. (2004), where they determining the performance of metal film in electronic circuits. They used metal and silicon to create stretchable material [10]. Stretchable electronic materials have been produced using conducting polymers like metal films on elastomeric substrate such as gold on PDMS. This application has proven to be the best option in terms of stability, conduction and tensile strength. Nevertheless, the main problem with these operations is that cracks were developed in the materials [10]. This is caused by a large thermo-mechanical difference between the materials and the strain ensuing from applied stress or after metal deposition. Further studies have focused on modelling and testing of optimized electronic systems in order to reduce in-plane strains [11], [12], [13]. There is therefore a need to research and develop techniques used to produce materials that are free of defects to be used for stretchable flexible electronic devices. Technical Progress The researcher began this work by studying different information for other researchers on the topic. Thus literature on stretchable electronics was used as a foundation for new data. These information assisted the researcher top develop research questions. Steps used to fabricate the circuit are similar to those producing flexible circuit. A number of steps used to fabricate stretchable circuit are similar to those of producing flexible circuit. For flexible circuits, the basic elements for construction are a) the polymer film that support electrical wires and electronic components, and the films may not possess adhesive layer, and b) a metal foil as a conductor through which the circuit paths are formed [14], [15]. Objectives To produce an electrode design that can exhibit minimum changes if it experience a strain. To analyse the flexibility of the flexible electronic elements Hypothesis The project is expected to produce stretchable flexible electronics. The project will determine the significance of using a simple and highly reliable technique of gold patterns on PDMS substrate the gold patterns. Methodology There are various steps involved in this practice. A stretchable flexible electronics can be made using photolithography method. In this process, light is used to define patterns in semiconductor materials that are used in electronic industry. Photolithography utilizes small semiconductor and ICS and uses the light. It involves the number of steps that are conducted using the technology. 1. The first step involves fabrication. In this process the wafer is cleaned to enhance stretchability of flexible electronics. In this process, the chemicals are used to remove particles or impurities on the wafer. Silicon dioxide is then applied on the wafer to provide a covering to the metallic impurities. 2. Another step is the coating of the wafer through application of resist using photoresist technique. It is characterised with stress-resistance measurements of materials which require the use of techniques which enhance transfer of geometrical shapes of semiconductors. In this application, a resist film of a known thickness is applied on the substrate. A positive pray coated photoresist was used. 3. The substrate is then baked to reduce the concentration of the solvent. This will produce the required a material with the required thickness. The substrate is then baked at temperature of 120o C for approximately two minutes to make it soft, before exposing it to ultra violet light for about 40 seconds in order to uncover the contact pads. 4. The last step is aligning the mask. A square glass plate with the wafer is used to transfer the material to the surface of the substrate. After the alignment has been produced, a high intensity ultra violet ray is directed on the substrate using printing projection. The method ensures that the substrate material is not damaged. The step by step fabrication of gold patterns using etch-back lift –off technique is shown in the figures below. Etch-back process Lift-off process Results All the results and the findings obtained from the study were documented. The results will form the basis of literature for future research conducted by other researchers who wish to explore the concept of improving the stretchability of flexible electronics. The final product was a 2 micrometer size patterns with dots and lines shapes on PDMS substrate. Discussion The lift-off process was used to fabricate the gold patterns due to the following reasons: a) to reduce critical isotropic metal over-etching issues, and b) to apply the same process to other metals such as copper, aluminium and gold, without application of metal etchants. Specifically, double layer lift off can be applied for reliable and uniform fabrication of patterns in a metal. A more reliable fabrication on three-inch layer was obtained by applying double layer lift off process [16]. The stretchable electronics produced through this method have features that can withstand repeated strain. These findings provide a simple cost-effective technique for producing stretchable electronics without the use of vacuum processing or organic solvents. The technology can be applied in bioelectronics and in large area electronics such as in stretchable displays and solar panels [17]. Risk analysis Any research activity has potential risks associated with project design. Therefore, the researcher conducts research in literature on experiments and similar research [18]. Failures committed by former researchers assist in mitigating risk related to the proposed design. For the current study, the risks that can be encountered in photolithography process are failure to distribute stress during the stretch and high temperatures. The risk can be mitigated by testing the operating temperature as the experiment is being conducted [18], [19]. Another risk may be caused due to omission errors related to chemicals used for cleaning wafer during the fabrication process. Chemicals may contaminate the eyes or may spill on the eyes of the researcher. Thus gloves and mask are used to protect against dangerous chemicals. In the photoresist application process, the risk may emanate from exposure to chemicals or evaporation of resist. It is desirable to use appropriate process like when applying the coat on the surface, as well as use protective gear [20]. Future work Further research is needed to find ways of reducing the spatial resolution in this technique, as well as to find out if the production of patterns together with exposure of electronic beam will enable stretchable submicron size features on PDMS. The integration technique and the stretchable materials used in this research can be used to develop other types of functional electronics that include elastic skin combined with stretchable pressure sensors or elastic stretchable actuators that can be sensitive to touch electrically. The combination of stretchable displays, sensors and actuators can be applied in the manufacturing of real, tangible displays and human friendly machine on different types of surfaces. The elastic conductor electronics produced in this study is printable and can be used to produce electronic integrated circuits that can spread on any surface, including movable parts and curved surfaces. This will increase the areas in which electronics devices can be used, such as display networks, sensors and actuators are expected to be friendly to human and can be use to improve convenience, security and safety. Conclusions This project involves the production of stretchable flexible electronics a simple and highly reliable technique of photolithography on the substrate. Stretchable electronic components can be used to manufacture artificial limbs. Enhancing stretchability of flexible electronics is one of the topics that haven studied widely. The results obtained from can be used by individuals such as military and doctors. To solve problems encountered. The main advantage for this approach include: a) reliability of fabrication on the entire wafer surface through the use of double layer lift-off process, b) reducing the contamination when transferring the patterns to PDMS using etchback process. References: [1] T. Someya. Stretchable Electronics. Stretchable Electronics. Weinheim: Wiley-VCH, 2013. [2] D. B. Mitzi. Solution Processing of Inorganic Materials. 2009. . [3] R. S. Dahiya, and V. Maurizio. Robotic Tactile Sensing Technologies and System. Dordrecht: Springer, 2013. . [4] A. Campo, and E. Arzt. Generating Micro- and Nanopatterns on Polymeric Materials. Generating Micro- and Nanopatterns on Polymeric Materials. Weinheim, Germany: Wiley-VCH, 2011. [5] H. Xinning & W. Jun. Films of Carbon Nanomaterials for Transparent Conductors. Materials; Volume 6; Issue 6; Pages 2155-2181. Multidisciplinary Digital Publishing Institute, 2013. . [6] F. Axisa, D. Brosteaux, E. De Leersnyder, F. Bossuyt, J. Vanfleteren, B. Hermans, and R. Puers. Biomedical Stretchable Sytems Using MID Based Stretchable Electronics Technology. ISBN: 9781424407873. IEEE, 2007. . [7] United States. Stretchable Form of Single Crystal Silicon for High Performance Electronics on Rubber Substrates. Washington, D.C: United States. Dept. of Energy, 2009. . [8] J. Brand, M. Kok, A. Sridhar, M. Cauwe, R. Verplancke, R. Bossuyt, J. Baets, and J. Vanfleteren. Flexible and Stretchable Electronics for Wearable Healthcare. IEEE Computer Society, 2014. [9] D. Y. Kahng J H.Q. Jiang $ J. A. Rogers, A stretchable form of single -crystal silicon for high performance electronics on rubber substrate, science, 2006, 311 [10] N. Fruehauf. Flexible Electronics 2004--Materials and Device, 2004 Technology Symposium Held April 13-16, 2004, San Francisco, California, U.S.A. Warrendale, Pa: Materials Research Society. [11] S. P., Lacour, J. Jones, Z. Suo, and S. Wagner. Design and Performance of Thin Metal Film Interconnects for Skin-Like Electronic Circuits. 2004. . [12] M. Gonzalez, F. Axisa, M. Bulcke, D. Brosteaux, B. Vandevelde, and J. Vanfleteren. "Design of Metal Interconnects for Stretchable Electronic Circuits." Microelectronics Reliability. 48.6 (2008): 825-832 [13] M. S. Shur, Electronics in Unconventional Substrates - Electrotextiles and Giant- Area Flexible Circuits: Symposium Held December 2 - 3, 2002, Boston, Massachusetts, U.S.A., [at the 2002 MRS Fall Meeting]. Warrendale, Pa: Materials Research Society, 2003. [14] F. Bossuyt, T. Vervust, F. Axisa, and J. Vanfleteren. A New Low Cost, Elastic and Conformable Electronics Technology for Soft and Stretchable Electronic Devices by Use of a Stretchable Substrate. 2009 European Microelectronics and Packaging Conference (EMPC 2009), Vols 1 and 2. IEEE, 2009. . [15] Materials Research Society. Materials and Devices for Flexible and Stretchable Electronics: April 13-17, 2009, San Francisco, Calfornia, USA. Warrendale, PA: Materials Research Society, 2010. [16] T. Takahashi. Nanomaterials Processing Toward Large-Scale Flexible/Stretchable Electronics. 2013. . [17] N.Maluf, and W. Kirt An Introduction to Microelectromechanical Systems Engineering. Norwood: Artech House, 2004. . [18]. Li, T., Huang, Z., Suo, Z., Lacour, S. P., and Wagner, S. Stretchability of Thin Metal Films on Elastomer Substrates. 2004. . G., [19] Cao, & Y. Wang, Nanostructures & nanomaterials: Synthesis, properties, and applications. New Jersey: World Scientific, 2011. [20] International Microelectronics Conference. Proceedings of the 6th International Microelectronics Conference, May 30-June 1, 1990, Tokyo, Japan. Tokyo, Japan: Society for Hybrid Microelectronics, 1990. Read More