Microfabrication of a MEMS piezoresistive flow sensor - materials and processes

Microelectromechanical systems (MEMS) based artificial sensory hairs for flow sensing have been widely explored, but the processes involved in their fabrication are lithography intensive, making the process quite expensive and cumbersome. Most of these devices are also based on silicon MEMS, which makes the fabrication of out-of plane 3D flow sensors very challenging. This thesis aims to develop new fabrication technologies based on Polymer MEMS, with minimum dependence on lithography for the fabrication of piezoresistive 3D out-of-plane artificial sensory hairs for sensing of air flow. Moreover, the fabrication of a flexible sensor array is proposed and new materials are also explored for the sensing application. Soft lithography based approaches are first investigated for the fabrication of an all elastomer device that is tested in a bench top wind tunnel. Micromolding technologies allow for the mass fabrication of microstructures using a single, reusable mold master that is fabricated by SU-8 photolithography, reducing the need for repetitive processing. Polydimethylsiloxane (PDMS) is used as the device material and sputter deposited gold is used as both the piezoresistive as well as the electrode material for collection of device response. The fabrication results of PDMS to PDMS metal transfer micromolding (MTM) are shown and the limitations of the process are also discussed. A dissolving mold metal transfer micromolding process is then proposed and developed, which overcomes the limitations of the conventional MTM process pertinent to the present application. Testing results of devices fabricated using the dissolving mold process are discussed with emphasis on the role of micro-cr  acking as one failure mode in elastomeric devices with thin film metal electrodes. Finally, a laser microfabrication based approach using thin film Kapton as the device material and an electrically conductive carbon-black elastomer composite as the piezoresistor is proposed and demonstrated. Laminated sheets of thick and thin Kapton form the flexible substrate on which the conductive elastomer piezoresistors are stencil printed. Excimer laser ablation is used to make the micro-stencil as well as to release the Kapton cantilevers. The fluid-structure interaction is improved by the deposition of a thin film of silicon dioxide, which produces a stress-gradient induced curvature, strongly enhancing the device sensitivity. This new approach also enables the fabrication of backside interconnects, thereby addressing the commonly observed problem of flow intrusion while using conventional interconnection technologies like wire-bonding. Devices with varying dimensions of the sensing element are fabricated and the results presented, with smallest devices having a width of 400 microns and a length of 1.5 mm with flow sensitivities as high as 60 Ohms/m/s. Recommendations are also proposed for further optimization of the device.M.S.Committee Chair: Allen, Mark; Committee Member: Allen, Sue Ann Bidstrup; Committee Member: Wong, C.P

Understanding the impact of polymer self-organization on the microstructure and charge transport in poly(3-hexylthiophene)

Conjugated polymers represent the next generation of conducting materials that will enable technological devices incorporating thin film transistors, photovoltaic cells etc., in a cost-effective roll-to-roll manner. Given the importance of microstructure on charge transport, ordered self-assembly in polymeric semiconductors assumes paramount relevance. This thesis thus focuses on a fundamental investigation of the correlations between the morphology and microstructure of the first high mobility solution processable semiconducting polymer, poly(3-hexylthiophene)(P3HT), and its corresponding charge transport properties. The evolution of polymer chain conformations is first studied, leading up to the formation of the conducting channel. An intermediate lyotropic liquid crystalline phase is identified, characterized by anisotropic ordering of the polymer chains. Methods for tuning the microstructure of P3HT thin films are also discussed, with an emphasis on understanding the role of molecular parameters, such as regioregularity and process parameters such as the film formation method. An ultrasound based technique for inducing the formation of ordered π-stacked molecular aggregates is also introduced. The results presented here not only provide understanding of microstructure-charge transport correlations, but also the very process of film formation in solution processable organic semiconductors, which could in turn hold the key to approaching the mobility benchmark represented by single crystals.PhDCommittee Chair: Elsa Reichmanis; Committee Member: Bernard Kippelen; Committee Member: Carson Meredith; Committee Member: David Collard; Committee Member: Dennis Hes

Tunable Crystallinity in Regioregular Poly(3‐Hexylthiophene) Thin Films and Its Impact on Field Effect Mobility

2011 Ziegler Award Winner, presented as a keynote address at the 2011 School of Chemical and Biomolecular Engineering Fourth Year Colloquium, Wednesday October 5, 2011 from 4-5 pm in room G011 of the Molecular Science and Engineering Building at Georgia Institute of Technology.The Waldemar T. Ziegler Awards were established by the family and friends of the late Dr. Waldemar T. Ziegler to honor his lifelong commitment to academic excellence and to research. Dr. Ziegler was a member of the faculty of the School of Chemical Engineering at Georgia Tech from 1946 until his retirement in 1978, when he was named Regents’ Professor Emeritus. He died in 1996, leaving behind a legacy of outstanding research in the fields of cryogenics and thermodynamics. Dr. Ziegler was instrumental in establishing both the School’s and Georgia Tech’s reputation for outstanding research. Currently, two individual Ziegler Awards are presented annually to graduate students.Avishek Aiyar was born and raised in Calcutta, West Bengal, India. In 2005, he received his B.Tech in chemical engineering from the AC College of Technology, Anna University, Chennai. Upon completing his undergraduate studies, Avishek enrolled in the graduate program in chemical engineering at Georgia Tech. He received his M.S. degree under the direction of Dr. Mark G. Allen, during which time Avishek’s research focused on microfabrication of polymer MEMS-based flow sensors for detection of trapped vorticity. In summer 2008, he joined Dr. Elsa Reichmanis’s research group to pursue his Ph.D. Avishek’s current research focuses on understanding fundamental correlations between microstructure in conjugated polymer thin films and macroscopic charge transport. Avishek Aiyar received the Best Paper Award.Runtime: 26:21 minutesThe properties of poly(alkylthiophenes) in solution are found to have a profound impact on the self-assembly process and thus the microstructural and electrical properties of the resultant thin films. Ordered supramolecular precursors can be formed in regioregular poly(3- hexylthiophene) (P3HT) solutions through the application of low intensity ultrasound. These precursors survive the casting process, resulting in a dramatic increase in the degree of crystallinity of the thin films obtained by spin coating. The crystallinity of the films is tunable, with a continuous evolution of mesoscale structures observed as a function of ultrasonic irradiation time. The photophysical properties of P3HT in solution as well in the solid state suggest that the application of ultrasound leads to a π stacking induced molecular aggregation resulting in field effect mobilities as high as 0.03 cm2V-1s-1. A multiphase morphology, comprising short quasi-ordered and larger, ordered nanofibrils embedded in a disordered amorphous phase is formed as a result of irradiation for at least 1 minute. We identify two distinct regions of charge transport characterized by an initial sharp increase in the field effect mobility by two orders of magnitude due to an increase in crystallinity up to the percolation limit, followed by a gradual saturation where the mobility becomes independent of the thin film microstructure.Dow Chemical Compan

Regioregularity and intrachain ordering: Impact on the nanostructure and charge transport in two-dimensional assemblies of poly(3-hexylthiophene)

The properties of supramolecular assemblies in conjugated macromolecular systems are strongly dependent on single chain effects. We report that differences in regioregularity (RR) of side chain attachment in poly(3-hexylthiophene) (P3HT) as small as ca. 4% are sufficient to induce dramatic changes in the electronic and morphological properties of the material. Casting the electronic absorption spectra in the framework of Spano's model reveals that the conjugation length is surprisingly sensitive to RR, with differences in free exciton bandwidth between the two P3HT samples approaching 73 meV. The enhanced main chain planarization motivates a concomitant increase in nanofibril width as well as crystallinity observed in thin films of the higher RR variant. This observation correlates well with the field effect mobilities that are attenuated by 1 to 2 orders of magnitude in the lower RR polymer film. We suggest that the increased intrachain order coupled with a reduced fraction of grain boundaries in the higher RR film is responsible for the reported differences. © 2012 American Chemical Society.

Regioregularity and Intrachain Ordering: Impact on the Nanostructure and Charge Transport in Two-Dimensional Assemblies of Poly(3-hexylthiophene)

The properties of supramolecular assemblies in conjugated macromolecular systems are strongly dependent on single chain effects. We report that differences in regioregularity (RR) of side chain attachment in poly­(3-hexylthiophene) (P3HT) as small as ca. 4% are sufficient to induce dramatic changes in the electronic and morphological properties of the material. Casting the electronic absorption spectra in the framework of Spano’s model reveals that the conjugation length is surprisingly sensitive to RR, with differences in free exciton bandwidth between the two P3HT samples approaching 73 meV. The enhanced main chain planarization motivates a concomitant increase in nanofibril width as well as crystallinity observed in thin films of the higher RR variant. This observation correlates well with the field effect mobilities that are attenuated by 1 to 2 orders of magnitude in the lower RR polymer film. We suggest that the increased intrachain order coupled with a reduced fraction of grain boundaries in the higher RR film is responsible for the reported differences

Ultrasound-Induced Ordering in Poly(3-hexylthiophene): Role of Molecular and Process Parameters on Morphology and Charge Transport

Facile methods for controlling the microstructure of polymeric semiconductors are critical to the success of large area flexible electronics. Here we explore ultrasonic irradiation of solutions of poly(3-hexylthiophene) (P3HT) as a simple route to creating ordered molecular aggregates that result in a one to two order of magnitude improvement in field effect mobility. A detailed investigation of the ultrasound induced phenomenon, including the role of solvent, polymer regioregularity (RR) and film deposition method, is conducted. Absorption spectroscopy reveals that the development of low energy vibronic features is dependent on both the regioregularity as well as the solvent, with the latter especially influential on the intensity and shape of the band. Use of either higher regioregular polymer or ultrasonic irradiation of lower regioregular polymer solutions results in high field effect mobilities that are nearly independent of the dynamics of the film formation process. Surprisingly, no distinct correlation between thin-film morphology and macroscopic charge transport could be ascertained. The relationships between molecular and process parameters are very subtle: modulation of one effects changes in the others, which in turn impact charge transport on the macroscale. Our results provide insight into the degree of control that is required for the development of reproducible, robust materials and processes for advanced flexible electronics based on polymeric materials. © 2013 American Chemical Society.

Ultrasound-Induced Ordering in Poly(3-hexylthiophene): Role of Molecular and Process Parameters on Morphology and Charge Transport

Facile methods for controlling the microstructure of polymeric semiconductors are critical to the success of large area flexible electronics. Here we explore ultrasonic irradiation of solutions of poly­(3-hexylthiophene) (P3HT) as a simple route to creating ordered molecular aggregates that result in a one to two order of magnitude improvement in field effect mobility. A detailed investigation of the ultrasound induced phenomenon, including the role of solvent, polymer regioregularity (RR) and film deposition method, is conducted. Absorption spectroscopy reveals that the development of low energy vibronic features is dependent on both the regioregularity as well as the solvent, with the latter especially influential on the intensity and shape of the band. Use of either higher regioregular polymer or ultrasonic irradiation of lower regioregular polymer solutions results in high field effect mobilities that are nearly independent of the dynamics of the film formation process. Surprisingly, no distinct correlation between thin-film morphology and macroscopic charge transport could be ascertained. The relationships between molecular and process parameters are very subtle: modulation of one effects changes in the others, which in turn impact charge transport on the macroscale. Our results provide insight into the degree of control that is required for the development of reproducible, robust materials and processes for advanced flexible electronics based on polymeric materials