Hot Embossing to Fabricate Parylene-Based Microstructures and Its Impact on the Material Properties

This study aims to establish and optimize a process for the fabrication of 3D microstructures of the biocompatible polymer Parylene C using hot embossing techniques. The different process parameters such as embossing temperature, embossing force, demolding temperature and speed, and the usage of a release agent were optimized, utilizing adhesive micropillars as a use case. To enhance compatibility with conventional semiconductor fabrication techniques, hot embossing of Parylene C was adapted from conventional stainless steel substrates to silicon chip platforms. Furthermore, this adaptation included an investigation of the effects of the hot embossing process on metal layers embedded in the Parylene C, ensuring compatibility with the ultra-thin Parylene printed circuit board (PCB) demonstrated previously. To evaluate the produced microstructures, a combination of characterization methods was employed, including light microscopy (LM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). These methods provided comprehensive insights into the morphological, chemical, and structural properties of the embossed Parylene C. Considering the improved results compared to existing patterning techniques for Parylene C like plasma etching or laser ablation, the developed hot embossing approach yields a superior structural integrity, characterized by increased feature resolution and enhanced sidewall smoothness. These advancements render the method particularly suitable for diverse applications, including but not limited to, sensor optical components, adhesive interfaces for medical wearables, and microfluidic systems

Solid-State pH Sensors for Mine Water Monitoring

Introduction The monitoring of water quality parameters such as pH, redox potential, and conductivity is essential in order to investigate the variety of hydrochemical processes occurring in aqueous environments. For a better understanding of these processes, measurements with high temporal resolution are needed, which cannot be achieved with manual sampling procedures. Hence, alternative technical analysis methods, such as sensors, are used to monitor relevant parameters in-situ. Unfortunately conventional probes often experience harsh or detrimental conditions during on-site deployment. All-solid-state potentiometric sensors have been proposed as robust alternatives for demanding environments that deliver results with sufficient accuracy, at relatively low-cost, with experimental and instrumental simplicity [1]. This work focuses on the development of a solid-state pH sensor for the monitoring of mine water and acid mine drainage (AMD). AMD is the consequence of uncontrolled sulfidic mineral oxidation, typically as a result of ore or coal mining practices. The resulting outflow of acidic water, which can contain high concentrations of metal ions, is potentially disastrous to aquatic life and also poses a great threat to water supplies [2]. Our aim is to develop a robust and cost-effective sensor that enables in-situ AMD monitoring in mines and surrounding waterways, as well as pit lakes formed as the result of flooded open-pit mines. With increased spatial and temporal resolution in comparison to manual sampling, these sensors will help to predict hazardous conditions and increase the understanding of the complex underlying geochemical mechanisms. Methods The proposed sensor consists of a pH sensitive iridium oxide (IrOx) layer in combination with a solid-state reference electrode. During sensor fabrication structured titanium and gold electrodes were deposited on oxidised silicon substrates using standard cleanroom techniques. IrOx was subsequently electrochemically coated on these electrodes via cyclic voltammetry. The deposition solution was prepared according to the protocol published by Marzouk [3]. In a three electrode configuration (reference: 3 M KCl Ag/AgCl, counter: platinum gauze) the prepared metal electrodes functioned as the working electrodes. Homogenous IrOx layers with a thickness around 80 nm were obtained after 100 cycles at 0.1 V/s between -0.5 V and 0.85 V or -0.7 V and 0.9 V for the gold and titanium electrodes respectively (see Figures 1 and 2). Several approaches for the construction of solid state reference electrodes (SSREs) have been presented in literature [4]. In this work, salt-saturated polymers were deposited on Ag/AgCl layers, similar to [5]. The selected polymers were either unsaturated polyester resins or UV-curing epoxy-based resins. These resins were mixed with up to 70 ma% of finely ground KCl. The resulting slurry was dropped on Ag/AgCl electrodes and cured to form the SSREs. Data collection was performed with EZO-circuits from Atlas Scientific in connection with a RaspberryPi or an ESP32-based microcontroller using I2C or serial protocols. The fabricated IrOx electrodes were characterised regarding their pH sensing performance. Their sensitivity was evaluated by recording the open circuit potential (OCP) vs. a commercial reference electrode (3 M KCl, Ag/AgCl) during a titration of a Briton-Robinson buffer with 0.5 M KOH at 25°C. Drift rates were measured in standard buffer solutions. In order to determine the SSRE’s electrochemical potential stability, OCP was recorded versus a conventional reference electrode in KCl solutions of varying concentration at 25°C. Results and Conclusions As shown in Figure 3, the IrOx electrodes exhibit a super-Nernstian pH sensitivity of around -73 mV/pH between pH 2 to 11.5. These values are comparable to published results [3]. Electrodes from several batches fabricated on different days show high reproducibility with respect to sensitivity and intercept, as indicated by the closely overlaying data points in Figure 3. These results suggest a high robustness of the coating process. The drift rate of the IrOx electrodes is less than 1 mV/h which is sufficient for the intended application. The functionality of the SSRE can be deducted from the data given in Figure 4: An uncoated Ag/AgCl electrode exhibits Nernstian behavior to changes in chloride ion concentration whereas the fabricated SSRE with a polymer coating shows a stable potential. The combination of SSRE and IrOx electrode results in a potentiometric pH sensor whose output changes with pH as shown in Figure 5. Outlook Future work will focus on improving the sensing characteristics as well as measurements in AMD samples. Eventually on-site deployment of the sensors is planned. Additionally, first experiments to transfer the electrodes to flexible and chemically inert substrates for ultimate robustness and potential applications in smart robotic skins have been conducted (see Figure 6). Acknowledgments This research has been funded by the European Social Fund (ESF) and the Free State of Saxony. (ARIDuA , project number 100310491). References: [1] M. Cuartero, All-solid-state potentiometric sensors: A new wave for in situ aquatic research, Current Opinion in Electrochemistry 10, 98-106 (2018); doi: 10.1016/j.coelec.2018.04.004 [2] D.K. Nordstrom, Hydrogeochemical processes governing the origin, transport and fate of major and trace elements from mine wastes and mineralized rock to surface waters, Applied Geochemistry 26, 1777-1791 (2011); doi: 10.1016/j.apgeochem.2011.06.002 [3] S.A.M. Marzouk, Improved Electrodeposited Iridium Oxide pH Sensor Fabricated on Etched Titanium Substrates, Analytic Chemistry 75, 1258-1266 (2003); doi: 10.1021/ac0261404 [4] U. Guth, Solid-state reference electrodes for potentiometric sensors, Journal Solid State Electrochemistry 13, 27-39 (2008); doi: 10.1007/s10008-008-0574-7 [5] F. Güth, Electrochemical Sensors Based on Printed Circuit Board Technologies, Procedia Engineering 168, 452-455 (2016); doi: 10.1016/j.proeng.2016.11.543 Figure 1 <jats:p /

Low-Temperature Parylene-Based Adhesive Bonding Technology for 150 and 200 mm Wafers for Fully Biocompatible and Highly Reliable Microsystems

Wafer bonding is a crucial process for fabricating microsystems. Within this study, the polymer parylene was used to establish a low-temperature adhesive wafer bonding process for wafers of 150 and 200 mm diameters. The bonding process was investigated for silicon and glass wafers with different additional coatings including silicon dioxide, silicon nitride, aluminum, and parylene C. Important process parameters such as bonding temperature and time were also investigated and the parylene adhesive was analyzed in detail with respect to its dimensions and type. The performance of the parylene bonding was characterized in different aspects, including mechanical tests, cross-sectional scanning electron microscopy, infrared light transmission, and different hermeticity tests. The reliability of the parylene bonded compounds was also investigated with respect to constant loading, mechanical shocking, and thermal cycling. As a result, the parylene bonding is feasible with various materials and shows high tensile and shear strengths of up to 35 MPa and 80 MPa, respectively. Hermeticity was excellent, with a helium leakage rate lower than 10‒7 mbar∙l s−1. The parylene bonded compounds were proven to feature high reliability. Finally, application of the superior properties of the parylene bonding was demonstrated with respect to the fabrication of different three-dimensional structures.</jats:p

(Invited) Parylene C Based Adhesive Bonding on 6” and 8” Wafer Level for the Realization of Highly Reliable and Fully Biocompatible Microsystems

For the realization of advanced bio-microsystems for medical applications such as implants, fabrication processes require the usage of biocompatible materials only. Especially for the encapsulation and hermetic sealing, e. g. of microfluidic structures, a biocompatible wafer bonding process is necessary. Additionally, the ongoing integration of new and temperature sensitive materials in micro systems, such as biomolecules, requires bonding processes with low bonding temperatures only. However, when choosing the wafer bonding process, additional criteria are a high reliability, since the bonding is critical for the operation of the chip, as well as the applicability on 6” and 8” wafer level in order to fabricate a high number of devices at low costs. In order to fulfill all these requirements, we propose an adhesive bonding process, which uses Parylene C as intermediate layer. Parylene C is a thermoplastic polymer, which is ISO 10993 certified as biocompatible and biostable, chemically inert against all common acids, bases and solvents, as well as a low permeable and optical transparent. In addition and in contrast to other established bond adhesives, such as SU8, the Parylene layers are deposited by CVD, and offer a very high three dimensional conformity as well as a very high homogeneity without any pinholes. Furthermore, due to its chemical inertness, Paryleneis compatible with a variety of established microtechnologies. Previous studies prove that adhesive bonding using Parylene C is feasible. However, these works are limited to 4” wafer level and the bonding provides only low tensile strengths, e. g. 10 MPa. [1-7] Our presented Parylene based adhesive bonding technology uses a 0.5 µm to 10 µm Parylene C layer, which is deposited on 6” and 8” wafers and can be patterned optionally by an oxygen plasma without leaving residues. A subsequent thermal and adhesive bonding process at temperatures of 280°C to 300°C as well as moderate wafer contact pressures are performed to successfully bond the wafers. Within our study, different materials combinations for the bond interface, e. g. silicon - silicon, silicon - silicon dioxide, silicon – glass, silicon – silicon nitride, and silicon – aluminum were investigated. Doing so, only one wafer was coated with Parylene. Additionally, bonding processes were performed with two Parylene coated wafers, and hence, a Parylene - Parylene bond interface. Since almost all materials, established in microsystem technologies can be coated with Parylene, this enables the usage of this bonding process for a high variety of material combinations and particularly materials, which were not investigated by our study. The focus of the experiments was the investigation and optimization of a feasible parameter range in respect of the bonding time, bonding temperature, wafer contact pressure, and Parylene thickness. The characterization of the bonding process was performed using Parylene test structures of squared geometry and different width. The bonding process was analyzed by IR imaging to determine voids and defects. Afterwards the wafers were diced into chips, centering the Parylene frame. The investigation of tensile and shear strengths proved a high reliability of the bonding but also a strong dependency on the bonding parameters. The tensile strength reached up to 35 MPa was improved by a factor of 3 compared to previous studies, the shear strength reached up to 80 MPa Furthermore, the impact of thermal and mechanical shocks as well as long-time load on the tensile strength was investigated. Finally, cross-sections of the bonded chips were analyzed by SEM. The results prove that the Parylene frame structure was still intact after bonding and not flattened due to bonding. Furthermore, the hermetic properties of the Parylene bonds were investigated. Doing so, the Helium leakage rate was determined to &lt; 1∙10-7 mbar ∙ l/s. The study was completed by the investigation of the bonding process on three dimensional structures, which refers to bonding over patterned metal but also bonding of cavities without previous pattern of the Parylene. Finally, after successfully bonding the test frames, the established bonding process was applied on a real microsystem. References: [1] M. Kurihara, et al., IEEE MEMS 2012 (Paris, France) pp 196-9 [2] Y.-C. Yen, et al., IEEE MEMS 2012 (Paris, France) pp 381-384 [3] Q. Shu, et al., International Conference on Solid-State and Integrated-Circuit Technology 2009 (Beijing, China) [4] D. P. Poenar, et al., Sens. Actuators, A 139, 2007, 162–71 [5] A. T. Ciftlik et al., J. Micromech. Microeng. 21, 2011, 35011 [6] D. Ziegler, et al., J. Microelectromech. Syst. 15 (6), 2006, 1477–82 [7] H.-S. Noh, et al., J. Microelectromech. Syst. 14, 2004, 625–31 </jats:p

(Invited) Parylene C Based Adhesive Bonding on 6” and 8” Wafer Level for the Realization of Highly Reliable and Fully Biocompatible Microsystems

The ongoing miniaturization and implementation of new functionalities into micro-electro-mechanical systems (MEMS) demand the development and application of new wafer bonding and encapsulation technologies with a high performance. Requirements are low process temperatures, high mechanical strengths of the bonded interface, as well as the applicability on large wafer sizes. Within the presented study, the polymer Parylene C was used as an adhesive for the bonding of 6” and 8” wafers. Doing so, the material combinations of the wafers, the Parylene thicknesses and geometries as well as the bonding parameters were varied. The properties of the wafer compounds were characterized with various methods, including mechanical tests, infrared imaging, cross-sections, hermeticity tests and the investigation of the thermal reliability. Using the Parylene C bonding process, tensile strengths of up to 35 MPa, and shear strengths of up to 80 MPa were realized. The determined helium leakage rate was lower than 1 ∙ 10-7 mbar ∙ l/s and the thermal reliability was verified to be excellent.</jats:p

Investigation of biocompatible Parylene as triboelectric layer for wearable energy harvesting

Abstract Triboelectric nanogenerators (TENGs) are energy converters or energy harvesters that convert mechanical motion into electrical energy on the basis of their material properties. A particular advantage of the TENG is its ability to convert small, low-frequency and random mechanical movements that are relevant for body movements and wearable applications. Within the presented study, different Parylene types were analysed as the dielectric material in TENG and found to be promising with respect to providing high output voltages and powers, respectively. Besides the verification of the usability of Parylene for TENG and its superior triboelectric properties, also significant differences were found between the Parylene types.</jats:p

Hot Embossing to Fabricate Parylene-Based Microstructures and Its Impact on the Material Properties

This study aims to establish and optimize a process for the fabrication of 3D microstructures of the biocompatible polymer Parylene C using hot embossing techniques. The different process parameters such as embossing temperature, embossing force, demolding temperature and speed, and the usage of a release agent were optimized, utilizing adhesive micropillars as a use case. To enhance compatibility with conventional semiconductor fabrication techniques, hot embossing of Parylene C was adapted from conventional stainless steel substrates to silicon chip platforms. Furthermore, this adaptation included an investigation of the effects of the hot embossing process on metal layers embedded in the Parylene C, ensuring compatibility with the ultra-thin Parylene printed circuit board (PCB) demonstrated previously. To evaluate the produced microstructures, a combination of characterization methods was employed, including light microscopy (LM) and scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Fourier-transform infrared spectroscopy (FTIR). These methods provided comprehensive insights into the morphological, chemical, and structural properties of the embossed Parylene C. Considering the improved results compared to existing patterning techniques for Parylene C like plasma etching or laser ablation, the developed hot embossing approach yields a superior structural integrity, characterized by increased feature resolution and enhanced sidewall smoothness. These advancements render the method particularly suitable for diverse applications, including but not limited to, sensor optical components, adhesive interfaces for medical wearables, and microfluidic systems

Paradigm Changing Integration Technology for the Production of Flexible Electronics by Transferring Structures, Dies and Electrical Components from Rigid to Flexible Substrates

Emerging trends like the Internet of Things require an increasing number of different sensors, actuators and electronic devices. To enable new applications, such as wearables and electronic skins, flexible sensor technologies are required. However, established technologies for the fabrication of sensors and actuators, as well as the related packaging, are based on rigid substrates, i.e., silicon wafer substrates and printed circuit boards (PCB). Moreover, most of the flexible substrates investigated until now are not compatible with the aforementioned fabrication technologies on wafers due to their lack of chemical inertness and handling issues. In this presented paper, we demonstrate a conceptually new approach to transfer structures, dies, and electronic components to a flexible substrate by lift-off. The structures to be transferred, including the related electrical contacts and packaging, are fabricated on a rigid carrier substrate, coated with the flexible substrate and finally lifted off from the carrier. The benefits of this approach are the combined advantages of using established semiconductor and microsystem fabrication technologies as well as packaging technologies, such as high precision and miniaturization, as well as a variety of available materials and processes together with those of flexible substrates, such as a geometry adaptivity, lightweight structures and low costs